Structure,insertion electrochemistry,and magnetic properties of a new type of substitutional solid solutions of copper,nickel, and iron hexacyanoferrates/hexacyanocobaltates

Substitutional solid solutions of metal hexacyanometalates in which low-spin iron(III) and cobalt(III) ions populate the carbon-coordinated sites were synthesized and studied by powder diffraction including Rietveld refinement, cyclic voltammetry of immobilized microparticles, diffuse reflection vis-spectrometry, and magnetization techniques. The continuous solid solution series of potassium copper(II), potassium nickel(II), and iron(III) [(hexacyanoferrate(III))(1-x)(hexacyanocobaltate(III))(x)] show that the substitution of low-spin iron(III) by cobalt(III) in the hexacyanometalate units more strongly affects the formal potentials of the nitrogen-coordinated copper(II) and high-spin iron(III) ions than those of the remaining low-spin iron(III) ions. In the case of copper(II) and iron(III) [(hexacyanoferrate(III))(1-x)(hexacyanocobaltate(III))(x)] the peak currents decrease much more than can be explained by stoichiometry, indicating that the charge propagation is slowed by the substitution of low-spin iron(III) by cobalt(III). The Rietveld refinement of all compounds confirmed the structure initially proposed by Keggin for Prussian blue and contradicts the structure described later by Ludi. The dependencies of lattice parameters on composition exhibit in all series of solid solutions studied similar, although small, deviations from ideality, which correlate with the electrochemical behavior. Finally, a series of solid solutions of the composition KNi(0.5)(II)Cu(0.5)(II)[Fe(III)(CN)(6)](1-x)[Co(III)(CN)(6)](x), where both the nitrogen- and carbon-coordinated metal ions are mixed populated and were synthesized and characterized. These are the first examples of solid solutions of metal hexacyanometalates with four different metal ions, where both the nitrogen- and the carbon-coordinated sites possess a mixed population.     

Formation of the second superhydrophobic shell around an encapsulated metal ion: synthesis, X-ray structure and electrochemical study of the clathrochelate and bis-clathrochelate iron(II) and cobalt(II, III) dioximates with ribbed perfluoroarylsulfide substituents

The nucleophilic substitution of six chlorine atoms of the n-butylboron-capped clathrochelate iron and cobalt(II) precursors with perfluoroarylthiolate anions afforded the hexaperfluoroarylsulfide macrobicyclic iron and cobalt(II) tris-dioximates. The complexes obtained are soluble in aromatic and aliphatic hydrocarbons as well as in polar aprotonic solvents due to the presence of the superhydrophobic fluorine-containing molecular periphery. As it follows from the X-ray data for five iron and cobalt mono- and bis-clathrochelates, the geometry of their macrobicyclic frameworks is affected by both the nature of an encapsulated metal ion and that of the ribbed substituents. Bis-capping fragment Co(II)O(6) of the Co(III)Co(II)Co(III) bis-clathrochelate possesses a trigonal antiprismatic geometry, all the Co(II)N(6) coordination polyhedra are trigonal-prismatic, and those of the encapsulated iron(II) and cobalt(III) ions are intermediate between them. The wide range of Co-N distances as well as the significant shifts of the encapsulated cobalt(II) ions from the centres of their N(6)-coordination polyhedra were explained by the Jahn-Teller distortion. The EPR and magnetometry data are also characteristic of the low-spin cobalt(II) complexes with this distortion. The parameters of the (57)Fe M?ssbauer spectra of the iron macrobicycles are characteristic of the low-spin iron(II) complexes. The cyclic voltammograms (CVs) for the complexes studied contain the one-electron oxidation and reduction waves assigned to metal-centered redox-processes. The Fe(2+/3+) and Co(2+/3+) oxidations are quasi-reversible or irreversible. The anionic clathrochelate species resulting from the reversible Co(2+/+) reductions are stable on the CV time scale, whereas their iron(I)-containing analogs are unstable.     

