Electronic structures and spectroscopic signatures of diiron intermediates generated by O2 activation of nonheme iron(II)-thiolate complexes.

The activation of O2 at thiolate-ligated iron(II) sites is essential to the function of numerous metalloenzymes and synthetic catalysts. Iron-thiolate bonds in the active sites of nonheme iron enzymes arise from either coordination of an endogenous cysteinate residue or binding of a deprotonated thiol-containing substrate. Examples of the latter include sulfoxide synthases, such as EgtB and OvoA, that utilize O2 to catalyze tandem S-C bond formation and S-oxygenation steps in thiohistidine biosyntheses. We recently reported the preparation of two mononuclear nonheme iron-thiolate complexes (1 and 2) that serve as structural active-site models of substrate-bound EgtB and OvoA (Dalton Trans. 2020, 49, 17745-17757). These models feature monodentate thiolate ligands and tripodal N4 ligands with mixed pyridyl/imidazolyl donors. Here, we describe the reactivity of 1 and 2 with O2 at low temperatures to give metastable intermediates (3 and 4, respectively). Characterization with multiple spectroscopic techniques (UV-vis absorption, NMR, variable-field and -temperature Mössbauer, and resonance Raman) revealed that these intermediates are thiolate-ligated iron(III) dimers with a bridging oxo ligand derived from the four-electron reduction of O2. Structural models of 3 and 4 consistent with the experimental data were generated via density functional theory (DFT) calculations. The combined experimental and computational results illuminate the geometric and electronic origins of the unique spectral features of diiron(III)-µ-oxo complexes with thiolate ligands, and the spectroscopic signatures of 3 and 4 are compared to those of closely-related diiron(III)-µ-peroxo species. Collectively, these results will assist in the identification of intermediates that appear on the O2 reaction landscapes of iron-thiolate species in both biological and synthetic environments.

Molecular and Electronic Structures and Single-Molecule Magnet Behavior of Tris(thioether)-Iron Complexes Containing Redox-Active α-Diimine Ligands.

Incorporating radical ligands into metal complexes is one of the emerging trends in the design of single-molecule magnets (SMMs). While significant effort has been expended to generate multinuclear transition metal-based SMMs with bridging radical ligands, less attention has been paid to mononuclear transition metal-radical SMMs. Herein, we describe the first α-diiminato radical-containing mononuclear transition metal SMM, namely, [κ2-PhTttBu]Fe(AdNCHCHNAd) (1), and its analogue [κ2-PhTttBu]Fe(CyNCHCHNCy) (2) (PhTttBu = phenyltris(tert-butylthiomethyl)borate, Ad = adamantyl, and Cy = cyclohexyl). 1 and 2 feature nearly identical geometric and electronic structures, as shown by X-ray crystallography and electronic absorption spectroscopy. A more detailed description of the electronic structure of 1 was obtained through EPR and Mössbauer spectroscopies, SQUID magnetometry, and DFT, TD-DFT, and CAS calculations. 1 and 2 are best described as high-spin iron(II) complexes with antiferromagnetically coupled α-diiminato radical ligands. A strong magnetic exchange coupling between the iron(II) ion and the ligand radical was confirmed in 1, with an estimated coupling constant J < -250 cm-1 (J = -657 cm-1, DFT). Calibrated CAS calculations revealed that the ground-state Fe(II)-α-diiminato radical configuration has significant ionic contributions, which are weighted specifically toward the Fe(I)-neutral α-diimine species. Experimental data and theoretical calculations also suggest that 1 possesses an easy-axis anisotropy, with an axial zero-field splitting parameter D in the range from -4 to-1 cm-1. Finally, dynamic magnetic studies show that 1 exhibits slow magnetic relaxation behavior with an energy barrier close to the theoretical maximum, 2|D|. These results demonstrate that incorporating strongly coupled α-diiminato radicals into mononuclear transition metal complexes can be an effective strategy to prepare SMMs.

Nonheme iron-thiolate complexes as structural models of sulfoxide synthase active sites.

