Spectroscopic and reactivity studies on graphite-conjugated salen complexes

Abstract

Metal oxo intermediates are evoked and observed in a wide range of natural and man-made catalytic system. These varied systems demonstrate divergent reactivity modes dependent on perturbations to the electronic structure and reaction conditions. It is necessary to uncover electronic structure characterization of reactive metal oxo intermediates to provide an improved means of understanding how perturbations to the electronic structure will direct towards these competing pathways. Herein, we have developed a platform for the study of site isolated metal oxo intermediates with a unique electronic structure derived from a conjugated linkage between graphitic carbon edge sites and a chelated metal center. This linkage results in a high degree of electronic coupling between the isolated metal site and the band structure of the graphitic carbon material, referred to as graphite-conjugated catalysts (GCCs). Specifically, we have synthesized a pyrazine-linked conjugated salen-type ligand platform which has been metallated with Mn²⁺ and used to functionalize graphitic carbon electrodes and powder. XPS surface characterization on the N and Mn chemical environments and relative abundance indicate the desired surface condensation to form a pyrazinic linkage has proceeded as desired. Electrochemical characterization of functionalized electrodes by cyclic voltammogram show three distinct features. Two reversible redox features consistent with proton-coupled electron transfers at the pyrizinic linkages and a third feature, putatively assigned to the metal site. Future work on this project will involve 1) the expansion of the scope of metallation to additional first row transition metals, 2) thermal reactivity studies, 3) in-situ XAS studies, and 4) electrochemical oxygen-reduction reactivity studies.