Mechanistic insight into N=N cleavage by a low-coordinate iron(II) hydride complex

The reaction pathways of high-spin iron hydride complexes are relevant to the mechanism of N2 reduction by nitrogenase, which has been postulated to involve paramagnetic iron-hydride species. However, almost all known iron hydrides are low-spin, diamagnetic Fe(II) compounds. We have demonstrated that the first high-spin iron hydride complex, LtBuFeH (LtBu = bulky beta-diketiminate), reacts with PhN=NPh to completely cleave the N-N double bond, giving LtBuFeNHPh. Here, we disclose a series of experiments that elucidate the mechanism of this reaction. Crossover and kinetic experiments rule out common nonradical mechanisms, and support a radical chain mechanism mediated by iron(I) species including a rare eta2-azobenzene complex. Therefore, this high-spin iron(II) hydride can break N-N bonds through both nonradical and radical insertion mechanisms, a special feature that enables novel reactivity.     

Low-spin→high-spin relaxation dynamics in the highly diluted spin-crossover system [Fe(x)Zn(1-x)(bbtr)3](ClO4)2

Whereas the neat polymeric iron(II) compound [Fe(bbtr)(3)](ClO(4))(2), bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, shows a quantitative spin transition triggered by a crystallographic phase transition centered at 107 K with a 13 K wide hysteresis, the iron(II) complexes in the diluted mixed crystals [Fe(x)Zn(1-x)(bbtr)(3)](ClO(4))(2), x = 0.02 and 0.1, stay predominantly in the (5)T(2) high-spin state down to cryogenic temperatures. However, the (1)A(1) low-spin state can be populated as metastable state via irradiation into the spin-allowed (5)T(2)→(5)E ligand-field transition of the high-spin species in the near-infrared. The quantum efficiency of the light-induced conversion is approximately 10% at low temperatures and decreases rapidly above 160 K. The lifetime of the light-induced low-spin state decreases from 15 days at 40 K to 30 ns at 220 K, that is, by 14 orders of magnitude. In the high-temperature regime the activation energy for the low-spin→high-spin relaxation is 1840(20) cm(-1).     

Rational tuning of the thiolate donor in model complexes of superoxide reductase: direct evidence for a trans influence in Fe(III)-OOR complexes

Iron peroxide species have been identified as important intermediates in a number of nonheme iron as well as heme-containing enzymes, yet there are only a few examples of such species either synthetic or biological that have been well characterized. We describe the synthesis and structural characterization of a new series of five-coordinate (N4S(thiolate))Fe(II) complexes that react with tert-butyl hydroperoxide ((t)BuOOH) or cumenyl hydroperoxide (CmOOH) to give metastable alkylperoxo-iron(III) species (N4S(thiolate)Fe(III)-OOR) at low temperature. These complexes were designed specifically to mimic the nonheme iron active site of superoxide reductase, which contains a five-coordinate iron(II) center bound by one Cys and four His residues in the active form of the protein. The structures of the Fe(II) complexes are analyzed by X-ray crystallography, and their electrochemical properties are assessed by cyclic voltammetry. For the Fe(III)-OOR species, low-temperature UV-vis spectra reveal intense peaks between 500-550 nm that are typical of peroxide to iron(III) ligand-to-metal charge-transfer (LMCT) transitions, and EPR spectroscopy shows that these alkylperoxo species are all low-spin iron(III) complexes. Identification of the vibrational modes of the Fe(III)-OOR unit comes from resonance Raman (RR) spectroscopy, which shows nu(Fe-O) modes between 600-635 cm(-1) and nu(O-O) bands near 800 cm(-1). These Fe-O stretching frequencies are significantly lower than those found in other low-spin Fe(III)-OOR complexes. Trends in the data conclusively show that this weakening of the Fe-O bond arises from a trans influence of the thiolate donor, and density functional theory (DFT) calculations support these findings. These results suggest a role for the cysteine ligand in SOR, and are discussed in light of the recent assessments of the function of the cysteine ligand in this enzyme.     

Ligand orientation control in low-spin six-coordinate (porphinato)iron(II) species

