Electronic Spectra of Iron-Sulfur Complexes Measured by 2p3d RIXS Spectroscopy.

Iron sulfur (FeS) proteins perform a wide range of biological functions including electron transfer and catalysis. Understanding the complex reactivity of these systems requires a detailed understanding of their electronic properties, which are encoded in the low-energy d-d excited states. Here we demonstrate that iron L-edge 2p3d resonant inelastic X-ray scattering (RIXS) can measure d-d excitation spectra in a series of monomeric, dimeric, and tetrameric FeS model complexes. RIXS provides advantages over traditional optical spectroscopies, because it is capable of measuring low-energy electronic excitations (0-10 000 cm-1) and spin-flip transitions. RIXS reveals the dense manifold of d-d excited states in dimeric [2Fe-2S] and tetrameric [MFe3S4]2+ (M = V or Mo) complexes resulting from covalency and exchange coupling. These results support recent ab initio theoretical predictions that FeS clusters possess a much greater number of low-lying excited states than predicted by model Hamiltonians.

Iron L2,3-Edge X-ray Absorption and X-ray Magnetic Circular Dichroism Studies of Molecular Iron Complexes with Relevance to the FeMoco and FeVco Active Sites of Nitrogenase.

Herein, a systematic study of a series of molecular iron model complexes has been carried out using Fe L2,3-edge X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopies. This series spans iron complexes of increasing complexity, starting from ferric and ferrous tetrachlorides ([FeCl4]-/2-), to ferric and ferrous tetrathiolates ([Fe(SR)4]-/2-), to diferric and mixed-valent iron-sulfur complexes [Fe2S2R4]2-/3-. This test set of compounds is used to evaluate the sensitivity of both Fe L2,3-edge XAS and XMCD spectroscopy to oxidation state and ligation changes. It is demonstrated that the energy shift and intensity of the L2,3-edge XAS spectra depends on both the oxidation state and covalency of the system; however, the quantitative information that can be extracted from these data is limited. On the other hand, analysis of the Fe XMCD shows distinct changes in the intensity at both L3 and L2 edges, depending on the oxidation state of the system. It is also demonstrated that the XMCD intensity is modulated by the covalency of the system. For mononuclear systems, the experimental data are correlated with atomic multiplet calculations in order to provide insights into the experimental observations. Finally, XMCD is applied to the tetranuclear heterometal-iron-sulfur clusters [MFe3S4]3+/2+ (M = Mo, V), which serve as structural analogues of the FeMoco and FeVco active sites of nitrogenase. It is demonstrated that the XMCD data can be utilized to obtain information on the oxidation state distribution in complex clusters that is not readily accessible for the Fe L2,3-edge XAS data alone. The advantages of XMCD relative to standard K-edge and L2,3-edge XAS are highlighted. This study provides an important foundation for future XMCD studies on complex (bio)inorganic systems.

Light-atom influences on the electronic structures of iron-sulfur clusters.

Ligand K-edge X-ray absorption spectroscopy was used to study dimeric and tetrameric Cl-terminated Fe-S clusters with variable numbers of S(2-) substituted by N(t)Bu(2-) (N(t)Bu(2-) = tertbutylimido) ligands to gain insights into the functional role of the interstitial light atom in the iron-molybdenum cofactor (FeMoco) of nitrogenase. These studies are complemented by time-dependent density functional theory analysis to quantify the relative effects on Fe-S and Fe-Cl bonding. The results show that N(t)Bu(2-) substitution dramatically affects the electronic structure of dimeric clusters, while the impact on tetrameric clusters is small. Strong agreement between experiment and theory merited extension of this analysis to hypothetical clusters with S(2-) substituted by N and C-atom donor ligands as well as FeMoco itself. These results show that very strong electron donors are required to appreciably modulate the electronic structure of tetrameric (or larger) iron sulfur clusters, pointing to a possible role of the central C(4-) in FeMoco.

Kinetics and mechanistic analysis of an extremely rapid carbon dioxide fixation reaction.

