The extradiol, aromatic ring-cleaving enzyme homoprotocatechuate 2,3-dioxygenase (HPCD) catalyzes a complex chain of reactions that involve second sphere residues of the active site. The importance of the second-sphere residue His200 was demonstrated in studies of HPCD variants, such as His200Cys (H200C), which revealed significant retardations of certain steps in the catalytic process as a result of the substitution, allowing novel reaction cycle intermediates to be trapped for spectroscopic characterization. As the H200C variant largely retains the wild-type active site structure and produces the correct ring-cleaved product, this variant presents a valuable target for mechanistic HPCD studies. Here, the high-spin FeII states of resting H200C and the H200C–homoprotocatechuate enzyme–substrate (ES) complex have been characterized with Mössbauer spectroscopy to assess the electronic structures of the active site in these states. The analysis reveals a high-spin FeII center in a low symmetry environment that is reflected in the values of the zero-field splitting (ZFS) (D ≈ – 8 cm–1, E/D ≈ 1/3 in ES), as well as the relative orientations of the principal axes of the 57Fe magnetic hyperfine (A) and electric field gradient (EFG) tensors relative to the ZFS tensor axes. A spin Hamiltonian analysis of the spectra for the ES complex indicates that the magnetization axis of the integer-spin S = 2 FeII system is nearly parallel to the symmetry axis, z, of the doubly occupied dxy ground orbital deduced from the EFG and A-values, an observation, which cannot be rationalized by DFT assisted crystal-field theory. In contrast, ORCA/CASSCF calculations for the ZFS tensor in combination with DFT calculations for the EFG- and A-tensors describe the experimental data remarkably well.

It was shown previously (J. Am. Chem. Soc. 2014, 136, 10846) that bubbling of O2 into a solution of FeII(BDPP) (H2BDPP = 2,6-bis[[(S)-2-(diphenylhydroxymethyl)-1-pyrrolidinyl]methyl]pyridine) in tetrahydrofuran at −80 °C generates a high-spin (SFe = 5/2) iron(III) superoxo adduct, 1. Mössbauer studies revealed that 1 is an exchange-coupled system, , where SR = 1/2 is the spin of the superoxo radical, of which the spectra were not well enough resolved to determine whether the coupling was ferromagnetic (S = 3 ground state) or antiferromagnetic (S = 2). The glass-forming 2-methyltetrahydrofuran solvent yields highly resolved Mössbauer spectra from which the following data have been extracted: (i) the ground state of 1 has S = 3 (J < 0); (ii) |J| > 15 cm–1; (iii) the zero-field-splitting parameters are D = −1.1 cm–1 and E/D = 0.02; (iv) the major component of the electric-field-gradient tensor is tilted ≈7° relative to the easy axis of magnetization determined by the MS = ±3 and ±2 doublets. The excited-state MS = ±2 doublet yields a narrow parallel-mode electron paramagnetic resonance signal at g = 8.03, which was used to probe the magnetic hyperfine splitting of 17O-enriched O2. A theoretical model that considers spin-dependent electron transfer for the cases where the doubly occupied π* orbital of the superoxo ligand is either “in” or “out” of the plane defined by the bent Fe–OO moiety correctly predicts that 1 has an S = 3 ground state, in contrast to the density functional theory calculations for 1, which give a ground state with both the wrong spin and orbital configuration. This failure has been traced to a basis set superposition error in the interactions between the superoxo moiety and the adjacent five-membered rings of the BDPP ligand and signals a fundamental problem in the quantum chemistry of O2 activation.

High-spin oxoiron(IV) species are often implicated in the mechanisms of nonheme iron oxygenases, their C–H bond cleaving properties being attributed to the quintet spin state. However, the few available synthetic S = 2 FeIV═O complexes supported by polydentate ligands do not cleave strong C–H bonds. Herein we report the characterization of a highly reactive S = 2 complex, [FeIV(O)(TQA)(NCMe)]2+ (2) (TQA = tris(2-quinolylmethyl)amine), which oxidizes both C–H and C═C bonds at −40 °C. The oxidation of cyclohexane by 2 occurs at a rate comparable to that of the oxidation of taurine by the TauD-J enzyme intermediate after adjustment for the different temperatures of measurement. Moreover, compared with other S = 2 complexes characterized to date, the spectroscopic properties of 2 most closely resemble those of TauD-J. Together these features make 2 the best electronic and functional model for TauD-J to date.

