Mössbauer spectroscopic studies of thermal decomposition of substituted pentacyanoferrate(II) complexes

The thermal behaviour of substituted pentacyanoferrates(II) of the type Na₃[Fe(CN₅)L]·xH₂O, whereL=n-, sec-, tert- oriso-butylamine,di-iso-butylamine ortri-n-butylamine, was investigated with the aid of Mössbauer spectroscopy, XRD and TG-DTG-DTA. The Mössbauer spectra of these complexes exhibit a quadrupole doublet with E _Q=0.70–0.83 mm s^–1 at room temperature. The isomer shift, =0.00±0.03 mm s^–1 suggests that the iron atom is in the +2 low-spin state. The complexes start to decompose at 50°C, yielding a residual mass of 5.8 –21.3% in the temperature range 900–950°C. The Mössbauer spectra recorded after heating at 150 and 300°C exhibit an asymmetric doublet, suggesting partial decomposition. The Mössbauer spectra at higher temperature are complex. At different stages of the thermal process, the presence of -Fe₂O₃, -Fe₂O₃, -Fe, Fe₃C and Fe₃O₄ was demonstrated.On leave from A. N. College, Anandwan-442 914, IndiaWe are grateful to the Monbusho (Ministry of Education, Science, Sports and Culture) for the award of a fellowship to RBL and for financial assistance for the research work. Thanks are also due to Dr. T. Nakamoto for valuable cooperation.     

Mössbauer and electronic spectral studies of iron(III) complexes of oximes

The hydroxo-bridge complexes of the type [Fe(2)(ligand-H)(4)(OH)(2)] with bidentate nitrogen-oxygen donor ligands, viz. 2-hydroxynaphthaldehydeoxime [hnoH(2)], 2-hydroxyacetphenoneoxime [haoH(2)], salicylaldooxime [SalH(2)], 2-hydroxypropiophenoneoxime [hnoH(2)] have been prepared. All the complexes have been characterized by elemental analysis, magnetic moments, electronic and M?ssbauer spectral studies. M?ssbauer parameters of the complexes clearly suggest high spin configuration of Fe(III) showing lower magnetic moment to that of the spin only value, i.e. 5.92 BM. It may be due to the antiferromagnetic interaction between Fe(III) centers.     

Synthesis,Mössbauer,and IP spectroscopic studies and thermal behavior of diorganotin(IV) complexes with carbohydrate ligands

A series of diethyltin(IV) complexes with carbohydrate ligands (aldoses, polyalcohols, sugar acids, sugar amines, and di- and trisaccharides] was prepared. The compositions of the complexes were determined by standard analytical methods. The results showed that complexes formed containing the diethyltin moiety and the carbohydrate ligand in 11 ratio. The IR spectra were consistent with the presence of tin—carbohydrate oxygen vibrations in the compounds. The comparison of experimental quadrupole splitting values with those calculated on the basis of the partial quadrupole splitting (PQS) concept revealed the compounds to be of three types: complexes in with the central tin(IV) atoms are present in purely trigonal bipyramidal surroundings, ones with the tin(IV) atoms present in purely octahedral surroundings, or ones with both octahedral and trigonal bipyramidal arrangements in approximately 11 ratio. The thermal decompositions showed that some of the complexes decompose via the reduction of tin(IV) to SnO as the final tin-containing product, some of them decompose without reduction, to give SnO₂, and some decompose to give indefinite final tin-containing products.     

A structural, magnetic and Mössbauer spectroscopic study of an unusual angular Jahn-Teller distortion in a series of high-spin iron(II) complexes

Single crystal X-ray structures and susceptibility data are described for six homoleptic iron(II) complex salts, of 2,6-di(pyrazol-1-yl)pyridine or a 3,3"-disubstituted derivative of it. Zero field Mossbauer spectroscopic data for four of the complexes, and one previously reported analogue, are also discussed. Four of these compounds exhibit an unusual angular Jahn-Teller distortion towards C(2) symmetry to differing degrees, while the other two exhibit structures close to the "ideal" D(2d) symmetry for this ligand set. This structural distortion has two components: a twisting of the plane of one ligand relative to the other about the N{pyridine}-Fe-N{pyridine} vector, so that the two ligands are no longer perpendicular; and a rotation of one ligand about the Fe ion, so that the N{pyridine}-Fe-N{pyridine} angle < 180 degrees. Susceptibility data show that all the complexes are fully high-spin between 5 and 300 K, but yield an unusually wide range of zero-field splitting parameters for the different compounds of between 2.6 and 13.4 cm(-1). Magnetostructural correlations suggest that a low value of |D| is diagnostic for a high degree of "rotation" distortion. The Mossbauer spectra imply that an increased quadrupole splitting might also be diagnostic for the presence of the angular distortion.     

