Adsorption/Deposition of the Ligands 9,10-Phenanthroline-5,6-dione and 9,10-Phenanthroline-5,6-diol and Their Metal Complexes on Pyrolytic Graphite Electrodes

Abstract

Metal complexes of the electroactive ligand 9,10-phenanthroline-5,6-dione (pdon) are receiving extensive study because of their demonstrated potential as electrocatalysts for the oxidation of NADH and other substrates. In aqueous media, the reduced ligand, 9,10-phenanthroline-5,6-diol (pdol), and its complexes with transition metals are only slightly soluble and tend to accumulate on the surfaces of electrodes, where they are generated by reduction of pdon. The adsorbed (or precipitated) molecular layers can interfere with the continued electroreduction of the more soluble, oxidized precursors from which they are generated. The use of freshly cleaved basal plane pyrolytic graphite electrodes allowed the surface coordination chemistry and electrochemistry of pdon, pdol, and their complexes to be inspected in more detail than in previous studies. The results revealed several general trends:  (i) Coordination of pdon to transition metals shifts its reduction potential to more positive values. (ii) The affinity of the dipositive form of transition metal redox couples for pdon is greater than that of the tripositive form by a factor that exceeds the corresponding affinities for 9,10-phenanthroline. (iii) Pdol and its complexes are much less soluble in aqueous media than are pdon and its complexes. (iv) Complexes of pdon are much less strongly adsorbed on graphite than is the free ligand in both its protonated and unprotonated forms.