The extradiol-cleaving dioxygenase homoprotocatechuate 2,3-dioxygenase (HPCD) binds substrate homoprotocatechuate (HPCA) and O2 sequentially in adjacent ligand sites of the active site FeII. Kinetic and spectroscopic studies of HPCD have elucidated catalytic roles of several active site residues, including the crucial acid–base chemistry of His200. In the present study, reaction of the His200Cys (H200C) variant with native substrate HPCA resulted in a decrease in both kcat and the rate constants for the activation steps following O2 binding by >400 fold. The reaction proceeds to form the correct extradiol product. This slow reaction allowed a long-lived (t1/2 = 1.5 min) intermediate, H200C-HPCAInt1 (Int1), to be trapped. Mössbauer and parallel mode electron paramagnetic resonance (EPR) studies show that Int1 contains an S1 = 5/2 FeIII center coupled to an SR = 1/2 radical to give a ground state with total spin S = 2 (J > 40 cm–1) in . Density functional theory (DFT) property calculations for structural models suggest that Int1 is a (HPCA semiquinone•)FeIII(OOH) complex, in which OOH is protonated at the distal O and the substrate hydroxyls are deprotonated. By combining Mössbauer and EPR data of Int1 with DFT calculations, the orientations of the principal axes of the 57Fe electric field gradient and the zero-field splitting tensors (D = 1.6 cm–1, E/D = 0.05) were determined. This information was used to predict hyperfine splittings from bound 17OOH. DFT reactivity analysis suggests that Int1 can evolve from a ferromagnetically coupled FeIII-superoxo precursor by an inner-sphere proton-coupled-electron-transfer process. Our spectroscopic and DFT results suggest that a ferric hydroperoxo species is capable of extradiol catalysis.

O2 bubbling into a THF solution of FeII(BDPP) (1) at −80 °C generates a reversible bright yellow adduct 2. Characterization by resonance Raman and Mössbauer spectroscopy provides complementary insights into the nature of 2. The former shows a resonance-enhanced vibration at 1125 cm–1, which can be assigned to the ν(O–O) of a bound superoxide, while the latter reveals the presence of a high-spin iron(III) center that is exchange-coupled to the superoxo ligand, like the FeIII–O2– pair found for the O2 adduct of 4-nitrocatechol-bound homoprotocatechuate 2,3-dioxygenase. Lastly, 2 oxidizes dihydroanthracene to anthracene, supporting the notion that FeIII–O2– species can carry out H atom abstraction from a C–H bond to initiate the 4-electron oxidation of substrates proposed for some nonheme iron enzymes.

Previous efforts to model the diiron(IV) intermediate Q of soluble methane monooxygenase have led to the synthesis of a diiron(IV) TPA complex, 2, with an O=FeIV–O–FeIV–OH core that has two ferromagnetically coupled Sloc = 1 sites. Addition of base to 2 at −85 °C elicits its conjugate base 6 with a novel O═FeIV–O–FeIV═O core. In frozen solution, 6 exists in two forms, 6a and 6b, that we have characterized extensively using Mössbauer and parallel mode EPR spectroscopy. The conversion between 2 and 6 is quantitative, but the relative proportions of 6a and 6b are solvent dependent. 6a has two equivalent high-spin (Sloc = 2) sites, which are antiferromagnetically coupled; its quadrupole splitting (0.52 mm/s) and isomer shift (0.14 mm/s) match those of intermediate Q. DFT calculations suggest that 6a assumes an anti conformation with a dihedral O═Fe–Fe═O angle of 180°. Mössbauer and EPR analyses show that 6b is a diiron(IV) complex with ferromagnetically coupled Sloc = 1 and Sloc = 2 sites to give total spin St = 3. Analysis of the zero-field splittings and magnetic hyperfine tensors suggests that the dihedral O═Fe–Fe═O angle of 6b is ∼90°. DFT calculations indicate that this angle is enforced by hydrogen bonding to both terminal oxo groups from a shared water molecule. The water molecule preorganizes 6b, facilitating protonation of one oxo group to regenerate 2, a protonation step difficult to achieve for mononuclear FeIV═O complexes. Complex 6 represents an intriguing addition to the handful of diiron(IV) complexes that have been characterized.

