Use of a macrocyclic tetracarbene ligand, which is topologically reminiscent of tetrapyrrole macrocycles though electronically distinct, has allowed for the isolation, X-ray crystallographic characterization and comprehensive spectroscopic investigation of a complete set of {FeNO}(x) complexes (x = 6, 7, 8). Electrochemical reduction, or chemical reduction with CoCp2, of the {FeNO}(7) complex 1 leads to the organometallic {FeNO}(8) species 2. Its crystallographic structure determination is the first for a nonheme iron nitroxyl {FeNO}(8) and has allowed to identify structural trends among the series of {FeNO}(x) complexes. Combined experimental data including (57)Fe Mössbauer, IR, UV-vis-NIR, NMR and Kβ X-ray emission spectroscopies in concert with DFT calculations suggest a largely metal centered reduction of 1 to form the low spin (S = 0) {FeNO}(8) species 2. The very strong σ-donor character of the tetracarbene ligand imparts unusual properties and spectroscopic signatures such as low (57)Fe Mössbauer isomer shifts and linear Fe-N-O units with high IR stretching frequencies for the NO ligand. The observed metal-centered reduction leads to distinct reactivity patterns of the {FeNO}(8) species. In contrast to literature reported {FeNO}(8) complexes, 2 does not undergo NO protonation under strictly anaerobic conditions. Only in the presence of both dioxygen and protons is rapid and clean oxidation to the {FeNO}(7) complex 1 observed. While 1 is stable toward dioxygen, its reaction with dioxygen under NO atmosphere forms the {FeNO}(6)(ONO) complex 3 that features an unusual O-nitrito ligand trans to the NO. 3 is a rare example of a nonheme octahedral {FeNO}(6) complex. Its electrochemical or chemical reduction triggers dissociation of the O-nitrito ligand and sequential formation of the {FeNO}(7) and {FeNO}(8) compounds 1 and 2. A consistent electronic structure picture has been derived for these unique organometallic variants of the key bioinorganic {FeNO}(x) functional units.

C-H bond activation mediated by oxo-iron (IV) species represents the key step of many heme and nonheme O-activating enzymes. Of crucial interest is the effect of spin state of the Fe(O) unit. Here we report the C-H activation kinetics and corresponding theoretical investigations of an exclusive tetracarbene ligated oxo-iron(IV) complex, [LFe(O)(MeCN)] (1). Kinetic traces using substrates with bond dissociation energies (BDEs) up to 80 kcal mol show pseudo-first-order behavior and large but temperature-dependent kinetic isotope effects (KIE 32 at -40 °C). When compared with a topologically related oxo-iron(IV) complex bearing an equatorial N-donor ligand, [LFe(O) (MeCN)] (A), the tetracarbene complex 1 is significantly more reactive with second order rate constants k' that are 2-3 orders of magnitude higher. UV-vis experiments in tandem with cryospray mass spectrometry evidence that the reaction occurs via formation of a hydroxo-iron(III) complex (4) after the initial H atom transfer (HAT). An extensive computational study using a wave function based multireference approach, viz. complete active space self-consistent field (CASSCF) followed by N-electron valence perturbation theory up to second order (NEVPT2), provided insight into the HAT trajectories of 1 and A. Calculated free energy barriers for 1 reasonably agree with experimental values. Because the strongly donating equatorial tetracarbene pushes the Fe-d orbital above d, 1 features a dramatically large quintet-triplet gap of ∼18 kcal/mol compared to ∼2-3 kcal/mol computed for A. Consequently, the HAT process performed by 1 occurs on the triplet surface only, in contrast to complex A reported to feature two-state-reactivity with contributions from both triplet and quintet states. Despite this, the reactive Fe(O) units in 1 and A undergo the same electronic-structure changes during HAT. Thus, the unique complex 1 represents a pure "triplet-only" ferryl model.

Flavodiiron nitric oxide reductases (FNORs) are a subclass of flavodiiron proteins (FDPs) capable of preferential binding and subsequent reduction of NO to NO. FNORs are found in certain pathogenic bacteria, equipping them with resistance to nitrosative stress, generated as a part of the immune defense in humans, and allowing them to proliferate. Here, we report the spectroscopic characterization and detailed reactivity studies of the diiron dinitrosyl model complex [Fe(BPMP)(OPr)(NO)](OTf) for the FNOR active site that is capable of reducing NO to NO [Zheng et al., J. Am. Chem. Soc. 2013, 135, 4902-4905]. Using UV-vis spectroscopy, cyclic voltammetry, and spectro-electrochemistry, we show that one reductive equivalent is in fact sufficient for the quantitative generation of NO, following a semireduced reaction mechanism. This reaction is very efficient and produces NO with a first-order rate constant k > 10 s. Further isotope labeling studies confirm an intramolecular N-N coupling mechanism, consistent with the rapid time scale of the reduction and a very low barrier for N-N bond formation. Accordingly, the reaction proceeds at -80 °C, allowing for the direct observation of the mixed-valent product of the reaction. At higher temperatures, the initial reaction product is unstable and decays, ultimately generating the diferrous complex [Fe(BPMP)(OPr)](OTf) and an unidentified ferric product. These results combined offer deep insight into the mechanism of NO reduction by the relevant model complex [Fe(BPMP)(OPr)(NO)] and provide direct evidence that the semireduced mechanism would constitute a highly efficient pathway to accomplish NO reduction to NO in FNORs and in synthetic catalysts.

