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

We wish to report here the first theoretical prediction that a unique nickel­(I) hydride complex Ni^IH­(bpy) (bpy = 2,2′-bipyridine) <b>1</b> 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 Ni^I–H bond of <b>1</b>, which occurs easily to afford Ni^I(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 Ni^I(η¹–OCOH)­(bpy) and HSiR₃ to produce H­(CO)­OSiR₃ and regenerate <b>1</b>. This is rate-determining (Δ<i>G</i>°^⧧ = 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 Δ<i>G</i>°^⧧ value of the metathesis, suggesting that Ni^IH­(dmbpy) is a better catalyst. In the dihydrogen generation via alcoholysis of hydrosilane by <b>1</b>, the first step is metathesis between the O–H bond of ROH (R= Me or H) and the Ni^I–H bond of <b>1</b> to afford Ni^I(OR)­(bpy) and dihydrogen molecule. Next step is the second metathesis between the Ni^I–OR bond and the Si–H bond of HSiR₃ to yield R₃Si­(OR) and regenerate <b>1</b>. The second metathesis occurs with a negligibly small Δ<i>G</i>°^⧧ value, indicating that various hydrosilanes can be employed for this reaction. On the basis of these computational findings, it is predicted that <b>1</b> can catalyze two completely different reactions, CO₂ hydrosilylation with hydrosilane and dihydrogen generation via alcoholysis/hydrolysis of hydrosilane