Carbon dioxide may react with free or metal-bound hydroxide to afford products containing bicarbonate or carbonate, often captured as ligands bridging two or three metal sites. We report the kinetics and probable mechanism of an extremely rapid fixation reaction mediated by a planar nickel complex [Ni(II)(NNN)(OH)](1-) containing a tridentate 2,6-pyridinedicarboxamidate pincer ligand and a terminal hydroxide ligand. The minimal generalized reaction is M-OH + CO(2) → M-OCO(2)H; with variant M, previous rate constants are ≲10(3) M(-1) s(-1) in aqueous solution. For the present bimolecular reaction, the (extrapolated) rate constant is 9.5 × 10(5) M(-1) s(-1) in N,N'-dimethylformamide at 298 K, a value within the range of k(cat)/K(M)≈10(5)-10(8) M(-1) s(-1) for carbonic anhydrase, the most efficient catalyst of CO(2) fixation reactions. The enthalpy profile of the fixation reaction was calculated by density functional theory. The initial event is the formation of a weak precursor complex between the Ni-OH group and CO(2), followed by insertion of a CO(2) oxygen atom into the Ni-OH bond to generate a four center Ni(η(2)-OCO(2)H) transition state similar to that at the zinc site in carbonic anhydrase. Thereafter, the Ni-OH bond detaches to afford the Ni(η(1)-OCO(2)H) fragment, after which the molecule passes through a second, lower energy transition state as the bicarbonate ligand rearranges to a conformation very similar to that in the crystalline product. Theoretical values of metric parameters and activation enthalpy are in good agreement with experimental values [ΔH(‡) = 3.2(5) kcal/mol].

Structural Analysis of Cubane-Type Iron Clusters.

The generalized cluster type [M4(µ3-Q)4L n ] x contains the cubane-type [M4Q4] z core unit that can approach, but typically deviates from, perfect Td symmetry. The geometric properties of this structure have been analyzed with reference to Td symmetry by a new protocol. Using coordinates of M and Q atoms, expressions have been derived for interatomic separations, bond angles, and volumes of tetrahedral core units (M4, Q4) and the total [M4Q4] core (as a tetracapped M4 tetrahedron). Values for structural parameters have been calculated from observed average values for a given cluster type. Comparison of calculated and observed values measures the extent of deviation of a given parameter from that required in an exact tetrahedral structure. The procedure has been applied to the structures of over 130 clusters containing [Fe4Q4] (Q = S2-, Se2-, Te2-, [NPR3]-, [NR]2-) units, of which synthetic and biological sulfide-bridged clusters constitute the largest subset. General structural features and trends in structural parameters are identified and summarized. An extensive database of structural properties (distances, angles, volumes) has been compiled in Supporting Information.

Iron-amide-sulfide and iron-imide-sulfide clusters: heteroligated core environments relevant to the nitrogenase FeMo cofactor.

Heteroligated cluster cores consisting of weak-field iron, strongly basic nitrogen anions, and sulfide are of interest with respect to observed and conjectured environments in the FeMo cofactor of nitrogenase. Selective syntheses have been developed to achieve such environments with tert-butyl-substituted amide and imide core ligands. A number of different routes were employed, including nominal ligand substitution and oxidative addition reactions, as well as novel transformations involving the combination of different cluster precursors. New cluster products include precursor Fe(2)(µ-NH(t)Bu)(2)[N(SiMe(3))(2)](2) (6), Fe(2)(µ-NH(t)Bu)(2)(µ-S)[N(SiMe(3))(2)](2) (7), which has a rare confacial bitetrahedral geometry previously unknown in iron chemistry, [Fe(2)(µ-N(t)Bu)(µ-S)Cl(4)](2-) (2), and cuboidal [Fe(4)(µ(3)-N(t)Bu)(3)(µ(3)-S)Cl(4)](-) (8), [Fe(4)(µ(3)-N(t)Bu)(2)(µ(3)-S)(2)Cl(4)](2-) (9), and [Fe(4)(µ(3)-N(t)Bu)(µ(3)-S)(3)Cl(4)](2-) (10), as well as selenide-substituted derivatives Fe(2)(µ-NH(t)Bu)(2)(µ-Se)[N(SiMe(3))(2)](2) (7-Se) and [Fe(4)(µ(3)-N(t)Bu)(µ(3)-Se)(3)Cl(4)](2-) (10-Se). The imide-sulfide clusters complete the compositional sets [Fe(2)(µ-N(t)Bu)(n)(µ-S)(2-n)Cl(4)](2-) (n = 0-2) and [Fe(4)(µ(3)-N(t)Bu)(n)(µ(3)-S)(4-n)Cl(4)](z) (n = 0-4), represented previously only by the all-imide and all-sulfide core congeners, and they share chemical and physical properties with the parent homoleptic core species. All imide-sulfide cores are compositionally stable and show no evidence of core ligand exchange over days in solution. Beyond structural differences, the impact of mixed core ligation is most evident in redox potentials, which show progressive decreases of -435 (for z = 1-/2-) or -385 mV (for z = 2-/3-) for each replacement of sulfide by the more potent imide donor, and a corresponding effect may be expected for the interstitial heteroligand in the FeMo cofactor. Cluster 10 presents an [Fe(4)NS(3)] core framework virtually isometric with the isostructural [Fe(4)S(3)X] subunit of the FeMo cofactor, thus providing a synthetic structural representation for this portion of the cofactor core.

