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

• Published in: Langmuir 1996-09, Vol.12 (18), p.4455-4462
• Main Authors: Maskus, Michael, Abruña, Héctor D
• Format: Article
• Language: English
• Subjects: Adsorption; Chemistry; Electrochemistry; Exact sciences and technology; General and physical chemistry; Study of interfaces
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la960308x

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.