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

Abstract

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.

A Nonheme, High-Spin {FeNO}8 Complex that Spontaneously Generates N2O

“…We previously showed that 1 exhibits spin-crossover behavior, with a low-spin ( S = 1/2) ground state and a thermally accessible high-spin ( S = 3/2) excited state. 8b The low-spin species exhibited a ν (N—O) at 1641 cm −1 at cryogenic temperatures, whereas the high-spin ( S = 3/2) ν (N–O) is observed at 1737 cm −1 above 150 K (CD 3 CN). The RR spectrum of 3 (Figure 4) (λ exc = 647 nm) reveals a prominent, broad band at 1588 cm −1 that shifts to 1518 cm −1 upon substitution with 15 N 18 O; this 70-cm −1 downshift is in agreement with an N-O harmonic oscillator model (calculated Δ ν = −71 cm −1 ), confirming the assignment of this band to the N-O stretch of the new {FeNO} 8 species.…”

“…Reduction of {FeNO} x (x = 6, 7) species to the reactive {FeNO} 8 or {FeN(H)O} 8 states is often invoked as a prerequisite for N 2 O formation in enzymatic systems. There has been considerable interest in the preparation and characterization of {FeNO} 8 /{FeN(H)O} 8 model complexes, 5 but their synthesis is challenging because of their instability.…”

“…There has been considerable interest in the preparation and characterization of {FeNO} 8 /{FeN(H)O} 8 model complexes, 5 but their synthesis is challenging because of their instability. A limited number of nonheme {FeNO} 8 complexes have been characterized, 5a–g and these complexes exhibit low-spin S = 0 ground states, 5a–f with one exception of a high-spin S = 1 complex.…”

“…This complex, [Fe(NO)(N3PyS)]BF 4 ( 1 ), prepared from addition of NO • (g) to the Fe II precursor [Fe II (CH 3 CN)(N3PyS)]BF 4 ( 2 ), 7 was crystallographically characterized and showed excellent reversibility for both one-electron oxidation and reduction processes by cyclic voltammetry. 8 Herein we describe the one-electron reduction chemistry of 1, which leads to the generation of a metastable, nonheme {FeNO} 8 complex, [Fe(NO)(N3PyS)] ( 3 ). This complex spontaneously generates N 2 O upon standing in solution at 23 °C and is the first example, to our knowledge, of a mononuclear {FeNO} 8 species that reacts to give N 2 O. Spectroscopic and computational investigations also indicate that complex 3 exhibits a rare high-spin S = 1 ground state.…”

“…8a The well-behaved reversibility of the redox couple centered at E 1/2 = −1.18 V suggested that the one-electron-reduced {FeNO} 8 species may be stable enough for synthesis and characterization by chemical methods. The one-electron reductant decamethylcobaltocene (CoCp * 2 ) has a redox potential of E 1/2 = −1.91 V (vs Fc + /Fc in CH 3 CN), 9 indicating that it should be thermodynamically competent to reduce the {FeNO} 7 complex.…”