Catechol-pendant terpyridine complexes: Electrodeposition studies and electrocatalysis of NADH oxidation

The synthesis, spectroscopic characterization, and electrochemical characte rization, including the electrodeposition onto glassy carbon (GC) and plati num (Pt) electrodes, of electroactive films of the homoleptic catechol-pend ant terpyridine complexes [M(L-2)(2)](2+) (where M = Co, Cr, Fe, Ni, Ru, an d Os, and L-2 = 4'-(3,4-dihydroxyphenyl)2,2':6',2 "-terpyridine) are descri bed. The potential dependence of the deposition was probed through electroc hemical quartz crystal microbalance (EQCM) studies. Multilayer equivalent f ilms were found to deposit at potentials less than that of the catechol oxi dation process. Whereas the heteroleptic ruthenium(II) complex [Ru(tpy)(L-2 )](2+) (tpy = 2,2':6',2 "-terpyridine) did not deposit onto electrode surfa ces, the corresponding osmium(II) heteroleptic complex [Os(tpy)(L-2)](2+) d eposited onto Pt and GC electrodes, suggesting that the metal center can pl ay an important role in the deposition process. The heteroleptic cobalt(II) complex [Co(v-tpy)(L-2)](2+) (v-tpy = 4'-vinyl-2,2':6',2 "-terpyridine) wa s found to deposit onto Pt or GC electrodes through either a catechol-based deposition or a v-tpy-based electropolymerization, depending on the potent ial range over which a homogeneous solution of the complex was cycled. The electrochemical response of [Co(L-2)(2)](2+)-modified GC electrodes in aque ous solution was robust and pH-dependent over the pH range 1-11, suggesting that the catechol moieties retain their pH-dependent redox activity upon i mmobilization. The application of these complexes, in solution and as elect rodeposited films, to the electrocatalytic oxidation of NADH war also probe d. [Co(L-2)(2)](2+) in solution plus [Co(L-2)(2)](2+)- and [Co(v-tpy)(L-2)] (2+)-modified GC electrodes were found to catalyze the oxidation of NADH in pH 7 phosphate buffer solution.