Synthesis and characterization of rhodium(I) and iridium(I) carbonyl phosphine complexes with bis(N-heterocyclic carbene)borate ligands

A series of iridium(I) and rhodium(I) carbonyl phosphine complexes bearing bis(N-heterocyclic carbene)borate ligands [H2B(ImtBu)2]Rh(CO)(PPh3) (5), [F2B(ImtBu)2]Rh(CO)(PPh3) (6), [F2B(ImtBu)2]Ir(CO)(PPh3) (7), [H2B(ImtBu)2]Rh(CO)(PCy3) (8), [F2B(ImtBu)2]Rh(CO)(PCy3) (9), and [F2B(ImtBu)2]Ir(CO)(PCy3) (10) (H2B(ImtBu)2 = dihydrobis(3-tert-butylimidazol-2-ylidene)borate; F2B(ImtBu)2 = difluorobis(3-tert-butylimidazol-2-ylidene)borate) have been prepared and characterized. IR stretching values of the CO ligands, the values of NMR coupling constants JRh–P, and electrochemical data are rationalized by the electronic effects of the bis(3-tert-butylimidazol-2-ylidene)borate and phosphine ligands. The results show that the σ donor capacity of bis(3-tert-butylimidazol-2-ylidene)borate is stronger than those of the analogous isoelectronic bis(pyrazolyl)borate (Bp−) and acetylacetonato (acac−) ligands. The molecular structures of complexes 6–10 have been determined by single-crystal X-ray diffraction, which showed square-plane geometries around the metal centers.