Electrochemical Characteristics of Self-Assembled Monolayers of a Novel Nickel(II) Pentaazamacrocyclic Complex on a Gold Electrode

Abstract

Ethyl disulfide possessing two nickel(II) pentaazamacrocyclic redox centers at its end carbon atoms, 1, adsorbs on gold electrodes from methanol solutions and yields stable, electroactive self-assembled monolayers (SAMs). Cyclic voltammetry of SAMs of 1 shows a stable redox wave at 0.55 V in aqueous 0.1 M Na2SO4 (pH 2), corresponding to the Ni2+/3+ redox reaction. For SAMs of 1 with the surface coverage (Γ) of nickel(II) redox centers of (1.41 ± 0.04) × 10-10 mol cm-2, the cyclic voltammograms (CV) show very small peak-to-peak separations (ΔEp < 20 mV) with the ΔEfwhm (full-width at half-maximum of CV peaks) values ranging from 140 to 150 mV, indicating electrostatic repulsive interactions among the surface-immobilized nickel(II) redox centers. The constructed SAM of 1 was defined as a square close-packed monolayer with the polyazamacrocyclic ring of the nickel(II) redox center oriented perpendicular to the electrode surface. Coadsorption of 1 with electroinactive alkyl/aryl disulfides furnishes mixed monolayers, and the Γ of the electroactive nickel(II) redox centers can be varied by the mole ratio of 1 and coadsorbate in the adsorption solution. In mixed monolayers, the ΔEfwhm decreases as the Γ of 1 decreases from the limiting monolayer surface coverage, suggesting a homogeneous distribution of 1 and coadsorbate in the constructed mixed monolayers. The formal potentials (E°‘) of the solution-dissolved 1 and the self-assembly of 1 with different Γ values, equivalent to 0.3−1 monolayer, are nearly equal, evidencing the same microenvironment around the nickel(II) redox centers in both the self-assembly and the solution phase. Exchange between the surface-immobilized gold−alkanethiolates with disulfides in solution has been observed and was found to accompany ordering of the monolayer. The apparent rate constant for the heterogeneous electron-transfer reaction, kapp, has been determined by fast-scan cyclic voltammetry to be 1.3 × 103 s-1 for the SAM of 1. An increase in the kapp with a decrease in the Γ of 1 in mixed monolayers was observed.