Dehydrogenation of formic acid catalyzed by magnesium hydride anions, HMgL2− (L = Cl and HCO2)

A two step gas-phase catalytic cycle for the dehydrogenation of formic acid was established using a combination of experiments carried out on a quadrupole ion trap mass spectrometer and DFT calculations. The catalysts are the magnesium hydride anions HMgL2− (L = Cl and HCO2), which are formed from the formate complexes, HCO2MgL2−, via elimination of carbon dioxide under conditions of collision induced dissociation. This is followed by an ion–molecule reaction between HMgL2− and formic acid, which yields hydrogen and also reforms the formate complex, HCO2MgL2−. A kinetic isotope effect in the range 2.3–2.9 was estimated for the rate determining decarboxylation step by carrying out CID on the (HCO2)(DCO2)MgCl2− and subjecting the resultant mixture of (H)(DCO2)MgCl2− and (HCO2)(D)MgCl2− ions at m/z 106 to ion–molecule reactions. DFT calculations (at the B3LYP/6−31 + G* level of theory) were carried out on the HMgCl2− system and revealed that: (i) the decarboxylation of HCO2MgCl2− is endothermic by 47.8 kcal mol−1, consistent with the need to carry out CID to form the HMgCl2−; (ii) HMgCl2− can react with formic acid via either a four centred transition state or a six centred transition state. The former reaction is favoured by 7.8 kcal mol−1.