Dimethylaluminium alkoxides: a physico-chemical investigation

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Abstract
Compounds of the type [Me2Al(µ-OR)]x(R = Me, Et, Prn, Pri, Bun, Bui, Bus, But, n-C5H11, CH2CH2Pri, CH2But, 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.

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