The synthesis of a low-spin six-coordinate iron(II) porphyrinate in which the two axial ligands are forced to have a relative perpendicular orientation has been successfully accomplished for the first time. The reaction of four-coordinate (tetramesitylporphinato)iron(II) with 2-methylimidazole leads to the preparation of [Fe(TMP)(2-MeHIm)(2)] which cocrystallizes with five-coordinate [Fe(TMP)(2-MeHIm)]. The six-coordinate complex accommodates the sterically crowded pair of imidazoles with a strongly ruffled core and relative perpendicular orientation. This leads to shortened equatorial bonds of 1.963(6) A and slightly elongated axial Fe-N bond lengths of 2.034(9) A that are about 0.04 A shorter and 0.03 A longer, respectively, in comparison to those of the bis-imidazole-ligated iron(II) species with parallel oriented axial ligands. The Mossbauer spectrum shows a pair of quadrupole doublets that can be assigned to the components of the cocrystallized crystalline solid. High-spin five-coordinate [Fe(TMP)(2-MeHIm)] has DeltaE(Q) = 2.25 mm/s and delta = 0.90 mm/s at 15 K. The quadrupole splitting, DeltaE(Q), for [Fe(TMP)(2-MeHIm)(2)] is 1.71 mm/s, and the isomer shift is 0.43 mm/s at 15 K. The quadrupole splitting value is significantly larger than that found for low-spin iron(II) derivatives with relative parallel orientations for the two axial ligands. Mossbauer spectra thus provide a probe for ligand orientation when structural data are otherwise not available.     

Correlation of spin states and spin delocalization with the dioxygen reactivity of catecholatoiron(III) complexes

A series of catecholatoiron(III) complexes, [Fe(III)L(4Cl-cat)]BPh4 (L = (4-MeO)2TPA (1), TPA (2), (4-Cl)2TPA (3), (4-NO2)TPA (4), (4-NO2)2TPA (5); TPA = tris(pyridin-2-ylmethyl)amine; 4Cl-cat = 4-chlorocatecholate), have been characterized by magnetic susceptibility measurements and EPR, 1H NMR, and UV-vis-NIR spectroscopies to clarify the correlation of the spin delocalization on the catecholate ligand with the O2 reactivity as well as the spin-state dependence of the O2 reactivity. EPR spectra in frozen CH3CN at 123 K clearly showed that introduction of electron-withdrawing groups effectively shifts the spin equilibrium from a high-spin to a low-spin state. The effective magnetic moments determined by the Evans method in a CH3CN solution showed that 5 contains 36% of low-spin species at 243 K, while 1-4 are predominantly in a high-spin state. Evaluation of spin delocalization on the 4Cl-cat ligand by paramagnetic 1H NMR shifts revealed that the semiquinonatoiron(II) character is more significant in the low-spin species than in the high-spin species. The logarithm of the reaction rate constant is linearly correlated with the energy gap between the catecholatoiron(III) and semiquinonatoiron(II) states for the high-spin complexes 1-3, although complexes 4 and 5 deviate negatively from linearity. The lower reactivity of the low-spin complex, despite its higher spin density on the catecholate ligand compared with the high-spin analogues, suggests the involvement of the iron(III) center, rather than the catecholate ligand, in the reaction with O2.     

Photomagnetic properties of an iron(II) low-spin complex with an unusually long-lived metastable LIESST state

A comprehensive study of the photomagnetic behavior of the [Fe(L222N5)(CN)2].H2O complex has been carried out. This complex is characterized by a low-spin (LS) iron(II)-metal center up to 400 K and exhibits at 10 K the well-known Light-Induced Excited Spin State Trapping (LIESST) effect. The critical LIESST temperature (T(LIESST)) has been measured to be 105 K. The kinetics of the transition from the metastable high-spin (HS) state to the low-spin state have been determined and used for reproducing the experimental T(LIESST) curve. This study represents a second example of a fully low-spin iron(II)-metal complex up to 400 K, which can be photoexcited at low temperature with an atypical long-lived metastable HS state. This underlines the preponderant role of the inner coordination sphere for stabilizing the lifetime of the photoinduced HS state.     

Ground-state singlet L3Fe-(mu-N)-FeL3 and L3Fe(NR) complexes featuring pseudotetrahedral Fe(II) centers

Pseudotetrahedral iron(II) coordination complexes that contain bridged nitride and terminal imide linkages, and exhibit singlet ground-state electronic configurations, are described. Sodium amalgam reduction of the ferromagnetically coupled dimer, {[PhBP(3)]Fe(mu-1,3-N(3))}(2) (2) ([PhBP(3)] = [PhB(CH(2)PPh(2))(3)](-)), yields the diamagnetic bridging nitride species [{[PhBP(3)]Fe}(2)(mu-N)][Na(THF)(5)] (3). The Fe-N-Fe linkage featured in the anion of 3 exhibits an unusually bent angle of approximately 135 degrees , and the short Fe-N bond distances (Fe-N(av) approximately equal to 1.70 A) suggest substantial Fe-N multiple bond character. The diamagnetic imide complex {[PhBP(3)]Fe(II)(triple bond)N(1-Ad)}{(n)()Bu(4)N} (4) has been prepared by sodium amalgam reduction of its low-spin iron(III) precursor, [PhBP(3)]Fe(III)(triple bond)N(1-Ad) (5). Complexes 4 and 5 have been structurally characterized, and their respective electronic structures are discussed in the context of a supporting DFT calculation. Diamagnetic 4 provides a bona fide example of a pseudotetrahedral iron(II) center in a low-spin ground-state configuration. Comparative optical data strongly suggest that dinuclear 3 is best described as containing two high-spin iron(II) centers that are strongly antiferromagnetically coupled to give rise to a singlet ground-state at room temperature.     

