Organometallic µ-Nitridodiiron Complexes in Oxidation States Ranging from (III/III) to (IV/IV).

Iron complexes with nitrido ligands are of interest as molecular analogues of key intermediates during N2-to-NH3 conversion in industrial or enzymatic processes. Dinuclear iron complexes with a bridging nitrido unit are mostly known in relatively high oxidation states (III/IV or IV/IV), originating from the decomposition of azidoiron precursors via high-valent Fe≡N intermediates. The use of a tetra-NHC macrocyclic scaffold ligand (NHC = N-heterocyclic carbene) has now allowed for the isolation of a series of organometallic µ-nitridodiiron complexes ranging from the mid-valent FeIII-N-FeIII (1) via mixed-valent FeIII-N-FeIV (type 4) to the high-valent FeIV-N-FeIV (type 5) species that are interconverted at moderate potentials, accompanied by axial ligand binding at the FeIV sites. Magnetic measurements and electron paramagnetic resonance spectroscopy showed the homovalent complexes to be diamagnetic and the mixed-valent system to feature an S = 1/2 ground state due to very strong antiferromagnetic coupling. The bonding in the Fe-N-Fe moiety has been further probed by crystallographic structure determination, 57Fe Mössbauer and UV-vis spectroscopies, as well as density functional theory computations, which revealed high covalency and nearly identical Fe-N distances across this redox series. The latter has been rationalized in terms of the nonbonding nature of the combination of Fe dz2 atomic orbitals from which electrons are successively removed upon oxidation, and these redox processes are best described as being metal-centered. The tetra-NHC-ligated µ-nitridodiiron series complements a set of related complexes with single-atom µ-oxido and µ-phosphido bridges, but the Fe-N-Fe core exhibits a comparatively high stability over several oxidation states. This promises interesting applications in view of the manifold catalytic uses of µ-nitridodiiron complexes based on macrocyclic N-donor porphinato(2-) or phthalocyaninato(2-) ligands.

Functional Models for the Mono- and Dinitrosyl Intermediates of FNORs: Semireduction versus Superreduction of NO.

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 [Jana et al. J. Am. Chem. Soc. 2017, 139, 14380]. 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.

Disproportionation Equilibrium of a µ-Oxodiiron(III) Complex Giving Rise to C-H Activation Reactivity: Structural Snapshot of a Unique Oxoiron(IV) Adduct.

µ-Oxodiiron(III) species are air-stable and unreactive products of autoxidation processes of monomeric heme and non-heme iron(II) complexes. Now, the organometallic [(LNHC )FeIII -(µ-O)-FeIII (LNHC )]4+ complex 1 (LNHC is a macrocyclic tetracarbene) is shown to be reactive in C-H activation without addition of further oxidants. Studying the oxidation of dihydroanthracene, it was found that 1 thermally disproportionates in MeCN solution into its oxoiron(IV) (2) and iron(II) components; the former is the active species in the observed oxidation processes. Possible cleavage scenarios for 1 are shown by scrambling experiments and structural characterization of an unprecedented adduct of 1 and oxoiron(IV) complex 2. Kinetic analysis gave an equilibrium constant for the disproportionation of 1, which is very small (Keq =7.5±2.5×10-8 m). Increasing Keq might by a useful strategy for circumventing the formation of dead-end µ-oxodiiron(III) products during Fe-based homogeneous oxidation catalysis.

Spin-State Variations of Iron(III) Complexes with Tetracarbene Macrocycles.

Starting from their six-coordinate iron(II) precursor complexes [L8R Fe(MeCN)]2+ , a series of iron(III) complexes of the known macrocyclic tetracarbene ligand L8H and its new octamethylated derivative L8Me , both providing four imidazol-2-yliden donors, were synthesized. Several five- and six-coordinate iron(III) complexes with different axial ligands (Cl- , OTf- , MeCN) were structurally characterized by X-ray diffraction and analyzed in detail with respect to their spin state variations, using a bouquet of spectroscopic methods (NMR, UV/Vis, EPR, and 57 Fe Mößbauer). Depending on the axial ligands, either low-spin (S=1/2) or intermediate-spin (S=3/2) states were observed, whereas high-spin (S=5/2) states were inaccessible because of the extremely strong in-plane σ-donor character of the macrocyclic tetracarbene ligands. These findings are reminiscent of the spin state patterns of topologically related ferric porphyrin complexes. The ring conformations and dynamics of the macrocyclic tetracarbene ligands in their iron(II), iron(III) and µ-oxo diiron(III) complexes were also studied.

