Fast CO2 hydrogenation to formic acid catalyzed by an Ir(PSiP) pincer hydride in a DMSO/water/ionic liquid solvent system

Abstract

Complex [IrClH{κP,P,Si-Si(Me)(C6H4–2-PiPr2)2}] (1) showed a remarkable catalytic activity for CO2 hydrogenation in a DMSO/H2O solvent system incorporating 1,2-dimethyl-3-butylimidazolium acetate ionic liquid (IL), producing 0.94 M formic acid with initial TOFs up to 1432 h−1 (CO2/H2 = 20/40 bar, 30 °C). While the hydrogenation outcome followed dependences upon gas composition, pressure and temperature similar to those of other efficient systems in DMSO/H2O, the kinetic dependence upon catalyst loading revealed non-linear effects suggestive of relevant IL-catalyst interactions. NMR speciation studies identified two major complexes, [Ir(OCHO)(H){κP,P,Si-Si(Me)(C6H4–2-PiPr2)2}] (2) and [Ir(H)2{κP,P,Si-Si(Me)(C6H4–2-PiPr2)2}(DMSO)] (3), potentially responsible for catalytic cycling though inactive outside the current solvent system.