Crystal structures and Mössbauer spectroscopic studies of oxo-centered trinuclear chloroacetate complexes

The crystal structures of oxo-centered trineclear cobalt-iron chloroacetate complex [Co^IIFe ₂ ^III O(CH₂ClCO₂)₆(H₂O)₃]·3H₂O (1) was compared with that of previously reported trinuclear iron complex [Fe^IIFe ₂ ^III O(CH₂ClCO₂)₆(H₂O)₃]·3H₂O (2) which has an isomorphous structure to 1. Compound 1 crystallizes in space group P2₁/n with Z=4 in a unit cell of a=14.826 (4) Å, b=4.536 (8) Å, c=14.000 (4) Å, =100.32 (2)⁰ and V=2968 (11) ų. The structure was refined to R=0.75 and R_w=0.82. The coordination geometries of the three iron atoms are observed equivalent in 1 indicating a static disorder of the position among cobalt and iron atoms. Two distinct Fe^III doublets observed in Mössbauer spectra of 1 become an indistinguishable broad doublet by dehydration of crystal water. On the other hand, no significant line-broadening is observed after the dehydration in complex 2. The results indicate that the dehydration in 2 induces a local environmental change reordering of an electronic configuration around iron atoms, whereas the remaining disordering is reflected in Mössbauer spectrum after the dehydration in 1.     

Synthesis and magnetic characterization of Ln(III)–Co(II) complexes with 1,10-phenanthroline ligands

Four heteronuclear complexes, [Ln₂Co₂L₁₀(H₂O)(phen)₂] · n(H₂O) (Ln = La 1, n = 2; Ln = Nd 2, Sm 3, Gd 4, n = 0; HL = α-methylacrylic acid, phen = 1,10-phenanthroline), have been synthesized and characterized by elemental analysis, IR and X-ray diffraction. The complexes with a discrete Co–Ln–Ln–Co tetranuclear molecule are isomorphous in the triclinic space group P 1 and Z = 1, in which all metal ions are bridged by bidentate α-methylacrylato groups. Magnetic measurements of 1, 2 and 3 show antiferromagnetic exchange interaction between paramagnetic centers.     

Mössbauer spectroscopic studies in alkali metal hexakis (trimethylacetato) ferrate(III) complexes and their thermal decomposition products

Mössbauer spectra of hexakis (trimethylacetato) ferrate(III) complexes with general formula M₃[Fe{OCOC(CH₃)₃}₆] (where M=H⁺, Li⁺, Na⁺, K⁺ and NH ₄ ⁺ ) exhibit a quadrupole doublet with E_Q=0.31–0.65 mms^–1 and =0.60–0.74 mms^–1 (with respect to S.N.P. as standard). Infrared studies suggest unidentate coordination of the carboxylate ligands. Anomalously high and E_Q values for H₃[Fe{OCOC(CH₃)₃}₆] have been explained in terms of possible hydrogen bonding. Thermal decomposition studies show fast and single stage decomposition yielding a constant weight at 320°C. Mössbauer spectra of intermediates after heating complexes at different temperatures indicate increasing E_Q values. At 350°C, all complexes exhibit six-line spectra, suggesting the formation of alkali metal ferrate (Na₂O·NaFeO₂) or Fe₂O₃.     