Iron-mediated hydrazine reduction and the formation of iron-arylimide heterocubanes.

The reaction of Fe(N{SiMe(3)}(2))(2) (1) with 1 equiv of arylthiol (ArSH) results in material of notional composition Fe(SAr)(N{SiMe(3)}(2)) (2), from which crystalline Fe(2)(µ-SAr)(2)(N{SiMe(3)}(2))(2)(THF)(2) (Ar = Mes) can be isolated from tetrahydrofuran (THF) solvent. Treatment of 2 with 0.5 equiv of 1,2-diarylhydrazine (Ar'NH-NHAr', Ar' = Ph, p-Tol) yields ferric-imide-thiolate cubanes Fe(4)(µ(3)-NAr')(4)(SAr)(4) (3). The site-differentiated, 1-electron reduced iron-imide cubane derivative [Fe(THF)(6)][Fe(4)(µ(3)-N-p-Tol)(4)(SDMP)(3)(N{SiMe(3)}(2))](2) ([Fe(THF)(6)][4](2); DMP = 2,6-dimethylphenyl) can be isolated by adjusting the reaction stoichiometry of 1/ArSH/Ar'NHNHAr' to 9:6:5. The isolated compounds were characterized by a combination of structural (X-ray diffraction), spectroscopic (NMR, UV-vis, Mössbauer, EPR), and magnetochemical methods. Reactions with a range of hydrazines reveal complex chemical behavior that includes not only N-N bond reduction for 1,2-di- and trisubstituted arylhydrazines, but also catalytic disproportionation for 1,2-diarylhydrazines, N-C bond cleavage for 1,2-diisopropylhydrazine, and no reaction for hindered and tetrasubstituted hydrazines.

Fast carbon dioxide fixation by 2,6-pyridinedicarboxamidato-nickel(II)-hydroxide complexes: influence of changes in reactive site environment on reaction rates.

The planar complexes [Ni(II)(pyN(2)(R2))(OH)](-), containing a terminal hydroxo group, are readily prepared from N,N'-(2,6-C(6)H(3)R(2))-2,6-pyridinedicarboxamidate(2-) tridentate pincer ligands (R(4)N)(OH), and Ni(OTf)(2). These complexes react cleanly and completely with carbon dioxide in DMF solution in a process of CO(2) fixation with formation of the bicarbonate product complexes [Ni(II)(pyN(2)(R2))(HCO(3))](-) having η(1)-OCO(2)H ligation. Fixation reactions follow second-order kinetics (rate = k(2)'[Ni(II)-OH][CO(2)]) with negative activation entropies (-17 to -28 eu). Reactions were monitored by growth and decay of metal-to-ligand charge-transfer (MLCT) bands at 350-450 nm. The rate order R = Me > macro > Et > Pr(i) > Bu(i) > Ph at 298 K (macro = macrocylic pincer ligand) reflects increasing steric hindrance at the reactive site. The inherent highly reactive nature of these complexes follows from k(2)' ≈ 10(6) M(-1) s(-1) for the R = Me system that is attenuated by only 100-fold in the R = Ph complex. A reaction mechanism is proposed based on computation of the enthalpic reaction profile for the R = Pr(i) system by DFT methods. The R = Et, Pr(i), and Bu(i) systems display biphasic kinetics in which the initial fast process is followed by a slower first order process currently of uncertain origin.

Selective syntheses of iron-imide-sulfide cubanes, including a partial representation of the Fe-S-X environment in the FeMo cofactor.