Two mononuclear iron(ii)-thiolate complexes have been prepared that represent structural models of the nonheme iron enzymes EgtB and OvoA, which catalyze the O2-dependent formation of carbon-sulfur bonds in the biosynthesis of thiohistidine compounds. The series of Fe(ii) complexes reported here feature tripodal N4 chelates (LA and LB) that contain both pyridyl and imidazolyl donors (LA = (1H-imidazol-4-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine; LB = N,N-bis((1-methylimidazol-2-yl)methyl)-2-pyridylmethylamine). Further coordination with monodentate aromatic or aliphatic thiolate ligands yielded the five-coordinate, high-spin Fe(ii) complexes [FeII(LA)(SMes)]BPh4 (1) and [FeII(LB)(SCy)]BPh4 (2), where SMes = 2,4,6-trimethylthiophenolate and SCy = cyclohexanethiolate. X-ray crystal structures revealed that 1 and 2 possess trigonal bipyramidal geometries formed by the N4S ligand set. In each case, the thiolate ligand is positioned cis to an imidazole donor, replicating the arrangement of Cys- and His-based substrates in the active site of EgtB. The geometric and electronic structures of 1 and 2 were analyzed with UV-vis absorption and Mössbauer spectroscopies in tandem with density functional theory (DFT) calculations. Exposure of 1 and 2 to nitric oxide (NO) yielded six-coordinate FeNO adducts that were characterized with infrared and electron paramagnetic resonance (EPR) spectroscopies, confirming that these complexes are capable of binding diatomic molecules. Reaction of 1 and 2 with O2 causes oxidation of the thiolate ligands to disulfide products. The implications of these results for the development of functional models of EgtB and OvoA are discussed.

Ferric uptake regulator (Fur) reversibly binds a [2Fe-2S] cluster to sense intracellular iron homeostasis in Escherichia coli.

The ferric uptake regulator (Fur) is a global transcription factor that regulates intracellular iron homeostasis in bacteria. The current hypothesis states that when the intracellular "free" iron concentration is elevated, Fur binds ferrous iron, and the iron-bound Fur represses the genes encoding for iron uptake systems and stimulates the genes encoding for iron storage proteins. However, the "iron-bound" Fur has never been isolated from any bacteria. Here we report that the Escherichia coli Fur has a bright red color when expressed in E. coli mutant cells containing an elevated intracellular free iron content because of deletion of the iron-sulfur cluster assembly proteins IscA and SufA. The acid-labile iron and sulfide content analyses in conjunction with the EPR and Mössbauer spectroscopy measurements and the site-directed mutagenesis studies show that the red Fur protein binds a [2Fe-2S] cluster via conserved cysteine residues. The occupancy of the [2Fe-2S] cluster in Fur protein is ∼31% in the E. coli iscA/sufA mutant cells and is decreased to ∼4% in WT E. coli cells. Depletion of the intracellular free iron content using the membrane-permeable iron chelator 2,2´-dipyridyl effectively removes the [2Fe-2S] cluster from Fur in E. coli cells, suggesting that Fur senses the intracellular free iron content via reversible binding of a [2Fe-2S] cluster. The binding of the [2Fe-2S] cluster in Fur appears to be highly conserved, because the Fur homolog from Hemophilus influenzae expressed in E. coli cells also reversibly binds a [2Fe-2S] cluster to sense intracellular iron homeostasis.

Characterization of Terminal Iron(III)-Oxo and Iron(III)-Hydroxo Complexes Derived from O2 Activation.

O2 activation at nonheme iron centers is a common motif in biological systems. While synthetic models have provided numerous insights into the reactivity of high-valent iron-oxo complexes related to biological processes, the majority of these complexes are synthesized using alternative oxidants. This report describes O2 activation by an iron(II)-triflate complex of the imino-functionalized tris(pyrrol-2-ylmethyl)amine ligand framework, H3[N(piCy)3]. Initial reaction conditions result in the formation of a mixture of oxidation products including terminal iron(III)-oxo and iron(III)-hydroxo complexes. The relevance of these species to the O2 activation process is demonstrated through reactivity studies and electrochemical analysis of the iron(III)-oxo complex.

Mössbauer Spectroscopy and Theoretical Studies of Iron Bimetallic Complexes Showing Electrocatalytic Hydrogen Evolution.