Syntheses,structural studies and spectroscopic characterisation of pyridyl–phthalimide complexes of fac-(CO)3ReI-diimines

The mono-dentate ligands, 3-aminomethyl-N-phthalimido-pyridine (L¹) and 3-amino-N-phthalimido-pyridine (L²), were synthesised using a solvent-free melt method. These ligands were then used to access three pairs of functionalised luminescent Re^I complexes of the generic type fac-{Re(CO)₃(diimine)(Lⁿ)}(BF₄) [where diimine = 4,4′-dimethyl-2,2′-bipyridine (dmb); 2,2′-bipyridine (bpy); 1,10-phenanthroline (phen)]. X-ray crystallography has been used to structurally characterise five of the complexes showing that in the cases of the L¹ species the phthalimide unit is adjacent to and co-planar with the coordinated diimine ligand. Solution state UV–Vis absorption, electrochemistry and IR studies confirm that the proposed formulations and coordination modes exist in solution. The photophysical studies show that the visible emission from each of the six complexes is ³MLCT at room temperature. Within each pair of complexes the precise energy of the emission was subtly dependent upon the axial ligand, Lⁿ with luminescence lifetimes in the range 121–288 ns.     

Effect of the sixth axial ligand in CS-ligated iron(II)octaethylporphyrinates: structural and Mössbauer studies

The effect of a sixth ligand in a series of low-spin thiocarbonyl-ligated iron(II)octaethylporphyrinates has been investigated. Six-coordinate complexes have been synthesized and characterized by M?ssbauer and infrared spectroscopy and single-crystal X-ray structure determinations. The results are compared with the five-coordinate parent complex. The crystal structures of [Fe(OEP)(CS)(1-MeIm)] and [Fe(OEP)(CS)(Py)] are reported and discussed. The 1-methylimidazole and pyridine derivatives exhibit Fe-C(CS) bond distances of 1.703(4) and 1.706(2) A that are significantly longer than the 1.662(3) A reported for five-coordinate [Fe(OEP)(CS)] (Scheidt, W. R.; Geiger, D. K. Inorg. Chem. 1982, 21, 1208). The trans Fe-N(ligand) distances of 2.112(3) and 2.1550(15) A observed for the 1-methylimidazole and pyridine complex are approximately 0.13 A longer than those observed for analogous bis-ligated complexes and are consistent with a significant structural trans effect for the CS ligand. M?ssbauer investigations carried out for five- and six-coordinate thiocarbonyl derivatives with several different sixth axial ligands reveal interesting features. All derivatives exhibit very small isomer shift values, consistent with a very strong interaction between iron and CS. The five-coordinate derivative has delta(Fe) = 0.08 mm/s, and the six-coordinate complexes exhibit delta(Fe) = 0.14 to 0.19 mm/s at 4.2 K. The five-coordinate complex shows a large quadrupole splitting (DeltaE(q) = 1.93 mm/s at 4.2 K) which is reduced on coordination of the sixth ligand (DeltaE(q) = 0.42-0.80 mm/s at 4.2 K). Addition of a sixth ligand also leads to a small decrease in the value of nu(CS). Correlations in structural, IR, and M?ssbauer results suggest that the sixth ligand effect is primarily induced by changes in sigma-bonding. The structure of [Fe(OEP)(CS)(CH(3)OH)] is briefly reported. Crystal data: [Fe(OEP)(CS)(1-MeIm)] crystallizes in the monoclinic system, space group P2(1)/n, Z = 4, a = 9.5906(5) A, b = 16.704(4) A, c = 23.1417(6) A, beta = 100.453(7) degrees. [Fe(OEP)(CS)(Py)] crystallizes in the triclinic system, space group P1, Z = 5, a = 13.9073(6) A, b = 16.2624(7) A, c = 22.0709(9) A, alpha = 70.586(1) degrees, beta = 77.242(1) degrees, gamma = 77.959(1) degrees. [Fe(OEP)(CS)(CH(3)OH)] crystallizes in the triclinic system, space group P1, Z = 1, a = 9.0599(5) A, b = 9.4389(5) A, c = 11.0676(6) A, alpha = 90.261(1) degrees, beta = 100.362(1) degrees, gamma = 114.664(1) degrees.     

