Synthesis and characterization of half-sandwich N-heterocyclic carbene complexes of cobalt and rhodium

A series of novel half-sandwich M(I) and M(III) complexes (M = Co, Rh) bearing the N-heterocyclic carbene ligand 1,3-dimesitylimidazol-2-ylidene (IMes) have been prepared and characterized. Thus, (eta(5)-C5R5) M(IMes)(C2H4) (M = Co, Rh; R = H, Me) were obtained from the corresponding bis(ethene) complexes (eta(5)-C5R5) M(C2H4)(2), except for CpRh(IMes)(C2H4) which was prepared via the novel 16-electron Rh(I) compound Rh(IMes)(C2H4)(2)Cl. The carbonyl compounds (eta(5)-C5R5) Co(IMes)(CO) (R = H, Me) were synthesized by thermal CO substitution of (eta(5)-C5R5) Co(CO)(2). A diamagnetic, apparently 16-electron Co(III) compound [CpCo(IMes)I](+)[I-3(-)] was obtained from CpCo(IMes)(CO) and I-2. Finally, Co(III) and Rh(III) complexes CpCo(IMes) Me-2 and Cp* Rh(IMes) Me-2 were prepared by methylation of [CpCo(IMes)I](+)[I-3(-)], and ligand exchange at Cp* Rh(Me2SO) Me-2, respectively. The molecular structures of CpCo( IMes)(CO), CpRh(IMes)(C2H4), Cp* Rh(IMes)(C2H4), and Cp* Rh( IMes) Me2 were determined by single crystal X-ray diffraction. Steric and electronic factors imposed by the strongly donating and sterically demanding IMes ligand are discussed on the basis of X-ray crystallographic, NMR, and IR spectroscopic analyses. Very poor correlations are found between values for (1)J(Rh-C(carbene)) and dRh-C(carbene) data for Rh(I) N,N-heterocyclic carbene complexes including literature data and this work.