Mössbauer spectroscopy and density functional theory (DFT) calculations are reported for the mononuclear Fe-nitrosyl complex [Fe( N, N'-bis(2-mercaptoethyl)-1,4-diazacycloheptane)NO] {[Fe(bme-dach)(NO)] (1)} and the series of dithiolate-bridged dinuclear complexes M-Fe(CO)Cp [M = Fe(bme-dach)(NO) (1-A), Ni(bme-dach) (2-A), and Co(bme-dach)(NO) (3-A)], in which M is a metallo-ligand to Fe(CO)Cp+ (Fe'Cp). The latter is an organometallic fragment in which Fe is coordinated by one CO and one cyclopentadienyl ligand. Complexes 1-A and 2-A were previously shown to have electrocatalytic hydrogen evolution activity. Mononuclear {Fe-NO}7 complex 1, with overall spin of 1/2, has an isomer shift of 0.23(2) mm/s [Δ EQ = 1.37(2) mm/s] and magnetic hyperfine couplings of {-38 T, -26.8 T, 8.6 T}. In complexes 2-A and 3-A, Fe'(CO)Cp+ has a diamagnetic ground state and δ = 0.33(2) mm/s (Δ EQ ≈ 1.78 mm/s), consistent with a low-spin FeII site. In contrast, in complex 1-A, M = Fe(bme-dach)(NO) (i.e., complex 1) the magnetic hyperfine interactions of both metallo-ligand, M, and low-spin Fe'Cp are perturbed and Fe'Cp exhibits small magnetic hyperfine interactions, although its isomer shift and quadrupole splittings are largely unaltered. The DFT calculations for 1-A are in agreement with the paramagnetism observed for the Fe'(CO)Cp+ iron site.

Radical Rebound Hydroxylation Versus H-Atom Transfer in Non-Heme Iron(III)-Hydroxo Complexes: Reactivity and Structural Differentiation.

The characterization of high-valent iron centers in enzymes has been aided by synthetic model systems that mimic their reactivity or structural and spectral features. For example, the cleavage of dioxygen often produces an iron(IV)-oxo that has been characterized in a number of enzymatic and synthetic systems. In non-heme 2-oxogluterate dependent (iron-2OG) enzymes, the ferryl species abstracts an H-atom from bound substrate to produce the proposed iron(III)-hydroxo and caged substrate radical. Most iron-2OG enzymes perform a radical rebound hydroxylation at the site of the H-atom abstraction (HAA); however, recent reports have shown that certain substrates can be desaturated through the loss of a second H atom at a site adjacent to a heteroatom (N or O) for most native desaturase substrates. One proposed mechanism for the removal of the second H-atom  involves a polar-cleavage mechanism (electron transfer-proton transfer) by the iron(III)-hydroxo, as opposed to a second HAA. Herein we report the synthesis and characterization of a series of iron complexes with hydrogen bonding interactions between bound aquo or hydroxo ligands and the secondary coordination sphere in ferrous and ferric complexes. Interconversion among the iron species is accomplished by stepwise proton or electron addition or subtraction, as well as H-atom transfer (HAT). The calculated bond dissociation free energies (BDFEs) of two ferric hydroxo complexes, differentiated by their noncovalent interactions and reactivity, suggest that neither complex is capable of activating even weak C-H bonds, lending further support to the proposed mechanism for desaturation in iron-2OG desaturase enzymes. Additionally, the ferric hydroxo species are differentiated by their reactivity toward performing a radical rebound hydroxylation of triphenylmethylradical. Our findings should encourage further study of the desaturase systems that may contain unique H-bonding motifs proximal to the active site that help bias substrate desaturation over hydroxylation.

Electronic, Magnetic, and Redox Properties and O2 Reactivity of Iron(II) and Nickel(II) o-Semiquinonate Complexes of a Tris(thioether) Ligand: Uncovering the Intradiol Cleaving Reactivity of an Iron(II) o-Semiquinonate Complex.