A Mössbauer and electrochemical study of mononuclear iron(III) complexes with benzimidazole-based flexible bidentate ligands

A series of FeIII complexes of stoichiometry [FeLX3].nH2O have been synthesized, where X=Cl–, HCO–2 and L is a flexible bidentate ligand, 1,3(bis-benzimidazolyl) propane and 1,4(bis-benzimidazolyl)butane. Mössbauer data reveals that the isomer shift values lie in the range typically observed for high spin FeIII complexes, while the slightly large quadrupole splitting parameter indicates a rhombically distorted FeIII centre. Cyclic voltammetric studies reveal that the E1/2 for the FeIII/FeII couple for the formate complex shifts more cathodically than for the chloride complex; implying that HCO–2 is more effective at lowering the Lewis acidity of the FeIII centre.     

Synthesis,spectroscopic, structural and magnetic studies of new binary Cr(III)–citrate pH-specific structural variants from aqueous media

In an attempt to understand the aqueous interactions of Cr(III) with the low molecular mass physiological ligand citric acid, the pH-specific synthesis in the binary Cr(III)–citrate system was pursued, leading to the new complexes Na₃[Cr(C₆H₅O₇)₂]·8.5H₂O (1) and (Hdmphen)₆[Cr(C₆H₅O₇)₂]·(NO₃)₃·14H₂O (2). Complexes 1 and 2 were characterized by elemental analysis, spectroscopic, structural, thermal, and magnetic susceptibility studies. The structures of 1 and 2 reveal a mononuclear octahedral complex of Cr(III) with two citrate ligands bound to it. Albeit of the same deprotonation state, the disposition of the two citrate ligands with respect to Cr(III) differs between 1 and 2 in the solid state, thus reflecting the presence of pH-structural variants in the requisite binary system. This conformational difference is lifted in aqueous solution, thus providing (a) comparative information on the distribution and diversity of species in the binary Cr(III)–citrate system, and (b) insight into the nature of interactions developing in the binary Cr(III)–hydroxycarboxylate systems in abiotic and biological applications.     

Self-assembled supramolecular M9 (Mn(II), Fe(III), Zn(II)), M5 (Fe(III)), and [M3]2 (Pb(II)) complexes: structural,magnetic, and Mössbauer properties

The tritopic ligand 2poap self-assembles in the presence of Zn(NO(3))(2) and Fe(NO(3))(3) to form homoleptic [3 x 3] nonanuclear M(9) (M = Zn(II), Fe(III)) square grid structures and with Pb(ClO(4))(2) to form a dimerized linear trinuclear [Pb(3)](2) structure. Cl2poap and Cl2poapz form self-assembled homoleptic [3 x 3] Mn(II)(9) square grids with Mn(ClO(4))(2) and Mn(NO(3))(2), respectively, but an unusual incompletely metalated Fe(III)(5) square grid is formed on reaction of Cl2poap with Fe(ClO(4))(3). X-ray structures are reported for [Mn(9)(Cl2poap-2H)(6)](ClO(4))(6).10H(2)O (3), [Mn(9)(Cl2poapz-2H)(6)] (NO(3))(6).22H(2)O (4), [Zn(9)(2poap-2H)(3)(2poap-H)(3)](NO(3))(9).24H(2)O (5), [Pb(3)(2poap-2H) (ClO(4))(4)](2).8H(2)O (6), and [Fe(5)(Cl2poap-H)(6)](ClO(4))(9).34.5H(2)O (7). Compound 3 crystallized in the monoclinic system, space group P(-)1, with a = 18.179(1) A, b = 18.857(1) A, c = 25.871(2) A, alpha = 70.506(2) degrees, beta = 86.440(1) degrees, gamma = 75.175(2) degrees, and z = 2. Compound 4 crystallized in the monoclinic system, space group P(-)1, with a = 16.900(2) A, b = 20.02393) A, c = 25.663() A, alpha = 84.743(3) degrees, beta = 84.885(2) degrees, gamma = 67.081(2) degrees, and z = 2. Compound 5 crystallized in the monoclinic system, space group P(-)1, with a = 18.482(1) A, b = 18.774(1) A, c = 28.112(2) A, alpha = 104.020(1) degrees, beta = 97.791(1) degrees, gamma = 117.036(1) degrees, and z = 2. Compound 6 crystallized in the monoclinic system, space group P(-)1, with a = 10.0513(6) A, b = 11.0958(6) A, c = 17.334(1) A, alpha = 100.932(1) degrees, beta = 100.387(1) degrees, gamma = 94.565(1) degrees, and z = 2. Compound 7 crystallized in the monoclinic system, space group P(-)1, with a = 19.164(1) A, b = 19.587(2) A, c = 26.673(2) A, alpha = 76.430(2) degrees, beta = 78.834(2) degrees, gamma = 64.973(1) degrees, and z = 2. Compound 3 exhibits intramolecular antiferromagnetic exchange within the nonanuclear [Mn(9)(mu-O)(12)] grid structure (J = -4.6 cm(-1)), while the analogous nonanuclear complex [Fe(9)(2poap-2H)(6)](NO(3))(15).18H(2)O (8) is dominated by intramolecular antiferromagnetic coupling at high temperatures but exhibits a low-temperature feature indicative of additional ferromagnetic interactions. The isolated pentanuclear Fe(5) [4 + 1] square grid in 7, with distant Fe-Fe bridging, exhibits very weak antiferromagnetic coupling (J = -0.2 cm(-1)). M?ssbauer spectroscopy data are consistent with high-spin Fe(III)(9) and Fe(III)(5) structures.     