The dinuclear precursors Fe(2)(N(t)Bu)(2)Cl(2)(NH(2)(t)Bu)(2), [Fe(2)(N(t)Bu)(S)Cl(4)](2-), and Fe(2)(NH(t)Bu)(2)(S)(N{SiMe(3)}(2))(2) allowed the selective syntheses of the cubane clusters [Fe(4)(N(t)Bu)(n)(S)(4-n)Cl(4)](z) with [n, z] = [3, 1-], [2, 2-], [1, 2-]. Weak-field iron-sulfur clusters with heteroleptic, nitrogen-containing cores are of interest with respect to observed or conjectured environments in the iron-molybdenum cofactor of nitrogenase. In this context, the present iron-imide-sulfide clusters constitute a new class of compounds for study, with the Fe(4)NS(3) core of the [1, 2-] cluster affording the first synthetic representation of the corresponding heteroligated Fe(4)S(3)X subunit in the cofactor.

[Fe(4)S(4)](q) cubane clusters (q = 4+, 3+, 2+) with terminal amide ligands.

Bis(trimethylsilyl)amide-ligated iron-sulfur cubane clusters [Fe(4)(mu(3)-S)(4)(N{SiMe(3)}(2))(4)](z) (z = 0, 1-, 2-) are accessible by the reaction of FeCl(N{SiMe(3)}(2))(2)(THF) (1) with 1 equiv of NaSH (z = 0), followed by reduction with either 0.25 (z = 1-) or 1 equiv (z = 2-) of Na(2)S as needed. The anionic clusters are obtained as the sodium salts [Na(THF)(2)][Fe(4)S(4)(N{SiMe(3)}(2))(4)] and [Na(THF)(2)](2)[Fe(4)S(4)(N{SiMe(3)}(2))(4)]; in the solid state, these two clusters both possess a unique contact ion pair motif in which individual sodium ions each coordinate to a cluster core sulfide, an adjacent amide nitrogen, and two THF donors. The monoanionic cluster can also be prepared as the lithium salt [Li(THF)(4)][Fe(4)S(4)(N{SiMe(3)}(2))(4)] by the reaction of 1 with 1:0.5 LiCl/Li(2)S. The characterization of the three-membered redox series allows an analysis of redox trends, as well as a study of the effects of the amide donor environment on the [Fe(4)S(4)] core. Bis(trimethylsilyl)amide terminal ligation significantly stabilizes oxidized cluster redox states, permitting isolation of the uncommon [Fe(4)S(4)](3+) and unprecedented [Fe(4)S(4)](4+) weak-field cores.

Calibration Bias and the Interpretation of Clinical Learning Environment Perceptions Surveys.

BACKGROUND: The clinical learning environment (CLE) is frequently assessed using perceptions surveys, such as the AAMC Graduation Questionnaire and ACGME Resident/Fellow Survey. However, these survey responses often capture subjective factors not directly related to the trainee's CLE experiences. OBJECTIVE: The authors aimed to assess these subjective factors as "calibration bias" and show how it varies by health professions education discipline, and co-varies by program, patient-mix, and trainee factors. METHODS: We measured calibration bias using 2011-2017 US Department of Veterans Affairs (VA) Learners' Perceptions Survey data to compare medical students and physician residents and fellows (n = 32 830) with nursing (n = 29 758) and allied and associated health (n = 27 092) trainees. RESULTS: Compared to their physician counterparts, nursing trainees (OR 1.31, 95% CI 1.22-1.40) and allied/associated health trainees (1.18, 1.12-1.24) tended to overrate their CLE experiences. Across disciplines, respondents tended to overrate CLEs when reporting 1 higher level (of 5) of psychological safety (3.62, 3.52-3.73), 1 SD more time in the CLE (1.05, 1.04-1.07), female gender (1.13, 1.10-1.16), 1 of 7 lower academic level (0.95, 1.04-1.07), and having seen the lowest tercile of patients for their respective discipline who lacked social support (1.16, 1.12-1.21) and had low income (1.05, 1.01-1.09), co-occurring addictions (1.06, 1.02-1.10), and mental illness (1.06, 1.02-1.10). CONCLUSIONS: Accounting for calibration bias when using perception survey scores is important to better understand physician trainees and the complex clinical learning environments in which they train.

A biomimetic approach to oxidized sites in the xanthine oxidoreductase family: synthesis and stereochemistry of tungsten(VI) analogue complexes.