The iron(II) semiquinonate character within the iron(III) catecholate species has been proposed by numerous studies to account for the O2 reactivity of intradiol catechol dioxygenases, but a well-characterized iron(II) semiquinonate species that exhibits intradiol cleaving reactivity has not yet been reported. In this study, a detailed electronic structure description of the first iron(II) o-semiquinonate complex, [PhTttBu]Fe(phenSQ) [PhTttBu = phenyltris(tert-butylthiomethyl)borate; phenSQ = 9,10-phenanthrenesemiquinonate; Wang et al. Chem. Commun. 2014, 50, 5871-5873], was generated through a combination of electronic and Mössbauer spectroscopies, SQUID magnetometry, and density functional theory (DFT) calculations. [PhTttBu]Fe(phenSQ) reacts with O2 to generate an intradiol cleavage product, diphenic anhydride, in 16% yield. To assess the dependence of the intradiol reactivity on the identity of the metal ion, the nickel analogue, [PhTttBu]Ni(phenSQ), and its derivative, [PhTttBu]Ni(3,5-DBSQ) (3,5-DBSQ = 3,5-di-tert-butyl-1,2-semiquinonate), were prepared and characterized by X-ray crystallography, mass spectrometry, 1H NMR and electronic spectroscopies, and SQUID magnetometry. DFT calculations, evaluated on the basis of the experimental data, support the electronic structure descriptions of [PhTttBu]Ni(phenSQ) and [PhTttBu]Ni(3,5-DBSQ) as high-spin nickel(II) complexes with antiferromagnetically coupled semiquinonate ligands. Unlike its iron counterpart, [PhTttBu]Ni(phenSQ) decomposes slowly in an O2 atmosphere to generate 14% phenanthrenequinone with a negligible amount of diphenic anhydride. [PhTttBu]Ni(3,5-DBSQ) does not react with O2. This dramatic effect of the metal-ion identity supports the hypothesis that a metal(III) alkylperoxo species serves as an intermediate in the intradiol cleaving reactions. The redox properties of all three complexes were probed using cyclic voltammetry and differential pulse voltammetry, which indicate an inner-sphere electron-transfer mechanism for the formation of phenanthrenequinone. The lack of O2 reactivity of [PhTttBu]Ni(3,5-DBSQ) can be rationalized by the high redox potential of the metal-ligated 3,5-DBSQ/3,5-DBQ couple.

Catalytic N2 Reduction to Silylamines and Thermodynamics of N2 Binding at Square Planar Fe.

The geometric constraints imposed by a tetradentate P4N2 ligand play an essential role in stabilizing square planar Fe complexes with changes in metal oxidation state. The square pyramidal Fe0(N2)(P4N2) complex catalyzes the conversion of N2 to N(SiR3)3 (R = Me, Et) at room temperature, representing the highest turnover number of any Fe-based N2 silylation catalyst to date (up to 65 equiv N(SiMe3)3 per Fe center). Elevated N2 pressures (>1 atm) have a dramatic effect on catalysis, increasing N2 solubility and the thermodynamic N2 binding affinity at Fe0(N2)(P4N2). A combination of high-pressure electrochemistry and variable-temperature UV-vis spectroscopy were used to obtain thermodynamic measurements of N2 binding. In addition, X-ray crystallography, 57Fe Mössbauer spectroscopy, and EPR spectroscopy were used to fully characterize these new compounds. Analysis of Fe0, FeI, and FeII complexes reveals that the free energy of N2 binding across three oxidation states spans more than 37 kcal mol-1.

Synthesis, Characterization, and Photophysical Studies of an Iron(III) Catecholate-Nitronylnitroxide Spin-Crossover Complex.

The synthesis and characterization of an Fe(III) catecholate-nitronylnitroxide (CAT-NN) complex (1-NN) that undergoes Fe(III) spin-crossover is described. Our aim is to determine whether the intraligand exchange coupling of the semiquinone-nitronylnitroxide Fe(II)(SQ-NN) excited state resulting from irradiation of the CAT → Fe(III) LMCT band would affect either the intrinsic photophysics or the iron spin-crossover event when compared to the complex lacking the nitronylnitroxide radical (1). X-ray crystallographic analysis provides bond lengths consistent with a ferric catecholate charge distribution. Mössbauer spectroscopy clearly demonstrates Fe(III) spin-crossover, hyperfine couplings, and a weak ferromagnetic Fe(III)-CAT-NN exchange, and spin-crossover is corroborated by variable-temperature magnetic susceptibility and electronic absorption studies. To explore the effect of the NN radical on photophysical processes, we conducted room-temperature transient absorption experiments. Upon excitation of the ligand-to-metal charge transfer band, an Fe(II)SQ state is populated and most likely undergoes fast intersystem crossing to the ligand field manifold, where it rapidly decays into a metastable low-spin Fe(III)CAT state, followed by repopulation of the high-spin Fe(III)CAT ground state. The decay components of 1-NN are slightly faster than those obtained for 1, perhaps due to the higher number of microstates present within the LMCT and LF manifolds for 1-NN. Although the effects of the NN radical are manifest in neither the spin-crossover nor the photophysics, our results lay the groundwork for future studies.

Redox active iron nitrosyl units in proton reduction electrocatalysis.

