Ni(I)-Hydride Catalyst for Hydrosilylation of Carbon Dioxide and Dihydrogen Generation: Theoretical Prediction and Exploration of Full Catalytic Cycle

Abstract

We wish to report here the first theoretical prediction that a unique nickel(I) hydride complex NiIH(bpy) (bpy = 2,2′-bipyridine) 1 is a good catalyst for hydrosilylation of CO2 with hydrosilane (HSiR3) and dihydrogen (H2) generation via alcoholysis/hydrolysis of hydrosilane. Full catalytic cycles of these reactions were elucidated by density functional theory (DFT) calculations. In the CO2 hydrosilylation, the first step is CO2 insertion into the NiI–H bond of 1, which occurs easily to afford NiI(η1–OCOH)(bpy) species, in which charge transfer (CT) occurs from singly occupied molecular orbital (SOMO) corresponding to Ni–H antibonding orbital to LUMO of CO2. The next step is metathesis between NiI(η1–OCOH)(bpy) and HSiR3 to produce H(CO)OSiR3 and regenerate 1. This is rate-determining (ΔG°⧧ = 19.5 kcal mol–1). In the transition state of the metathesis, the Si atom takes a hypervalent trigonal-bipyramidal structure, suggesting that hydrosilane with electron-withdrawing substituents is good for this reaction. The use of 6,6′-dimethyl-2,2′-bipyridine (dmbpy), which has methyl groups at the ortho position to the N of bpy, substantially decreases the ΔG°⧧ value of the metathesis, suggesting that NiIH(dmbpy) is a better catalyst. In the dihydrogen generation via alcoholysis of hydrosilane by 1, the first step is metathesis between the O–H bond of ROH (R= Me or H) and the NiI–H bond of 1 to afford NiI(OR)(bpy) and dihydrogen molecule. Next step is the second metathesis between the NiI–OR bond and the Si–H bond of HSiR3 to yield R3Si(OR) and regenerate 1. The second metathesis occurs with a negligibly small ΔG°⧧ value, indicating that various hydrosilanes can be employed for this reaction. On the basis of these computational findings, it is predicted that 1 can catalyze two completely different reactions, CO2 hydrosilylation with hydrosilane and dihydrogen generation via alcoholysis/hydrolysis of hydrosilane.