Examination of the Coordination Sphere of AlIII in Trifluoromethyl-Heteroarylalkenolato Complex Ions by Gas-Phase IRMPD Spectroscopy and Computational Modelling

A series of aluminium complex ions with trifluoromethyl-heteroarylalkenolato (TMHA) ligands are studied by gas-phase infrared multiphoton-dissociation (IRMPD) spectroscopy and computational modelling. The selected series of aluminium TMHA complex ions are promising species for the initial study of intrinsic binding characteristics of AlIII cations in the gas phase as corresponding molecular ions. They are readily available for examination by (+) and (-) electrospray ionization mass spectrometry (ESI-MS) by spraying of [Al3+center dot(L-)3] solutions. The complex ions under investigation contain trivalent Al3+ cations with two chelating anionic enolate ligands, [Al3+center dot(L-)2]+, providing insights in the nature of the heteroatom-Al bonds. Additionally, the structure of a deprotonated benzimidazole ligand, L-, and an anionic complex ion of AlIII with two doubly deprotonated benzimidazole ligands, [Al3+center dot(L2-)2]-, are examined by (-)ESI-IRMPD spectroscopy. Experimental and computational results are highly consistent and allow a reliable identification of the ion structures. In all complex ions examined the planar TMHA ligands are oriented perpendicular to each other around the metal ion, leading to a tetrahedral coordination sphere in which aluminium interacts with the enolate oxygen and heteroaryl nitrogen atoms available in each of the bidentate ligands.