Electron and Oxygen Atom Transfer Chemistry of Co(II) in a Proton Responsive, Redox Active Ligand Environment.

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The bis-pyrazolato pyridine complex LCo(PEt3)2 serves as a masked form of three-coordinate CoII and shows diverse reactivity in its reaction with several potential outer sphere oxidants and oxygen atom transfer reagents. N-Methylmorpholine N-oxide (NMO) oxidizes coordinated PEt3 from LCo(PEt3)2, but the final cobalt product is still divalent cobalt, in LCo(NMO)2. The thermodynamics of a variety of oxygen atom transfer reagents, including NMO, are calculated by density functional theory, to rank their oxidizing power. Oxidation of LCo(PEt3)2 with AgOTf in the presence of LiCl as a trapping nucleophile forms the unusual aggregate [LCo(PEt3)2Cl(LiOTf)2]2 held together by Li+ binding to very nucleophilic chloride on Co(III) and triflate binding to those Li+. In contrast, Cp2Fe+ effects oxidation to trivalent cobalt, to form (HL)Co(PEt3)2Cl+; proton and the chloride originate from solvent in a rare example of CH2Cl2 dehydrochlorination. An unexpected noncomplementary redox reaction is reported involving attack by 2e reductant PEt3 nucleophile on carbon of the 1e oxidant radical Cp2Fe+, forming a P-C bond and H+; this reaction competes in the reaction of LCo(PEt3)2 with Cp2Fe+.