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

The nonheme iron complex, [Fe­(NO)­(N3PyS)]­BF4, 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 CD3CN for [Fe­(NO)­(N3PyS)]­BF4 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, [FeIII(NO2)­(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 O2 is incorporated in the FeIII–NO2 product. The O2 reactivity of [Fe­(NO)­(N3PyS)]­BF4 in methanol is dramatically different from CH3CN, 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 FeIII-superoxo and FeIII-peroxynitrite intermediates and takes into account the experimental observations. The stability of the FeIII-nitrite complex is limited, and decay of [FeIII(NO2)­(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.