To obtain structural and spectroscopic models for the diiron(II,III) centers in the active sites of diiron enzymes, the (μ-alkoxo)(μ-carboxylato)diiron(II,III) complexes [FeIIFeIII(N-Et-HPTB)(O2CPh)(NCCH3)2](ClO4)3 (1) and [FeIIFeIII(N-Et-HPTB)(O2CPh)(Cl)(HOCH3)](ClO4)2 (2) (N-Et-HPTB = N,N,N′,N′-tetrakis(2-(1-ethyl-benzimidazolylmethyl))-2-hydroxy-1,3-diaminopropane) have been prepared and characterized by X-ray crystallography, UV–visible absorption, EPR, and Mössbauer spectroscopies. Fe1–Fe2 separations are 3.60 and 3.63 Å, and Fe1–O1–Fe2 bond angles are 128.0° and 129.4° for 1 and 2, respectively. Mössbauer and EPR studies of 1 show that the FeIII (SA = 5/2) and FeII (SB = 2) sites are antiferromagnetically coupled to yield a ground state with S = 1/2 (g= 1.75, 1.88, 1.96); Mössbauer analysis of solid 1 yields J = 22.5 ± 2 cm–1 for the exchange coupling constant ( = JSA·SB convention). In addition to the S = 1/2 ground-state spectrum of 1, the EPR signal for the S = 3/2 excited state of the spin ladder can also be observed, the first time such a signal has been detected for an antiferromagnetically coupled diiron(II,III) complex. The anisotropy of the 57Fe magnetic hyperfine interactions at the FeIII site is larger than normally observed in mononuclear complexes and arises from admixing S > 1/2 excited states into the S = 1/2 ground state by zero-field splittings at the two Fe sites. Analysis of the “D/J” mixing has allowed us to extract the zero-field splitting parameters, local g values, and magnetic hyperfine structural parameters for the individual Fe sites. The methodology developed and followed in this analysis is presented in detail. The spin Hamiltonian parameters of 1 are related to the molecular structure with the help of DFT calculations. Contrary to what was assumed in previous studies, our analysis demonstrates that the deviations of the g values from the free electron value (g = 2) for the antiferromagnetically coupled diiron(II,III) core in complex 1 are predominantly determined by the anisotropy of the effective g values of the ferrous ion and only to a lesser extent by the admixture of excited states into ground-state ZFS terms (D/J mixing). The results for 1 are discussed in the context of the data available for diiron(II,III) clusters in proteins and synthetic diiron(II,III) complexes.

The trigonal-bipyramidal high-spin (S = 2) oxoiron(IV) complex [FeIV(O)(TMG2dien)(CH3CN)]2+ (7) was synthesized and spectroscopically characterized. Substitution of the CH3CN ligand by anions, demonstrated here for X = N3– and Cl–, yielded additional S = 2 oxoiron(IV) complexes of general formulation [FeIV(O)(TMG2dien)(X)]+ (7-X). The reduced steric bulk of 7 relative to the published S = 2 complex [FeIV(O)(TMG3tren)]2+ (2) was reflected by enhanced rates of intermolecular substrate oxidation.

