Mediated electrocatalysis with polyanthraquinone-functionalized monolayer-protected clusters

This paper describes the reduction of 1,1-dinitrocyclohexane by electrogenerated anthraquinone radical anions that are ligated to monolayer-protected gold clusters. Each cluster bears multiple (ca. 18) anthraquinone units. The clusters are prepared by place-exchange of a fraction of the cluster's original octanethiolate ligands with 1-(1,3-dithiapropyl)anthracene-9,10-dione. The electrocatalytic reduction currents are compared to those observed for anthraquinone monomer at an equivalent concentration. The electrocatalytic efficiency is larger for cluster-bound anthraquinone. The results are analyzed by digital simulation and by a discrete mathematical approach so as to determine the rate constant for electron transfer between the substrate and anthraquinone radical anion (cluster-bound and free). The rate constant is slightly larger for the free anthraquinone monomer. The enhancement in catalytic efficiency for cluster-bound anthraquinone arises from the smaller diffusion coefficient of cluster-bound anthraquinone, relative to that of the monomer, resulting in spatial compression of the reaction zone next to the electrode.