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

The bis-pyrazolato pyridine complex LCo­(PEt₃)₂ serves as a masked form of three-coordinate Co^II and shows diverse reactivity in its reaction with several potential outer sphere oxidants and oxygen atom transfer reagents. <i>N</i>-Methylmorpholine <i>N</i>-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<sup>+