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Seven new di-, tri-, tetra-, and hexanuclear iron complexes that incorporate a polydentate Schiff base and variously substituted catecholate ligands have been synthesized from the trinuclear precursor [Fe(3)(OAc)(3)(L)(3)] (1), where LH(2) = 2-[[(2-hydroxyethyl)imino]phenylmethyl]-phenol. These were isolated as the compounds [Fe(3)(OAc)(Cat)(L)(3)] (2), [Fe(6)(OAc)(2)(Cat)(4)(L)(4)] (3), [Fe(4)(3,5-DBCat)(2)(L)(4)] (4), [Bu(4)N][Fe(4)(OAc)(3,5-DBCat)(4)(L)(2)] (5a, 5(-) is the complex monoanion [Fe(4)(OAc)(3,5-DBCat)(4)(L)(2)](-)), [Fe(4)(OAc)(3,5-DBCat)(3)(3,5-DBSQ)(L)(2)] (6), [Fe(2)(Cl(4)Cat)(2)(L)(LH(2))(H(2)O)] (7), and [Et(3)NH](2)[Fe(2)(Cl(4)Cat)(2)(L)(2)] (8a, 8(2-) is the complex dianion [Fe(2)(Cl(4)Cat)(2)(L)(2)](2-)), where CatH(2) = catechol; 3,5-DBCatH(2) = 3,5-di-tert-butyl-catechol; 3,5-DBSQH = 3,5-di-tert-butyl-semiquinone, and Cl(4)CatH(2) = tetrachlorocatechol. While compounds 2-4, 5a, 7, and 8a were obtained by directly treating 1 with the appropriate catechol, compound 6 was synthesized by chemical oxidation of 5a. These compounds have been characterized by single crystal X-ray diffraction, infrared and UV-visible spectroscopy, voltammetry, UV-visible spectroelectrochemistry, and magnetic susceptibility and magnetization measurements. An electrochemical study of the three tetranuclear complexes (4, 5(-), and 6) reveals multiple reversible redox processes due to the o-dioxolene ligands, in addition to reductive processes corresponding to the reduction of the iron(III) centers to iron(II). A voltammetric study of the progress of the chemical oxidation of compound 5a, together with a spectroelectrochemical study of the analogous electrochemical oxidation, indicates that there are two isomeric forms of the one-electron oxidized product. A relatively short-lived neutral species (5) that possesses the same ligand arrangement as complex 5(-) is the kinetic product of both chemical and electrochemical oxidation. After several hours, this species undergoes a significant structural rearrangement to convert to complex 6, which appears to be largely driven by the preference for the 3,5-DBSQ(-) ligand to bind in a non-bridging mode. Variable temperature magnetic susceptibility measurements for compounds 3, 4, 5a, 6, 7, and 8a reveal behavior dominated by pairwise antiferromagnetic exchange interactions, giving rise to a poorly isolated S = 0 ground state spin for compound 3, well-isolated S = 0 ground state spins for complexes 4, 5(-), 7 and 8(2-), and a well-isolated S = 1/2 ground state spin for complex 6. The ground state spin values were confirmed by low temperature variable field magnetization measurements. The thermal variation of the magnetic susceptibility for compounds 3, 4, 5a, 6, 7, and 8a were fitted and/or simulated using the appropriate Hamiltonians to derive J values that are consistent with magnetostructural correlations that have been reported previously for alkoxo-bridged ferric complexes.

We report the synthesis and structural characterisation of a family of finite molecular chains, specifically [{[R2NH2](3) [Cr6F11 (O2CCMe3)(10)]}(2)] (in which R=nPr 1, Et 2, nBu 3), [{Et2NH}(2) {[Et2NH2](3)[Cr7F12(O2C-CMe3)(12)][HO2CCMe3](2)}(2)] (4), [{[Me2NH2](3)[Cr6F11(O2CCMe3)(10)]center dot 2.5 H2O}(4)] (5) and [I{iPr(2)NH(2)](3)[Cr7F12(O2CCMe3)(12)]}(2)] (6). The structures all contain horseshoes of chromium centres, with each Cr center dot center dot center dot Cr contact within the horseshoe bridged by a fluoride and two pivalates. The horseshoes are linked through hydrogen bonds to the secondary ammonium cations in the structure, leading to di- and tetrahorseshoe structures. Through magnetic measurements and inelastic neutron scattering studies we have determined the exchange coupling constants in 1 and 6. In 1 it is possible to distinguish two exchange interactions, J(A) = -1.1 meV and J(B) = -1.4 meV; J(A) is the exchange interactions at the tips of the horseshoe and JB is the exchange within the body of the horseshoe (1 meV = 8.066 cm(-1)). For 6 only one interaction was needed to model the data: J=-1.18meV The single-ion anisotropy parameters for Cr-III were also derived for the two compounds as: for 1, D-Cr = -0.028 meV and vertical bar ECr vertical bar = 0.005meV; for 6, D-Cr=-0.031meV. Magnetic-field-dependent inelastic neutron scattering experiments on I allowed the Zeeman splitting of the first two excited states and level crossings to be observed. For the tetramer of horseshoes (5), quantum Monte Carlo calculations were used to fit the magnetic susceptibility behaviour, giving two exchange interactions within the horseshoe (-1.32 and -1.65 meV) and a weak inter-horseshoe coupling of +0.12meV. Multi-frequency variable-temperature EPR studies on 1, 2 and 6 have also been performed, allowing further characterisation of the spin Hamiltonian parameters of these chains.

