Regeneration and Degradation in a Biomimetic Polyoxometalate Water Oxidation Catalyst

Abstract

Complete understanding of catalytic cycles is required to advance the design of water oxidation catalysts, but it is difficult to attain, due to the complex factors governing their reactivity and stability. In this study, we investigate the regeneration and degradation pathways of the highly active biomimetic water oxidation catalyst [Mn3+ 2Mn4+ 2V4O17(OAc)3]3–, thereby completing its catalytic cycle. Beginning with the deactivated species [Mn3+ 4V4O17(OAc)2]4– left over after O2 evolution, we scrutinize a network of reaction intermediates belonging to two alternative water oxidation cycles. We find that catalyst regeneration to the activated species [Mn4+ 4V4O17(OAc)2(OH)­(H2O)]− proceeds via oxidation of each Mn center, with one water ligand being bound during the first oxidation step and a second water ligand being bound and deprotonated during the final oxidation step. ΔΔG values for this last oxidation are consistent with previous experimental results, while regeneration within an alternative catalytic cycle was found to be thermodynamically unfavorable. Extensive in silico sampling of catalyst structures also revealed two degradation processes: cubane opening and ligand dissociation, both of which have low barriers at highly reduced states of the catalyst due to the presence of Jahn–Teller effects. These mechanistic insights are expected to spur the development of more efficient and stable Mn cubane water oxidation catalysts.

Closing Kok’s cycle of nature’s water oxidation catalysis

“…Ligand dissociations are ubiquitous in organometallic chemistry, mostly acting as pre-steps in substitution reactions for formation of catalytically active species as necessary intermediates 58 . The obtained barrier height for W2 detaching from Mn4(III) is quite similar to a recent report on a biomimetic polyoxometalate water oxidation catalyst 59 , and the reactivity is enabled by the presence of Jahn–Teller (J–T) effect at Mn(III) which extends the bonding distance of the J–T axial ligand and thereby facilitates its de-coordination. Besides, the coincidence of W2 protonation and W7 binding synergistically further weakens the W2 coordination to Mn4, because of the further elongated Mn4–W2 bond and the “structural trans effect” 60 , 61 in octahedral transition-metal complexes.…”

Coordination Tuning of Metal Porphyrins for Improved Oxygen Evolution Reaction

“…[21][22][23][24][25][26][27] As the coordination environment of a metal ion controls the metal-oxo reactivity, tuning the coordination to make metaloxo more active towards nucleophilic attack is an intriguing way to improve OER. However, although many transition metal complexes, including those of Mn, [26,[28][29][30] Fe, [27,[31][32][33][34] Co, [35][36][37][38][39][40] Ni, [41][42][43] and Cu, [44][45][46][47] have been identified as molecular OER catalysts, few studies have been shown to improve OÀ O bond formation via rational coordination tuning of metal ions. [27] Axial ligands can significantly affect the reactivity of its trans metal-oxo units via an electronic "push effect" (Figure 1a).…”