Beyond Conventional NHCs
-abnormal NHC-  

Sep. 29, 2012!
Kumiko Yamamoto	Contents	
1. introduction	

13

shape of the C NMR signal, as well as the calculated chemical shift (32) for the C5 carbon of
the free aNHC 9 (205 ppm), suggested that the
new compound was the aNHC lithium 7. Similarcomplexation has previously been observed forother singlet carbenes such as small NHCs (35)and bis(diisopropylamino)cyclopropenylidene(36, 37); in both cases, coordination of the lithium
cation was apparent from the broadening and theupfield shift of the carbene 13C NMR resonance.
Subsequently, we sought to sequester the lithium
cation through addition of excess [12]crown-4 to adiethylether solution of 7 (where X is Br). This
treatment induced a clean rearrangement to generate 8, which was isolated in 45% yield. This
product formally results from the deprotonation of
an isopropyl substituent of the Dip group by thecarbene center of the aNHC lithium adduct 7,
followed by nucleophilic addition of the resulting
benzyl anion to C2. However, calculations indicated that the rearrangement of 9 into 8 is
exothermic by only 6.1 kcal mol–1 and involves
an activation barrier of 20.3 kcal mol–1 for the
proton transfer. Therefore, we hypothesized thatthe observed rearrangement was catalyzed by a

2. N-heterocyclic carbenes (NHCs)	

first metal-NHC complex	
first free, isolable NHC	NHCs as ligands for transition metals	
Fig. 3. Molecular views
NHCs as organocatalysts	
(50% thermal ellipsoids

component of the (crown)LiBr system, and thatthe formation of 8 did not imply that the free
aNHC 9 was too reactive to be isolated.
Sodium and potassium bases have provenmore appropriate than lithium bases in generating
free carbenes (35–37), because the corresponding
carbon-heavy alkali metal bonds are more labile,which favors precipitation of the salt. When the
deprotonation of imidazolium 6 (HX•X–, where
X is Cl or Br) was performed with two equivalents (34) of potassium hexamethyldisilazide(KHMDS) in tetrahydrofuran, a clean reaction
occurred, with the 13C NMR spectrum of the resulting product showing a very sharp signal at
201.9 ppm. After the products were worked up,
the free aNHC 9 was isolated as a green powder(480 mg, 68% yield), and single crystals wereobtained by recrystallization from a dry hexane
solution at –78°C (Fig. 3, right).
In the solid state, both the free aNHC 9 andthe imidazolium salt 6 (Br–) feature a fully planarring (maximum deviation for N1–C2–N3–C4–C5–
C21–C31–C41–C53 was 1.9 and 6.3 pm for 9 and6, respectively), confirming the delocalization of
the p system. This electronic structure is corrobo-

In the solid state, both the free aNH
the imidazolium salt 6 (Br–) feature a furing (maximum deviation for N1–C2–N3
C21–C31–C41–C53 was 1.9 and 6.3 pm
6, respectively), confirming the delocali
the p system. This electronic structure is1	

and double bonds. The carbene bondangle
N1– views
Fig. 3.
Molecular
C5–C4 for 9 [101.03 T 17°] is morethan
(50% acutethermalellipsoids
the corresponding angle in the cationicprecursor
are shown) of imidazolium–
6 [108.0 T 3°]. This feature is bromide
consistent
6 (Brwith) (left) andincreased s character of the s lone-pair
onthe solid
aNHC 9orbital(right) inthe carbene atom in 9 as comparedwith
C–H+H atoms
state(fortheclarity,
bonding orbital in 6. A similararerelationshipisomitted, except
for+the ringhydrogen). Bondobserved in NHCs and their NHC(H) precursors
lengths and angles for(3, 4).
–
are kcalas follows:
Calculations predict aNHC 96to(Brbe )14.1
C5–N1,
1.368
mol–1 less stable than its isomericnormalNHCT 4 Å;
N1–C2,
1.334
with the phenyl group bonded to C5 instead ofT 4 Å;
C2–N3,occupied
1.363 T 4 Å;
C2. Figure 4 shows the two highest
1.408 T 4 Å;molecular orbitals (HOMOs) ofN3–C4,
9. The HOMO
C4–C5,(–4.403 eV) is a s-type lone-pairorbital1.355
at C5;T 5 Å;
N1–C5–C4,
108.0 T 3°;
the HOMO-1 (–4.879 eV) is a C5–C4
p-bonding
C5–C4–N3, 106.0 T 3°;
orbital, which exhibits antibonding conjugation
C4–N3–C2, 108.8 T 3°;
with the p orbital of the phenyl substituent at C4.
N3–C2–N1, 106.9 T 3°;
These molecular orbitals are muchhigher in110.4
en- T 3°.
C2–N1–C5,
ergy than those of the isomeric NHC
Bond(–5.000
lengths andand angles for 9 are as follows: C5a–N1a, 1.417 T 2 Å; N1a–C2a, 1.357 T 2 Å; C2a––5.279 eV, respectively), whichN1a–C5a–C4a,
indicates that101.03 T 17°; C5a–C4a–N3a, 111.01 T 16°; C4a–N3a–C2a, 107.97 T 15°; N3aFig. 4. Plot of the calculated two highest-lying
are shown) of imidazolium–
occupied orbitals HOMO
bromide 6 (Br ) (left) and(left) andHOMO-1(right)
aNHC 9 (right) in the solidof the aNHC 9.
state (for clarity, H atoms
are omitted, except for
Published on Web 04/06/2004
the ring hydrogen). Bond
lengths and angles for–
6 (Br ) are as follows:
Structure and Reactivity of “Unusual” N-Heterocyclic Carbene (NHC)
C5–N1, 1.368 T 4 Å;Palladium Complexes Synthesized from Imidazolium Salts
N1–C2, 1.334 T 4 Å;
C2–N3, 1.363 T 4 Å;Hélène Lebel,*,‡ Marc K. Janes,‡ André B. Charette,‡ and Steven P. Nolan§
N3–C4, 1.408 T 4 Å;Département de chimie, UniVersité de Montréal, C.P. 6128, Succursale Centre-Ville,
C4–C5, 1.355 T 5 Å;Montréal, Québec, Canada, H3C 3J7, and Department of Chemistry, UniVersity of New Orleans,
N1–C5–C4, 108.0 T 3°;
New Orleans, Louisiana 70148View Online
C5–C4–N3, 106.0 T 3°;
C4–N3–C2, 108.8 T 3°;
Received January 14, 2004; E-mail: lebelhe@chimie.umontreal.ca
N3–C2–N1, 106.9 T 3°;
For 6b: 1H NMR (CDCl3, 298 K): d 9.09 (s, 1H, NCHN), 8.51 (d, 3JHH
4.6 Hz, 1H, py-H), 7.73 (dt, 3JHH 7.7, 4JHH 1.8 Hz , 1H, py-H), 7.58 (d, 3JHH
C2–N1–C5, 110.4 T 3°.
Recently,1.345
a numberof 7.8
newHz,
transition
an N-Heterocyclic
Os T 2 Å; C2a–N3a,Vol.processes
26,
No.(s,26,
1H,Organometallics,py-H),
7.53(s, 1H, imid-H),
1H, 2007
imid-H),6557
7.27 (dd, 3JHH
Bond lengths and angles forCoordination
9 are as follows:ofC5a–N1a,
1.417 T 2 Å;Carbene
N1a–C2a,to
1.357
T 2 Å; N3a–C4a,
1.412
T 3metal-catalyzed
Å;
C4a–C5a,
1.383
T 37.32
Å;3J
3
23 OCTOBER 2009 VOL 326 SCIENCE w
1H,558py-H),
5.49(s, 4H, CH
havebeen developed
andoptimized
throughhigh-throughput
and
HH 4.8 Hz,2), 4.19 (t, JHH 7.5 Hz, 2H, BuN1a–C5a–C4a, 101.03 T 17°; C5a–C4a–N3a, 111.01 T 16°; C4a–N3a–C2a, 107.97 T 15°;
N3a–C2a–N1a,106.25
T7.7,
16°;
C2a–N1a–C5a,113.72
T 15°.
1H),
1.89–1.81(m,
2H,
Bu-H),
1.39–1.30(m,
2H,Bu-H),
0.93(t, 3JHH 7.5
Chart 1
combinatorial methods. Such strategies13are1 particularly efficientHz, 3H, Bu-H). C{ H} NMR (CDCl3, 298 K): d 152.25 (Cpy), 149.77
for the screening of a large
number of transition metal-ligand
Fig. 4. Plot of the cal(Cpy), 137.86 (Cpy), 136.37 (NCN), 124.06 (Cpy), 123.80 (Cpy), 122.93
combinations under various
setsof reaction
culated two highest-lying), 121.75(Cimid),conditions.
54.00 (CH2),However,
49.93 (CBu), 31.81 (CBu), 19.34 (CBu),(Cimidunder these in situ conditions,13.30 (Ctheoccupied orbitals HOMO
Bu). exact nature of the catalytic
1H NMR (CDCl , 298 K): d 8.72 (s, 1H, NCHN), 8.23 (d, 1H,
For 7a:3 assumption of(left) andHOMO-1(right)
species is not always precisely
known.
Indeed, the33
5.5 Hz,ligandpy-H), 7.37–7.14(m, 32H,
py-H,of the aNHC 9.
a normal mode of bindingJHH
betweenand transition
metal
is Ph-H), 6.07 (t, JHH 6.3 Hz,py-H), 5.17 (s, 1H, imid-H), 4.70 (s, 2H, CH2), 4.25 (septet, 1H, 3JHH 6.5
most often assumed and maybemisleading.
3
2
Hz, CH), 1.19 (d, 6H, JHH 6.5 Hz, CH3), 210.83 (dt, JPH 19.6, 3JHH 4.9
3J
During the course of ouron 2ligands
to Hz, Ir–H). 13C{1H} NMR
JPH 18.6,suitable
Hz,studies
Ir-H), focusing
221.49 (dt,
HH 4.92 ), 141.07 (t, J
d 161.78 (Cpy),processes,
153.10 (Cinduce enantioselectivity (CDClin transition
metal-catalyzed
3, 298 K):
py
PC 7.1 Hz,
C),
137.04(C),
134.93(t,J=
26.3
Hz, CPh), 133.55 (t, JPC 6.0 Hz,
carbene N-heterocyclicpy
we became interested in testing
carbenePC(NHC)CPh), 132.40 (CPh), 129.55 (NCN), 127.84 (t, JPC 5.8, CPh), 125.87 (Cimid),
metal complexes. A numberof simple NHC palladium-based
124.15 (Cimid), 123.56 (Cpy), 55.03 (CH2), 50.47 (CPr ), 22.94 (CPr ).
31P{1H} NMR
complexes have recently emerged
as effective
a variety(CDCl3catalysts, 298 K): ford 21.36.
3
1H NMR7b: studyK): d 8.71of(s, 1H, NCHN), 8.19 (d, 3JHH
of cross-coupling reactions.ForOurof (CDClthe exact
3, 298structure
5.1
Hz,
1H,
py-H),
7.37–7.15
(m,
32H,
py-H,
various palladium NHC species reveals that the metalation site
on Ph-H), 6.07 (t, 3JHH 5.9 Hz,
1H, py-H), 5.03 (s, 1H, imid-H), 4.72 (s, 2H, CH2), 3.63 (t, 3JHH 6.9 Hz, 1H,
the imidazolium salt is strongly
influenced by the presence of base.
CH2), 1.47 (m, 2H, CH2), 1.17 (m, 2H, CH2), 0.90 (t, 3JHH 7.7 Hz, CH3),
products also depends upon the counterion of the used We
salt. now
This report the synthesis
2J
3J
210.89and(dt, 2structure
JPH 19.6, 3of
JHHnovel
5.1 Hz,palladiumIr-H), 219.61 (dt,Figure
1. ORTEP
diagram of palladium complex 2. Selected bond len
PH 18.4,
HH 5.1
surprising and interesting fact certainly needs additional
studies bearing NHCHz,ligands
1H} NMR (CDClcomplexes
and “abnormal”(Cpyand), 153.07
Ir-H). 13inC{“normal”
angles(C(deg):
Pd(1)-C(12), 2.019(13); Pd(1)-C(25), 2.021(
3, 298 K): d 161.77(Å)py),
with other systems where abnormal binding also occurs(t, Jof
6.9
Ccarbene), 137.09(C),2a.
134.96(t, JPCbond= 26.3,CPh), Pd(1)-Cl(2), 2.302(4); C(12)-Pd(1)-C(25), 17
Pd(1)-Cl(1),2.289(4);Figure1.141.30
MolecularofcationofpytoSelectedbinding tomotifs. The
bindingmode
the Hz,NHC
to thePd
isshown
PC structure
JPCpalladium
6.0, C)129.59127.86133.79(5);
C(12)-Pd(1)-Cl(1),93.3(4); C(25)-Pd(1)-Cl(1), 87.3(3); C(1establish how often it takes place.
Ph), 133.61
Ph),
lengths(Å)
and (C
angles(deg):
Os-P(1)2.3605(14),Os-P(2))(t, JPC 4.7,
substantially affectthe catalytic
behaviorof(t,thecom- (NCN),CPh),Os-N(3)126.13 (C)), 125.90 (C(C2.123(6),(CH(CBu), C(25)-Pd(1)-Cl(2), 90.9(3); Cl(1)-Pd(1)-C2
imid2.213(4),py), 124.18)py), 55.08Pd(1)-Cl(2),2), 47.89 88.6(4);2.3683(14),Os-C(1)
H(1A)The complex
OsH6(PiPr
which is3)2 is a d polyhydride plexes.
1H} NMR (CDCl , 298 K): d
558
23 OCTOBER2009VOL
SCIENCE), 19.36 (CBu), 13.37(CBu). 31P{P(1)-Os-N(3)31.96(CBuP(1)-Os-P(2)Fig.
1 Molecular
structureofthe 326
cation of
7a showing www.sciencemag.org
50% probability
177.23(18).
3
H(1B)
)
1.44(5);)164.39(5),)
obtained inthermalhigh ellipsoids.
yield fromcompound- positions)Cl2been
It 2hasreported thatpalladium (II)-NHC complexes could
21.39.
Onlythethe known
metal-boundhydrogensOsH(calculated
99.70(11),P(2)-Os-N(3)) 93.44(11),P(1)-Os-C(1) ) 98.58(14),(PiPr3)2,7 via
the trihydride
tetrahydrideborate‡ Crystal (II)dataacetatefor 7a:andC48H
are shown.
Selected distances(Å) and angles (°):intermediate
Ir(1)–P(1)2.287(2),Ir(1)–
be easily
preparedfrom palladiumthecorresponding
47BF
4IrN3P2, M = 1006.89, colorless crystal,displayed two doublets at 6.57 and 7.47 ppm (J ) 1.7
P(2)-Os-C(1))monoclinic
90.78(14),2 P(2) 2.301(2),i84primitive
cellN(3)-Os-C(1)with dimensions:a)=85.40(18).
9.4493(2), b = 20.6687(6),Ir(1)–N(3)2.193(5),Ir(1)–C(7)2.100(6),N(1)–C(6)OsH3(η -H2BH2)(P Pr3)2. This hexahydride reacts with
Lewis salts. Metal binding
imidazoliumat the C(2)positionis usuallycorrespondingto H(3) and the carbene H(1) of a possibly C
c = 22.0865(6) Å, b = 90.160(2)° and V = 4313.6(2)Å3; Z = 4; m(Mo–1.461(7),N(1)–C(7) 1.404(7),N(1)–C(9)1.335(8),N(2)–C(8) 1.390(8),bases to afford
d4 polyhydride
derivatives,
wheretheobserved,
hydrideand complexes
bearing
tworeaction
NHCs
canpotentially
exist
13
21
23 (iii) the preparation
NHC ligand
in theproducts,
and3. abnormal N-heterocyclic carbenes (aNHCs)	
first metal-aNHC complex	
first free aNHC!
C2- vs C4- bound carbene!
NHC vs aNHC!
application to catalysts	

i

18 September 2012
ubs.rsc.org | doi:10.1039/B107881J

4. summary	benzyl anion to C2. However, calculations indicated that the rearrangement of 9 into 8 is
–1
exothermic
rated by the values of the endocyclic
C–Nby[6:only 6.1 kcal mol and–1involves
barrier of 20.3 kcal mol for the
1.335 T 5 to 1.409 T 5; 9: 1.354 T 2antoactivation
1.408 T 3 Å]
and C–C bond lengths [6: 1.351proton
T 5, 9:transfer.
1.385 TTherefore, we hypothesized thatobserved
rearrangement was catalyzed by a3 Å], which are halfway betweenthe
thoseof single

Downloaded from www.sciencemag.org on September 17, 2012REPORTSi

2	

Carbenes	
carbenes!
The electrically neutral species H2C: and its derivatives, in which the carbon iscovalently bonded to two univalent groups of any kind or a divalent group andbears two nonbonding electrons, which may be spin-paired (singlet state) or
spin-non-paired (triplet state).!
IUPAC Gold Book-carbenes (http://goldbook.iupac.org/C00806.html)	

3	

Carbenes	
singlet vs triplet	Figure 1. Triplet carbenes I and singlet carbenes II-XIV.

Figure 1. Triplet carbenes I and singlet carbenes II-XIV.

be easily analyzed in
effects.

be easilyin
II.1.1.
Electanalyzed
ronic Effects

effects.
II.1.1.1. Inductive
substituents’
II.1.1. Electronicelectron
Effects
tiplicity was recogniz
II.1.1.1. Inductive
reexaminedrecently.1
Figure 2. Relationship between the carbene bond angle
substituents’ electron
σ-electron-withdrawin
and the nature of the frontier orbitals.
tiplicitythewasrecognizversustripletsta
111a,creexaminedrecently.
al.
showed that the
Figure 2. Relationship between the carbene bond angle
σ-electron-withdrawin
and the nature of the frontier orbitals.
tosinglet when the sversusthe triplet
staelectropositive
lithium
11a,cal.
showedth
gativefluorinethat(althoto singlet
alsoplay a whenrole forthetheselectropositive
lithium
effectis easily rational
gative diagrams
fluorine (altho
orbital(C2v
Figure 3. Electronic configurations of carbenes.
also play a role
for thewithdrawing
substitu
effect is easily
rationanonbonding
orbitalbyirreducible representation!
10
ground state. There are some similarities with theorbitaldiagrams
(Cun
2v
leave
the
pπ orbital
Figure3.
Electronic
configurationsofcarbenes.
crystal-field theory (strong-field low-spin and weakwithdrawing
increased
andsubstitu
the sing
field
nonbonding
orbital byIn
contrast, σ-electron
triplethigh-spinstate! 10 configurations).
singlet state!
groundstate.
There
are
some
similarities
withthetheσ-ppπ orbital
aleavesmall
w
Given these statements, the influence of the subπ gap uncrystal-field
theory(strong-field
low-spinandweakincreased
and
the
sing
(Figure 5b).
stituents on the carbene ground-state multiplicity canfield high-spin configurations).
In contrast, σ-electron
a small σ-pπ gap w
Given these statements, the influence of the subD. Bourissou,
O. Guerret,
F. P. Gabbaȉ,G. Bertrand,
Rev. 2000,(Figure5b).100, 39.	
stituentson the carbene
ground-state
multiplicity
can Chem.

). Sincethenation of thenation
substituent
pairs (b1loneof the lone
substituent
pairs(b1σ). Sinceorbital remains
almostunchanged,
σ-pπ gapthe
is σ-porbitalremains
almost theunchanged,
π g4	
Carbenes	
increased andthe singlet
state
is favored(Figure6a). (Figur
increased
andthe singlet
state
is favorednfluence
substituents’Figure of
4. theInfluenceof the electronegativity
substituents’ electronegativity
Note that theσ orbitalandσ thenonsymmetric
comNotethat theorbitaland the nonsymmetricdonstate
carbenespin multiplicity.
theground statecarbenespin effect-	
multiplicity.
electronic
effect-inductive
bination of thesubstituent
pairs (alone
close(a2) arebinationof the lone
substituent
pairs2) arein energy, theirrelativetheirposition
dependingthein energy,
relative
positionondependingoMesomericEffects. Although
inductive
II.1.1.2. Mesomeric
Effects.
Although inductive
electronegativityof X comparedthat of carbon.
electronegativityofXtocomparedto that of ca
ate
the ground-state
multiplicity
of a few
effects
dictate the ground-state
multiplicityofa few
σ-electron-withdrawing
substituents	
σ-electron-donating
substituents	
11c mesomeric
11c
Donation of the
X-substituent
lone pairs results
in a resultuch
as the triplet
Donationof the X-substituent
lone pairs
carbenessuch asLi-C-Li,
the triplet Li-C-Li, mesomeric

X

C

X

X C X
X!

high HOMO!

low LUMO!
X!

Figure 5. orbital
Pertubation
orbitaldiagrams
showing theinfluenceof theinductive effects.
Pertubation
diagrams
showing
the influenceof theinductive
effects.
σ-electron withdrawing substituents favor the singlet state.	

D. Bourissou, O. Guerret, F. P. Gabbaȉ, G. Bertrand, Chem. Rev. 2000, 100, 39.	

5	

Carbenes	
electronic effect -mesomeric effect-	
X: π-electron-donating substituents!
F, Cl, Br, I, NR2, PR2, OR, SR, SR3, …!

X

C

X

X: π-electron-withdrawing substituents!
COR, CN, CF3, BR2, SiR3, PR3+, …!

Z C Z
bent singlet carbene!
linear singlet carbene!
Figure 5. Pertubation orbital diagrams showing the influence of the inductive effects.
pπ

pπ

σ

σ

high!
HOMO!

pπ

low!
pπ LUMO!

σσbent carbene!
with π-EWG substituents!

▶ mesomeric effects can play a more significant role.!
▶ (X,X)-carbenes are predicted to be bent singlet carbenes.!
▶ (Z,Z)-carbenes are predicted to be linear singlet carbenes.!
Figure 6. Pertubation orbital diagrams showing the influence of the mesomeric effects.
D. Bourissou, O. Guerret, F. P. Gabbaȉ, G. Bertrand, Chem. Rev. 2000, 100, 39.!
大学院講義, pp.284.	

6	

Carbenes	
steric effect	▶ bulky substituents kinetically stabilize all types of carbenes!
▶ if electronic effects are negligible…!
Figure 1. Triplet carbenes I and singlet carbenes II-XIV.

be easily analyzed in terms of electro
effects.

II.1.1. Electronic Effects
A linear geometry favors

II.1.1.1.
Inductive
the triplet
state.! Effects. The insubstituents’ electronegativity on the
tiplicity was recognized relatively ea
reexamined recently.13 It is now well es
Figure 2. Relationship between the carbene bond angle
σ-electron-withdrawing substituents fa
and the nature of the frontier orbitals.
versus the triplet state. In particularal.11a,c showed that the ground state go
to singlet when the substituents areelectropositive lithium to hydrogen an
111º
143º
152º
gative fluorine (although mesomericBulky
substituentsfavor element) (Falso playa rolefor the latter
Me Me tBu
tBu
thetripletrationalizedstate.!
effect iseasilyon the basis
orbital diagrams (C2v symmetry). Inde
Figure3.
Electronic
configurationsofcarbenes.
singlettripletwithdrawing substituents inductively
nonbonding orbital by increasing its s
10ground state. There are some similarities with theleave the pπ orbital unchanged. The σcrystal-field theory (strong-field low-spin and weakincreased and the singlet state is favore
field high-spin configurations).
In contrast, σ-electron-donating substi
σ-p
which
the
Given these statements,
the influenceof thesubπ gapRev.
D. Bourissou,
O. Guerret,
F. P.
Gabbaȉ, aG. small
Bertrand,Chem.
2000,favors
100, 39.!

7	

Carbenes	
carbenes!
The electrically neutral species H2C: and its derivatives, in which the carbon iscovalently bonded to two univalent groups of any kind or a divalent group andbears two nonbonding electrons, which may be spin-paired (singlet state) or
spin-non-paired (triplet state).!
IUPAC Gold Book-carbenes (http://goldbook.iupac.org/C00806.html)	

known before 2000	

R2P

SiR'3 R N
Dipp NSPh

N R' R N

iPr2N

ODtbp

N
N R' Ph N N Ph tBuH C N
2(iPr2N)2P

PH(NiPr2)2

iPr2N

N CH tBu2

NiPr2

discovered after 2000	

CF3(iPr2N)2P

NCy2

CF3
NiPr2 iPr2N

P(NiPr2)2
CF3

CF3

tBu
and so on...

M. Melaimi, M. Soleilhavoup, G. Bertrand, Angew. Chem. Int. Ed. 2010, 49, 8810.	

Contents	
1. introduction	

13

shape of the C NMR signal, as well as the calculated chemical shift (32) for the C5 carbon of
the free aNHC 9 (205 ppm), suggested that the
new compound was the aNHC lithium 7. Similarcomplexation has previously been observed forother singlet carbenes such as small NHCs (35)and bis(diisopropylamino)cyclopropenylidene(36, 37); in both cases, coordination of the lithium
cation was apparent from the broadening and theupfield shift of the carbene 13C NMR resonance.
Subsequently, we sought to sequester the lithium
cation through addition of excess [12]crown-4 to adiethylether solution of 7 (where X is Br). This
treatment induced a clean rearrangement to generate 8, which was isolated in 45% yield. This
product formally results from the deprotonation of
an isopropyl substituent of the Dip group by thecarbene center of the aNHC lithium adduct 7,
followed by nucleophilic addition of the resulting
benzyl anion to C2. However, calculations indicated that the rearrangement of 9 into 8 is
exothermic by only 6.1 kcal mol–1 and involves
an activation barrier of 20.3 kcal mol–1 for the
proton transfer. Therefore, we hypothesized thatthe observed rearrangement was catalyzed by a

2. N-heterocyclic carbenes (NHCs)	

first metal-NHC complex	
first free, isolable NHC	NHCs as ligands for transition metals	
Fig. 3. Molecular views
NHCs as organocatalysts	
(50% thermal ellipsoids

component of the (crown)LiBr system, and thatthe formation of 8 did not imply that the free
aNHC 9 was too reactive to be isolated.
Sodium and potassium bases have provenmore appropriate than lithium bases in generating
free carbenes (35–37), because the corresponding
carbon-heavy alkali metal bonds are more labile,which favors precipitation of the salt. When the
deprotonation of imidazolium 6 (HX•X–, where
X is Cl or Br) was performed with two equivalents (34) of potassium hexamethyldisilazide(KHMDS) in tetrahydrofuran, a clean reaction
occurred, with the 13C NMR spectrum of the resulting product showing a very sharp signal at
201.9 ppm. After the products were worked up,
the free aNHC 9 was isolated as a green powder(480 mg, 68% yield), and single crystals wereobtained by recrystallization from a dry hexane
solution at –78°C (Fig. 3, right).
In the solid state, both the free aNHC 9 andthe imidazolium salt 6 (Br–) feature a fully planarring (maximum deviation for N1–C2–N3–C4–C5–
C21–C31–C41–C53 was 1.9 and 6.3 pm for 9 and6, respectively), confirming the delocalization of
the p system. This electronic structure is corrobo-

