Heme-Like {CoNO}⁹, (STTP^•-2){CoNO}⁹, and {CoNO}¹⁰ Complexes Supported by 21-Thiaporphyrin Macrocycle

Using a weaker ligand field thiaporphyrin macrocycle (STTP) as a heme-like ligand, we succeeded in the isolation of an unprecedented complete series of {CoNO}ⁿ (n = 9 and 10 in the Enemark-Feltham notation) and a (STTP^•-2){CoNO}⁹ species. Specifically, electrochemical or chemical reduction of a {CoNO}⁹ (S_T = 1/2) species by potassium graphite (KC₈) results in first ligand-based reduction leading to a {CoNO}⁹ moiety antiferromagnetically coupled to a thiaporphyrin dianionic radical yielding an overall S_T = 0 ground state, and subsequent metal-based reduction affording an S_T = 1/2 complex comprised of a genuine {CoNO}¹⁰ unit ligated by a thiaporphyrin radical. Multiple analytical and spectroscopic measurements using SXRD, IR, NMR, SQUID, and EPR coupled to detailed DFT calculations support the assignment of a high-spin Co^II center in {CoNO}⁹ and (STTP^•-2){CoNO}⁹ and a high spin Co^I center in {CoNO}¹⁰. Furthermore, the reaction of the {CoNO}⁹ complex with Ph₃CSNO and PhNO was found to furnish cobalt C-diazeniumdiolate heme-like complexes, which provides an alternative route to accessing a hyponitrite-like intermediate in heme models.