The all-ferrous, carbene-capped Fe4S4 cluster, synthesized by Deng and Holm (DH complex), has been studied with density functional theory (DFT). The geometry of the complex was optimized for several electronic configurations. The lowest energy was obtained for the broken-symmetry (BS) configuration derived from the ferromagnetic state by reversing the spin projection of one of the high spin (Si = 2) irons. The optimized geometry of the latter configuration contains one unique and three equivalent iron sites, which are both structurally and electronically clearly distinguishable. For example, a distinctive feature of the unique iron site is the diagonal Fe···S distance, which is 0.3 Å longer than for the equivalent irons. The calculated 57Fe hyperfine parameters show the same 1:3 pattern as observed in the Mössbauer spectra and are in good agreement with experiment. BS analysis of the exchange interactions in the optimized geometry for the 1:3, MS = 4, BS configuration confirms the prediction of an earlier study that the unique site is coupled to the three equivalent ones by strong antiferromagnetic exchange (J > 0 in J ∑j<4Ŝ4·Ŝj) and that the latter are mutually coupled by ferromagnetic exchange (J′ < 0 in J′ ∑i<j<4Ŝi·Ŝj). In combination, these exchange couplings stabilize an S = 4 ground state in which the composite spin of the three equivalent sites (S123 = 6) is antiparallel to the spin (S4 = 2) of the unique site. Thus, DFT analysis supports the idea that the unprecedented high value of the spin of the DH complex and, by analogy, of the all-ferrous cluster of the Fe-protein of nitrogenase, results from a remarkably strong dependence of the exchange interactions on cluster core geometry. The structure dependence of the exchange-coupling constants in the FeII-(μ3-S)2-FeII moieties of the all-ferrous clusters is compared with the magneto-structural correlations observed in the data for dinuclear copper complexes. Finally, we discuss two all-ferric clusters in the light of the results for the all-ferrous cluster.

The newly synthesized dinuclear complex [FeIII2(μ-OH)2(bik)4](NO3)4 (1) (bik, bis(1-methylimidazol-2-yl)ketone) shows rather short Fe···Fe (3.0723(6) Å) and Fe–O distances (1.941(2)/1.949(2) Å) compared to other unsupported FeIII2(μ-OH)2 complexes. The bridging hydroxide groups of 1 are strongly hydrogen-bonded to a nitrate anion. The 57Fe isomer shift (δ = 0.45 mm s–1) and quadrupole splitting (ΔEQ = 0.26 mm s–1) obtained from Mössbauer spectroscopy are consistent with the presence of two identical high-spin iron(III) sites. Variable-temperature magnetic susceptibility studies revealed antiferromagnetic exchange (J = 35.9 cm–1 and H = JS1·S2) of the metal ions. The optimized DFT geometry of the cation of 1 in the gas phase agrees well with the crystal structure, but both the Fe···Fe and Fe–OH distances are overestimated (3.281 and 2.034 Å, respectively). The agreement in these parameters improves dramatically (3.074 and 1.966 Å) when the hydrogen-bonded nitrate groups are included, reducing the value calculated for J by 35%. Spontaneous reduction of 1 was observed in methanol, yielding a blue [FeII(bik)3]2+ species. Variable-temperature magnetic susceptibility measurements of [FeII(bik)3](OTf)2 (2) revealed spin-crossover behavior. Thermal hysteresis was observed with 2, due to a loss of cocrystallized solvent molecules, as monitored by thermogravimetric analysis. The hysteresis disappears once the solvent is fully depleted by thermal cycling. [FeII(bik)3](OTf)2 (2) catalyzes the oxidation of alkanes with t-BuOOH. High selectivity for tertiary C–H bond oxidation was observed with adamantane (3°/2° value of 29.6); low alcohol/ketone ratios in cyclohexane and ethylbenzene oxidation, a strong dependence of total turnover number on the presence of O2, and a low retention of configuration in cis-1,2-dimethylcyclohexane oxidation were observed. Stereoselective oxidation of olefins with dihydrogen peroxide yielding epoxides was observed under both limiting oxidant and substrate conditions.

Two all-ferrous, edge-bridged 8Fe−8S clusters, one capped with carbenes (2) and the other with phosphenes (3), have been characterized by 57Fe Mössbauer spectroscopy. The clusters have diamagnetic ground states that yield spectra consisting of one quadrupole doublet with a large splitting (25% of absorption) and one (3) or two (2) doublets with much smaller splittings (75% of absorption). These patterns closely resemble those observed for all-ferrous 4Fe−4S clusters. Structurally, the 4Fe−4S fragments of 2 and 3 are remarkably similar to all-ferrous 4Fe−4S clusters, sharing with them the characteristic 3:1 pattern of the iron sites, a differentiation that has been shown previously to reflect spontaneous distortions of the cluster core. These spectroscopic and geometric similarities suggest that the diamagnetic ground state of the 8Fe−8S cluster results from antiferromagnetic exchange coupling of two identical 4Fe−4S modules, each carrying spin S4Fe = 4. The iron atoms with the largest quadrupole splittings are located at the opposite ends of the body diagonal containing the bridging sulfides.

