Catalytic properties of nickel bis(phosphinite) pincer complexes in the reduction of CO2 to methanol derivatives

A new nickel bis(phosphinite) pincer complex [2,6-(R2PO)2C6H3]NiCl (LRNiCl, R = cyclopentyl) has been prepared in one pot from resorcinol, ClP(C5H9)2, NiCl2, and 4-dimethylaminopyridine. The reaction of this pincer compound with LiAlH4 produces a nickel hydride complex, which is capable of reducing CO2 rapidly at room temperature to give a nickel formate complex. X-ray structures of two related nickel formate complexes LRNiOCHO (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 LRNiOCHO (R = cyclopentyl, isopropyl, and tert-butyl) with catecholborane has suggested that the reaction is favored by a bulky R group. LRNiOCHO (R = tert-butyl) does not react with PhSiH3 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 CO2 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 CO2 with PhSiH3.

Catalytic reduction of CO2 with catecholborane is catalyzed by nickel bis(phosphinite) pincer hydride complexes and the most efficient catalyst contains bulky groups on the phosphorus donor atoms. Nickel pincer hydride complexes are inactive catalysts for the hydrosilylation of CO2 with PhSiH3.Figure optionsDownload as PowerPoint slideHighlights
► One-pot synthesis of a nickel bis(phosphinite) pincer chloride complex.
► Crystal structures of two new nickel pincer formate complexes.
► Stoichiometric reaction of formate complexes with different boranes and PhSiH3.
► Catalytic hydroboration of CO2 with different nickel hydride complexes.
► Experimental data in support of a previous computational study.