Elucidating the effect of the nucleophilicity of the silyl group in the reduction of CO2 to CO mediated by silyl-copper(I) complexes

The reduction of CO2 to CO with silyl-copper(I) complexes bearing various silyl groups has been investigated. The silyl-copper(I) complexes [LSi(X)Cu(IPr)] 2-5 (X = OtBu (2), OH (3), H (4), OC6F5 (5); L = CH{C=CH2}(CMe)(NAr)2, IPr = (CHNAr)2C:, Ar = 2,6-iPr2C6H3) bearing OtBu, OH, H, and OC6F5 as functional groups are readily accessible by the activation of the Cu-O and Cu-H bonds in (IPr)CuX with silylene LSi: (1). These complexes are not readily accessible by the commonly used transmetallation reaction, rendering this methodology rather unique and facile in synthesizing silicon-functionalized silyl-metal complexes. The functional groups at the silicon atoms in compounds 2-5 enable the silyl groups to feature different nucleophilicity, which affords different activities toward CO2 reduction to CO. The silyl moieties of complexes 2 and 3, containing electron-donating groups (i.e., OtBu and OH) at the silicon centers, are more nucleophilic than that of compound 4 and 5, bearing a hydride and the electron-withdrawing group OC6F5 at the silicon centers, respectively. Consistent with this observation, compounds 2 and 3 show higher activity in CO2 reduction to CO compared to compounds 4 and 5, and the latter cases are zero-order reactions with respect to 4 and 5 (4: k = 7.8×10(-6) mol L(-1) s(-1); 5: 2.7×10(-8) mol L(-1) s(-1)). This suggests that the more nucleophilic the silyl moiety in a silyl-copper(I) complex is, the higher is the efficiency in CO2 reduction to CO. In addition, the siloxyl-copper(I) complexes [LSi(X)OCu(IPr)] 6-9 [X = OtBu (6), OH (7), H (8), OC6F5 (9)] were isolated as the products from the corresponding reduction reactions. Complexes 2-4 and 6-8 were characterized by spectroscopic and structural means.