Catechol-Pendant Terpyridine Complexes: Electrodeposition Studies and Electrocatalysis of NADH Oxidation

The synthesis, spectroscopic characterization, and electrochemical characterization, including the electrodeposition onto glassy carbon (GC) and platinum (Pt) electrodes, of electroactive films of the homoleptic catechol-pendant terpyridine complexes [M(L(2))(2)](2+) (where M = Co, Cr, Fe, Ni, Ru, and Os, and L(2) = 4'-(3,4-dihydroxyphenyl)-2,2':6',2' '-terpyridine) are described. The potential dependence of the deposition was probed through electrochemical quartz crystal microbalance (EQCM) studies. Multilayer equivalent films were found to deposit at potentials less than that of the catechol oxidation process. Whereas the heteroleptic ruthenium(II) complex [Ru(tpy)(L(2))](2+) (tpy = 2,2':6',2' '-terpyridine) did not deposit onto electrode surfaces, the corresponding osmium(II) heteroleptic complex [Os(tpy)(L(2))](2+) deposited onto Pt and GC electrodes, suggesting that the metal center can play 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) was found to deposit onto Pt or GC electrodes through either a catechol-based deposition or a v-tpy-based electropolymerization, depending on the potential range over which a homogeneous solution of the complex was cycled. The electrochemical response of [Co(L(2))(2)](2+)-modified GC electrodes in aqueous solution was robust and pH-dependent over the pH range 1-11, suggesting that the catechol moieties retain their pH-dependent redox activity upon immobilization. The application of these complexes, in solution and as electrodeposited films, to the electrocatalytic oxidation of NADH was also probed. [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.