Structure, spectroscopy, and metal exchange among M(III) clusters with salicylidene-α-hydroxy acid chelates (M = Al, Ga, In, Fe)

The tetradentate salicylidene-α-hydroxy acid chelate, 3,5-diCl-Sal-AHA, was previously shown to form trinuclear clusters with the similarly-sized Fe(III) and Ga(III) ions, but a binuclear structure was observed with the much larger uranyl ion. To determine the structural effect of ionic size, complexes of 3,5-diCl-Sal-AHA with the smaller Al(III) ion and larger In(III) ion were prepared. X-ray data show that Al(III) forms a trinuclear cluster similar to those of Fe(III) and Ga(III). However, ESI-MS data show that Al(III) trinuclear clusters exist in equilibrium with binuclear clusters, with basic conditions favoring increased binuclear cluster formation. X-ray data from a crystal obtained from basic solution of Al(III) with 3,5-diCl-Sal-AHA show a binuclear, carbonato-bridged complex. ESI-MS data for the In(III) complex indicate that it forms a trinuclear cluster. The Fe(III), Ga(III), Al(III), and In(III) complexes all have strong circular dichroism spectra, while only the Al(III), Ga(III), and In(III) complexes are fluorescent. When the Group 13 metal complexes are treated with ferric chloride, Fe(III) displaces Al(III), Ga(III), and In(III) in aqueous methanol, but no metal exchange occurs in dry methanol.