In the solid state, both the free aNH
the imidazolium salt 6 (Br–) feature a furing (maximum deviation for N1–C2–N3
C21–C31–C41–C53 was 1.9 and 6.3 pm
6, respectively), confirming the delocali
the p system. This electronic structure is8	and double bonds. The carbene bondangle
N1– views
Fig. 3.
Molecular
C5–C4 for 9 [101.03 T 17°] is morethan
(50% acutethermalellipsoids
the corresponding angle in the cationicprecursor
are shown) of imidazolium–
6 [108.0 T 3°]. This feature is bromide
consistent
6 (Brwith) (left) andincreased s character of the s lone-pair
onthe solid
aNHC 9orbital(right) inthe carbene atom in 9 as comparedwith
C–H+H atoms
state(fortheclarity,
bonding orbital in 6. A similararerelationshipisomitted, except
for+the ringhydrogen). Bondobserved in NHCs and their NHC(H) precursors
lengths and angles for(3, 4).
–
are kcalas follows:
Calculations predict aNHC 96to(Brbe )14.1
C5–N1,
1.368
mol–1 less stable than its isomericnormalNHCT 4 Å;
N1–C2,
1.334
with the phenyl group bonded to C5 instead ofT 4 Å;
C2–N3,occupied
1.363 T 4 Å;
C2. Figure 4 shows the two highest
1.408 T 4 Å;molecular orbitals (HOMOs) ofN3–C4,
9. The HOMO
C4–C5,(–4.403 eV) is a s-type lone-pairorbital1.355
at C5;T 5 Å;
N1–C5–C4,
108.0 T 3°;
the HOMO-1 (–4.879 eV) is a C5–C4
p-bonding
C5–C4–N3, 106.0 T 3°;
orbital, which exhibits antibonding conjugation
C4–N3–C2, 108.8 T 3°;
with the p orbital of the phenyl substituent at C4.
N3–C2–N1, 106.9 T 3°;
These molecular orbitals are muchhigher in110.4
en- T 3°.
C2–N1–C5,
ergy than those of the isomeric NHC
Bond(–5.000
lengths andand angles for 9 are as follows: C5a–N1a, 1.417 T 2 Å; N1a–C2a, 1.357 T 2 Å; C2a––5.279 eV, respectively), whichN1a–C5a–C4a,
indicates that101.03 T 17°; C5a–C4a–N3a, 111.01 T 16°; C4a–N3a–C2a, 107.97 T 15°; N3aFig. 4. Plot of the calculated two highest-lying
are shown) of imidazolium–
occupied orbitals HOMO
bromide 6 (Br ) (left) and(left) andHOMO-1(right)
aNHC 9 (right) in the solidof the aNHC 9.
state (for clarity, H atoms
are omitted, except for
Published on Web 04/06/2004
the ring hydrogen). Bond
lengths and angles for–
6 (Br ) are as follows:
Structure and Reactivity of “Unusual” N-Heterocyclic Carbene (NHC)
C5–N1, 1.368 T 4 Å;Palladium Complexes Synthesized from Imidazolium Salts
N1–C2, 1.334 T 4 Å;
C2–N3, 1.363 T 4 Å;Hélène Lebel,*,‡ Marc K. Janes,‡ André B. Charette,‡ and Steven P. Nolan§
N3–C4, 1.408 T 4 Å;Département de chimie, UniVersité de Montréal, C.P. 6128, Succursale Centre-Ville,
C4–C5, 1.355 T 5 Å;Montréal, Québec, Canada, H3C 3J7, and Department of Chemistry, UniVersity of New Orleans,
N1–C5–C4, 108.0 T 3°;
New Orleans, Louisiana 70148View Online
C5–C4–N3, 106.0 T 3°;
C4–N3–C2, 108.8 T 3°;
Received January 14, 2004; E-mail: lebelhe@chimie.umontreal.ca
N3–C2–N1, 106.9 T 3°;
For 6b: 1H NMR (CDCl3, 298 K): d 9.09 (s, 1H, NCHN), 8.51 (d, 3JHH
4.6 Hz, 1H, py-H), 7.73 (dt, 3JHH 7.7, 4JHH 1.8 Hz , 1H, py-H), 7.58 (d, 3JHH
C2–N1–C5, 110.4 T 3°.
Recently,1.345
a numberof 7.8
newHz,
transition
an N-Heterocyclic
Os T 2 Å; C2a–N3a,Vol.processes
26,
No.(s,26,
1H,Organometallics,py-H),
7.53(s, 1H, imid-H),
1H, 2007
imid-H),6557
7.27 (dd, 3JHH
Bond lengths and angles forCoordination
9 are as follows:ofC5a–N1a,
1.417 T 2 Å;Carbene
N1a–C2a,to
1.357
T 2 Å; N3a–C4a,
1.412
T 3metal-catalyzed
Å;
C4a–C5a,
1.383
T 37.32
Å;3J
3
23 OCTOBER 2009 VOL 326 SCIENCE w
1H,558py-H),
5.49(s, 4H, CH
havebeen developed
andoptimized
throughhigh-throughput
and
HH 4.8 Hz,2), 4.19 (t, JHH 7.5 Hz, 2H, BuN1a–C5a–C4a, 101.03 T 17°; C5a–C4a–N3a, 111.01 T 16°; C4a–N3a–C2a, 107.97 T 15°;
N3a–C2a–N1a,106.25
T7.7,
16°;
C2a–N1a–C5a,113.72
T 15°.
1H),
1.89–1.81(m,
2H,
Bu-H),
1.39–1.30(m,
2H,Bu-H),
0.93(t, 3JHH 7.5
Chart 1
combinatorial methods. Such strategies13are1 particularly efficientHz, 3H, Bu-H). C{ H} NMR (CDCl3, 298 K): d 152.25 (Cpy), 149.77
for the screening of a large
number of transition metal-ligand
Fig. 4. Plot of the cal(Cpy), 137.86 (Cpy), 136.37 (NCN), 124.06 (Cpy), 123.80 (Cpy), 122.93
combinations under various
setsof reaction
culated two highest-lying), 121.75(Cimid),conditions.
54.00 (CH2),However,
49.93 (CBu), 31.81 (CBu), 19.34 (CBu),(Cimidunder these in situ conditions,13.30 (Ctheoccupied orbitals HOMO
Bu). exact nature of the catalytic
1H NMR (CDCl , 298 K): d 8.72 (s, 1H, NCHN), 8.23 (d, 1H,
For 7a:3 assumption of(left) andHOMO-1(right)
species is not always precisely
known.
Indeed, the33
5.5 Hz,ligandpy-H), 7.37–7.14(m, 32H,
py-H,of the aNHC 9.
a normal mode of bindingJHH
betweenand transition
metal
is Ph-H), 6.07 (t, JHH 6.3 Hz,py-H), 5.17 (s, 1H, imid-H), 4.70 (s, 2H, CH2), 4.25 (septet, 1H, 3JHH 6.5
most often assumed and maybemisleading.
3
2
Hz, CH), 1.19 (d, 6H, JHH 6.5 Hz, CH3), 210.83 (dt, JPH 19.6, 3JHH 4.9
3J
During the course of ouron 2ligands
to Hz, Ir–H). 13C{1H} NMR
JPH 18.6,suitable
Hz,studies
Ir-H), focusing
221.49 (dt,
HH 4.92 ), 141.07 (t, J
d 161.78 (Cpy),processes,
153.10 (Cinduce enantioselectivity (CDClin transition
metal-catalyzed
3, 298 K):
py
PC 7.1 Hz,
C),
137.04(C),
134.93(t,J=
26.3
Hz, CPh), 133.55 (t, JPC 6.0 Hz,
carbene N-heterocyclicpy
we became interested in testing
carbenePC(NHC)CPh), 132.40 (CPh), 129.55 (NCN), 127.84 (t, JPC 5.8, CPh), 125.87 (Cimid),
metal complexes. A numberof simple NHC palladium-based
124.15 (Cimid), 123.56 (Cpy), 55.03 (CH2), 50.47 (CPr ), 22.94 (CPr ).
31P{1H} NMR
complexes have recently emerged
as effective
a variety(CDCl3catalysts, 298 K): ford 21.36.
3
1H NMR7b: studyK): d 8.71of(s, 1H, NCHN), 8.19 (d, 3JHH
of cross-coupling reactions.ForOurof (CDClthe exact
3, 298structure
5.1
Hz,
1H,
py-H),
7.37–7.15
(m,
32H,
py-H,
various palladium NHC species reveals that the metalation site
on Ph-H), 6.07 (t, 3JHH 5.9 Hz,
1H, py-H), 5.03 (s, 1H, imid-H), 4.72 (s, 2H, CH2), 3.63 (t, 3JHH 6.9 Hz, 1H,
the imidazolium salt is strongly
influenced by the presence of base.
CH2), 1.47 (m, 2H, CH2), 1.17 (m, 2H, CH2), 0.90 (t, 3JHH 7.7 Hz, CH3),
products also depends upon the counterion of the used We
salt. now
This report the synthesis
2J
3J
210.89and(dt, 2structure
JPH 19.6, 3of
JHHnovel
5.1 Hz,palladiumIr-H), 219.61 (dt,Figure
1. ORTEP
diagram of palladium complex 2. Selected bond len
PH 18.4,
HH 5.1
surprising and interesting fact certainly needs additional
studies bearing NHCHz,ligands
1H} NMR (CDClcomplexes
and “abnormal”(Cpyand), 153.07
Ir-H). 13inC{“normal”
angles(C(deg):
Pd(1)-C(12), 2.019(13); Pd(1)-C(25), 2.021(
3, 298 K): d 161.77(Å)py),
with other systems where abnormal binding also occurs(t, Jof
6.9
Ccarbene), 137.09(C),2a.
134.96(t, JPCbond= 26.3,CPh), Pd(1)-Cl(2), 2.302(4); C(12)-Pd(1)-C(25), 17
Pd(1)-Cl(1),2.289(4);Figure1.141.30
MolecularofcationofpytoSelectedbinding tomotifs. The
bindingmode
the Hz,NHC
to thePd
isshown
PC structure
JPCpalladium
6.0, C)129.59127.86133.79(5);
C(12)-Pd(1)-Cl(1),93.3(4); C(25)-Pd(1)-Cl(1), 87.3(3); C(1establish how often it takes place.
Ph), 133.61
Ph),
lengths(Å)
and (C
angles(deg):
Os-P(1)2.3605(14),Os-P(2))(t, JPC 4.7,
substantially affectthe catalytic
behaviorof(t,thecom- (NCN),CPh),Os-N(3)126.13 (C)), 125.90 (C(C2.123(6),(CH(CBu), C(25)-Pd(1)-Cl(2), 90.9(3); Cl(1)-Pd(1)-C2
imid2.213(4),py), 124.18)py), 55.08Pd(1)-Cl(2),2), 47.89 88.6(4);2.3683(14),Os-C(1)
H(1A)The complex
OsH6(PiPr
which is3)2 is a d polyhydride plexes.
1H} NMR (CDCl , 298 K): d
558
23 OCTOBER2009VOL
SCIENCE), 19.36 (CBu), 13.37(CBu). 31P{P(1)-Os-N(3)31.96(CBuP(1)-Os-P(2)Fig.
1 Molecular
structureofthe 326
cation of
7a showing www.sciencemag.org
50% probability
177.23(18).
3
H(1B)
)
1.44(5);)164.39(5),)
obtained inthermalhigh ellipsoids.
yield fromcompound- positions)Cl2been
It 2hasreported thatpalladium (II)-NHC complexes could
21.39.
Onlythethe known
metal-boundhydrogensOsH(calculated
99.70(11),P(2)-Os-N(3)) 93.44(11),P(1)-Os-C(1) ) 98.58(14),(PiPr3)2,7 via
the trihydride
tetrahydrideborate‡ Crystal (II)dataacetatefor 7a:andC48H
are shown.
Selected distances(Å) and angles (°):intermediate
Ir(1)–P(1)2.287(2),Ir(1)–
be easily
preparedfrom palladiumthecorresponding
47BF
4IrN3P2, M = 1006.89, colorless crystal,displayed two doublets at 6.57 and 7.47 ppm (J ) 1.7
P(2)-Os-C(1))monoclinic
90.78(14),2 P(2) 2.301(2),i84primitive
cellN(3)-Os-C(1)with dimensions:a)=85.40(18).
9.4493(2), b = 20.6687(6),Ir(1)–N(3)2.193(5),Ir(1)–C(7)2.100(6),N(1)–C(6)OsH3(η -H2BH2)(P Pr3)2. This hexahydride reacts with
Lewis salts. Metal binding
imidazoliumat the C(2)positionis usuallycorrespondingto H(3) and the carbene H(1) of a possibly C
c = 22.0865(6) Å, b = 90.160(2)° and V = 4313.6(2)Å3; Z = 4; m(Mo–1.461(7),N(1)–C(7) 1.404(7),N(1)–C(9)1.335(8),N(2)–C(8) 1.390(8),bases to afford
d4 polyhydride
derivatives,
wheretheobserved,
hydrideand complexes
bearing
tworeaction
NHCs
canpotentially
exist
13
21
23 (iii) the preparation
NHC ligand
in theproducts,
and3. abnormal N-heterocyclic carbenes (aNHCs)	
first metal-aNHC complex	
first free aNHC!
C2- vs C4- bound carbene!
NHC vs aNHC!
application to catalysts	

i

18 September 2012
ubs.rsc.org | doi:10.1039/B107881J

4. summary	benzyl anion to C2. However, calculations indicated that the rearrangement of 9 into 8 is
–1
exothermic
rated by the values of the endocyclic
C–Nby[6:only 6.1 kcal mol and–1involves
barrier of 20.3 kcal mol for the
1.335 T 5 to 1.409 T 5; 9: 1.354 T 2antoactivation
1.408 T 3 Å]
and C–C bond lengths [6: 1.351proton
T 5, 9:transfer.
1.385 TTherefore, we hypothesized thatobserved
rearrangement was catalyzed by a3 Å], which are halfway betweenthe
thoseof single

Downloaded from www.sciencemag.org on September 17, 2012REPORTSiim Hochvakuum
aufwhich120’ aresublimierte
entacarbonylchromat(-II)
Nucleophiliccarbenes,
similar in CCI.80% Ausbeutecentral atom [2,31. (I)ru9	
N-Heterocyclic(NHCs)	
ne
blassgelbe
Substanz Carbenes
dershould
Zusammensetzung
eHaCrN205
forCidirect
synthesismitof einem Schmelzpunkt
to isocyanides[41,be suitable7).
on 100.5“;und massenspektroskopischkonntedass bier nachsuchNMRcomplexes.
That is the case, as wehave bewiesen
shown bywerden,afirstfirstmetal-NHC
leichung(2) example:
ein neuercomplexes	
Inaergangsmetallcarbonyl-Carben-Komplex
view of the results obtained by Schoilkopf (II) entstanden war:l , treated 1,3-diet a!., with Hgzc as central a t o m [ 3 ~ 3 ~wephenylimidazolium perchlorate ( CIH)3 with mercury(i1) acetate
pale yellow crystalline!
in dimethyl sulfoxide (DMSO) Iand obtained the carbene
air and thermally stable!
complex (3) 151. It was unnecessary to CrK0)5
add a base
+ Hz to produce(2)
mp.di100.5 ℃!
the carbene in situ. Bis(l,3-diphenylimidazolio)mercury
1H NMR (d -acetone) δ = 7.24, 3.87 ppm!
6
perchlorate (3) [crystallizes from DMSO as colorless plates,
at‘=H3CH3m.p. ca. 370 "C (decomp.), which darken above ca. 250 "C] ishat(I)K. Öfele, J. Organomet. Chem. 1968, 12, P42.	
acid in almost quantitative
formed with liberation of acetic (III)ralyield. It is stable to acids but is immediately reconverted intoity
Das NMR-Spektrum(gemessen
in De-Aceton) enth& nur 2 Signale bei 2.76 Tm.
) und 6.13 T (s) mit einem IntensitMsverh&ris
von I:3 entsprechend den beiden Pro1ith
nen an den C-Atomen 4 bzw. 5 und den 6 Methylprotonen.
Ahnlich wie bei itemof
+ H2S--colorless rectangular plates!
OrganometaL Chem. 12 (1968)c
P42-P43
stable to acids!
mp. ca. 370 ℃ (decomp.)!

of
ric
was

-

+ H~(OAC)~- 2 AcOH

H.-W. Wanzlick, H.-J. Schönherr, Angew. Chem. Int. Ed. Engl. 1968, 7, 141.	

141

10	

N-Heterocyclic Carbenes (NHCs)	
first free,NHC	
362
J. Am.isolable
Chem. Sor..
Vol. 113. No. I . 1991

0Communications to thcolorless rectangular prism!
stable in the absence of oxygen and moisture!
mp. 240-241 ℃!

362 J. Am. Chem. Sor.. Vol. 113. No. I . 1991③	
δ(H) = 7.9	

a

0

2

a

Communications to the Editor

δ(H) = 6.9	

@
1

@

push

of this
NHC!with that of the initially isolateda 'Hfeatures
N M R spectrum
identical① smallangle! data of 1 in benzene-d, are as follows:carbene.
The N-C-N
U MR spectral
longerC-Nppm
'H ②
NMR
6 1.58(s. Ad,,,,,,. I2 H).1 2.01 (s. Ad,,,,,, 6 H),
2bonds!
diminishedAdz.8z. 12 π-delocalization	
H). 6.91 (s. NCH. 2 H); 'IC N M R ['HI 6
2.29③(s.
a 'H N M R spectrum identical with that of the initially isolated
211.43 (carbene.
s . C d . The
113.88lJCH
Hz. 'JCH =follows:
13.4 Hz.
U MR(dd.
spectral
data=of 185.6
1 in benzene-d, are as pull(s. Ad,).
(tm.
lJCH I2=H).
131.3
Hz.Ad,,,,,,
Adz.8.p).
C4,d. 55.99'H NMR
6 1.58 44.8
ppm (s.
Ad,,,,,,.
2.01 (s.
6 H), 37.7
Table 1. Selected Bond Lengths (pm) and Angles (deg) in I
Adz.8z.Hz.
12 H). 6.91 (s. NCH.
2 H); 'ICN M=R 128.9['HI 6 Hz.
30.3 (dm.
llCH(tm. 'JCH2.29=(s.126.8
211.43(s.
Cd.
113.88
(dd.
lJCH
=
185.6
Hz.
'JCH
=
13.4
Hz.
Ad],,.,):"NmadeNMR 6this-160.5(ref NH,NOl).
The electron-impact
What
NHC
isolable?!
bond leneth
bond aC4,d. 55.99 (s. Ad,). 44.8 (tm. lJCH = 131.3 Hz. Adz.8.p). 37.7
PaveHz.a base oeakm l rllCH= =336.26.
corre- Table 1. Selected Bond Lengths (pm)masssoectrumand(2)
Angles (deg) in I
▶ electronic
136.7= 1126.8
30.3at(dm.
128.9 Hz.
(tm. 'JCHofstabilization!
sponding
IO
themoleblarion.
Theelemental
analysisof1gaveAd],,.,):"N
NMR
6-160.5(ref
NH,NOl).
Theelectron-impactbondlenethbond anele
137.3
(2)
π-donation into the carbene p orbital !
1 Pavea base oeakat m l r H.
= 336.26.
C. 82.13;mass
H. soectrum
9.64; N.of8.36(theory
C, 82.09;
9.58; correN, 8.32).
133.8 (3)136.7 (2) ①	
imidazolium!
effectsofthenitrogens!
sponding IO themoleblarTheelemental
analysisof 1 gave1504,②	
137.3 (2) 138.2 (2)IR spectrum(KBr)of 1 ion.
shows
absorbances
at 2920.
Theσ-electronegativity
salts!
C. 82.13;
H. 9.64; N. 8.36 (theory C, 82.09; H. 9.58; N, 8.32).
133.8 (3) 138.6 (2)▶ steric
stabilization!
1448.
1378.
1350.
1303.
1213.
1291.
833.
695.
and
495
Em-'.
ca.
109°	
imidazolium!
The IR spectrum (KBr) of 1 shows absorbances at 2920. 1504,
138.2 (2)148.2
(2)
A single
crystalsuitablefor
X-raydiffraction
studieswasgrown
salts!
1448. 1378. 1350. 1303. 1213. 1291. 833. 695. and 495 Em-'.
138.6 (2)148.2 (2) 148.5 (2)1.32 Å!
crystalsuitable fordiffraction
was grown
by coolingAa single
toluenesolutionof X-ray
1. TheX-raystudies
crystalstructure
148.5 (2)by coolingina tolueneof 1. Thebond
X-raylengths
crystal structure
Figuresolution
I . Selected
and anglesof 1 is depicted
is depicted in Figure I . Selected bond lengths and angles
out-of-planeorbitalby theelectron-richare givenof
in1given
Table
I . Thefinal
wereR R= =0.038
out-of-planep orbital by pthe
electron-richr-systemarein Table
I . ThefinalR Rfactors
factors were
0.038
and and carbenecarbene(N-C=C-N)and Chem.
a a-electronegativity
effect.
The
R, = 0.034.
The largest
residualdensityin in
the
final
(N-C=C-N)
and a a-electronegativity
effect.
The *-interR, = 0.034.
The largest
residualelectron
electrondensity
theIII,
finalR.
A. J. Arduengo
L. Harlow,
M. Kline,
J. Am.
Soc. 1991,
113,
361.	
~

~

N-Heterocyclic Carbenes (NHCs)	
4, 749-750
2001
749-750
3,
NHCs as ligands for transition metals	

ne Exchange

Sq (cal-1 K-1)∆Gq (298 K)(kcal mol-1)

2(2
3(6

19.88 ( 0.06
23.0 ( 0.4
Scheme 4

eK)
1.
1
)

1st generation!
Grubbs catalyst	

74974911	

Communications to the Editor

Scheme 4strong σ-donation!
→ enhance the dissociation of PCy3!
Communications to the Editor→ stabilize the 14-e intermedaite!
steric bulk!
ndCommunications
the Editor
2 generation!
→tostabilize
the 14-e intermedaite!
Grubbs catalyst	

Scheme 4
06
me
1
1
4

vent. Catalysts 1 and 2
exclusive formation of
Table 2. Values for the k-1/k2 Ratio in Catalysts 1 and 2d2
HOEt.
This reaction isof“the activity
1 and 2 is
only
Table 2. Values for the k-1/k2 Ratio in Catalysts 1 and 2-1) of kcatalyst-1notcatalyst
T(°C)
k(intercept)(s(predicted)(s)
k-1/k2
1
B
istitativecatalyst
and essentiallyrelated to the phosphine dissociation rate
-1
-1
T
(°C)
k
(intercept)(s)
k(predicted)
(s
)
k
/k
1
B
-1 2
lly
k1, but also to the k-1/k0.16
mely
rapid
(significantly
2 ratio which
1
37
0.26
15300
tly
1
37
0.26
0.16
15300
determines
whetherthecatalyst
binds
olefin2
50provide0.0031
1.25 0.0031
ide and should
2 0.003 50
0.003
1.25
ns),
or returns to its resting state”!
ms.
rates
in these systems. binding are fast).21 Under pseudo-first1
me
2onH 2 the steps following olefin
1
21 Under pseudo-firstconditionsby
in olefin,
affords
the relationH this approximation
er wasorderstudiedthesteps
following
olefinbindingarefast).
ndof
2 generation
Grubbscatalyst;
Scholl,S.19Ding, C. W. Lee, R. H. Grubbs, Org. Lett. 1999, 1, 953.!
which is M.
shownin eq 1.
ship between
1/kobs and [PCy3]/[olefin]
onditions,theinitiation
order
conditions
inolefin,this approximation
affords
the123,
relationuiv
mechanistic study; M. S. Sanford, M. Ulman,
R. H. Grubbs,
J. Am. Chem. Soc.
2001,
749. !
-1
1/kobs ) of
k-1[PCy3]/kshipk[olefin]
+
1/k
(1)
ults
s ) isTable
independent[PCywhichSoc.
is shown
in eq
1.19
betweenJ.1 1/k
Sanford,
A. Love,
R. H.
2001, 123,6543.	
obs and
2. Values for theM.1 k2S.
1 Grubbs,
and3]/[olefin]
2 J. Am. Chem.
-1/k2 Ratio in Catalysts

finreported the synthesis of the triazolin-5-ylidene 22 startinand derivatives, e.g., 16, are today often called Wanzlick
ketones. Aliphatic aldehydes showed the36best results withfrom the triazole precursor 20. The crystalline carbene 2
carbenes (Figure 7).
catalyst 10a, whereas for aromatic substrates, the thiazoliumwasobtainedfrom catalysts
20 by additionof 5).
sodium22,23 methanolate
salts 10b or 10c
were better(Figuregive the stable adduct 21 followed by R-elimination of methanol, either at 80 °C under vacuum or in refluxing toluen
NHCs as organocatalysts	
Compound 22 proved to be stable at temperatures up to 15°C in the absence of air and moisture.37 Compound 22 wa
the first carbene to be commercially available (Scheme 5

12	

N-Heterocyclic Carbenes (NHCs)	

Figure 7. Typical Wanzlick carbenes.

Ph
Figure 5. N
Thiazolium
utilizedMechanism
by Stetter etofal.the Formoin
Scheme 6. salts
Postulated
N Condensation Reported by Teles and Co-workers

Figure 3. Structure of the transketolase enzyme isolated from
3.2. First Asymmetric Benzoin Condensations
SaccharomycesHowever,
cereVisiae.the carbene character of Bertrand’s phosphiClO

4
nocarbene 17 was initially doubted, as its chemical reactivity
As the N
product of the benzoin condensation is an R-hyPh32
resemblea phosphaacetylene
Scheme 1. appeared
Catalytic to
Cycleof the more
BenzoinCondensation as 18. Recent
droxyketonePh
8, a new stereogenic center is formed. ConseProposed by
Breslowstudiesseem to support its carbene character again (Scheme
quently, many chemists have tried to develop heterazolium3).33
catalyzed asymmetric benzoin condensations and, later, othercoenzyme thiamine!
Scheme 3. Phosphinocarbene Reported by Bertrand andnucleophilic acylation reactions (Scheme 2).

iamine (vitaminB1), a Bcoenzyme.
(vitamin)	
1Co-workers

Scheme 2. Asymmetric Benzoin Condensation

59%	
Thus, the imidazolin-2-ylidene 15a synthesized by Arduengo et al. in 1991 is now referred to as the firstIn 1966, Sheehan and Hunneman presented the first results
N-heterocyclic carbene to be isolated and characterized. Itsof an asymmetric benzoin condensation utilizing the chiralpreparation was achieved by deprotonation of the corresalt 11 as precatalyst.24 However, the enantiomericsponding imidazolium salt 19. Later on, Arduengo etthiazoliumal.
excess of the synthesized benzoin was only 22%. Some years
could also synthesize stable carbenes with less bulky
later, they were able to obtain enantiomeric excesses of up
substituents. It should be noted that the structure of theseto 51% with modified thiazolium salts such as 12. Unforcarbenes is identical with that of the Wanzlick carbenes.
the products were formed in low yields (6%).25
Progress in laboratory methods was finally decisive fortunately,
theMany years later, Dvorak and Rawal improved the protocol
successful isolation of these carbenes (Scheme 4).
of Sheehan and co-workers, obtaining 48% enantiomerictructure of a transketolase enzyme determined by
Scheme 4. Preparation of Stable Imidazolin-2-ylidenes byexcess (ee) with better yields.26 Menthyl-substituted thiazo. Reproduced with kind permissionof Prof.
Schneider.
Arduengo andCo-workers
lium salts such as 13 were developed by Takagi et al. andcatalyzed the formation of benzoin in a micellar two-phaseoriginallydrawn asanda mesomericzwitterion. Nucleophilicve been largely
elucidated,the enzymes
reaction
systemJ.-P.
with anenantiomeric
excess
35% and an E.
T. Ugai,
S.ofTanaka,
S. Dokawa,!an aldehyde 4 then genJ. H. Teles,
Melder,K. Ebel,R.ofSchneider,attackthecarbonylfunctionof2
used as synthetic tools. Much research has
improved yield of 20%.27 Zhao et al. combined the Sheehan
Pharm.salt
Soc.
Jpn.
63, 296.!
Gehrer, W. Harder, S. Brode, D. Enders, K. Breuer,
erates the J.
thiazolium
adduct5. 1943,
Deprotonation/reprotocatalysts with the Takagi reaction conditions and obtained
to determinenation
the leads
actualcatalytically
activeto theChem.
active aldehyde
in the formof the resoR. Breslow, J. Am.
Soc. 1958,
80, 3719.	
G. Raabe,Helv. Chim.
Acta.
1996, 79,
moderateenantiomeric
excessesof 47-57%
and 61.	
yields of

13	

Cyclic Non-NHCs	
N

P

N

NHC!

P

N

PHC!
diphosphinocarbene!

N

N

CAAC!
(alkyl)(amino)carbene!

P

N-PHC!
(amino)(phosphino)carbene!

E

N-YHC!
(amino)(ylidic)carbene!

CP!
cyclopropenylidene!

N
P
CBA!
cyclic bent allene!

P

CCDP!
cyclic carbodiphosphorane!

P
CVP!
vinylidenephosphorane!

NaNHC!
abnormal-NHC!

M. Melaimi, M. Soleilhavoup, G. Bertrand, Angew. Chem. Int. Ed. 2010, 49, 8810.	

Contents	
1. introduction	

13

shape of the C NMR signal, as well as the calculated chemical shift (32) for the C5 carbon of
the free aNHC 9 (205 ppm), suggested that the
new compound was the aNHC lithium 7. Similarcomplexation has previously been observed forother singlet carbenes such as small NHCs (35)and bis(diisopropylamino)cyclopropenylidene(36, 37); in both cases, coordination of the lithium
cation was apparent from the broadening and theupfield shift of the carbene 13C NMR resonance.
Subsequently, we sought to sequester the lithium
cation through addition of excess [12]crown-4 to adiethylether solution of 7 (where X is Br). This
treatment induced a clean rearrangement to generate 8, which was isolated in 45% yield. This
product formally results from the deprotonation of
an isopropyl substituent of the Dip group by thecarbene center of the aNHC lithium adduct 7,
followed by nucleophilic addition of the resulting
benzyl anion to C2. However, calculations indicated that the rearrangement of 9 into 8 is
exothermic by only 6.1 kcal mol–1 and involves
an activation barrier of 20.3 kcal mol–1 for the
proton transfer. Therefore, we hypothesized thatthe observed rearrangement was catalyzed by a

2. N-heterocyclic carbenes (NHCs)	

first metal-NHC complex	
first free, isolable NHC	NHCs as ligands for transition metals	
Fig. 3. Molecular views
NHCs as organocatalysts	
(50% thermal ellipsoids

component of the (crown)LiBr system, and thatthe formation of 8 did not imply that the free
aNHC 9 was too reactive to be isolated.
Sodium and potassium bases have provenmore appropriate than lithium bases in generating
free carbenes (35–37), because the corresponding
carbon-heavy alkali metal bonds are more labile,which favors precipitation of the salt. When the
deprotonation of imidazolium 6 (HX•X–, where
X is Cl or Br) was performed with two equivalents (34) of potassium hexamethyldisilazide(KHMDS) in tetrahydrofuran, a clean reaction
occurred, with the 13C NMR spectrum of the resulting product showing a very sharp signal at
201.9 ppm. After the products were worked up,
the free aNHC 9 was isolated as a green powder(480 mg, 68% yield), and single crystals wereobtained by recrystallization from a dry hexane
solution at –78°C (Fig. 3, right).
In the solid state, both the free aNHC 9 andthe imidazolium salt 6 (Br–) feature a fully planarring (maximum deviation for N1–C2–N3–C4–C5–
C21–C31–C41–C53 was 1.9 and 6.3 pm for 9 and6, respectively), confirming the delocalization of
the p system. This electronic structure is corrobo-

In the solid state, both the free aNH
the imidazolium salt 6 (Br–) feature a furing (maximum deviation for N1–C2–N3
C21–C31–C41–C53 was 1.9 and 6.3 pm
6, respectively), confirming the delocali
the p system. This electronic structure is14	

