Direct electrochemical reduction of hemin in imidazolium-based ionic liquids

The direct electrochemical reduction of hemin, protoporphyrin(IX) iron(III) chloride, ligated with strong or weak heterocyclic bases, was investigated in the ionic liquids (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([omim][PF6]), using cyclic voltammetry and chronocoulometry. Hemin complexed with N-methylimidazole (NMI) or with pyridine had E-1/2 values slightly (4-59 mV) more positive in IL (without electrolyte) than in methanol (1.0 M electrolyte) using a gold electrode. NMI-ligated hemin had a lower E-1/2 than pyridine-ligated hemin in either IL, consistent with the stronger electron donor characteristic of NMI. [Bmim][PF6] solutions consistently yielded E-1/2 values 30 mV more negative than [omim][PF6] solutions. The diffusion coefficients D-o of hemin in the IL ranged between 1.50 and 2.80 x 10(-7) cm(2) s(-1), while the heterogeneous electron-transfer rate constants k(s) ranged between 3.7 and 14.3 x 10(-3) cm s(-1). Cyclic voltammetry of hemin adsorbed to a gold surface through 4,4'-bispyridyl disulfide (AT4) linkages showed a large positive shift in the oxidation wave, indicating that adsorption stabilizes the reduced hemin state. The surface concentration To of the adsorbed hemin was determined to be 1.21 x 10(-10) mol cm(-2), indicating the presence of one or more complete monolayers of hemin. These findings suggest that while hemin is electrochemically active in IL, its behavior is modified by the ligand field strength and surface adsorption phenomena. Published by Elsevier Science B.V.