A review on bio-functional models of catechol oxidase probed by less explored first row transition metals

Catecholase activity is a biological process in which type-3 active site of a copper enzyme, named catechol oxidase, catalyzes oxidation of o-diphenolic substrates to the corresponding o-quinones coupled with reduction of molecular dioxygen into water. Presence of a hydroxo bridged dicopper(II) core in the structure of catechol oxidase is responsible for its catalytic activity. Interest in mimicking the physicochemical properties of catechol oxidase led to synthesis of a number of model complexes utilizing other transition metal ions, however, the number of studies is remarkably less compared to their copper analogues. This review focuses on catecholase activity of the first row transition metals other than copper to highlight relatively less recognized synthetic and functional models. It includes (i) presentations of some of the proposed mechanisms, (ii) structural descriptions of the relevant catalysts and an attempt to establish a structure-function correlation and (iii) a comparative discussion on catalytic efficiencies of homo and heterometallic complexes. In the summary, this review may provide inspiration for the development of new catalysts and for a more effective approach to draw structure-activity correlations.