and double bonds. The carbene bondangle
N1– views
Fig. 3.
Molecular
C5–C4 for 9 [101.03 T 17°] is morethan
(50% acutethermalellipsoids
the corresponding angle in the cationicprecursor
are shown) of imidazolium–
6 [108.0 T 3°]. This feature is bromide
consistent
6 (Brwith) (left) andincreased s character of the s lone-pair
onthe solid
aNHC 9orbital(right) inthe carbene atom in 9 as comparedwith
C–H+H atoms
state(fortheclarity,
bonding orbital in 6. A similararerelationshipisomitted, except
for+the ringhydrogen). Bondobserved in NHCs and their NHC(H) precursors
lengths and angles for(3, 4).
–
are kcalas follows:
Calculations predict aNHC 96to(Brbe )14.1
C5–N1,
1.368
mol–1 less stable than its isomericnormalNHCT 4 Å;
N1–C2,
1.334
with the phenyl group bonded to C5 instead ofT 4 Å;
C2–N3,occupied
1.363 T 4 Å;
C2. Figure 4 shows the two highest
1.408 T 4 Å;molecular orbitals (HOMOs) ofN3–C4,
9. The HOMO
C4–C5,(–4.403 eV) is a s-type lone-pairorbital1.355
at C5;T 5 Å;
N1–C5–C4,
108.0 T 3°;
the HOMO-1 (–4.879 eV) is a C5–C4
p-bonding
C5–C4–N3, 106.0 T 3°;
orbital, which exhibits antibonding conjugation
C4–N3–C2, 108.8 T 3°;
with the p orbital of the phenyl substituent at C4.
N3–C2–N1, 106.9 T 3°;
These molecular orbitals are muchhigher in110.4
en- T 3°.
C2–N1–C5,
ergy than those of the isomeric NHC
Bond(–5.000
lengths andand angles for 9 are as follows: C5a–N1a, 1.417 T 2 Å; N1a–C2a, 1.357 T 2 Å; C2a––5.279 eV, respectively), whichN1a–C5a–C4a,
indicates that101.03 T 17°; C5a–C4a–N3a, 111.01 T 16°; C4a–N3a–C2a, 107.97 T 15°; N3aFig. 4. Plot of the calculated two highest-lying
are shown) of imidazolium–
occupied orbitals HOMO
bromide 6 (Br ) (left) and(left) andHOMO-1(right)
aNHC 9 (right) in the solidof the aNHC 9.
state (for clarity, H atoms
are omitted, except for
Published on Web 04/06/2004
the ring hydrogen). Bond
lengths and angles for–
6 (Br ) are as follows:
Structure and Reactivity of “Unusual” N-Heterocyclic Carbene (NHC)
C5–N1, 1.368 T 4 Å;Palladium Complexes Synthesized from Imidazolium Salts
N1–C2, 1.334 T 4 Å;
C2–N3, 1.363 T 4 Å;Hélène Lebel,*,‡ Marc K. Janes,‡ André B. Charette,‡ and Steven P. Nolan§
N3–C4, 1.408 T 4 Å;Département de chimie, UniVersité de Montréal, C.P. 6128, Succursale Centre-Ville,
C4–C5, 1.355 T 5 Å;Montréal, Québec, Canada, H3C 3J7, and Department of Chemistry, UniVersity of New Orleans,
N1–C5–C4, 108.0 T 3°;
New Orleans, Louisiana 70148View Online
C5–C4–N3, 106.0 T 3°;
C4–N3–C2, 108.8 T 3°;
Received January 14, 2004; E-mail: lebelhe@chimie.umontreal.ca
N3–C2–N1, 106.9 T 3°;
For 6b: 1H NMR (CDCl3, 298 K): d 9.09 (s, 1H, NCHN), 8.51 (d, 3JHH
4.6 Hz, 1H, py-H), 7.73 (dt, 3JHH 7.7, 4JHH 1.8 Hz , 1H, py-H), 7.58 (d, 3JHH
C2–N1–C5, 110.4 T 3°.
Recently,1.345
a numberof 7.8
newHz,
transition
an N-Heterocyclic
Os T 2 Å; C2a–N3a,Vol.processes
26,
No.(s,26,
1H,Organometallics,py-H),
7.53(s, 1H, imid-H),
1H, 2007
imid-H),6557
7.27 (dd, 3JHH
Bond lengths and angles forCoordination
9 are as follows:ofC5a–N1a,
1.417 T 2 Å;Carbene
N1a–C2a,to
1.357
T 2 Å; N3a–C4a,
1.412
T 3metal-catalyzed
Å;
C4a–C5a,
1.383
T 37.32
Å;3J
3
23 OCTOBER 2009 VOL 326 SCIENCE w
1H,558py-H),
5.49(s, 4H, CH
havebeen developed
andoptimized
throughhigh-throughput
and
HH 4.8 Hz,2), 4.19 (t, JHH 7.5 Hz, 2H, BuN1a–C5a–C4a, 101.03 T 17°; C5a–C4a–N3a, 111.01 T 16°; C4a–N3a–C2a, 107.97 T 15°;
N3a–C2a–N1a,106.25
T7.7,
16°;
C2a–N1a–C5a,113.72
T 15°.
1H),
1.89–1.81(m,
2H,
Bu-H),
1.39–1.30(m,
2H,Bu-H),
0.93(t, 3JHH 7.5
Chart 1
combinatorial methods. Such strategies13are1 particularly efficientHz, 3H, Bu-H). C{ H} NMR (CDCl3, 298 K): d 152.25 (Cpy), 149.77
for the screening of a large
number of transition metal-ligand
Fig. 4. Plot of the cal(Cpy), 137.86 (Cpy), 136.37 (NCN), 124.06 (Cpy), 123.80 (Cpy), 122.93
combinations under various
setsof reaction
culated two highest-lying), 121.75(Cimid),conditions.
54.00 (CH2),However,
49.93 (CBu), 31.81 (CBu), 19.34 (CBu),(Cimidunder these in situ conditions,13.30 (Ctheoccupied orbitals HOMO
Bu). exact nature of the catalytic
1H NMR (CDCl , 298 K): d 8.72 (s, 1H, NCHN), 8.23 (d, 1H,
For 7a:3 assumption of(left) andHOMO-1(right)
species is not always precisely
known.
Indeed, the33
5.5 Hz,ligandpy-H), 7.37–7.14(m, 32H,
py-H,of the aNHC 9.
a normal mode of bindingJHH
betweenand transition
metal
is Ph-H), 6.07 (t, JHH 6.3 Hz,py-H), 5.17 (s, 1H, imid-H), 4.70 (s, 2H, CH2), 4.25 (septet, 1H, 3JHH 6.5
most often assumed and maybemisleading.
3
2
Hz, CH), 1.19 (d, 6H, JHH 6.5 Hz, CH3), 210.83 (dt, JPH 19.6, 3JHH 4.9
3J
During the course of ouron 2ligands
to Hz, Ir–H). 13C{1H} NMR
JPH 18.6,suitable
Hz,studies
Ir-H), focusing
221.49 (dt,
HH 4.92 ), 141.07 (t, J
d 161.78 (Cpy),processes,
153.10 (Cinduce enantioselectivity (CDClin transition
metal-catalyzed
3, 298 K):
py
PC 7.1 Hz,
C),
137.04(C),
134.93(t,J=
26.3
Hz, CPh), 133.55 (t, JPC 6.0 Hz,
carbene N-heterocyclicpy
we became interested in testing
carbenePC(NHC)CPh), 132.40 (CPh), 129.55 (NCN), 127.84 (t, JPC 5.8, CPh), 125.87 (Cimid),
metal complexes. A numberof simple NHC palladium-based
124.15 (Cimid), 123.56 (Cpy), 55.03 (CH2), 50.47 (CPr ), 22.94 (CPr ).
31P{1H} NMR
complexes have recently emerged
as effective
a variety(CDCl3catalysts, 298 K): ford 21.36.
3
1H NMR7b: studyK): d 8.71of(s, 1H, NCHN), 8.19 (d, 3JHH
of cross-coupling reactions.ForOurof (CDClthe exact
3, 298structure
5.1
Hz,
1H,
py-H),
7.37–7.15
(m,
32H,
py-H,
various palladium NHC species reveals that the metalation site
on Ph-H), 6.07 (t, 3JHH 5.9 Hz,
1H, py-H), 5.03 (s, 1H, imid-H), 4.72 (s, 2H, CH2), 3.63 (t, 3JHH 6.9 Hz, 1H,
the imidazolium salt is strongly
influenced by the presence of base.
CH2), 1.47 (m, 2H, CH2), 1.17 (m, 2H, CH2), 0.90 (t, 3JHH 7.7 Hz, CH3),
products also depends upon the counterion of the used We
salt. now
This report the synthesis
2J
3J
210.89and(dt, 2structure
JPH 19.6, 3of
JHHnovel
5.1 Hz,palladiumIr-H), 219.61 (dt,Figure
1. ORTEP
diagram of palladium complex 2. Selected bond len
PH 18.4,
HH 5.1
surprising and interesting fact certainly needs additional
studies bearing NHCHz,ligands
1H} NMR (CDClcomplexes
and “abnormal”(Cpyand), 153.07
Ir-H). 13inC{“normal”
angles(C(deg):
Pd(1)-C(12), 2.019(13); Pd(1)-C(25), 2.021(
3, 298 K): d 161.77(Å)py),
with other systems where abnormal binding also occurs(t, Jof
6.9
Ccarbene), 137.09(C),2a.
134.96(t, JPCbond= 26.3,CPh), Pd(1)-Cl(2), 2.302(4); C(12)-Pd(1)-C(25), 17
Pd(1)-Cl(1),2.289(4);Figure1.141.30
MolecularofcationofpytoSelectedbinding tomotifs. The
bindingmode
the Hz,NHC
to thePd
isshown
PC structure
JPCpalladium
6.0, C)129.59127.86133.79(5);
C(12)-Pd(1)-Cl(1),93.3(4); C(25)-Pd(1)-Cl(1), 87.3(3); C(1establish how often it takes place.
Ph), 133.61
Ph),
lengths(Å)
and (C
angles(deg):
Os-P(1)2.3605(14),Os-P(2))(t, JPC 4.7,
substantially affectthe catalytic
behaviorof(t,thecom- (NCN),CPh),Os-N(3)126.13 (C)), 125.90 (C(C2.123(6),(CH(CBu), C(25)-Pd(1)-Cl(2), 90.9(3); Cl(1)-Pd(1)-C2
imid2.213(4),py), 124.18)py), 55.08Pd(1)-Cl(2),2), 47.89 88.6(4);2.3683(14),Os-C(1)
H(1A)The complex
OsH6(PiPr
which is3)2 is a d polyhydride plexes.
1H} NMR (CDCl , 298 K): d
558
23 OCTOBER2009VOL
SCIENCE), 19.36 (CBu), 13.37(CBu). 31P{P(1)-Os-N(3)31.96(CBuP(1)-Os-P(2)Fig.
1 Molecular
structureofthe 326
cation of
7a showing www.sciencemag.org
50% probability
177.23(18).
3
H(1B)
)
1.44(5);)164.39(5),)
obtained inthermalhigh ellipsoids.
yield fromcompound- positions)Cl2been
It 2hasreported thatpalladium (II)-NHC complexes could
21.39.
Onlythethe known
metal-boundhydrogensOsH(calculated
99.70(11),P(2)-Os-N(3)) 93.44(11),P(1)-Os-C(1) ) 98.58(14),(PiPr3)2,7 via
the trihydride
tetrahydrideborate‡ Crystal (II)dataacetatefor 7a:andC48H
are shown.
Selected distances(Å) and angles (°):intermediate
Ir(1)–P(1)2.287(2),Ir(1)–
be easily
preparedfrom palladiumthecorresponding
47BF
4IrN3P2, M = 1006.89, colorless crystal,displayed two doublets at 6.57 and 7.47 ppm (J ) 1.7
P(2)-Os-C(1))monoclinic
90.78(14),2 P(2) 2.301(2),i84primitive
cellN(3)-Os-C(1)with dimensions:a)=85.40(18).
9.4493(2), b = 20.6687(6),Ir(1)–N(3)2.193(5),Ir(1)–C(7)2.100(6),N(1)–C(6)OsH3(η -H2BH2)(P Pr3)2. This hexahydride reacts with
Lewis salts. Metal binding
imidazoliumat the C(2)positionis usuallycorrespondingto H(3) and the carbene H(1) of a possibly C
c = 22.0865(6) Å, b = 90.160(2)° and V = 4313.6(2)Å3; Z = 4; m(Mo–1.461(7),N(1)–C(7) 1.404(7),N(1)–C(9)1.335(8),N(2)–C(8) 1.390(8),bases to afford
d4 polyhydride
derivatives,
wheretheobserved,
hydrideand complexes
bearing
tworeaction
NHCs
canpotentially
exist
13
21
23 (iii) the preparation
NHC ligand
in theproducts,
and3. abnormal N-heterocyclic carbenes (aNHCs)	
first metal-aNHC complex	
first free aNHC!
C2- vs C4- bound carbene!
NHC vs aNHC!
application to catalysts	

i

18 September 2012
ubs.rsc.org | doi:10.1039/B107881J

4. summary	benzyl anion to C2. However, calculations indicated that the rearrangement of 9 into 8 is
–1
exothermic
rated by the values of the endocyclic
C–Nby[6:only 6.1 kcal mol and–1involves
barrier of 20.3 kcal mol for the
1.335 T 5 to 1.409 T 5; 9: 1.354 T 2antoactivation
1.408 T 3 Å]
and C–C bond lengths [6: 1.351proton
T 5, 9:transfer.
1.385 TTherefore, we hypothesized thatobserved
rearrangement was catalyzed by a3 Å], which are halfway betweenthe
thoseof single

Downloaded from www.sciencemag.org on September 17, 2012REPORTSi• 3-pyrazolylid
cycloaddition to Fischer carbenes15	
• 4-imidazolyl• 4-triazolylide• 2-pyridyliden

Reviews
ws
What is “Abnormal”?	

carbeneamongst
ligands theamongstmo
8. Abnormal
most thebasicn. Abnormal
NHCs NHCs
heteroatom carbene
alkylationligands
abnormal
NHC (aNHC)!
heteroatom alkylation• 3-pyrazolylidaIncludesthus
NHCs
with
twofar”.
different
stabilizing
heterothus
Therefore,
the4-pyri
synt
far”.
Therefore,
theof•synthesis
2-, 3-,
Those NHC ligands for which a canonical valence bondrepresentation
exciting
challenge.
8.1. requires
Backgroundexciting
challenge.
1. Backgroundthe introduction of additional formal
chargesonsomenuclei.!
cycloaddition
toFischercarbenes
cycloaddition to Fischer carbenesBefore our•free
2-imidazolyl
Beforeourno
studies,no freestudies,aNHCs
O. Schuster, L. Yang, H. G. Raubenheimer, M. Albrecht, Chem. Rev. 2009, 109, 3445.	
• 3-pyrazolylid
even characterizedin andsolution,manyourgroup
researchhas been interested
evenincharacterized
in solution,
but4-pyria rep
For many For
years,
ouryears,
research
hasgroup
been interestedin•2[163c][163c]
co-workers
the synthesis
stable, isomers
unusual ofisomers
of well-known
is •noteworthy.
co-workers
he synthesisof stable,ofunusual
well-known
is noteworthy.
They sho
expanded
rin
[222]
[222]
compounds.
tion ofimidazo[1,5-a]pyridinium
Cyclicallenesfrom
derivedfrompyrazolium
ompounds.
tion
imidazo[1,5-a]pyridinium
salts aNp
Cyclic bent
allenes
derivedpyrazoliumaa bentIncludes
NHCs
with
twoof
different
stabilizing
Includes NHCs withtwo different
stabilizing
heteroatoms
in a heteroatoms
position R tointhe
11
N N ofare constitutional
isomersof imidazol-2-ylidenes,
the that
NHC
that
can be (Scheme
isolated (Schalts are salts
constitutional
isomers
imidazol-2-ylidenes,
NHC
can beisolated
57)
N N thewell-known
unsaturated
B 56).
(Scheme56). C5-Depronot theobservethe corresponding
ell-known
unsaturated
NHCs B NHCs
(SchemeC5-Depronot observe
corresponding
free aN
NHC!
aNHC!
Asuch
fin
C2-substituted
precursors
C2-substituted
precursors
such as aNHC
classific
were ablewere
to isolate
corresponding
able totheisolatethe corresclassific
by performing
the deprotonation
reactiby performing
the deprotonati
all insta[{RhCl(cod)}result[{RhCl(cod)}latterclearlres2]. The latter2]. Theingofa1Figure 1. N-heterocycliccarbenes,
including
the
“cla
1
aNHC hasa reasonable
lifetime, lifetim
and t
aNHC
has a reasonablesimilarl
representative (A) and representatives of subclasses
attemptthepreparation
andisolation
attemptthepreparation
andshoisoreduced heteroatom stabilization (B-K); allnegativearecarbene form.
canonic

limiting
8,9
ongoing work of Bertrand and co-workers.
whetherheteroatom-freecycliccarbenes
are not
furth
Figure 1. N-heterocyclic
includingthe“classical”
NHC
Figure 1.carbenes,
N-heterocycliccarbenes,
includingthe “classical”
NHC
semanti10
representative (A)and representatives
subclasses comprisinghere.(A) and ofrepresentative
representativesof subclasses comprisingexperimreduced heteroatom
stabilization(B-K);
all
are
shown
reduced heteroatom(B-K);inalltheirare showninthei
In the stabilization
literature, different
terms
have
been
a
relativ
carbene form. carbene form.
describe the bonding of such less-stabilized) carbenelectfragments. For example, terms such as “wrs, so the molecule is fluxional on the NMR time scale atFirst Exampleof Metal-aNHC
Complex	
probably
via a boat-boat
six-memberedring-flip.

Once the normal or abnormal c
16	
do not easily interconvert. Attemp
the mixture of 12d and 13 inδ(H) = 8.7 (s)	solution at 100 °C failed. After seHunchanged, and even on prolocolorless solid!
decomposition products were ob
stable toward air and moisture!
THF!
obtained when a sample was hereflux, 2 h	Hof 5 equiv of CF3COO①DMSO, 100 ℃, 1presenceh!
δ(H) = 5.2 (s)	②DMSO, 160 ℃!
Nonclassical①no reaction! Carbene Forma②gradualdecomp.!
pertinent NMR R	
spectroscopicof the complexes
Case.
The ligand
precursors 10byield (%)	 data
mp. (℃)!
For 6bare similar
andmesityl	
consistentrigid abnormalcarbene(eq 4) like their methylene-linke
4.6 Hz, 1
12a	
79	with255-257(dec)	
7.8 Hz, 1
1H NMR features of 12b are repre7.7, J
g in solution.
The
reactionconditions
theproduct
12b	
 iPr	
85	
246-248 (dec)	
H), 1.89–
Hz, 3H,ve: a low-fieldpeak at 8.72
ppm is229
assignedto the acidic(C ), 13complexes 14b-d. The reaction
12c	
 nBu	
68	
(dec)	(C ), 113.30 (Cton; the proton
at C5 is highfield shifted to 5.17 ppm,
substituted 10a with IrH5(PPh
3)7a
For
12d	
 Me	
75 (C5/C2 = 1.2:1)	
J
5.5
C4-Ir	e CH2 linker group appears as a singlet at 4.70 ppm,py-H), 5.
we did notsee full conversiont
Hz, CH),Hz, Ir-H①DMSO,
100conformation℃, several h! and an effective
ent with a nonrigid
ligandfrom CHCl3/pentane or THF/pen(CDCl ,②DMSO, 100 ℃, 3 d!
C),mmetry in solution. The two inequivalent hydrides have), 132
C °C
colorless, high-melting (>200124.15(
P{ H}
eld resonances at -10.83①C4/C2
ppm (probably
trans
to
N)
and
= 1.2:1!
iPr is not adjacentwhere a mixture was seen prev
For 7b
5.1 Hz, 1
14②decomp.!
to
the
M-C
bond	
1H, py-H
9 ppm (trans to C) both split into doublets of triplets
carbene was now observed. ThesCH ), 1.4
215
210.89 (ual coupling ( JHH ) 4.9 Hz, typical for cis hydrides )Hz, Ir-HfK) for 14 are very similar to that141.30 (t2(Cupling to apparently equivalent phosphorus nuclei ( JPHbinding. For example, in the133.79
C ),ca126
31.96 (C
1 Molecular structure of the cation of 7a showing 50% probabilityand 18.6 Hz, respectively). The singlet at 21.4 ppm in Fig.
21.39.
thermal ellipsoids.
Only the metal-boundhydrogens(calculated
positions)
protonsappeartobewell-separat‡ Crystal
are shown. Selected distances (Å) and angles (°): Ir(1)–P(1) 2.287(2), Ir(1)–primitive
P(2)H.
2.301(2),Ir(1)–N(3)13
2.193(5),1 Ir(1)–C(7)2.100(6), 2001,
N(1)–C(6) 2274.!
Kovacevic, M.
Albrecht,
J. W. Faller, R.
Crabtree,Chem.
Commun.
P{1H} NMRS. Gründemann,
spectrum isA.consistentwithmagnetically
in
the
H}
NMR
c = 22.0
1.461(7), N(1)–C(7)1.404(7), C{
N(1)–C(9)1.335(8),N(2)–C(8) C2
1.390(8),appear
S. Gründemann, A. Kovacevic, M. Albrecht, J. W. Faller, R. H. Crabtree, J. Am. Chem. Soc. 2002, 124, 10473.!
3

HH

py

imid

3

HH

3

carbenePhn 18 September 2012//pubs.rsc.org | doi:10.1039/B107881J

31

1

2

Ph

N(2)–C(9) 1.327(8), N(2)–C(10) 1.459(8); N(3)–Ir(1)–C(7) 89.3(2), P(1)–Ka) = 3accelerate
C–H bond cleava
those heterolyticof single
Sodium and potassium bases have proven 3 Å], which are halfway between
new compound was the aNHC lithium 7. SimilarAll
to deprotonate–the i3 Å],andwhich
are halfway
betweenthoseof attempts
singleand potassium
bases bases
have inprovennew complexation
compound washas
thepreviously
aNHC lithium
Similar for Sodium
doublebonds. The
carbene
bond
angle
N1–
more appropriate
than lithium
generating
been7.observed
tetrafluoroborate
salt
6
(BF
) has
fa
hydrogenbonding,andthiseffect–4
and double
The carbenebondangleacuteN1– than salt 6 (BF–4 ) failed
morefreeappropriate
lithiumbases incomplexation
has carbenes
previously
been
for (35)
tetrafluoroborate–
C5–C4bonds.
for 9 [101.03
T 17°]
is morecarbenesthan(35–37),
becausethegeneratingcorresponding
other singlet
such
as observed
small NHCs
small
anionssuch as –Cl andC2C5-unsubstituted
imidaz–Br
C5–C4
9 [101.03 Tangle17°] iniswithmore
acute andthan
(35–37),
becausecorresponding
otherandsingletcarbenes such as small NHCs (35) free carbenes
theforcorresponding
thecationic
precursor
carbon-heavy
alkalimetal thebonds
are more labile,bis(diisopropylamino)cyclopropenylidene
smallanions
such
as
Cl
and
Br
ar
the corresponding
angle
in thecationic
precursor
carbon-heavy
alkaliprecipitation
metal bondsofarethemoreand (36,bis(diisopropylamino)cyclopropenylidene
accelerateheterolytic
C–Hbondtoaccelerate
favor
metallation
ofC–H
C2
(with
theclm
6 [108.0 T 3°].
Thisfeatureisconsistentwithwhich favors
salt.labile,
When the37); in both cases, coordination of the lithium
heterolyticbond cleav
6[108.0T
3°].
Thisfeatureisconsistent
with
which
favorsprecipitation
of
the
salt.
When
(36, 37);in
bothcases,
coordinationofthelithium–thes lone-pairorbitalon(33). We
cation was apparent from the broadening and the deprotonation of imidazolium 6 (HX•X, where increased s character of theproton)hydrogenbonding,
andreasoned
this effectth
over
C5
–
increaseds character
ofinthes comparedlone-pairorbitalonC–H+
ofBr)
imidazolium
6 (HX•X
cationupfield
was apparent
fromcarbene
the broadeningandthe deprotonation, where13hydrogenbonding,
and this
effect hathecarbene
atom
9
as
with
the
shift of the
X
is
Cl
or
was
performed
withtwoequivC
NMR
resonance.
13
+6,
with
C2and
C5-unsubstituted
imp
protected
insmall
anionsshouldthecarbene
atom
in
9
as
compared
with
the
C–H
upfield
shift
of
the
carbene
X
is
Cl
or
Br)
was
performed
withtwoequivC
NMR
resonance.
yellow
crystal!
with C2and C5-unsubstituted
imida
Subsequently, we sought to sequester the lithium alents (34) of potassium hexamethyldisilazide bonding orbital in 6. A similar
relationshipis(34) of potassium
hexamethyldisilazide
Subsequently,
we sought
to ofsequester
the lithium to alents
bonding
in 6. Aandsimilarrelationshipis+ deprotonation
tofavor
metallation
of
(with
t
attheC
toorbital
air!
in tetrahydrofuran,a clean sensitive
reaction
cation throughadditionexcess [12]crown-4
a (KHMDS)
observedin NHCs
theirdesired
NHC(H) precursors
+
to
favor
metallationof reaction
C2C2
(with
–
(KHMDS)intetrahydrofuran,aclean
reaction
cationdiethylether
through additionofexcess[12]crown-4
toaobserved
in
NHCs
and
their
NHC(H
)
precursors
13
solution of 7 (where X is Br). THF!
This occurred, with13the C NMR spectrum of(3, 4).for a few days!
the re- atwhen
6 (HCl•Cl)(33).
wastreatedwith tproton)
over
C5
(33).
We
reasonestableproton)over
C5Wereasoned
occurred,with
the Cshowing
diethylether
solutionofa7clean(whereX is Br). This(3, 4). rtCalculations
NMR spectrumof thesignal
re- attreatment
inducedrearrangement
to genpredict aNHC
9
to
be
14.1
kcal
sulting
producta very sharp
alents
of
ain
base
such
as
n-b
6,
small
anionsshou－78
30 min!
treatment
cleanisolated
rearrangement
gen-℃,
predictthan
aNHC
9protected
to protectedbe (34)14.1
kcal
sulting
productvery sharp
signalat up,
mp.
65Calculations℃ –1(dec)!
in NHC
6,lithiumsmallanionsshoulderate induced
8, whicha wasin 45%toyield.
This
201.9
ppm.showing
After theaproducts
wereworkedless stable
its isomericnormal–1mol→rt, 201.9
2 h	
 ppm. After the products were worked up, mol less stable than its isomericerateproduct
8, which
was isolated
in 45%
yield. This
normalNHC(nBuLi)lithium
desireddeprotonation
reactionat aC
formally
results fromthe deprotonationof thefree aNHC 9 was isolated as a green powder with the phenyl group bonded
to C5orinsteadof diisopropylamide
desireddeprotonation
reaction1–
product
formally
resultsfromthedeprotonation
of
the
free
aNHC
9
was
isolated
asagreen
powderwith
thephenylgroup
bondedtoC5insteadofnuclearmagnetic(with
HwN– ) resonance
an isopropyl substituent of the Dip group by the (480 mg, 68% yield), and single crystals were C2. Figure 4 shows the proton
twohighestoccupiedwhen
6(HCl•Clwastreatedwhen
6(HCl•Cl)wastreatedan isopropylof theDip group
the 7,(480 obtained
mg,
68%by
yield),
and =singlecrystals
were
C2. Figure
4 shows
two highest
occupied
68%
yield
(X
Cl)!from
carbene substituent
center of theaNHClithiumbyadductrecrystallization
a dryhexanemolecular
orbitalsthe(HOMOs)
of
9.
The
HOMO
trum
of(34)
theproduct
showed(34)
oflithiuma lithium
base
such
as
carbenecenterbyofnucleophilic
the aNHC addition
lithium ofadduct7, obtainedby recrystallizationa dry hexane molecularorbitals
9. alents
The
HOMO
alentsofresulting
base
such
as nfollowed
the resulting
solutionat –78°C(Fig. 3,fromright).
(–4.403eV) is(HOMOs)a s-type oflone-pairorbitalat aC5;green
powder	pearance
thelithium
singletat 8.7 parts
followed by nucleophilic addition of the resulting solution at –78°C (Fig. 3, right).
(–4.403 eV) is a s-type lone-pairorbitalatof(nBuLi)diisopropylam
benzyl anion to C2. However, calculations inIn the solid state, both the free aNHC 9 and the HOMO-1 (–4.879 eV) is(nBuLi)
a C5–C4
p-bondingorC5;orlithium
diisopropylamide
benzyl anion to C2. However, calculations inIn the solid state, both the –free aNHC 9 and the HOMO-1 (–4.879 eV) is a C5–C4
p-bonding(ppm)
arisingfromC5(H)of 6. In(1thedicated that the rearrangement of 9 into 8 is the imidazolium salt 6– (Br ) feature a fully planar orbital, which exhibits antibonding
conjugation
protonnuclearmagnetic
resonance
protonnuclearmagnetic
resonanceH(dicated that the rearrangement of 9 –1into 8 is the imidazolium salt 6 (Brorbital,which
exhibits
antibonding
conjugation) feature a fully planarfrom
recrystallization
Hex.
at
－78
℃	
exothermic by only 6.1 kcal–1mol and involves ring (maximum deviation for N1–C2–N3–C4–C5– with the p orbital of the phenyl
substituent
atC4.
spectrum,theC5
carbon
gives
rise
to
trum
of
the
resultingproduct
shoa
exothermic by only 6.1 kcal mol and involves
atC4.
ring (maximum deviation for N1–C2–N3–C4–C5– with the p orbital of the phenyl substituent
trum
ofhighertheresultingproduct
showe
much
in
enan activation barrier of 20.3 kcal–1mol–1 for the C21–C31–C41–C53 was 1.9 and 6.3 pm for 9 and These molecular orbitals areresonance
at of
190
ppm,
ispas
higher
in ofenwas
1.9 and 6.3 pm for 9 and These molecular orbitals are muchan activation barrier of 20.3 kcal mol for the C21–C31–C41–C53
pearance
singletat
pearancethesingletatwhich8.78.7parts
yellow
crystal	
NHC (–5.000
andtheconfirming the delocalization of ergy than those of the isomeric
proton transfer. Therefore, we hypothesized that 6, respectively),(–5.000 arisingandproton transfer. Therefore, we hypothesized that 6, respectively), confirming the delocalization of ergy than those of the isomeric NHCof the(ppm)
from C5(H) ofresona
6. In
which
indicatesthatcorresponding
the observed rearrangement was catalyzed by a the p system. This electronic structure is corrobo- –5.279 eV, respectively),downfieldarising
indicatesthat from C5(H) of 6. In th
the observed rearrangement was catalyzed by a the p system. This electronic structure is corrobo- –5.279 eV, respectively), which(ppm)
6
(124
ppm).
AlthoughtheFig. 2. Synthesis of aNHC lithium adduct 7, rearrangement product 8, free aNHC 9, and its ensuing precursor
spectrum,carbongives
spectrum,
thetheC5C5carbongives
riserisetoFig. 3. Molecular views
Fig. (50%
3. Moleculargold(I)
complex
10 and CO2-adduct 11.
dicatedthatata at190
deprotonation
had isociresonance
190ppm,ppm,
whichthermalviews
ellipsoids
resonance
which(50%arethermalellipsoids
shown) of imidazolium
downfield
corresponding
redownfield
of ofthethecorrespondingreson
are shown)of imidazolium–
bromide–6 (Br ) (left) andbromide6(Br)(left)
andprecursor
6 (124ppm).
AlthoughFig.
Synthesis
aNHClithium
lithiumadductadduct7,7,
rearrangement
product
free
aNHC
9,
its
ensuing precursor
ppm).
Although
th
Fig.
2.2.
ofofaNHC
rearrangement
product
8, 8,
free
aNHC
9,326
andandits23
ensuingaNHC
9Synthesis(right)in the solid
www.sciencemag.org
SCIENCEVOL
OCTOBER
20096 (124aNHCstate
9 (right)
in
the
solid
(for
clarity, H 10
atoms
gold(I)
complex
10andandCOCO11.
dicateda deprotonation
gold(I)
complex11.
dicatedthatthata deprotonation
hadhado2-adduct2-adductstate(for clarity,
H atomsareomitted,except forare omitted,
except forBond
the ring hydrogen).
the ring hydrogen). Bond
lengths and angles forlengths andangles forwww.sciencemag.org
326
OCTOBER
2009
www.sciencemag.org SCIENCESCIENCE VOL
VOL
326 2323
OCTOBER2009
6 (Br–) are as follows:
6 (Br–) are as follows:
C5–N1, 1.368 T 4 Å;
C5–N1, 1.368 T 4 Å;
N1–C2, 1.334 T 4 Å;
N1–C2, 1.334 T 4 Å;
C2–N3, 1.363 T 4 Å;
C2–N3, 1.363 T 4 Å;
N3–C4, 1.408 T 4 Å;
N3–C4, 1.408 T 4 Å;
∠N1-C5-C4!
C4–C5, 1.355 T 5 Å;
∠N1-C5-C4!
C4–C5, 1.355 T 5 Å;
N1–C5–C4, 108.0 T 3°;
101.03 ± 17°	N1–C5–C4, 108.0 T 3°;
108.0 ± 3°	
C5–C4–N3, 106.0 T 3°;
C5–C4–N3, 106.0 T 3°;
C4–N3–C2, 108.8 T 3°;
C4–N3–C2, 108.8 T 3°;
N3–C2–N1, 106.9 T 3°;
N3–C2–N1, 106.9 T 3°;
C2–N1–C5, 110.4 T 3°.
C2–N1–C5, 110.4 T 3°.
Bond lengths and angles for 9 are as follows: C5a–N1a, 1.417 T 2 Å; N1a–C2a, 1.357 T 2 Å; C2a–N3a, 1.345 T 2 Å; N3a–C4a, 1.412 T 3 Å; C4a–C5a, 1.383 T 3 Å;
Bond lengths and angles for 9 are as follows: C5a–N1a, 1.417 T 2 Å; N1a–C2a, 1.357 T 2 Å; C2a–N3a, 1.345 T 2 Å; N3a–C4a, 1.412 T 3 Å; C4a–C5a, 1.383 T 3 Å;
N1a–C5a–C4a, 101.03 T 17°; C5a–C4a–N3a,111.01 T 16°;A.
C4a–N3a–C2a,107.97 T 15°;
N3a–C2a–N1a, 106.25
16°; C2a–N1a–C5a, 113.72
15°.
E. Aldeco-Perez,
J. Rosenthal,
B. Donnadieu,
P. TParameswaran,
G.T Frenking,G. Bertrand, !
N1a–C5a–C4a, 101.03 T 17°; C5a–C4a–N3a, 111.01 T 16°; C4a–N3a–C2a, 107.97 T 15°; N3a–C2a–N1a, 106.25 T 16°; C2a–N1a–C5a, 113.72 T 15°.

