Literature Seminar(D1 Part)

Organometallics for Sustainability- Exploring Organometallics with P. J. Chirik's Work as Central Themes -N=N Bond Cleavage by a Low-Coordinate
Iron(II) Hydride Complex

P. L. Holland et al.
J. Am. Chem. Soc.
2003, 125, 15752

Cross-Coupling of sp3 C-H Bonds in Alkenes:
Catalytic Cyclization of Alkene-Amide Substrates

Y. Kimura (D1)
21st April 2010

D. Sames et al.
J. Am. Chem. Soc
2004, 126, 6556

Contents[1] Dinitrogen Cleavage and Functionalization by Carbon Monoxide Promoted by a Hafnium Complex

P. J. Chirik et al.
Nature Chemistr y
2010, 2, 30

N2 cleavage

C-N and C-C bond formation[2] Iron-Catalyzed, Hydrogen-Mediated Reductive Cyclization of 1,6-Enynes and Diynes:
Evidence for Bis(imino)pyridine Participation

P. J. Chirik et al.
J. Am. Chem. Soc.
2009, 131, 8772

1

[1] Dinitrogen Cleavage and Functionalization by Carbon Monoxide Promoted by a Hafnium Complex

P. J. Chirik et al.
Natur e Chemistr y
2010, 2, 30(1) Overview

A new method for the cleavage of atmospheric N2 and homologation by a hafnium complex that operates at ambient temperature and mild pressures

2(2) BackgroundN278 % of the Earth's atmosphere"Chatt-mechanism"

its non-polality, high ionizing potential, strong triple bondcoordinated dinitrogen is functionalized by addition of strong electrophiles,

render it a typically inert molecule

typically proton donors such as mineral acids.

efficient chemical methods for the assemble of N-C bonds

Chirk's group sought to develop methods to elaborate coordinated N2

using N2 are attractivewith non-polar reagents, such as dihydrogen

Haber-Bosch process: N2 (g) + 3H2 (g)2NH3 (g)

140 million tons/year, sustaining 40% of the world population

molecular nitrogen is not a good ligand
even binding N2 to metal complex in solution was a decades-long challenge

harsh conditions: 400 ~ 650 °C, 200 ~ 400 atm(3) Precedents and Related Studies[Related Study 1]
Preparation of a Dinitrogen Complex of Bis(pentamethylcyclopentadienyl)zirconium(II).
Isolation and Protonation Leading to Stoichiometric Reduction of Dinitrogen to Hydrazine

ZrCl4 + 2 LiCp*
2 Cp*2ZrCl2 + 4 Na(Hg)J. E. Bercaw et al.
J. Am. Chem. Soc.
1974, 96, 6229

Cp*2ZrCl2 + 2 LiCl2
1 atm N2(Cp*2Zr)(N2)3 + 4 NaCl

first zirconocene dinitrogen complex (1-N2 = (Cp*2Zr)(N2)3 )both terminal and bridging, end-on coordinated N2 ligand
N N bond lengths only slightly elongated from free N2
neutral [N2]0 formulation was most appropriate
exposure 1-N2 to H2 induces N2 loss and formation of the zirconocene-H2 complex[Related Study 2-1]
Synthesis of Singly and Doubly Bridged ansa-Zirconocene Hydrides.
Formation of an Unusual Mixed Valence Trimeric Hydride by Reaction of H2 with {(Me2Si)2( 5-C5H3)2}Zr(CH3)2 andGeneration of a Dinitrogen Complex by Reaction of N2 with a Zirconocene Dihydrideansa-zirconocene complex:

J. E. Bercaw et al.
Or ganometallics
2001, 20, 534

N-N bond length inidicates an important contribution from [N2]2-

two cyclopentadienyl rings are tethered by a bridging moiety

resonance form

configurational stability, control of the substituents' orientation

antiferromaganetic d1, Zr (III) centers must be invoked to accountansa-ziconocene complex dihydride in the presence of N2

for the apparent diamagnetism

undergoes facile thermal reductive elimination of H2 andslow, thermal reductive elimination of dihydrogen followed byforms the side-on bound dinitrogen complex

rapid trapping by dinitrogen

conversion of an early transition metal hydride to a dinitrogenansa-bridge promotes reductive elimination

complex is rather rare

(in a non-bridged analogue, no reductive elimination of H2 observed)
probably due to reduced electron-donating ability2

