The speciation of aluminum in aqueous solutions of aluminum carboxylates. Part I. X-ray molecular structure of Al[OC(O)CH(OH)CH3]3

The crystal structure of tris(2-hydroxypropanoate-(L))aluminum(III) was determined by X-ray crystallography and refined to R=0.044. The molecular structure is uncomplicated and contains monomeric [Al(lact)3]entities, in which the metal atom is surrounded by a distorted octahedral {O6} coordination sphere where the oxygen atoms of the carboxylate and of the hydroxyl group are the donating sites. The major source of the geometrical distortion is in the OAlO angles of the metallo-organic ring (c83°. The AlO distances range from 1.86 to 1.91 Å. All hydroxyl hydrogen atoms are involved in very tight intermolecular hydrogen bonds (c. 2.50 Å) which involve the carbonyl groups of adjacent Al(lact)3 unities. The solid state IR spectrum of Al(lact)3 is interpreted in terms of monohapto ‘end on’ coordination of the carboxylate donating site to AlIII.