Single-crystal EPR Measurements Research Articles

Synthesis and characterization of heterometallic rings templated through alkylammonium or imidazolium cations.

We report the synthesis and structural characterization of a series of heterometallic rings templated via alkylammonium or imidazolium cations. The template and preference of each metal's coordination geometry can control the structure of heterometallic compounds, leading to octa-, nona-, deca-, dodeca-, and tetradeca-metallic rings. The compounds were characterized by single-crystal X-ray diffraction, elemental analysis, magnetometry, and EPR measurements. Magnetic measurements show that the exchange coupling between metal centres is antiferromagnetic. EPR spectroscopy shows that the spectra of {Cr7Zn} and {Cr9Zn} have S = 3/2 ground states, while the spectra of {Cr12Zn2} and {Cr8Zn} are consistent with S = 1 and 2 excited states. The EPR spectra of {(ImidH)-Cr6Zn2}, {(1-MeImH)-Cr8Zn2}, and {(1,2-diMeImH)-Cr8Zn2} include a combination of linkage isomers. The results on these related compounds allow us to examine the transferability of magnetic parameters between compounds.

Electron Paramagnetic Resonance Characteristics and Crystal Structure of a Tutton Salt Analogue: Copper-Doped Cadmium Creatininium Sulfate.

Electron paramagnetic resonance and crystallographic studies on copper-doped cadmium creatininium sulfate (CdCrnS) were undertaken to study the characteristics of a copper-hexahydrate complex in an organic analogue of Tutton's salt. X-ray diffraction experiments determined the crystal structure of CdCrnS at both 100 and 298 K. CdCrnS, like Tutton salt, crystallizes in the monoclinic space group P21/n. The unit cell contains two cadmium hexahydrate complexes, four creatininium ions, four sulfates, and four additional solvation waters. Both crystallography and EPR find that the doped copper replaces the cadmium in the structure. Single-crystal EPR measurements at room temperature determined the g and copper hyperfine (ACu) tensors (principal values: g = 2.437, 2.134, and 2.080 and ACu = -327, -84.8, and 7.33 MHz). EPR spectra of the powder at room temperature gave g = 2.448, 2.125, and 2.085 and ACu = -315, -75.0, and 35.0 MHz and at 110 K gave g = 2.462, 2.116, and 2.077 and ACu = -340, -30.0, and 35.0 MHz. The room-temperature tensors are close to the "rigid lattice limit" values found in copper-doped Tutton salts but with a higher gmin and weaker ACux coupling than average. A small but measurable temperature dependency of the tensors indicated the presence of a dynamic Jahn-Teller (JT) effect. In addition, the EPR line width changed dramatically with temperature, which is like that found in all copper-doped Tutton crystals. Utilizing the model of Silver-Getz for the g-value variation gave an estimate for the energy difference (δ12 = 640 cm-1) between the ground and next highest JT configurations. An empirical correlation appears to exist between δ12 and gmin and ACux for the copper hexahydrates studied in similar crystals. This suggests a relationship between the amount of unpaired spins in the copper d-orbital x lobe and the gap between wells of the adiabatic potential surface.

Crystal structure and physical properties of 1-methyl-3-(carboxymethyl)benzimidazolium betaine·CuBr2 in crystal and water solution

Planar CuO2Br2 complex exists in a triclinic unit cell. Single crystal EPR and MO-theory give electron spin density delocalization parameters.

Magnetic Properties of Weakly Exchange-Coupled High Spin Co(II) Ions in Pseudooctahedral Coordination Evaluated by Single Crystal X-Band EPR Spectroscopy and Magnetic Measurements

