Organometallic Study Meeting #11

2011/6/18 K.Oisaki

Chapter 5. Ligand Substitution Reactions
5.1 Introdution
Ligand Substitution = A reaction in which a free ligand replaces a coordinated ligand<Interchange Mechanism>
Dessociativeinter change

Dissociative Mechanism
cf. SN1 reaction

XMXnrds

M-L is longer

Associativeinterchange
M-L' is longer

TS

cf. SN2 reaction

YY
MXn-1

Associative Mechanism
XY

MXn

MXn-1Y

coordinatively
unsaturated metalMX
Xn-1

partial bondingtransition state

18e-type of complex

16e- or 17e-

0th order in Y
1st order overallrate law

1st order in Y
2nd order overall

small positive S‡
small positive V ‡activation parameterslarge negative S‡
large negative V ‡more e-rich metal
more e-poor metalsterically hindered metalfavorable
electronic & steric factors

more basic Y
more e-poor metalsterically accessible metal

affected strongly
by the strength of M-X bondothersreduction weakens M-X bond(acceleration)

5.2. Thermochemical Considerations
MXn +

Y

BDE(M-Y) > BDE(M-X)

MXn-1Y+X

thermodynamically favorable

BDE = bond dissociation energyenergy needed for homolytic cleavage (two radical generation) for charged X-type ligand
energy needed for heterolytic cleavage (unsaturated metal + Lewis base) for neutral L-type ligandcf) BDE (M-CO) = 25-46 kcal/mol = 50-150oC heating leads to cleave
BDE (CH3-CH3) = 88 kcal/mol
BDE (CH3-H) = 104 kcal/molcf) BDE (polyhapto) > BDE (monohapto)cf) BDE (M-L) tend to be first-row < second-row < third-row (but not always)
BDE(Cr(CO)5-CO) < BDE (Mo(C)5-CO) < BDE (W(CO)5-CO)
3740
46 kcal/molcf) BDE (M- acidic) > BDE (M- neutral) if M is low valentcf) BDE (less hindered) > BDE (more hindered)MXn-1Y5.3. Mechanisms of Ligand Substitutions of 16e- and 17e- complexes> Associative mechanisms of square-planer d8 complexes
rate = kobs[L3MX] = (k1 + k2[Y])[L3MX]# "solvent assistance" = two associative substitution

non-solvent
assisted pathsolvent
assisted pathscheme 5.2 / figure 5.1

often occurs in parallel# stereochemistry = retentive (but not always) // cf. analogy of SN2X

XcLXt

M

Lct

LY

Lc

LM

XcLM

Lc

ct

LL

Lc

X

M

Lt
Lc-X

Lc
YM

Lt
Lc

Y

Y

if pentacoordinate intermediate is long-lived,
stereochemistry is lost via Berry pseudorotation.
# other factors affecting to rate
PPh3 >> SCN- >> I-, Br- > N3- > Cl- > NO2- ~ NH3

softness
nucleophilicity

cf.
ClPt
cf.

leavingability

Py

Pt

H
N
N
H2Y

ClClPtPt

2Pt

Y

HNPy

NH2Py

N
H2X+ Cl2Pt

NH2+X

Py

NO3- > H2O > Cl- > Br- > I- > N3- > SCN- > NO2- > CN(but less dominant than effect of Y)
BDE

cf.

o-tol
Et3PM

PEt3

PyClo-tol
Et3PM

PEt3
Py

+ Cl

Ni > Pd > Pt

trans effect = trans ligand affect the rate of ligand substitutions = transition state/dynamism/kinetic(cf. trans infulence = related to bond length = ground state/structure/thermodynamic)
4H- > Me- > Ph- > Cl- spans a f actor of x10

cf.
Lt
Et3PPt

PEt3

PyClLtEt3PPt

PEt3Py+ Cl

cis effect = similar effect by cis ligand, but usually smaller contributioncf.

Et3PLc

Pt

PEt3
Cl

PyPt
Lc

Pt

Fig 5.2

PEt3
Py+ Cl

Me- > Ph- > Cl- spans a f actor of x3
larger when Lt is -donating (weakens M-X bond), or -accepting (stabilizing 5-coordinate intermediate).

> When dessociative mechanism of square-planer d8 complexes is dominant ?
# weakly bound X# strong trans influence ligand# unsaturated metal site is covered by polydentate coordination# axial association of Y is prohibited# metal geometry is distorted from square planer# coordination number > 4# very fast association generate super leaving groupOcf.
PhPhPtOSPhDMSO-d 6Pt

PhSO
strong trans influence

OPhSPh

SPt

CD3
CD3

SO
covered vacant site
cf.

cf.

t

O P Bu2R

O P Bu2
70-110oC

Ir

distoted geometryt

R

Ph2MeSi
Ph2MeSiIr

O PtBu2Pt

O PtBu2

PMe2PhPMe2Phstrong trans influencestrong trans influence
cf.

DNNPt

MeDOTfMefastNNPt

DMeNMeNMe

dessociativeexchange

Pt
Me> Associative mechenism of 17e- complexes
substitution rate:
17e complexes > 18e complexescf.) Poe's experimentref)
Dessociative mech.

