Quo Vadis CO2 Activation: Catalytic Reduction of CO2 to Methanol Using Aluminum and Gallium/Carbon-based Ambiphiles

We report on so-called "hidden FLPs" (FLP: frustrated Lewis pair) consisting of a phosphorus ylide featuring a group 13 fragment in the ortho position of a phenyl ring scaffold to form five-membered ring structures. Although the formation of the Lewis acid/base adducts was observed in the solid state, most of the title compounds readily react with carbon dioxide to provide stable insertion products. Strikingly, 0.3-3.0 mol% of the reported aluminum and gallium/carbon-based ambiphiles catalyze the reduction of CO2 to methanol with satisfactory high selectivity and yields using pinacol borane as stoichiometric reduction equivalent. Comprehensive computational studies provided valuable mechanistic insights and shed more light on activity differences.

Accessing Cationic α-Silylated and α-Germylated Phosphorus Ylides

The synthesis and full characterization of α-silylated (α-SiCPs; 1-7) and α-germylated (α-GeCPs; 11-13) phosphorus ylides bearing one chloride substituent R3 PC(R1 )E(Cl)R2 2 (R=Ph; R1 =Me, Et, Ph; R2 =Me, Et, iPr, Mes; E=Si, Ge) is presented. The molecular structures were determined by X-ray diffraction studies. The title compounds were applied in halide abstraction studies in order to access cationic species. The reaction of Ph3 PC(Me)Si(Cl)Me2 (1) with Na[B(C6 F5 )4 ] furnished the dimeric phosphonium-like dication [Ph3 PC(Me)SiMe2 ]2 [B(C6 F5 )4 ]2 (8). The highly reactive, mesityl- or iPr-substituted cationic species [Ph3 PC(Me)SiMes2 ][B(C6 F5 )4 ] (9) and [Ph3 PC(Et)SiiPr2 ][B(C6 F5 )4 ] (10) could be characterized by NMR spectroscopy. Carrying out the halide abstraction reaction in the sterically demanding ether iPr2 O afforded the protonated α-SiCP [Ph3 PCH(Et)Si(Cl)iPr2 ][B(C6 F5 )4 ] (6 dec) by sodium-mediated basic ether decomposition, whereas successfully synthesized [Ph3 PC(Et)SiiPr2 ][B(C6 F5 )4 ] (10) readily cleaves the F-C bond in fluorobenzene. Thus, the ambiphilic character of α-SiCPs is clearly demonstrated. The less reactive germanium analogue [Ph3 PC(Me)GeMes2 ][B{3,5-(CF3 )2 C6 H3 }4 ] (14) was obtained by treating 11 with Na[B{3,5-(CF3 )2 C6 H3 }4 ] and fully characterized including by X-ray diffraction analysis. Structural parameters indicate a strong CYlide -Ge interaction with high double bond character, and consequently the C-E (E=Si, Ge) bonds in 9, 10 and 14 were analyzed with NBO and AIM methods.