Metal-Free Reduction of CO2 to Methanol Using Boranes at Ambient Conditions Promoted by Boron-Appended N-Heterocyclic Nitrenium Catalysts.

Abstract

The chemical reduction of carbon dioxide (CO2) into value-added C1 building blocks, particularly under sustainable, metal-free conditions, remains a profound challenge in modern catalysis. Herein, we report robust boron-tethered N-heterocyclic nitrenium (NHN) catalysts for the reduction of CO2 under ambient conditions to produce methoxide-level products. The developed protocol works under room temperature and 1 bar CO2 pressure, with a low catalyst loading. The borane reductant governs the selectivity of the hydroboration reaction. Employing catecholborane (HBcat) or borane dimethylsulfide complex (BH3·SMe2) drives rapid, selective, complete reduction to methoxide-level products. In contrast, the use of 9-borabicyclo[3.3.1]nonane (9-BBN) under identical conditions yields a mixture of acetal-level (CH2(OBBN)2) and methoxyborane (MeOBBN) species, while pinacolborane (HBpin) exhibits lower reactivity, generating a distribution across all three reduction states. Overall, the catalytic platform exhibits excellent turnover numbers (TON). This work establishes a sustainable, metal-free methodology for tunable reduction of CO2 to methanol equivalents, highlighting the versatility of main-group catalysis.