Oxidative addition of MeI to a rhodium(I) N-heterocyclic carbene complex. A kinetic study

The Rh(l) iodocarbonyl complexes [Rh(CO)(L-Me)(2)I] (3a) and [Rh(CO)(L-Mes)(2)I] (3f) (L-Me = 3,5-dimethylimidazoline-2-ylidene; L-Mes = 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(L-Mes)(2)I-2(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 (DeltaH = -51 +/- 3 kJ mol(-1)) but disfavored entropically (DeltaS = -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)(2)I] 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(L-Me)(2)I-2-(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 L-Me ligands and formation of [Rh(CO)(2)I-2](-).