Structure–Activity and Stability Relationships for Cobalt Polypyridyl‐Based Hydrogen‐Evolving Catalysts in Water

Abstract

A series of eight new and three known cobalt polypyridyl‐based hydrogen‐evolving catalysts (HECs) with distinct electronic and structural differences are benchmarked in photocatalytic runs in water. Methylene‐bridged bis‐bipyridyl is the preferred scaffold, both in terms of stability and rate. For a cobalt complex of the tetradentate methanol‐bridged bispyridyl–bipyridyl complex [CoIIBr(tpy)]Br, a detailed mechanistic picture is obtained by combining electrochemistry, spectroscopy, and photocatalysis. In the acidic branch, a proton‐coupled electron transfer, assigned to formation of CoIII−H, is found upon reduction of CoII, in line with a pKa(CoIII−H) of approximately 7.25. Subsequent reduction (−0.94 V vs. NHE) and protonation close the catalytic cycle. Methoxy substitution on the bipyridyl scaffold results in the expected cathodic shift of the reduction, but fails to change the pKa(CoIII−H). An analysis of the outcome of the benchmarking in view of this postulated mechanism is given along with an outlook for design criteria for new generations of catalysts.

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

“…[4] Unfortunately,c obaloxime catalysts are not very stable in water and degradationo f the catalystt hrough nanoparticle formation was reported in electrochemical assays. [14][15][16][17][18][19][20] The [Co II (aPPy)] catalyst has ap entadentatep olypyridyl ligand.I nt he initial Co II state (Scheme 1) in water,t he metal is coordinated by the ligand, consisting of the two 2,2'-bipyridine-6-yl( bipy) subunitsa nd one single pyridin-2-yl (py), which are bridged by am ethanol group, and as olvent water mole- Figure 1. AT ON of 30 000 [13] and at urnover frequency (TOF) of 10 000 h À1 have been reported [14] for as ystem based on [Co II (aPPy)] ( Figure 1).…”

“…[14][15][16][17][18][19][20] The [Co II (aPPy)] catalyst has ap entadentatep olypyridyl ligand.I nt he initial Co II state (Scheme 1) in water,t he metal is coordinated by the ligand, consisting of the two 2,2'-bipyridine-6-yl( bipy) subunitsa nd one single pyridin-2-yl (py), which are bridged by am ethanol group, and as olvent water mole- Figure 1. [20,23] In this mechanism, the first intermediate is a Co I complex, which can be protonated to form aC o III Hh ydride (Scheme1,t op). The concentration of [Co II (aPPy)] is 0.8 mm.Inset showsthe DFT-optimized structure.…”

“…[20] Below pH 5, the effective concentration of ascorbate drops and limits the supply of electrons from the photochemical reaction. [Co II (aPPy)] has the highest rate of H 2 productiona tp H5.0.…”

“…Such progress was rendered possible by an empiricals earch for optimal ligands and conditions by various research groups. [14][15][16][17][18][19][20] The [Co II (aPPy)] catalyst has ap entadentatep olypyridyl ligand.I nt he initial Co II state (Scheme 1) in water,t he metal is coordinated by the ligand, consisting of the two 2,2'-bipyridine-6-yl( bipy) subunitsa nd one single pyridin-2-yl (py), which are bridged by am ethanol group, and as olvent water mole- The concentration of [Co II (aPPy)] is 0.8 mm.Inset showsthe DFT-optimized structure. Cgray;Hwhite; Ored;Nlight blue;Codark blue.…”

“…[14] One possible mechanismf or hydrogen evolution by the [Co II (aPPy)] catalysti sa nalogoust ot he mechanism suggested for cobaloxime [21,22] and cobalt tetrapyridyl complexes. [20,23] In this mechanism, the first intermediate is a Co I complex, which can be protonated to form aC o III Hh ydride (Scheme1,t op). Further reaction paths are under debate in the literature and depend on the precise experimental conditions, such as catalyst concentration, solvent, and pH.…”