Stereospecific alkane hydroxylation by non-heme iron catalysts: mechanistic evidence for an Fe(V)=O active species.

High-valent iron-oxo species have frequently been invoked in the oxidation of hydrocarbons by both heme and non-heme enzymes. Although a formally Fe(V)=O species, that is, [(Por(*))Fe(IV)=O](+), has been widely accepted as the key oxidant in stereospecific alkane hydroxylation by heme systems, it is not established that such a high-valent state can be accessed by a non-heme ligand environment. Herein we report a systematic study on alkane oxidations with H(2)O(2) catalyzed by a group of non-heme iron complexes, that is, [Fe(II)(TPA)(CH(3)CN)(2)](2+) (1, TPA = tris(2-pyridylmethyl)amine) and its alpha- and beta-substituted analogues. The reactivity patterns of this family of Fe(II)(TPA) catalysts can be modulated by the electronic and steric properties of the ligand environment, which affects the spin states of a common Fe(III)-OOH intermediate. Such an Fe(III)-peroxo species is high-spin when the TPA ligand has two or three alpha-substituents and is proposed to be directly responsible for the selective C-H bond cleavage of the alkane substrate. The thus-generated alkyl radicals, however, have relatively long lifetimes and are susceptible to radical epimerization and trapping by O(2). On the other hand, 1 and the beta-substituted Fe(II)(TPA) complexes catalyze stereospecific alkane hydroxylation by a mechanism involving both a low-spin Fe(III)-OOH intermediate and an Fe(V)=O species derived from O-O bond heterolysis. We propose that the heterolysis pathway is promoted by two factors: (a) the low-spin iron(III) center which weakens the O-O bond and (b) the binding of an adjacent water ligand that can hydrogen bond to the terminal oxygen of the hydroperoxo group and facilitate the departure of the hydroxide. Evidence for the Fe(V)=O species comes from isotope-labeling studies showing incorporation of (18)O from H(2)(18)O into the alcohol products. (18)O-incorporation occurs by H(2)(18)O binding to the low-spin Fe(III)-OOH intermediate, its conversion to a cis-H(18)O-Fe(V)=O species, and then oxo-hydroxo tautomerization. The relative contributions of the two pathways of this dual-oxidant mechanism are affected by both the electron donating ability of the TPA ligand and the strength of the C-H bond to be broken. These studies thus serve as a synthetic precedent for an Fe(V)=O species in the oxygen activation mechanisms postulated for non-heme iron enzymes such as methane monooxygenase and Rieske dioxygenases.     

Variable pi-bonding in iron(II) porphyrinates with nitrite, CO, and tert-butyl isocyanide: characterization of [Fe(TpivPP)(NO2)(CO)]-

