An ab initio molecular orbital study of the grignard reagents CH3MgCl and [CH3MgCl]2: The Schlenk equilibrium

Abstract

Ab initio molecular orbital calculations are used to study the modified Schlenk equilibrium: 2RMgCI ,-~ (RMgCI) 2 ~ MgR2 + MgCI2 "-' Mg(CI2)MgR2 with R = H and CH 3. In the absence of any solvents, calculations indicate that the formation of the various possible bridged dimers (RMgC1)2 is substantially exothermic. However, using dimethylether as a model solvent, we show that the formation of the dimer (Me20)(CH3)Mg(~CI2)Mg(CH3)(OMe2) is exothermic only when entropic effects are included.

Synthesis, characterization and Schlenk equilibrium studies of methylmagnesium compounds with O- and N-donor ligands – the unexpected behavior of [MgMeBr(pmdta)] (pmdta=N,N,N′,N″,N″-pentamethyldiethylenetriamine)

“…Addition of N,N,N 0 ,N 0 -tetramethylethylenediamine (tmeda) to a diethyl ether solution of MgMeBr in an equimolar ratio proved to be strongly exothermic and resulted in precipitation of MgMeBr(tmeda) (7) also as a white powder in nearly quantitative yield. From a thf solution of 7 well shaped colorless crystals were crystallized at À40°C; singlecrystal X-ray diffraction analysis revealed the formation of a thf adduct [MgMeBr(thf)(tmeda)] (8). Drying these crystals in vacuo at room temperature for 1/2 h resulted in removing the thf molecule yielding again 7.…”

“…8). Cooling Table 2 The differences of 1 H and 13 C resonances (Dd in ppm) and of 1 J(C,H) coupling constants (DJ in Hz) for coordinated ligands in methylmagnesium compounds 2, 5-7, 9, 12 and corresponding free ligands in thfd 8 thf-d 8 solutions of 9 from room temperature down to À100°C did not result in signal splitting but only in a slight high-field shift of the signal by about 0.1 ppm. Furthermore, as for [MgMe 2 (pmdta)] (12) a slight line broadening was observed at À100°C, which may be attributed to changes in solvent viscosity and to decreased quadrupolar relaxation time at lower temperatures [29].…”

“…Thus, our investigations gave evidence that MgMe 2 in thf remains to be monomeric even at À80°C at a concentration of 0.04 mol/l. To gain an insight into the dynamic behavior of [MgMeBr(tmeda)] (7), the dependence of the methyl proton resonance on concentration as well as on temperature was investigated in thf-d 8 (Fig. 6).…”

“…Furthermore, they are critically dependent on the solvent with the generalization that in better coordinating solvents the monomeric species predominate and the position of the Schlenk equilibrium a tends to be shifted to the dismutation products [6,7]. In the last years highlevel ab initio molecular orbital and density-functional studies provided further insight into the solvation effects on the Schlenk equilibrium [8][9][10][11].…”

“…Hz, CH 3 Mg).1 H NMR (500 MHz, thf-d8 ), temperature dependence of d(CH 3 ); concentration 0.04 mol/l: À1.695 (27°C), À1.769 (À20°C), À1.748/À1.800 (À40°C), À1.767/À1.817 (À60°C), À1.786/À1.844 (À80°C).3.2.4. MgMeBr(diglyme) (5)A solution of MgMeBr (18.6 mmol) in diethyl ether (6 ml) was concentrated in vacuo up to ca.…”