J. B. Kerister et al.
J. Am. Chem. Soc.
2002, 124, 9525

[Related Study 2-2]
The Electronic Influence of Ring Substituents and Ansa Bridges in Zirconocene Complexes as Probed byInfrared Spectroscopic, Electrochemical, and Computational Studies

CO stretching frequency of a metal carbonyl complex provides a simple

RnZr

CO

means of assessing the electron density on the metal center

COoverall electron-donating influence of the substituents follows the sequence;
t-Bu> i-Pr > Et > Me > Me3Si > H

CO(cm-1)
for unbridged substituents:
CO on a per substituent basis correlates well with Hammet

meta parametes

suggestig the influence of these substituents is via a simple inductive effecthowever, for bridged systems....
MeMeZrCOCO[Me2C] and [Me2Si] ansa bridges resulted in an increase inCO

these substituents have electron withdrawing influence on the Cp ringsin contrast to the case of non-bridged substituentsCp'

ZrBridge

Parameters to define metallocene geometries

interpretation by MO caluculations
Figure 15+ Zr dx2 interacts with Cp's acceptor orbital+ this interaction represents

back-donation+ bridging reinforces this interaction
Figure 17+ antibonding orbital on bridging atom decrese
the energy a1 acceptor orbital

Figure 15
Back-bonding interaction between"the filled dx2 orbital" and"the cyclpentadienyl ligand acceptor orbital"

Figure 17
Stabilization of the combined cyclopentadienyl
ligand a1 acceptor orbital by the bridging atomansa-type zirconocene
increased reductive elimination of H2
due to electron-withdrawing effect by the bridging
less favorable for N2 activation itself[Related Study 3-1]
Hydrogenation and Cleavage of Dinitrogen to Ammonia with a Zirconium Complex

P. J. Chirik et al.
Natur e 2004, 427, 527

going back to unbridged zirconocene complex
systematic variation of the cyclopentyadienyl methyl substituents, expecting
altered reactivity
attempt with the complex laking one methyl from Cp* moiety
affording side-on dinitrogen complex; N-N length 1.38 Å (cf. free N2 1.10 Å)
consistent with four electron reduction
65.3 twist of the dimer - the dihedral angle between two planes formed

dinitrogen hydrogenation with 2-N2 was attempted
1 atm H2, 23 C; formation of hydrido zirconocene
diazenido complex 2-N2H4
23 C

continued hydrgenation at 85 C afforded ca. 10 % NH3
along with zirconocene dihydride complex
3

P. J. Chirik et al.
J. Am. Chem. Soc.
2004, 126, 14326[Related Study 3-2]
On the Origin of Dinitrogen Hydrogenation Promoted by [( 5-C5Me4H)2Zr]2( 2, 2, 2-N2)

N2
how about the reaction with "mixed ring" zirconocene complex 4 ?

Cp*(C5Me4H)ZrI2

KC8=> end-on complex formed, no reduction of N2 with H2 gas"mixed r ing"
interpretation by MO caluculations
6: model complex [( 5-C5H5)2Zr]2( 2, 2, 2-N2)
where the dihedral angle was confined to 0
LUMO: in-phase liniar combination (LC) of two zirconocene 1a1 orbitals
which is suited to incoming H2
HOMO: out-of-phase LC of two zirconocene 1a1 orbitals engaged in-bonding with a * orbital of the N2 ligand
reministent of group 4 transition metal imido complex, Zr=N character
HOMO of 6: does not have appropiate symmetry to engage in significant
backbonding with N2 ligand[Related Study 3-3]
How Many Methyl Groups in [( 5-C5MenH5-n)2Zr]2( 2, 2, 2-N2) Are Needed for Dinitrogen Hydrogenation ?
A Theoretical Study

K. Morokuma
Angew. Chem. Int. Ed.
2005, 44, 7101

previous experimental results
n = 5 end-on no hydrogenation Bercaw JACS 1974
n = 4 side-on hydrogenation
Chirik Natur e 2004
end-on= H or R

caluculations in this researchn = 1 ~ 4 side-on complex is more stablen=5
end-on complex is more stable owing tosteric repulsion between five methyl groups

side-on[Related Study 3-4]
Kinetics and Mechanism of N2 Hydrogenation in Bis(cyclopentadienyl) Zirconium Complexes andDinitrogen Functionalization by 1,2-Adition of a Saturated C-H Bond