The synthesis and crystallographic characterization of a new family of M(μ-CN)Ln complexes are reported. Two structural series have been prepared by reacting in water rare earth nitrates (LnIII = La, Pr, Nd, Sm, Eu, Gd, Dy, Ho) with K3[M(CN)6] (MIII = Fe, Co) in the presence of hexamethylenetetramine (hmt). The first series consists of six isomorphous heterobinuclear complexes, [(CN)5M-CN-Ln(H2O)8]·2hmt ([FeLa] 1, [FePr] 2, [FeNd] 3, [FeSm] 4, [FeEu] 5, [FeGd] 6), while the second series consists of four isostructural ionic complexes, [M(CN)6][Ln(H2O)8]·hmt ([FeDy] 7, [FeHo] 8, [CoEu] 9, [CoGd] 10). The hexamethylenetetramine molecules contribute to the stabilization of the crystals by participating in an extended network of hydrogen bond interactions. In both series the aqua ligands are hydrogen bonded to the nitrogen atoms from both the terminal CN− groups and the hmt molecules. The [FeGd] complex has been analyzed with 57Fe Mössbauer spectroscopy and magnetic susceptibility measurements. We have also analyzed the [FeLa] complex, in which the paramagnetic GdIII is replaced by diamagnetic LaIII, with 57Fe Mössbauer spectroscopy, electron paramagnetic resonance (EPR), and magnetic susceptibility measurements, to obtain information about the low-spin FeIII site that is not accessible in the presence of a paramagnetic ion at the complementary site. For the same reason, the [CoGd] complex, containing diamagnetic CoIII, was studied with EPR and magnetic susceptibility measurements, which confirmed the S = 7/2 spin of GdIII. Prior knowledge about the paramagnetic sites in [FeGd] allows a detailed analysis of the exchange interactions between them. In particular, the question of whether the exchange interaction in [FeGd] is isotropic or anisotropic has been addressed. Standard variable-temperature magnetic susceptibility measurements provide only the value for a linear combination of Jx, Jy, and Jz but contain no information about the values of the individual exchange parameters Jx, Jy, and Jz. In contrast, the spin-Hamiltonian analysis of the variable-field, variable-temperature Mössbauer spectra reveals an exquisite sensitivity on the anisotropic exchange parameters. Analysis of these dependencies in conjunction with adopting the g-values obtained for [FeLa], yielded the values Jx = +0.11 cm−1, Jy = +0.33 cm−1, and Jz = +1.20 cm−1 (Ŝ1·J·Ŝ2 convention). The consistency of these results with magnetic susceptibility data is analyzed. The exchange anisotropy is rooted in the spatial anisotropy of the low-spin FeIII ion. The condition for anisotropic exchange is the presence of low-lying orbital excited states at the ferric site that (i) effectively interact through spin−orbit coupling with the orbital ground state and (ii) have an exchange parameter with the Gd site with a value different from that for the ground state. Density functional theory (DFT) calculations, without spin−orbit coupling, reveal that the unpaired electron of the t2g5 ground configuration of the FeIII ion occupies the xy orbital, that is, the orbital along the plane perpendicular to the Fe···Gd vector. The exchange-coupling constants for this orbital, jxy, and for the other t2g orbitals, jyz and jxz, have been determined using a theoretical model that relates them to the anisotropic exchange parameters and the g-values of FeIII. The resulting values, jyz = −5.7 cm−1, jxz = −4.9 cm−1, and jxy = +0.3 cm−1 are quite different. The origin of the difference is briefly discussed.

