Exploring the mechanisms of aqueous methanol dehydrogenation catalyzed by defined PNP Mn and Re pincer complexes under base-free as well as strong base conditions

Abstract

The mechanisms of aqueous methanol dehydrogenation reaction [CH₃OH + H₂O = 3H₂ + CO₂] catalyzed by conjugated PNP pincer amido M(CO)₂(PNP) and amino ^HM(CO)₂(PN^HP) complexes [M = Mn, Re; and PNP = N(CH₂CH₂P(isopropyl)₂)₂] under base-free and strong base conditions as well as the K⁺ promotion effect were studied at the B3PW91 level of density functional theory. Benchmark calculations including dispersion and/or solvation corrections validated the computed gas phase data to be closest to the available kinetic and thermodynamic data from experiments. Under base-free conditions, the innocent mechanism is kinetically more favorable than the non-innocent mechanism. Under strong base conditions, KOH plays a dual role: deprotonating the substrate by OH⁻ and stabilizing the rate-determining transition state by K⁺ by lowering the free energy barrier for H₂ formation by N⋯K⁺⋯O interaction. Considering the special role of formic acid in H₂ storage and CO₂ hydrogenation, formic acid dehydrogenation should be accessible under base-free and strong base conditions.