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Isolation and Study of Ruthenium–Cobalt Oxo Cubanes Bearing a High-Valent, Terminal RuV–Oxo with Significant Oxyl Radical Character

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posted on 05.12.2019 by Jaruwan Amtawong, David Balcells, Jarett Wilcoxen, Rex C. Handford, Naomi Biggins, Andy I. Nguyen, R. David Britt, T. Don Tilley
High-valent RuV–oxo intermediates have long been proposed in catalytic oxidation chemistry, but investigations into their electronic and chemical properties have been limited due to their reactive nature and rarity. The incorporation of Ru into the [Co3O4] subcluster via the single-step assembly reaction of CoII(OAc)2(H2O)4 (OAc = acetate), perruthenate (RuO4), and pyridine (py) yielded an unprecedented Ru­(O)­Co33-O)4(OAc)4(py)3 cubane featuring an isolable, yet reactive, RuV–oxo moiety. EPR, ENDOR, and DFT studies reveal a valence-localized [RuV(S = 1/2)CoIII3(S = 0)O4] configuration and non-negligible covalency in the cubane core. Significant oxyl radical character in the RuV–oxo unit is experimentally demonstrated by radical coupling reactions between the oxo cubane and both 2,4,6-tri-tert-butylphenoxyl and trityl radicals. The oxo cubane oxidizes organic substrates and, notably, reacts with water to form an isolable μ-oxo bis-cubane complex [(py)3(OAc)4Co33-O)4Ru]–O–[RuCo33-O)4(OAc)4(py)3]. Redox activity of the RuV–oxo fragment is easily tuned by the electron-donating ability of the distal pyridyl ligand set at the Co sites demonstrating strong electronic communication throughout the entire cubane cluster. Natural bond orbital calculations reveal cooperative orbital interactions of the [Co3O4] unit in supporting the RuV–oxo moiety via a strong π-electron donation.