We report single-crystal X-band EPR and magnetic measurements of the coordination polymer catena-(trans-(μ2-fumarato)tetraaquacobalt(II)), 1, and the Co(II)-doped Zn(II) analogue, 2, in different Zn:Co ratios. 1 presents two magnetically inequivalent high spin S = 3/2 Co(II) ions per unit cell, named A and B, in a distorted octahedral environment coordinated to four water oxygen atoms and trans coordinated to two carboxylic oxygen atoms from the fumarate anions, in which the Co(II) ions are linked by hydrogen bonds and fumarate molecules. Magnetic susceptibility and magnetization measurements of 1 indicate weak antiferromagnetic exchange interactions between the S = 3/2 spins of the Co(II) ions in the crystal lattice. Oriented single crystal EPR experiments of 1 and 2 were used to evaluate the molecular g-tensor and the different exchange coupling constants between the Co(II) ions, assuming an effective spin S′= 1/2. Unexpectedly, the eigenvectors of the molecular g-tensor were not lying along any preferential bond direction, indicating that, in high spin Co(II) ions in roughly octahedral geometry with approximately axial EPR signals, the presence of molecular pseudo axes in the metal site does not determine preferential directions for the molecular g-tensor. The EPR experiment and magnetic measurements, together with a theoretical analysis relating the coupling constants obtained from both techniques, allowed us to evaluate selectively the exchange coupling constant associated with hydrogen bonds that connect magnetically inequivalent Co(II) ions (|JAB(1/2)| = 0.055(2) cm(–1)) and the exchange coupling constant associated with a fumarate bridge connecting equivalent Co(II) ions (|JAA(1/2)| ≈ 0.25 (1) cm(–1)), in good agreement with the average J(3/2) value determined from magnetic measurements.

Energy-level crossing of atomic hydrogen spin states

Atomic hydrogen1H0has four spin states. It is not well known that the upper two, in its electronic ground state, cross at a sufficiently high externally applied magnetic field Bx(ca. 17 T). The magnetic resonance spectra show particular effects in the crossing region. In the present work, we predict the relevant field-swept spectra at selected fixed frequencies, for1H0encapsulated in crystalline alpha-quartz powder, using the known spin Hamiltonian parameters derived from single-crystal EPR measurements. It was found that there is a 5 GHz frequency region in which spectral effects from the crossing can be observed.

Single Crystal EPR Study of the Dinuclear Cu(II) Complex [Cu(tda)(phen)]2·H2tda (tda = Thiodiacetate, phen = Phenanthroline): Influence of Weak Interdimeric Magnetic Interactions

We report powder and single crystal EPR measurements of [Cu(tda)(phen)](2)·H(2)tda (tda = thiodiacetate, phen = phenanthroline) at 9.7 GHz. This compound consists of centrosymmetric copper(II) ion dimers, weakly ferromagnetically exchange-coupled (J = +3.2 cm(-1)), in which the dimeric units are linked by hydrophobic chemical paths involving the phen molecules. EPR revealed that the triplet spectra are collapsed by interdimeric exchange interactions mediated by that chemical path. Analysis and simulation of the single crystal EPR spectra were performed using Anderson's exchange narrowing model, together with statistical arguments. This approach allowed us to interpret the spectra modulated by the interdimeric interactions in situations of weak, intermediate, and strong exchange. We evaluated an interdimeric exchange constant J' = 0.0070(3) cm(-1), indicating that hydrophobic paths can transmit weak exchange interactions between centers at relatively long distances of the order of ∼10 Å.

Copper(II) complexes containing tetrathiafulvalene substituted by a bipyridine ligand: Synthesis, X-ray diffraction and EPR characterizations

We report a preliminary study of two novel complexes combining a paramagnetic transition metal (Cu(II)) and a cyclophane-type TTF-bipyridine ligand TTF-bipy = ( cis)2,7-bis(pentylsulfanyl)-3,6-[2,2′-bipyridine-bis(4,4′-methanediyl)bis-sulfanyl]tetrathiafulvalene. The syntheses of the complexes Cu(hfac) 2(TTF-bipy) (hfac = hexafluoroacetylacetonate) and Cu(NO 3) 2(TTF-bipy)·H 2O are reported. The results of single-crystal EPR measurements are discussed on the basis of structural features deduced from preliminary X-ray diffraction data and indicate that these neutral molecular complexes behave as isolated paramagnetic units.

Complexes based on asymmetrically substituted pyridine–dithiolene ligands [M(4-pedt) 2] (M = Au, Cu, Ni; 4-pedt = 1-(pyridin-4-yl)-ethylene-1,2-dithiolate): Synthesis, structure and physical properties

The monoanionic Ni, Au and Cu bisdithiolene complexes based on the 1-(pyridin-4-yl)-ethylene-1,2-dithiolate were prepared and isolated as [PPh 4] +, [ n-Bu 4N] + and [Na(18C6)] + salts which were characterised by single-crystal X-ray diffraction, cyclic voltammetry, EPR and magnetic susceptibility measurements. All the complexes adopt a square planar coordination geometry with a trans arrangement of the substituent pyridine rings whose planes are rotated from 13° to 33° relatively to the central MS 4 core. The C–C and C–S bond lengths are consistent with ene-1,2-dithiolate form of the ligand and M III oxidation state. The Ni complexes are paramagnetic with an effective magnetic moment of ∼1.7 BM.

