Efficient Reduction of Carbon Dioxide to Methanol Equivalents Catalyzed by Two-Coordinate Amido-Germanium(II) and -Tin(II) Hydride Complexes
Résumé
The bulky amido-germanium(II) and -tin(II) hydride complexes, (LEH)-E-dagger [E = Ge or Sn; L-dagger = -N(Ar-dagger) (SiPr3i); Ar-dagger = C6H2Pri\C(H)Ph-2\(2)-4,2,6], which are two-coordinate in solution, are shown to be efficient and highly selective \textquotedblleftsingle site\textquotedblright catalysts for the reduction of CO2 to methanol equivalents (MeOBR2), using HBpin or HBcat as the hydrogen source. (LSnH)-Sn-dagger is the most active non-transition metal catalyst yet reported for such reductions, yielding turnover frequencies of up to 1188 at room temperature. Computational studies have identified two thermodynamically and kinetically viable catalytic pathways by which these reductions may operate. Spectroscopic investigations have identified several reaction intermediates, which leads to the conclusion that one of these reaction pathways predominates in the experimental situation. Stoichioinetric reactivity studies have shed further light on the reaction mechanisms in operation and indicate that the involvement of the second reaction pathway cannot be ruled out. This study highlights the potential of relatively cheap, main group complexes as viable alternatives to transition metal-based systems in the catalytic transformation of small molecules.