Adsorption of solvent and oxidant molecules on magnesium surface: effect on the electronic structure and reactivity of magnesium in reactions of Grignard reagent formation

Abstract

The effect of adsorption of the oxidant (EtBr) and aprotic dipolar solvent molecules on the electronic structure and properties of Mg n (n ≤ 50) clusters simulating the surface of metallic magnesium was studied by the B3PW91/6 31G(d) density functional method. It was found that the work function of an electron from the cluster monotonically decreases as the donor ability of the adsorbed molecules increases. The experimentally measured rate of the magnesium oxidation by EtBr correlates with the negative Mulliken charge density of the first coordination sphere of the surface adsorption site. The results obtained are in agreement with the experimentally determined work function of electron from metals during adsorption from the gas phase. Presumably, the rate limiting step of the Grignard reagent formation is the surface oxidation reaction.Reactions of oxidative dissolution of metals in aprotic media have been the subject of considerable litera ture. 1-7 A classical example of this reaction in a system including a metal, an organic or organoelement oxidant, and an aprotic dipolar solvent is the formation of the Grignard reagent RMgX:Although reaction (1) was discovered and quantita tively characterized more than a hundred years ago, its mechanism is still the matter of discussion. 8-15 In partic ular, the role of aprotic solvent that significantly facili tates the course of reaction (1) is still to be clarified. 16 Donor polar solvents can dissolve films of magnesium hydroxide 5 and/or products of reactions occurring on the metal surface, 9 thus making surfaces accessible to reac tants. In addition, this type of solvents can stabilize both intermediates and organometallic reaction products.The assumption that acceleration of oxidation reactions in the presence of a solvent is due to an increase in the solubility of the halide of the metal oxidized was also not substantiated. 16 The catalytic activity of the solvent in this reaction is also associated 17,18 with its ability to ac tivate the metal surface by affecting active sites on the surface.Numerous experimental data 7,19 obtained in the stud ies of metal oxidation by compounds with different chem ical composition and structure in aprotic solvents re vealed extremal dependences of the reaction rate on the donor numbers (DN SbCl 5 ) of the solvents. 20 The sol vent effect on the rate of oxidation reactions of various metals can be described 21,22 by a multiparameter equa tion including macroscopic and physicochemical char acteristics of the medium (in particular, the Kop pel-Palm nucleophilicity, polarity, the Hildebrandt sol ubility parameter, and the van der Waals radius of free solvent molecule 23 ). These correlations are based on the linear free energy relationship and the Hammett-Taft linear relations were proposed 12-14 in studies of the oxi dation of magnesium by p substituted phenyl halides; however, the mechanism of the process is still unknown.It is assumed 9 that the solvent adsorbed on a metal surface causes the work function of an electron from the m...