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-<i>i</i>Pr₂C₆H₃) <b>1</b> 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 <b>2</b> 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 <b>1</b>, because their π*-antibonding MOs exist at higher energy than that of CF₃PhCO. Though <b>2</b> is easily formed, the catalytic hydrogenation of CF₃PhCO by <b>1</b> is difficult because the reaction of <b>2</b> with a dihydrogen molecule needs a large activation energy. On the other hand, our calculations clearly show that the catalytic hydrogenation of ketone by <i>cis</i>-RhH­(PPh₃)₂ <b>4</b> easily occurs, as expected. The comparison of catalytic cycle between <b>1</b> and <b>4</b> suggests that the strong Ge–O bond of <b>2</b> is the reason of the very large activation energy for the hydrogenation by <b>1</b>. 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.