17	

Downloaded from www.sciencemag.org on September 17, 2012

Downloaded from www.sciencemag.org on September 17, 2012

First Example of Free aNHC	

Science, 2009, 326, 556.!

prepared in good yie

of 20.3 kcal mol–1 for the C21–C31–C41–C53 was 1.9 and 6.3 pm for 9 and These molecular orbitals are much higher in enfor thebromideknown
syntheticprergyaNHC
than thoseof the isomeric
NHC(–5.000 and C2 and C5 metal
6, (Top)respectively),
confirming
the delocalizationof andefore, we hypothesized Fig.
that 1.
Resonance
structures
for
NHC(I/I′)(III/III′),
and
their
correspondingesAll
attempts
ChemieeV,
respectively),which
indicatesthat
gement was catalyzed by a the p system. This electronic structure is corrobo- –5.279
fullycharacterized
Thus,
Thus,
for forthe thefirstfirsttime,time,
we prove
we proverigorously
rigorously
much
much
of whatof whatb

Downloaded from www.sciencemag.org on Septem

Angewandte 17-1	
First Example
of
Free
aNHC	
complexes, II and IV, respectively. (Center) Synthesis of aNHC metal complex 1 by Crabtree and co-workers

tetrafluoroborate

x-ray(15). (Bottom) Synthesis of metal-free NHC 3, and aNHC
metal
complexonly
5 byonly
Lassaletta
and co-workers (27). a single-crystal
ws
waswaspreviously
previously
speculation.
speculation.
small
anionsds
for the bromide su
sa
counter anion and base effects	
accelerateheteroum
solution, it rearranges quantitatively to 58 upon heating inAllattempts
tondhydrogenbondibenzene at 50 8C for 48 h (Scheme 59). This fused heterocycle tetrafluoroborate sa
lidwith C2- and C5
Results
Results
andandDiscussion
ms
most
likely results from the deprotonationofanDiscussion
isopropyl small anions such
nBuLi!
!
−	to favorheterolyt
metallafor
HX・X
ndGeometries
Geometries
and and
Energies
Energies accelerateor LDA	
proton)bonding,over C5hydrogenorprotected
in 6, sws:
with C2and
C5-u
The
The
optimized
optimized
geometries
geometries
of
the
of
the
free
free
imidazole
imidazole
isomersisomers
desired deproto
fÅ;free aNHC2 and of its lithium adductto favor
metallation
nNHC,
nNHC,
aNHC,
aNHC,
and and
IMID
IMID
at theat theBP86/TZ2P
BP86/TZ2P
levellevel
are
Å;
when
6 are
(HCl•CÅ;
proton)
over
C5
(3
shown
shown
in Figurein Figure1, along
1, along
withwiththe themostmost
importantimportantalents bond(34) bondof a li
Å; X = BF − → all attempts failed!
Li
base!
protected
in 6,comsma
4−−Å;lengths
lengths
and andanglesanglesand andthe therelative
relativeenergies
energiesof (nBuLi)
theof thecomor lithiu
X
=
Cl
,
Br
(smallanion)!
→
Li-aNHC
complex!
desired deprotonat
3°;
nuclearmpounds.
pounds.
The
The
full
full
set
of
set
geometricalofgeometrical
data
data
is
listedislistedinprotonthein SuptheSuponsknown
toaccelerateheterolytic
3°; are→ promote the deprotonation !
Na, K base!
when 6 (HCl•Cl–)
trumof
the resu
3°;
porting
Information,
Information,
Table
Table
S1. S1.
TheTheimidazole
imidazole
molecule
molecule
throughreactionhydrogenatbonding,and interest- → free aNHC!porting
alents(34)
of a lithiu
C5	
3°;
!1
!1
2
pearanceof the
IMID
IMID
is of
26.7
isaNHC
26.7
kcal
kcal
molmollower
lower
in energy
in energy
thanthan
the
the
nNHC
nNHC
Scheme 59. Rearrangement
uponheatingin solution.
(nBuLi)or lithium
been
used with C2- and C5-unsubstituted
3°.
(ppm) arising fr
les for 9 are as follows: C5a–N1a, 1.417 T 2 Å; N1a–C2a, 1.357 T 2 Å; C2a–N3a, 1.345 T 2 Å; N3a–C4a, 1.412 T 3 Å; C4a–C5a, 1.383 T 3 Å;
proton nuclear magn
favor
metalation
at
C2
(with
the
more
HOMO
of
aNHC	
relativeenergy	spectrum, the C5
3 T 17°; C5a–C4a–N3a, 111.01 T 16°; C4a–N3a–C2a, 107.97 T 15°; N3a–C2a–N1a, 106.25 T 16°; C2a–N1a–C5a, 113.72 T 15°.

trum of the resultin
er C5.[231] Since the C2 position ofresonance at 19
pearanceof the si
alsubstituent of the Dipp group by the carbene center, followed
ected, small anions should promote thedownfield of theng(ppm) arising fromby nucleophilic
addition of productthe resulting
benzyl9,anion
C2.
on reaction at C5. Indeed,
deprotonationprecursor 6 (124Fig. 2. Synthesisof aNHC lithium
adduct 7, rearrangement
8, free aNHC
and itstoensuingMO
spectrum,the C5 ca!
2+ht) (H ) with HClgold(I)
10 and11.
dicated that a d
Theresults reported above, including the latter rearrangeHC
as complex
the anion
wasCO2-adduct2resonance at 190
ment, gives aPhgood indication of the substituent patterns that downfield of the coquivalents of a lithium base such as n- Dip
Nallow for the isolation of other aNHCs. Taking into
will
A, but, as alreadyFig.
observedwith diaminoprecursor2009
6 (124 p
2. Synthesisof aNHC lithium
adduct7, rearrangement product SCIENCE
8, free aNHC VOL
9, and326
its ensuing
www.sciencemag.org
23 OCTOBER
22
2
N
account
CP and cyclicgold(I)bent-allene
the2-adductcomplex CBA
10 and, CO
11. that aNHC is stable despite the presence of a simple dicated that a dep
PhDipon the carbon atom adjacent to the carbene
phenyl group
was formed. Potassium bases, especiallycenter, steric bulk does NHC
not seembe a requirement.
is kcal/mol)	
proven here to be more appropriate for(0.0 tokcal/mol)	
aNHCThis(17.4
www.sciencemag.org SCIENCE VOL 326 23 OCTOBER 20092
in line with the absence of a possible dimerization pathway.
ed free aNHC , which was isolated after[232]
However, accessible C!H bonds spatially close to the carbene
owder in 68 % yield.
centershould G.
beHeydenrych,
avoided to G.
prevent
the Chem.
intramolecular, free aNHC2 features a fully planar ring,
R. Tonner,
Frenking,
Asian J. 2007, 2, 1555.!
as Donnadieu,
observed in therearrangement leadingtooms also being in a planar
environment A. deprotonation
E. Aldeco-Perez,
J. Rosenthal, B.
P. Parameswaran,
G. Frenking,G. Bertrand, !
58. Notably, the substituent at C4 undergoes conjugation
with 2009, 326, 556.!
, 1.9 pm; Figure 15). The endocyclic C!NScience,Zr (1Zr)
aNHC form.
This is in (Lagreementwiththe previous
workbyCentral atomsW
!
nNHC,
aNHC,
IMID).
full(2W)
was also found by SEC, who calculated
was also
17found
TMA[ClTM(L)]CHTUNGRE(mol
nNHC)
was
by!1SEC,
also
comfound
who=calculated
byone
SEC,
metal
who
17 TMAcalculated
fragment,
CHTUNGRE(nNHC)The
17
PtCl
comTMA[14a]
3 ,CHTUNGRE(
Hf
(1Hf)
).
Bonding in NHC Tautomers and Phosphine Ligands
who metrical
reported datathat isthegiven
relative
are IMIDin energies
the SupportingIn18	
C2- vs C4-bound Carbene	
 Ti (1Ti) Fe (3Fe)SEC,!1Figure 2 shows the optimized!1geometriesofth(0.0 kcal mol) <S2.
nNHC (28.9 kcal mol ) < aNHC (48.9 kcal
Zr (1Zr)
Cr (2Cr)
Table
3
Ru (3Ru) !1 [14a] complexes [Cl TM(L)], [(CO) TM(L)], [(CO) T
4
5
Hf (1Hf)
Mo (2Mo)2mol ).
Thebondequilibriumgeometries
ofthe EGroup
44en
(3Os)
kinetic vs thermodynamic-calculation-	
Table 2. Metal–ligand
bond lengths Os
dAW
CHTUNGRE(TM–L),
Table
2.
relative
Metal–ligand
energies
Table
2.
E
Metal–ligand
lengthsofthecomplexes,
dA
(
E
R
G
N
U
T
H
C
TM–L),
bondlengths
relativebond-dissociationdA
energies
(
E
R
G
N
U
T
H
C
TM–L),
energies
relative
of
the
D
c
rel
rel
[ClTM(L)]
(L
=
nNHC,
aNHC,
IMID).
The
full(2W) Figure 2 shows the optimized geometries of the calculated[a]!1 (TM = T
TMA
(
E
R
G
N
U
T
H
C
nNHC)]
and
[Cl
TMA
(
E
R
G
N
U
T
H
C
aNHC)]Cu (4Cu)
4
4ligands.[a]investigated.
of the ligands.[a] [Cloftheligands.
Cr (2Cr)
Table 1. Overview of ofthethecomplexes
more staI
isomer, which
inis turnis 17.4kcal
metricaldata
given
in [(CO)the molSupporting
complexes have
[Cl
[(CO)
and
4TM(L)],
5TM(L)],
4TM(L)],
Ag
(4Ag)
4
Fe
(3Fe)
Mo
(2Mo)
the
carbeneligandintheequatorial
[a]
aNHC
form.
is in agreementwith thepositio
previ
LnNHC Central
L
L
nNHCThisaNHC
nNHC
Descriptor
Complex
atoms
Table
S2.
Au (4Au)[ClTM(L)](L =imidazole
nNHC,
aNHC,
IMID).
fullofaxialgeo3(3Ru)
298 The298 settheligandIMID
occupies
andACHTUNGRE(TM–L) W (2W)
Erel
Erel dACHTUNGRE(reported
TM–L)
ED
DRu
D0298 dACHTUNGRE(TM–L)
dACHTUNGRE(TM–L)
DEe rel (kcal/mol)	
D
dACHTUNGRE(TM–L)
D0298
dACHTUNGRE(Terel
0 e theDrelative
eD0
SEC,
who
that
energieTheequilibriumgeometries
of
the
Group
4
Ti (1Ti)
Os(3Os)
metrical data
is
given
in
the
Supporting
Information,
!1
!1
[Cl
TMA
(
E
R
G
N
U
T
H
C
IMID)](Figure2).
Theenergydifference
4Free L
0.0
Free
L 0.0
(0.0
kcal
mol ) <17.4
nNHC0.0(28.9 kcal mol ) < aNH
Fe (3Fe)1
Zr Free(1Zr)L[Cl
Cu(4Cu)
4TMACHTUNGRE(nNHC)] and [Cl4TMACHTUNGRE(aNHC)] (TM = T
Table
S2.
!1
[14a]
the
axial
and
equatorialisomersof 2.213
the
1Ti
2.230
0.0
26.5
1Ti
12.0
2.230
1Ti
2.213
0.0
2.230
26.5
11.9
0.0
32.0
26.5
17.4
12.0[Cl4TM
2.3
Ru (3Ru)
Hf (4Ag)(1Hf)
mol the). carbene ligand12.0
Ag
4
have
in
the
equatorial
positi
The equilibriumgeometries
the42.358complexes
1Zr
2.378
0.0
33.5 1Zr 18.8
2.378
1Zr 2.358
0.0
2.378 33.5
11.7 of Geometry
18.8
0.0
39.1 Group
33.5
24.4
18.8
2.4o
[a]
Os (3Os)
plexesaresmall.
optimization
Au (4Au)
Figure2quiteshows
the optimized
geometries
of th
the
imidazole
ligandIMID
occupies
an
axial
Cr
(2Cr)
1Hf
2.346
0.0
34.0
1Hf
19.3
2.346
1Hf
2.327
0.0
2.346
34.0
11.7
19.3
0.0
39.8
34.0
25.0
2.327
19.3
2.3
[Cl4TMACHTUNGRE(nNHC)]and[Cl
CHTUNGRE(aNHC)](TM=
Ti,CHTUNGRE(aNHC)]
Zr, Hf)Cu (4Cu)
4TMA
Appelhans et al.
form
of
[Cl
TMA
(
E
R
G
N
U
T
H
C
nNHC)]
and
[Cl
TMA
g
4
4
complexes[Cl
TM(L)],[(CO)
TM(L)],[(CO)452.2
5 41.7
4T[Cl
CHTUNGRE(IMID)](Figure2).
energydifferenc
Mo2Cr
(2Mo)
2
2Cr
2.090
0.0
52.2
36.6 the2.090
2Cr
2.108
0.0 2.090
13.7 equatorial
36.6
0.0
55.9 The
52.2
2.108
36.6
2.1
4TMA
Ag (4Ag)!
havecarbeneligandintheposition,whereas
tures
that,
in
cases, 36.5
areless49.4
than
1the
kcal
mol
[ClTM(L)](Lall=nNHC,
aNHC,
IMID).
The
ful
W 2Mo
(2W)36.5 2.238
2Mo
2.238
0.0
49.4
2Mo2.252
0.0and
2.238
49.4
13.6
0.0
53.1
39.9
36.5
2.2
the
axial
equatorialisomersof2.252[Cl
Au (4Au)
4TM
NHC, however, the abnormal
C5
isomerwastheimidazole
ligandIMID
occupiesanaxial
positioninenergythantheequatorial
energyminima.
The
2W
2.228
0.0
55.7
2W
42.7
2.228
2W
2.245
0.0
2.228
55.7
13.5
42.7
0.0
59.6
55.7
46.4
2.245
42.7
metricaldata
issmall.
given
in the optimization
Supporting2.2a[a]plexesarequite
Geometrynormal isomer could still beif the C4 0.0
Fe 3Fe(3Fe)[Cl45.4
IMID)](Figure2). 60.1
The
differences
between
3Feobtained1.968
60.1
3Fe 1.993
0.0
13.7 energy
45.4
0.0
63.8 imaginary
60.1
48.3 1.993
45.4
2.0
4TMACHTUNGRE(1.968
haveoneor1.968
sometimes
two
modes
wi
Table
S2.
form
of
[Cl
TMA
(
E
R
G
N
U
T
H
C
nNHC)]
and
[Cl
TMA
(
E
R
G
N
U
T
H
C
aNHC)]
4
4
3
Ru3Ru
(3Ru)
s were blocked,
as in the benzimidazolium
3Ru
2.108 salts. 0.0
48.0
36.0
2.108
3Ru
2.129
0.0
2.108
48.0
13.4
36.0
0.0
52.0
48.0
39.3
2.129
36.0
2.1
the axial andequatorialisomerstheof equatorial
the [Cl4TM(L)]com-oquencies.
Likewise,isomersThe
equilibriumgeometriesof the
Group
4
Os3Os
(3Os)
tures
that,
in allcases,
are
less
than
1
kcal
mol
53.8
40.9
2.119
3Os
2.141
0.0
2.119
53.8
13.2
40.9
0.0
58.0
53.8
44.1
2.141
40.9
2.1
tip groups at N3, such as 3Os
methyl, a 2.119
mixture of 0.0!1
plexesare ACHTUNGRE(quite
small.
Geometry
optimization
ofstablethe axial
IMID)]
are
less
than
1
kcal
mol
Thus, for the first 4Cutime, we proverigorously
much
of
what
less
tha
[Cl
TMAthan
CHTUNGRE(0.0nNHC)]
and 56.0[Cl
CHTUNGRE(aNHC)]
=
Cu4Cu
(4Cu)
the67.9equatorial
energyThe
4TMA
1.8461). For 0.0
67.9
56.0 1.846energy
4Cu41.850
1.846
13.4
0.0
71.9
67.9
59.9minima.
1.850
56.0(TM
1.8
abnormal (3) NHCs was seen (Table
form
of
[Cl
TMA
(
E
R
G
N
U
T
H
C
nNHC)]
and
[Cl
TMA
(
E
R
G
N
U
T
H
C
aNHC)]
gave
struc44was
speculation.
forms.
Ag4Ag(4Ag)
4 previously only4Ag
2.050
0.0
53.4
42.0 2.050
4Ag2.054
2.050
53.4
12.8
42.0
0.0
57.9
53.4
45.9
2.054
42.0 posit
2.1
have
the0.0orcarbeneligandin theequatorial
haveonesometimes
two
imaginary!1 modes wa
45:55
action of eq 1 (R ) Me, A4Au
) BF4) gave
tures
that,
in
all
cases,
are
less
than
1
kcal
mol
higher
in
Au4Au
(4Au)
!L 1.986
1.978
0.0
76.3
64.4 1.978 the
4Au
1.986
1.978the76.3
13.2optimized
64.4
0.0
80.6 TM
76.3
68.0
64.4
2.0
Table
20.0
gives
bond
imidazole
ligandIMID
occupies
an length
axial
quencies.
Likewise,theequatorialisomersts 2d and 3d (R ) Me, A ) BF4).
energythaninthewereequatorial
energyminima.
The
axial
forms
!1
!1isomers,!1!1
energies
of
the
and
the
calculat. whatAll calculations
at energies
the.1BP86/TZ2P
Alllevel.
were. All
performe
calculati
Bondwelengthsin !,energies in much
kcal mol[a]Bondlengthstive[a]!,4Bond
energies
lengthskcal
in (Figure
!,
mol
inThe
kcal
mol
TMA
CHTUNGRE(performedIMID)]
2).calculations
energydifferen
ACHTUNGRE([ClIMID)]
areinless
than
kcal
mol
Thus, for the first [a]
time,
prove
rigorouslyofHeydenrych,
lessstableth
R. Tonner,
G.
G.
Frenking,
Chem.
Asian
J.
2007,
2,
1555.!
have one orsometimes
two
imaginary
modes
with
low
freBDEs.
The
calculated
data
revealinteresting
Results
andDiscussion
the axial and equatorial isomers of the [Cl4Tt
was previously only speculation.
forms.
quencies.
Likewise,theisomersof [Cl4TM[a]
aNHC
ligandisequatorial
alwaysmorestrongly
bondedplexesarequite
small.
Geometry
optimization
!L
Table
2
gives
the
optimized
TM
bond
lengt
nNHC
(2)!
aNHC
(3)!
!1
and Energies
(
E
R
G
N
U
T
H
C
A
IMID)]
are
less
than
1
kcal
mol
s, for the first time, we Geometries
prove
rigorously
much
of
what
less
stable
than
the
axial
Chem. Asian
J. 2007, 2,
1555
–
1567
Chem.
"
Asian
2007
Wiley-VCH
J.
2007,
Chem.
2,
1555
Verlag
Asian
–
1567
J.
GmbH
2007,
&
2,
Co.
1555
KGaA,
–
"
1567
2007
Weinheim
Wiley-VCH"
Verlag
2007
GmbH
Wiley
ww
E (kcal/mol)	
 E (kcal/mol)	
nNHC
ligand.
The
lies
betweeformenergies
of[Cl4TMACHTUNGRE(nNHC)]
and DD[ClCHTUNGRE(aNHC)]
tiveof
thedifferenceisomers,and4eTMA
thecalcula!1!1previously
speculation.
forms.
mol
3FeL
andcases,
2CrL
andless
5.8 thankcalmol
The only
optimized
geometries
of freethe free 0	
imidazole 14.5	
isomerstures forthat,
incalculatedallare
1 kcal for
mol
BDEs.
The
data
revealinteresting
Results andDiscussion
	
!
Table
2
gives
the
optimized
TML
bond
lengths,therelastronger
TM–aNHCbondsmore
maketheminima.
energydiffnNHC, aNHC, and IMID at the BP86/TZ2P level areenergy ligandthantheenergyThe
aNHC
is equatorial
alwaysstronglybonde
Ir	
 Energies0	
10.1	
Geometries
and!L*didn’tintroduce
anion
effect.!
tive
energiesoftheisomers,
andthecalculated
TM
tween
the
complexes
TMAtwoCHTUNGRE(nNHC)
andTMA
CHTUNGRE(betwe
aNH
shown in Figure 1, along with the most important bondhave oneor sometimes
imaginary
modes
w
nNHC
ligand.
The differenceDDe liesBDEs.
calculated
data
revealinteresting
trends.
The
Results
andDiscussion
!1
!1
than
those
the
free
ligands,
so
that
the
T
lengths
and angles
and therelativeenergies
theisomers
com-A. The
quencies.
Likewise,theequatorialisomersmolforA.between
3FeL
and
2CrL
5.8 kcalmolfor
The optimized
geometries
thefreeimidazole
L. ofN.
Appelhans,
D. of
Zuccaccia,
Kovacevic,
R.
Chianese,
J.and
R. Miecznikowski,
!
!1
!1
aNHC
ligandisalwaysmorestrongly
bondedthanthecomplexes
areonly
11.7–13.7
kcalthemolpounds.
The
fullfirst
setof
geometricaldata
is listedin levelthelower
A.IMID
Macchioni,
E.
Clot,
O. Eisenstein,
R. H. Crabtree,
J.
Am.
Chem.
2005,
127,
6299.!
ACHTUNGRE(IMID)] TM–aNHC
are
less
than
1Soc.
kcalmakemolThus, Geometries
for
the
time,
we proverigorously
muchofSupwhatlessstablethstrongerbonds
energy
dif
nNHC,
aNHC,
and
at theBP86/TZ2P
areandEnergiesnNHC
ligand.
ThedifferenceDD
lies
between
3.7
kcal
e CHTUNGRE(nNHC)
ratio isporting
kinetic
becausetheratio with
isS1.
invariant
Table
The most
imidazole
moleculethanthetheTMA
CHTUNGRE(aNHC) species.
For andthe BDEso
was previously
only
speculation.
forms.
tween
complexes
TMA
TMACHTUNGRE(aNH
shown
inInformation,
Figure
1,2/3
along
the
important
bond
!1
!1
!1
mol
and
and
5.8
kcal
mol
1HfL.
The
optimized
geometries
ofmol
the
free
imidazole
isomers▶ C2
is26.7more
stablethanchangeof theanion.
Thermalinterconversion
ofenergies
!Lso
IMID
is andkcal
ACHTUNGRE(than
nNHC)
and
TMA
CHTUNGRE(IMID)
compounds,
much
lower
in energy
than ofthethenNHC
Table
22CrL
gives
the
optimized
TMfor
bond
leng
those
betweenthefreeligands,
that large
thelengths
anglesandthe C4!
relativecom- for 3FeL
!1stronger
bonds
make
the
energydifference
beHC,
and
IMID
at oftheBP86/TZ2P
level
are
▶ the
energydifferences
between
M-NHC
andinM-aNHC
decreased
relative
toasthefreeligands.!
ever aNHC,
seen
even
after
prolonged
reactionstimes.
ceswere
calculated,thelattermolecules
have
tiveenergies
theisomers,
andthecalculcomplexes
are ofonly
11.7–13.7
kcal
mol
pounds.
The
full
set
geometricaldata
is listedtheSup- areTM–aNHC
lower
tween the ly
complexes
TMAcalculatednNHC) bonds
andCHTUNGRE(aNHC)
smallerwn
in Figure
1, Information,
along
with
the
most
importantbond molecule
he 45:55
mixturefrom 1d
(R )
Me,
A
BF
weakermetal–ligand
thantheformer.
Tho
porting
Table
S1.)
The4) imidazole
thantheTheTMA
CHTUNGRE(CHTUNGRE(aNHC)
species.
For
the
BDEs
BDEs.
dataTMA
revealinteresting
Results
andDiscussion

riptor1

Complexwherethe hydride)-3-methylimidazolium
tetraphenylborate,
6 h,theproNHCligandtheoverreaction
products,(iii) tothethepreparation
gnge
anprocesses
NHC andNHC complexes
2ainand3a,
accordingto eq and1.cation
importabnormalnormal one (eq 6), which is isolatedtherelease
of
2.0
equiv
of
molecularhydrogen
and
the
and
X-ray
structure
of
a
novel
bis(normal
NHC)Oscompound.
19	
C2vs
C4-boundCarbene	nce ofC-H
the bonds
abnormas the tetrafluoroborate salt 3b. Treatment of the latter withvates
ion ofmixture of the abnormal- and normalodeof anthe84:16
carbonsodium tetraphenylborate affords 3a as a pure yellow solid. In
ive
species
remicomplexes
2a
and
3a,
accordingtoeq
1.
Resultstetrahydrofuran
and Discussion
vs13 thermodynamic -experimental-	an abnunder reflux, complex 3a is stable and does noturai’s kinetic
reactions.
14
ligandisomerize into 2a ofafterh.
. A.; Meyer, rare,
extremely
1. Preparation and Characterization
an6Abnormal
than inutomers, and we
Os-NHC Complex. Treatment under reflux of tetrahydrofuranA.; Ruiz, N.;interachehexahydride
solutions of OsH6(PiPr3)2 (1) with
1.1 equivof vs
1-(2-pyridyl; Esteruelas,
Figure3. Timecomposition of the reaction mixture of 1 withorbitals
thylimidazolium
C.;
Volatron,methyl)-3-methylimidazolium tetraphenylborate,over 6 h, pro1-(2-pyridylmethyl)-3-methylimidazolium
tetraphenylborate.
coordinduces the release of 2.0 equiv of molecular hydrogen and theate, E.; Oro,abnorm20
heformationformation of an 84:16 mixture ofinto
thebenzene
abnormalnormalandand
triphenylborane,
and the isomerization of
5, 7811.
(b) and
THF!
ntaining
an NHC
m. Soc. 1997,
NHC complexes
2a and 3a, according
to eq 1.cation to the normal one (eq 6), which is isolatedregard
theabnormalreflux, 6 h	Figu
F.influence
J.; Lledós, of theas the tetrafluoroborate salt 3b. Treatment of the latter withN.; Sola, E.
functioion
mode of thesodium tetraphenylborate
affords
3a
as
a
pure
yellow
solid.
In
A.; Martín,
Organometallics, Vol. 26, No. 26, 2007 6557
(1)
treatme63% (2a:3a = 5.3:1)!
cs 2001, 20,
tetrahydrofuran under reflux, complex 3a is stable and(6)does notylimidaisomerize
intoseparated
2a after 6 h.
López, J. A.; Meyer, The major product, the abnormal complex
2a,
was
As is
A.88M.;
The coordination of the normal carbon atom of the five. Oñate,(1) crystals suitable for an
13
1
;ro,
Esteruelas,fromitsnormalisomer3aasyellow
formatmembered ringis supported byBF
the C{ H} NMR spectrum in
L. A.; Ruiz, N.;
BF4!
4!
achy, I.; Esteruelas,X-ray diffraction analysis, by crystallization
with reCD2Cl2 in
atdichloromethane–diroom temperature, which contains the OsC resonance
Lledós,C.;
A.;
major
product,theabnormalcomplex2a,wasseparated
Valero,Volatron,
ethyl ether. Figure 1 shows a viewatof182.0
the cationof thesalt. The
ppm, asa tripletwith a C-P coupling constant of 6.8
2. (b) Barrio,tsnormalisomer3aasyellow
crystalssuitableforanstructure
provestheabnormalcoordinationoftheimidazolium
tallics
2001,
M.; Oñate,analysis,E.; Oro, by crystallization in dichloromethane–di(21)iffraction
isCooper,a hydride-elongated1996, 35, 7811. (b)ring and suggests that this complex
126,
824
(20)
J.
N.;
Powell,
R.
E.
J.
Am.
Chem.
Soc.
1963,
85,
1590
.
ics
23,
15 of the salt. The
ther.
Figureshows
a view ofspecies.
the cation
m. 2004,
Chem.
Soc.11997,
dihydrogenreprovesabnormalof thethe
imidazolium
Lahoz,
F. J.;theLledós,
The coordinationgeometry aroundosmium atom can be rationalized
26
.
;
Ruiz,
N.;
Sola,
E.
2b	
3b	
nd
suggeststhat
thiscomplexisahydride-elongated
Chem.
as
a
distorted
pentagonal
bipyramid
withthephosphine
ligands
Lledós,Soc.,
A.; Martín,
15
2b:3b = 1.3:1	
(1)
ogen
Chem.species.
Soc.,
(6)
ometallics
2001, 20,occupying axial positions (P(1)-Os-P(2) ) 164.39(5)°). The
Fischer, J.;around the osmium atom can be rationalized
geometry
metal coordination
sphereis completed
by thepyridinic
starlenas,
R.;
Thewithmajor
product,
theligands
abnormalcomplex2a,nitrogenwas separated
López,
A.pentagonal
M.; Oñate, bipyramid
stortedthephosphineThecoordinationof the normal carbon atom of the fiveatom
N(3)
and
the
abnormalcarbon
atom
C(1)
of
the
chelate(e)Cabeza,arrio, P.; Esteruelas,from its normal
isomer 3a
as yellow
crystalsisomerized
suitable
fortoanC4-boundcarbene
its
ying
axial
positions(P(1)-Os-P(2))
164.39(5)°).
The
memberedring
is supported
bythermodynamicallythe 13C{1H} NMR spectrum inTrans. 2006,
group X-ray
and thehydrogenatoms
H(1A),
H(1B),andH(1C)ofdiffraction
analysis,bycrystallizationindichloromethane–dieroy,
R. Lledós,K.; spherefavored CD
C2-boundcarbenein acidicwhich
condition.!
oordinationis completed
by the pyridinic nitrogentemperature,contains the OsC resonanceM.
A.;A.; (yellow
2Cl2 at room
2acrystal)	
gure
1.
Molecular
structureofthecationof
2a.
Selectedbond
ethyl
ether.
Figure
1
shows
a
view
of
the
cation
of
the
salt.
The
.;
Esteruelas,
1,
20,
442.
(b)
Barrio,N(3)andthe abnormalcarbonatom C(1)of the) chelate at 182.0 ppm, as a triplet with a C-P coupling constant of 6.8
gths (Å)and(deg):(15)Os-P(1)) 2.3605(14),Os-P(2)Barrio,P.;proves
Esteruelas,M.
A.; Lledós,A.; Oñate, ofE.;theTomàs,
J.
tarlenas,
R.;angles
structuretheabnormalcoordination
imidazolium
ganometallics
2001,
683(14),Os-N(3))
2.213(4),Os-C(1))2.123(6),H(1A)andhydrogenOrganometallics
atoms H(1A),
H(1B),
and
H(1C)
of
2004,
23,
3008
and
references
therein.1B) )the.
1.44(5); P(1)-Os-P(2) ring
) 164.39(5),P(1)-Os-N(3)and suggeststhat) this complex is a hydride-elongated(20) Cooper, J. N.; Powell, R. E. J. Am. Chem. Soc. 1963, 85, 1590.
ometallics
2004, 23, ) 93.44(11), P(1)-Os-C(1) ) 98.58(14),1570(11),
P(2)-Os-N(3)dihydrogenspecies.
M.Lledós,
Baya,)A.;
B.
Eguillor,
A. Esteruelas,
M. Oliván, E. Qñate, Organometallics, 2007, 26, 6556.!
2)-Os-C(1)90.78(14),M.
N(3)-Os-C(1)85.40(18).
Barrio, P.; )Esteruelas,
A.;
Oñate, E.; M.
Tomàs,J.

