Gas phase models of hydride transfer from divalent alkaline earth metals to CO 2 and CH 2 O.

The reactivity of HMg + , HMgCl, and HMgCl 2 - in hydride transfer reactions with CO 2 and CH 2 O were studied by means of the reverse reactions-decarboxylation of HCO 2 MgCl n +/0/- and deformylation of CH 3 OMgCl n +/0/- ( n = 0-2)-by a combination of quantum chemical computations and mass spectrometry experiments. HCO 2 Mg + , HCO 2 MgCl and HCO 2 MgCl 2 - display similar energetics for unimolecular carbon dioxide loss; for CH 3 OMg + , CH 3 OMgCl and CH 3 OMgCl 2 - , formaldehyde loss is more favourable for the cationic species than for the anionic one, with the neutral species found in-between. Despite very similar overall thermochemistry for each of the charge states of the CO 2 and CH 2 O systems, the intermediate reaction barriers are higher for the CO 2 eliminations due to a more complex and demanding reaction mechanism. Exothermic ligand exchange is observed for CH 3 OMg + reacting with CO 2 , forming HCO 2 Mg + and CH 2 O. Both the thermochemistry and the presence of intermediate energy barriers slightly disfavour this type of ligand exchange for CH 3 OMgCl and CH 3 OMgCl 2 - . It was also found that CH 3 OMg + reacts readily with H 2 O to eliminate H 2 , whereas quantum chemical computations predict that the corresponding neutral and anionic species suffer unfavourable reaction thermochemistry. A corresponding reaction for the magnesium formate compounds was not observed. Quantum chemical computations were performed to investigate periodic trends in reactivity. The energetic requirements for decarboxylation of HCO 2 M + , HCO 2 MCl and HCO 2 MCl 2 - increase in the order M = Be, Mg, Ca, Sr and Ba; the only exception is the cationic Be species for which the reaction is more endothermic than for the corresponding Mg species. For deformylation of CH 3 OM + , CH 3 OMCl and CH 3 OMCl 2 - the trends are more irregular and less pronounced than for the decarboxylation reactions; however, the Be species was found to have higher reaction energies than the Mg species for all the methoxy-compounds irrespective of charge state. The effect on decarboxylation and deformylation upon replacing Cl with F, Br or I was found to be minimal for all aforementioned species. The consequences of these observations on the reverse reactions, hydride transfer to CO 2 and CH 2 O, are discussed, and the effects of systematic structural changes on reactivity are rationalized on the basis of thermochemical cycles and well-known periodic trends.