The addition of the strongly pi-bonding ligands CO or tert-butyl isocyanide to the low-spin five-coordinate iron(II) nitrite species [Fe(TpivPP)(NO2)]- (TpivPP = picket fence porphyrin) gives two new six-coordinate species [Fe(TpivPP)(NO2)(CO)]- and [Fe(TpivPP)(NO2)(t-BuNC)]-. These species have been characterized by single-crystal structure determinations and by UV-vis, IR, and M?ssbauer spectroscopies. All evidence shows that in the mixed-ligand iron(II) porphyrin species, [Fe(TpivPP)(NO2)(CO)]-, the two trans, pi-accepting ligands CO and nitrite compete for pi density. The CO ligand however dominates the bonding. The Fe-N(NO2) bond lengths for the two independent anions in the unit cell at 2.006(4) and 2.009(4) A are lengthened compared to other nitrite species with either no trans ligands or non-pi-accepting trans ligands to nitrite. The Fe-C(CO) bond lengths are 1.782(4) A and 1.789(5) A for the two anions. The two Fe-C-O angles at 175.5(4) and 177.5(4) degrees are essentially linear in both anions. The quadrupole splitting for [Fe(TpivPP)(NO2)(CO)]- was determined to be 0.32 mm/s, and the isomer shift was 0.18 mm/s at room temperature in zero applied field. Both of the M?ssbauer parameters are much smaller than those found for six-coordinate low-spin iron(II) porphyrinates with neutral nitrogen-donating ligands as well as iron(II) nitro complexes. However, the M?ssbauer parameters are typical of other six-coordinate CO porphyrinates signifying that CO is the more dominant ligand. The CO stretching frequency of 1974 cm(-1) is shifted only slightly to higher energy compared to six-coordinate CO complexes with neutral nitrogen-donor ligands trans to CO. Crystal data for [K(222)][Fe(TpivPP)(NO2)(CO)].1/2C6H5Cl: monoclinic, space group P2(1)/c, Z = 8, a = 33.548(6) A, b = 18.8172(15) A, c = 27.187(2) A, beta = 95.240(7) degrees, V = 17091(4) A3.     

Solution behavior of iron(III) and iron(II) porphyrins in DMSO and reaction with superoxide. Effect of neighboring positive charge on thermodynamics, kinetics and nature of iron-(su)peroxo product

The solution behavior of iron(III) and iron(II) complexes of 5(4),10(4),15(4),20(4)-tetra-tert-butyl-5,10,15,20-tetraphenylporphyrin (H(2)tBuTPP) and the reaction with superoxide (KO(2)) in DMSO have been studied in detail. Applying temperature and pressure dependent NMR studies, the thermodynamics of the low-spin/high-spin equilibrium between bis- and mono-DMSO Fe(II) forms have been quantified (K(DMSO) = 0.082 ± 0.002 at 298.2 K, ΔH° = +36 ± 1 kJ mol(-1), ΔS° = +101 ± 4 J K(-1) mol(-1), ΔV° = +16 ± 2 cm(3) mol(-1)). This is a key activation step for substitution and inner-sphere electron transfer. The superoxide binding constant to the iron(II) form of the studied porphyrin complex was found to be (9 ± 0.5) × 10(3) M(-1), and does not change significantly in the presence of the externally added crown ether in DMSO (11 ± 4) × 10(3) M(-1). The rate constants for the superoxide binding (k(on) = (1.30 ± 0.01) × 10(5) M(-1) s(-1)) and release (k(off) = 11.6 ± 0.7 s(-1)) are not affected by the presence of the external crown ether in solution. The resulting iron(II)-superoxide adduct has been characterized (mass spectrometry, EPR, high-pressure UV/Vis spectroscopy) and upon controlled addition of a proton source it regenerates the starting iron(II) complex. Based on DFT calculations, the reaction product without neighboring positive charge has iron(II)-superoxo character in both high-spin side-on and low-spin end-on forms. The results are compared to those obtained for the analogous complex with covalently attached crown ether, and more general conclusions regarding the spin-state equilibrium of iron(II) porphyrins, their reaction with superoxide and the electronic structure of the product species are drawn.     

Korrelation zwischen Mössbauer-Isomerieverschiebungen und ESCA-Bindungsenergien

Correlation between Mössbauer Isomer Shifts and ESCA Binding Energies The correlation between core-electron binding energies and Mössbauer isomer shifts is investigated and discussed for low-spin pentacyanoferrates(II), high-spin iron(III) compounds and high-spin iron(II) halides. The Fe2p_3/2 binding energies of the investigated pentacyano ferrates(II) increase with decreasing isomer shifts as a consequence of the increasing π acid strengths of the sixth ligands. In contrast, the electron binding energies in high spin iron(III) compounds and iron(II) halides increase with increasing isomer shifts. This correlation is caused by the σ donor properties and the electronegativity of the ligands.     

π Back-bonding of iron(II) complexes supported by tris(pyrid-2-ylmethyl)amine and its nitro-substituted derivatives

The electronic and geometric structures of a series of iron(II) complexes supported by tetradentate tris(pyrid-2-ylmethyl)amine-type ligands with different numbers of 4-nitropyridine groups, [(PyCH(2))(3-n)(4-NO(2)PyCH(2))(n)N] (n = 0-3), were examined by X-ray absorption fine-structure and variable-temperature (1)H NMR spectroscopies and theoretical calculations to reveal how the low-spin state is stabilized through π back-bonding interactions between iron(II) and 4-nitropyridine donor group(s).     

