Mechanistic Studies of Hydrogen Evolution in Aqueous Solution Catalyzed by a Tertpyridine–Amine Cobalt Complex

Abstract

The ability of cobalt-based transition metal complexes to catalyze electrochemical proton reduction to produce molecular hydrogen has resulted in a large number of mechanistic studies involving various cobalt complexes. While the basic mechanism of proton reduction promoted by cobalt species is well-understood, the reactivity of certain reaction intermediates, such as Co(I) and Co(III)-H, is still relatively unknown owing to their transient nature, especially in aqueous media. In this work we investigate the properties of intermediates produced during catalytic proton reduction in aqueous solutions promoted by the [(DPA-Bpy)Co(OH2)](n+) (DPA-Bpy = N,N-bis(2-pyridinylmethyl)-2,20-bipyridine-6-methanamine) complex ([Co(L)(OH2)](n+) where L is the pentadentate DPA-Bpy ligand or [Co(OH2)](n+) as a shorthand). Experimental results based on transient pulse radiolysis and laser flash photolysis methods, together with electrochemical studies and supported by density functional theory (DFT) calculations indicate that, while the water ligand is strongly coordinated to the metal center in the oxidation state 3+, one-electron reduction of the complex to form a Co(II) species results in weakening the Co-O bond. The further reduction to a Co(I) species leads to the loss of the aqua ligand and the formation of [Co(I)-VS)](+) (VS = vacant site). Interestingly, DFT calculations also predict the existence of a [Co(I)(κ(4)-L)(OH2)](+) species at least transiently, and its formation is consistent with the experimental Pourbaix diagram. Both electrochemical and kinetics results indicate that the Co(I) species must undergo some structural change prior to accepting the proton, and this transformation represents the rate-determining step (RDS) in the overall formation of [Co(III)-H](2+). We propose that this RDS may originate from the slow removal of a solvent ligand in the intermediate [Co(I)(κ(4)-L)(OH2)](+) in addition to the significant structural reorganization of the metal complex and surrounding solvent resulting in a high free energy of activation.

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

“…The μs time constants for formation of the Co( i ) species in both biohybrids are similar to what has been observed in the literature for Co( i ) species in multimolecular systems with Ru and Re PS. 30,48,49 …”

“…Axially-linked or equatorially-linked Ru–Co supramolecular complexes form Co( i ) species that exhibit short lifetimes of 20–70 ps due to rapid charge-recombination in small synthetic architectures that cannot provide opportunities to capture or accumulate electrons needed for catalysis. 5,6,50 In multimolecular systems, the Co( i ) state of Co–polypyridyl catalysts with Ru PSs have reported lifetimes of 30–60 μs, 30,49 while the Co( i ) state of another Co–polypyridyl complex with a Re PS has been proposed to last from 2–200 ms. 48 Time-resolved X-ray absorption techniques using the CoBF 2 catalyst, [Ru(bpy) 3 ] 2+ as a PS, and methyl viologen as an electron relay show transient formation of a Co( i ) species over the time regime of 0.5–100 μs, 51 which is consistent with the time regime for the Co( i ) species in our protein hybrids.…”

Nickel Complexes of Pyridine‐Functionalized N‐Heterocyclic Carbenes: Syntheses, Structures, and Activity in Electrocatalytic Hydrogen Production

“…Dihydrogen is an environmentally friendly energy carrier as upon combustion it only produces H 2 O, In order to enable the establishment of a society based on dihydogen as an energy source, for many years researchers have focused on the search for efficient proton‐reduction catalysts, notably for catalysts that are not based on noble metals . In the field of bioinorganic chemistry the synthesis of structural models of hydrogenases is a common strategy to devise molecular catalysts for proton reduction , .…”

Application of Pulse Radiolysis to Mechanistic Investigations of Catalysis Relevant to Artificial Photosynthesis

“…Combining all reaction pathways into as ingle kinetic expression results in the dependence of the observed second-order rate constant (k obs )f or the disappearance of the singly reduced species as af unction of protonconcentration[Eq. (29)]:…”

“…[29] Kinetic resultso btained from PR experimentsa nd electrochemical data indicate that the Co I species must undergo some structuralc hange prior to accepting ap roton and this transformation represents the rate-determining step (RDS) in the overall formation of [Co …”