Cu(i) complex bearing a PNP-pincer-type phosphaalkene ligand with a bulky fused-ring Eind group: properties and applications to FLP-type bond activation and catalytic CO2 reduction

Abstract

Herein, we report the synthesis of [Cu(Eind₂-BPEP)][PF₆] (2) (Eind₂-BPEP = 2,6-bis(2-Eind-2-phosphaethenyl)pyridine, Eind = 1,1,3,3,5,5,7,7-octaethyl-1,2,3,5,6,7-hexahydro-s-indacen-4-yl), a three-coordinated Cu(I) complex bearing a PNP-pincer-type phosphaalkene ligand with bulky fused-ring Eind groups. The Gutmann–Beckett test revealed that complex 2 is highly Lewis acidic and comparable in strength to B(C₆F₅)₃, which is a relatively strong Lewis acid. In addition, 2 is more Lewis acidic than [Cu(Mes*₂-BPEP)][PF₆] (3), the analogous complex with less-bulky Mes* instead of Eind groups. DFT calculations using model compounds revealed that the higher Lewis acidity of 2 compared to 3 is not due to the electronic effects of the ligand, but due to a reduction in the LUMO energy caused by the steric effect of the bulky Eind groups. When combined with a tertiary amine, the highly Lewis acidic and bulky 2 exhibits the reactivity of a frustrated Lewis pair (FLP) and can activate hydrogen and phenylacetylene. Complexes 2 and 3 were found to catalyze the hydrogenation and hydrosilylation of CO₂ in the presence of DBU under relatively mild conditions.