Two series of square pyramidal (SP) monodithiolene complexes, [M (VI)O 3- n S n (bdt)] (2-) and their silylated derivatives [M (VI)O 2- n S n (OSiR 3)(bdt)] (-) ( n = 0, M = Mo or W; n = 1, 2, M = W), synthesized in this and previous work, constitute the basic molecules in a biomimetic approach to structural analogues of the oxidized sites in the xanthine oxidoreductase enzyme family. Benzene-1,2-dithiolate (bdt) simulates native pyranopterindithiolene chelation in the basal plane, tungsten instead of the native metal molybdenum was employed in sulfido complexes to avoid autoreduction, and silylation models protonation. The complexes [MO 3(bdt)] (2-) and [MO 2(OSiR 3)(bdt)] (-) represent inactive sites, while [MO 2S(bdt)] (2-) and [MOS(OSiR 3)(bdt)] (-), with basal sulfido and silyloxo ligands, are the first analogues of the catalytic sites. Also prepared were [MOS 2(bdt)] (2-) and [MS 2(OSiR 3)(bdt)] (-), with basal sulfido and silyloxo ligands. Complexes are described by angular parameters which reveal occasional distortions from idealized SP toward a trigonal bipyramidal (TBP) structure arising from crystal packing forces in crystalline Et 4N (+) salts. Miminized energy structures from DFT calculations are uniformly SP and reproduce experimental structures. For example, the correct structure is predicted for [WO 2S(bdt)] (2-), whose basal and apical sulfido diastereomers are potentially interconvertible through a low-lying TBP transition state for pseudorotation. The lowest energy tautomer of the protonated form is calculated to be [WOS(OH)(bdt)] (-), with basal sulfido and hydroxo ligands. Computational and experimental structures indicate that protein sites adopt intrinsic coordination geometries rather than those dictated by protein structure and environment.

Reactivity of a sterically hindered Fe(II) thiolate dimer with amines and hydrazines.

The sterically hindered Fe(II) thiolate dimer Fe(2)(mu-STriph)(2)(STriph)(2) (1; [STriph](-) = 2,4,6-triphenylbenzenethiolate) reacts with primary amines ((t)BuNH(2), aniline) and N(2)H(4) to form the structurally characterized addition complexes Fe(STriph)(2)(NH(2)(t)Bu)(2), Fe(2)(mu-STriph)(2)(STriph)(2)(NH(2)Ph)(2), and Fe(2)(mu-eta(1):eta(1)-N(2)H(4))(2)(N(2)H(4))(4)(STriph)(4) in high yield. Chemical and NMR spectroscopic evidence indicate that the binding of these nitrogen donors is labile in solution and multispecies equilibria are likely. With arylhydrazines, 1 catalytically disproportionates 1,2-diphenylhydrazine to aniline and azobenzene, and it rearranges 1-methyl-1,2-diarylhydrazines to give, after treatment with alumina, mononuclear, trigonal bipyramidal Fe(III) complexes of composition Fe(ISQ)(2)(STriph), where [ISQ](-) denotes an appropriately substituted bidentate o-diiminobenzosemiquinonate ligand. Complex 1 shows no reaction with hindered 1,2-dialkylhydrazines (isopropyl or tert-butyl) or tetrasubstituted 1,2-dimethyl-1,2-diphenylhydrazine.

Multiple-Edge XAS Studies of Synthetic Iron-Copper Bridged Molecular Assemblies Relevant to Cytochrome c Oxidase. Structure Determination Using Multiple-Scattering Analysis with Statistical Evaluation of Errors.

