Abstract
The crystal structure of [Al(tBu-salen)]2O·HCl shows major changes compared to that of [Al(tBu-salen)]2O. The additional proton is localized on the bridging oxygen atom, making the aluminium atoms more electron deficient. As a result, a water molecule coordinates to one of the aluminium atoms, which becomes six-coordinate. This pushes the salen ligand associated with the six-coordinate aluminium ion closer to the other salen ligand and results in the geometry around the five-coordinate aluminium atom becoming more trigonal bipyramidal. These results experimentally mirror the predications of DFT calculations on the interaction of [Al(tBu-salen)]2O and related complexes with carbon dioxide. Variable temperature NMR studies of protonated [Al(tBu-salen)]2O complexes revealed that the structures were dynamic and could be explained on the basis of an intramolecular rearrangement in which the non-salen substituent of a five-coordinate aluminium(tBu-salen) unit migrates from one face of a square based pyramidal structure to the other via the formation of structures with trigonal bipyramidal geometries. Protonated [Al(tBu-salen)]2O complexes were shown to have enhanced Lewis acidity relative to [Al(tBu-salen)]2O, coordinating to water, dioxane and 1,2-epoxyhexane. Coordinated epoxyhexane was activated towards ring-opening, to give various species which remained coordinated to the aluminium centers. The protonated [Al(tBu-salen)]2O complexes catalysed the synthesis of cyclic carbonates from epoxides and carbon dioxide both in the presence and absence of tetrabutylammonium bromide as a nucleophilic cocatalyst. The catalytic activity was principally determined by the nature of the nucleophilic species within the catalyst structure rather than by changes to the Lewis acidity of the metal centers.
Highlights
Line shape analysis§ of the tert-butyl signals in the variable temperature NMR spectra of 1·TFA was carried out to allow the rate constant of the exchange process and the Gibbs energies of activation (ΔG‡) to be determined at each temperature (see Electronic supplementary information (ESI)†)
There are a number of stereochemical aspects of such salen ligands and their metal complexes which can have an important influence on the structure and catalytic activity of metal complexes
That sample was prepared by treatment of the corresponding salen ligand with aluminium triethoxide followed by aqueous work-up (Scheme 2)
Summary
Line shape analysis§ of the tert-butyl signals in the variable temperature NMR spectra of 1·TFA was carried out to allow the rate constant of the exchange process and the Gibbs energies of activation (ΔG‡) to be determined at each temperature (see ESI†). The minor peaks present in the 1H NMR spectrum are consistent with complex 1 in which one of the salen ligands is in a trigonal bipyramidal geometry as observed for one of the two independent molecules in the crystal structure of complex 1.19 This interpretation was supported by a DOSY experiment (in CDCl3) which showed that the major and minor species had very similar diffusion coefficients of 5.4 × 10−10 m2 s−1 and 5.1 × 10−10 m2 s−1 respectively (see ESI†).
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