57Fe Mössbauer spectroscopic study of some clusters related to Fe3(CO)12

The ⁵⁷Fe Mössbauer spectra of Fe₃(CO)₁₂-related clusters [Fe₃(CO)₁₁]²⁻, [Fe₂Ru(CO)₁₂], [FeRu₂(CO)₁₂], [Fe₃(CO)₁₁PPh₃], [Fe₃(CO)₁₁PPh₂Me], [Fe₃(CO)₁₁PPhMe₂], [Fe₃(CO)₉(PPh₂Me)₃], [Fe₂Ru(CO)₁₁P(OMe)₃], [FeRu₂(CO)₁₁PPh₃] and [FeRu₂(CO)₁₀(PPh₃)₂] have been recorded at 78 K. The data are compared with published data for other M₃ clusters.Generally, the isomer shifts (δ) fall within a narrow range, for example with compounds containing Fe or Fe and Ru and four or five CO ligands per metal, all δ values lie between 0.29 and 0.36 mm s⁻¹ even though the ligands may be terminal, doubly bridging or triply bridging. Values of quadrupole splitting (Δ) are much more susceptible to changes in the Fe environment, for example the Fe(CO)₄ sites have Δ values from about zero {Fe(CO)₄^t in [Fe₃(CO)₁₂)]} to 1.52 {Fe(CO)₃^tCO^tbr in [Fe₃(CO)₁₁]²⁻}. The quadrupole splitting of the Fe site in [FeRu₂(CO)₁₂] (0.77 mm s⁻¹) clearly indicates that the structure of this cluster is not exactly similar to that of [Ru₃(CO)₁₂]. Substitution of CO by phosphine in general leads to small changes in Δ and Δ if the geometry of the Fe site is unaltered. However, Δ especially can be affected if phosphine substitution cause changes in geometry or if there is multiple substitution.     

Mössbauer and electronic spectral characterization of homo-bimetallic Fe(III) complexes of unsymmetrical [N10] and [N12] macrocyclic ligands

The homo-bimetallic complexes of stoichiometry Fe2(L)ClO4(ClO4)2 where L are novel unsymmetrical [N10] (L1.2HClO4) and [N12] (L2.2HClO4) macrocyclic ligands, have been prepared. The ligands were obtained from an in situ capping reaction of the reactive substrate, N,N'-bis(N-ethylaniline)hydrazine-1,2-diimine with a mixture of aniline or 1,3-diaminopropane and HCHO in presence of HClO4. The compounds have been characterized by elemental analyses, conductometric, IR, FAB-mass and electronic spectral studies. IR data of complexes suggest coordination from unsymmetrical aza sites as a tridentate (N,N,N) or tetradentate (N,N,N,N) ligand. mu(eff) values of the complexes suggest presence of antiferromagnetically coupled (Fe3+-Fe3+=S5/2-S5/2) spin exchange. M?ssbauer parameters of the complexes support (+/-3/2)-->(+/-1/2) nuclear transition in high-spin configurations of Fe(III) nuclei of the homo-bimetallic complexes with the presence of Kramer's double degeneracy.     

