Investigating the Role of Ligand Electronics on Stabilizing Electrocatalytically Relevant Low-Valent Co(I) Intermediates

Abstract

Cobalt complexes have shown great promise as electrocatalysts in applications ranging from hydrogen evolution to C−H functionalization. However, the use of such complexes often requires polydentate, bulky ligands to stabilize the catalytically active Co(I) oxidation state from deleterious disproportionation reactions to enable the desired reactivity. Herein, we describe the use of bidentate electronically asymmetric ligands as an alternative approach to stabilizing transient Co(I) species. Using disproportionation rates of electrochemically generated Co(I) complexes as a model for stability, we measured the relative stability of complexes prepared with a series of N,N-bidentate ligands. While the stability of Co(I)Cl complexes demonstrates a correlation with experimentally measured thermodynamic properties, consistent with an outer-sphere electron transfer process, the set of ligated Co(I)Br complexes evaluated was found to be preferentially stabilized by electronically asymmetric ligands, demonstrating an alternative disproportionation mechanism. These results allow a greater understanding of the fundamental processes involved in the disproportionation of organometallic complexes and have allowed the identification of cobalt complexes that show promise for the development of novel electrocatalytic reactions.

Nickel‐Catalyzed Chain‐Walking Cross‐Electrophile Coupling of Alkyl and Aryl Halides and Olefin Hydroarylation Enabled by Electrochemical Reduction

“…Further investigation of the nickel complex 5 were conducted. CV studies in DMA as a solvent and KBr as a supporting electrolyte showed more complex behavior (see the Supporting Information) . Thus, DFT calculations for the oxidative addition step at Ni 0 were conducted (Figure a; see the Supporting Information computational methods).…”

Electroreductive Cobalt‐Catalyzed Carboxylation: Cross‐Electrophile Electrocoupling with Atmospheric CO2

“…This could be seen as there are two reduction peaks and they could be plausibly assigned as E = À1.70 V vs. SCE for the reduction of cobalt(II) to cobalt(I) and E = À1.95 V vs. SCE for the reduction of cobalt(I) to cobalt(0) (Figure 3 b). [24] The reduction of cobalt(III) to cobalt(II) was not observed as it has a much higher potential, usually in the positive range. [25] These results indicate that the oxidative addition of the substrate onto the active cobalt catalyst is possibly not involved in the rate-determining step.…”

New Redox Strategies in Organic Synthesis by Means of Electrochemistry and Photochemistry

“…Minteer and Sigman utilized cyclic voltammetry (CV) and square wave voltammetry (SWV) to study the thermodynamic and kinetic profile of a series of Co I complexes with bidentate N , N -ligands that are relevant to electrocatalysis ( Figure 12 ). 50 In combination with multivariate linear regression analysis and various other experimental and theoretical techniques, the mechanism of Co I disproportionation—an undesired catalyst decomposition pathway—was studied in great detail. This study also led to the identification of an electronically asymmetric pyrox ligand ( 12 - 1 ) that both promotes desired oxidative addition of Co I to benzyl bromide and suppresses the deleterious disproportionation pathway.…”