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

Abstract

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, (η⁵-C₅R₅)M(IMes)(C₂H₄) (M = Co, Rh; R = H, Me) were obtained from the corresponding bis(ethene) complexes (η⁵-C₅R₅)M(C₂H₄)₂, except for CpRh(IMes)(C₂H₄) which was prepared via the novel 16-electron Rh(I) compound Rh(IMes)(C₂H₄)₂Cl. The carbonyl compounds (η⁵-C₅R₅)Co(IMes)(CO) (R = H, Me) were synthesized by thermal CO substitution of (η⁵-C₅R₅)Co(CO)₂. A diamagnetic, apparently 16-electron Co(III) compound [CpCo(IMes)I]⁺[I₃⁻] was obtained from CpCo(IMes)(CO) and I₂. Finally, Co(III) and Rh(III) complexes CpCo(IMes)Me₂ and Cp*Rh(IMes)Me₂ were prepared by methylation of [CpCo(IMes)I]⁺[I₃⁻], and ligand exchange at Cp*Rh(Me₂SO)Me₂, 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 ¹J_{Rh–C(carbene)} and dRh–C(carbene) data for Rh(I) N,N-heterocyclic carbene complexes including literature data and this work.