Folding under pressure: Crystallographic studies on a Mn6 single-molecule magnet under high pressure conditions show the drastic structural changes of the magnetic core (see picture, Mn purple, O red, N blue), which impact on the magnetic properties of ferromagnetic exchange between the metal atoms will be in booster weaker, and under extremely high pressure, a transition to antiferromagnetic behavior.

The reaction of N-(2-hydroxy-5-nitrobenzyl)iminodiethanol (= H 3 (5-NO 2 -hbide)) with Mn(OAC) 2 · 4H 2 O in methanol, followed by recrystallization from 1,2-dichloroethane, yielded a wheel-shaped single-molecule magnet (SMM) of [Mn II 3 Mn III 4 (5-NO 2 -hbide) 6 ]·5C 2 H4Cl 2 (1). In 1, seven manganese ions are linked by six tri-anionic ligands and form the wheel in which the two manganese ions on the rim and the one in the center are Mn II and the other four manganese ions are Mn III ions. Powder magnetic susceptibility measurements showed a gradual increase with Χm T values as the temperature was lowered, reaching a maximum value of 53.9 emumol -1 K. Analyses of magnetic susceptibility data suggested a spin ground state of S=19/2. The zero-field splitting parameters of D and B 0 4 were estimated to be -0.283(1) K and -1.64(1) x10 -5 K, respectively, by high-field EPR measurements (HF-EPR). The anisotropic parameters agreed with those estimated from magnetization and inelastic neutron scattering experiments. AC magnetic susceptibility measurements showed frequency-dependent in- and out-of-phase signals, characteristic data for an SMM, and an Arrhenius plot of the relaxation time gave a re-orientation energy barrier (ΔE) of 18.1 K and a pre-exponential factor of 1.63 ×10 -7 s. Magnetization experiments on aligned single crystals below 0.7 K showed a stepped hysteresis loop, confirming the occurrence of quantum tunneling of the on magnetization (QTM). QTM was, on the other hand, suppressed by rapid sweeps of the magnetic field even at 0.5 K. The sweep-rate dependence of the spin flips can be understood by considering the Landau-Zener-Stuckelberg (LZS) model.

Na(10)Co(4)O(10) was investigated by neutron powder diffraction at 230, 70, and 4 K. The crystal structure, determined previously by X-ray diffraction on single crystals, was confirmed. Na(10)Co(4)O(10) orders magnetically below 37 K. All observed magnetic reflections could be indexed by integers (hkl) with respect to the chemical unit cell and the magnetic propagation vector q=0. The refinement was performed in the Shubnikov space group C2/c and indicated a collinear antiferromagnetic spin structure. The determined spin arrangement is consistent with the magnetic intratetramer interactions suggested previously from the analysis of magnetic susceptibility data: the magnetic moments of the central Co(III) ions of the Co(4)O(10) tetramer lie parallel to each other and couple in an antiparallel fashion to the terminal Co(II) moments. The Rietveld analysis shows that the net moments of 0.64 mu(B) per tetramer form ferromagnetic layers parallel to the ab plane. Adjacent layers are coupled antiferromagnetically along c. The spins are aligned in the ac plane along the line connecting adjacent Co(II) and Co(III) ions of the tetramer. We have determined unusually low values for the ordered magnetic moments of 2.43(5) mu(B) and 2.11(6) mu(B) for Co(III) and Co(II), respectively. The occurrence of spontaneous magnetization below 37 K indicates a slight canting of 2.2 degrees of the antiferromagnetic structure. A representation analysis shows that a weak ferromagnetic component along b is compatible with the determined antiferromagnetic structure.