The Semireduced Mechanism for Nitric Oxide Reduction by Non-Heme Diiron Complexes: Modeling Flavodiiron Nitric Oxide Reductases.

Flavodiiron nitric oxide reductases (FNORs) are a subclass of flavodiiron proteins (FDPs) capable of preferential binding and subsequent reduction of NO to N2O. 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 [Fe2(BPMP)(OPr)(NO)2](OTf)2 for the FNOR active site that is capable of reducing NO to N2O [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 N2O, following a semireduced reaction mechanism. This reaction is very efficient and produces N2O with a first-order rate constant k > 102 s-1. 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 [Fe2(BPMP)(OPr)2](OTf) and an unidentified ferric product. These results combined offer deep insight into the mechanism of NO reduction by the relevant model complex [Fe2(BPMP)(OPr)(NO)2]2+ and provide direct evidence that the semireduced mechanism would constitute a highly efficient pathway to accomplish NO reduction to N2O in FNORs and in synthetic catalysts.

Functional Mononitrosyl Diiron(II) Complex Mediates the Reduction of NO to N2O with Relevance for Flavodiiron NO Reductases.

Reaction of [Fe2(N-Et-HPTB)(CH3COS)](BF4)2 (1) with (NO)(BF4) produces a nonheme mononitrosyl diiron(II) complex, [Fe2(N-Et-HPTB)(NO)(DMF)3](BF4)3 (2). Complex 2 is the first example of a [FeII{Fe(NO)}7] species and is also the first example of a mononitrosyl diiron(II) complex that mediates the reduction of NO to N2O. 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.

Nonclassical Single-State Reactivity of an Oxo-Iron(IV) Complex Confined to Triplet Pathways.

C-H bond activation mediated by oxo-iron (IV) species represents the key step of many heme and nonheme O2-activating enzymes. Of crucial interest is the effect of spin state of the FeIV(O) unit. Here we report the C-H activation kinetics and corresponding theoretical investigations of an exclusive tetracarbene ligated oxo-iron(IV) complex, [LNHCFeIV(O)(MeCN)]2+ (1). Kinetic traces using substrates with bond dissociation energies (BDEs) up to 80 kcal mol-1 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, [LTMCFeIV(O) (MeCN)]2+ (A), the tetracarbene complex 1 is significantly more reactive with second order rate constants k'2 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-dx2-y2 orbital above dz2, 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 FeIV(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.

Model of the MitoNEET [2Fe-2S] Cluster Shows Proton Coupled Electron Transfer.

MitoNEET is an outer membrane protein whose exact function remains unclear, though a role of this protein in redox and iron sensing as well as in controlling maximum mitochondrial respiratory rates has been discussed. It was shown to contain a redox active and acid labile [2Fe-2S] cluster which is ligated by one histidine and three cysteine residues. Herein we present the first synthetic analogue with biomimetic {SN/S2} ligation which could be structurally characterized in its diferric form, 52-. In addition to being a high fidelity structural model for the biological cofactor, the complex is shown to mediate proton coupled electron transfer (PCET) at the {SN} ligated site, pointing at a potential functional role of the enzyme's unique His ligand. Full PCET thermodynamic square schemes for the mitoNEET model 52- and a related homoleptic {SN/SN} capped [2Fe-2S] cluster 42- are established, and kinetics of PCET reactivity are investigated by double-mixing stopped-flow experiments for both complexes. While the N-H bond dissociation free energy (BDFE) of 5H2- (230 ± 4 kJ mol-1) and the free energy ΔG°PCET for the reaction with TEMPO (-48.4 kJ mol-1) are very similar to values for the homoleptic cluster 4H2- (232 ± 4 kJ mol-1, -46.3 kJ mol-1) the latter is found to react significantly faster than the mitoNEET model (data for 5H2-: k = 135 ± 27 M-1 s-1, ΔH‡ = 17.6 ± 3.0 kJ mol-1, ΔS‡ = -143 ± 11 J mol-1 K-1, and ΔG‡ = 59.8 kJ mol-1 at 293 K). Comparison of the PCET efficiency of these clusters emphasizes the relevance of reorganization energy in this process.

