Realization of artificial photosynthesis requires fast and durable water splitting catalysts enabling to promote the most challenging multielectron water oxidation reaction (WOR) step 2H2O → O2 + 4e- + 4H+ for hydrogen production. For advanced design and development of a catalyst the knowledge of the reaction mechanisms and catalytic pathways is crucial. Here we report in situ X-ray spectroscopy study of recently reported pentanuclear water oxidation electrocatalyst [FeII4FeIII(μ-3-O)(μ-L)6]3+ (LH = 3,5-bis(2-pyridyl)pyrazole) (1) (Figure 1). We observed formation of Fe(V) species during bulk electrolysis in a custom-made electrochemical cell under conditions of electrocatalytic water oxidation using in situ X-ray absorption (XAS) and X-ray emission (XES) spectroscopy. Spectroscopically characterized Fe(V) catalytic species are exceedingly rare. The high -valent metal intermediates suggested to initiate the O-O bond formation in WORs are often remain undetected, presumably due to their high reactivity. During prolonged (&gt; 8 hours) bulk electrolysis we have observed gradual shifting of the Fe K-edge toward more oxidized state with no signs of the catalyst degradation. Our EXAFS data indicate that under applied potential (~ 1.8V vs NHE) the electrochemical transformations of (1) do not stop on Fe(IV) species but go further with formation of five-coordinated Fe(V) intermediates at concentration ~ 50%. These highly oxidized Fe(V) centers undergo WNA with subsequent formation of peroxo- species followed by oxygen release. For the first time, in situ spectroscopy of the electrolyzed solution detected the presence of an ultrashort, FeV=O moiety in this cluster, a crucial intermediate for O-O bond formation. Detected Fe(V) center with FeV=O distance at ~1.60Å is in good agreement with Fe(V) complexes characterized by XAS previously, including [FeV=O,OH(pytacn)] water oxidation catalyst trapped in liquid N2. Thus, we reveal a key specie preceding O-O formation in WOR catalyzed by (1).

Formation of Fe(V) intermediates in water oxidation reaction promoted by pentanuclear iron catalyst

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.