P. J. Chirik et al.
J. Am. Chem. Soc.
2005, 127, 14051

several mechanistic studies
kinetics: first-order dependence on H2 concentration
KIE: normal primary isotope effectlarge negative entropies of activation=> first H2 addition being rate determining step
through an ordered transition structure where H-H bondscission is simutaneous with Zr-H and N-H bond formation

imido-like character of the side on N2 zirconocene complex was confirmed
also by the 1,2-addition of a saturated C-H bond in the ligand

4

P. J. Chirik et al.
J. Am. Chem. Soc.
2006, 128, 10696[Related Study 4]
N-C Bond Formation Promoted by a Hafnocene Dinitrogen Complex:
Comparison of Zirconium and Hafnium Congeners
attempt to expand the scope of 1,2-addition to include
nitrogen-carbon forming reactions
treatment of 1-N2 with PhNCO produced complex mixture
87 % of the gas expected for the loss of 1 equiv of free N2
competitive ligand-induced side-on, end-on isomerization proceeded?
addition of 2 equiv of PMe3 to 1-N2 afforded end-on N2 complex
more strongly activated side-on N2 complex was targeted
Hf congeners 2-N2
addition of 10 equiv of PMe3 to 1-N2 resulted in no change
greater N2 reduction imparted by the more reducing third-row metal
with 2-N2, C-N bond formation was successful[ 2 + 2]
cycloaddition[ 2 + 2]
cycloaddition

C=N cycloaddition to Zr=N bond
C=O insertion into the newly formed Hf-N bond
C=Oinsertion

potentially rich dinitrogen functionalization chemistry is possible
via [2 + 2] cycloaddition of activated -systems
if deleterious side-on, end-on isomerization can be suppressed.

(4) Thematic Research
Dinitrogen Cleavage and Functionalization by Carbon Monoxide Promoted by a Hafnium Complex

P. J. Chirik et al.
Nature Chemistr y 2010, 2, 30

metal: more reducing Hf instead of Zr
ansa-complex substituted with electro-donating groups
reduction of 2-I2 with Na(Hg) under 1 atm N2 afforded
targeted side-on N2 complex 2-N2 (dihedral angle = 54 )4 atm CO, toluene as solvent,following protonolysis also confirmed N-N bond cleavage coupled withC2 symmetric hafnium oxiamidide 2-(N2C2O2)-C2 (y. 82%)

N-C and C-C bond formation

reducing the CO pressure to 1 atm

2-(N2C2O2) + HCl -> 2-Cl2 + H2NC(O)-C(O)NH2 (oxiamide)

an isomeric hafnium product (C1 symmetry) 2-(N2C2O2)-C2 (y. 73%)

free oxiamide could also be liberated using weaker acids such as ethanol

the oxamidide ligand arises from cleavage of N2 coupled with C-Nbond formation and homologation of COa slight excess CO: cyclometallated complex 3 was formed

unprecedent transformation: strong-field CO ligand to metal N2 complexes

N-N bond cleavage, N-C bond formation and C-H bond activation

typically results in N2 loss and formation of the corresponding metal

in a single step(di)carbonyl derivative

protonolysis of 3 with gaseous HCl yielded NH4Cl5

formal insertion of CO into one of the hafnium-nitrogen bondhafnium-nitrogen multiple bond promotes the intramolecularmost likely one that is distal from the t-Bu group

C-H activation of t-Bu substituents on Cp' ring by 1,2-addition

retro [2 + 2] cycloaddition effects N-N bond cleavage andto furnish 3generates isocyanate ligand, along with a bridging hafnocene nitride(5) Appendix
Structure and Chemistry of Bis(cyclopentadienyl)-MLn Complexes
R. Hoffmann et al. J. Am. Chem. Soc. 1976, 98, 1729

Theoretical Prediction of a New Dinitrogen Reduction Process:Utilization of Four Dihydrogen Molecules and a Zr2Pt2 Cluster
D. G. Musaev J. Phys. Chem. B 2004, 108, 10012

6[2] Iron-Catalyzed, Hydrogen-Mediated Reductive Cyclization of 1,6-Enynes and Diynes:
Evidence for Bis(imino)pyridine Ligand Participation(1) Overview

