Mechanism of the Grignard reagent formation - Part 1 - Theoretical investigations of the Mg-n and RMgn participation in the mechanism

Electronic-structure calculations on neutral and cationic Mg-n clusters (n less than or equal to 8) have been carried out in order to study the struct ures and properties of a potential intermediate RMgn(I) in the Grignard rea ction. We find that the neutral clusters favor the maximum coordination num ber. In cationic clusters, bonds are reinforced. This result suggests that the cationic clusters are more stable than their parent neutral clusters an d that a Mg atom transfer from the metallic surface to the solution under t he Mg+ form should be more difficult (provided that solvation does not reve rt the picture). Our studies on RMgn(I) species show that these clusters ar e stable enough to intervene as intermediates in the mechanism of Grignard reagent formation. They are also better reducing species than their homolog ous clusters Mg-n((0)) without an alkyl group. Hence, in the Grignard reage nt formation, after the initiation step, the organic halide RX could receiv e more rapidly an electron from RMgn(I) rather than from the Mg-n((0)) clus ters.