Amide-Based Nonheme Cobalt(III) Olefin Epoxidation Catalyst: Partition of Multiple Active Oxidants CoVO, CoIVO, and CoIIIOO(O)CR

A mononuclear nonheme cobalt(III) complex of a tetradentate ligand containing two deprotonated amide moieties, [Co(bpc)Cl2][Et4N] (1; H2bpc=4,5-dichloro-1,2-bis(2-pyridine-2-carboxamido)benzene), was prepared and then characterized by elemental analysis, IR, UV/Vis, and EPR spectroscopy, and X-ray crystallography. This nonheme CoIII complex catalyzes olefin epoxidation upon treatment with meta-chloroperbenzoic acid. It is proposed that complex 1 shows partitioning between the heterolytic and homolytic cleavage of an OO bond to afford CoVO (3) and CoIVO (4) intermediates, proposed to be responsible for the stereospecific olefin epoxidation and radical-type oxidations, respectively. Moreover, under extreme conditions, in which the concentration of an active substrate is very high, the CoOOC(O)R (2) species is a possible reactive species for epoxidation. Furthermore, partitioning between heterolysis and homolysis of the OO bond of the intermediate 2 might be very sensitive to the nature of the solvent, and the OO bond of the CoOOC(O)R species might proceed predominantly by heterolytic cleavage, even in the presence of small amounts of protic solvent, to produce a discrete CoVO intermediate as the dominant reactive species. Evidence for these multiple active oxidants was derived from product analysis, the use of peroxyphenylacetic acid as the peracid, and EPR measurements. The results suggest that a less accessible CoVO moiety can form in a system in which the supporting chelate ligand comprises a mixture of neutral and anionic nitrogen donors.