A pyrazine-bridged linear pentanuclear copper(II) complex and related tri- and dinuclear complexes showing various coordination structures and magnetic couplings

Sterically bulky pyrazines have been successfully utilized for the preparation of discrete oligo-nuclear TBP (trigonal bipyramidal), SqP (square pyramidal), and Oh (octahedral) copper(II) complexes. We have synthesized a unique linear pentanuclear complex [{Cu(hfac) 2} 5(μ-2-butyl-3-methylpyrazine) 4]. The two terminal copper(II) ions have a SqP structure while the three inner ions have an Oh one. The solvent molecule was incorporated in the clearance of the lattice. From another reaction under harsh conditions, we separated [{Cu(hfac) 2} 3(μ-2-butyl-3-methylpyrazine) 2], which can be regarded as the central moiety of the pentanuclear one. We also prepared a dinuclear complex [{Cu(hfac) 2} 2(μ-tetramethylpyrazine)], in which the pyrazine nitrogen atoms were located at TBP equatorial positions. Single-crystal EPR measurements supported its compressed TBP structure. The exchange coupling was antiferromagnetic with J TBP–TBP/ k B = −3.6 K. The linear trinuclear [{Cu(hfac) 2} 3(μ-2,3,5-trimethylpyrazine) 2], having two TBP Cu ions with an intervening Oh Cu ion, showed very weak antiferromagnetic coupling. DFT calculations on these compounds indicated that the σ-type orbital overlap between the Cu and N atoms is essential for superexchange interactions.

Single Crystal 55Mn ENDOR of Concanavalin A: Detection of Two Mn2+ Sites with Different 55Mn Quadrupole Tensors

Concanavalin A is a member of the plant hemeagglutinin (or plant lectin) family that contains two metal binding sites; one, called S1, is occupied by Mn2+ and the other, S2, by Ca2+. 55Mn electron-nuclear double resonance (ENDOR) measurements were performed on a single crystal of concanavalin A at W-band (95 GHz, ~3.5 T) to determine the 55Mn nuclear quadrupole interaction in a protein binding site and its relation to structural parameters. Such measurements are easier at a high field because of the high sensitivity for size-limited samples and the reduction of second-order effects on the spectrum which simplifies spectral analysis. The analysis of the 55Mn ENDOR rotation patterns showed that two chemically inequivalent Mn2+ types are present at low temperatures, although the high-resolution X-ray structure reported only one site. Their quadrupole coupling constants, e2Qq/h, are significantly different; 10.7 +/- 0.6 MHz for Mand only -2.7 +/-0.6 MHz for M. The ENDOR data also refined the hyperfine coupling determined earlier by single-crystal EPR measurements, yielding a small but significant difference between the two: -262.5 MHz for M and -263.5 MHz for M. The principal z-axis for M is not aligned with any of the Mn-ligand directions, but is 25 off the Mn-asp10 direction, and its orientation is different than that of the zero-field splitting (ZFS) interaction. Because of the small quadrupole interaction of M the orientation dependence was very mild, leading to larger uncertainties in the asymmetry parameter. Nonetheless, there too z is not along the Mn-ligand bonds and is rotated 90 with respect to MnA. These results show, that similar to the ZFS, the quadrupolar interaction is highly sensitive to small differences in the coordination sphere of the Mn2+, and the resolution of the two types is in agreement with the earlier observation of a two-site conformational dynamic detected through the ZFS interaction, which is frozen out at low temperatures and averaged at room temperature. To account for the structural origin of the different e2Qq/h values, the electric field gradient tensor was calculated using the point-charge model. The calculations showed that a relatively small displacement of the oxygen ligand of asp10 can lead to differences on the order observed experimentally.

Single crystal X- and Q-band EPR spectroscopy of a binuclear Mn(2)(III,IV) complex relevant to the oxygen-evolving complex of photosystem II.

