Amine-tethered N-heterocyclic carbene complexes of rhodium(I)

Article abstract of DOI:10.1139/V08-082

A new family of rhodium-diene based complexes has been developed that incorporates an N-heterocyclic carbene ligand with an N-donor tether. The ligand is denoted ^Mes[CNH] 2 (where ^Mes[CNH] is 2,4,6-Me ₃C₆H₂NC₃H₂NCH ₂CH₂NH-2,4,6-Me₃C₆H₂) and ^Mes[CN] for the amido form. The synthesis of the ^Mes[CNH] ligand involves reaction of N-mesitylimidazole with 2-chloroethyl-N- mesitylamine under melt conditions, followed by deprotonation with KN(SiMe ₃)₂. The reaction of ^Mes[CNH] with [(diene)RhCl]₂ results in the formation of the monodentate complexes, ^Mes[CNH]Rh(diene)Cl (where diene = 1,5-cyclooctadiene (COD): 3a; diene = 2,5-norbornadiene (NBD): 3b). Bidentate variants could be isolated as either a neutral species, ^Mes[CN]Rh(diene) 4a-4b, via deprotonation, or an ionic analogue such as [^Mes[CNH]Rh(diene)]BF₄ 5a-5b by reaction with NaBF₄. Compounds 4-5 are the first examples of rhodium compounds that contained a bidentate NHC ligand with a pendant amino or amido donor. Complexes 3-5 were characterized fully and the solid-state single crystal X-ray structures of 3a, 4a, and 5b are discussed. The utility of these complexes as catalyst precursors for hydrogenation reactions was examined and it was determined that these systems are not significantly more active than colloidal rhodium when parallel reactions were run. Various methods of transfer hydrogenations were also investigated with 3a, which did not yield an appreciable conversion of either benzophenone or N-benzylideneaniline as substrates.