Near thermal, selective liberation of hydrogen from formic acid catalysed by copper hydride ate complexes

Abstract

A near thermal two-step catalytic cycle for the selective release of hydrogen from formic acid by mononuclear cuprate anions was revealed using multistage mass spectrometry experiments, deuterium labelling and DFT calculations. In gas-phase ion-molecule reactions, mononuclear copper hydride anions [(L)Cu(H)]⁻ (where L = H⁻, O₂CH⁻, BH₄⁻ and CN⁻) were found to react with formic acid (HCO₂H) to yield [(L)Cu(O₂CH)]⁻ and H₂. The copper formate anions [(L)Cu(O₂CH)]⁻ can decarboxylate via collision-induced dissociation (CID) to reform the copper hydride [(L)Cu(H)]⁻, thereby closing the two-step catalytic cycle. Analogous labelling experiments with d₁-formic acid (DCO₂H) reveal that the decarboxylation process also occurs spontaneously. A kinetic study was carried out to provide further insights into the species involved in this reaction. Energetics from density functional theory (DFT) calculations show that the key decarboxylation step can occur without CID, thus in support of experimental observations.