Dimethylaluminium alkoxides: a physico-chemical investigation

Abstract

Compounds of the type [Me₂Al(µ-OR)]_x(R = Me, Et, Prⁿ, Prⁱ, Buⁿ, Buⁱ, Bu^s, Bu^t, n-C₅H₁₁, CH₂CH₂Prⁱ, CH₂Bu^t, n-C₆H₁₃, n-C₈H₁₇, n-C₁₀H₂₁ or n-C₁₂H₂₅) have been synthesised and studied by ¹H, ¹³C, ¹⁷O and ²⁷Al NMR, IR and mass spectrometry. With the exception of R = CH₂CH₂Prⁱ, all the compounds with branched-chain alkoxide substituents are purely dimeric (x= 2), while the former and the n-alkyl derivatives exist in solution as an equilibrium between dimeric (x= 2) and trimeric (x= 3) forms. Equilibrium constants and ΔH and ΔS for the trimer–dimer equilibria in solution were obtained for R = Prⁿ. These results, and the NMR spectroscopic data, are interpreted on the basis of steric interactions and ring strain. The kinetics of conversion of [Me₂Al(OPrⁿ)]₃ into [Me₂Al(OPrⁿ)]₂ has been investigated, ΔH^‡ and ΔS^‡ determined, and the identity of the reaction intermediates probed by NMR and mass spectrometry. Ab initio molecular orbital calculations have been carried out on the model compounds [H₂Al(µ-OH)]₂ and [H₂Al(µ-OH)]₃. Their relative energies and calculated structures are considered in relation to experimental data.