In biology, high valent oxo-iron(IV) species have been shown to be pivotal intermediates for functionalization of C-H bonds in the catalytic cycles of a range of O-activating iron enzymes. This work details an electronic-structure investigation of [Fe(O)(L)(NCMe)] (L = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex 1) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based multireference calculations. The IRPD spectrum of complex 1 reveals the Fe-O stretching vibration at 832 ± 3 cm. By analyzing the Franck-Condon progression, we can determine the same vibration occurring at 616 ± 10 cm in the E(d → d) excited state. Both values are similar to those measured for [Fe(O)(TMC)(NCMe)] (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The low-temperature MCD spectra of complex 1 exhibit three pseudo A-term signals around 12 500, 17 000, and 24 300 cm. We can unequivocally assign them to the ligand field transitions of d → d, d → d, and d → d, respectively, through direct calculations of MCD spectra and independent determination of the MCD C-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [Fe(O) (SR-TPA)(NCMe)] (SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine), the excitations within the (FeO) core of complex 1 have similar transition energies, whereas the excitation energy for d → d is significantly higher (∼12 000 cm for [Fe(O)(SR-TPA)(NCMe)]). Our results thus substantiate that the tetracarbene ligand (L) of complex 1 does not significantly affect the bonding in the (FeO) unit but strongly destabilizes the d orbital to eventually lift it above d. As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet-triplet energy separation for complex 1, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed.

Reaction of [Fe(N-Et-HPTB)(CHCOS)](BF) (1) with (NO)(BF) produces a nonheme mononitrosyl diiron(II) complex, [Fe(N-Et-HPTB)(NO)(DMF)](BF) (2). Complex 2 is the first example of a [Fe{Fe(NO)}] species and is also the first example of a mononitrosyl diiron(II) complex that mediates the reduction of NO to NO. This work describes the selective synthesis, detailed characterization and NO reduction activity of 2 and thus provides new insights regarding the mechanism of flavodiiron nitric oxide reductases.

The iron(II) complex 1 of a macrocyclic tetracarbene binds NO to form a low-spin (S = (1)/2) {FeNO}(7) complex (2) with a linear FeNO unit and a short Fe-NO bond. IR, electron paramagnetic resonance, and Mössbauer spectroscopies as well as density functional theory calculations suggest some Fe(I)NO(+) character and reveal that the singly occupied molecular orbital of 2, resulting from the σ-antibonding interaction of Fe dz(2) and the NO lone pair, is largely iron-based. Reduction yields a quite stable {FeNO}(8) species (3); both 2 and 3 feature very low Mössbauer isomer shifts (∼0.0 mm·s(-1)).

The reduction of NO to N2O by flavodiiron nitric oxide reductases (FNORs) is related to the disruption of the defense mechanism in mammals against invading pathogens. The proposed mechanism for this catalytic reaction involves both nonheme mono- and dinitrosyl diiron­(II) species as the key intermediates. Recently, we reported an initial account for NO reduction activity of an unprecedented mononitrosyl diiron­(II) complex, [Fe2(N-Et-HPTB)­(NO)­(DMF)3]­(BF4)3 (1) (N-Et-HPTB is the anion of N,N,N′,N′-tetrakis­(2-(l-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane; DMF = dimethylformamide) with [FeII{FeNO}7] formulation [J. Am. Chem. Soc.201713914380]. Here we report the full account for the selective synthesis, characterization, and reactivity of FNOR model complexes, which include a dinitrosyl diiron­(II) complex, [Fe2(N-Et-HPTB)­(NO)2(DMF)2]­(BF4)3 (2) with [{FeNO}7]2 formulation and a related, mixed-valent diiron­(II, III) complex, [Fe2(N-Et-HPTB)­(OH)­(DMF)3]­(BF4)3 (3). Importantly, whereas complex 2 is able to produce 89% of N2O via a semireduced mechanism (1 equiv of CoCp2 per dimer = 50% of NO reduced), complex 1, under the same conditions (0.5 equiv of CoCp2 per dimer = 50% of NO reduced), generates only ∼50% of N2O. The mononitrosyl complex therefore requires superreduction for quantitative N2O generation, which constitutes an interesting dichotomy between 1 and 2. Reaction products obtained after N2O generation by 2 using 1 and 2 equiv of reductant were characterized by molecular structure determination and electron paramagnetic resonance spectroscopy. Despite several available literature reports on N2O generation by diiron complexes, this is the first case where the end products from these reactions could be characterized unambiguously, which clarifies a number of tantalizing observations about the nature of these products in the literature.

A disulfide-bridged diiron complex with [Fe-S-S-Fe] core, which represents an isomer of the common biological [2Fe-2S] ferredoxin-type clusters, was synthesized using strongly σ-donating macrocyclic tetracarbene capping ligands. Though the complex is quite labile in solution, single crystals were obtained, and the structure was elucidated by X-ray diffraction. The electron-rich iron-sulfur core is found to show rather unusual magnetic and electronic properties. Experimental data and density functional theory studies indicate extremely strong antiferromagnetic coupling (-J > 800 cm(-1)) between two low-spin iron(III) ions via the S2(2-) bridge, and the intense near-IR absorption characteristic for the [Fe-S-S-Fe] core was assigned to a S → Fe ligand-to-metal charge transfer transition.