Distinct Proton and Water Reduction Behavior with a Cobalt(III) Electrocatalyst Based on Pentadentate Oximes

Abstract

A new pentadentate oxime has been designed to drive the preferential coordination favored by Co(I) in catalysts used for proton/water reduction. The ligand incorporates water upon metal coordination and is water soluble. This Co(III) species is doubly reduced to Co(I) and exhibits H(+) reduction activity in the presence of weak acids in MeCN and evolves H2 upon protonation suggesting that the ligand design increases catalyst effectiveness. Superior catalysis is observed in water with a turnover number (TON) of 5700 over 18 h. However, the catalyst yields Co-based nanoparticles, indicating that the solvent media may dictate the nature of the catalyst.

Deactivation of a Cobalt Catalyst for Water Reduction through Valence Tautomerism

“…This corresponds to an overpotential of 0.54 V. Moreover, the onset potential of −0.95 V NHE closely resembles that of the Co II /Co I couple observed at E 1/2 =−1.08 V NHE obtained in acetonitrile. This observation confirms that the active species in catalysis is the Co I complex in accordance with the accepted mechanisms for proton reduction using cobalt metal complexes; catalysis is initiated by the reaction of Co I with a proton to form a Co III −H − hydride intermediate …”

“…Such catalysts must withstand drastic electronic and structural changes from high to low redox states required for the hydride formation that precedes H 2 evolution. To this end, cobalt complexes have been extensively studied because of the energetically affordable stepwise conversions from 3d 6 Co III to 3d 8 Co I and back to Co III −H − and Co II −H − hydride species . As such, mechanistic understanding of catalytic pathways, including those of deactivation, becomes a necessary condition to the development of robust catalysts.…”

“…Our group has studied the mechanisms of several proton and water reduction cobalt catalysts, including some phenolate‐rich Co III [N 2 O 3 ] catalysts that served as the stepping stone to much improved pyridine‐rich Co II/III [N 2 N py 3 ] catalysts for water reduction that display a turnover number (TON) greater than 7000 mol −1 . We have gathered evidence that some molecular catalysts such as cobalt oximes are converted into nanoparticulates through ligand hydrolysis; triggered by radical‐based mechanisms . Therefore, although the involvement of ligands in the catalytic cycle has been reported, we conclude that radical formation may have deleterious effects on H 2 production .…”

Bimetallic Cooperativity in Proton Reduction with an Amido‐Bridged Cobalt Catalyst

“…To date it is unclear what factors control metal cooperativity in proton reduction, and this lack of understanding prevents a more rational design of Co 2 catalysts. Continuing our long‐standing interest in the mechanisms of H 2 generation by Co catalysts, we hypothesize that cooperativity will be dependent on 1) the distance between the Co centers, 2) the relative topology of the coordination environments, and 3) the degree of orientation and overlap between redox‐active orbitals. To evaluate this hypothesis, we analyzed the catalytic potential of the bimetallic complex [Co II 2 (L 1 )(bpy) 2 ]ClO 4 ( 1 ), in which (L 1 ) 3− is the triply deprotonated ligand shown in Figure a, by means of electrochemical, spectroscopic, and computational methods.…”

Efficient water oxidation with electromodified Langmuir–Blodgett films of procatalytic [CoIII(N2O3)] metallosurfactants on electrodes

“…We have recently observed a similar situation where a pentadentate cobalt(III) [N 5 ] oxime species that retains its molecular nature in acetonitrile, is converted into nanoparticles in water within 1 h of electrocatalysis. [24] This prompted us to study the effect of cycling two electrodes; the first is coated with a monolayer of [Co III (L N2O3 )H 2 O], while the second was coated with nine layers of [Co III (L N2O3 )H 2 O]. For the 1-layer electrode there was a slight increase in current density (~20%) after 100 cycles (Figure 2).…”