Magnetic Circular Dichroism Evidence for an Unusual Electronic Structure of a Tetracarbene-Oxoiron(IV) Complex.

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 O2-activating iron enzymes. This work details an electronic-structure investigation of [FeIV(O)(LNHC)(NCMe)]2+ (LNHC = 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-1. By analyzing the Franck-Condon progression, we can determine the same vibration occurring at 616 ± 10 cm-1 in the E(dxy → dxz,yz) excited state. Both values are similar to those measured for [FeIV(O)(TMC)(NCMe)]2+ (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-1. We can unequivocally assign them to the ligand field transitions of dxy → dxz,yz, dxz,yz → dz2, and dxz,yz → dx2-y2, 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 [FeIV(O) (SR-TPA)(NCMe)]2+ (SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine), the excitations within the (FeO)2+ core of complex 1 have similar transition energies, whereas the excitation energy for dxz,yz → dx2-y2 is significantly higher (∼12 000 cm-1 for [FeIV(O)(SR-TPA)(NCMe)]2+). Our results thus substantiate that the tetracarbene ligand (LNHC) of complex 1 does not significantly affect the bonding in the (FeO)2+ unit but strongly destabilizes the dx2-y2 orbital to eventually lift it above dz2. 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.

Complete Series of {FeNO}(8), {FeNO}(7), and {FeNO}(6) Complexes Stabilized by a Tetracarbene Macrocycle.

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 Kß 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.

A trans-1,2 End-On Disulfide-Bridged Iron-Tetracarbene Dimer and Its Electronic Structure.

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.

An exclusively organometallic {FeNO}(7) complex with tetracarbene ligation and a linear FeNO unit.

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)).

Activation of sulfur dioxide by bis[N,N'-diisopropylbenzamidinato(-)]silicon(II): synthesis of neutral six-coordinate silicon(IV) complexes with chelating O,O'-Sulfito or O,O'-dithionito ligands.

The neutral six-coordinate silicon(IV) complexes 2 and 3 (mixture of cis-3 and trans-3) were synthesized by reaction of the donor-stabilized silylene bis[N,N'-diisopropylbenzamidinato(-)]silicon(II) (1) with SO2 . Compounds 2 and 3 are the first silicon(IV) complexes with chelating sulfito or dithionito ligands, and 3 is even the first molecular compound with a chelating dithionito ligand. Compounds 2 and 3 were structurally characterized by crystal structure analyses and multinuclear NMR spectroscopic studies in the solid state and in solution.

Substitution effects on the formation of T-shaped palladium carbene and thioketone complexes from Li/Cl carbenoids.

The preparation of palladium thioketone and T-shaped carbene complexes by treatment of thiophosphoryl substituted Li/Cl carbenoids with a Pd(0) precursor is reported. Depending on the steric demand, the anion-stabilizing ability of the silyl moiety (by negative hyperconjugation effects) and the remaining negative charge at the carbenic carbon atom, isolation of a three-coordinate, T-shaped palladium carbene complex is possible. In contrast, insufficient charge stabilization results in the transfer of the sulfur of the thiophosphoryl moiety and thus in the formation of a thioketone complex. While the thioketones are stable compounds the carbene complexes are revealed to be highly reactive and decompose under elimination of Pd metal. Computational studies revealed that both complexes are formed by a substitution mechanism. While the ketone turned out to be the thermodynamically favored product, the carbene is kinetically favored and thus preferentially formed at low reaction temperatures.