Electrocatalytic Behavior of Self-assembled Monolayer of a Novel Tetraazamacrocyclic Copper(II) Complex in Ascorbate Oxidation

Abstract

Self-assembled monolayers (SAMs) of alkanethiols on gold have been used as a powerful method to prepare a chemical interface which is stable and structurally welldefined monolayer with a controllable thickness and desirable function. These characteristics of a SAM make them ideal model systems to study both fundamental and practical issues such as catalysis, electro-optic devices, sensors, corrosion, lubrication, adhesion, molecular recognition, and electron/energy transfer. In particular, a SAM can offer specific properties and selectivity to the chemical interface if a terminal group of the SAM is redox active for the functions of the monolayer-modified electrode surface. The transition metal complexes of macrocyclic ligands have been extensively investigated in solution because they are very useful in design and development for anion recognition recepter as well as effective electrocatalyst. In addition, they play an important role in biological process catalyzing the dismutation of a toxic superoxide radical (O2 −) to hydrogen peroxide and water. However, there have been only a few studies on the SAMs of transition metal-macrocyclic ligand complex. Ohsaka et al. reported an electrocatalytic oxidation of hydrogen peroxide using SAM of Ni(II)-pentaazamacrocyclic complex on gold. Bilewitz and co-workers studied the role of SAM of a Cu(II)-tetraazamacrocyclic complex in ascorbate oxidation catalysis. With a view to developing new type of dismutase mimic and catalase active site containing transition metal ion, we have synthesized a novel tetraazamacrocyclic ligand L (shown in Figure 1) which can not only complex with a transition metal(II) ion in solution but also contains long alkylthiol as a head group for anchoring the electroactive macrocyclic complex to the gold electrode surface. In this communication, we report elecctrocatalytic behavior of the SAM consisted of Cu(II) complex of the ligand L in oxidation of ascorbic acid. There are considerable interests for this type of copper(II) complexes because of development of an effective superoxide dismutase mimic and in their immobilization on electrode in order to obtain new catalytically active interface. First of all, the electroactive Cu(II) complex of L (abbreviated as CuL) has been easily synthesized in situ by dropping 5 mM CuCl2 methanol solution into 1 mM L in methanol. In order to find out the ratio of metal to ligand in the CuL complex formation reaction, the absorbance of CuL was monitored and the complex showed an absorbance maximum at 600 nm. In the spectroscopic titration, the absorbance was increased up to the equivalent point and then was not changed to the continuous addition of Cu(II) solution. We have observed the stoichiometric equivalents of Cu cation and L at the equivalent point. This exactly indicates that Cu(II) complexes of 1 : 1 Cu(II) : L are produced in the titration reaction. In order to modify gold electrode with the SAM of CuL, a clean keyhole-shaped Au thin film electrode was immersed for 12 h in the methanolic mixture solution containing the same equivalents of CuCl2 and L (1 mM) after the solution was stirred for a few minutes. As an alternative to prepare the SAM of CuL on gold, we have tried to make coordination of Cu into the macrocyclic ring of L which has been already covalently immobilized onto the gold surface via Au-thiolate bond. From cyclic voltammetric measurement, this method was found to be not effective to obtain a full coverage of Cu-complexation at the interface of SAM of L. It might be due to compact two-