C2vs C4-boundCarbene	1
Organometallics,
Vol. 27,spectra
No. 3, (4002008 MHz,ble-temperature
H NMR

gh-field region of [OsH5(1-mesityl-3-methylimiPiPr3)2wingtip]BPh4 (2).groups	
Scheme 1

Figure 4. Variable-temperature 1H NMR spectra (400 MHz, C7D8)20	
etEguillorin the high-field region of OsH4(1-mesityl-3-ethylimidazol-4ylidene)(PiPr3)2 (5).
Scheme 2

Appelhans et al.

the normal C2 NHC, however, the abnormal C5 isomer was
69% normal	
obtained. The normal isomer could still be obtained if the C4
71% as(2)inabnormal!
and C5 positions were blocked,
the benzimidazolium salts.
67%(3)
abnormal	For small wingtip groups at N3, such as methyl, a mixture of
normal (2) and abnormal (3) NHCs was seen (Table 1). For
B. Eguillor, M. A. Esteruelas, M. Oliván, M. Peurta, Organometallics, 2008, 27, 445.!
example, the reaction of eq 1 (R ) Me, A ) BF4) gave a 45:55Figure 1. Molecular diagram of the cation of 2. Selected bo
lar diagram
complex2d
4. andSelectedbondlengths
ratio of of
products
3d (R
) Me,
A ) BF4).
lengths (Å) and angles (deg): Os-P(1) ) 2.3716(10), Os-P(2)eg): Os-P(1) ) 2.319(2), Os-P(2) ) 2.321(3),2.3709(11), Os-C(19) ) 2.120(4), H(02) · · · H(03) ) 1.25(
9); P(1)-Os-P(2) ) 170.98(10), P(1)-Os-C(1)R	A	
2 (normal)	
) 3151.72(4),(abnormal)!
P(1)-Os-P(2)
P(1)-Os-C(19) ) 95.92(1Os-C(1) ) 94.0(2).
P(2)-Os-C(19) ) 92.83(12).
Me	
 Br	
91	
9	
) 170.98(10)°). The metal coordination sphere
NHC osmiumcomplexes
reported (2.108(2)–1.993(9) Å),17
iPr
	
Br
84	
16	
	
(Scheme 2), which
is isolated
as of
a whitesolid in 69% yield.
thehydride ligands
ligands are(H-H> 1.70 σ-donorsÅ) and theand poor
the NHtautomer
8-methylquinoline
OsCl2(η2-H2){κ
abnormalconsidered
π-acThe normal
coordination
ligand is supported
byi NHC55	
19
Me	
 BF
45	
of theof the
NHC1,6ligand,whichlies between
H(01)
4	
(HNC
tor
groups.
However,
NHC
complexesof late transition
8H6Me)}(P Pr3)2 (2.005(6) Å), and the NH tautomer
13
1
the C{ H} NMR
spectrum, whichshows
the OsC resonance2iOs-C(1)
distanceof 2.087(9)is aboutstate
0.03have notbenzo[h]quinoline
OsCl
(η
-H
2100	
2){κC-[HNbq]}(P Pr3)2 (2.055(1ments withthe metal
in a high Åoxidation
been
iPr
	
BF
0	
4	
20
at 163.9 ppm, Å).
shifted
ppm towith
lower
in comparison
to the metalat
he
Os-NHC separation in 2, suggesting that
In27.6agreement
thefieldabnormalnature6 oforted.
that of 2. In agreement
the3,X-ray
structureof thelatter13
f the2 hydrogen atom from the metal
centercarbons
ofwith2 andtheirresonancesin Miecznikowski,
the
C{1H} NMi N. Appelhans, D. Zuccaccia,
L.
A.
Kovacevic,
A.
R.
Chianese,
J.
R.
!
1
The
d
hexahydridecomplex
OsH
(P
Pr
)
(1)
has
been
shown
The
productratio
iskinetic
because
the
2/3
ratio
is
invariant
6 This3 is
2 alsocompound, atspectra
200 K,appear
the high-fieldregionof theHrespectively.
NMR
ack-bonding ability of the metal.
at
136.3
and
137.1
ppm,
A.
E. containing
Clot,
O. Eisenstein,
R. H. Crabtree, J. Am. Chem. Soc. 2005, 127, 6299.!
13
1 subsequent
activate
C-H spectrabonds
of 4oforganic
molecules
a of ofchangetheMacchioni,
anion.
Thermalinterconversion
spectrum6 shows
three
in aatoms
1:2:2H(01),intensity
ratio H(03), H(0C{onH} NMR
of
and
5,
which
show
At 100K, resonances
the hydrogen
H(02),
15
eroatomgroup.
Ineven
this
work
we
that,
in times.
andleading3154
wasppm:neverafter
prolonged
at-6.22,-7.55,
and -11.09
ppm.
In addition,
the spectrum
e at2abouti.e.,seenshiftedabout
18
ppmprovereactionsand H(05)of 2 werelocatedin the difference
Fourier maps aLargewingtipgroupsfavor
C4-boundcarbene.!
eementwiththethetrendof mixture
contains
low
intensity,
which
may
be
due
For example,
45:55
from 1dcompounds(R ) Me,
Aof) resonances
BF4)refined ofasverythregardto thoseof
2 and
3.the hydrogenisotropic atoms together with the remaining no
ctropositive
metalsto liberate
H2thein contact
with
asalt
small
of the abnormal isomer (about 7%).
can be converted
eithertoBrtothefairly
SbFto6acidic
by amountioncoordination
certainly
minimizes
theorstericinfra)wingtip
and with
literatureThis
is perhaps
For smallgroups
at N3,expectations.
such as methyl,a mixtureof most2, 12, 6, 14, m, and, likely, o (overlapped with 1
easilyshown
bythesensitivityoftheprotonNMR
shiftof
normal(2)
and abnormal (3)NHCs was seenA R(Table
contact is observed with the two hydrides. 21	
The saT I C L1).E SFor
C2vs
C4-boundCarbene	
C2-H
to
the
natureoftheanion.
For
example,
for
the
model
)
gave
a
45:55
example, the reaction of eq 1 (R ) Me, A ) BF
are also observed for both abnormal and norma
A 4R T I C L E S
imidazolium
salt
4,
the
resonance
shifts
(CDCl
)
are
as
3
Table
1.
Ratio
of
2
to
3
for
Various
R and
A in Eq 1AR
ratio
of products
2deffect	and 3d (R ) Me, A ) BF
the norm
4).T I C L E S
CD2ClA2 (Figure S4 in Supporting Information).
counteranion
follows: A ) Br, δ 10.45; BF4, 9.85; Table
PF6, 8.79;
SbF
,
8.50.
6 2 to 3 for AVarious
1. Ratio
R andA in
the wing
norm
obtainedR of
2 (normal)(abnormal) of the n-Bu
also detected
at Eq
the1R 3position
1. Ratioof 2 to3,
3 for Various R and A in Eq 1
thenorm
The same interaction is detected in theTable
abnormalcarbeneandC5R
2 (normal)
3 (abnormal)
projectsa
Me
Br A in the abnormal
91 case, where
9the R group obtained
obtained
where the acidic CH is directed away from themetal.
The
range
R
A
2
(normal)3(abnormal)
and C5
For
sma
ba
Me
OAc
80
209 detailed discussio
where the anion
binds. A more
Br
91
a
and
C5
of shifts seen is very similar: A ) Br, δ 9.71
(3a);
BF
,
8.66
Me 4
OCOAr
70
30
For
sma
ac
Br
91is available for
9a closely related
b
OAc
80
20
normalexperiment
co
a
dc
BF4
45
55
For
sma
(3d); PF6, 8.44 (3e); SbF6, 8.38 (3g). BromidecanMe readilyOCOAr
70
30
b
OAc
80
20
normalexamplea Ion-Pair50
From thPF50
dce in 4, asMe
BF64
45
55
70Structure-Computational.
30
normalreplace SbF6 (or BF4) as ion-pairing partner
shown byOCOAr
example
ratio
of
PF
50
def
BF64 on the products,
45
55
Me
OCOPh
the anion50
is essentiallyexample
locatedthe change in chemical shift of the C2-H geproton
from
that
of
of
f
Me
OCOPh
PF
SbF6 6 of the most50
11protonic C-H50
89bond: C2-Hratio
for theratio
ofthe SbF6 salt to that of the Br salt on addition
of
NBu
Br
in
Me 4
SbF
11
89
OCOPh
50
50
hgf
i-Pr
Br 6
84
16
C5-H
for
the
normalisomer.
However,IP
con
Br 4 684
16
Me of Br.SbF
11
89
i-Pr
BF
0
100
CDCl3. The titration curve suggests Keq ishgi1.6 in favor
seen
with
the
protons
of
the
i
i-Pr
BF
0
100
hj issalts
Br84
16 methylene linker an
n-Bu
BF44
5
95
TheSimilar
production-pairingratio is kinetic
becausethe 2/3 ratio
invarianteffects
in imidazolium
have
been
ji
n-Bu
95
4
i-Pr ion-pair
BFenergetics!
100
imidazolium
1 ion-pairsolution
structure!
the pyridine50ring. To bettercharacterize the netw
on subsequentchangeof the anion.
Thermalinterconversionofreported.38
a jO CAr )n-Bu
BF
5
95
4
2,4,6-triisopropylbenzoate.
operating in these systems, ONIOM (B3PW91/UF
a O2CAr )2 and 3 was never seen even after prolonged reactions
times.
2,4,6-triisopropylbenzoate.
2
were 2!
carried out. In the
a O CArFor example, the 45:55 mixture from 1d (R ) Me,) BF
2,4,6-triisopropylbenzoate.
3! system chosen, the wing
4)
2 A )anion	
obtained(Nchanging
thethecounterions
along theatMMlevel (UFF)
together with the
can be converted either to the Br or to the SbF
by path),ion while
Etreated(kcal/mol)	
E (kcal/mol)	
obtainedpath),
while changing
the counterions
along the6 salt (Nseries
Br,
BF
,
PF
,
SbF
progressively
switchesthekineticA	δ(H)(ppm)	
4, PF6while, whereas
the rest
ofkinetic
thethemolecule, inclu
on PPh3theexchange. The products retain the 45:55 ratioseriesof theobtained(N
counterions
along
Br,starting
BFpath),SbF66changing
progressively
switchesthe46,none0	
10.1	
	
product
totheC5
species
(AN
Such
C5
NHCs
is treatedat path).
the
DFTmaterial even on heating Br(5	
 h, refluxing
The ionstronger! present
seriesBr,
BF4abnormal, PF6, SbF
switchestheproducttotheabnormal
C5
species(ANpath).
Such(B3PW91)
C5kinetic
NHCslevel. The ani
9.71	 thf).
6 progressively
The
are
still
but
other
examplesnow
beenandhave
OAc,
Figure 2 shows
are BFBr	
C5 species
0	
3.8	
The p
p
H⋯A
bond	
4, Br,
during the synthesis therefore decides the ratio,
not
any
anion
product
torare,theabnormal(ANpath).
Such
C5and
NHCs
are
still
rare,
but several
severalotherexamples
have
now
been
BF4	
8.66	
21-23
on
subs
reported.
onThesubsp(
geometries
for thenormaland abnormal
subsequently introduced.
are still 21-23
rare, but several
exampleshavenow (N)been
reported.
OAc	
 other
0	
0.9	
2
and
33
21-23of
Formationoxidative
proposed
onand
subs
2
respectively.
Selectedwas
geometricalarePF6	
obtained
8.44	
The abnormal isomer wasin pure formby synthesis
reported.
Formationof NHCs
NHCs byby C-H
C-H
oxidative addition
additionwas
proposed parameters
24
For
exa
0	
was
1.6	
proposed
24 but
For
exa3
2 and
this
pathwayauthenticatedresome
years
ago,
4	
C-H
2. oxidativewith the BF4 salt, followedby recrystallization,and fully
butBF
this
pathway
was only
only
authenticatedresomeyears
ago,
Formation
of
NHCs
by
addition
was
SbF
8.38	
19F,1H-HOESY NMR
6	
 spectra of 1i (R ) i-Pr and A ) BF )can exa
be t
Forcentlyby Peristheoxidativeprocharacterized. The
in years
pureform
but4whothis observed
pathwaywas
only authenticatedresomeago,et24 al.
In theabsenceof addition
anion, thegeometries
of
R normal isomer was obtained
CDCl
25 Previous(R ) i-Pr salt
and and
A in)[IrH
BF4(Me) pyridyl-imidazolium
salts
exchang
can simibe
duct
directly.
computational
work
has
analyzedfrom and
theH5imidazolium
centlyby Peris
et al.
who
observedthe(DFT)
oxidative
pro-are very
and abnormal(AN)additionisomers
2 3	
 2CO)
2L
2]BF
4 (seeN
26-32
recordedSection).
in CD2ClThedipolarinteractions
betweenmaterial
25
2 show strong
exchangthebonding
associatedwith the NHC-metalinteraction.
Experimental
different
spectroscopic
duct
directly.charPrevious computational(DFT) work
has analyzed
H cationvery
pK
33anion 19F anda =the2, 13, fully
12, CHcase of1i), and
26-32
2 (in
material
Fewtheexampleshave
yet beenon thepathwayswhichacteristics of theNtwo types of NHC,
discussed
previously
thebonding
associatedwith reported
the NHC-metalinteraction.
(39)(a) Hahn,
E. L.
Phys.
ReV.by1950,
80, 580. (b) during
Stejskal, t1
11 H nuclei(FigureS1 in SupportingThis
J. Chem.
Phys.
1965,imidazolium
42,by
288.
(c) Stilbs, P.subsequ
Prog. NtpK
during
NHC
complexes
areformedby reactionofparenta = 24±1
by comparison
with crystallographicallyauthenticatedexamples,FewInformation).
exampleshave
yetbeen reported
on thepathwayswhich
Spectrosc.
1987,
19,
1.
(d)
Price,W.
S.
Concepts
Mag
R the pyridine nitrogen is predominantly
25,33,34
that
oriented
36 In particular,
The10a!
allowsindicatessecure identification
even
in Appelhans,mixtures.salts
metalcomplexes.
now
that
NHC
complexes
arebyreactionofChianese,parent299.
(e)R.Price,
W.findS. imidazolium
Concepts
Magn.
Reson.subsequ
1998,
L. N.
D.withZuccaccia,
A.formed
Kovacevic,
A.We
J. R.such
Miecznikowski,
C. S., Jr. Prog. Nucl. Magn. Reson. Spectrosc.
1999,
25,33,34
Thetha3
the adjacentimidazole C4
C5
aromatic
protons
of normal
A.and
Macchioni,
O.
Eisenstein,
R. H. Crabtree,
Am.now
Chem.
Soc.
6299.!
salts
metal
complexes.
We
find
that
such
calculations
haveJ. allowedus(40)
tounderstand
the
anion
effectas 127, withFor J.
reviews
onapplications
to 2005,
organometallic
chemistry(37) Smidt, S. P.; Zimmermann,N.; Studer,E.
M.;Clot,
Pfaltz,A.withChem.-Eur.
2004,
characte
withaacalculations
resultN
andpaths
havingdifferentmechanisms
characte
isomer 2d10,are4685.
relatively close
proton NMR
chemicalshift
M.;Rüegger,H.; anion
Pregosin,
P.2009,
S. as
HelV.
Chim.
Actath2
O. in
Schuster,L. Yang.
H.of
Raubenheimer,
M.understandAlbrecht,
Chem.
Rev.
109,
3445.!
havetotheeffectresultofG.thethe
N allowed
and ANAN us
paths
havingdifferentmechanisms

For small wingtip groups at N3, such as methyl, a mixture of
normaland abnormal (3)NHCs was seen (Table 1). For
C2vs(2)C4-boundCarbene	
example, the reaction of eq 1 (R ) Me, A ) BF4) gave a 45:55
ratio
of products
and 3d (R ) Me, A ) BF4).
counteranion2deffect	

Anion-Dependent Switch in Selectivity
Anion-Dependent Switch in Selectivity

21-1	

free 2	
 BF4-2	
 free 3	 BF4-3	
Ir-H (Å)! 1.600	
 1.608	ARTICLES
ARTIC
LES
1.623	
1.630	

cis significantly elongated(1.097 vs 1.085 Å) as a result ofissignificantly
elongated(1.097vsBF
1.085
as a result-	
ofcounteranion
stronger H-bonding.
with
Ir-C2	
As observed
2.115	
2.114	
-	4, theÅ)counteranion
stronger
As observed
with
BF4,totheshiftscloser H-bonding.
to the imidazolium
ion nodoubt
enhancetheThe product ratio is kinetic because the 2/3 ratio is invariant
Ir-C5	
-	 (Ir-C2‚‚‚Br
2.107	
shifts closer
to theinteraction
imidazoliumion -	
no doubt
to enhancethepreferred
H-bonding) 2.098	
121.5°,
N-Br;
on subsequent change of the anion. Thermal interconversionofpreferred
H-bondinginteraction(Ir-C2‚‚‚Br)
121.5°,
N-Br;
Ir-C5‚‚‚Br ) 124.5°, AN-Br). However, the Br anion stillC2-H	-	just with-	
1.079	
1.092	
2 and 3 was never seen even after prolonged reactionsinteracts
times.
Ir-C5‚‚‚Br) 124.5°,
AN-Br).
However,
Br anionwith multiple
Hs, not
C2-H inthea linear
C2-stillinteracts
multipleHs, not just with C2-H in a linear C2H‚‚‚Br
bond. 1.079	
For example, the 45:55 mixture from 1d (R ) Me, A )BF) with4hydrogenC5-H	
1.082	
-	-	H‚‚‚Br tobond.
a chelatinganion, acetate CH3COO (OAc), allows
can be converted either to the Br or to the SbF6 salt byMoving
ion hydrogen
Movingto aa chelatinganion, acetate
CH3COO(OAc),
allowsinteraction
withgreater numberof H atoms.
In thenormalexchange. The products retain the 45:55 ratio of the startinginteraction
withagreaternumberofH
atoms.
Inthenormalisomer N-OAc, the very short contact with H-C5 (1.842 Å)
aNHC
vs NHC!
material even on heating (5 h, refluxing thf). The ion present
isomerformation
N-OAc,
thea strong
very shortcontact
with
H-C5
indicatesofhydrogenbond.
The
C5-H(1.842
bond Å)free 2 (R = iPr)	
free 3 (R = iPr)	
▶significantly
the higher
electron-donating
power
of
aNHC!
indicates
formationofastronghydrogen
bond.
The
C5-H
bond
during thesynthesis therefore decides
the ratio, not anyis anion
more elongated (1.106 Å) than in the case ofissignificantly
moreelongated(1.106
Å)
than
in
the
case
→shorterbond(2.098vs 2.115)!
BF4 (1.082
Å) or BrIr-C(1.085
Å). Also,
the O‚‚‚H-C5
atoms are ofsubsequently introduced.
BF→higher(1.082 Å)(170°)orBr (1.085
Å).(1.623
Also,O‚‚‚H-C5
atoms are4 collinear
almostaseffect
expected
for thea vsclassicalhydrogentrans1.600)!
The abnormal isomer was obtained in pure form by synthesis
almostcollinear(170°)
asexpectedforaclassicalhydrogenbond. Nevertheless, the chelating ability of OAc means thatthewith the BF4 salt, followed by recrystallization, andformation
fullybond. Nevertheless,thechelatingability
of
OAc
means
that
of this strong H-bond is not made at the expense of theformation
thisthestrongH-bondis notmadeproton
at the andexpensecharacterized. The normal isomer was obtained in pureform
the interactionofwith
equally
acidicpyridine
the ofcounteranion
the interaction
witheffect!
the equally acidic pyridine proton and thefrom the imidazolium salt and [IrH2(Me2CO)2L2]BFcontact
4 (seewith pyridine is also the shortest among the series ofcontactwithpyridinealso theshortest
series▶chartheisomers
presence
H-bondinginteractions!
(2.109ofÅ,isN-BF
Å, among
N-Br; the
2.019
Å, of
4; 2.740
Experimental Section). The very different spectroscopicnormal
normalisomersC-H(2.109Å, N-BF
Å, N-Br;2.019 Å,4; 2.740
N-OAc).
→longerbond(1.082
vs 1.079/ 1.092
vs 1.079)!
acteristics of the two types of NHC, fully discussed previously
N-OAc).
In the abnormal isomer AN-OAc, the more acidic H-C2
by comparison with crystallographically authenticated examples,
the abnormal
the more
acidicH-C2
centerInattracts
the OAcisomer
so that AN-OAc,
the O‚‚‚H-C2
contact
is very
36
centerattracts
the
OAc
so
that
the
O‚‚‚H-C2
contact
is
BF4-2 (R= iPr)	
BF4in-3 (R= iPr)	
allows secure
identificationeven
mixtures.
In particular,
short (1.659 Å) and the elongation of the H-C2 bond is veryvery
short(1.659Å)thisandcase,the elongationof theH-C2isbond
is verylarge(1.135
Å). Kovacevic,
Inalmostlinearityachieved
the adjacent imidazole C4 and C5
aromatic
protons
normalL. N.
Appelhans,
D. of
Zuccaccia,
A.
A. perfect
R.perfect
Chianese,J.
R. Miecznikowski, !
large(1.135
Å).
In
this
case,
almostlinearityisachieved(172.3°)
as expected
for a strongJ.H-bond.
A contactwith 2005,
theisomer 2d are relatively A.
close
in protonE.NMR
shift
Macchioni,
Clot,chemical
O. Eisenstein,
H. Crabtree,
Chem.
Soc.
(172.3°) R.
as expected
for a strongAm.
H-bond.
A contactwith the 127, 6299.!

22	

C2- vs C4-bound Carbene	n-Dependent
Switchin Selectivity
counteranion
effect	ARTICLE

proposed mechanism!

oxidative addition!
▶ non-H-bonding anion !
▶ large wingtip group!

C-H heterolysis!
with proton transfer!
▶ H-bonding anion !
▶ small wingtip group!

ure 3. The proposed normal (N) and abnormal(AN) pathwaysD.
(LZuccaccia,) PMe3).
L. N. Appelhans,
A. Kovacevic, A. R. Chianese, J. R. Miecznikowski, !

A. Macchioni, E. Clot, O. Eisenstein, R. H. Crabtree, J. Am. Chem. Soc. 2005, 127, 6299.!

C2- vs C4-bound Carbene	

Anion-Dependent Switch in Selectivity

Anion-Dependent Switch in Selectivity

22-1	

ARTICLES

BF4 interact
with two H	

oxidative addition!

C-H heterolysis!

4. Optimized geometries for the σ-complex, transition state, and intermediate located along the N and AN pathways with BFwith
4 as counteranion.
proton Intransfer!
rtant difference between the two pathways, the N intermediate is an ▶
IrIIInon-H-bonding
complex of H2 and the
AN intermediate
is an IrV polyhydride. Energyanion!
▶ H-bonding anion !
(kcal mol-1) relative to the σ-complex: N pathway, 2.6 (TS), -17.5 (intermediate); AN pathway, 2.0 (TS), -19.9 (intermediate).

▶ large wingtip group!

▶ small wingtip group!

on state, the Br anion is near Hun (Br‚‚‚Hun ) 3.00 Å).
in the gas phase and 2 kcal mol-1 in the THF solvent with BF4
dihydrogen complex formed from the N transition state,as the counteranion). In the absence of better information, we
anion is very close to the dihydrogen ligand (shortest
have analyzed the computational profiles by focusing on the
distance ) 2.528 Å). These geometrical features for theenergy barriers ∆Eq from each of the σ-complexes (Table 4).
a and transition state (extrema) on the two paths show
The activation energies ∆Gq, also given in Table 4, show similar
Br anion interact
Br always prefers to be near H-C2 rather than H-C5,
trends, and so we have chosen to discuss the results based onqstrongly with C2-H	
s preference is enhanced when H-C2 is the bond
being
∆E .
Appelhans
et al.
Figure 5. Optimized geometries for the σ-complex, transitioned. pentagonal
The fact that we could not locate a transition state forintermediates are IrIII (N) and IrV (AN) as in the BF4 pathway-18.6 (intermediate); AN pathway, 3.8 (TS), -16.5 (interme
Ir(V)	
rgybipyramidal
Profiles for C-H
Bond Activation and Discussion.
activation of the C2-H bond in absence of counteranion is in
Figure 5. Optimized geometries for the σ-complex, transit
nitial H2 loss, the reaction continues with the coordinationline with a previous finding that the activation of the C2-H
-1V) Associated with the N
Table
4. Energetics
molIr
intermediates
are IrIII(kcal(N) and(AN) as in the BF4 pathw
bond of an imidazolium salt, in the absence of the counteranion,
imidazolium/anion ion-pair to IrH3(PMe3)2 via a C-H
PathwaysCalculated for,N!-Dialkyl-imidazoliumo-18.6 (intermediate);
ANNpathway,
3.8 (TS), -16.5Salts(interma
Br Reacting with IrH3(PMe3)2
locatedAlthough
along the N
AN pathways
4 as counteranion.
by a Pd(0) diphosphine species occurs without any energy
plex.
theandresulting
pair withof σBF
complexes
could In
Table 4. Energetics (kcal mol-1q) Associated with theplex of forH2 coordinationand the AN intermediateanand
IrV polyhydride.
Energyanion
subst.
path
∆E
∆E
∆Gq
barrier.24a The energy barrier found in the present study for theulatedat C2-H is(N)
at C5-H (AN),Pathways Calculated for N,N!-Dialkyl-imidazolium Salt
diate); AN pathway, 2.0 (TS), -19.9 (intermediate).
-1
ative free energies of these two species versus theactivation of the C5-H was 0.2 kcal mol . In the presence of
Br 4Reactingwith IrH3N(PMe3)2a2.6
BF
Me/Me
-17.5
1.9
-1 in(PMeAN
2.0 q -19.9
1.5 q
solvent with
BF4
eedgasphase and 2 kcal andmolIrH
imidazolium/anion
anions, both energy barriers are nonzero but very small. Afterare not
3 the THF
3)2 fragment
anion
subst.
path
∆E
∆E
∆G
BF4
Me/iPr
N
2.3
-16.7
1.7
eevaluate.
counteranion).
In theabsencebetterpassage of the transition state, the energy profile goes sharply
The anionis pairedwithofthe
moreinformation,
acidic C2-Hwe2.72.6 -18.3
3.11.9BF4Me/Me ANN
-17.5
analyzed the computational
byhydrogen
focusingbond.
on thedownward. In the case of Br, the energy barrier for the AN
midazolium/anion
ion-pair in aprofiles
C-H‚‚‚A
iPr/MeBF4
N AN 4.22.0 -11.9
3.31.5-19.9
gybarriers∆Eq from
σ-complexes(Table 4).
path is 3.8 kcal mol-1 and only 2.5 kcal mol-1 for the N path.
nationto C5-Hthus eachdoes ofnottherequiremuch structural
BrBF4
Me/Me
NN
2.52.3 -18.6
0.31.7
Me/iPr
-16.7
q
ANAN 3.82.7 -16.5
3.23.1-18.3
given
infromTable
show
activationbecause
energies∆Gnizationthisbond
is remotethe4,anion.
This
With BF
respective(Lvaluesare3).
2.0 and 2.6 kcal mol-1 (Me/4, theFigure3., also
The
proposednormal(N)similar
and abnormal(AN)pathways
) PMe
i
i
BrBF4
Me/Pr/Me
Pr
NN
2.54.2 -18.5
0.83.3-11.9
s,wecoordination
have chosenattoC2-H
discuss
the results
based
heand
casesofor
where
the anion
hason
Me entries in Table 4). These results show that there is no great
3.52.5 -16.4
3.60.3Br
Me/Me ANN
-18.6
splaced. These changes, which of course occur in a polar
kinetic preference for activating either the C2-H or the C5-H
AN
3.8
-16.5
3.2
with
thethemetal
has some
Lewis
C-H
bondoncleavage
is morethe
N pathway
iPrin (Ra The
iPr;
Bradvanced
Me/R/R!
N
2.5R! ) -18.5
0.8
notation) Me,
Me, iPr) indica
fact that
we could
notlocateaLUMO,
transitionstate,e cannot
be associatedeasilyevaluatedwithpresent theleveloffor fragmentbonds despite
the fact
that in manyThedecadesof workthenot Ir(V)!
process(C5-H
where theANmigratingis adjacent to
3.5proton
3.6t
base
activity
associatedwiththeisbasal
hydrides.
) 1.429Å) than in theoxidative
AN path)1.384Å),-16.4
ationof the C2-H
bond populationin absenceof thecounteranion
in
Therefore,
therelativeoftwo σ-commetalation of imidazolium salts up (C2-H
to 2002, only
the normal(see Scheme 1). The activation barriers (∆Eq and ∆Gq) and reactbut
Ir(III)
dihydrogencomplex	
L. N.
Zuccaccia,
Kovacevic,
A. R. (∆E
Chianese,
J.
R.
Miecznikowski,
!i
with
a be
previous
finding
the activation
the C2-H
cannotevaluated
and isthat
certainly
not well of
C2 NHC productsD.
ever observed,
presumably
becausea The
iPr; R! ) Me,
iPr) ind
and
∆G)
arein
evaluatedfrom
σ-complex
and A.
theIr-C bondis accordingly
shorterthecase(Ir-C2
notation
R/R!N
(R
) the
Me,correspondingThe
reactionstartswith represented
formationof aAppelhans,
σ-complex of thewere
C-H
N oxidative(respectivelyAN) where
refers theto theN (respectivelyAN)pathwaof an imidazolium
salt,
the absenceof theA.
counteranion,
calculateddifferencein inelectronicenergies
between
the
NHCs
were
only
very
rarelysynthesized
by
C-H
bond
process
migratingprotonisadjacent
Macchioni,
E.
Clot,
O. apical
Eisenstein,
R. H.Å)Crabtree,Am.caseChem.
2005,) 2.157
than in theJ.AN(Ir-C5Soc.
) 2.188Å).127,
The 6299.! t
be cleaved,
C-Hanywhich
transto results
theac(see Scheme 1). The activation barriers (∆Eq and ∆Gq) and rePd(0)(6diphosphine
without
energy binds
stems
kcalbond
mol-1 toactivation.
The
also suggest that the nature of the anion
inspecies
favor
ofoccurs
the σ-complex
at, H-C2

A Comparison of aNHCs with NHC	
were

23	

2464 Organometallics, Vol. 23, No. 10, 2004
▶ aNHCs are stronger σ-electron donors than NHCs. !
carbonat room
temperature,
as indicated by broad▶ aNHCs tend to cleave
frombond
the metal
moreeasily than NHCs.!
Organisopropyl
ometallics,methyl
Vol. 23,resonances
No. 10, 2004
2463
in the 1H NMR spectrum.

c Carbenes

Tolman’s electronic parameter!

