Strong Electronic Coupling of Molecular Sites to Graphitic Electrodes via Pyrazine Conjugation

Abstract

Glassy carbon electrodes were functionalized with redox-active moieties by condensation of o-phenylenediamine derivatives with o-quinone sites native to graphitic carbon surfaces. Electrochemical and spectroscopic investigations establish that these graphite-conjugated catalysts (GCCs) exhibit strong electronic coupling to the electrode, leading to electron transfer (ET) behavior that diverges fundamentally from that of solution-phase or surface-tethered analogues. We find that (1) ET is not observed between the electrode and a redox-active GCC moiety regardless of applied potential. (2) ET is observed at GCCs only if the interfacial reaction is ion-coupled. (3) Even when ET is observed, the oxidation state of a transition metal GCC site remains unchanged. From these observations, we construct a mechanistic model for GCC sites in which ET behavior is identical to that of catalytically active metal surfaces rather than to that of molecules in solution. These results suggest that GCCs provide a versatile platform for bridging molecular and heterogeneous electrocatalysis.

Computational Insights on the Electrocatalytic Behavior of [Cp*Rh] Molecular Catalysts Immobilized on Graphene for Heterogeneous Hydrogen Evolution Reaction

“…All results are compiled in Table 1. As can be noted, the DFT calculations afforded E red o of −1.07 and −1.05 V for complexes A1 and A2, respectively, which are in good agreement with the experimental values of −1.19 and −1.21 V, respectively 18,21,36 . However, a notable discrepancy for the calculated E red o of the heterogenized analogous is observed.…”

“…This new SOMO in turn may shield the SOMO-1 orbital, which is of the same characters as that of the HOMO of the A(2e)-Gr-H + , and hence may retard further processes for the HER. In fact, this non-metallic centered SOMO is more likely consistent with the non-metallic electrochemical one-electron reduction of the heterogenized catalyst that was observed experimentally 18 .…”

“…It must be mentioned that performing such evaluating protocols requires initially performing accurate DFT-based geometry optimizations for all species that are involved in the electrochemical reduction and catalytic cycle of interest. According to previously reported experimental results concerning the redox behavior of complexes of [Rh(Cp*) (L)Cl] + (L= bpy or phen), such complexes commonly undergo a metal-centered two-electron reduction (Rh III/I ) at approximately −1.19 V vs Fc +/0 ,with simultaneous dissociation of chloride ion (Cl − ) 18,21,22 ; whereas the heterogenized analogue undergoes a nonmetal-centered one-electron reduction at −1.29 V vs Fc +/0 with alike dissociation of Cl 18 . As such, we optimized the geometries of these species that are involved in the electrochemical reduction using DFT calculations.…”

“…This can be attributed to various reasons that could be either computational or even experimental such as scanning rate and type of electrodes and concentration of the supporting electrolytes. Nevertheless, the majority of halide-bound diimine complexes if [Cp*Rh III ] can generally undergo two-electron electrochemical reduction that is observed experimentally as a single process 18,21,22,36,41 . Further attempts were conducted to reduce the discrepancy between the calculated and the experimental value of the 1-electron reduction process.…”

“…covalent attachment of the molecular homogeneous catalysts to the surface of electrochemically active carbon materials. For example, Surendranath et al have reported on the immobilized heterogeneous catalyst of Rh metal complexes on graphite-conjugated catalyst linked via pyrazine groups 18 . In their work, they demonstrated experimentally that the homogeneous molecular catalysts [Rh III (cp*)(bpy)Cl] 1+ can exhibit different redox behavior compared to the heterogenized analogue in acetonitrile as they undergo two-electron metal-centered and one-electron non-metal-centered reduction pathways, respectively.…”