5 mol% 1-(N2)24 atm H2
C6H6, 23 °C

P. J. Chirik et al
J. Am. Chem. Soc.
2009, 131, 8772

Iron-catalyzed hydrogen-mediatedreductive cyclization of enynes and diynes
with turnover frequencies comparable tothose of established precious metal catalysts(2) Background+ Catalytic bond forming reaction mediated by homogenous transition metal complex: indispensable tool
+ In many cases, the best catalysts rely on the scarest elements (Rh, Ir, Pt, etc...)+ Attempts to replace noble metal catalysts to more abundant and benign metal catalysts.
(3) Related Studies[Related Study 1]
Pentacarbonyliron(0) Photocatalyzed Hydrogenation and Isomerization of Olefins

M. S. Wrighton et al.
J. Am. Chem. Soc.
1976, 98, 551(b) A Mechanistic Study

Appearance of deuterium in alkenes
demands a mechanism featuring
a reversible hydrogen transfer.
(c) Proposed Mechanism

conditions: near-UV, 25 C, benzene as solvent, H2 at 10 psi(a) General Reactivity of Transition Metal-Carbonyl Complexes

Unsaturated iron-carbonyl complex Fe(CO)3 is the proposed active species:=> introduction of a terdentate bis(imino)pyridine chelate
may allow thermal access to a catalytically active iron complex [Fe(iPrPDI)]
Initial expectations:N+ ease of synthesis, modulality+ well-documented success in catalysis+ stabilization of electron-rich Fe(0) complex by -acidity of the ligand

N

Ar =N

Ar

[Fe(iPrPDI)][Related Study 2]
Preparation and Molecular Electronic Structure of Iron(0) Dinitrogen and Silane Complexes andTheir Application to Catalytic Hydrogenation and Hydrosilation

highly active catalyst for the polymerization of ethylene
M. Brookhart et al. JACS 1998, 120, 4049

FeArP. J. Chirik et al.
J. Am. Chem. Soc.
2004, 126, 13794

7

• conditions: 4 atm H2, 22 °C, 1.25 M in toluene• the actitvity for 1,5-hexadiene is slightly reduced from 1-hexane• preferential hydrogenation of the gem-disubstituted over the trisubstituted• high activity in non-polar solvents• exposure of 1-N2 to 1 atm H2 afforded iron(0) dihydrogen complex=> the electon withdrawing character of the i-PrPDI ligand renders the ironcenter sufficiently electron deficient to inhibit oxidative addition of H2?

[Related Study 3]
Electronic Structure of Bis(imino)pyridine Iron Dichloride, Monochloride, and Neutral Ligand Complexes:
A Combined Structual Spectroscopic and Computational Study

P. J. Chirik et al.
J. Am. Chem. Soc.
2006, 128, 13901• one electron reduction of 1-Cl in the presence ofweak fields ligands such as DMAP or N2 results in
reduction of the bis(imino)pyridine chelate,furnishing dianionic [PDI]2- fragment complexed to
an intermediate ferrous ion
8[Related Study 4]
Iron-Catalyzed [2 + 2 ] Cycloaddition of-Dienes : The Imporatance of Redox-Active Supporting Ligands

P. J. Chirik et al.
J. Am. Chem. Soc.
2006, 128, 13340(a) Main Experimental Results• some substrates, such as E = SiMe2, and those with internal olefins(allyl-ter t-butylcrotylamine or tertbutyldicrtylamine): not successful• the stereochemistry of the catalytic [2 + 2 ] reaction

no isotopic scrambling, affording only one isotopomer product(b) Mechanistic Study (1)• only stoichiometric cyclization, cleanly affording 1-(HNC6H10)• E = Nt-Bu: 1-(N2)2 was recovered

Entry 3• E = NBn: 1-(BnNC6H10) was identified following turnover=> product inhibition dictates the rate of catalytic turnover• the metrical parameters and NMR spectroscopic data of 1-(HNC6H10)
are analogous to those 1-DMAP, where computational, spectroscopic,
1-(HNC6H10)

and structural data established an intermediate spin (SFe = 1) ferrous
center complexed by dianion [i-PrPDI]2-(c) Mechanistic Study (2)• the reaction with 1,3-butadiene which would unlikely to undergo cycloaddition• the iron-butadiene compound 1-C4H6 was formed• a rare example of trans-butadiene• two electron reduced bis(imino)pyridine complex=> these data support Fe(II) complexed by [iPrPDI]2rather than an Fe(0) ion with a neutral chelate.