Base metal, molecular catalysts for the fundamental process of conversion of protons and electrons to dihydrogen, remain a substantial synthetic goal related to a sustainable energy future. Here we report a diiron complex with bridging thiolates in the butterfly shape of the 2Fe2S core of the [FeFe]-hydrogenase active site but with nitrosyl rather than carbonyl or cyanide ligands. This binuclear [(NO)Fe(N2S2)Fe(NO)2](+) complex maintains structural integrity in two redox levels; it consists of a (N2S2)Fe(NO) complex (N2S2=N,N'-bis(2-mercaptoethyl)-1,4-diazacycloheptane) that serves as redox active metallodithiolato bidentate ligand to a redox active dinitrosyl iron unit, Fe(NO)2. Experimental and theoretical methods demonstrate the accommodation of redox levels in both components of the complex, each involving electronically versatile nitrosyl ligands. An interplay of orbital mixing between the Fe(NO) and Fe(NO)2 sites and within the iron nitrosyl bonds in each moiety is revealed, accounting for the interactions that facilitate electron uptake, storage and proton reduction.

Dioxygen reactivity of biomimetic Fe(II) complexes with noninnocent catecholate, o-aminophenolate, and o-phenylenediamine ligands.

This study describes the O2 reactivity of a series of high-spin mononuclear Fe(II) complexes each containing the facially coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine ((Ph2)TIP) ligand and one of the following bidentate, redox-active ligands: 4-tert-butylcatecholate ((tBu)CatH(-)), 4,6-di-tert-butyl-2-aminophenolate ((tBu2)APH(-)), or 4-tert-butyl-1,2-phenylenediamine ((tBu)PDA). The preparation and X-ray structural characterization of [Fe(2+)((Ph2)TIP)((tBu)CatH)]OTf, [3]OTf and [Fe(2+)((Ph2)TIP)((tBu)PDA)](OTf)2, [4](OTf)2 are described here, whereas [Fe(2+)((Ph2)TIP)((tBu2)APH)]OTf, [2]OTf was reported in our previous paper [Bittner et al., Chem.-Eur. J. 2013, 19, 9686-9698]. These complexes mimic the substrate-bound active sites of nonheme iron dioxygenases, which catalyze the oxidative ring-cleavage of aromatic substrates like catechols and aminophenols. Each complex is oxidized in the presence of O2, and the geometric and electronic structures of the resulting complexes were examined with spectroscopic (absorption, EPR, Mössbauer, resonance Raman) and density functional theory (DFT) methods. Complex [3]OTf reacts rapidly with O2 to yield the ferric-catecholate species [Fe(3+)((Ph2)TIP)((tBu)Cat)](+) (3(ox)), which undergoes further oxidation to generate an extradiol cleavage product. In contrast, complex [4](2+) experiences a two-electron (2e(-)), ligand-based oxidation to give [Fe(2+)((Ph2)TIP)((tBu)DIBQ)](2+) (4(ox)), where DIBQ is o-diiminobenzoquinone. The reaction of [2](+) with O2 is also a 2e(-) process, yet in this case both the Fe center and (tBu2)AP ligand are oxidized; the resulting complex (2(ox)) is best described as [Fe(3+)((Ph2)TIP)((tBu2)ISQ)](+), where ISQ is o-iminobenzosemiquinone. Thus, the oxidized complexes display a remarkable continuum of electronic structures ranging from [Fe(3+)(L(2-))](+) (3(ox)) to [Fe(3+)(L(•-))](2+) (2(ox)) to [Fe(2+)(L(0))](2+) (4(ox)). Notably, the O2 reaction rates vary by a factor of 10(5) across the series, following the order [3](+) > [2](+) > [4](2+), even though the complexes have similar structures and Fe(3+/2+) redox potentials. To account for the kinetic data, we examined the relative abilities of the title complexes to bind O2 and participate in H-atom transfer reactions. We conclude that the trend in O2 reactivity can be rationalized by accounting for the role of proton transfer(s) in the overall reaction.

Synthetic, spectroscopic, and DFT studies of iron complexes with iminobenzo(semi)quinone ligands: implications for o-aminophenol dioxygenases.

