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

The mechanisms of aqueous methanol dehydrogenation reaction [CH 3 OH + H 2 O = 3H 2 + CO 2 ] catalyzed by conjugated PNP pincer amido M(CO) 2 (PNP) and amino H M(CO) 2 (PN H P) complexes [M = Mn, Re; and PNP = N(CH 2 CH 2 P(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 H 2 formation by N⋯K + ⋯O interaction. Considering the special role of formic acid in H 2 storage and CO 2 hydrogenation, formic acid dehydrogenation should be accessible under base-free and strong base conditions.