Compound 9a
providesa way
to measure the electron
R. H. (eqCrabtree,J.prepared
Organomet. Chem. 2005, 690, 5451.!
2,4-blocked imidazolium
salt
7a‚HI
4)
was
cis to the NHC. The
donor strength of the abnormal carbene ligand 7a, using
the Ir-P bonds, which
the CO stretching frequencies observed by infraredn of equivalentisopro- ability -experimental-	electron-donating
spectroscopy. To compare the donor strength of the
Figure 2. ORTEP diagram of 8a, showing
50%carbene
probability
spectroscopy.
abnormalwith that of standard NHCs, a series
ation of ellipsoids.
Iridium(I)from[LIr(CO)2Cl] (L = NHC,
PR3)!
of analogous
normal carbene-iridium complexes wasor
preparingtransitionr) orprepared.
xpected to give normaltion).
Imidazolium salts 7b‚HI and 7c,d‚HBr (eq 5) wereeeshigher
acidity of thisofstituentsuch as Me or
alkyl
urage
rties C5 binding byategy of blocking theacing
ubstitution
at C2 is of
were
McLachanet al. have
ilver
7a!
7b：R1 = Me, R2 = Ph!
ond
at
C2
by
Pd(0)
in
step
7c：R1 = iPr, R2 = Ph!
d we have initial data
were
electron-donating!
37 by successive N-alkylation
7d：R1 = Ph, R2 = H!
from 2,4-diphenylimidazole
by Rh(I) in a 2-methylwith isopropyl bromide followed by methyl iodide (seeFigurfound thate previously
C2Experimental Section).
The
stericallycontrolled
regioellipso
prepared using standard N-alkylation methods, fromving a proton at bothgandchemistry of alkylation
shown4,5-diphenylimidazole
was confirmed by X-ray
commercial(7b‚HI, 7c‚HBr) or
abnormal C5 bindinge‚HI,
diffraction
analysisofthederivediridiumcompound8a
experimental
data!
Figure 3. Correlationof theaverageν(CO)valuesfor
1-phenylimidazole (7d‚HBr) (see Experimental Section).
etrawork reveals
generalbelow).
compounds Ir(CO)(see
Cl(L)
with
the
Tolmanelectronic
pa-model the electronic properties of2
Ligands
7b
and 7cA.
R.
Chianese,A.
Kovacevic,
B.
M.
Zeglis,J.
W.
Faller,R. H. Crabtree,
Organometallics, 2004, 23, 2461.!
ofagevarietyofpossiblerameters (TEP). Extrapolated
positionsofwiththe NHC
ligands
We attemptedtopreparean abnormal
iridium-NHC
7a,two
phenylC-substituentsand two alkyl
ransorce deprotonation
at indicatedstudied are
by arrows(tmiyby)transmetalation
1,3-di(4-tolyl- from the silver
complexfrom 7a‚HI
N-substituents. Ligand 7d models the steric properties
s2 withde- a phenylgroup.
methyl)imidazolin-2-ylidene,biy ) 1,3-dibutylimidazolincomplex. Stirringwithsilver oxideinof7a‚HI7a, witha phenylgroup forand1.5an hisopropylgroup facingucts.
is readily
N,N!-di2-ylidene).
dichloromethane theat room
temperaturegave a compounds
ratheriridium
center. Iridium
8b-d were
tegivert- 1,3-diisopropyltoOrbital[a]

Table 3. (LUMO)
Kohn–Shamofvalenceorbitals (BP86/TZ2P) of the ligands
nNHC,
aNHC,
and (!6.33
IMID.
Figure3 gandsshows
schematicallythe relevant
orbitalinteraclyingeV) hasthanthecarbons>lone-pairorbitalsof nNHC (!4.97 eV) and aNHC (!4.25 eV). Th
molecular orbital
the molecules.
the
trend
aNHC
nNHC
IMID.
Figure3 shows
schematicallytherelevantgenslone-pairorbitalimidazole
aNHC,
IMID.
3 the
shows
the five
highest>occupied
lyingand(!6.33eV)Tablethancarbons lone-pairorbitalsofmolecular orbital(LUMO)of these
orbitalsthatthes-donorstrengthof thelitions
betweena suggesttransition
metal
andofthethenNHC
ligandaNHC, and
IMID.[a] of the molecules.
tions
betweena transition
metal
andeV).
the ThenNHC
ligandby ofinteracnNHC(!4.97eV)
and
aNHC(!4.25energylevelsTable 3. Kohn–Shamvalenceorbitals(BP86/TZ2P)
oftheligandsnNHC,
Figure3showsschematicallytherelevant
orbitaltheseorbitalssuggestthat
thes-donor
HOMO-1.
Notably,
thisorbitalisenergeticalmolecular
orbitals(HOMOs)andthelowestunoccupied
tions
betweenatransition
metal
and
the strenN
nNHC(!4.97eV)
and
aNHC(!4.25eV).
Theenergylevels
gands
has
the
trend
aNHC
>
nNHC
>
IMID.
using
C
symmetry.
Asimilaroutlineapplies
for
the
oth
2v
[a]
OrbitalTable
3.theseKohn–Shamvalenceorbitals(BP86/TZ2P)
of theligands
nNHC,
aNHC, and IMID.
using
C2vorbitals
symmetry.
A similar
outlineappliesforthetheotherofsuggestthat
thes-donorstrengthoftheli- ligand
Orbitaltions
betweenathe
transition
metal
and
nNHC
by2v symmetry.
gands
has
theeV)
trendthanaNHC> carbonnNHC>sIMID.
lying(!6.33thelone-pmolecular
orbitalofmolecules.
using
C
A
similaroutlineapplie[a] (LUMO)
oftheseorbitalssuggestthat
thes-donorstrengthoftheliFigure3
shows
schematicallytherelevant
orbitalinteraTable 3. Kohn–Sham valence orbitals (BP86/TZ2P) of the ligands nNHC, OrbitalaNHC,
and has
IMID.
gands
the
trend
> nNHC> IMID.
LUMO
Table
3. Kohn–Sham[a] valence orbitals (BP86/TZ2P) of the ligands nNHC,
using
C2vaNHCsymmetry.
A similar
outline tions
applies
for theFigure3 shows
schematicallythe relevant
nNHC(!4.97eV)andandaNHC
eV).
Th
aNHC, and IMID.
Orbitalgands has the
trend
aNHC
> nNHC> IMID.
betweenaothertransitionmetal
the (!4.25
nNHC
ligandaNHC, and IMID.[a]Figure 3 shows schematically the relevant orbital interac- tions
betweenatransition
metal
and
the stre
nN
of
these
orbitalssuggestthat
thes-donorFigure3showsschematicallytherelevant
orbitalinteracusingCsymmetry.
Asimilaroutlineapplies
for
the
oth
LUMO
2v
Orbital
LUMO
Table
3. Kohn–Shamorbitals metal(BP86/TZ2P)
of theligandsligand
nNHC, by using C symmetry. A similar outline applie
tions
between valencea transition
and the
nNHC
gands
has
the
trend
aNHC
>
nNHC
>
IMID.
2v
!1.25 (a’’)!0.57 (a’’)
tionsa transition metal and the nNHC ligand by
Orbital[a]!0.32 (a2)aNHC,
andbetween
IMID.
using
C
symmetry. A similar outline applies for the other
OrbitalFigure 3 shows schematically the relevant
using C2v
2v symmetry. A similar outline applies for the other
Orbital
LUMO!1.25 (a’’)!0.57 (a’’)!0.32 (a2)!1.25 (a’’)!0.57 (a’’)
tions between a transition metal and the nN!0.32 (a2)
LUMO
LUMO
using C2v symmetry. A similar outline applie
E/eV	
Orbital24	

A Comparison of aNHCs with NHC	electron-donating ability -calculation-	
LUMO

HOMO-2!7.50 (a2)!5.85 (b1)!5.85 (b1)

HOMO-1!7.03 (a’’)!5.49 (a’’)

	HOMO-2!7.76 (a’’)!6.33 (a’)!6.33 (a’)!7.50 (a2)
!5.49 (a’’)!5.49 (a’’)

twoligands.
Figure3 aonly
andbFiguredisplaysthenNH? p interactions
backdonation.
The nNHC
ligand also
a second
pwhereas
orbitalrelevant
as
itthe
the
p
in-planewith
3 dorbital
shows
t
that
is in theplane
the ring.
Thismbene complexes.
The in-planep k interactions
arehasusuallyab-k notp
donations,Figure3iscofmetal.
showstheout-of-planenNHC
TM
in-plane p thatinteractions
with theFigure3 d showstheis consideredas pmetal.
bondingin mostdiscussionsof car-two
ligands.
Figure33 theaa and
b
the
s
and
p
donations,whereas3 c in
shows
the
!6.33
(a’)nNHC!TM
iss!5.49
ininteractions.
plane
ofWe
the
ring.
This
orbital
give
rise
that
is to
consideredaspFigurebonding
most
discn
!5.85
(b
nNHC
TM
pThe
This
interaction,
however,twoligands.
Figureandb displays
displays
thenNHC!TMsmay
and1)sorbedin kthatthefollow
thisconvention.
in-planep k interactions
withmetal.
k backdonation.
backdonation.
nNHC
ligandalso
has thea second
pFigur
orbi
!7.03
(a’’)
(a’’)
!6.33
(a’) areisusually
!7.03
(a’’)
!5.85
(bbackdonation.
bene
The
in-planep k interactions
ab1)
nNHC
TM
p
This
interaction,
however,pdonations,whereasFigure
3
c
shows
the
nNHC
TM
p
backdonation.
ThenNHC
ligand
also
has
a se
k complexes.
!5.49
(a’’)
in-planepinteractions
withthemetal.
Figure
3
d
shows
the
bene
complexes.
The
in-planepinteractions
!7.76
not
relevantas
it isofonly
theout-of-plane
pEDA
donations,whereas3 c showstheCnNHC
pthenNHC
calculations
carriedout withk FigureTM
This
interactio? contributi
that TMis inforplane
the pring.
This orbital kp
may
give
rise
s symmetry
k backdonation.
!7.50(a2) were
HOMO-1	!

HOMO-1HOMO-1!5.49 (a’’)!5.85 (b1)!7.76 (a’’)!

HOMO-2!5.76 (a’’)!5.76 (a’’)!6.33 (a’)!
!

	

!5.85 (b1)!

HOMO-1!

!5.49 (a’’)
HOMO-1!

1!4.25 (a’)!4.25 (a’)!

HOMO-2!5.76 (a’’)!4.25 (a’)!6.33 (a’)

	
HOMO	
!5.85
(b )

!4.97 (a1)!4.97 (a1)HOMO

!4.97 (a1)!5.49 (a’’)

transition-metalFigure
fragment
and the nNHC
ligand byC2v symme
3. Schematic
representationof using
the orbital
interaLUMO
HOMO
symmetry.
transition-metalfragment and the nNHC ligand by using Ca)
2v L!TM
!4.97(a1)
donation.
c)
TM!Lp ? backdosdonation.
b)
L!TM
p
?
transition-metalfragment
and
the
nNHC
ligandby us
Figure3.
Schematic
representationoftheorbitalinteractions
between
a
!4.25
(a’)
two
ligands.
Figure b)
3 aL!TMand bpdisplaysthec) nNHC!TMs anddonation.
TM!Lp
backdonaa)
L!TMs donation.
?
?
tion.
TM!L
p L!TMk backdonation.
!4.97 (a1)s donation. b) L!TM p ? donation. c) TM!
transition-metalfragmentthe(a’’)
nNHC
ligand
by d)
using
!5.76
!4.25
(a’)
p donations,Figure3 candshows
the nNHC
pC2v a)symmetry.
tion.
d) TM!L pwhereas
FigureTM
3. Schematic
representation of the orbital interactions between
k backdonation.
!1.25
(a’’)
tion.
p k backdonation.
!0.57
(a’’)
two
ligands.
Figure3aandbdisplays
thenNHC!TMsd)
anddonation.
c)
TM!Lpbackdonaa)
L!TMsdonation.
b)
L!TMp
HOMO-1
!0.32
(a
)
?
?
2
!5.76
(a’’)
ligands.
3 a anddisplaysnNH
Figure3.TM!L
Schematic
representationof using
the orbital
interaHOMO
transition-metalfragment
andFigurethe nNHC
ligandb byCthe
backdonation. The
nNHC ligand also has a second
p orbital two
2v symmetion.
d)representation
TM!L pwhereas
k backdonation.
p
donations,Figure
3
c
shows
the
nNHC
TM
p
transition-metalfragment
and
the
nNHC
ligandby usn!5.76(a’’)
Figure3.
Schematicoftheorbitalinteractions
between
a
!4.97
(ais
p
donations,whereasFigure
3
c
shows
the
donation.
c)
TM!Lpbackdoa)
L!TMs
donation.
b)
L!TMp1) in the plane of the ring. This orbital may give
?
?
that
rise
to
!4.25
(a’)
3. Schematic
representationof the orbital interactions between a!6.33 Figure
(a’)
!4.97
(a
a)
L!TM
s donation. b) L!TM p ? donation. c) TM!
!6.33
symmetry.
transition-metalfragment
and theligandby usingCbackdonation.
ThenNHCnNHC
ligandalso
hasad)secondp!5.85(b11))
2v (a’)tion.
TM!Lbackdonation.
porbital(a’)
k backdonation.
nNHCtheligandalso has as seligands.
Figure3 a andp bThe
displaysnNHC!TM
a
!5.76
(a’’)
symmetry.
transition-metalfragmentandthe !4.25
nNHC
ligandby using3C
!5.49
(a’’)
in-planep donation.
withthemetal.
Figuredtwo
shows
the tion.
k interactions
	d) TM!Ldonation.
c) nNHC!TM
TM!L
p 2v
backdonaa)
L!TM
s
b)
L!TM
p
k backdonation.
? of? (a’’)
twoligands.
Figure3aandbdisplays
the
s
and
!5.76
that
is
in
the
plane
the
ring.
This
orbital
may
give
rise
to
HOMO	
donation.
c)
TM!Lpbackdonaa)
L!TMsdonation.
b)
L!TMp
HOMO-1
that
is
in
the
plane
of
the
ring.
This
orbitalm
Figure3.
Schematic
representationoftheorbitalintera? This interaction, however,? donations,pwhereasFigure
3
c
shows
the
nNHC
TM
two
ligands.
Figure3aandbdisplays
the
nNH
HOMO
nNHC
TM
p
backdonation.
istion. d) TM!L p k kbackdonation.
p donations,Figure 33 ca andshowsthemetal.
nNHC
TM3 p
!6.33
in-planep k interactions
withtheFigured shows
thebackdonation.
tion.
d)1)TM!L
pwhereastransition-metalfragment
andtheligandbyusn
k ligands.
!5.85
two
b(a’)
displaysnNHC!TMs andin-planep k interactions
withthemetal.
backdonation.
ThenNHC
ligand also
hasnNHCa3 csecond
pFigureorbi
pdonations,whereasFigureshowsthenot(brelevant
as
it is (a’’)
only
the out-of-plane
p ? thecontribution!5.49
	Figure!4.97The(a1) nNHCa) L!TMs donation. b) L!TM p ? donation. c) TM!
backdonation.
ligandalso
hasasecondp
orbitalnNHCTMpbackdonation.
Thisinteraction,
however,
is
!6.33
(a’)
k
!4.25
(a’)
!5.85
(b
)
p
donations,whereasFigure
3
c
shows
the
nNHC
TM
p
backdonation.
Thisthat ofis inplanetheppring.
This orbital
may
give
backdonation.
ThenNHC
ligandalsointeractio
has riseas sek displaysthat is considered1 as p bonding
car-thenNHC
twoligands.
Figure3ofaTM
andb
thenNHC!TMa!5.49 in(a’’)most discussionstion.
d) TM!Lk backdonation.
HOMO-2!6.33(a’) is in the
HOMO-1!5.76(a’’)rise
thatof themay
give
notplanerelevant
as ring.
it is This
only orbital
the out-of-plane
p tocontribution!

HOMO-1!5.85 (b1)!0.57 (a’’)!0.57 (a’’)!

HOMO-1HOMO
HOMO!1.25 (a’’)!1.25 (a’’)!4.25 (a’)!0.32 (a2)!0.32 (a2)HOMO
!4.97 (a1)backdonaa)
L!TMs donation. b) L!TM p ? donation. c) TM!L p ? a)
!4.97
(a
L!TM s donation. b) L!TM p ? donation. c) TM!
1)
!1.25
(a’’)
!0.57 (a’’)!4.25 (a’)!0.32 (a2)Figure 3. Schematic representation of the orbital interactions between
tion. d) TM!L p k backdonation.
tion.
d) TM!L p k backdonation.
!5.76 Figure
(a’’)
!1.25
(a’’)
!0.57
(a’’)
3. Schematicof the orbital interactions
between a!0.32 (arepresentation!5.76 (a’’)2)!

HOMO-1

HOMO

	

!

HOMO

!1.25 (a’’)!5.76 (a’’)!4.25 (a’)!1.25 (a’’)(a’’)by using C symmetry.
!0.32
(a2)
transition-metalfragment
and the nNHC!0.57
ligand
2v
!1.25
(a’’)
Figure3.2)Schematic
representationof TM!L
the orbital
interactions
betweena!0.57
donation.
c)p ? (a’’)backdona- Figure
a)
L!TM
s
donation.
b)
L!TM
p
!0.32
(a
LUMO	
?
3. Schematic
representation of the orbital intera
HOMO
LUMO
symmetry.
transition-metalfragment and the nNHC ligand by using C2v transition-metal!0.57 (a’’)
tion. d) TM!Lp k backdonation.
fragment and the nNHC ligand by us!

	

!0.57 (a’’)
LUMO

!

LUMO
LUMO

Figure 3. Schematic representation of the orbital interactions between a!1.25 (a’’)
(a’)
LUMO	
!0.32 (a2!4.25)!4.97 (a1)!0.32 (a2)
!4.97 (a1)
HOMO

!

HOMO

sorbedthep ligand
stheinteractions.
this convention.
HOMO-2
not relevant
asThe
it in
is
only
out-of-plane
p ?follow
contribution
backdonation.
nNHC
also has aWe
secondp orbital
two
Figure3ofa andb displays
the nNH
that ligands.
is in
plane
the ring.
This follow
orbitalmnNHC
TMbackdonation.
Thisinteraction,
however,
isinthesorbedthes interactions.
We
th

that
is considered
as p bondinginmost
discussionsof p
cabackdonation.
ThewhichnNHC
alsothehasa’’a second
pgive
orbitalmost molecules,
means
that
orbitalsthekligandnot relevant
as ittheis metal.
only
theout-of-plane
in-planep k interactions
with
Figure3 d showst!7.03(a’’) discussions
EDA
calculations
werecarriedout withCp
symmetry
for
that
is
consideredaspbonding
in
most
of
that
is
in
the
plane
of
the
ring.
This
orbital
may
give
rise
to
p
donations,whereasFigure
3
c
shows
the
n
in-planepinteractions
withthemetal.
Figurs carknotrelevant
as
it
is
only
the
out-of-plane
contribution
EDA
calculations
werecarried
out
with
C
bene
complexes.
The
in-planepinteractions
areusually
!7.76
(a’’)
that
is) in the planewhereasof the ring.
orbitalmaytop ? (acontribution,
the a’This
orbitalsgive
thegives(a’)+risep k? inHOMO-3
thatisconsidered
as
in
most
disc
sa
k p bonding
nNHC
TM
p
backdonation.
Thisinteraction,
however,!7.03 (a’’)
k
!7.50
!6.33
2
!5.85
(b
1)
!7.03
(a’’)orbitals
mostmolecules,
which
means
that
a’’
give
the
bene
complexes.
The
in-plane
pcomplexesinteractions
areusuallyab!7.76 (a’’)
in-planepinteractions
withthemetal.
Figure
33the
d
shows
the
backdonation.
ThenNHC
ligandalso
hasa’’a or
se
nNHC
TM
p
backdonation.
Thisinteractio
HOMO-2
!5.49
(a’’)
kp
kthat
k
most
molecules,
which
means
that
the
is
consideredasbondingin
most
discussionsofcarsorbed
inthesinteractions.
We
follow
thisconventio!7.50 (a2)in-planepinteractions
withthemetal.
Figuredshows
theteractions.
For
the
Group
11
4TM(L)
withL=
in-plane p p!7.76 (a’’)
not relevantbene
as itcomplexes.
is only theThe
out-of-planekk interactions
HOMO-2? contributi
(a
!7.03
(a’’)
2)interactions.
p!7.50whereasthe pa’ orbitals
givecalculations
thesp+ pcontribution,
inHOMO-3
sorbedintheWe
follow
thisC however,convention.
thatisabin thecarriedplanethewithring.
ThisorbitalmfnNHC
TM
backdonation.
Thisinteraction,
is
not
as
isofout
only
the a’
out-of-planep?pcontribution,
krelevant
the
give
bene
complexes.
in-plane!7.76 (a’’)
HOMO-3EDA
were
C
symmetrynNHCTM
p kk sreport
backdonation.
Thisinteraction,
however,is usuallynNHC, wealso
EDAThecalculations
withsymmetry,?
sorbedinp thesitwhereasinteractions.
We
followth
k interactions
s orbitals
thatis areconsideredasbondingin mostdiscussionsofca
2v
HOMO-2!10.68 (b1) !7.50 (a2)
HOMO-2
teractions.
For
Group
11
complexes
4TM(L)
with
L
=
	
EDA
calculations
werecarriedout
withCsymmetryforin-planepinteractions
withthemetal.
Figur
notrelevant
as
it
is
only
the
out-of-planepcontributionthatisconsideredaspbonding
in
most
disc
!7.03
(a’’)
k For
teractions.
thepthat
Group
11
4TM
sorbedin (a’’)
thes interactions.
We
thisconvention.
most
molecules,
which
means
the
a’’ complexes
orbitalsgive!9.99 (a’)
which
make
itaspossible
to distinguishbetweenpfollow
andpcomplexes.
notrelevant
it is
only
the
out-of-plane
ps ??!7.03
contribution!9.79 (a’)
EDAThecalculations
werecarriedout usually
with
Ctas
in-planeinteractions
are
!7.76
? benek in(a’’)
k
!7.50
(ais
	
bonding
nNHC,whichwealso
reportEDA
withC2) considered
nNHC
TM
pthe
backdonation.
interactio
most
means
that
the calculations
a’’discussions
orbitals
give
the
bene
complexes.
The
in-planep This!7.76(a’’)most
that
as p
in
of
2v symmetry,
k report
k interactions!10.68 (b1)
nNHC,
we
also
EDA
calculations
with
EDA
calculations
werecarriedout with
Cs carsymmetryforpcontribution,a’We
orbitalsgive
thethes +a’’
p kori
HOMO-3
teractions.
thatismolecules,consideredinmost
discussions
car!7.50
most
molecules,
which
means
that
? of
2)1as
sorbedin thes whereasinteractions.
follow
thisconventio!10.68(a(b) p bonding
HOMO-2!7.03 (a’’)!7.03 (a’’)!9.99 bene
(a’)
which
make
it
possibletodistinguishbetweenpandp
in!9.79 (a’)
notrelevant
as
it
is
only
the
out-of-planeppcontribution,
whereasthea’orbitals
give
the
s
+
p
insorbedinthesinteractions.
We
followth0
HOMO-3complexes.
The
in-planepinteractions
areusuallyab!7.76 (a’’)?
k?Thek ab- Fork interactions
!9.99
(a’)
which
make
it
possibletodistinguishbetween!9.79(a’)
mostresults
molecules,
which
means
that
the
a’’
orbitalsgivetheteractions.
the
Group
11
complexes
4TM(L)
with
EDA
for
Group
4
d
metal
compounds
bene
complexes.
The
in-planepareusually!7.76 (a’’)!7.50 (a2)pcontribution,
whereasthea’orbitals
give
HOMO-3
EDA calculations
were carried out with Cs symmetry Lf
k?
!7.50 (a2)	teractions.
	
that
is
consideredaspbonding
in
most
disc
EDA
calculations
werecarriedout
withCteractions.
For
the
Group
11
complexes
4TM(L)
withL=
sorbedinthesinteractions.
We
follow
thisconvention.
pwhereastheorbitalsgive
thesteractions.
+ preport
in- EDAnNHC,
we alsocalculations
with
C2v symmet
HOMO-3(Tablethat the nature
thea’Cl
interacsorbed
in suggest
thes interactions.
We offollow
thisconvention.
teractions.
For
the
Group
complexes
4TM? contribution,
kwhich
4TM–L
!7.03
most
means
that
the 11a’’orbitals
givets!10.68(b1)4)
0 (a’’) molecules,
HOMO-4The(aEDA
results
for withGroup
4symmetrymetalcompounds
0 a’’ orbene
complexes.
The
in-plane
pthat
!7.76
(a’’)
most
molecules,
which
means
the
nNHC,
we
also
reportEDA
calculations
withC2vdwhichsymmetry,
EDA
calculations
werecarriedoutCfork interactions
	s nNHC,!9.99(a’)!10.68 (b1)The
EDA
results
for
Group
4
d
meta
make
it
possibletodistinguishbetweenpandpteractions.
For
the
Group
11
complexes
4TM(L)
with
L
=
!7.50
)
!9.79
(a’)
tions
is
quite
similaramong
theligands
L
=
aNHC,
EDA
calculations
werecarriedout
withCsymmetry
for
2
?
k
nNHC,
we
also
reportEDA
calculationswiths
p ? contribution, whereas the a’ orbitals give the s + p k ii
HOMO-3!10.68 (b4)
1) suggest that the nature of the(Table
Clp4TM–L
interacsorbed
insuggest
the s whereasinteractions.
thT!9.99 (a’)
p
the
a’We
orbitals
give	toHOMO-3which
make
it possible
distinguishbetweenporbitaland!9.79 (a’)
mostmolecules,
which
means
that
the
a’’
orbitals
give? contribution,? teractions.
k theHOMO-4(Table4)
thenatureoffollow
the
nNHC,
we
also
reportEDA
calculations
with
CinandIMID.
The
percentage
contributionoftheintermost
molecules,
which
means
that
the
a’’
orbitals
give
the
!9.99
(a’)
4
which
it possible
to distinguishbetween2v symmetry,
!9.79
(a’)
teractions.
ForthemakeGroup
11 thatcomplexes
4TM(L)
withClL!10.68 (b1)
HOMO-4!10.67 (a’’)
tions
is
quite
similaramong
theligands
L
=
nNHC,
aNHC,
!11.10 (a’’)
EDAcalculations
werecarriedout
withC
011 complexes
teractions.
For
the
Group
4TM
teractions.
pcontribution,
whereasthea’orbitals
give
the
s
+
p
inHOMO-3?
k!10.71 (a1)
tions
among
the ligands
L =sn!9.99 p
(a’)
The
EDA
results
for similarGroup
4 d with
metalcompounwhich
makeit30.8possibleto distinguishbetweenp
preportin-quite
actionsvariesbetween34.7%,give
which
means
that
!9.79 (a’)
whereastheanda’ orbitals
the
s
+
p
inHOMO-3
teractions.
? and
k is
? contribution,
k
nNHC,
we
also
EDA
calculations
Csymmet2v
0!10.68(b1) EDA
and
IMID.
The
of theLorbitalintermost
molecules,
whichofmeans
the0 intera
a’’
or
nNHC,
we thealso
reportEDA
calculations
withThe
results
for percentageGroup
4 dcontribution
metalcompounds
teractions.
For
the
11
complexes
4TM(L)
with=!10.68(b1) Group
and
IMID.
The
percentage
contribution(Table4)thatnaturethe
Clthatteractions.
the[a] Orbital energies are given below in eV.
L bondsaretwo-thirds
electrostatic
and
For
the
Group
11
complexes
with makeL=suggest!10.67teractions.
(a’’) Cl4TM!
4TM–L
EDA
results
forbetweenGroup
d ofmet!9.99 (a’) 4TM(L)which
it Thepossibletodistinguishp4? andpth!11.10 HOMO-4
(a’’)
!9.79
(a’) roughly
k i!10.67(a’’)!10.71 (a1)
!11.10(a’’)
pcontribution,
whereasthea’orbitals
give
actionsvariesbetween
30.8
and
34.7
%,
which
means
that
HOMO-3
!9.99
(a’)
which
make
it
possibletodistinguishbetween
0symmetry,!9.79 (a’)(Table4)suggestthat
thenature
of
the
Cl
TM–L
interac?
nNHC,
we
also
reportEDA
calculations
withC4!10.71(a)
2v
actionsvariesbetween
30.8
and
34.7
%,
whi
tions
is
quite
similaramong
theligands
L
=
nNHC,
aNH
1EDA
!10.68(b)
Theresults
for
Group
4
d
metal
compounds
HOMO-4
nNHC,
we
also
reportEDA
calculations
withC
symmetry,
1(Table4)suggestthat
thenatureoftheCl
2v
teractions.
!10.68 (b1)4
HOMO-4!amongteractions.
For
the Group
11 complexes
4TM
theClgiven[a] Orbitalenergies are given
below in eV.
Ltobonds
roughly
two-thirds
electrostaticandtions energies
ismakequite
similartheareligands
L=
nNHC,
aNHC,
!9.99
4TMbelowwhichitpossiblepandp
in!9.79
!
and
IMID.
The
percentage
contribution
theorbitalintetheCl[a] Orbitalin eV.
L bondsareroughly
two-thirds
el!9.99 (a’)(a’)(Table4) suggest
thatthebetween
natureofClinteracwhich
make
itare
possibleto distinguishdistinguishbetweenp ??theandp4kkTM–L
in!9.79 (a’)
(a’)
tions
is4TM
quite
similaramong
theligands
The
EDA
results
for
Group
4EDA
d0 of
metal
compoun
HOMO-40 L=!10.67(a’’)!11.10
(a’’)
nNHC,
we
also
reportcalculations
with
The
EDA
for%,Group
4 meansd metand (aIMID.
The Weinheim
percentage
contributionof theorbital1560
www.chemasianj.org! 2007 Wiley-VCH Verlag!10.71
GmbH&1)Co. KGaA,
Chem. AsianJ.
2007,
2, 1555–nNHC,
1567
teractions.
!10.68)
actions
between
30.8results
and 34.7
which
th
tions
is(b1quite
similaramong
theligands
L =interaNHC,
teractions.
and
IMID.
The
percentage
contributionof th!10.67 (a’’)(Table4)variessuggestthat
thenatureof
Cl4TM–Lintera!11.10 (a’’)
0HOMO-4
!9.99
(a’)
make
it possible
to thedistinguishbetween!9.79(a’)!10.67(a’’) that! which!10.71 (a1)
(Table4)suggestthat
thenatureoftheCl
0 metalactionsvariesbetween
30.8
and
34.7
%,
which
means
EDA
results
for
Group
4
d
compounds!11.10(a’’)
theCl[a] Orbital energies
areThe
given
below
in
eV.
TM
L
bonds
are
roughly
two-thirds
electrostatic
a4
HOMO-4
and
IMID.
The
percentageof theorbitalinterThe
EDA
results
for Weinheim
Group 4 dcontribution
metalcompounds!10.71
4 quitebetween30.8 Land
34.7 %,aNH
whi1) KGaA,similaramong
the ligands= nNHC,1560
www.chemasianj.org!11.10 (a’’) ! 2007 Wiley-VCH
Verlag GmbH&(aCo.
Chem. tions
Asian J.is
2007, 2, actions
1555– 1567varies!10.67(a’’) Cl
teractions.
tions
among
the
ligands
L=!
1560
www.chemasianj.org
! Cl
2007
Wiley-VCH
VerlagGmbH&isCo.quite
KGaA,similar
Weinheim
Chem. AsianJ.
the[a] Orbital energies are given below
in(aeV.
TM
L
bonds
are
roughly
two-thirds
electrostatic
and(Table4)suggestthat
thenatureofthe
TM–L
interac4
4!10.71)
actionsvaries
30.8
%,
which
that
(Tableenergies4) suggest
thatthebetween
oftheandCl434.7
TM–L
1
HOMO-4the percentage
Cl[a] Orbital
are given
below
innatureeV.
are roughly
two-thirds
elandinteracIMID.means
Thecontributionof theinte
4TM!L bonds
HOMO-4The
EDA
for contribution
Group
4orbital
d0 of
met
and
IMID.
Theresults
percentageth!10.67 (a’’)
tions
among
the
ligands
L
nNHC,
aNHC,
(a’’)
thesimilar
Cl!11.10[a] Orbital energies are given below in eV.
L bonds
roughlyelectrostaticand 30.8 and 34.7 %, which means thtions(aisis1) quite
quite
similaramong
theareligands
L==two-thirds
nNHC,
aNHC,
!10.67(a’’)!10.71
4TM!
actionsvariesbetween!11.10(a’’)(Table4)variessuggestthatChem.
thenatureof 2,theCl(a1)actions
between
30.8
and
34.7
%,
1560
www.chemasianj.org!
2007 Wiley-VCH
GmbH &
Co. KGaA,
Weinheim
Asian
J. 2007,1555whi– 14
and IMID.
IMID. !10.71
The percentage
percentage
contributionof Verlag
the orbital
orbitalinterHOMO-4
andThe
contributionofthe!Ltions
the
ClinterOrbitalenergies
are given
below
in Weinheim
eV.
bonds
are
roughly
two-thirds
electrostatic!10.67(a’’)
4TM!11.10 (a’’)
(a’’)
1560
www.chemasianj.org! 2007[a]Wiley-VCH
Verlag
GmbH
&
Co.
KGaA,
Chem.
Asian
J.
2007,
2,
1555
–
1567
!10.67
(a’’)
is
quite
similaramong
theligands
L
=a
!
the
Cl
[a]
Orbitalenergies
are
given
below
in
eV.
TM
L
bonds
are
roughly
two-thirds!11.10!10.71(a)
actions varies
variesbetween 30.8
30.8 andand 34.7
34.7 %,%,! whichwhichmeans Verlag
that GmbH &4Co. KGaA, Weinheim
1560actions
www.chemasianj.org
2007 Wiley-VCH
Chem. Asian elJ.
!10.71 (a11)betweenmeans
that
The percentageof thTonner,G.
Heydenrych,
G. Frenking,
Chem.
Asian.
J. 2007,contribution
2, 1555.!
!
the ClCl[a] Orbital
Orbital energies
energies
are givengiven below
below
in eV.
eV.
TMGmbH
L bondsbonds
are roughlyroughly
two-thirds
electrostaticand1560 arewww.chemasianj.org! 2007R.
Wiley-VCH
Verlag&Co. KGaA,
Weinheim
Chem.
Asian
2007,
2,and
1555 –IMID.
1567
4TM
!L
!10.67(a’’)J. and
the
[a]
in
are
two-thirds
electrostatic
!

!

!
!

!

HOMO-2

N

H N

H

H N

N H

H NMR spectrum from dH = 5.37 to 5.53 ppm, probably due
AbnormalC4-Bondingin N-Heterocyclic
Complexes
intramolecularhydrogenbonding ofCarbene
the isopropyl
proAtoComparison
ofaNHCs[19b] with NHC	
tons to the metal-bound halides.
The structure of 13 a
was further confirmed by crystallographic analysis.[23] Mechcleavage from the metal	
anistically, formation of the chloride complex may be rationalized by an oxidative addition of Cl2 to the palladium
center followed by reductive I2 or ICl elimination. In thepresence of excess Cl2, the postulated palladium(IV) oxidative addition product [PdCl4(dicarbene)] is apparently unstable and reverts back to complex 13 a.
In contrast, the C4-bound carbene complex 8 b afforded(92%)	
the dichloro dimidazolium salt 14 upon exposure
to Cl2(Scheme4). Whileof theremovalof Icomplexes
Scheme3. Reactivity
normaldicarbene
towardsaccording
molecular
2 may proceed25	
FULL PAP

chlorine.

Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2009, 15, 9375 – 9386

(48%)	

(50%)	

Figure 8. Molecular structure of a) one of the crystallographi
pendent dications of 14, and b) one of the two cationic portion
pound 16, (ORTEP representation, 50 % probability, [PdC
[PdCl3ACHTUNGRE(DMSO)]! ions omitted for clarity). Selected bond lengt
14: C(18)!C(27) 1.341(14), C(20)!C(21) 1.340(14), C
1.692(10), C(20)!C(l4) 1.668(10). Selected bond lengths [!] a[8] for 16: C(3)!C(4) 1.336(13), C(4)!C(5) 1.433(14),1.345(15); N(4)-C(1)-N(1) 99.3(7), C(3)-C(4)-N(1) 107.4(8),C(5) 147.3(9), C(5)-C(4)-N(1) 105.3(8), C(6)-C(5)-N(4) 108.3
C(5)-C(4) 145.4(9), C(4)-C(5)-N(4) 106.3(8).

Scheme 4. Reactivity of abnormal dicarbene complexes towards molecular chlorine (X! = [PdCl3ACHTUNGRE(DMSO)]! or 0.5 ACHTUNGRE[PdCl4]2!).

come of the reductive elimination processes. The
groupsM.
atAlbrecht,
C5 in complex
a andthe 15,putative
N. Heckenroth, A. Neels, M. G. Garnier, P. Aebi, A. W. Ehlers,
Chem.8Eur.
J. in
2009,
9375.!PdMontréal, Québec, Canada, H3C 3J7, and Department of Chemistry, UniVersity of New Orleans,

Weskamp,
T. J. Organomet.
Weskamp
Chem.
Louisiana70148
also unambiguously establishedalso unambiguously
by single-crystal
established
X-ray analysis
by single-crystal(see New Orleans,
X-rayanalysis(see
A.;
Hillier,A.
C.;
Nolan,
S.
A.;
P.
Org.
HillieL
Received January 14, 2004; E-mail: lebelhe@chimie.umontreal.ca
26	
aNHCs
as
Ligands
forTransition
Metals	
Grasa,S. P.
S.;OrgGr
Supporting Information).
Supporting
Attempts Information).
to convert complex
Attempts
2 intoto convert
thecomplexM.2 S.;
into
the G. A.; Nolan, M.
(f)Grasa,
G.
A.;
Viciu,
M.
(f)
S.;
Grasa,
Huan
Recently, a number of new transition metal-catalyzed
processes
99C(2) isomer 1 in the presenceC(2) isomerof a base
1 inthewerepresence
unsuccessful.
of a basewere unsuccessful.
have been developed and optimized through high-throughput andNolan, S. P. Organometallics
Nolan,
2002,
S.
Suzuki–Miyaura coupling	combinatorial methods. Such strategies are particularly efficient1

Grasa, G. A.; Viciu, M. S.;Grasa,
Lee, GH

for the screening of a large number of transition metal-ligand
Organomet. Chem. 2002, 653,
Organom
69-8
combinations under various sets of reaction conditions. However,under these in situ conditions, the exact nature of the catalytic
F.; Navarro-Fernandez, O.; F.;Stevens,Navar
species is not always precisely known. Indeed, the assumption of
2002,
21,
5470-5472.
(i)
Pytkowicz,
2002,
21,
a normal mode of binding between ligand and transition metal isG.;
Jutand,A.
J.
Organomet.
G.;
Chem.
Jutan
most often assumed and may be misleading.
During the course of our studies focusing on ligands suitable toM.; Fu, G. C. J. Am. Chem.M.;
Soc.
Fu,20G
induce enantioselectivity in transition metal-catalyzed processes,2 (4) Herrmann, W. A.; Elison,(4)
HerrmannM.;
Fischwe became interested in testing N-heterocyclic carbene (NHC)Angew.
Chem.,
Int.
Ed.
Engl.
Angew.
1995,
C
metal complexes. A number of simple NHC palladium-based
a C-5
ForIMes,the onscomplexes have recently emerged as effective catalysts for a variety (5) For the only example of (5)of cross-coupling reactions.3 Our study of the exact structure ofA.; Albrecht, M.; Faller, J. A.;
W.;AlbreCrabvarious palladium NHC species reveals that the metalation site on
2274-2275. (b) Gründemann,
2274-22S.; Kothe imidazolium salt is strongly influenced by the presence of base.
W.; Crabtree, R. H. J. Am. W.;
Chem.
Crab
S
We now report the synthesis and structure of novel palladium
Figure 1. ORTEP diagram of palladium complex 2. Selected bond lencomplexes bearing NHC ligands in “normal” and “abnormal” (6)(Å) Forand angles(deg): Pd(1)-C(12),Pd(1)-C(25),selected
early 2.019(13);examples(6)Forof selec
a2.021(C(
Pd(1)-Cl(1), 2.289(4); Pd(1)-Cl(2), 2.302(4); C(12)-Pd(1)-C(25), 17(1)binding(a) Arduengo,
A.J. III. US
patent
5 077 414,mode
1991; (b)ofArduengo,
A.J. III; toKrafczyk,
motifs.
Thebinding
the NHC
Pd isR.;shown toE. D.;
Petersen,
J. L.;
Stevens,
Nolan
Schmutzler, R. Tetrahedron, 1999, 55, 14523-14534.
(5); Stevens,
C(12)-Pd(1)-Cl(1),93.3(4);
C(25)-Pd(1)-Cl(1),87.3(3);C(1E
affectthe M.
catalytic
behaviorof thepalladiumcom(2)substantially(a) Schmid, M.; Eberhardt,
R.; Klinga,
Leskelä, Rieger,
B. Organometallics,
2001,
20, 2321C
O
Pd(1)-Cl(2),88.6(4);
C(25)-Pd(1)-Cl(2),90.9(3);
Cl(1)-Pd(1)-C2330; (b) Steele, M.; Watkinson, M.; Whiting, A. J. Chem. Soc., Perkin Trans. 1, 2001, 6, 5882674-2678. (b) Huang, J.;2674-26
Schanz,plexes.
598.
177.23(18).
(3) Serra, A.C.; da Silva Corrêa, C.M.M.; do Vale, M.L.C. Tetrahedron, 1991, 47, 9463-9488.
It has been reported that palladium (II)-NHC complexes could
Organometallics 1999, 18,Organom
2370-2(4) Farina, V.; Krishnan, B.; Marshall, D.R.; Roth, G.P. J. Org. Chem. 1993, 58, 5434-5444.
G.A.; prepared
Viciu, M.S.; Huang,
Zhang, C.; Trudell,M.L.;
Nolan, S.P.
Organometallics,
beGrasa,
easilyfromJ.; palladium(II)
acetateandthe corresponding
Palladium-Catalyzed (5)Suzuki-Miyaura
Reactions
2002, 21, 2866-2873.
displayedtwodoublets
at
6.57
and
7.47
ppm
(J ) 1.7co
Stevens,
E.
D.;
Nolan,S.
P.
Stevens,
Organoshowed a difference in reactivity when
4 Metal(6)imidazolium
Feuerstein, M.; Doucer,
H.; Santelli,
M. J. Org.
Chem. 2001,
5923-5925.
salts.
binding
at66,theC(2) position is usuallycorresponding
to
H(3)
and
the
carbeneH(1)ofapossiblyC(7) Yang, C.; Nolan, S.P. Synlett, 2001, 10, 1539-1542.
(7) The catalyst,
related unusual
C(2)-C(5)(7)toThecom
it wasthefoundbe arelatmo
13C NMR
2 observed, and complexes bearing two NHCs
2 can potentially exist generated
spectrumshowbound IMes ligand. Furthermore,as trans or cis isomers, depending on the steric hindrance of thebis(2,6-di
twobis(2,6-diisopropylphenyl)imidazoliu
carbon175.9 and 150.7
ppm that
we Work
assigned
andsignals
Heckat coupling
reactions.
nitrogen substituent R. We first attempted to synthesize the Suzuki(8)C(2)The
normalcomplex(8) Two
1The
and
norm
(IM
and C(5)bound to C(2)the palladium
center.
carbonsign
palladium complex 1 derived from 2 equiv of N,N!-bis(2,4,6- natureof andcatalytically
activespecies
inand
in 12sfor inseparable
C(4)
C(5), andmixture.
for C(3),at respectively
122.8
inseparabtrimethylphenyl)imidazolium chloride (IMes‚HCl) (1) and 1 equiv
were also observed. On the basis of the NMR data,
ismechanism
ongoing. for the (9)(9)ppmThe
formation
The mechofof palladium (II) acetate under the standard reaction conditions systems
proposed the reaction product to have the structure 2 (eq 1).
(dioxane, 80 °C, 6 h). The reaction proceeded smoothly leading toously established.
However,
ously
there
esta
ar
unambiguously
establish this structure,single crystals
were
gro
S5
the isolation of a single palladium-containing
product.
Acknowledgment.
Francineby slow diffusion of hexanesWe
into athanksaturated
solution of B
2
ORTEP representation of complex 1 (elipsoid drawn at 30% propability level)

The reactivity
2 wasreactivity
studied ofand1 comparedand 2 was withstudiedthe andin compared with the in
Tableof1. 1 and The
situ-formed catalyst (fromsitu-formed
Pd(OAc)catalyst(fromPd(OAc)(1 equiv)
andIMes‚HCl(1 (2equiv) and IMes‚HCl (2equiv)) for the Suzuki-Miyaura
equiv)) for (Table
the Suzuki-Miyaura
1) and Heck reactions(Table 1) and Heck reactions(Table 2).
(Table 2).
Complex 1 proved to beComplexan inactive
1 provedcatalyst
to be
for anboth
inactivecoupling
catalyst for both coupling
viaSuzukithe formation of a palladium-ca
via the fo
reactions, while complex
reactions,
2 lead towhile
the desiredcomplexproduct.
2 lead In
to Suzukithe desired product.acetone.
In
The single-crystal X-ray analysis provided the ORT
insertion
intotheC(5)-Hinsertion
bondworkof
X-ray
crystalstructure
analysis.
This
diagramshown
inFigure 1. The ORTEP reveals that the pallad
Miyaura reactions,
Miyaura2 wasreactions,
notcomplex2(°C)wasthe innotsituastheinsituentry complex
catalyst(2 mol%) as efficient
tempasyield(%) efficient as
instance,
theanduseligandof weaker
instance,base,sucis C(2)bound
to oneNHC(the normal
bindingmoNational
ScienceEngineering
Researc
10highest
whereasthesecondligandisattached
throughtheC(5)carbonformationofcomplex2.
Conversely
formation
formed catalyst.110 In sharp
formedcatalyst.
contrast,
the
In
sharp
isolated
contrast,
yield
the
for
highestisolatedyield
forcomplex 1
80<5
Canada,theFond FCAR
ofdistancesQuébec
and thethe second
imidazolium.
Both chloride,
Pd-C
arefromequivalent(2.
frompalladium
which
sugg
pall
2
complex2
80
44 with 2.
and
2.021
Å)
and
are
consistent
with
Pd-C
singlebonds.
the Heck coupling reaction
the Heck
was obtained
coupling withreaction
2. was obtained
S.P.N.baseacknowledges
support fromthecarben
Natiofora square-planar
the formation
ofaround
the
base
for
3
IMes‚HCl, Pd(OAc)2 (2:1)
80
76
ORTEP shows
coordination
the palladfor
work
performed
atUNO.
usefulcommentsandsuggestions
usefulco
In conclusion,4 these IMes‚HCl,findings
In conclusion,
highlight
thesethefindings
importancehighlight
of
the
the
importanceof
thecenter,
with
the
two
chlorine
atoms
bound
to
the
palladium.
H
Pd(OAc)2 (1:1)
80
28
again,
no distortion theinofthefirst
Pd-Cl
distancesisobserved(2.289formation2.
formation
report of theprocedure used for the procedure
generation usedof a forcatalyticallythe generation
activeofspecies
a catalytically active
2.302 Å). species
Complex2, isolated in good
is aSupporting
Information
ChaItshouldbe
noted
thatAvailable:(10)
inyields
Suzuki-Mi
It(74%),
shouldcatalytic
activityofaNHC	
The NMR data suggested an unusual coordination mode for the (10)example of C(5) coordination of an IMes ligand and constitu
NHC
ligands,
indicatedItby appears
11 proton and22
carbonthe
signals.
In compounds
andexperimental
procedures(CIin cross-coupling
reactions.
in cross-coupling
It appears Heckthatreactions.
theas catalytically
activethatcatalytically
activedefinedcomplexes
bearingdefinedonly onecC
the first
exampleof anorganometallic
complexcontaining
one
Table 2.
Palladium-Catalyzed
Reactionsaddition to the singlet at 6.85 ppm for H(4) and H(5) of a
NHC
ligandandone
C(5)-boundimidazolium
ligand.
bearing
two.
See
ref
3f.
bearingavailable
free ofthe Internet at htw
presumably
C(2)-boundligand, reactions
the 1
H in
NMR spectrum
species precursor in cross-coupling
species precursor
reactionsin cross-coupling
is not IMes
complexis notcomplex
1 charge
ina mixturevia
Surprisingly, whenof cesium carbonate, pallad
5-7

1

H. Lebel, M. K. Janes,
A. B. Charette, S. P. Nolan, J. Am.
Chem.
Soc. 2004,
5046.!
(II) acetate,
and IMes‚HCl
was stirred126,
at 80 °Cin dioxane,
Université de Montréal.
‡27	

aNHCs as Ligands for Transition Metals	
ArticleOrganometallics, Vol. 29, No. 23

Suzuki–Miyaura coupling	

ArticleTable 2. NHC-Promoted Suzuki-Miyaura Cross-Coupling

Reactions (NHC:Pd = 2:1)

asubstrate
scope! Suzuki-Miyaura Cross-Coupling
Table
2. NHC-Promoted
Reactions (NHC:Pd = 2:1)aOrganometallics,
Vol.
29, No. 23, 2010 Suzuki-Mi
6345
Table
3. Scope of
aNHC-Promoted
Coupling Reactions (NHC:Pd = 2:1

Table 3. Scope of aNHC-Promoted Suzuki-Miyaura Crossa
Organometallics,
Vol.(NHC:Pd
29, No. =23,2:1)2010
Coupling Reactions

6344

Figure 2. Normal and abnormal NHC precursors.

6346
5
1
entry
entry R

2
R1 R

R2 A

AB

there is no example of using abnormal binding carben

6

5
AB

Organometallics, Vol. 29, No. 23, 2010

6 precursors for similar in situ generation of catalyst specie
BA

B
Duringour recent research focus on the NHC-based trans1OMe
Me
60 (57)23
76 (73)<0.1
tion
metal complexes for efficient organic transformations
OMeMe Me79 (76) 60 (57)
23
76
(73)
<0.1
2 1 H
14
90 (89)<0.1
we<0.1
became interested in abnormal NHC as a new ligan
H Me Me75 (72) 79 (76)
14
90
(89)
3 2 F
28
84 (76)<0.1
scaffold
4 3 Me F OMe Me41 (34) 75 (72)10
64 (61)
<0.1
28
84
(76)
<0.1 in homogeneous catalysis. Herein, we report the fir
5 4 Me Me H
30
(25)
4
66
(62)
2
OMe
41 (34)10
64 (61)<0.1
exampleof using a C2-protected imidazolium salt as a
6 5 Me Me F
4
29 (24)H 6 (4) 30 (25)
4
661.3
(62)
2
aNHC precursor in a Pd-catalyzed cross-coupling reactio
7HOMe
67 (63)7
78 (70)
<0.1
6
Me
F
6
(4)
4
29
(24)
1.3
8
H
F
30 (25)3
50 (41)<0.1
To<0.1
confirm the observation from in situ catalysis, we als
7
H
OMe
67
(63)
7
78
(70)
a
GC
screened
the activity of the related well-defined aNHC an
8 yields using
H dodecaneFas an internal
30standard;(25) isolated
3 yields
50are
(41)
<0.1
shown in parentheses, average of at least two runs.
nNHC Pd complexes.
a
GC yields using dodecane as an internal standard; isolated yields areboronicacid
has often been
observed
inleastthe Suzuki-Miyaura
shown
in parentheses,
averageof at
two runs.
reactions.17
Results and Discussion
The
substrateof the abnormalcarbene6boronic
acidscope
has often
been observed
inprecursor
the Suzuki-Miyaura
17
promoted
Suzuki-Miyaura
coupling reactions was explored
reactions.
NHC Precursor. Two imidazolium salts, 5 and 6, wer(TableThe3). Itsubstrate
showed excellent
activity
withelectron-deficient
scope of the abnormal carbene precursor
6- as NHC precursors to investigate the activity betwee
chosenaryl bromides with good functional group tolerance. Howpromoted
Suzuki-Miyaura
coupling
was exploredthe normal and the abnormal carbenes (Figure 2). Bot
ever,reduced yields
or no activity wasobservedreactions
with unacti-

X. Xu, B. Xu,
Y. Li, S. have
H. Hong,
Organometallics,
29,as6343.!
(Table 3). It showed excellent activity with electron-deficient
precursors
isopropyl
and phenyl2010,groupswingtirole as theactivator,
butB.notas
spectator
ligandinligandthecatalyticallycoordination
silvernucleus.
Removalof
D. Santarsiero,P. Schaefer,
J. E.
Bercaw,
J. Am.
roleasbut
not as W.
spectatorinthe catalyti
olefins under mild conditions (RT,
0.1 MPa geometryH2) only aroundthe roleofanactivator,
activator.
+
activespecies.
28	
aNHCs as Ligands forTransition
Metals	
1987, 109,5203;
the Ag ion to afford the bissolvento
complexcanc) J.
beY. K. Tsang, M. S. A. Buschhaus, P
active species.
J. Am. Chem. Soc. 2007,palladium
129, 5372. electrons in 2 aoctane [Eq. (1)] is observed with the bissolvento complex 5,[3] a) A. E. Shilov, G. B. Shul!pin,
Activation and com
Catahydrogenation	
other palladium(II)duction[1]
J.
A.
Labinger,
J.
E.
Bercaw,Nature2002,
417,
tions of Saturated Hydrocarbons
in the Presence
while the neutral
homologue
2 was haveinactivetheseand discussion
conditions.
Comparative
experiments
beenunder
performed
electronbondingreflect
Results
Crabtree,Chem.
Rev.Dordrecht,
1995,
95, 987.
Results
anddiscussion
Complexes,
Kluwer,2000;b) “Activation
with the bissolvento complex of 3 b, that is, the C(2)-boundboundcarbenesandcor[2] tionalizationa) P. L. Watson,
J.
Am.
Chem.ACS
Soc.Symp.
1983,Ser.
105,2004,
649
!H
of
C
Bonds”:
etal-catalyzedhydrogenationofunsaturated
substrates[12]
Structural
aspects
complex 6. Irrespective of the solvent, the conversions Structuralareaspects
fromvibrationsin reThompson,
S. M.
A. R.CO
Bulls,
B. J. Burger,
M
K.
I. Goldberg,
A.Baxter,
S. Goldman).
ng beensubstantiallya classic domaintransition
complexes
lowerofwith
6 than metal
with theC(4)-boundcomplexWe
exploited
B. D.
Santarsiero,W.
P. Schaefer,
J.haveE. Bercaw,
J. Am.T
Complexes
1–4[4]were
accordingtoGamble,
known
proce[1]R.
A.prepared
Periana,D.
J. Taube,
S.
H.
Taube,
Complexes
1–4
were
preparedaccording
to
known
pr
carbene (NHC)
ising phosphine
5 (Table ligands.
1, entriesN-heterocyclic
1–5). The hydrogenationof cyclooctene
16]
1987, 109,
203;1998,c) J.2Y.
K.complexes
Tsang,M.
S.in
A.
Buschhaus,ol
catalytic
280,
560.
dures.[11,dures.
The[11,solvento
complexes
and
4
a–c
were
analyzed
16] Fujii, Science
[2]
[3]
The
solvento Soc.
complexes129,
2 and
a–c were anals, once considered as substitutes of phosphines, have
J. Am.
5372.4223,
the activation
of
H2 b)
as C.
ak
[5] a)
R. G.Chem.
Bergman, 2007,
Science1984,902;[4] by X-ray diffraction (Figure 2, Table 1). Comparison of the strucby[a]X-ray [3]diffraction(Figure
2,
Table
1).
Comparisonofthesconsiderably less successful in hydrogenation catalysis.
a)A.
E.
Shilov,
G.
B.
Shul!pin,
Activation
andCata
Chem. Commun. 1999,have
2443;beenc) I. carriedG"ttker-Schnout u
Table 1: Catalytic hydrogenation of cyclooctene with complexes 5 and 6.
tions5	
ofPresence
mitation conditions.
of NHC complexes
may bedue in partto theComparative
experiments
havebeen performed6	
 Chem. inWhite,
M.Saturated
Brookhart,Hydrocarbons
J. precursors
Am.
Soc.the
2004,
126,
under
mild
con1
[b]
[c]
Entry
Solventt [h]cat. loading
Conversion [%]Complexes,
Kluwer,Dordrecht,
2000;b)“Activation
nsity of thecarbeneligand to undergo
reductive
eliminawith
the bissolvento
complexof2.3 Formationb, thatis,oftheC(2)-boundcomplexes[6] a)4 D.
O. Guerret,
P. Gabbai,
G. Bertraand 1F.ACS
atm
H2. Thehydr
SchemeandBourissou,
5.
5 the bissolvento
6!
tionalizationofC
H
Bonds”:
Symp.
Ser.
2004,
[12]
[5]
Rev.
2000,
100,
39;
b)
W.
A.
Herrmann,Angew.
Chem
Irrespective
theNHCsolvent,
conversions areactions, complex
despite 6.the factthat the of
M!Cbond isthe
genK. I. Goldberg,
A. S. Goldman).
1342;
Angew. Chem.
Int. Ed. 2002, 41, 1290; c) N-H
1substantially
MeOH
8 (24)6 character
1 %with and
24
lower
with
than
the C(4)-boundconsidered
to havehighcovalent
thus78to(100) complex
6294
www.angewandte.org! 2007[4]Wiley-VCH
VerlagGmbH&Taube,Co. KGaA, Weinheim H. Taube, T
R.
A.
Periana,D.
J.
25 (TableEtOH
4.5
1%
100
19
Carbenes in Synthesis (Ed.: S.S.P.Gamble,
Nolan), Wiley-VCH,
1,
entries
1–5).
Thehydrogenationofcyclooctene
atively strong. Reductive elimination pathways are particFujii,
Science
1998,
280,
560.
3
THF
8 (24)
1%
50 (100)
92006.
relevant4in catalytictransformations
involvingintermedi[5]
a) V.
R. G.
Bergman,Science 1984,
223, B.
902;Donnad
b) C.
8
(24)
1
%
30
(100)
0
CH2Cl
[7] a)
Lavallo,J. Mafhouz,
Y. Canac,2[6, 7][8]
Chem. Commun.
1999,
2443;Chem.
c) I. Soc.
G"ttker-Schn
hat compriseor complexes
aryl
Catalytic
5 and5Table 1: metal-boundtoluene hydrogenation
8hydrides,(24) of1cyclooctene% alkyl, with
0 (34)
0 6.[a]Schoeller,
G. Bertrand,
J. Am.
2004, 126,[d]
[9]White,
M.
Brookhart,
J.
Am.
Chem.
Soc.
2004,
126,G1
6EntryEtOH
3cat.
% loading[b]100s.
Lavallo, Y. Canac, C. Pr#sang, B. Donnadieu,
Solventt2.5[h]Conversion[c]n.d.
[%][6] Angew.
a) D. Bourissou,
O. Guerret,
P. Gabbai,
G. Int.
Bertr
EtOHthat abnormal
26
0.1 %
66
n.d.
Chem. 2005,
117, 5851;F.Angew.
Chem.
E
recently7 discovered
dicarbene
palladium
5
6
Rev.
2000,
100,
39;
b)
W.
A.
Herrmann,Angew.
Chem
8
EtOH
72
0.01 %< 5n.d.
5705; c) V. Lavallo, Y. Canac, A. Dehope, B. Don
exes, that is, NHC-type ligands that coordinate to the1342; Angew.
Chem.
Int.2005,
Ed. 2002,
41, 1290;c) N-H
1
MeOH
8 (24)
1%
78 (100)
24
Bertrand,Angew.
Chem.
117, 7402;
Angew.
Che[a] General conditions:
cyclooctene(2.0 mmol),(1 mol %),center abnormallythrough theC4Pdorcomplex
C5
2
EtOH
4.5 imidazolylidene
1%
100
19
Carbenesin
Synthesis
(Ed.:
S.
P.
Nolan),
Wiley-VCH,
2005, 44, 7236.
EtOH(6 mL),
1 atm Hprecursor 6 is the product from an[10]2 ; catalyst
providethan theC2RT,
position,
3 normalTHF
8 (24)
1 % catalyst precur50 (100)92006.
[8] a)J. S. Owen, J. A. Labinger, J. E. Bercaw, J. Am.
from 3 b (Scheme 1, see also
AgBF4-mediated halide abstraction[11]
8 (24)The 1abnormal%
30 (100)Figure01. C2-bound
4
CH2Clof2 olefins.
[7]dicarbenea) V. 126,
Lavallo,
B.C4Donnadbondingr the hydrogenation
complexes
andandY.
theCanac,abnormal
2004,
8247;J.b)1Mafhouz,W.
H.2 Meyer,M.
Deetlefs,
M. Po
Ref. [12]). [b] In mol equiv relative to cyclooactene. [c]Determined
by
[12]
5
toluene
8
(24)
1
%
0
(34)
0
Schoeller,
G.
Bertrand,
J.
Am.
Chem.
Soc.
2004,
126,
bound
carbenehomologues
3and4.
was assumed
to beessential for this catalytic activity,
Scholtz, M. W. Esterhuysen, G. R. Julius, H. G. Rau
GC. [d] Not
determined.
6
EtOH
2.5
3%
100
n.d.[d]Lavallo,Trans.
Y. Canac,
Pr#sang,
B. Donnadieu,G
Dalton2004, C.
413;c) M. Albrecht,
H. StoeC4-bound NHCs are considerably stronger donors than
7
EtOH[10, 13] 26
0.1 %
66
n.d.
Angew.Commun.
Chem. 2005,
117,
5851;
Chem. Int.
Chem.
2005,
4705;
d)Angew.
S. K. Schneider,
G.ER
normal counterparts.
Thisenhanced
donor ability
8
EtOH
72
0.01
%
<
5
n.d.
[a]
Dr.
M.
Heckenroth,
Prof.
Dr.
M.
Albrecht
5705; c) V.
Lavallo,
Y. Canac, A. Frenking,Dehope, B.
with 5 in polar solvents such as EtOH is complete within2. less
Loschen,G. Raubenheimer,
W.Don
A.
ORTEP representationof H.
complexes
2 (a), 4 a (b), 4G.
b (c), and 4 c (d).
es access to new reactivity pathways. Specifically, the FigureDepartment
Figure 2.ofORTEP
representationof Chem.
complexes
2 (a),
4 7402;a (b), 4Angew.
b (c), andChemistry,
UniversityofFribourg
Bertrand,Angew.
2005,
117,
Ch4
[a]
Generalconditions:
cyclooctene(2.0mmol),
Pd
complex(1mol%),
than 5 h. Longer reaction times are required to achieve
full
Dalton Trans.
1226; e)S. K.
Schneider,
P. Roem
Thermal ellipsoids
are calculated
at 50 %2006,
probability
level;hydrogenatoms
Thermalellipsoids
arecalculated
at50%
probability
level;hydrogenato
CheminduMus!e9,
CH-1700
Fribourg(Switzerland)
lectron density
theis surmised
2005,
44,
7236.
EtOH (6impartedmL), whenRT, 1 toatm
H2 ;metal
catalystcenter
precursor
6 isthe
productfromanand cocrystallized
solvent
molecules
areomitted for clarity.
Julius,H.
G.
Raubenheimer,
W.
A.
Herrmann,Adv.
S
conversion
thecatalyst
loadingisreducedbelow
andM.
cocrystallized
solvent
molecules
areInt.
omittedfor[14]
M.
Heckenroth,E.b Kluser,
Neels,
Albrecht,
Angew.
Chem.
Ed.J.2007,
46, 6293.!
[b]
V. Khlebnikov,
Prof.
Dr.J.M.
Albrecht[8] 2006,
a)S.348,Owen,
J. A. Labinger,
E. clarity.
Bercaw,
J. Am.
ilitate oxidative
additionreactions,
which
are
a(Scheme
key A.
-mediated
halideabstraction
from
3
1,
see
also
AgBF
1862.
4
1 mol
%
(Table 1, entries
6–8). Catalytic
activity
essentially
SchoolChemistryM.
& Chemical
Biology
Heckenroth,A. Neels,
G. Garnier,P. Determined
Aebi,
A. W.byofEhlers,
Albrecht,
Chem.
Eur.
J. 2009,15,
9375.!
2004,
126, 8247;b) W.
Meyer,M. M.
Deetlefs,
M. PoJ
Ref.M.
[12]).
[b] Inactivation
mol equiv
relativeto cyclooactene.
[c]n the metal-mediatedof
H .[15]M.
[9] a)S. Gr$ndemann,
A.H.Kovacevic,
Albrecht,2

29	

aNHCs as Ligands for Transition Metals	
hydrogenation	

alytic Hydrogenation Using Abnormal NHC!Pd Complexes
about mechanism!

not filtered	filtered after 2 h	

20% conv.	

6% conv.	

Figure 5. Correlation of particle size RH (grey columns) and conversion (^) for catalytic runs that were not filtered (a), and tho
Figure 5. Correlation of particle size RH (grey columns) and conversion (^) for catalytic run
2 h (c), and after 4 h (d).
2 h (c), and after 4 h (d).

“formation of submicrometer size particles is essential for the catalytic activity,
hence indicating a heterogeneous mechanism.”!

