Electrochemical Detection of Transient Cobalt Hydride Intermediates of Electrocatalytic Hydrogen Production

Abstract

A large variety of molecular cobalt complexes are used as electrocatalysts for H2 production, but the key cobalt hydride intermediates are frequently difficult to detect and characterize due to their high reactivity. We report that a combination of variable scan rate cyclic voltammetry and foot-of-the-wave analysis (FOWA) can be used to detect transient Co(III)H and Co(II)H intermediates of electrocatalytic H2 production by [Co(II)(P(tBu)2N(Ph)2)(CH3CN)3](2+) and Co(II)(dmgBF2)2(CH3CN)2. In both cases, reduction of a transient catalytic intermediate occurs at a potential that coincides with the Co(II/I) couple. Each reduction displays quasireversible electron-transfer kinetics, consistent with reduction of a Co(III)H intermediate to Co(II)H, which is then protonated by acid to generate H2. A bridge-protonated Co(I) species was ruled out as a catalytic intermediate for Co(II)(dmgBF2)2(CH3CN)2 from voltammograms recorded at 1000 psi of H2. Density functional theory was used to calculate Co(III)-H and Co(II)-H bond strengths for both catalysts. Despite having very different ligands, the cobalt hydrides of both catalysts possess nearly identical heterolytic and homolytic Co-H bond strengths for the Co(III)H and Co(II)H intermediates.

Ru–protein–Co biohybrids designed for solar hydrogen production: understanding electron transfer pathways related to photocatalytic function

“…52–54 It is particularly unlikely that a CoBF 2 catalyst bound to a protein will diffusionally interact with another Co( iii )–H to proceed through the homo-coupling mechanism. 55,56 Protein aggregates can be excluded as intact complexes can be observed by ICP-AES after photocatalysis or transient optical experiments with similar metal binding before and after catalysis.…”

“…Thus, our experimental results favour the Co( iii )–H scheme for catalysis, which is supported by our results with PSI–Co biohybrids 57 as well as recent electrochemical studies on cobaloxime complexes. 53,54 We believe the addition of the second electron and proton to our system to be the rate limiting step in catalysis and further investigation of Co–hydride intermediates should help to determine if this is limited by the time needed to regenerate the Ru PS.…”

Directing the reactivity of metal hydrides for selective CO 2 reduction

“…As a result, additional reduction of [HPt(dmpe) 2 ](PF 6 ) (2) to another intermediate is not possible at the electrochemical potential required to reduce 3. Several other group 10 metal hydride complexes (26) along with multiple cobalt hydride complexes (84,88,(92)(93)(94) have the same property; reduction of the hydride complex ([LMH] n in Eq. 7) is more challenging than reduction of the parent complex ([LM] n+1 in Eq.…”

SNS versus SNN Pincer Ligands: Electrochemical Studies and Their Palladium(II) Complexes as Electro‐Catalyst for Proton Reduction

“…The overpotentials associated with the catalysed proton reductions are calculated (Eq. 2 [ii] ) to be 97 mV and 156 mV for 1 and 2 , respectively. …”

“…The overpotentials associated with the catalysed proton reductions are calculated (Eq. 2 [ii] ) [77][78] to be 97 mV and 156 mV for 1 and 2, respectively.…”