Electrochemistry of Polymeric Cobaloxime-Containing Assemblies in Organic and Aqueous Solvents

We report on the structural and electrochemical properties of a heterogeneous-homogeneous assembly composed of molecular cobaloxime catalysts immobilized onto graphite electrodes via an intervening polyvinylpyridine surface coating. When these modified electrodes are immersed in an organic solvent (propylene carbonate containing 0.1 M tetrabutylammonium perchlorate as a supporting electrolyte) or basic aqueous solutions (0.1 M NaOH), cyclic voltammetry measurements enable determination of the CoIII/II peak potentials and CoII/I midpoint potentials of cobaloximes embedded within the polymeric architectures. Additionally, voltammetry measurements recorded using pH neutral aqueous solutions (0.1 M phosphate buffer) confirm the immobilized cobaloximes remain catalytically active for hydrogen production and operate at a turnover frequency of 1.6 s-1 when polarized at -0.35 V vs the H+/H2 equilibrium potential. Waveform analysis of redox features associated with immobilized cobaloximes indicates more repulsive interactions within the polymer film at pH neutral vs basic conditions, which is attributed to the increased fraction of pyridinium species at lower pH values. Our measurements also show the number of electrochemically active sites changes when measured in different solvent environments, indicating that electroactive loadings determined under non-catalytic solvent conditions are not necessarily representative of those under catalytic conditions and could thereby lead to misrepresentations of catalytic turnover frequencies.