A new nonheme iron(II) complex, FeII(Me3TACN)((OSiPh2)2O) (1), is reported. Reaction of 1 with NO(g) gives a stable mononitrosyl complex Fe(NO)(Me3TACN)((OSiPh2)2O) (2), which was characterized by Mössbauer (δ=0.52 mm s−1, |ΔEQ|=0.80 mm s−1), EPR (S=3/2), resonance Raman (RR) and Fe K‐edge X‐ray absorption spectroscopies. The data show that 2 is an {FeNO}7 complex with an S=3/2 spin ground state. The RR spectrum (λexc=458 nm) of 2 combined with isotopic labeling (15N, 18O) reveals ν(N‐O)=1680 cm−1, which is highly activated, and is a nearly identical match to that seen for the reactive mononitrosyl intermediate in the nonheme iron enzyme FDPnor (ν(NO)=1681 cm−1). Complex 2 reacts rapidly with H2O in THF to produce the N‐N coupled product N2O, providing the first example of a mononuclear nonheme iron complex that is capable of converting NO to N2O in the absence of an exogenous reductant.

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Addition of dioxygen at low temperature to the nonheme ferrous complex Fe II (Me 3 TACN)((OSi Ph2 ) 2 O) (1) in 2-MeTHF produces a peroxo-bridged diferric complex Fe 2 III (μ-O 2 )(Me 3 TACN) 2 ((OSi Ph2 ) 2 O) 2 (2), which was characterized by UV−vis, resonance Raman, and variable field Mossbauer spectroscopies. Illumination of a frozen solution of 2 in THF with white light leads to homolytic O−O bond cleavage and generation of a Fe IV (O) complex 4 (ν(Fe=O) = 818 cm −1 ; δ = 0.22 mm s −1 , ΔE Q = 0.23 mm s −1 ). Variable field Mossbauer spectroscopy measurements show that 4 is a rare example of a high-spin S = 2 Fe IV (O) complex and the first synthetic example to be generated directly from O 2 . Complex 4 is highly reactive, as expected for a high-spin ferryl, and decays rapidly in fluid solution at cryogenic temperatures. This decay process in 2-MeTHF involves C−H cleavage of the solvent. However, the controlled photolysis of 2 in situ with visible light and excess phenol substrate leads to competitive phenol oxidation, via the proposed transient generation of 4 as the active oxidant.

Dinuclear iron centers with a bridging hydroxido or oxido ligand form active sites within a variety of metalloproteins. A key feature of these sites is the ability of the protein to control the structures around the Fe centers, which leads to entatic states that are essential for function. To simulate this controlled environment, artificial proteins have been engineered using biotin− streptavidin (Sav) technology in which Fe complexes from adjacent subunits can assemble to form [Fe III −(μ-OH)−Fe III ] cores. The assembly process is promoted by the site-specific localization of the Fe complexes within a subunit through the designed mutation of a tyrosinate side chain to coordinate the Fe centers. An important outcome is that the Sav host can regulate the Fe•••Fe separation, which is known to be important for function in natural metalloproteins. Spectroscopic and structural studies from X-ray diffraction methods revealed uncommonly long Fe•••Fe separations that change by less than 0.3 Å upon the binding of additional bridging ligands. The structural constraints imposed by the protein host on the di-Fe cores are unique and create examples of active sites having entatic states within engineered artificial metalloproteins.

Transition-metal-mediated reductive coupling of nitric oxide (NO (g) ) to nitrous oxide (N 2 O (g) ) has significance across the fields of industrial chemistry, biochemistry, medicine, and environmental health. Herein, we elucidate a density functional theory (DFT)-supplemented mechanism of NO (g) reductive coupling at a copper-ion center, [(tmpa)Cu I (MeCN)] + (1) {tmpa = tris(2pyridylmethyl)amine}. At −110 °C in EtOH (<−90 °C in MeOH), exposing)] 2+ (OO Xray ) complex. Complementary stopped-flow kinetic analysis of the reaction in MeOH reveals an initial mononitrosyl species [(tmpa)Cu(NO)] + (1-(NO)) that binds a second NO molecule, forming a dinitrosyl species [(tmpa)Cu II (NO) 2 ] (1-(NO) 2 ). The decay of 1-(NO) 2 requires an available starting complex 1 to form a dicopper-dinitrosyl species hypothesized to be [{(tmpa)Cu} 2 (μ-NO) 2 ] 2+ (D) bearing a diamond-core motif, en route to the formation of hyponitrite intermediate 2. In contrast, exposing 1 to NO (g) in 2-MeTHF/THF (v/v 4:1) at <−80 °C leads to the newly observed transient metastable dinitrosyl species [(tmpa)Cu II (NO) 2 ] (1-(NO) 2 ) prior to its disproportionation-mediated transformation to the nitrite product [(tmpa)Cu II (NO 2 )] + . Our study furnishes a near-complete profile of NO (g) activation at a reduced Cu site with tripodal tetradentate ligation in two distinctly different solvents, aided by detailed spectroscopic characterization of metastable intermediates, including resonance Raman characterization of the new dinitrosyl and hyponitrite species detected.