Rhodium(III) Complexes Containing C4-Bound N-Heterocyclic Carbenes: Synthesis, Coordination Chemistry, and Catalytic Activity in Transfer Hydrogenation

Direct metalation of C2-protected diimidozolium salts with RhCl 3 or [RhCl(cod)] 2 and KI afforded a series of new rhodium(III) complexes with abnormally C4-bound, cis -chelating NHC ligands. The complexes were isolated as dimetallic species containing two (μ 2 -I) 3 -bridged rhodium(III) centers. In the presence of coordinating ligands such as CH 3 CN, PPh 3, or dppe, the dimeric structure is readily cleaved and yields monometallic complexes. Crystallographic analysis of representative structures indicates a higher trans influence of abnormally C4-bound carbenes as compared to normal NHCs. The exceptionally strong donor ability of carbenes in such a C4 coordination mode increases the catalytic activity of the rhodium center and allows for efficient transfer hydrogenation of ketones in i PrOH/KOH.