Add time:08/02/2019         Source:sciencedirect.com

Inspired by the active site structure of Co-HPCD (HPCD = homoprotocatechuate 2,3-dioxygenase), in which the catechol binds to the cobalt(II) center in its monoanionic form in a bidentate fashion, we report the synthesis and characterization of [PhTttBu]Co(3,5-DBCatH) [PhTttBu = phenyltris(tertbutylthiomethyl)borate; 3,5-DBCatH = 3,5-di-tert-butyl-catecholate monoanion], the first example of a well-characterized mononuclear cobalt(II) complex of a monoanionic catecholate ligand. The net reaction of hydrogen atom transfer (HAT) occurs immediately when [PhTttBu]Co(3,5-DBCatH) is exposed to O2, generating a previously reported five-coordinate Co(II)-semiquinonate complex, [PhTttBu]Co(3,5-DBSQ) (3,5-DBSQ = 3,5-di-tert-butyl-1,2-semiquinonate). At low temperature, O2 uptake by [PhTttBu]Co(3,5-DBSQ) was observed by electronic absorption spectroscopy. Upon warming to room temperature, [PhTttBu]Co(3,5-DBSQ) decays slowly under an O2 atmosphere releasing muconic anhydride, an intradiol cleavage product, in 16% yield as previously reported. Kinetic measurements reveal that the decay of [PhTttBu]Co(3,5-DBSQ) under excess O2 exhibits pseudo-first-order behavior. Mechanistic considerations involving Co(III)-superoxo and Co(III)-alkylperoxo intermediates are presented based on the above-mentioned data and the metal ion effect on the O2 reactivity.

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