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

Abstract

Formation of chloro-complexes of AlIII 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 AlIII–Cl- system were obtained spectrophotometrically on the basis of those of the NiII–Cl- system by means of competitive complexation between AlIII and NiII ions. All data can be explained well in terms of formation of mono- and dichloro-complexes of AlIII 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 AlIII in DMF are significantly less than the corresponding values of GaIII and InIII. 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.