Preparation, characterisation, and crystal and molecular structure of a novel tetrameric aluminium phosphate complex: [{Al(PO4)(HCl)(C2H5OH)4}4]

Abstract

A tetrameric complex of aluminium phosphate has been prepared by reaction of aluminium trichloride with phosphoric acid in ethanol. It is a crystalline solid, soluble in water and most simple alcohols, readily decomposes to AlPO₄ on heating, and has empirical formula corresponding to Al(PO₄)(HCl)(EtOH)₄. The crystal structure has been determined from three-dimensional X-ray diffractometer data, and refined by block-diagonal least-squares to R 0.090 for 1 607 independent reflections. Crystals are tetragonal, with unit-cell dimensions a=b= 15.45, c= 14.44 Å, space group I, Z= 2 tetramers. The structure consists of aluminium phosphate tetramers of approximately cubic shape. Each aluminium atom is octahedrally co-ordinated, carrying oxygens from three ethanol ligands, in addition to three phosphate oxygens. There are four further ethanol molecules hydrogen bonded to each tetramer unit. The fourth oxygen of each phosphate tetrahedron is hydrogen bonded to a chlorine atom. These chlorine atoms are also hydrogen bonded to other, adjacent, tetramer units and serve to hold the structure together. Because of its ready conversion to aluminium phosphate, the complex can be used to form thin, glassy, films of AlPO₄. Various related solid complexes have also been prepared.