Structural insights into nonheme alkylperoxoiron(III) and oxoiron(IV) intermediates by X-ray absorption spectroscopy

Transient mononuclear low-spin alkylperoxoiron(III) and oxoiron(IV) complexes that are relevant to the activation of dioxygen by nonheme iron enzymes have been generated from synthetic iron(II) complexes of neutral tetradentate (TPA) and pentadentate (N4Py, Bn-TPEN) ligands and structurally characterized by means of Fe K-edge X-ray absorption spectroscopy (XAS). Notable features obtained from fits of the EXAFS region are Fe-O bond lengths of 1.78 A for the alkylperoxoiron(III) intermediates and 1.65-1.68 A for the oxoiron(IV) intermediates, reflecting different strengths in the Fe-O pi interactions. These differences are also observed in the intensities of the 1s-to-3d transitions in the XANES region, which increase from 4 units for the nearly octahedral iron(II) precursor to 9-15 units for the alkylperoxoiron(III) intermediates to 25-29 units for the oxoiron(IV) species.     

Sulfoxide as a ligand in iron(II) porphyrinates: S- or O-bound?

In order to study the bonding of sulfoxides to iron(II) porphyrinates, an equilibrium study of Fe(TPP) with tetramethylenesulfoxide (TMSO) has been performed. UV-vis spectra at different concentrations of TMSO have shown distinct character belonging to three species: four-coordinate Fe(II)(TPP), five-coordinate [Fe(II)(TPP)(TMSO)], and six-coordinate [Fe(II)(TPP)(TMSO)2]. The isosbestic points for the low TMSO concentrations suggest that the equilibrium constant K1 is much larger than K2. Analysis of spectral data by the nonlinear least-squares program SQUAD gives K1 = 267 and K2 approximately 1. Even though the five-coordinate species is the dominant species under the synthetic conditions, only the six-coordinate species was crystallized and characterized by an X-ray diffraction study. [Fe(TPP)(TMSO)2] (C52H44Fe-N4O2S2): monoclinic, P2(1)/c, a = 11.2580(3) A, b = 15.9262(5) A, c = 12.3930(4) A, beta = 116.246(1) degrees , V = 1992.95(10) A3, Z = 2. X-ray crystallography demonstrates the complex is a low-spin bis-TMSO ligated species. The average Fe-Np distance is 1.999(4) A. The most important feature is that TMSO is coordinated to iron(II) by the sulfur donors, not oxygen. The Fe-S distance is 2.2220(3) A.     

Iron(II) complexes of new hexadentate 1,1,1-tris-(iminomethyl)ethane podands, and their 7-methyl-1,3,5-triazaadamantane rearrangement products

New iron(II) podand complexes have been prepared, by condensation of 2-(aminomethyl)-2-methyl-1,3-diaminopropane with 3 equiv of a heterocyclic aldehyde in the presence of hydrated Fe[BF(4)](2) or Fe[ClO(4)](2) as templates. The 2-(aminomethyl)-2-methyl-1,3-diaminopropane is prepared in situ by deprotonation of its trihydrochloride salt. The chloride must be removed from these reactions by precipitation with silver, to avoid the formation of the alternative 2,4,6-trisubstituted-7-methyl-1,3,5-triazaadamantane condensation products, or their FeCl(2) adducts. The crystal structures of two 2,4,6-tri(pyridyl)-7-methyl-1,3,5-triazaadamantane-containing species are presented, and contain two different geometric isomers of this tricyclic ring with three equatorial, or two equatorial and one axial, pyridyl substituents. Both structures feature strong C-HX (X = Cl or F) hydrogen bonding from the aminal C-H groups in the triazaadamantane ring. Five iron(II) podand complexes were successfully obtained, all of which contain low-spin iron centres.     