The oxidative C-C bond cleavage of o-aminophenols by nonheme Fe dioxygenases is a critical step in both human metabolism (the kynurenine pathway) and the microbial degradation of nitroaromatic pollutants. The catalytic cycle of o-aminophenol dioxygenases (APDOs) has been proposed to involve formation of an Fe(II)/O2/iminobenzosemiquinone complex, although the presence of a substrate radical has been called into question by studies of related ring-cleaving dioxygenases. Recently, we reported the first synthesis of an iron(II) complex coordinated to an iminobenzosemiquinone (ISQ) ligand, namely, [Fe((Ph2)Tp)((tBu)ISQ)] (2a; where (Ph2)Tp=hydrotris(3,5-diphenylpyrazol-1-yl)borate and (tBu)ISQ is the radical anion derived from 2-amino-4,6-di-tert-butylphenol). In the current manuscript, density functional theory (DFT) calculations and a wide variety of spectroscopic methods (electronic absorption, Mössbauer, magnetic circular dichroism, and resonance Raman) were employed to obtain detailed electronic-structure descriptions of 2a and its one-electron oxidized derivative [3a](+). In addition, we describe the synthesis and characterization of a parallel series of complexes featuring the neutral supporting ligand tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine ((Ph2)TIP). The isomer shifts of about 0.97 mm s(-1) obtained through Mössbauer experiments confirm that 2a (and its (Ph2)TIP-based analogue [2b](+)) contain Fe(II) centers, and the presence of an ISQ radical was verified by analysis of the absorption spectra in light of time-dependent DFT calculations. The collective spectroscopic data indicate that one-electron oxidation of the Fe(II)-ISQ complexes yields complexes ([3a](+) and [3b](2+)) with electronic configurations between the Fe(III)-ISQ and Fe(II)-IBQ limits (IBQ=iminobenzoquinone), highlighting the ability of o-amidophenolates to access multiple oxidation states. The implications of these results for the mechanism of APDOs and other ring-cleaving dioxygenases are discussed.

Hyperfine interactions and electron distribution in Fe(II)Fe (I) and Fe (I)Fe (I) models for the active site of the [FeFe] hydrogenases: Mössbauer spectroscopy studies of low-spin Fe(I.).

Mössbauer studies of [{µ-S(CH2C(CH3)2CH2S}(µ-CO)Fe(II)Fe(I)(PMe3)2(CO)3]PF6 (1 OX ), a model complex for the oxidized state of the [FeFe] hydrogenases, and the parent Fe(I)Fe(I) derivative are reported. The paramagnetic 1 OX is part of a series featuring a dimethylpropanedithiolate bridge, introducing steric hindrance with profound impact on the electronic structure of the diiron complex. Well-resolved spectra of 1 OX allow determination of the magnetic hyperfine couplings for the low-spin distal Fe(I) ([Formula: see text]) site, A x,y,z  = [-24 (6), -12 (2), 20 (2)] MHz, and the detection of significant internal fields (approximately 2.3 T) at the low-spin ferrous site, confirmed by density functional theory (DFT) calculations. Mössbauer spectra of 1 OX show nonequivalent sites and no evidence of delocalization up to 200 K. Insight from the experimental hyperfine tensors of the Fe(I) site is used in correlation with DFT to reveal the spatial distribution of metal orbitals. The Fe-Fe bond in [Fe2{µ-S(CH2C(CH3)2CH2S}(PMe3)2(CO)4] (1) involving two [Formula: see text]-type orbitals is crucial in keeping the structure intact in the presence of strain. On oxidation, the distal iron site is not restricted by the Fe-Fe bond, and thus the more stable isomer results from inversion of the square pyramid, rotating the [Formula: see text] orbital of [Formula: see text]. DFT calculations imply that the Mössbauer properties can be traced to this [Formula: see text] orbital. The structure of the magnetic hyperfine coupling tensor, A, of the low-spin Fe(I) in 1 OX is discussed in the context of the known A tensors for the oxidized states of the [FeFe] hydrogenases.

Ambidentate thiocyanate and cyanate ligands in dinitrosyl iron complexes.

To explore the effect of delocalization in the Fe(NO)(2) unit on possible linkage isomerism of ambidentate ECN(-) ligands, E = S and O, anionic DNICs, dinitrosyl iron complexes, (SCN)(2)Fe(NO)(2)(-) (1) and (OCN)(2)Fe(NO)(2)(-) (2) were synthesized by the reaction of in situ-generated [Fe(CO)(2)(NO)(2)](+) and PPN(+)ECN(-). Other {Fe(NO)(2)}(9) (Enemark-Feltham notation) complexes, (N(3))(2)Fe(NO)(2)(-) and (PhS)(2)Fe(NO)(2)(-), were prepared for comparison. The X-ray diffraction analysis of 1 and 2 yielded the typical tetrahedral structures of DNICs with two slightly bent Fe-N-O oriented toward each other, and linear FeNCE units. The ν(NO) IR values shift to lower values for 1 > 2 > (N(3))(2)Fe(NO)(2)(-) > (PhS)(2)Fe(NO)(2)(-), reflecting the increasing donor ability of the ancillary ligands and consistent with the redox potentials of the complexes, and the small trends in Mössbauer isomer shifts. Computational studies corroborate that the {Fe(NO)(2)}(9) motif prefers N-bound rather than E-bound isomers. The calculated energy differences between the linkage isomers of 1 (Fe-NCS preferred over Fe-SCN by about 6 kcal/mol) are smaller than those of 2 (Fe-NCO preferred over Fe-OCN by about 16 kcal/mol), a difference that is justified by the frontier molecular orbitals of the ligands themselves.