An X-ray absorption spectroscopy study has been carried out at the Fe and Cu K-edges for two bridged molecular assemblies, both of which contain an Fe-X-Cu (X = O(2)(-), OH(-)) bridge unit, some of whose features are relevant to the binuclear site of cytochrome c oxidase. The two complexes [(OEP)Fe-O-Cu(Me(6)tren)](1+) and [(OEP)Fe-(OH)-Cu(Me(5)tren)(OClO(3))](1+) have similar structural fragments around the metal centers except that they differ significantly in the bridge structure (the former contains a linear oxo bridge while the latter has a bent hydroxo bridge). We report a comparative study of these complexes using multiple-scattering (MS) EXAFS analysis and the program package GNXAS. It is found that there is a dramatic increase in the amplitude of the Fe-X-Cu MS pathway as the bridge unit approaches linearity. Full EXAFS MS analysis enables accurate quantitation of bridge metrical details and geometry for both complexes. These studies were done with an expanded version of GNXAS, which allows for simultaneous multiple-edge fitting. Such multiple-edge analysis (using both Fe and Cu edge data) allows common pathways (in this case involving the Fe-X-Cu bridge) to be constrained to be the same, thus improving the observation/variable ratio and enhancing sensitivity for determination of the bridge structure. The accuracy of the structural determination for the bridge units is evaluated by a statistical analysis methodology in which correlations among fitting parameters are identified and contour plots are used to determine random error. The overall error in the EXAFS structural determination is found by establishing the variance with the crystallographically determined values: for the EXAFS-determined parameters at distances below 4 Å, distances and angles deviated on average from crystallographic values by 0.014 Å and 1.5 degrees, respectively. It is also established that structural features in the Fe absorption preedge are diagnostic of oxo vs hydroxo ligation. The relevance of this study to the structural definition of binuclear bridged sites in cytochrome c oxidase and other metalloenzymes is considered.

Synthesis and elaboration of the dinuclear iron-imide cluster core [Fe2(mu-NR)2]2+.

The protolysis of mononuclear ferric amide precursors FeCl[N(SiMe3)2]2(THF) (1) or [FeCl2{N(SiMe3)2}2]- (2) by primary amines provides, under suitable conditions, an effective route to dinuclear weak-field ferric-imide clusters with [Fe2(mu-NR)2]2+ cores. In the synthesis of known arylimide clusters [Fe2(mu-NAr)2Cl4]2- (Ar = Ph, p-Tol, Mes) from 2, the counterion has a major effect on selectivity and yield, and the use of quaternary ammonium salts affords a substantial improvement over earlier, Li+-based chemistry. The new tert-butylimide core is obtained by protolysis of 1 with excess tBuNH2 to give crystalline cis-Fe2(mu-NtBu)2Cl2(NH2tBu)2 (9). Complex 9 can be transformed to other dinuclear species through substitution of the terminal amines by pyridines, PEt3, or chloride, or through protolysis of bridging alkylimides by arylamines, allowing isolation of trans-Fe2(mu-NtBu)2Cl2(DMAP)2 (DMAP = 4-dimethylaminopyridine), cis-Fe2(mu-NtBu)2Cl2(PEt3)2, [Fe2(mu-NtBu)2Cl4]-, and trans-Fe2(mu-NPh)2Cl2(NH2tBu)2. The susceptibility of alkyl substituents to beta-elimination appears to limit the general applicability of protolytic cluster assembly using alkylamines. The dinuclear clusters have been characterized by X-ray, spectroscopic, and electrochemical analyses.

Speculative synthetic chemistry and the nitrogenase problem.

There exist a limited but growing number of biological metal centers whose properties lie conspicuously outside the realm of known inorganic chemistry. The synthetic analogue approach, broadly directed, offers a powerful exploratory tool that can define intrinsic chemical possibilities for these sites while simultaneously expanding the frontiers of fundamental inorganic chemistry. This speculative application of analogue study is exemplified here in the evolution of synthetic efforts inspired by the cluster chemistry of biological nitrogen fixation.

3,3'-Bis(triphenylsilyl)biphenoxide as a sterically hindered ligand on Fe(II), Fe(III), and Cr(II).

