Discovery of low energy pathways to metal-mediated BN bond reduction guided by computation and experiment

Abstract

This manuscript describes a combination of DFT calculations and experiments to assess the reduction of borazines (B–N heterocycles) by η⁶-coordination to Cr(CO)₃ or [Mn(CO)₃]⁺ fragments. The energy requirements for borazine reduction are established as well as the extent to which coordination of borazine to a transition metal influences hydride affinity, basicity, and subsequent reduction steps at the coordinated borazine molecule. Borazine binding to M(CO)₃ fragments decreases the thermodynamic hydricity by >30 kcal mol⁻¹, allowing it to easily accept a hydride. These hydricity criteria were used to guide the selection of appropriate reagents for borazine dearomatization. Reduction was achieved with an H₂-derived hydride source, and importantly, a pathway which proceeds through a single electron reduction and H-atom transfer reaction, mediated by anthraquinone was uncovered. The latter transformation was also carried out electrochemically, at relatively positive potentials by comparison to all prior reports, thus establishing an important proof of concept for any future electrochemical BN bond reduction.