Aggregation properties of bis(salicylaldiminato)zinc(II) Schiff-base complexes and their Lewis acidic character

The synthesis, characterization, H-1 NMR, optical absorption and fluorescent properties of a series of amphiphilic Schiff-base bis(salicylaldiminato)zinc(II) complexes are reported. Detailed H-1 NMR, DOSY NMR, optical absorption and fluorescence spectroscopy studies indicate the existence of aggregate species in solutions of non-coordinating solvents. The degree of aggregation is related to the nature of the bridging diamine. Chloroform solutions of complexes where the bridging diamine contains a naphthalene or the pyridine nucleus are always characterized by the presence of defined dimer aggregates, whereas oligomeric aggregates are likely formed by complexes where the bridging diamine contains a benzene ring. In coordinating solvents or in the presence of coordinating species, a complete deaggregation of the complexes occurs, because of the axial coordination to the Zn-II ion, accompanied by considerable changes in the H-1 NMR and optical absorption spectra. The effect of the alkyl chains length seems to play a minor role in the aggregation properties, as noticed by H-1 NMR data, optical absorption and fluorescence spectra, which remain almost unaltered on changing the chain length.