Structural and spectroscopic features of mixed valent Fe(II)Fe(I) complexes and factors related to the rotated configuration of diiron hydrogenase.

The compounds of this study have yielded to complementary structural, spectroscopic (Mössbauer, EPR/ENDOR, IR), and computational probes that illustrate the fine control of electronic and steric features that are involved in the two structural forms of (µ-SRS)[Fe(CO)2PMe3]2(0,+) complexes. The installation of bridgehead bulk in the -SCH2CR2CH2S- dithiolate (R = Me, Et) model complexes produces 6-membered FeS2C3 cyclohexane-type rings that produce substantial distortions in Fe(I)Fe(I) precursors. Both the innocent (Fc(+)) and the noninnocent or incipient (NO(+)/CO exchange) oxidations result in complexes with inequivalent iron centers in contrast to the Fe(I)Fe(I) derivatives. In the Fe(II)Fe(I) complexes of S = 1/2, there is complete inversion of one square pyramid relative to the other with strong super hyperfine coupling to one PMe3 and weak SHFC to the other. Remarkably, diamagnetic complexes deriving from isoelectronic replacement of CO by NO(+), {(µ-SRS)[Fe(CO)2PMe3] [Fe(CO)(NO)PMe3](+)}, are also rotated and exist in only one isomeric form with the -SCH2CR2CH2S- dithiolates, in contrast to R = H ( Olsen , M. T. ; Bruschi , M. ; De Gioia , L. ; Rauchfuss , T. B. ; Wilson , S. R. J. Am. Chem. Soc. 2008 , 130 , 12021 -12030 ). The results and redox levels determined from the extensive spectroscopic analyses have been corroborated by gas-phase DFT calculations, with the primary spin density either localized on the rotated iron in the case of the S = 1/2 compound, or delocalized over the {Fe(NO)} unit in the S = 0 complex. In the latter case, the nitrosyl has effectively shifted electron density from the Fe(I)Fe(I) bond, repositioning it onto the spin coupled Fe-N-O unit such that steric repulsion is sufficient to induce the rotated structure in the Fe(II)-{Fe(I)((•)NO)}(8) derivatives.

Analysis of a pentacoordinate iron dicarbonyl as synthetic analogue of the Hmd or mono-iron hydrogenase active site.

Pentacoordinate iron dicarbonyls, (NS)Fe(CO)(2)P (NS=2-amidothiophenylate, P=PCy(3) (4), PPh(3), (5), and P(OEt)(3) (6)) were prepared as potential biomimetics of the active site of the mono-iron hydrogenase, [Fe]-H(2)ase. Full characterization including X-ray diffraction, density functional theory (DFT) computations, and Mössbauer studies for complexes 5 and 6 find that, despite similar infrared v(CO) pattern and absorption frequencies as the active site of the [Fe]-H(2)ase, the geometrical distortions towards trigonal bipyramidal, the negative isomer shift parameters, and the differences in CO-uptake reactivity are due to the "non-innocence" of the NS ligand. Ligand-based protonation with a strong acid, HBF(4).Et(2)O, interrupted the extensive pi-delocalization over Fe and NS ligand of complex 4 and switched on CO uptake (1 bar) to form a CO adduct, mer-[(H-NS)Fe(CO)(3)(PCy(3))](+) or 4(CO)-H(+). The extrinsic CO is reversibly removed on deprotonation with Et(3)N to regenerate complex 4. In a (13)CO atmosphere, concomitant CO uptake by 4-H(+) and exchange with intrinsic CO groups provide a facile route to (13)C-labeled 4(CO)-H(+) and, upon deprotonation, (13)C-labeled complex 4. DFT calculations show substantial Fe character in the LUMO of 4-H(+) typical of the d(6) Fe(II) in a regular square-pyramidal geometry. Thus, the Lewis acidity of 4-H(+) makes it amenable for CO binding, whereas the dianionic NS ligand renders the iron center of 4 insufficiently electrophilic and largely of Fe(I) character.

