Thermodynamics and structure of chloro-complexes of aluminium(III) in N,N-dimethylformamide and N,N-dimethylacetamide

Abstract

Formation of chloro-complexes of Al^III has been studied by spectrophotometry, calorimetry and ²⁷Al NMR spectroscopy in N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) at 298 K. Formation constants of the Al^III–Cl^- system were obtained spectrophotometrically on the basis of those of the Ni^II–Cl^- system by means of competitive complexation between Al^III and Ni^II ions. All data can be explained well in terms of formation of mono- and dichloro-complexes of Al^III in DMF, and mono- and trichloro-complexes in DMA, and their formation constants, formation enthalpies and entropies were determined. Formation entropies of chloro-complexes of Al^III in DMF are significantly less than the corresponding values of Ga^III and In^III. It is thus suggested that the formation of inner-sphere AlCl²⁺ and AlCl₂⁺ is strongly suppressed, and the chloro-complexes are present mainly as the outer-sphere [Al(DMF)₆]³⁺Cl^- and [Al(DMF)₆]³⁺2Cl^- complexes in DMF. This is indeed evidenced by the ²⁷Al NMR. On the other hand, in DMA, the large and positive enthalpy and entropy values indicate that the mono- and trichloro-complexes are inner-sphere ones, AlCl²⁺ and AlCl₃⁰. The ²⁷Al NMR signal ascribed to AlCl₃⁰ is largely shifted to a lower magnetic field indicating that the complex is tetrahedrally four-coordinated, [AlCl₃(DMA)]⁰, and that a geometry change takes place upon complexation of the trichloro-complex in DMA.