Formation of Methylmagnesium or Coordinated Ylide? Competition between Decarboxylation of Acetate and Betaine Ligands in [CH3CO2MgO2CCH2X(CH3)2]+(where X = NCH3and S)

Abstract

Decarboxylation of the ligands in the magnesium cations [CH3CO2MgO2CCH2X(CH3)2]+ (where X = NCH3, 1a; and S, 1b) can give rise to either an organometallic cation [CH3MgO2CCH2X(CH3)2]+, 2, or a coordinated ylide [CH3CO2MgCH2X(CH3)2]+, 3. Collision-induced dissociation of a precursor complex in which the acetate ligand was 13C labeled allowed the direct determination of the relative ratio of formation of 2 and 3 via a comparison of the losses of 13CO2 and CO2. For the nitrogen betaine, 1a, both 2a and 3a are formed, in a ratio of ca 1:3, respectively. In contrast, for the sulfur betaine, 1b, the coordinated ylide, 3b, is almost exclusively formed. These striking differences are confirmed by DFT calculations at the B3LYP/6-31+G(d) level of theory, which show that the activation energies associated with the formation of 2a and 3a are close (2.33 eV versus 2.28 eV), but that formation of 3b (Ea = 1.97 eV) is favored over formation of 2b (Ea = 2.33 eV). Use of the 13C-labeled acetate ligand also allowed the bimolecular hydrolysis reactions of isomerically pure populations of mass-selected 2 and 3 to be examined. Ion−molecule reactions with water proceeded via addition/elimination reactions, but the nature of the magnesium hydroxide is different for each isomer. Thus 2a reacted to yield the charged magnesium hydroxide [HOMgO2CCH2N(CH3)3]+, plus methane, while 3a and 3b reacted to yield the neutral magnesium hydroxide [CH3CO2MgOH] plus the charged onium ions (CH3)3X+. Kinetic measurements reveal a reactivity order of 2a > 3a ≈ 3b, consistent with DFT calculations on the potential energy surfaces associated with these hydrolysis reactions.