Nitric oxide dioxygenation (NOD) reactions of CoIII-peroxo and NiIII-peroxo complexes: NOD versus NO activation

Abstract

Here, we report the nitric oxide dioxygenation (NOD) reactions of Ni^III-peroxo, [(12-TMC)Ni^III(O₂²⁻)]⁺ (Ni^III–O₂²⁻, 1), and Co^III-peroxo, [(12-TMC)Co^III(O₂²⁻)]⁺ (Co^III–O₂²⁻, 2), complexes bearing N-tetramethylated cyclam (TMC) ligands. The reaction of complex 1 with nitric oxide (NO) generates Ni^II-nitrato complex [(12-TMC)Ni^II(NO₃⁻)]⁺ (Ni^II-NO₃⁻, 3). However, in contrast to the NOD reaction of 1, compound 2 produces Co^II-nitrito complex, [(12-TMC)Co^II(NO₂⁻)]⁺ (Co^II-NO₂⁻, 4), with O₂ gas formation. Results suggested the formation of a presumptive Ni-peroxynitrite ([Ni-PN]⁺, [Ni-ONOO^–]⁺) intermediate, in the NOD reaction of 1, prior to the generation of Ni^II-NO₃⁻ (3). Although the end product of the reaction of 2 with NO is different from that of 1, the mechanistic study explored by using 2,4-di-tert-butylphenol (2,4-DTBP) suggested that the reaction of 2 with NO also occurs via a presumed Co-peroxynitrite ([Co-PN]⁺, [Co-ONOO⁻]⁺) intermediate. Furthermore, exploring these NOD reactions and tracking the oxygen atom using ¹⁸O-labeled hydrogen peroxide (H₂¹⁸O₂) revealed that two oxygen atoms of 3 (Ni^II-18O₂N¹⁶O⁻) and one oxygen atom of 4 (Co^II-18ON¹⁶O⁻) are derived from the peroxo (O₂²⁻) moieties of 1 and 2, respectively. Furthermore, we have explored the M-peroxo regeneration from NOD products (3 and 4), and for the first time, we have observed the formation of M^III–O₂²⁻ complexes, 1 and 2, upon treatment with H₂O₂ and triethylamine (TEA), respectively. This work is the first-ever report which showed the exact NO dioxygenase (NOD) enzyme catalytic cycle. Furthermore, we have also explored the NO activation of 3 and 4, and surprisingly, we observed the formation of Co-nitrosyl (5) from 4 upon reaction with NO; in contrast, 3 was unreactive towards NO. Evidence supporting the formation of [Ni-PN]⁺ and [Co-PN]⁺ intermediates is also presented.