Crystal structures and spectroscopic properties of copper(II)–dipicolinate complexes with benzimidazole ligands

The syntheses, crystal structures and spectroscopic properties of three Cu(II)–dipicolinate complexes with benzimidazole ligands, namely [Cu(bzim)(dipic)(MeOH)] (1), [Cu₂(2-Etbzim)₂(dipic)₂]_n·0.5nH₂O (2) and [Cu₂(2-ⁱPrbzim)₂(dipic)₂]_n (3), where dipic?=?dipicolinate, bzim?=?1-H-benzimidazole, 2-Etbzim?=?2-ethyl-1-H-benzimidazole and 2-iPrbzim?=?2-isopropyl-1-H-benzimidazole, are reported. Crystal structure studies revealed different coordination modes of the dipicolinate ligands; tridentate chelating for monomeric complex 1, and both tridentate chelating and bridging for similar polymeric complexes 2 and 3. Polymers 2 and 3 both contain two units, in which the Cu(II) central atoms Cu1 and Cu2 have different coordination polyhedra. The first unit {Cu(dipic)₂} with Cu1 is connected to the second via two bidentate carboxylate groups of an μ₃-bridging dipicolinate. In the second unit, Cu2 is coordinated by two imidazole nitrogen atoms from 2-ethyl-1-H-benzimidazole (2) or 2-isopropyl-1-H-benzimidazole (3) ligands. Complex 2 is of higher symmetry and has a localized Cu(II) atom Cu2 in a special position on the twofold axis. EPR spectra of all three Cu(II) complexes, which were measured at both room temperature and 98 K, indicate distorted tetragonal coordination spheres for all the Cu(II) atoms. The g-factor relation (g_//>?g_┴?>?2.0023) is consistent with a \(d_{{x^{2} - y^{2} }}\) ground electronic state in each case.     

Mössbauer spectroscopic studies of molecule-based magnets: NBu4[Fe(II) x Mn(II)1-x Cr(III)(ox)3] and NBu4[Fe(II) x Ni(II)1-x Fe(III)(ox)3]

The internal magnetic field (H _n ) at⁵⁷Fe nucleus was investigated for the mixed crystals, NBu₄[Fe(II) _x Mn(II)_1-x Cr(III) (ox)₃] (x=0.03?1) and NBu₄[Fe(II) _x Ni(II)_1-x Fe(III)(ox)₃]) (x=0?1) using Mössbauer spectroscopy, where NBu₄/⁺=tetra(n-butyl)ammonium ion and ox^2?=oxalate ion. With the decrease ofx, the direction ofH _n at Fe(II) in NBu₄[Fe(II) _x Mn(II)_1-x Cr(III)(ox)₃] changed gradually from parallel to perpendicular, to the honeycomb layers consisting of an alternate array of the bivalent and tervalent ions through ox^2? ligands. A variation of ca. 50° in direction was observed for theH _n at Fe(III) in NBu₄[Fe(II) _x Ni(II)_1-x Fe(III)(ox)₃].     

A structural and Mössbauer study of complexes with Fe(2)(micro-O(H))(2) cores: stepwise oxidation from Fe(II)(micro-OH)(2)Fe(II) through Fe(II)(micro-OH)(2)Fe(III) to Fe(III)(micro-O)(micro-OH)Fe(III)

