Metal-catalyzed hydrogen atom transfer (MHAT) reactions transform alkenes to a diverse set of hydrofunctionalized products. Many of these catalytic reactions are hypothesized to go through a radical-polar crossover mechanism, with a postulated salen cobalt(IV) alkyl species facilitating carbon-heteroatom bond formation. To date, only indirect characterization of the one-electron oxidation products of alkylcobalt(III) chelates exists, and there is scant evidence supporting the intermediacy of salen cobalt(IV) alkyls in MHAT catalysis. Here, we describe the preparation and solution spectroscopic characterization of a catalytically relevant salen-supported cobalt(IV) sec-alkyl complex and demonstrate its reactivity with nucleophiles. Using EPR and ENDOR spectroscopies, we verify that the species has an intact cobalt-carbon bond and that the best description is as a low-spin cobalt(IV) complex. Surprisingly, the cobalt-carbon bond is strongerin the +4 than the +3 oxidation state, and it is easier to oxidize the cobalt(III) alkyl than the square-planar cobalt(II) precatalyst. Therefore, electron transfer between an alkylcobalt(III) and a cationic cobalt(III) complex is thermodynamically favorable, and is a viable mechanism for the formation of alkylcobalt(IV) during catalysis. We additionally demonstrate that under catalytically relevant conditions, these complexes react with alcohols to produce ethers as in MHAT catalysis. Our findings give support to the intermediacy of cobalt(IV) alkyl species in MHAT catalysis, and provide a stepping stone to further mechanistic studies.

Salen cobalt(IV) sec-alkyl complexes: Characterization and relevance to MHAT catalysis

ACS Spring 2022

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