Photoinduced spin transition for Iron(II) compounds with liquid-crystal properties

The iron(II) compounds [Fe(3Cn-L)2(NCS)2] (n = 6 (1), n = 8 (2), n = 10 (3), n = 12 (4), n = 14 (5), n = 16 (6), n = 18 (7), n = 20 (8), and n = 22 (9)) were synthesized and their physical properties characterized by polarizing optical microscopy, differential scanning calorimetry, and powder X-ray analysis, where 3Cn-L denotes bidentate Schiff-base ligands formed from the corresponding aniline derivatives and pyridine-2-carboxyaldehyde. The iron(II) compounds 4-8 exhibited crystal to liquid-crystal transitions at 318, 334, 345, 338, and 347 K, respectively. Variable-temperature magnetic susceptibility measurements revealed that the compounds 1-9 exhibit spin-crossover behavior between the high-spin and low-spin states and a photoinduced spin transition from a low-spin state to a metastable high-spin state. Therefore, the iron(II) compounds 4-8 can undergo spin-crossover and photoinduced spin transition as well as have liquid-crystal properties all in a single molecule. Compounds with multifunctions are important in the development of molecular switches and optical materials.     

A synthetic analogue of the active site of Fe-containing nitrile hydratase with carboxamido N and thiolato S as donors: synthesis, structure, and reactivities.

As part of our work on models of the iron(III) site of Fe-containing nitrile hydratase, a designed ligand PyPSH(4) with two carboxamide and two thiolate donor groups has been synthesized. Reaction of (Et(4)N)[FeCl(4)] with the deprotonated form of the ligand in DMF affords the mononuclear iron(III) complex (Et(4)N)[Fe(III)(PyPS)] (1) in high yield. The iron(III) center is in a trigonal bipyramidal geometry with two deprotonated carboxamido nitrogens, one pyridine nitrogen, and two thiolato sulfurs as donors. Complex 1 is stable in water and binds a variety of Lewis bases at the sixth site at low temperature to afford green solutions with a band around 700 nm. The iron(III) centers in these six-coordinate species are low-spin and exhibit EPR spectra much like the enzyme. The pK(a) of the water molecule in [Fe(III)(PyPS)(H(2)O)](-) is 6.3 +/- 0.4. The iron(III) site in 1 with ligated carboxamido nitrogens and thiolato sulfurs does not show any affinity toward nitriles. It thus appears that at physiological pH, a metal-bound hydroxide promotes hydration of nitriles nested in close proximity of the iron center in the enzyme. Redox measurements demonstrate that the carboxamido nitrogens prefer Fe(III) to Fe(II) centers. This fact explains the absence of any redox behavior at the iron site in nitrile hydratase. Upon exposure to limited amount of dioxygen, 1 is converted to the bis-sulfinic species. The structure of the more stable O-bonded sulfinato complex (Et(4)N)[Fe(III)(PyP[SO(2)](2))] (2) has been determined. Six-coordinated low-spin cyanide adducts of the S-bonded and the O-bonded sulfinato complexes, namely, Na(2)[Fe(III)(PyP[SO(2)](2))(CN)] (4) and (Et(4)N)(2)[Fe(III)(PyP[SO(2)](2))(CN)] (5), afford green solutions in water and other solvents. The iron(II) complex (Et(4)N)(2)[Fe(II)(PyPS)] (3) has also been isolated and structurally characterized.     

mer-[Fe III(bpca)(CN)3]-: a new low-spin iron(III) complex to build heterometallic ladder-like chains

The novel mononuclear complex PPh(4)-mer-[Fe(III)(bpca)(3)(CN)(3)].H(2)O (1) [PPh(4)(+) = tetraphenylphosphonium cation and bpca = bis(2-pyridylcarbonyl)amidate anion] and ladder-like chain compound [[Fe(III)(bpca)(micro-CN)(3)Mn(II)(H(2)O)(3)] [Fe(III)(bpca)(CN)(3)]].3H(2)O (2) have been prepared and characterized by X-ray diffraction analysis. Compound 1 is a low-spin iron(III) compound with three cyanide ligands in mer arrangement and a tridentate N-donor ligand building a distorted octahedral environment around the iron atom. Compound 2 is an ionic salt made up of cationic ladder-like chains [[Fe(III)(bpca)(micro-CN)(3)Mn(II)(H(2)O)(3)]](+) and uncoordinated anions [Fe(III)(bpca)(3)(CN)(3)](-). The magnetic properties of 2 correspond to those of a ferrimagnetic chain with significant intrachain antiferromagnetic coupling between the low-spin iron(III) centers and the high-spin manganese(II) cations. This compound exhibits ferrimagnetic ordering below 2.0 K.