Observation of O–O Bond Forming Step of Molecular Co₄O₄ Cubane Catalyst for Water Oxidation by Rapid-Scan FT-IR Spectroscopy

Abstract

Monitoring of water oxidation by the established Co cubane catalyst Co4O4(OAc)4(py)4 (abbrev. Co4O4, OAc = acetate, py = pyridine) by time-resolved rapid-scan ATR FT-IR spectroscopy revealed the O-O bond forming intermediate by its O-O stretch mode at 833 cm-1. The catalysis was either driven in alkaline solution by a visible light sensitizer, or in hydroxide (OH-) containing acetonitrile solution using the one-electron oxidized cubane as the sole source of charge. The μ-peroxido structure of the intermediate was established by 18 O isotopic labeling, and its catalytic relevancy and role in the O2 evolving cycle revealed by the kinetic relationship with decreasing cubium ([Co4O4] +) and recovery of Co4O4 complex.

Formation of CoIV=O intermediate at the Boundary of the “Oxo-wall” Induces Water Oxidation

“…Co III -OOH/Co III (CH 3 CO 2 H) mono ] 0 peroxide is ~762 cm -1 and is insensitive to O-OH to O-OD exchange(Table 2). Thus, this type of end-on peroxide also agrees with reported FT-IR29 . O-OH vibration (~807 cm -1 ) of the oxidized [Co III 2 O 4 Co IV -OOH/Co III (CH 3 CO 2 H) mono ] + form is highly plausible under applied potential and better agrees with the FT-IR experiment.…”

“…Recent time-resolved rapid-scan IR observation of photo-oxidized cubane at pH=12.3 indicates no free acetate at reaction conditions, suggesting that all acetates remain at least as the terminal ligands to the cubane. 29 However, this observation contradicts the paper's following interpretations: assigning ~833 cm -1 band to Co-OO-Co peroxo species. Thus, we explored [Co III 2 O 4 Co IV =O/Co IV (CH 3 CO 2 ) mono ] 0 reaction with water via WNA mechanism which has negative (-0.27 eV) DG in the calculations done with 15% of Hartree Fock exchange (Table 1, Fig.…”

“…We computed bridging peroxide (Co III 2 O 4 Co III -OO-Co III ) that was proposed to explain FT-IR results 29 but obtained ~718 cm -1 . This O-O bond frequency signi cantly deviates from the experimental ~833 cm -1 value.…”

“…15,25,26,27,28 The peroxo-intermediates of Co and other transition metal oxides participating in WOR were detected using mostly different IR spectroscopic techniques. 29 Besides the Co IV /Co IV formation 21 the possibilities of Co III /Co V and cofacial hydroxo-oxospecies Co III (OH)/Co IV have been considered. 21,23 A similar conformational motif of the edge sites was proposed for the electrocatalytic intermediates of the Co oxides and Co-Pi catalytic lms.…”

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

“…The in situ rapid-scan FT-IR spectroscopic investigations by Frei and co-workers indicated that the photo-driven water oxidation by Co 4 O 4 (Py) 4 (OAc) 4 cubane involves a Co III -(mperoxide)-Co III species at the O-O bond formation step, which is preceded by a Co III Co IV intermediate. 127 In 2017, our group reported a highly intact and stable Co-dpk (dpk: di(2-pyridyl)ketone) cubane WOC 45 ( Fig. 5), displaying the characteristic {H 2 O-Co 2 (OR) 2 -OH 2 } edge-site motif that catalyses photo-driven water oxidation.…”