Theoretical Study of Reactivity of Ge(II)-hydride Compound: Comparison with Rh(I)-Hydride Complex and Prediction of Full Catalytic Cycle by Ge(II)-hydride

The reaction of a Ge­(II) hydride compound HC­{CMeArN}₂GeH (Ar = 2,6-iPr₂C₆H₃) 1 with 2,2,2-trifluoroacetophenone (CF₃PhCO) is theoretically investigated with density functional theory and spin-component-scaled second-order Møller–Plesset methods. This reaction easily occurs with moderate activation barrier and considerably large exothermicity, to afford a Ge­(II) alkoxide 2 through a four-membered transition state. In the transition state, the charge transfer from the Ge–H σ-bonding molecular orbital (MO) to the CO π*-antibonding MO of CF₃PhCO plays an important role. Acetone ((CH₃)₂CO) and benzophenone (Ph₂CO) are not reactive for 1, because their π*-antibonding MOs exist at higher energy than that of CF₃PhCO. Though 2 is easily formed, the catalytic hydrogenation of CF₃PhCO by 1 is difficult because the reaction of 2 with a dihydrogen molecule needs a large activation energy. On the other hand, our calculations clearly show that the catalytic hydrogenation of ketone by cis-RhH­(PPh₃)₂ 4 easily occurs, as expected. The comparison of catalytic cycle between 1 and 4 suggests that the strong Ge–O bond of 2 is the reason of the very large activation energy for the hydrogenation by 1. To overcome this defect, we investigated various reagents and found that the catalytic cycle can be completed with the use of SiF₃H. The product is silylether CF₃PhCHOSiF₃, which is equivalent to alcohol because it easily undergoes hydrolysis to afford CF₃PhCHOH. The similar catalytic cycles are also theoretically predicted for hydrosilylations of CO₂ and imine. This is the first theoretical prediction of the full catalytic cycle with a heavier main-group element compound.