Gas phase models of hydride transfer from divalent alkaline earth metals to CO2 and CH2O

Abstract

The reactivity of HMg⁺, HMgCl, and HMgCl₂⁻ in hydride transfer reactions with CO₂ and CH₂O were studied by means of the reverse reactions—decarboxylation of HCO₂MgCl_n^+/0/− and deformylation of CH₃OMgCl_n^+/0/− (n = 0–2)—by a combination of quantum chemical computations and mass spectrometry experiments. HCO₂Mg⁺, HCO₂MgCl and HCO₂MgCl₂⁻ display similar energetics for unimolecular carbon dioxide loss; for CH₃OMg⁺, CH₃OMgCl and CH₃OMgCl₂⁻, 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 CO₂ and CH₂O systems, the intermediate reaction barriers are higher for the CO₂ eliminations due to a more complex and demanding reaction mechanism. Exothermic ligand exchange is observed for CH₃OMg⁺ reacting with CO₂, forming HCO₂Mg⁺ and CH₂O. Both the thermochemistry and the presence of intermediate energy barriers slightly disfavour this type of ligand exchange for CH₃OMgCl and CH₃OMgCl₂⁻. It was also found that CH₃OMg⁺ reacts readily with H₂O to eliminate H₂, 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 HCO₂M⁺, HCO₂MCl and HCO₂MCl₂⁻ 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 CH₃OM⁺, CH₃OMCl and CH₃OMCl₂⁻ 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 CO₂ and CH₂O, are discussed, and the effects of systematic structural changes on reactivity are rationalized on the basis of thermochemical cycles and well-known periodic trends.