Dimerization of lawsone occurs upon reaction with Co(BF4)2 center dot 6H(2)O and N,N'-bis(pyridin-2-ylmethyl)ethylenediamine (py(2)en) to produce the mononuclear complex [Com(bhnq)(py(2)en)]BF4 center dot H2O (1). This complex has been investigated as a prototype of a bioreductive prodrug, where the bhnq(2-) ligand acts as a model for cytotoxic naphthoquinones. Cyclic voltammetry data in aqueous solution have shown a quasi-reversible Co-III/Co-II process at E-1/2 = -0.26 V vs Fc/Fc(+). Reactivity studies revealed the dissociation of bhnq(2-) from the complex upon reduction of 1 with ascorbic acid, and a dependence of the reaction rate on the oxygen concentration suggests the occurrence of redox cycling.