Revisiting CO2 Reduction with NaBH4 under Aprotic Conditions: Synthesis and Characterization of Sodium Triformatoborohydride

The reduction of CO2 to formate using sodium borohydride was originally investigated in the 1950s. Despite this clue from the chemical literature, many recent publications describe catalytic CO2 hydroboration methods leading to formate or methoxide with more expensive and less reactive boranes such as pinacolborane. Herein we describe the uptake of 3 equiv of CO2 by NaBH4, along with full spectroscopic and crystallographic characterization of the resulting triformatoborohydride, Na­[HB­(OCHO)3]. Conducting the synthesis in acetonitrile under 300 psi of CO2 constitutes a new preparative procedure for generating Na­[HB­(OCHO)3]. This reaction does not require the presence of a strongly coordinating alkali metal cation, as evidenced by the analogous reactivity of [NEt4]­[BH4]. Even at 1 atm pressure and without using rigorously dry solvent, treatment of NaBH4 with CO2 and subsequent quenching gave formic acid (1.5 equiv based on B).