Oxidative Addition of MeI to a Rhodium(I) N-Heterocyclic Carbene Complex. A Kinetic Study

The Rh(I) iodocarbonyl complexes [Rh(CO)(LMe)2I] (3a) and [Rh(CO)(LMes)2I] (3f) (LMe =
3,5-dimethylimidazoline-2-ylidene; LMes = 3,5-dimesitylimidazoline-2-ylidene) have been
prepared by the reaction of [Rh(CO)2(OAc)]2 or [Rh(CO)2(acac)] with an imidazolium salt in
the presence of Cs2CO3. Complex 3a reacts with MeI to give a Rh(III) acetyl complex, [Rh(LMe)2I2(COMe)] (5a), which is unstable and decomposes by elimination of MeI to regenerate
3a. Rate and equilibrium constants have been measured for the reversible reaction 3a +
MeI → 5a. Thermodynamic parameters indicate that the forward reaction is exothermic
(ΔH = −51 ± 3 kJ mol-1) but disfavored entropically (ΔS = −159 ± 12 J mol-1 K-1). Complex
3f does not react with MeI. The results are compared with data reported for [Rh(CO)(PEt3)2I],
and the differences in behavior of the N-heterocyclic carbene and PEt3 systems are interpreted
on the basis of ligand steric effects. An X-ray crystal structure is presented for [Rh(LMe)2I2(OAc)] (6a), and a mechanism is proposed for its formation from 5a. Tests on 3a as a catalyst
for the carbonylation of methanol indicate sequential loss of the two LMe ligands and formation
of [Rh(CO)2I2]-.