The anisotropic g and hyperfine tensors of the Mn di-micro-oxo complex, [Mn(2)(III,IV)O(2)(phen)(4)](PF(6))(3).CH(3)CN, were derived by single-crystal EPR measurements at X- and Q-band frequencies. This is the first simulation of EPR parameters from single-crystal EPR spectra for multinuclear Mn complexes, which are of importance in several metalloenzymes; one of them is the oxygen-evolving complex in photosystem II (PS II). Single-crystal [Mn(2)(III,IV)O(2)(phen)(4)](PF(6))(3).CH(3)CN EPR spectra showed distinct resolved (55)Mn hyperfine lines in all crystal orientations, unlike single-crystal EPR spectra of other Mn(2)(III,IV) di-micro-oxo bridged complexes. We measured the EPR spectra in the crystal ab- and bc-planes, and from these spectra we obtained the EPR spectra of the complex along the unique a-, b-, and c-axes of the crystal. The crystal orientation was determined by X-ray diffraction and single-crystal EXAFS (Extended X-ray Absorption Fine Structure) measurements. In this complex, the three crystallographic axes, a, b, and c, are parallel or nearly parallel to the principal molecular axes of Mn(2)(III,IV)O(2)(phen)(4) as shown in the crystallographic data by Stebler et al. (Inorg. Chem. 1986, 25, 4743). This direct relation together with the resolved hyperfine lines significantly simplified the simulation of single-crystal spectra in the three principal directions due to the reduction of free parameters and, thus, allowed us to define the magnetic g and A tensors of the molecule with a high degree of reliability. These parameters were subsequently used to generate the solution EPR spectra at both X- and Q-bands with excellent agreement. The anisotropic g and hyperfine tensors determined by the simulation of the X- and Q-band single-crystal and solution EPR spectra are as follows: g(x) = 1.9887, g(y) = 1.9957, g(z) = 1.9775, and hyperfine coupling constants are A(III)(x) = |171| G, A(III)(y) = |176| G, A(III)(z) = |129| G, A(IV)(x) = |77| G, A(IV)(y) = |74| G, A(IV)(z) = |80| G.

Effect of lattice defects in divalent lattices by incorporating trivalent ions in biominerals: An EPR study of Cr(III) in zinc(II) potassium phosphate hexahydrate

AbstractIn order to understand the effect of charge compensating vacancies on the number, orientation and magnitude of D‐tensors, single crystal EPR measurements have been carried out with Cr(III) in zinc(II) potassium phosphate hexahydrate at room temperature. During crystal rotations, a maximum of 6/7 resonance lines or a minimum of 2 is noticed. Crystal rotations done in the three orthogonal planes have yielded spin Hamiltonian parameters for one site with: gxx=1.969, gyy= 1.995, gzz= 2.107; Dxx= 66.2 mT; Dyy= 91.7 mT, and Dzz=‐157.9 mT. The powder EPR spectrum indicates a very broad line at g = 8, without any extra resonances from 0 to 650 mT both at room temperature and 77 K. The reason may be that due to large zero field splitting, the resonance at g = 2 is shifted to g = 8 position and the other resonances are below 0 or above 650 mT. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ni(II), Cu(II), and Zn(II) dinuclear metal complexes with an aza-phenolic ligand: crystal structures, magnetic properties, and solution studies.

The basicity behavior and ligational properties of the ligand 2-((bis(aminoethyl)amino)methyl)phenol (L) toward Ni(II), Cu(II), and Zn(II) ions were studied by means of potentiometric measurements in aqueous solution (298.1 +/- 0.1 K, l = 0.15 mol dm-3). The anionic L-H- species can be obtained in strong alkaline solution; this species behaves as tetraprotic base (log K1 = 11.06, log K2 = 9.85, log K3 = 8.46, log K4 = 2.38). L forms mono- and dinuclear complexes in aqueous solution with all the transition metal ions examined; the dinuclear species show a [M2(L-H)2]2+ stoichiometry in which the ligand/metal ratio is 2:2. The studies revealed that two mononuclear [ML-H]+ species self-assemble, giving the dinuclear complexes, which can be easily isolated from the aqueous solution due to their low solubility. This behavior is ascribed to the fact that L does not fulfill the coordination requirement of the ion in the mononuclear species and to the capacity of the phenolic oxygen, as phenolate, to bridge two metal ions. All three dinuclear species were characterized by determining their crystal structures, which showed similar coordination patterns, where all the single metal ions are substantially coordinated by three amine functions and two oxygen atoms of the phenolate moieties. The two metals in the dinuclear complexes are at short distance interacting together as shown by magnetic measurements performed with Ni(II) and Cu(II) complexes, which revealed an antiferromagnetic coupling between the two metal ions. The [Cu2(L-H)2]2+ cation shows a phase transition occurring by the temperature between 100 and 90 K; the characterization of the compounds existing at different temperatures was investigated using X-ray single-crystal diffraction, EPR, and magnetic measurements.

