Mechanism of the Grignard reagent formation - Part 2 - An ab initio study of the RMgx formation from the RMgn(I) clusters

The results of density functional calculations are reported for CH3Mg2, CH3 Mg4T and for CH3Mg5TBCl model clusters, with T = tetrahedral and TB = trigo nal bipyramid These calculations aim at a simulation of the migration of a methyl group in the proposed intermediates RMgn(I) (n = 2 and 4) and of the succession of steps from the substrate to RMgX. The mono-coordination of t he methyl group in the clusters CH3Mgn (n = 2 or 4) represents the most sta ble structure. The energy barrier to pass from poly-coordination structure to mono-coordination structure is low (1.213 eV = 27.9 kcal/mol). It is suf ficient, however, to prevent the methyl migration from a magnesium atom to another one but is probably frozen at lower temperatures. We also present a Grignard reagent formation proposal different from the classical linear me chanism generally proposed. This mechanism implies a neutral species ((RMgX )-X-(II))Mgn-1 ((RMgX)-X-(II) linked to the metallic surface) as intermedia te. The reaction would evolve then toward the Grignard reagent RMgX and a m agnesium cluster with n-1 magnesium atoms in an irreversible pathway.