1-C4H6On trans-butadiene iron complex:
Or ganometallics, 2006, 25, 2705(d) Proposed Mechanism• substitution of the N2 ligands by the diene• based on the properties of 1-(C4H6),
the oxidation states of the iron are maintained as ferrous
throughout the process• C-C coupling to form the matallocycle• formal reductive elimination

9(4) Thematic Research
Iron-Catalyzed, Hydrogen-Mediated Reductive Cyclization of 1,6-Enynes and Diynes:
Evidence for Bis(imino)pyridine Ligand Participation(a) Preliminary Study:

P. J. Chirik et al.
J. Am. Chem. Soc.
2009, 131, 8772

• stoichiometric reaction of 1,6-enynes with 1-(N2)2• conditions: 23 °C, 3hr, in benzene-d 6• complete consumption of both starting materials• product: 3,4-disubstituted pyrrolidine with only one (Z)-alkene
the intramolecular olefin complex 2• overall, dehydrogenation of an i-Pr aryl substituents promotes
iron-mediated reductive enyne cyclization.

(b) Further Attempt: H2 as the Stoichometric Terminal Reductant• 2 reacts cleanly with H2 to yield a catalytically active iron-H2 complex• facile hydrogen-mediated cyclization with TOF comparable to thoseof Rh catalysts• R = H; continued stirring led to the reduction of the exo-methylene,
preference for the formation of cis-diastereomers over trans ones
•RH; only the unsaturated products were observed(reductance of 1-(N2)2 to hydrogenate unactivated trisubstituted alkenes
was previously observed)(c) Extention to Diyne Substrates(d) Mechanistic Studies

MeNTs• the catalytic reaction with D2 gas• enyne (R = Me): labels exclusively in the methyl and vinyl
position of the exo alkenyl substituents• enyne (R = H) : d 4-isotopologue, additional deuterium incorporation
arising from deuteration of the exo methylene pyrrolidine
intermediate• diynes: pyrrolidine-d 2 with the two labels in the vinyl positions of
the product• the stoichiometric reaction with the labelled iron dinitrogen complex
1*-(N2)2 where the isopropyl methyl substituents were deuterated• reaction with enyne A resulted in complete and exclusive conversion
into two pyrrolidine-d 1 isotopers=> transfer hydrogenation from isopropyl aryl substituted• paramagnetic (S = 2) intermediate, iron-metallocycle 3* was observed.
at this point, no isotpoic scrambling from isopropyl methyl groups• degradation studies of 3*;• Comparison of the reactions with 3 and 3*; KIE kH/kD = 6.0(2)=> C-H bond breaking event in the turnover-limiting stepwith NaOH/H2O: no deuterium incorporation in the productwith NaOD/D2O: deuterium incorporation

10

(e) Proposed Mechanism• coordination of the substrate to the iron complex• cyclometallation to form 3(cyclization of the substrate upon addition of the catalyst is rapid)• on the basis of the previous study, they believe reductive
cyclization to form the C-C bond involves electron transfer
and formal oxidation of the bis(imino)pyridine chelaterather than the iron center• observation of 3 and the KIE results suggest that the nexthydrogenation step is turnover limiting• hydrogenation of 3 can occur either at alkyl or alkenyl position• H2 addition step can occur by OA/RE or by -bond metathesis
to form new C-H and Fe-H bonds• reductive elimination of a carbon-hydrogen bond forms the product• ferrous oxidation state is preserved throughout the catalytic cycle• the redox activity of the [iPrPDI] ligand preserves the ferrous oxidation state throughout the cycle and may prevent complications from Fe(0)precipitation that are observed with other (e.g., Ni) metallocycles.

P. H. M. Budzelaar
Dalton Tr ans. 2006, 5442

(f) On PDI ligands
Ligand-centered reactivity in diiminepyridine complexes• extended -system• two low-lying *-orbitals which are basically the combination of the twoimine * orbital, with siginificant participation of the central pyridine ring• different nature of metal-ligand from that of classical -acceptor like CO• in the case of CO* LUMO is concentrated on the carbon atom directly bound to the metallarge overlap between metal d and liand * orbitals, resulting in a mainly
covalent backdonation-type interaction• in the case of PDI ligand* orbitals are very localized, direct d- * overlap is much smaller=> the interaction will have the character of an intramolecular singleelectron transfer

11