Definitive Assignment of the g Tensor of [Fe(OEP)(NO)] by Single-Crystal EPR

Single-crystal EPR measurements have been performed on the triclinic form of [Fe(OEP)(NO)] (Ellison, M. K.; Scheidt, W. R. J. Am. Chem. Soc. 1997, 119, 7404) and on the isomorphous cobalt derivative [Co(OEP)(NO)] (Ellison, M. K.; Scheidt, W. R. Inorg. Chem. 1998, 37, 382) which has been doped with [Fe(OEP)(NO)]. Principal values of the g tensor determined at room temperature are gmax = 2.106, gmid = 2.057, and gmin = 2.015. The principal direction associated with the minimum g value lies 8 degrees from the Fe-N(NO) direction, 2 degrees from the normal to the heme plane, and 42 degrees from the N-O direction. The direction associated with the maximum g value lies 9 degrees from the normal to the Fe-N-O plane. The fact that the direction of gmin is near the Fe-N(NO) direction is consistent with the dominant role of spin-orbit coupling at the iron atom in determining the g tensor and with the picture of the electronic structure of the compound from restricted calculations, which makes the half-filled orbital mostly dz2 on the iron atom. The hyperfine tensor is nearly isotropic and was only resolved in the doped samples. Principal values of the A tensor determined at room temperature are 40.9, 49.7, and 42.7 MHz. Principal values of the g tensor determined from the doped samples at 77 K are gmax = 2.110, gmid = 2.040, and gmin = 2.012. Principal values of the A tensor are 42.5, 52.8, and 44.1 MHz at 77 K. The small change in g values with temperature is in contrast to the large temperature dependence on g values observed in samples of MbNO (Hori et al. J. Biol. Chem. 1981, 256, 7849).

Crystal Structures and Magnetic Properties of CuX(2)(pdmp)(2) Complexes (X = Br, Cl).

We report the synthesis and the structural and magnetic characterization of two new compounds: dibromobis(pdmp)copper(II), CuBr(2)C(22)H(24)N(4) (1), and dichlorobis(pdmp)copper(II), CuCl(2)C(22)H(24)N(4) (2), where pdmp = 1-phenyl-3,5-dimethylpyrazole. The structures were refined by full-matrix least-squares techniques to R1 = 0.0620 and 0.0777, respectively. Compound 1 belongs to the space group P2(1)/n with a = 8.165(5) Å, b = 10.432(3) Å, c = 13.385(4) Å, beta = 100.12(4) degrees, and Z = 2. Compound 2 belongs to the space group P2(1)/c with a = 8.379(2) Å, b = 22.630(2) Å, c = 12.256(2) Å, beta = 98.43(3) degrees, and Z = 4. It has the same molecular formula as a compound reported previously but a different crystal structure. Detailed single-crystal EPR measurements were performed for single-crystal samples of 1 and 2 at 9 and 35 GHz and at room temperature. The positions and line widths of the EPR lines were measured as a function of the magnetic field orientation in three orthogonal planes. The data were used to study the electronic properties of the copper ions and to evaluate the exchange interactions between them. Our results are discussed in terms of the electronic pathways for superexchange between copper ions, which are provided by the stacking of pyrazole and phenyl rings of neighboring molecules and by hydrogen-halogen bonds.

Structure, Single Crystal EPR Spectra, and Exchange Interactions in [Cu(L-proline)(2)](2).5H(2)O and Cu(D,L-proline)(2).2H(2)O.

A new copper(II) compound, [Cu(L-proline)(2)](2).5H(2)O (C(20)Cu(2)H(42)N(4)O(13)) (called compound I) was synthesized and crystallized, and its structure was solved using X-ray methods. It is monoclinic, space group P2(1), with a = 11.187(1) Å, b = 12.172(3) Å, c = 11.661(1) Å, beta = 114.96(1) degrees, and Z = 2. There are two chemically different copper molecules (labeled A and B), both with the copper atom in a N(2)O(2) square planar coordination. Molecule type A has one water molecule in an apical position. Molecule B has water molecules in each of the two apical positions. Single-crystal EPR measurements have been performed in I and also in Cu(D,L-proline)(2).2H(2)O (compound II). From the similar angular variations of the position of the single resonance observed in both compounds, we evaluated the molecular g tensors. Interpretation of the molecular g tensors resulted in d(x)()()2(-)(y)()()2 orbital ground states. From the angular variations of the line width we calculated the magnitude of the exchange interactions coupling neighbor copper ions in each compound. In I copper ions type A at 7.25 Å are arranged in chains coupled through axial-equatorial bonds. The exchange coupling within these chains is |J/k| = 118 mK. The coupling between copper ions type B is weaker. However, the interactions between copper ions type A and B generate a three-dimensional magnetic network. Our data in compound II indicate that a superexchange pathway containing a weak hydrogen bond C-H- - -O is the path for an exchange interaction with |J'/k| = 48 mK between coppers in neighbor layers at 9.75 Å.

