Replacing Pyridine with Pyrazine in Molecular Cobalt Catalysts: Effects on Electrochemical Properties and Aqueous H2 Generation

Abstract

Four new molecular Co(II)tetrapyridyl complexes were synthesized and evaluated for their activity as catalysts for proton reduction in aqueous environments. The pyridine groups around the macrocycle were substituted for either one or two pyrazine groups. Single crystal X-ray analysis shows that the pyrazine groups have minimal impact on the Co(II)–N bond lengths and molecular geometry in general. X-band EPR spectroscopy confirms the Co(II) oxidation state and the electronic environment of the Co(II) center are only very slightly perturbed by the substitution of pyrazine groups around the macrocycle. The substitution of pyrazine groups has a substantial impact on the observed metal- and ligand-centered reduction potentials as well as the overall H2 catalytic activity in a multimolecular system using the [Ru(2,2′-bipyridine)3]Cl2 photosensitizer and ascorbic acid as a sacrificial electron donor. The results reveal interesting trends between the H2 catalytic activity for each catalyst and the driving force for electron transfer between either the reduced photosensitizer to catalyst step or the catalyst to proton reduction step. The work presented here showcases how even the difference of a single atom in a molecular catalyst can have an important impact on activity and suggests a pathway to optimize the photocatalytic activity and stability of molecular systems.

Recent findings and future directions in photosynthetic hydrogen evolution using polypyridine cobalt complexes

“…, 5 vs. 8 for tetradentate complexes, 22 , 23 , and 28 vs. 18–20 for pentadentate examples, and 29–31 vs. 33–41 for amino–polypyridyl complexes); and (iii) the exploitation of chelating macrocyclic ligands ( 11–17 ). 55–57 Interestingly, the observation 70,71 of the largest maximum TONs when using complexes with hexadentate ligands ( 42–46 , Fig. 6 and Table 3) is fully consistent with this notion.…”

“…46 Furthermore, the presence of two bipyridine units in the chelating macrocyclic ligands of complexes 11–17 is presumably responsible for the highest performances being shown by these examples within the class of tetradentate complexes. 55–57 A similar trend is also seen when pentadentate polypyridine catalysts are considered. As a matter of fact, complexes featuring a bipyridine unit ( 22 and 23 ) clearly perform better than their corresponding molecular analogues with only a single pyridine ( 18–20 ).…”

“…3 and Table 1); 46,54 (ii) an increase in the steric hindrance of the tetradentate ligand decreases the activity of the corresponding cobalt complex ( 5 > 1 > 7 and 8 > 9 > 10 ), 46 supporting the hypothesis previously envisioned that a large deviation from (pseudo)octahedral geometry is detrimental to efficient catalysis; (iii) chelating macrocyclic ligands yield highly active catalysts ( 11–17 ); (iv) in examples utilizing macrocycles, the catalytic efficiencies depend on the presence/absence of open positions ( e.g. , 12 vs. 13 ) and the structural distortion of the resulting cobalt complex; 55–57 and (v) the introduction of non-coordinating groups capable of hydrogen bonding, thus possibly acting as intramolecular proton shuttles, does not seem to be a valuable strategy for improving the catalytic activity, as proved by complexes 33–37 . 42…”

“…Subsequently, a series of similar complexes ( 14–17 ) was prepared featuring redox-active pyrazine groups in place of the pyridine moieties. 57 Photolysis under comparable conditions as those used for the parent complex 13 yielded hydrogen with maximum TONs of 629, 1569, 272, and 324 and TOFs of 450, 3419, 376, and 538 h −1 for 14 , 15 , 16 , and 17 , respectively.…”

“…Interestingly, comparable performances as those obtained using such amino–polypyridyl cobalt complexes can also be attained when using polypyridine-only analogues featuring flexible ligands, such as complexes 13 , 15 , 22 , 23 , and 28 . 35,56,57,62…”