27Al NMR study of the trimethylaluminum monomer-dimer equilibrium

Abstract

The 27Al NMR signal from trimethylaluminum (TMA) in solution is shifted to lower field with: (i) increasing temperature; (ii) decreasing concentration of TMA; and (iii), more markedly, the increasing solvating power of the solvent. This shift reflects an increase in the degree of dissociation of the dimer to the monomer. Thermodynamic data ( K d, Δ H d, Δ S d) of the TMA dissociation have been calculated from the observed values of the NMR chemical shift by use of the shifts estimated for the separate dimeric (Al 2Me 6) and monomeric (AlMe 3) species. In both solvents employed (n-heptane and mesitylene) the derived Δ H d values lie between 11.2 and 14.7 kcal/mol for Al 2Me 6, values in good agreement with the Δ H d value of 13.4 kcal/mol previously determined experimentally. (These experimental values are lower than previously estimated Δ H d values, which lay between 14.2 and 19.7 kcal/mol.) The 27Al NMR spectra of TMA solutions recorded over the range of 25–100°C unambiguously proved the exchange of methyl substituents between bridge and terminal positions in the TMA dimer. In both aromatic and aliphatic solvents, this exchange occurs via the “intermolecular mechanism”, i.e. via the separated monomeric form of TMA; experimentally determined Δ H d values are in accord with this mechanism.