The Cl2O2+ Cation: Preparation and Structural Investigation of Cl2O2+SbF6- and Cl2O2+Sb2F11-

Cl2 reacts with O2+SbF6- in anhydrous HF, forming subsequently a violet, a blue, and a yellow solution. Crystals from the violet solution are identified as Cl2O2+SbF6- or Cl2O2+Sb2F11- by means of single-crystal structural determinations, Raman spectroscopy, magnetic and EPR measurements, and ab initio calculations. The cation has a planar trapezoid structure with Cl−Cl distances of 190.9(1)−191.6(5) pm, O−O distances of 118.5(11)−120.7(5) pm, and Cl−O distances of 240.8(4)−242.8(4) pm. The bonding between the two parts of the cation may be described as a π*−π* interaction, resulting in a charge transfer of about 0.6 electrons with respect to isolated Cl2+ and O2 fragments. The bonding of O2 to Cl2+ is reversible, as is shown by 18O isotope exchange. The nature of the blue solution remains unknown. Both deeply colored compounds turn into yellow Cl3+ if warmed to room temperature, especially in the presence of excess Cl2. The Cl3+ cation is symmetrical and bent with Cl−Cl = 197.23−199.44 pm and Cl−Cl−Cl = 104.51(10)−105.62(7)°.

Ferromagnetic Mn(II).Cu(II) Exchange in the New Bimetallic Quasi-2-D Compound Cu(op)(2)MnCl(4) (op = 1,4-Diazacycloheptane).

The synthesis, crystal structure, and magnetic properties are reported for the new bimetallic compound Cu(op)(2)MnCl(4), where op = HN(CH(2))(5)NH. The compound, C(10)H(24)N(4)Cl(4)CuMn, crystallizes in the monoclinic space group P2(1)/n. Cell dimensions are as follows: a = 15.316(3) Å, b = 16.608(3) Å, c = 7.141(2) Å, beta = 100.01(5) degrees, Z = 4. The structure consists of well-separated and magnetically equivalent layers which are composed of chloride-bridged Cu(op)(2)MnCl(4) binuclear units connected by rather loose Cu-N-H.Cl-Mn contacts. The MnCl(4) fragment approximates tetrahedral symmetry. The Cu(II) geometry is (4 + 1) square-pyramidal with the apical position occupied by a bridging chloride ligand and the basal ones by the nitrogen atoms from the organic ligands. The shortest interlayer M.M separations, approximately 7 Å, are of the Mn.Cu type. Magnetic susceptibility and single-crystal EPR measurements for the compound have been carried out over the range 4-300 K. At room temperature the chiT product (per MnCu unit) has a value of 4.84 emu.mol(-)(1).K, close to that expected for uncoupled S = (5)/(2) and S = (1)/(2) spins. When the temperature is lowered, chiT remains almost constant until 80-90 K, slightly increases to reach a maximum at approximately 13 K (5.21 emu.mol(-)(1).K), and then rapidly decreases. Comparison between theory and experiment, made with use of both a mean field corrected dimer model and an approximate 2-D model, indicates that Mn(II).Cu(II) exchange is ferromagnetic within the dimers (J(1) approximately 2.6 cm(-)(1)) and antiferromagnetic among dimers, with J values between -0.07 and -0.03 cm(-)(1) (the interaction Hamiltonian is of the form H = -2JS(A).S(B)). Single-crystal EPR spectra recorded along the a, b, and c axes show a large temperature dependence of the g factors: at 4.2 K, g(a) = 2.10, g(b) = 1.96, and g(c) = 2.01. This pattern substantiates the presence of a 2-D magnetic structure with ferromagnetic intradimer exchange and interdimer antiferromagnetic exchange of weaker magnitude. The opposite signs of the interactions are ascribed to the local symmetries of the Cu(II) and Mn(II) ions.

