A mononuclear cobalt(III) carboxylate complex with a fully O-based coordination sphere: Co III -O bond homolysis and controlled radical polymerisation from [Co(acac) 2 (O 2 CPh)].

The addition of benzoyl peroxide to [Co(acac)] in a 1 : 2 ratio selectively produces [Co(acac)(OCPh)], a diamagnetic (NMR) mononuclear Co complex with an octahedral (X-ray diffraction) coordination geometry. It is the first reported mononuclear Co derivative with a chelated monocarboxylate ligand and an entirely O-based coordination sphere. The compound degrades in solution quite slowly by homolytic Co-OCPh bond cleavage upon warming above 40 °C to produce benzoate radicals and can serve as a unimolecular thermal initiator for the well-controlled radical polymerisation of vinyl acetate. Addition of ligands (L = py, NEt) induces benzoate chelate ring opening and formation of both and isomers of [Co(acac)(OCPh)(L)] for L = py under kinetic control, then converting quantitatively to the isomer, whereas the reaction is less selective and equilibrated for L = NEt. The py addition strengthens the Co-OCPh bond and lowers the initiator efficiency in radical polymerisation, whereas the NEt addition results in benzoate radical quenching by a redox process. In addition to clarifying the mechanism of the radical polymerisation redox initiation by peroxides and rationalizing the quite low efficiency factor for the previously reported [Co(acac)]/peroxide-initiated organometallic-mediated radical polymerisation (OMRP) of vinyl acetate, this investigation provides relevant information on the Co-O homolytic bond cleavage process.