The structural coordination chemistry of the sterically hindered 3,3'-bis(triphenylsilyl)-1,1'-bi-2-phenoxide ligand ([(TPS)LO(2)](2)(-)), an isosteric homologue of a chiral binaphthoxide ligand used in asymmetric induction, has been investigated on Fe(II), Fe(III), and Cr(II). The ligand diol ((TPS)L(OH)(2), 2) can be prepared on a multigram scale from 2,2'-dimethoxybiphenyl via a convenient, two-step ortho-metalation/silylation/deprotection sequence; the sodium salt of the ligand dianion (Na(2)[(TPS)LO(2)], 3) can be obtained by NaH deprotonation and was crystallographically characterized with two THF ligands bound to each cation. Protonolysis of Fe[N(SiMe(3))(2)](2) or Cr[N(SiMe(3))(2)](2)(THF)(2) with biphenol 2 in arene solvent gives [Fe(mu-(TPS)LO(2))](2) (4) or ((TPS)LO(2))Cr(THF)(2) (9), respectively, while anion metathesis of FeCl(2) or FeCl(3)/bipy with biphenoxide salt 3 in THF yields ((TPS)LO(2))Fe(THF)(2) (8) or ferric monomer ((TPS)LO(2))FeCl(bipy) (10), respectively. Dimer 4 reacts with exogenous ligands to form monomeric ligand adducts of Fe(II): with py, ((TPS)LO(2))Fe(py)(2) (5); with bipy, ((TPS)LO(2))Fe(bipy) (6); with XyNC (Xy = 2,6-xylyl), ((TPS)LO(2))Fe(CNXy)(4) (7); and with THF, 8. Complexes were characterized in solution by (1)H NMR and in the solid state by single-crystal X-ray diffraction. The 4-coordinate complexes (5, 6, 8, 9) adopt skew-distorted tetrahedral (for Fe(II)) or square planar (for Cr(II)) geometries; the 5- and 6-coordinate complexes (10 and 7) assume more typical distorted square pyramidal/trigonal bipyramidal and cis-octahedral stereochemistries. Dimer 4 possesses an unusual structure where each Fe(II) center is pseudo-4-coordinate and each biphenoxide ligand provides one terminal phenoxide donor, one bridging phenoxide, and a weak aromatic pi-interaction from one of the phenyl groups of a SiPh(3) substituent. The steric influence of the hindered biphenoxide ligand within the coordination sphere is revealed structurally through distortion of coordination polyhedra in the 4-coordinate species and through conformational and deformational changes within the biphenoxide ligand itself.

Iron-arylimide clusters [Fe(m)()(NAr)(n)Cl(4)](2)(-) (m, n = 2, 2; 3, 4; 4, 4) from a ferric amide precursor: synthesis, characterization, and comparison to Fe-S chemistry.

Tetrahedral FeCl[N(SiMe(3))(2)](2)(THF) (2), prepared from FeCl(3) and 2 equiv of Na[N(SiMe(3))(2)] in THF, is a useful ferric starting material for the synthesis of weak-field iron-imide (Fe-NR) clusters. Protonolysis of 2 with aniline yields azobenzene and [Fe(2)(mu-Cl)(3)(THF)(6)](2)[Fe(3)(mu-NPh)(4)Cl(4)] (3), a salt composed of two diferrous monocations and a trinuclear dianion with a formal 2 Fe(III)/1 Fe(IV) oxidation state. Treatment of 2 with LiCl, which gives the adduct [FeCl(2)(N(SiMe(3))(2))(2)](-) (isolated as the [Li(TMEDA)(2)](+) salt), suppresses arylamine oxidation/iron reduction chemistry during protonolysis. Thus, under appropriate conditions, the reaction of 1:1 2/LiCl with arylamine provides a practical route to the following Fe-NR clusters: [Li(2)(THF)(7)][Fe(3)(mu-NPh)(4)Cl(4)] (5a), which contains the same Fe-NR cluster found in 3; [Li(THF)(4)](2)[Fe(3)(mu-N-p-Tol)(4)Cl(4)] (5b); [Li(DME)(3)](2)[Fe(2)(mu-NPh)(2)Cl(4)] (6a); [Li(2)(THF)(7)][Fe(2)(mu-NMes)(2)Cl(4)] (6c). [Li(DME)(3)](2)[Fe(4)(mu(3)-NPh)(4)Cl(4)] (7), a trace product in the synthesis of 5a and 6a, forms readily as the sole Fe-NR complex upon reduction of these lower nuclearity clusters. Products were characterized by X-ray crystallographic analysis, by electronic absorption, (1)H NMR, and Mössbauer spectroscopies, and by cyclic voltammetry. The structures of the Fe-NR complexes derive from tetrahedral iron centers, edge-fused by imide bridges into linear arrays (5a,b; 6a,c) or the condensed heterocubane geometry (7), and are homologous to fundamental iron-sulfur (Fe-S) cluster motifs. The analogy to Fe-S chemistry also encompasses parallels between Fe-mediated redox transformations of nitrogen and sulfur ligands and reductive core conversions of linear dinuclear and trinuclear clusters to heterocubane species and is reinforced by other recent examples of iron- and cobalt-imide cluster chemistry. The correspondence of nitrogen and sulfur chemistry at iron is intriguing in the context of speculative Fe-mediated mechanisms for biological nitrogen fixation.