Effects of CoS torsion angle variation in a cobalt(II)-thiolate complex: X-ray crystal structure analysis, single-crystal EPR measurements and ligand-field calculations

X-ray structural determination and single-crystal EPR measurements were performed for (Me 4N) 2[Co(SPh) 4]. The compound crystallizes in the tetragonal space group P4 12 12 with a = 10.220(6), c = 32.316(16) A ̊ and Z = 4 . It was found that the Co(SPh) 4 2− anion has an approximate S 4 geometry: the four S′CoSC torsion angles are nearly the same (the mean value is 57.2°), where the S′CoS angle is bisected by the approximate S 4 axis, and the CoSPh units are almost planar. Analysis of the single-crystal EPR results provides effective g values of g′ x = 2.34, g′ y = 5.68 and g′ z = 1.64. The g′ z axis is most likely parallel to the approximate S 4 axis, and such a g′ z axis direction was found to be consistent with the single-crystal EPR results. The g′ values correspond to E/D = 0.30 and g ‖ = 2.09, g ⊥ = 2. in the S = 3 2 spin Hamiltonian description. These spin Hamiltonian parameters are contrasted with those of the D 2 d conformation anion in (Ph 4P) 2[Co(SPh) 4], which demonstrates the striking effects of the variation of the CoS torsion angles. Ligand-field calculations based on the angular overlap model were made with the aim of understanding the effects of torsion angle variation. It is shown that the striking torsion angle dependence can be understood in terms of the torsion angle dependent nature of the directions of CoS π and off-axis σ interactions. The calculations semi-quantitatively reproduce the contrasted magnetic properties of the two compounds.

Variable magnetic interactions in an organic radical system of (m-N-methylpyridinium alpha -nitronyl nitroxide)

In this paper we study an organic system of geometrical spin frustration. m-N-methylpyridinium \ensuremath{\alpha}-nitronyl nitroxide (m-${\mathrm{MPYNN}}^{+}$) is a spin-1/2 organic radical. The simple salt, m-${\mathrm{MPYNN}}^{+}$\ensuremath{\cdot}${\mathrm{ClO}}_{4}^{\mathrm{\ensuremath{-}}}$\ensuremath{\cdot}1/3 (acetone), crystallizes in a trigonal P3c1 space group, where the m-${\mathrm{MPYNN}}^{+}$ molecules exist as a dimer and the dimer units form a two-dimensional (2D) triangular lattice. One-third of the ${\mathrm{C}1\mathrm{O}}_{4}^{\mathrm{\ensuremath{-}}}$ ions are in the organic layer, joining the m-${\mathrm{MPYNN}}^{+}$ molecules, and the remainder is between the layers, compensating the excess of positive charge in the organic layers. The single-crystal EPR measurements clearly indicate a 2D Heisenberg character of the magnetic system in it. m-${\mathrm{MPYNN}}^{+}$ makes a crystalline solid-solution system, m-${\mathrm{MPYNN}}^{+}$\ensuremath{\cdot}(${\mathrm{ClO}}_{4}^{\mathrm{\ensuremath{-}}}$${)}_{\mathit{x}}$\ensuremath{\cdot}${\mathrm{I}}_{1\mathrm{\ensuremath{-}}\mathit{x}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\cdot}1/3 (acetone) (0\ensuremath{\le}x\ensuremath{\le}1), which also belongs to the trigonal system. Both the a and c axes are slightly lengthened with increasing the ratio of the ${\mathrm{C}1\mathrm{O}}_{4}^{\mathrm{\ensuremath{-}}}$ ion, x, in the solid solution: The unit-cell volume is increased by 3.2% when x runs from 0 to 1. The temperature dependence of the magnetic susceptibilities of the solid solutions can be well interpreted in terms of a strong ferromagnetic intradimer interaction ${\mathit{J}}_{1}$ forming a triplet state and a weak antiferromagnetic interdimer interaction ${\mathit{J}}_{2}$ which is expected to give rise to spin frustration among the triplet spin species on each side of the triangles. It is found that ${\mathit{J}}_{2}$ quickly weakens with an increase in x, while ${\mathit{J}}_{1}$ shows little dependence. There is a possibility that this organic system can be characterized as a spin-1 kagom\'e antiferromagnet at very low temperatures.