Previously we have characterized two high-valent complexes [LFeIV(μ-O)2FeIIIL], 1, and [LFeIV(O)(μ-O)(OH) FeIVL], 4. Addition of hydroxide or fluoride to 1 produces two new complexes, 1-OH and 1-F. Electron paramagnetic resonance (EPR) and Mössbauer studies show that both complexes have an S = 1/2 ground state which results from antiferromagnetic coupling of the spins of a high-spin (Sa = 5/2) FeIII and a high-spin (Sb = 2) FeIV site. 1-OH can also be obtained by a 1-electron reduction of 4, which has been shown to have an FeIV═O site. Radiolytic reduction of 4 at 77 K yields a Mössbauer spectrum identical to that observed for 1-OH, showing that the latter contains an FeIV═O. Interestingly, the FeIV═O moiety has Sb = 1 in 4 and Sb = 2 in 1-OH and 1-F. From the temperature dependence of the S = 1/2 signal we have determined the exchange coupling constant J (ℋ = JŜa·Ŝb convention) to be 90 ± 20 cm−1 for both 1-OH and 1-F. Broken-symmetry density functional theory (DFT) calculations yield J = 135 cm−1 for 1-OH and J = 104 cm−1 for 1-F, in good agreement with the experiments. DFT analysis shows that the Sb = 1 → Sb = 2 transition of the FeIV═O site upon reduction of the FeIV−OH site to high-spin FeIII is driven primarily by the strong antiferromagnetic exchange in the (Sa = 5/2, Sb = 2) couple.

It is well established that the cysteinate-coordinated [Fe4S4] cluster of the iron protein of nitrogenase from Azotobacter vinelandii (Av2) can attain the all-ferrous core oxidation state. Mössbauer and electron paramagnetic resonance (EPR) studies have shown that the all-ferrous cluster has a ground-state spin S = 4 and an effective 3:1 site symmetry in the spin structure and 57Fe quadrupole interactions. Recently, Deng and Holm reported the synthesis of [Fe4S4(Pri2NHCMe2)4],2 (1; Pri2NHCMe2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) and showed that the all-ferrous carbene-coordinated cluster is amenable to physicochemical studies. Mössbauer and EPR studies of 1, reported here, reveal that the electronic structure of this complex is strikingly similar to that of the protein-bound cluster, suggesting that the ground-state spin and the 3:1 site ratio are consequences of spontaneous distortions of the cluster core. To gain insight into the origin of the peculiar ground state of the all-ferrous clusters in 1 and Av2, we have studied a theoretical model that is based on a Heisenberg−Dirac−van Vleck Hamiltonian whose exchange-coupling constants are a function of the Fe−Fe distances. By combining the exchange energies with the elastic deformation energies in the harmonic approximation, we obtain for a T2 distortion a minimum with spin S = 4 and a C3v core structure in which one iron is unique and three irons are equivalent. This minimum has all of the spectroscopic and structural characteristics of the all-ferrous clusters of 1 and Av2. Our analysis maps the unique spectroscopic iron site to a specific site in the X-ray structure of the [Fe4S4]0 core both in complex 1 and in Av2.

Recently, we reported the characterization of the S = 1/2 complex [FeV(O)B*]−, where B* belongs to a family of tetraamido macrocyclic ligands (TAMLs) whose iron complexes activate peroxides for environmentally useful applications. The corresponding one-electron reduced species, [FeIV(O)B*]2− (2), has now been prepared in >95% yield in aqueous solution at pH > 12 by oxidation of [FeIII(H2O)B*]− (1), with tert-butyl hydroperoxide. At room temperature, the monomeric species 2 is in a reversible, pH-dependent equilibrium with dimeric species [B*FeIV−O−FeIVB*]2− (3), with a pKa near 10. In zero field, the Mössbauer spectrum of 2 exhibits a quadrupole doublet with ΔEQ = 3.95(3) mm/s and δ = −0.19(2) mm/s, parameters consistent with a S = 1 FeIV state. Studies in applied magnetic fields yielded the zero-field splitting parameter D = 24(3) cm−1 together with the magnetic hyperfine tensor A/gnβn = (−27, −27, +2) T. Fe K-edge EXAFS analysis of 2 shows a scatterer at 1.69 (2) Å, a distance consistent with a FeIV═O bond. DFT calculations for [FeIV(O)B*]2− reproduce the experimental data quite well. Further significant improvement was achieved by introducing hydrogen bonding of the axial oxygen with two solvent–water molecules. It is shown, using DFT, that the 57Fe hyperfine parameters of complex 2 give evidence for strong electron donation from B* to iron.