Dinuclear non-heme iron clusters containing oxo, hydroxo, or carboxylato bridges are found in a number of enzymes involved in O(2) metabolism such as methane monooxygenase, ribonucleotide reductase, and fatty acid desaturases. Efforts to model structural and/or functional features of the protein-bound clusters have prompted the preparation and study of complexes that contain Fe(micro-O(H))(2)Fe cores. Here we report the structures and spectroscopic properties of a family of diiron complexes with the same tetradentate N4 ligand in one ligand topology, namely [(alpha-BPMCN)(2)Fe(II)(2)(micro-OH)(2)](CF(3)SO(3))(2) (1), [(alpha-BPMCN)(2)Fe(II)Fe(III)(micro-OH)(2)](CF(3)SO(3))(3) (2), and [(alpha-BPMCN)(2)Fe(III)(2)(micro-O)(micro-OH)](CF(3)SO(3))(3) (3) (BPMCN = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane). Stepwise one-electron oxidations of 1 to 2 and then to 3 demonstrate the versatility of the Fe(micro-O(H))(2)Fe diamond core to support a number of oxidation states with little structural rearrangement. Insight into the electronic structure of 1, 2', and 3 has been obtained from a detailed M?ssbauer investigation (2' differs from 2 in having a different complement of counterions). Mixed-valence complex 2' is ferromagnetically coupled, with J = -15 +/- 5 cm(-)(1) (H = JS(1).S(2)). For the S = (9)/(2) ground multiplet we have determined the zero-field splitting parameter, D(9/2) = -1.5 +/- 0.1 cm(-)(1), and the hyperfine parameters of the ferric and ferrous sites. For T < 12 K, the S = (9)/(2) multiplet has uncommon relaxation behavior. Thus, M(S) = -(9)/(2) <--> M(S) = +(9)/(2) ground state transition is slow while deltaM(S) = +/-1 transitions between equally signed M(S) levels are fast on the time scale of M?ssbauer spectroscopy. Below 100 K, complex 2' is trapped in the Fe(1)(III)Fe(2)(II) ground state; above this temperature, it exhibits thermally assisted electron hopping into the state Fe(1)(II)Fe(2)(III). The temperature dependence of the isomer shifts was corrected for second-order Doppler shift, obtained from the study of diferrous 1. The resultant true shifts were analyzed in a two-state hopping model. The diferric complex 3 is antiferromagnetically coupled with J = 90 +/- 15 cm(-)(1), estimated from a variable-temperature M?ssbauer analysis.     

Dinuclear (Fe(II), Gd(III)) complexes deriving from hexadentate Schiff bases: synthesis,structure, and Mössbauer and magnetic properties

The dinuclear (Fe(II), Gd(III)) complexes studied in this report derive from hexadentate Schiff base ligands abbreviated H(2)L(i)() (i = 1, 2, 3). H(2)L(1) = N,N'-bis(3-methoxysalicylidene)-1,3-diamino-2,2'-dimethyl-propane, H(2)L(2) = N,N'-bis(3-methoxysalicylidene)-1,2-diamino-2-methylpropane, and H(2)L(3) = N,N'-bis(3-methoxysalicylidene)-1,2-diaminoethane. The crystal and molecular structures of three complexes have been determined at 160 K. Depending on the solvent used in the preparation, L(1)Fe(CH(3)OH)Gd(NO(3))(3)(CH(3)OH)(2), 1, or L(1)Fe((CH(3))(2)CO)Gd(NO(3))(3), 1', is obtained from H(2)L(1). A similar complex, L(2)Fe((CH(3))(2)CO)Gd(NO(3))(3), 2, is obtained from H(2)L(2). Complex 1 crystallizes in the orthorhombic space group Pca2(1) (No. 29): a = 22.141(3) A, b = 9.4159(16) A, c = 15.2075(17) A, V = 3170.4(7) A(3), Z = 4. Complexes 1' and 2 crystallize in the monoclinic space group P2(1)/c (No. 14): 1', a = 9.6264(17) A, b = 19.662(3) A, c = 16.039(3) A, beta = 95.15(2) degrees, V = 3023.6(9) A(3), Z = 4; 2, a = 9.7821(13) A, b = 18.7725(17) A, c = 16.100(2) A, beta = 96.497(16) degrees, V = 2937.5(6) A(3), Z = 4. Complexes 1, 1', and 2 possess an Fe(O(phenoxo))(2-)Gd core. The mononuclear L(3)Fe complex could be prepared from H(2)L(3) but not the related dinuclear (Fe, Gd) species. M?ssbauer spectroscopy evidences that the iron center is in the +2 oxidation state for the six complexes. The experimental magnetic susceptibility and magnetization data of complexes 1, 1', and 2 indicate the occurrence of weak Fe(II)-Gd(III) ferromagnetic interactions. Single ion zero-field splitting of the iron(II) must be taken into account for satisfactorily fitting the data by exact calculation of the energy levels associated to the spin Hamiltonian through diagonalization of the full matrix for axial symmetry (1, J = 0.50 cm(-1), D = 2.06 cm(-1); 1', J = 0.41 cm(-1), D = 3.22 cm(-1); 2, J = 0.08 cm(-1), D = 4.43 cm(-1)).