Redox-active ligands provide a promising strategy to overcome the intrinsic one-electron redox behavior of first-row transition metal complexes. The efficient storage or release of electrons can be facilitated by ligand-based molecular orbitals that are energetically accessible, or the redox-active metal and ligand can work synergistically to promote multi-electron transfers. In this way, higher energy intermediates may be avoided and electrocatalytic transformations, such as formate or methanol oxidation, may be mediated at lower overpotentials. Herein, we seek to combine the concepts of redox-active ligands and pendent proton-active groups for the design of new cobalt electrocatalysts capable of rapid proton and multi-electron transfers. A series of air and moisture stable cyclopentadienyl cobalt complexes [CpCo(R,RL2–)] have been synthesized bearing a dianionic ortho-phenylenediamido ligand, which can undergo oxidation to the semiquinonediiminato radical and quinonediimine forms. The addition of ureanyl arms to the phenylenediamido structure introduces tunable proton relay sites. Cyclic voltammetry studies with the [CpCo(Ph,PhL2–)] complex demonstrate the redox non-innocence of the ligand as revealed by a quasi-reversible two-electron oxidation around 0.7 V vs. [Cp*2Fe]+/0 in tetrahydrofuran. Comparatively, the reversible Co(III/II) couple appears at -0.71 V vs. [Cp*2Fe]+/0. This unusual electrochemical behavior may promote multi-electron transfer reactions, which, in tandem with the pendent ureanyl groups, may facilitate the oxidation of formate or methanol. In the presence of Brønsted acids, the CpCo(Ph,PhL2–) complex shows redox features consistent with reductive protonation to generate a hydride species and subsequent H2 evolution. Further investigations into the activity of these complexes for electrocatalytic H2 evolution and formate and methanol oxidation are underway.

Revealing reactivity of cobalt complexes bearing redox- and proton-active ligands based on the phenylenediamine backbone

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.