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 CoCl2 of a gold electrode whose surface had been previously modified with the tpy-SH ligand ostensibly gives rise to Au/S-tpy-CoCl2. 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.