Visible absorption spectra of metal–catecholate and metal–tironate complexes

Interactions between metals and catechol (1,2-dihydroxybenzene) or other ortho-dihydroxy moieties are being found in an increasing number of biological systems with functions ranging from metal ion internalization to biomaterial synthesis. Although metal–catecholate interactions have been studied in the past, we present the first systematic study of an array of these compounds, all prepared under identical conditions. We report the ultraviolet–visible absorption (UV–vis) spectra for catecholate and tironate complexes of the first row transition elements. Generation and identification of these species were accomplished by preparing aqueous solutions with varied ligand ∶ metal ratios and subsequently titrating with base (NaOH). Controlled ligand deprotonation and metal binding resulted in sequential formation of complexes with one, two, and sometimes three catecholate or tironate ligands bound to a metal ion. We prepared the mono-, bis- and tris-catecholates and -tironates of Fe3+, V3+, V4+and Mn3+, the mono- and bis-catecholates and -tironates of Cu2+, Co2+, Ni2+, Zn2+, Cr2+ and Mn2+, and several Ti4+ and Cr3+ species. The UV–vis spectra of each complex are described, some of which have not been reported previously. These data can now be applied to characterization of biological metal–catecholate systems.