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(O₂CPh)]

The addition of benzoyl peroxide to [Co-II(acac)(2)] in a 1 : 2 ratio selectively produces [Co-III(acac)(2)(O2CPh)], a diamagnetic (NMR) mononuclear Co-III complex with an octahedral (X-ray diffraction) coordination geometry. It is the first reported mononuclear Co-III derivative with a chelated monocarboxylate ligand and an entirely O-based coordination sphere. The compound degrades in solution quite slowly by homolytic Co-III-O2CPh bond cleavage upon warming above 40 degrees 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, NEt3) induces benzoate chelate ring opening and formation of both cis and trans isomers of [Co-III(acac)(2)(O2CPh)(L)] for L = py under kinetic control, then converting quantitatively to the cis isomer, whereas the reaction is less selective and equilibrated for L = NEt3. The py addition strengthens the Co-III-O2CPh bond and lowers the initiator efficiency in radical polymerisation, whereas the NEt3 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-II(acac)(2)]/peroxide-initiated organometallic-mediated radical polymerisation (OMRP) of vinyl acetate, this investigation provides relevant information on the Co-III-O homolytic bond cleavage process.