Synthesis and Characterization of Redox-Active Metal Complexes Sequentially Self-Assembled onto Gold Electrodes via a New Thiol−Terpyridine Ligand

We report on the synthesis of a new thiol-modified terpyridine ligand 4‘-(5-mercaptopentyl)-2,2‘:6‘,2‘‘-terpyridinyl ( tpy − SH ) as well as of its complexes with Co, Cr, and Os. We have prepared complexes of the type M( tpy − SH ) 2 n + and M( tpy − SH )L n + (L = terpyridine, tetrapyridylpyrazine (tppz)). The free ligand as well as the tpy − SH -containing metal complexes adsorb strongly onto gold electrode surfaces, and in the case of the metal complexes, they retain their redox-active responses at potentials very close to those of nonadsorbing analogs in homogeneous solution. In the case of Co( tpy − SH )(tpy) 2+ adsorbed onto a gold electrode surface, the cyclic voltammetric response exhibits an aging process where it sharpens significantly over long times (days), suggesting a slow reorganization of the adsorbed complex on the electrode surface. Reaction with CoCl 2 of a gold electrode whose surface had been previously modified with the tpy − SH ligand ostensibly gives rise to Au/S−tpy−CoCl 2 . Upon cycling the potential of such a modified electrode over the potential region of +0.40 to −0.40 V, we observe a voltammetric wave at about −0.20 V which initially decreases rapidly with time but much more slowly afterward. Concomitantly, a new reversible voltammetric wave appears at +0.13 V which we ascribe to the formation of the surface-immobilized Co( tpy − SH ) 2 2+ complex via a surface chelate formation (Au−S−tpy−Co−tpy−S−Au). The presence of sharp isopotential points suggests a simple transformation between these two surface species. Gold electrodes could also be modified with Os( tpy − SH )(tpy) 2+ which, as in the case of the analogous cobalt complex, exhibited a redox wave associated with the surface immobilized complex at a potential very similar to that of Os(tpy) 2 2+ in solution. Reaction of a gold electrode surface, previously modified with Os( tpy − SH )tppz 2+, with Co(tppz)(Cl) 2 gives rise to the appearance of a new voltammetric wave at +0.26 V, which we ascribe to binding of the cobalt to the pendant tppz to give rise to a structured interface of the type Au−S−tpy−Os−tppz−Co−tppz. An electrode modified with a layer of Cr( tpy − SH ) 2 3+ exhibits electrocatalytic behavior toward the reduction of nitric oxide (NO) in solution.