Examination of the coordination sphere of Al(III) in trifluoromethyl-heteroarylalkenolato-complex-ions by gas-phase IRMPD spectroscopy and computational modelling

摘要

The IRMPD spectroscopic results combined with careful computational analysis of the gas-phase ion structures allows an unambiguous structure elucidation of aluminium TMHA complex ions. According to our results cationic [Al~(3+)·(L~-)_2]~+ and anionic [Al~(3+)·(L~(2-))_2]~- species adopt tetrahedral arrangements with two planar bidentate ligands arranged around the central main group metal cation. In the examined complex ions the negatively charged enolate oxygen is coordinated to aluminium, however, experimental evidence suggests that the second binding site of the bidentate ligands is determined by the aromatic character of the heterocycle i.e. imidazole, oxazole and thiazole of the ligands. We only find those Lewis-basic nitrogens interacting with Al(III) whose free electron pairs are not directly involved in the delocalized aromatic π-system. Although this straight forward explanation serves well to understand the complete outcome of this study, it is remarkable that the aromaticity of the heterocycles governs the Al coordination sphere and outweighs the strong affinity of aluminium to oxygen. To the best of our knowledge this is the first time that the coordination sphere of such a highly charged, small and strongly polarizing, main group metal ion as Al(III) is investigated in the gas phase. The spectroscopic data set interpreted on the basis of in-depth computations provides a detailed view on the structure and the nature of cationic and anionic aluminium complexes which could serve as a blueprint for other hard main group metal cations with Lewis basic ligands of any kind, e.g. bio-molecules, such as proteins.