Characterization of the High-Spin Co(II) Intermediate Species of the O₂-Evolving Co₄O₄ Cubic Molecules

Abstract

An artificial oxygen-evolving complex (OEC) contains a tetranuclear CoO cubic cluster ligated with acetate and pyridine molecules, a light-activated Ru(bpy) moiety (bpy = 2,2'-bipyridine), and the sacrificial electron acceptor SO side. A recent EPR investigation of this system showed the formation of the high-spin Co(II) ion under visible-illumination conditions. It has been supported that this center originates from the cluster and is involved in the oxygen-evolving process. The present report is focused on the further characterization of the high-spin d configuration of the Co(II) species. The measurement of the EPR spectrum at a wider magnetic field range in comparison to that reported recently shows the presence of an additional signal that contributes to the spectrum of the Co(II) center. The theoretical simulation of this spectrum reveals that an isotropic g value and considerably small zero-field splitting parameters describe the high-spin Co(II) ion in a unique way, which supports a tetrahedral crystal field symmetry. On the basis of the spin-Hamiltonian parameters, the looping transitions that lead to the experimental EPR signals are determined. Additionally, a possible role of the symmetry of the Co(II) species and a proposed model that explains its formation during the O-evolving process of the CoO cubic molecules are discussed.

Recent Advances in First‐Row Transition Metal Clusters for Photocatalytic Water Splitting

“…This impurity oxidation to obtain the well‐known heterogeneous Co‐based OEC catalyst, which was an active and stable structure. The work showed that the existence of the Co(II) impurity was the predominant factor of the water oxidation activity of the Co 4 O 4 molecular cubane complexes . Meanwhile, Patzke and co‐workers introduced the active and stable [Co II 4 (dpy{OH}O) 4 (OAc) 2 (H 2 O) 2 ](ClO 4 ) 2 (Co 4 O 4 ‐dpk) compounds for the first time in 2017 .…”

“…From the view of the coordination chemistry, the cubane unit is the most widespread molecular structure type, and many of the 3d-metal clusters show the cubane unit. Therefore, a large series of metal clusters, such as Co/Cu/Ni/Ti-based cluster photocatalysts, [18,[28][29][30][31][32][33][34][35] were studied as photocatalytic water splitting catalysts.…”

“…The work showed that the existence of the Co (II) impurity was the predominant factor of the water oxidation activity of the Co 4 O 4 molecular cubane complexes. [29] Meanwhile, Patzke and co-workers introduced the active and stable Rong [49] The work indicated that the Co(II) centers of Co 4 O 4 -dpk oxidated to Co(III) is not a primary premise for photocatalytic oxygen evolution in the system. Therefore, the mechanism understanding of the Co 4 O 4 cubane complexes remains to be discussed.…”

“…[13] Co-based clusters have been widely used for photocatalytic water oxidation in recent years. [18,50,53,[112][113][114][115] {Co III 4 O 4 } cubane and Co-POM WOCs have increased the demand of synthesizing targeted clusters to study structure-activity relationship. The {Co II 3 Ln(OR) 4 } combined Ln 3 + centers with flexible aqua/acetate ligands to promote bioinspired WOC design and formed an active and stable WOCs (maximum TON/TOF values of 211/9 s À 1 ).…”

“…In 2015, Patzke and co‐workers prepared the [Co II 3 Ln(hmp) 4 (OAc) 5 H 2 O]({Co II 3 Ln(OR) 4 }; Ln=Ho−Yb, hmp=2‐(hydroxymethyl)pyridine) cubane series, which were bioinspired catalysts with active and stable water oxidation performances . Co‐based clusters have been widely used for photocatalytic water oxidation in recent years . {Co III 4 O 4 } cubane and Co‐POM WOCs have increased the demand of synthesizing targeted clusters to study structure‐activity relationship.…”