On the Mechanism of CO2 Insertion into the Mg‐N Bond of Molecular Mono‐ and Dinuclear Magnesium Compounds: A Quantum Chemical Study

Abstract

AbstractHerein the mechanism for CO2 fixation by alkylmagnesium amides and magnesium bisamides is analyzed on the basis of DFT (density functional theory) calculations relying on the functionals BP86 and B3LYP in combination with a TZVPP basis set. Monomeric as well as dimeric compounds were considered, and prior to the CO2 reactions the thermodynamics of several equilibria (monomer‐dimer, disproportionation) is analyzed. Additional calculations were performed to study the mechanism for CO2 fixation in the presence of a chelating donor molecule, namely ethylene diamine, which preferentially complexes the monomeric alkylmagnesium amide. On the basis of the calculations it is suggested that (i) for monomeric magnesium compounds CO2 insertion into the Mg‐N bonds proceeds without a barrier to an intermediate featuring a four‐membered MgOCN ring and then with a relatively small barrier to the carbamate, (ii) for dinuclear magnesium compounds CO2 insertion is opposed by a significant barrier, and (iii) the addition of donor ligands which coordinate to the monomeric species shifts the monomer‐dimer equilibrium to the monomer side, and in addition leads to direct carbamate formation without any intermediate stage and with no reaction barrier. The studies might be of interest for a better understanding of enzymatic reactivity.