Electrochemistry of molybdenum(VI)–catecholamide siderophore complexes in aqueous solution

Complexes of molybdenum(VI) with the catecholamide siderophore ligands aminochelin, azotochelin and protochelin undergo electrochemical reduction by sequential Mo(VI)→Mo(V) and Mo(V)→Mo(III) electron transfers in pH 5.40–6.74 pyridine (py) buffer in a manner closely analogous to the cis-dioxo Mo(VI)–catechol complex, MoO2(cat)2 2−. The pH dependence of the electrode half-reaction potentials for Mo–azotochelin and Mo–aminochelin reveals that between one and two protons are transferred in conjunction with Mo(VI) to Mo(V) reduction and between two and three protons in conjunction with Mo(V) to Mo(III) reduction. It appears that the acidity of the protons on the coordinated H2O in the MoVO(OH2)(L)2 − product is enhanced by the amide substituents on the siderophore ligands. Electrode potentials shift in the positive direction with increasing py concentration, consistent with coordination of one py to Mo(V) and two py to Mo(III). The Mo(VI)/Mo(V) and Mo(V)/Mo(III) potentials of Mo–siderophore complexes fall in the ranges of −0.45–−0.55 and −0.75–−0.8 V versus Ag/AgCl (−0.2–−0.3 and −0.5–−0.55 V vs. NHE), respectively, in pH 6.74 py buffer.