CNC-pincer iron complexes containing a bis(N-heterocyclic carbene)Amido ligand: Synthesis and application to catalytic hydrogenation of alkenes

Abstract

This work studied preparation and catalytic application of CNC-pincer Fe complexes containing a bis(NHC)amido ligand (NHC: N-heterocyclic carbene). Deprotonation of bis(3-isopropylimidazoliumyl)amine salt [(CNCiPr)H3]2+[I−]2 (1a) with lithium hexamethyldisilazide (LiHMDS) afforded the corresponding bis(NHC)amido-Li complex 2a. Treatment of in-situ generated 2a with FeI2(thf)2 gave a CNC-pincer Fe(II) iodide complex Fe(CNCiPr)I (3a) and a cationic homoleptic Fe(III) complex [Fe(CNCiPr)2]+I− (4a). Reaction of in-situ generated 2a with Fe[N(SiMe3)2]2 produced the corresponding amido complex Fe(CNCiPr)[N(SiMe3)2] (5a). Similarly, deprotonation of a less hindered methyl analogue [(CNCMe)H3]2+[I−]2 (1b) with LiHMDS followed by treatment of Fe[N(SiMe3)2]2 gave an amido complex Fe(CNCMe)[N(SiMe3)2] (5b). Molecular structures of 3a, 5a and 5b, which were confirmed by X-ray diffraction study, showed a distorted tetrahedral geometry. Complexes 3a and 5b were found to be active in hydrogenation of alkenes. Reaction mechanism was investigated by density functional theory (DFT) calculations.