Synthesis and Mössbauer characterization of octahedral iron(II) carbonyl complexes FeI2(CO)3L and FeI2(CO)2L2: developing models of the [Fe]-H(2)ase active site.

A series of mono- and disubstituted complexes, FeI(2)(CO)(x)L(4-x), x = 2 or 3, is conveniently accessed from simple mixing of N-heterocyclic carbenes, phosphines, and aromatic amines with FeI(2)(CO)(4), first reported by Hieber in 1928. The highly light sensitive complexes yield to crystallization and X-ray diffraction studies for six complexes showing them to be rudimentary structural models of the monoiron hydrogenase, [Fe]-H(2)ase or Hmd, active site in native (Fe(II)(CO)(2)) or CO-inhibited (Fe(II)(CO)(3)) states. Diatomic ligand (nu(CO)) vibrational and Mossbauer spectroscopies are related to those reported for the Hmd active site. The importance of a serial approach for relating such parameters in model compounds to low spin Fe(II) in the diverse ligation of enzyme active sites is stressed.

A high-spin organometallic Fe-S compound: structural and Mössbauer spectroscopic studies of [phenyltris((tert-butylthio)methyl)borate]Fe(Me).

The synthesis and structure of the pseudotetrahedral, sulfur-rich, high-spin organoiron(II) [phenyltris((tert-butylthio)methyl)borate]Fe(Me), [PhTt(tBu)]Fe(Me), 1, are reported. Low-temperature Mössbauer spectroscopic studies reveal an isomer shift of delta = 0.60(3) mm/s and DeltaE(Q) = 0.00(1) mm/s and an S = 2 ground multiplet with a negative zero-field splitting, D = -33(3) cm(-1), E/D approximately = 0.01. The small separation of the ground doublet, Delta approximately = 0.01 cm(-1), allows for observation of X-band EPR signals at g(eff) approximately = 10 (g(z) = 2.6, g(x,y) = 2.00). The relatively large negative zero-field splitting and a highly anisotropic magnetic hyperfine tensor, containing a large orbital z component, {-10(4), -10(4), +33.8(2) MHz}, are concordant with the presence of unquenched orbital angular momentum. Density functional theory (DFT) calculations predict that the lowest-lying orbitals have predominantly d(xy)- and d(x(2)-y(2))-like character, separated by an energy gap small enough to allow mixing through spin-orbit coupling, to generate a negative zero-field splitting, consistent with the experimental observations. The experimental and DFT-calculated isomer shifts are in good agreement (delta(calcd) = 0.5 mm/s). The unusual (for a high-spin ferrous site) null electric field gradients can be qualitatively explained in the frame of the spin-orbit coupling mixing. The very small Fermi contact component of the magnetic hyperfine tensor (A(FC)(exp) = -9 MHz) is not well described by the DFT approach (A(FC)(calcd) = +2 MHz). To our knowledge, this is the first study of a sulfur-coordinated high-spin organoiron(II) complex.

A Series of Cyanide-Bridged Binuclear Complexes.

A series of cyanide-bridged binuclear complexes, ('S(3)')Ni-CN-M[Tp(tBu)] ('S(3)' = bis(2-mercaptophenyl)sulfide, Tp(tBu) = hydrotris(3-tert-butylpyrazolyl)borate, M = Fe (2-Fe), Co (2-Co), Ni (2-Ni), Zn (2-Zn)) was prepared by the coupling of K[('S(3)')Ni(CN)] with [Tp(tBu)]MX. The isostructural series of complexes was structurally and spectroscopically characterized. A similar coupling strategy was used to synthesize the anionic copper(I) analogue, Et4N{('S3')Ni-CN-Cu[Tp(tBu)]}, 2-Cu.An alternative synthesis was devised for the preparation of the linkages isomers of 2-Zn, i.e. of cyanide-bridged linkage isomers. X-ray diffraction, (13)C NMR and IR spectral studies established that isomerization to the more stable Ni-CN-Zn isomer occurs. DFT computational results buttressed the experimental observations indicating that the cyanide-bridged isomer is ca. 5 kcal/mol more stable than its linkage isomer.