Re2(CO)10
associative mech. leads to energetic gains.

PPh3

Re(CO)4Cl
16eh
PPh3

Re(CO)517e

Re(CO)4(PPh3)Cl + Re(CO)5C
l 18e
18e

CCl4

PPh3

CCl4
CORe(CO)5 (PPh3)
Re(CO)4(PPh3)
19e
17e
Associative mech.
excess CO had no effect on the ratio
dessociative path was deniedFig. 5.3

cf.

R affects reaction rate
V(CO)617eCO

PR3

V(CO)5(PR3)hexane, rt

rate = k[V(CO)6][PR3]negative S‡

5.4. Mechanisms of Ligand Substitutions of 18e- complexes> Dessociative mechanism
k1

LnM-X

k2

LnM

k-1

k1k2 [LnMX][Y]rate =LnM-YY

fig 5.4

k1 [LnMX]

k-1 [X] + k2[Y](usually k2[Y] >> k-1[X])cf.
Ni(CO)4

COL

Ni(CO)3(solv)

solvent

BDE((OC)3Ni-CO) = 25 kcal/mol
positive S‡ (= +8~+13 eu)

Ni(CO)3L# Steric bulkiness of ligand (Tolman's cone angle) largely affects the rate.
R = Et << p-tol < i Pr < o-tol << PPh3cf.
KD
Ni(P(OR)3)4

PhH, rtNi(P(OR)3)3 + P(OR)3# stereochemistry = easily lost //cf. analogy to SN1
L
M
L

L-L

M

L

L

L

L
L

L

L
flip of 16e- intermediate

L
M

M

L

L
18eL

L

L

M

X

L
L

L

L

M

X

L
18e-L
X

L

L
pseudorotation# cis effect in (pseudo)octahedral complexes= X makes dessociation of Lcis faster than that of Ltrans
Lc
Lc

Mc

Lc
Lc

Lt

Lt

Lc

X

L

M
Lc

Lc

c

Lc

Lt

LX

M
Lc

LcX

NO3- > OAc-, HCO2-, RC(O)- > NHR > SH- > OR > Cl- > Br - > I - > carbene > PPh3 > H-, COlarger in case of chelating, -donating, less -donating ligand
cf.

COOCW

OCCOOAcCO

13CO

stablization of unsaturated metal
CO

OC

13

W

OCCOOAcCO

> Reagent and catalyst induced dessociation of CO# photo-inducedcf.

hHRe(CO)5

rt, 20 minn

# redox, SET-induced

PBu3

HRe(CO)4(PBu3)

w/o light: NR at 75 oC for 60 days*(M-L)

cf. analogous to radical chain rxnscheme 5.8# ligand oxidation-induced (review: Coord. Chem. Rev. 1984, 53, 227.)
cf. N-oxide promoted Pauson-Khand rxn (Schreiber, S. L. et al. TL 1990, 31, 5289)
M(CO)
x

R3N-O
R3N
O(CO)x-1M
O
M(CO)x-1(NR3) + CO2(faster if CO is more electrophilic)5.4. Mechanisms of Substitutions Involving Polyhapto Ligands> Diene and Triene# 18e- complex + strong dative L = competitive pathways
rate = (k1 + k2[L])[M(CO4)(diene)]Fig5,43

dissociativepath

associativepath> Arene# stepwise ring slip ( 6 - 4 - 2) occurs
on the occasion when solvent or ligand income.
faster in case of donar solvent, weakly bindin arenes
cyclohexanone (1300) > PhCN (600) > THF (30)
~ Cr(CO)3(C6Me6) (30) > uncatalyzed (1)scheme 5.9
stability# photoirradiation, redox, SET also accelerate the process.
cf.) acceleration by ligand design (Semmelhack, M. F. et al. JACS 2001,123, 8438, JACS 2005, 127, 7759.)<concept>= -acidic ligand(comparable to CO)

> Cp ligand# associative substitution of ligand other than Cp ring
stepwise ring slip ( 5 - 3) occurs on theoccasion when solvent or ligand income.

scheme 5.48
Fig.5.7

Table 5.11

aromaticity is not preserved during ring slip, in case of rightside Cp families.
as other ratioinalization, ground state energy of

5-indenyl is higher (because of ring dissymmetry) than5-Cp.

5.6 Ligand Substitutions in Multimetallic Clusters
All associative, dessociative, reagent-induced subsitutions are allowed as mononuclear complexes.

> Complex possessing metal-metal bond
L(OC)4M

M(CO)4L-COM(CO)4L

associative pathway(see 17e- complex)

L(OC)4M M(CO)3L

dessociative pathway(see 18e- complex)

depends relative strength between BDE(M-M) and BDE(M-CO)
weaker in case of first-row transition metalscf.)
eq 5.53

relative rate ,1st : 2nd : 3rd = 1 : 30 : 920
presence of bridging CO ligand affects largely.

1st: associative
2nd: dessociative
3rd: dessociative