Structure of the Co^I Intermediate of a Cobalt Pentapyridyl Catalyst for Hydrogen Evolution Revealed by Time‐Resolved X‐ray Spectroscopy

Abstract

Cobalt polypyridyls are highly efficient water-stable molecular catalysts for hydrogen evolution. The catalytic mechanism explaining their activity is under debate and the main question is the nature of the involvement of pyridyls in the proton transfer: the pentapyridyl ligand, acting as a pentadentate ligand, can provide stability to the catalyst or one of the pyridines can be involved in the proton transfer. Time-resolved Co K-edge X-ray absorption spectroscopy in the microsecond time range indicates that, for the [Co (aPPy)] catalyst (aPPy=di([2,2'-bipyridin]-6-yl)(pyridin-2-yl)methanol), the pendant pyridine dissociates from the cobalt in the intermediate Co state. This opens the possibility for pyridinium to act as an intramolecular proton donor. In the resting state, the catalyst returns to the original six-coordinate high-spin Co state with a pentapyridyl and one water molecule coordinating to the metal center. Such a bifunctional role of the polypyridyl ligands can be exploited during further optimization of the catalyst.

Insights into the mechanism of photosynthetic H2 evolution catalyzed by a heptacoordinate cobalt complex

“…Considering the pKa of pyridine in water (pKa = 5.3), protonation of the pendent pyridine is expected to occur at acidic pH resulting in a favourable proton relay for facilitating H-H bond formation, 13,42 as recently proposed by Smolentsev and co-workers for a similar Co-based polypyridyl catalyst. 22 The other two possible intermediates originated by the reduction of the two Co(I)-BPYH species have a positive charge of 1.38 a.u. on the Cobalt centre, indicating that only a very small fraction of the second electron reduces the metal, while, as expected, it largely delocalizes on the bi-coordinated BPY ligand that is not protonated.…”

“…Another important aspect is the role of the ligand, that can be protonated and therefore can actively participate in the catalytic process as observed for a cobalt pentadentate ligand synthesized by Wang and co-workers 21 and by a recent transient X-ray absorption study. 22 The investigation of the mechanistic profile of each catalyst is imperative for a comprehensive understanding of all the factors that influence the catalytic activity. Cobalt hydrides such as Co(III)-H or Co(II)-H generated from the protonation of Co(I) have been generally acknowledged as the key intermediates for the production of hydrogen, which can occur through either a heterolytic (a proton reacting with a cobalt hydride to evolve H2) or a homolytic (two cobalt hydrides giving reductive elimination to produce H2) mechanism.…”

Structure‐Functional Analysis of Hydrogen Production Catalyzed by Molecular Cobalt Complexes with Pentadentate Ligands in Aqueous Solutions

“…Long, Chang, Castellano, and co‐workers reported the detection of an active Co I species of 2a which displays a weak and broad absorption band between 450–700 nm during photocatalytic HER. [25b] Using time‐resolved Co K‐edge X‐ray absorption spectroscopy, Smolentsev and co‐workers reported the characterizations of the Co I intermediate of catalyst 2c during photocatalytic HER . Upon reduction, the pyridine group in the hexa‐coordinated high‐spin Co II form dissociates to yield the intermediate Co I species as a tetracoordinated Co center with a square‐planar geometry.…”

“…Upon reduction, the pyridine group in the hexa‐coordinated high‐spin Co II form dissociates to yield the intermediate Co I species as a tetracoordinated Co center with a square‐planar geometry. The resulting pyridinium from the protonation of the dissociated pyridine group was proposed as an active intramolecular proton donor to facilitate the formation of Co–H intermediates and subsequent H–H bond formation …”

Polar Substituents Enable Efficient Catalysis for a Class of Cobalt Polypyridyl Hydrogen Evolving Catalysts

“…The resulting tetradentate structure then enters the catalytic cycle. [36] In this study, we aim at understanding the influence of the dangling pyrid-6-yl and the hydroxy group. In catalysts 2, 3 and 5, the pyridyl group was replaced by an inactive hydrogen, a methyl or a phenyl group, respectively, whereas the À OH group was left in place.…”