Aluminium–ligand cooperation promotes selective dehydrogenation of formic acid to H2 and CO2

Abstract

Herein, we report that molecular aluminium complexes of the bis(imino)pyridine ligand, (^PhI₂P²⁻)Al(THF)X, X = H (1), CH₃ (2), promote selective dehydrogenation of formic acid to H₂ and CO₂ with an initial turnover frequency of 5200 turnovers per hour. Low-temperature reactions show that reaction of 1 with HCOOH affords a complex that is protonated three times: twice on the ^PhI₂P²⁻ ligand and once to liberate H₂ or CH₄ from the Al-hydride or Al-methyl, respectively. We demonstrate that in the absence of protons, insertion of CO₂ into the Al-hydride bond of 1 is facile and produces an Al-formate. Upon addition of protons, liberation of CO₂ from the Al-formate complex affords an Al-hydride. Deuterium labelling studies and the solvent dependence of the reaction indicate that outer sphere β-hydride abstraction supported by metal–ligand cooperative hydrogen bonding is a likely mechanism for the C–H bond cleavage.