Topological Redox Isomers: Surface Chemistry of Zeolite-Encapsulated Co(salen) and [Fe(bpy)3] Complexes

The electroactivity of zeolite-encapsulated redox-active transition metal complexes, {M(L)}Z, was explored for Co(salen) and [Fe(bpy)3] formed in NaY zeolite (where salen ) N,N′-bis(salicylidene)ethylenediamine and bpy ) 2,2′-bipyridine). The zeolite boundary was characterized via X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry in nonaqueous electrolyte at either zeolite-modified electrodes (ZMEs) or a stirred microheterogeneous dispersion of the redox-modified zeolite. Voltammetric incongruities arising for {M(L)}Z studied as a ZME rather than as a dispersion are attributed to changes imposed on the redoxmodified zeolite by the mechanical force used to prepare a ZME. An increase in the time in which a mixture of {[Fe(bpy)3]}NaY and carbon are either ground or pressed produces improved peak resolution and an initial but short-lived increase in the magnitude of the voltammetric peak currents. Cyclic voltammetry of a stirred dispersion of {M(L)}Z particles at a large surface area electrode yields fewer complications attributable to the electrode binders, carbons, or mechanical handling necessary to prepare a zeolite-modified electrode. Unlike its response in a ZME, {Co(salen)}NaY gives stable voltammetry for hours when characterized in a microheterogeneous dispersion. Using terminology analogous to that established in the study of zeoliteassociated photochemical probes, we reconcile the range of voltammetric responses observed for a given redox-modified zeolite, both in our results and those in the literature, as due to the type of topological redox isomer expressing the electroactivity. The voltammetry obtained with both ZMEs and heterogeneous dispersions of zeolite-encapsulated transition metal complexes provides evidence for electroactivity restricted to boundary-associated complexes.