Modeling Solvation of Magnesium Centers by Ether Ligands: Gas-Phase Synthesis and Hydrolysis of the Organomagnesium Cations [CH3Mg(3X-crown-X)]+(X= 4–6)

Abstract

Gas-phase decarboxylation of the acetate ligand in the magnesium acetate crown complex cations [CH3CO2Mg(3X-crown-X)]+ (where X = 4–6) has been explored as a means of synthesizing the corresponding organomagnesium cations as models for the organomagnesium core CH3Mg+, which is solvated by ether ligands. Low-energy collision-induced dissociation (CID) of these complexes in ion trap mass spectrometers gives rise to a range of product ions. High-resolution mass measurements reveal the formation of isobaric product ions arising from decarboxylation, [CH3Mg(3X-crown-X)]+, and from loss of C2H4O from the crown ether ligand. The use of low-energy CID of the isotopically labeled complexes [CH313CO2Mg(3X-crown-X)]+ allowed for (i) energy-resolved CID studies, which demonstrated that [CH313CO2Mg(12-crown-4)]+ is more easily decarboxylated than [CH313CO2Mg(15-crown-5)]+ and [CH313CO2Mg(18-crown-6)]+ and (ii) the separation and isolation of the methyl magnesium crown ether cations [CH3Mg(3X-crown-X)]+ for subsequent reactivity studies with water. Rate constants for the hydrolysis of [CH3Mg(3X-crown-X)]+ were experimentally determined to follow the reactivity order [CH3Mg(12-crown-4)]+ > [CH3Mg(18-crown-6)]+ > [CH3Mg(15-crown-5)]+. DFT calculations at the B3LYP/6-31+G(d) level of theory in conjunction with RRKM modeling are consistent with the experimentally determined reactivity orders for decarboxylation of [CH3CO2Mg(3X-crown-X)]+ and hydrolysis of [CH3Mg(3X-crown-X)]+ and highlight that reactivity generally decreases with increasing solvation of the organomagnesium core CH3Mg+.