C!H bond elimination of the imidaz
ent after about 6 h, when conversion reached about 40 %. FilC!H bo
ent
after
about
6
h,
when
conversion
reachedaboutspecies
40 %. Fildium
hydrideas effective
tration of identical solutions through a nanoporous membrane[24]
dium hidentical solutions
membranecolloidal palladium.
after 1, 2, and 4 h indicatedtrationa clearofcorrelation
betweenthroughcon- aofnanoporous
Oxidativeof collo
1, 2,(Figureand 4 5hb–d).
indicateda clear correlation
betweencon- density
version and the presence of afterparticles
For examtated by thehigh electron
Pd-aNHC
complexdidn’tactivate
H-Hreaction
versionandanalysis
thepresence
ofbond…!
particles parted(Figure by
5 b–d).
exam- oftatedple, filtration after
2 h (Figure5 c) andof thethe ForC4-bonding
the bcple,efficient
filtration
after 2ofh all(Figure5 c) andreductive
analysis of
the reaction
partedmixture after 2.5 h showed theremovalnanopareliminationof C4-boundmixtureafter
2.5
h showedresumed
theAlbrecht,
efficient
removalall nanoparM. Heckenroth,V. notKhlebnikov,
A. Neels,
Schurtenberger,
M.
ChemCatChem,
2011, 3, reducti
167.!
ticles, butof the complex,
as P.
particleformation
fasterthanofthatof C2-boundNHCssmall anions such as Cl and B
accelerate heterolytic C–H bond30	aNHCs as an Organocatalyst	
hydrogen bonding, and this effewith C2- and C5-unsubstituted iring opening polymerization of cyclic esters	
to favor metallation of C2 (withTable 2. Results of the ROPproton)of ε-CLover
withC5Various(33). We reaso
Multifunctional Initiators at Room Temperature in THF Usingin 6, small anions sho
Carbeneprotected4
desired deprotonation
reactionbtime conversiona–Mn) was treated
ROH [M]0 (M) M/I I/C (min)when 6(%)(HCl•Cl(g/mol)
PDIb V
alents (34)of a lithium
base1.37
such/19/2002
O
5
1
100
2
60
95
28 800V
diisopropyla
6
0.5
100
2
100 (nBuLi)90or lithium
28 800
1.15
O
7 the presence
0.5100
45 proton90
21 500of 1 2indicatesthe nuclear
formationof resonancea 1.24hydr
Table 1 Polymerization data of cyclic estersaNHC(initiator)
1 as catalyst
magnetic
BnOH
O usingbenes
as Catalysts for Living
7
0.5
100
8
45
60
14
600
1.09
O
trum
theofresulting
bondedadduct.
formation
this ofproducttypeoshaCyclic
presenceof 1 The
indicates
formationa hydrTable 1 Polymerization
data of cyclic
aNHC
V
a the
HtheofBnO
Opening
Polymerizationof estersMusing[M]0/[1]/Conv.
Mn 1 as catalyst
Determinedby 1H NMR. b Determinedby
GPC
versuspolystyrenenpearanceofthesinglet
at
8.7
nNHC!
!
!alcoholhydrogenbondedadducthas
been
rep
a25 ºC, 1b hnbonded adduct. The formationof this type oV(%) a (NMR) (theory) PDI standards.
M
Solvent [BnOH](ppm)
arisingfrom
C5(H) offro
6.
M[M]0/[1]/
Conv.
M
n
n
earlierwhich
demonstrated
a shift ofadduct–OH
resonance
ε-caprolactonenNHC!
!
!alcoholhydrogenbonded
has
been
rep
a
b
(%)
(NMR)
(theory)
PDI Scheme
M
Solvent[BnOH]
e-CL
Toluene99
10500 aNHC
11
400
1.09
the
ofMechanism
1 indicates
theNHC-Catalyzed
formationof ROP
agiveshydr
Table 1 Polymerization
data of cyclic
estersusing
1 as catalyst
spectrum,the C5 carbon
3presence. Proposed
for
the
ofris
, AndreasMöck, 100/1/1
and James
L. Hedrick*
3.3
to
9.2 ppmdue
to hydrogenbondedadductform
earlierwhich
demonstrated
a
shift
of
–OH
resonancefrothepresenceof1
indicates
theformationofahydr
Table 1 Polymerization
cyclicesters1 as catalyst
rac-LA
Toluene 100/1/1data of(ε-CL)
99
13using
400 aNHC
14 4001.27
ε-CLresonanceat 190which
15bondedadduct.andTheformationofthisppm,
typeod, SanJose,Toluene
California
95120 99
betweennNHC
alcohol.
NMR
calculation
usin
e-CL
100/1/1
11
1.09
e-CL
THF
100/1/1
99
11
300
11
400
1.07
Mn500
[M]0/[1]/Conv.a 10
Mn400
3.3
to
9.2
ppm
due
to
hydrogenbondedadductform
bondedadduct.
The
formation
of
this
type
o
downfieldadductof the corresponding
nNHC! !(Gauge-Independent!alcohol hydrogen bonded
has beenreparac-LA
Toluene99
14M[M]0/[1]/Conv.a 13
Mn400 b 1.27
GIAO
AtomicOrbital)methode-CL
THF
100/1/2
95
5100
5500
1.12
(%)
(NMR)
(theory)
PDI
M
Solvent 100/1/1[BnOH]
n400
15
9, 2001
nNHC!
! !alcoholhydrogenbondedhas
been
rep
betweennNHC
alcohol.
NMR
calculation
usin
a
6–OH
(124
ppm).
AlthougSynthesis100/1/1
of aNHC lithium
7,700
rearrangement
aNHC
9,
and
itsandensuinge-CL
THF
100/1/1
99
11
300
11
400 bproduct1.07
earlierwhich
demonstrated
aprecursor
shift
ofadductresonancefro(%) adduct11(NMR)(theory)
PDI 8, free
MFig. 2. THF
Solvent[BnOH]
rac-LA
91
13
200
1.23
reveals
a
shift
of
10.8
ppm
after
the
hydrogenbondedadgold(I)
complex10
andCO-adduct
11.
dicatedthatadeprotonationhToluene100/1/1
99
10
500
11
400
1.09
earlierwhich
demonstrated
a
shift
of
–OH
resonance
fro
GIAO(Gauge-Independent
AtomicOrbital)methode-CL
THF
100/1/2
95
5100
5500
1.12
2
d-VL
Toluene 100/1/1
96
8000
9700
1.22
16
3.3
to
9.2
ppm
due
to
hydrogenbondedadductform
Further,avariabletemperature
study
up
to
formation.
e-CL
10
500
11
1.09
heme
1
Toluene100/1/1
99
13
400
14
400
1.27
rac-LA
THF
91
11
700
13
200
1.23
d-VL
100/1/1
92
8000
9300
1.18
3.3
to 9.2ppm
hydrogenadductform
15 thebonded
revealsa nNHCshiftof due10.8 toppm
afterhydrogenbondedad
betweenandalcohol.
NMR
calculationusin
rac-LA
Toluene100/1/1
99
13
14
1.27
e-CLc
THF
99
11
300
11
400
1.07
indicatesthaton
increasingthetemperature
this
hydr
15
d-VL
Toluene100/1/1
96
8000
9700
1.22
16
THF
500/1/1
55
23 400200
31 400
500
1.11
e-CL
between(Gauge-IndependentnNHC
and aalcohol.
NMR Orbital)
calculation
Further,variabletemperature
study
upusintoformation.
e-CL
99
11
11
1.07
GIAO
Atomicmethode-CL
THF
100/1/2
95
5100
5500
1.12
d-VL
100/1/1
92
8000
9300
1.18
www.sciencemag.org
SCIENCEVOL
326
23 OCTOBER
2009
Toluene 100/1/1
500/1/1
60
40 300300
43 400300
1.21
rac-LAc THF
bonded–OH
protongraduallymoves
towards
the
shi
c
GIAO
AtomicOrbital)bonded
methode-CL
THF
100/1/2
95
5100
5500
1.12
indicatesa(Gauge-Independentthat ofon10.8increasingthetemperature
this
hydr
rac-LA
THF
91
11
13
1.23
55
23 700200
31 200
500
1.11
e-CL
Toluene 100/1/1
500/1/1
75
30
100
37
600
1.17
d-VLc
reveals
shift
ppm after
thehydrogenadregionindicating
weakeningofthehydrogenbonding(Fic
rac-LA
THF
100/1/1
91
11
700
13
200
1.23
d-VL
Toluene100/1/1
96
8000
9700
1.22
reveals
a
shift
of
10.8
ppm
after
the
hydrogenbonded16
500/1/1
60
40
300
43
300
1.21
rac-LAbonded –OHFurther,
proton agradually
moves towards
the
a
1
b
variable temperature
study
up shitoad
formation.
Determinedby
H
NMR
spectroscopy.
Calculated
from
d-VL
Toluene100/1/1
96
8000
9700
1.22
c
ESIw).
Allthese
observations
supporttheformation
16
d-VL
THF
100/1/1
92
8000
9300
1.18
Toluene 500/1/1
75
30 10037 6001.17
d-VLc
Further,a variabletemperature
study up (Fitoformation.
regionindicating
ofthe
the
hydrogen bonding
d-VL
THF
9300
1.18
indicatesthat
onweakening
increasing
temperature
this hydr" (% 100/1/1
Conv.) " ([M]92)+
MWBnOH. c Reaction
MWmonomer
0/[BnOH]08000
THF
500/1/1
55
23
200
31
500
1.11
e-CL
hydrogenbondedadductbetween
1
and
BnOH.
c c
a
1
indicatesthaton observations
increasing thesupport
temperature
this hydr
THF
500/1/1
55 spectroscopy.
23 300200 b Calculated
31 300
500
1.11
e-CL
Determinedby
H 1NMR
fromESIw).
Allthese
the formationtime
2 hc insteadof500/1/1
usual
h reaction
time.40
Toluene6043
1.21
rac-LAbonded–OH
protongraduallymoves
towards
themonshi
Further,thecontrolled
additionofacyclicester
ccToluene500/1/1
60
40
300
43
300
1.21
rac-LAbonded–OH
protongraduallymoves
towards
the
shi
"
(%
Conv.)"([M]/[BnOH])+MW.
ReactionMW
Toluene500/1/1
75
30
100
37
600
1.17
d-VLmonomer
00
BnOH
hydrogenbondedadductbetween1 andBnOH
BnOH.
cregionindicating
weakeningof the andhydrogenbonding(Fi
toareaction
mixture
of
aNHC
was
done
Toluene500/1/1
75
30
100
37
600
1.17
d-VL
with	
time
2 h instead of usual
1 h reaction time.
regionindicating
weakeningof the hydrogenbonding(Fiab
1 controlled
Further,theadditionof aThis
cyclicester monThe
progress ofpolymerization
reaction wasmonitoredfrom
byDetermined
bythe11H
NMR spectroscopy.
Calculated
ESIw).
All bytheseobservations
supporttheexperimentformationabmonitored
H
NMR
spectroscopy.
s
by Conv.)
H "NMR /[BnOH]
spectroscopy.
Calculatedfrom
1 Determined
ESIw).
All these
observations
supportthe formation" (%MW. cc ReactionMW
H NMR
at ([M]different
timeintervals.
The
result istoareaction
mixture
of
aNHC
and
BnOH
was
done
monomerspectroscopy
0
0) +
BnOH
hydrogenbondedadductbetween
1
and
BnOH.
that
the hydrogenbondedbetween–OH resonancegradually shi
Conv.)([M]. Reaction
MW
24
h, "
rt	
(%
0/[BnOH]0) + MW
BnOH
hydrogenbondedadduct
1 and
BnOH.
1
The monomerof1A
polymerization
reactionwasmonitoredbytime
2progress
hininstead
ofthe
usual
1"hreveals
reaction
time.
shown
Fig.
which
a
first
order
kinetics
forringmonitoredby
H
NMR
spectroscopy.
This
Further,of a cyclic
ester monhighfield. theForcontrolled
example, addition
after addition
ofexperiment
three
equis
time
2 h THF	
instead ofusual 1 h reaction
1
120/1/2	
99	
 time.
1.30	
Further,thecontrolled
additionofacyclic
ester
mon
H
NMR
spectroscopy
atdifferent
timeintervals.
The
resultismolecular
weights
andthe polydispersities
increase(Table1polymerizationof rac-lactide.
The rateof polymerizationthatthehydrogenbonded–OH
resonance
graduallyshi
to arac-lactide
reaction
mixtureof aNHC
andresonance
BnOH
was
done
bleopening
1. Selected
Polymerization
Data of Cyclic
EstersUsing
themonomer,the –OH
moves
59
to
a
reaction
mixture
of
aNHC
and
BnOH
was
done
shown
in Carbene
Fig.
1A
revealsorder
kinetics
for A.
ring
and high
Figure2),
implicating
theonset additionof competitivetranses1
The progressthepolymerization
reactionmonitored
byHeterocyclic
1whichCatalyst
T.as
K.a inSen,
S.aaC.first
Sau,
A.was
Mukherjee,
Modak,S. K.
Mandal,Chem.
Commun.
2010,
11972.!
was
found
toof
fastertoluene
than
THF
under
identical
field.
For
example,
after
of47,
three
monitored
by
NMR
spectroscopy.
Thisexperiment
s
d
11.5
ppm
to
d
10.3
ppm
and
to
d
10.2
ppm
after
theequi
add
1H
60-65
The
progressofbethepolymerization
reactionwasmonitoredby1monitoredby
H
NMR
spectroscopy.
Thisexperiments
terification
reactionscatalyzed
by
the
NHC
catalyst.
openingpolymerizationof
rac-lactide.
The
rate
of
polymerization
H
NMR
spectroscopy
atdifferent
timeintervals.
The
resultis
E.The
F. bConnor,
G.c W. Nyce,
M.DP
Myers,
J.eight
L. Hedrick,J.bondedAm.ofChem.
Soc.
2002,
124,ESIw).
914.!
de n(NMR)]conditionsof conv.
molecular
weight[M
1 monomer (Fig. 1A).
therac-lactide
monomer,the –OH
resonance
I/(Cat./I)(M/I)
PDIf A.isMöck,thatthehydrogen–OH
resonanceshiofequivalentsmonomer(Fig. graduallyS5, moves
H NMR spectroscopy
atplotdifferent
time intervals.
The
resultaNHCs as an Organocatalyst	

31	

ring opening polymerization of cyclic esters	

Financial
Technology
highly ackn
UGC, resp
IISER-Kolk

ber 2012| doi:10.1039/C1CC15032D

Reference

1 R. Breslo
2 (a) A. J. A
Soc., 199
R. L. H
5530–553
3 (a) D. E
534–541;
Chem., 2
2988–300
Rev., 200
Synlett, 2
Fig. 2 Frontier KS-molecular orbitals for hydrogen bonded adducts
4 V. Nair,with benzyl alcohol: (a) 1a (aNHC" " "BnOH adduct) and (b) 2a
2691–269(nNHC" " "BnOH adduct). The orbital energies are in eVs.
5 (a) E. F
L. H
T.
K. Sen, S.
C. Sau,Mukherjee,
A. Modak,S. mechanism
K. Mandal, Chem.
Commun.
throughattackbyA.theactivatedalcohol
pathway;a 2010, 47,J. 11972.!

Scheme S1. Plausible mechanism of ring opening polymerization using abnormal carbene 1.

32	

aNHCs as an Organocatalyst	
ring opening polymerization of cyclic esters	O H

plausible mechanism!
N

BnOH

: N

:
N

Formation of
adductN

O

1aO

Ring Opening ofr ac- Lactide

O

O

O
O

N

O
Bn O

O

O
O

OH
O

: N
H

N

O

2

O

E

N

O

Formationof new
alcoholBinsertion of
BnOH

O

Bn O

O

O
O

H
O

O
O

O

N

O

N

O

D

N

O

: N

O

ad
an dition
eq other ofmo uival
no entm e of
r

O
Bn

O

O

O

H

C

O

T. K. Sen, S. C. Sau, A. Mukherjee, A. Modak, S. K. Mandal, Chem. Commun. 2010, 47, 11972.!

Contents	
1. introduction	

13

shape of the C NMR signal, as well as the calculated chemical shift (32) for the C5 carbon of
the free aNHC 9 (205 ppm), suggested that the
new compound was the aNHC lithium 7. Similarcomplexation has previously been observed forother singlet carbenes such as small NHCs (35)and bis(diisopropylamino)cyclopropenylidene(36, 37); in both cases, coordination of the lithium
cation was apparent from the broadening and theupfield shift of the carbene 13C NMR resonance.
Subsequently, we sought to sequester the lithium
cation through addition of excess [12]crown-4 to adiethylether solution of 7 (where X is Br). This
treatment induced a clean rearrangement to generate 8, which was isolated in 45% yield. This
product formally results from the deprotonation of
an isopropyl substituent of the Dip group by thecarbene center of the aNHC lithium adduct 7,
followed by nucleophilic addition of the resulting
benzyl anion to C2. However, calculations indicated that the rearrangement of 9 into 8 is
exothermic by only 6.1 kcal mol–1 and involves
an activation barrier of 20.3 kcal mol–1 for the
proton transfer. Therefore, we hypothesized thatthe observed rearrangement was catalyzed by a

2. N-heterocyclic carbenes (NHCs)	

first metal-NHC complex	
first free, isolable NHC	NHCs as ligands for transition metals	
Fig. 3. Molecular views
NHCs as organocatalysts	
(50% thermal ellipsoids

component of the (crown)LiBr system, and thatthe formation of 8 did not imply that the free
aNHC 9 was too reactive to be isolated.
Sodium and potassium bases have provenmore appropriate than lithium bases in generating
free carbenes (35–37), because the corresponding
carbon-heavy alkali metal bonds are more labile,which favors precipitation of the salt. When the
deprotonation of imidazolium 6 (HX•X–, where
X is Cl or Br) was performed with two equivalents (34) of potassium hexamethyldisilazide(KHMDS) in tetrahydrofuran, a clean reaction
occurred, with the 13C NMR spectrum of the resulting product showing a very sharp signal at
201.9 ppm. After the products were worked up,
the free aNHC 9 was isolated as a green powder(480 mg, 68% yield), and single crystals wereobtained by recrystallization from a dry hexane
solution at –78°C (Fig. 3, right).
In the solid state, both the free aNHC 9 andthe imidazolium salt 6 (Br–) feature a fully planarring (maximum deviation for N1–C2–N3–C4–C5–
C21–C31–C41–C53 was 1.9 and 6.3 pm for 9 and6, respectively), confirming the delocalization of
the p system. This electronic structure is corrobo-

In the solid state, both the free aNH
the imidazolium salt 6 (Br–) feature a furing (maximum deviation for N1–C2–N3
C21–C31–C41–C53 was 1.9 and 6.3 pm
6, respectively), confirming the delocali
the p system. This electronic structure is33	

and double bonds. The carbene bondangle
N1– views
Fig. 3.
Molecular
C5–C4 for 9 [101.03 T 17°] is morethan
(50% acutethermalellipsoids
the corresponding angle in the cationicprecursor
are shown) of imidazolium–
6 [108.0 T 3°]. This feature is bromide
consistent
6 (Brwith) (left) andincreased s character of the s lone-pair
onthe solid
aNHC 9orbital(right) inthe carbene atom in 9 as comparedwith
C–H+H atoms
state(fortheclarity,
bonding orbital in 6. A similararerelationshipisomitted, except
for+the ringhydrogen). Bondobserved in NHCs and their NHC(H) precursors
lengths and angles for(3, 4).
–
are kcalas follows:
Calculations predict aNHC 96to(Brbe )14.1
C5–N1,
1.368
mol–1 less stable than its isomericnormalNHCT 4 Å;
N1–C2,
1.334
with the phenyl group bonded to C5 instead ofT 4 Å;
C2–N3,occupied
1.363 T 4 Å;
C2. Figure 4 shows the two highest
1.408 T 4 Å;molecular orbitals (HOMOs) ofN3–C4,
9. The HOMO
C4–C5,(–4.403 eV) is a s-type lone-pairorbital1.355
at C5;T 5 Å;
N1–C5–C4,
108.0 T 3°;
the HOMO-1 (–4.879 eV) is a C5–C4
p-bonding
C5–C4–N3, 106.0 T 3°;
orbital, which exhibits antibonding conjugation
C4–N3–C2, 108.8 T 3°;
with the p orbital of the phenyl substituent at C4.
N3–C2–N1, 106.9 T 3°;
These molecular orbitals are muchhigher in110.4
en- T 3°.
C2–N1–C5,
ergy than those of the isomeric NHC
Bond(–5.000
lengths andand angles for 9 are as follows: C5a–N1a, 1.417 T 2 Å; N1a–C2a, 1.357 T 2 Å; C2a––5.279 eV, respectively), whichN1a–C5a–C4a,
indicates that101.03 T 17°; C5a–C4a–N3a, 111.01 T 16°; C4a–N3a–C2a, 107.97 T 15°; N3aFig. 4. Plot of the calculated two highest-lying
are shown) of imidazolium–
occupied orbitals HOMO
bromide 6 (Br ) (left) and(left) andHOMO-1(right)
aNHC 9 (right) in the solidof the aNHC 9.
state (for clarity, H atoms
are omitted, except for
Published on Web 04/06/2004
the ring hydrogen). Bond
lengths and angles for–
6 (Br ) are as follows:
Structure and Reactivity of “Unusual” N-Heterocyclic Carbene (NHC)
C5–N1, 1.368 T 4 Å;Palladium Complexes Synthesized from Imidazolium Salts
N1–C2, 1.334 T 4 Å;
C2–N3, 1.363 T 4 Å;Hélène Lebel,*,‡ Marc K. Janes,‡ André B. Charette,‡ and Steven P. Nolan§
N3–C4, 1.408 T 4 Å;Département de chimie, UniVersité de Montréal, C.P. 6128, Succursale Centre-Ville,
C4–C5, 1.355 T 5 Å;Montréal, Québec, Canada, H3C 3J7, and Department of Chemistry, UniVersity of New Orleans,
N1–C5–C4, 108.0 T 3°;
New Orleans, Louisiana 70148View Online
C5–C4–N3, 106.0 T 3°;
C4–N3–C2, 108.8 T 3°;
Received January 14, 2004; E-mail: lebelhe@chimie.umontreal.ca
N3–C2–N1, 106.9 T 3°;
For 6b: 1H NMR (CDCl3, 298 K): d 9.09 (s, 1H, NCHN), 8.51 (d, 3JHH
4.6 Hz, 1H, py-H), 7.73 (dt, 3JHH 7.7, 4JHH 1.8 Hz , 1H, py-H), 7.58 (d, 3JHH
C2–N1–C5, 110.4 T 3°.
Recently,1.345
a numberof 7.8
newHz,
transition
an N-Heterocyclic
Os T 2 Å; C2a–N3a,Vol.processes
26,
No.(s,26,
1H,Organometallics,py-H),
7.53(s, 1H, imid-H),
1H, 2007
imid-H),6557
7.27 (dd, 3JHH
Bond lengths and angles forCoordination
9 are as follows:ofC5a–N1a,
1.417 T 2 Å;Carbene
N1a–C2a,to
1.357
T 2 Å; N3a–C4a,
1.412
T 3metal-catalyzed
Å;
C4a–C5a,
1.383
T 37.32
Å;3J
3
23 OCTOBER 2009 VOL 326 SCIENCE w
1H,558py-H),
5.49(s, 4H, CH
havebeen developed
andoptimized
throughhigh-throughput
and
HH 4.8 Hz,2), 4.19 (t, JHH 7.5 Hz, 2H, BuN1a–C5a–C4a, 101.03 T 17°; C5a–C4a–N3a, 111.01 T 16°; C4a–N3a–C2a, 107.97 T 15°;
N3a–C2a–N1a,106.25
T7.7,
16°;
C2a–N1a–C5a,113.72
T 15°.
1H),
1.89–1.81(m,
2H,
Bu-H),
1.39–1.30(m,
2H,Bu-H),
0.93(t, 3JHH 7.5
Chart 1
combinatorial methods. Such strategies13are1 particularly efficientHz, 3H, Bu-H). C{ H} NMR (CDCl3, 298 K): d 152.25 (Cpy), 149.77
for the screening of a large
number of transition metal-ligand
Fig. 4. Plot of the cal(Cpy), 137.86 (Cpy), 136.37 (NCN), 124.06 (Cpy), 123.80 (Cpy), 122.93
combinations under various
setsof reaction
culated two highest-lying), 121.75(Cimid),conditions.
54.00 (CH2),However,
49.93 (CBu), 31.81 (CBu), 19.34 (CBu),(Cimidunder these in situ conditions,13.30 (Ctheoccupied orbitals HOMO
Bu). exact nature of the catalytic
1H NMR (CDCl , 298 K): d 8.72 (s, 1H, NCHN), 8.23 (d, 1H,
For 7a:3 assumption of(left) andHOMO-1(right)
species is not always precisely
known.
Indeed, the33
5.5 Hz,ligandpy-H), 7.37–7.14(m, 32H,
py-H,of the aNHC 9.
a normal mode of bindingJHH
betweenand transition
metal
is Ph-H), 6.07 (t, JHH 6.3 Hz,py-H), 5.17 (s, 1H, imid-H), 4.70 (s, 2H, CH2), 4.25 (septet, 1H, 3JHH 6.5
most often assumed and maybemisleading.
3
2
Hz, CH), 1.19 (d, 6H, JHH 6.5 Hz, CH3), 210.83 (dt, JPH 19.6, 3JHH 4.9
3J
During the course of ouron 2ligands
to Hz, Ir–H). 13C{1H} NMR
JPH 18.6,suitable
Hz,studies
Ir-H), focusing
221.49 (dt,
HH 4.92 ), 141.07 (t, J
d 161.78 (Cpy),processes,
153.10 (Cinduce enantioselectivity (CDClin transition
metal-catalyzed
3, 298 K):
py
PC 7.1 Hz,
C),
137.04(C),
134.93(t,J=
26.3
Hz, CPh), 133.55 (t, JPC 6.0 Hz,
carbene N-heterocyclicpy
we became interested in testing
carbenePC(NHC)CPh), 132.40 (CPh), 129.55 (NCN), 127.84 (t, JPC 5.8, CPh), 125.87 (Cimid),
metal complexes. A numberof simple NHC palladium-based
124.15 (Cimid), 123.56 (Cpy), 55.03 (CH2), 50.47 (CPr ), 22.94 (CPr ).
31P{1H} NMR
complexes have recently emerged
as effective
a variety(CDCl3catalysts, 298 K): ford 21.36.
3
1H NMR7b: studyK): d 8.71of(s, 1H, NCHN), 8.19 (d, 3JHH
of cross-coupling reactions.ForOurof (CDClthe exact
3, 298structure
5.1
Hz,
1H,
py-H),
7.37–7.15
(m,
32H,
py-H,
various palladium NHC species reveals that the metalation site
on Ph-H), 6.07 (t, 3JHH 5.9 Hz,
1H, py-H), 5.03 (s, 1H, imid-H), 4.72 (s, 2H, CH2), 3.63 (t, 3JHH 6.9 Hz, 1H,
the imidazolium salt is strongly
influenced by the presence of base.
CH2), 1.47 (m, 2H, CH2), 1.17 (m, 2H, CH2), 0.90 (t, 3JHH 7.7 Hz, CH3),
products also depends upon the counterion of the used We
salt. now
This report the synthesis
2J
3J
210.89and(dt, 2structure
JPH 19.6, 3of
JHHnovel
5.1 Hz,palladiumIr-H), 219.61 (dt,Figure
1. ORTEP
diagram of palladium complex 2. Selected bond len
PH 18.4,
HH 5.1
surprising and interesting fact certainly needs additional
studies bearing NHCHz,ligands
1H} NMR (CDClcomplexes
and “abnormal”(Cpyand), 153.07
Ir-H). 13inC{“normal”
angles(C(deg):
Pd(1)-C(12), 2.019(13); Pd(1)-C(25), 2.021(
3, 298 K): d 161.77(Å)py),
with other systems where abnormal binding also occurs(t, Jof
6.9
Ccarbene), 137.09(C),2a.
134.96(t, JPCbond= 26.3,CPh), Pd(1)-Cl(2), 2.302(4); C(12)-Pd(1)-C(25), 17
Pd(1)-Cl(1),2.289(4);Figure1.141.30
MolecularofcationofpytoSelectedbinding tomotifs. The
bindingmode
the Hz,NHC
to thePd
isshown
PC structure
JPCpalladium
6.0, C)129.59127.86133.79(5);
C(12)-Pd(1)-Cl(1),93.3(4); C(25)-Pd(1)-Cl(1), 87.3(3); C(1establish how often it takes place.
Ph), 133.61
Ph),
lengths(Å)
and (C
angles(deg):
Os-P(1)2.3605(14),Os-P(2))(t, JPC 4.7,
substantially affectthe catalytic
behaviorof(t,thecom- (NCN),CPh),Os-N(3)126.13 (C)), 125.90 (C(C2.123(6),(CH(CBu), C(25)-Pd(1)-Cl(2), 90.9(3); Cl(1)-Pd(1)-C2
imid2.213(4),py), 124.18)py), 55.08Pd(1)-Cl(2),2), 47.89 88.6(4);2.3683(14),Os-C(1)
H(1A)The complex
OsH6(PiPr
which is3)2 is a d polyhydride plexes.
1H} NMR (CDCl , 298 K): d
558
23 OCTOBER2009VOL
SCIENCE), 19.36 (CBu), 13.37(CBu). 31P{P(1)-Os-N(3)31.96(CBuP(1)-Os-P(2)Fig.
1 Molecular
structureofthe 326
cation of
7a showing www.sciencemag.org
50% probability
177.23(18).
3
H(1B)
)
1.44(5);)164.39(5),)
obtained inthermalhigh ellipsoids.
yield fromcompound- positions)Cl2been
It 2hasreported thatpalladium (II)-NHC complexes could
21.39.
Onlythethe known
metal-boundhydrogensOsH(calculated
99.70(11),P(2)-Os-N(3)) 93.44(11),P(1)-Os-C(1) ) 98.58(14),(PiPr3)2,7 via
the trihydride
tetrahydrideborate‡ Crystal (II)dataacetatefor 7a:andC48H
are shown.
Selected distances(Å) and angles (°):intermediate
Ir(1)–P(1)2.287(2),Ir(1)–
be easily
preparedfrom palladiumthecorresponding
47BF
4IrN3P2, M = 1006.89, colorless crystal,displayed two doublets at 6.57 and 7.47 ppm (J ) 1.7
P(2)-Os-C(1))monoclinic
90.78(14),2 P(2) 2.301(2),i84primitive
cellN(3)-Os-C(1)with dimensions:a)=85.40(18).
9.4493(2), b = 20.6687(6),Ir(1)–N(3)2.193(5),Ir(1)–C(7)2.100(6),N(1)–C(6)OsH3(η -H2BH2)(P Pr3)2. This hexahydride reacts with
Lewis salts. Metal binding
imidazoliumat the C(2)positionis usuallycorrespondingto H(3) and the carbene H(1) of a possibly C
c = 22.0865(6) Å, b = 90.160(2)° and V = 4313.6(2)Å3; Z = 4; m(Mo–1.461(7),N(1)–C(7) 1.404(7),N(1)–C(9)1.335(8),N(2)–C(8) 1.390(8),bases to afford
d4 polyhydride
derivatives,
wheretheobserved,
hydrideand complexes
bearing
tworeaction
NHCs
canpotentially
exist
13
21
23 (iii) the preparation
NHC ligand
in theproducts,
and3. abnormal N-heterocyclic carbenes (aNHCs)	
first metal-aNHC complex	
first free aNHC!
C2- vs C4- bound carbene!
NHC vs aNHC!
application to catalysts	

i

18 September 2012
ubs.rsc.org | doi:10.1039/B107881J

4. summary	benzyl anion to C2. However, calculations indicated that the rearrangement of 9 into 8 is
–1
exothermic
rated by the values of the endocyclic
C–Nby[6:only 6.1 kcal mol and–1involves
barrier of 20.3 kcal mol for the
1.335 T 5 to 1.409 T 5; 9: 1.354 T 2antoactivation
1.408 T 3 Å]
and C–C bond lengths [6: 1.351proton
T 5, 9:transfer.
1.385 TTherefore, we hypothesized thatobserved
rearrangement was catalyzed by a3 Å], which are halfway betweenthe
thoseof single

Downloaded from www.sciencemag.org on September 17, 2012REPORTSi

34	

Summary	
1. introduction	
2. N-heterocyclic carbenes (NHCs)	
first metal-NHC complex	
first free, isolable NHC	NHCs as ligands for transition metals	
NHCs as organocatalysts	

3. abnormal N-heterocyclic carbenes (aNHCs)	
first metal-aNHC complex	
first free aNHC!
C2- vs C4- bound carbene!
NHC vs aNHC!
application to catalysts	

4. summary	▶ C4-carbenes are kinetically favored products. !
▶ Large wingtip groups and non-H-bound anions !
favor C4-bound carbene.!
▶ aNHCs are stronger σ-electron donors than NHCs. !
▶ aNHCs tend to cleave from the metal more easily than NHCs.!
This may limit their utility in catalysis…!

References	P. L. Arnold, S. Pearson, Coord. Chem. Rev. 2007, 251, 596.!
M. Albrecht, Chem. Commun. 2008, 3601.!
O. Schuster, L. Yang, H. G. Raubenheimer, M. Albrecht, Chem. Rev. 2009, 109, 3445.!
M. Melaimi, M. Soleilhavoup, G. Bertrand, Angew. Chem. Int. Ed. 2010, 49, 8810.	

35