Simultaneous Two-Hydrogen Transfer as a Mechanism for Efficient CO2 Reduction

Two-hydrogen transfer (simultaneous protic and hydridic hydrogen transfer) is examined as a potentially efficient mechanism for the selective reduction of CO2 to methanol. High-level ab initio CCSD(T) coupled-cluster theory simulations of ammonia-borane (AB), which contains both protic and hydridic hydrogen, show the effectiveness of this mechanism. AB demonstrates how simultaneous two-hydrogen transfer is kinetically efficient because (1) two-hydrogen transfer avoids high-energy single-electron-reduced intermediates, (2) the CO2's HOMO is protonated while the LUMO is concurrently reduced by a hydride, and (3) complementary charge polarities around the six-membered-ring transition-state structures stabilize the transition states. This study suggests that an effective mechanism for the reduction of CO2 to methanol proceeds through three two-hydrogen-transfer steps and that suitable catalysts should be developed that exploit two-hydrogen transfer without the use of AB.