Photoinitiated Reactivity of a Thiolate-Ligated, Spin-Crossover Nonheme {FeNO} ⁷ Complex with Dioxygen

The nonheme iron complex, [Fe(NO)(N3PyS)]BF ₄ , is a rare example of an {FeNO} ⁷ species that exhibits spin-crossover behavior. The comparison of X-ray crystallographic studies at low and high temperatures and variable-temperature magnetic susceptibility measurements show that a low-spin S = 1/2 ground state is populated at 0-150 K, while both low-spin S = 1/2 and high-spin S = 3/2 states are populated at T > 150 K. These results explain the observation of two N-O vibrational modes at 1737 and 1649 cm ^-1 in CD ₃ CN for [Fe(NO)(N3PyS)]BF ₄ at room temperature. This {FeNO} ⁷ complex reacts with dioxygen upon photoirradiation with visible light in acetonitrile to generate a thiolate-ligated, nonheme iron(III)-nitro complex, [Fe ^III (NO ₂ )(N3PyS)] ⁺ , which was characterized by EPR, FTIR, UV-vis, and CSI-MS. Isotope labeling studies, coupled with FTIR and CSI-MS, show that one O atom from O ₂ is incorporated in the Fe ^III -NO ₂ product. The O ₂ reactivity of [Fe(NO)(N3PyS)]BF ₄ in methanol is dramatically different from CH ₃ CN, leading exclusively to sulfur-based oxidation, as opposed to NO· oxidation. A mechanism is proposed for the NO· oxidation reaction that involves formation of both Fe ^III -superoxo and Fe ^III -peroxynitrite intermediates and takes into account the experimental observations. The stability of the Fe ^III -nitrite complex is limited, and decay of [Fe ^III (NO ₂ )(N3PyS)] ⁺ leads to {FeNO} ⁷ species and sulfur oxygenated products. This work demonstrates that a single mononuclear, thiolate-ligated nonheme {FeNO} ⁷ complex can exhibit reactivity related to both nitric oxide dioxygenase (NOD) and nitrite reductase (NiR) activity. The presence of the thiolate donor is critical to both pathways, and mechanistic insights into these biologically relevant processes are presented.