Catalytic properties of nickel bis(phosphinite) pincer complexes in the reduction of CO 2 to methanol derivatives

Abstract

A new nickel bis(phosphinite) pincer complex [2,6-(R 2PO) 2C 6H 3]NiCl (L RNiCl, R = cyclopentyl) has been prepared in one pot from resorcinol, ClP(C 5H 9) 2, NiCl 2, and 4-dimethylaminopyridine. The reaction of this pincer compound with LiAlH 4 produces a nickel hydride complex, which is capable of reducing CO 2 rapidly at room temperature to give a nickel formate complex. X-ray structures of two related nickel formate complexes L RNiOCHO (R = cyclopentyl and isopropyl) have shown an “in plane” conformation of the formato group with respect to the coordination plane. The stoichiometric reaction of nickel formate complexes L RNiOCHO (R = cyclopentyl, isopropyl, and tert-butyl) with catecholborane has suggested that the reaction is favored by a bulky R group. L RNiOCHO (R = tert-butyl) does not react with PhSiH 3 at room temperature; however, it reacts with 9-borabicyclo[3.3.1]nonane and pinacolborane to generate a methanol derivative and a boryl formate species, respectively. The catalytic reduction of CO 2 with catecholborane is more effectively catalyzed by a more sterically hindered nickel pincer hydride complex with bulky R groups on the phosphorus donor atoms. The nickel pincer hydride complexes are inactive catalysts for the hydrosilylation of CO 2 with PhSiH 3.