Spectroelectrochemistry of nickel(II) complexes of N,N′-bis(salicylaldehyde)–o-phenylenediamine and N,N-bis(2-hydroxy- 1-naphthaldehyde)–o-phenylenediamine using an optically transparent thin-layer electrode

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Abstract
The Schiff base nickel(II) complexes of N,N′-bis(salicylaldehyde)–o-phenylenediamine and N,N-bis(2-hydroxy-1-naphthaldehyde)–o-phenylenediamine provide an opportunity for the spectroelectrochemical investigation of the azomethazine and transition-metal chromophores using a gold-mesh optically-transparent thinlayer electrode (OTTLE) in protic solvents. Cyclic voltammetry of these compounds showed a well ordered electrochemical behaviour with reductions in both the metal and ligand. All complexes showed two well defined electrode couples, NiI/NiII and NiII/NiIII. The dependence of the electrochemical parameters on the solvent, supporting electrolyte, scan rate and the nature of the ligands is discussed. Application of the OTTLE cell technique to the study of the redox chemistry of the nickel(II) pigments tested proved efficient for the spectral and electrochemical characterization of the various oxidation states. The spectropotenotiostatic experiments provided redox potentials, n values, close to those obtained by controlled potential coulometry and in agreement with those values predicted from the bulk cyclic voltammetry. The estimated standard electrode potential (E0) values agreed well with those obtained by Nernstian analysis, and facilitated the acquisition of E and n values, which are not readily obtained by conventional techniques. This was possible even for quasireversible reactions where the peak separation does not yield accurate n values. Exhaustive electrolysis of the small volume of solution in the OTTLE ensured the quantitative generation of a particular redox state without interference from extraneous redox reactions. The effect of an additional aromatic ring is to provide an additional redox couple. The electrochemical and spectral behaviours were in good agreement, which suggests that the phenomenon of colour fading in the complex and electrochemical changes produce similar trends.

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