The iron(II) complex LFeCl2Li(THF)2 (L = β-diketiminate), 1, has been studied with variable-temperature, variable-field Mössbauer spectroscopy and parallel mode electron paramagnetic resonance (EPR) spectroscopy in both solution and the solid state. In zero applied field the 4.2 K Mössbauer spectrum exhibits an isomer shift δ = 0.90 mm/s and quadrupole splitting ΔEQ = 2.4 mm/s, values that are typical for the high-spin (S = 2) state anticipated for the iron in 1. Spectra recorded in applied magnetic fields yield an anisotropic magnetic hyperfine tensor with Ax = +2.3 (+1.0) T, Ay = Az = −21.5 T (solution) and a nearly axial zero-field splitting of the spin quintet with D = Dx ≈ −14 cm−1 and rhombicity E/D ≈ 0.1. The small, positive value for Ax results from the presence of residual orbital angular momentum along x. The EPR analysis gives gx ≈ 2.4 (and gy ≈ gz ≈ 2.0) and reveals a split “MS = ± 2” ground doublet with a gap distributed around Δ = 0.42 cm−1. The Mössbauer spectra of 1 show unusual features that arise from the presence of orientation-dependent relaxation and a distribution in the magnetic hyperfine field along x. The origin of the distribution has been analyzed using crystal field theory. The analysis indicates that the distribution in the magnetic hyperfine field originates from a narrow distribution, σϕ ≈ 0.5°, in torsion angle ϕ between the FeN2 and FeCl2 planes, arising from minute inhomogeneities in the molecular environments.

Recently, the synthesis, crystallographic structure, and Mössbauer characterization of the first example of an [(FeIVTAML)2O]2− (TAML = tetra-amido microcyclic ligand) complex were reported. Here, we elucidate the prominent structural, electronic, and magnetic properties of this complex on the basis of density functional theory (DFT) calculations. While the torsion between the molecular halves is caused by hydrogen bonding between the TAML moieties, the bending of the Fe−O−Fe unit is an intrinsic property of the bridge. The values for the 57Fe isomer shift and quadrupole splitting obtained with DFT are in good agreement with experimental results and indicate that the irons have intermediate spin states (S1 = S2 = 1). The iron spins are coupled by strong antiferromagnetic exchange to yield a ground state with system spin S = 0. The Fe−O distances in the excited S > 0 states are significantly longer than in the ground state. Since the wave function of the ground state, in which the iron spins are antiferromagnetically coupled to give system spin S = 0, is a linear combination of Slater determinants that cannot be treated with existing DFT codes, the Fe−O distance for the S = 0 state has been estimated by extrapolation from the optimized geometries for the ferromagnetic state (S = 2) and the broken symmetry state to be 1.748 Å, in good agreement with the crystallographic distance 1.728 Å. To accommodate the spin-dependent reorganization energies, the conventional bilinear spin Hamiltonian has been extended with a biquadratic coupling term: Ĥex = c′ + j0Ŝ1·Ŝ2 + j1(Ŝ1·Ŝ2)2. A computational scheme is presented for estimating the exchange parameters, yielding the values j0 = 199 cm−1 and j1 = −61 cm−1 for [(FeIVB*)2O]2−. Two mechanisms for biquadratic exchange are discussed.

We have determined the equilibrium conformations of the diiron(III) cluster [2Fe–2S–4(SCH3)]2− using density functional theory. The conformers have dihedral Fe–Fe–S–C angles of ∼0° and ±120°. The relative energies of the conformers can be accurately parameterized with a small number of side-chain repulsion parameters. Of the 17 conformers identified on the basis of the ideal values for the dihedrals, 10 conformers are stable in both the ferromagnetic and broken symmetry state for the cluster. The exchange coupling constants for the seven energetically lowest conformers are predicted to belong to a narrow range, 150 cm−1 ⩽ J ⩽ 178 cm−1. The cluster conformers found in proteins do not coincide with any of the intrinsic ones, due to distortion of one of the dihedral angles under the influence of the protein scaffold.The equilibrium conformations of diiron(III) cluster [2Fe–2S–4(SCH3)]2− have been determined using density functional theory. The relative conformer energies depend on a small number of side-chain repulsion parameters. Ten of the 17 possible conformations are stable in both the ferromagnetic and broken symmetry state. The cartoon shows the conformation with the lowest energy. Conformation has a moderate influence on J.