Ligand structural influences upon electrochemical reactivity: organic substituent effects upon (carboxylato)pentaamminecobalt(III) reductions at mercury and gold electrodes

Abstract

Abstract : The electroreduction kinetics of 33 carboxylatopentaamminecobalt(III) complexes containing a variety of aliphatic, aromatic, and heterocyclic substituents have been examined at mercury- and gold-aqueous interfaces and compared with the corresponding homogeneous reduction rates with Ru(NH3)6(2+) in order to examine the relationships between the substituent structure and electrochemical reactivity. Complexes having acyclic aliphatic groups yielded 'normal' outersphere reactivities on the basis of their similar relative rate constants at a given electrode potential in comparison with the corresponding homogeneous rate ratios. However, electrochemical reactivities that are enhanced by ca 10 to 10,000 fold on this basis were observed for complexes with ring-containing substituents. Similar unimolecular rate constants were nonetheless observed for the electroreduction of several of these complexes when electrostatically adsorbed at chloride-coated silver. This indicates that the observed catalysis at mercury and gold surfaces arises from reactant adsorption, i.e., from increased precursor stability, presumably associated with 'hydrophobic' or van der Waals ligand-surface interactions. Originator supplied keywords include: ligand-surface interactions, substituent effects, nonaqueous media.