We feature our recent work in the field of single-molecule magnets (SMMs) using inelastic neutron scattering (INS). The term "pressure" in the title has a triple meaning. First, there is the expectation from research-funding agencies and the public to make some significant steps towards applications. Second, the synthesis of new compounds and the applications of physical techniques for their understanding are being pushed to their limits. And third, by applying hydrostatic pressure, valuable insight into the mechanisms behind the SMM phenomena can be gained. Examples from our research have been taken to illustrate these points. After a brief introduction to the technique, the strength of INS for the accurate determination of exchange and anisotropy interactions in SMMs is highlighted. We hope to demonstrate that, by pushing INS measurements to their limits, i.e., using the best available neutron sources and instrumentation, combined with high quality samples, new insights into the relevant physical processes in SMMs can be gained.

A new family of tetranuclear Ni complexes [Ni(4)(ROH)(4)L(4)] (H(2)L = salicylidene-2-ethanolamine; R = Me (1) or Et (2)) has been synthesized and studied. Complexes 1 and 2 possess a [Ni(4)O(4)] core comprising a distorted cubane arrangement. Magnetic susceptibility and inelastic neutron scattering studies indicate a combination of ferromagnetic and antiferromagnetic pairwise exchange interactions between the four Ni(II) centers, resulting in an S = 4 spin ground state. Magnetization measurements reveal an easy-axis-type magnetic anisotropy with D approximately -0.93 cm(-)(1) for both complexes. Despite the large magnetic anisotropy, no slow relaxation of the magnetization is observed down to 40 mK. To determine the origin of the low-temperature magnetic behavior, the magnetic anisotropy of complex 1 was probed in detail using inelastic neutron scattering and frequency domain magnetic resonance spectroscopy. The spectroscopic studies confirm the easy-axis-type anisotropy and indicate strong transverse interactions. These lead to rapid quantum tunneling of the magnetization, explaining the unexpected absence of slow magnetization relaxation for complex 1.

International audience; [(iC3H7)2NH2][Cr8NiF9(O2CCMe)18], or Cr8Ni, is a prominent example of an odd-membered antiferromagnetic "wheel." A detailed characterization of the magnetic properties of Cr8Ni has been conducted. Inelastic neutron scattering (INS) is used to investigate the energy and momentum transfer dependence of the low-lying spin excitations, including excited states inaccessible by other experimental techniques. The richness of the INS data, in conjunction with microscopic spin Hamiltonian simulations, enables an accurate characterization of the magnetic properties of Cr8Ni. Nearest-neighbor exchange constants of JCrCr = 1.31 meV and JCrNi = 3.22 meV are determined, and clear evidence of axial single-ion anisotropy is found. The parameters determined by INS are shown to fit magnetic susceptibility. The spectroscopic identification of several successive S=1 excited total spin states and lowest spin band excitations show that the rotational band picture, valid for bipartite AFM wheels, breaks down for this odd-numbered wheel. The exchange constants determined here differ from previous efforts based on bulk measurements, and possible reasons are discussed. The large JCrNi/JCrCr ratio in Cr8Ni puts this wheel into a regime with strong quantum fluctuations in which the ground state can be described with a valence bond solid state picture.

The antiferromagnetic molecular finite chain Cr6 was studied by inelastic neutron scattering. The observed magnetic excitations at 2.6 and 4.3 meV correspond, due to the open boundaries of a finite chain, to standing spin waves. The determined energy spectrum revealed an essentially classical spin structure. Hence, various spin-wave theories were investigated in order to assess their potential for describing the elementary excitations of finite spin systems., Comment: 6 pages, 6 figures, accepted for publication in Europhysics Letters

The Q dependence of the inelastic neutron scattering (INS) intensity of transitions within the ground-state spin multiplet of single-molecule magnets (SMMs) is considered. For these transitions, the Q dependence is related to the spin density map in the ground state, which in turn is governed by the Heisenberg exchange interactions in the cluster. This provides the possibility to infer the exchange-coupling constants from the Q dependence of the INS transitions within the spin ground state. The potential of this strategy is explored for the M = +-10 -> +- 9 transition within the S = 10 multiplet of the molecule Mn12 as an example. The Q dependence is calculated for powder as well as single-crystal Mn12 samples for various exchange-coupling situations discussed in the literature. The results are compared to literature data on a powder sample of Mn12 and to measurements on an oriented array of about 500 single-crystals of Mn12. The calculated Q dependence exhibits significant variation with the exchange-coupling constants, in particular for a single-crystal sample, but the experimental findings did not permit an unambiguous determination. However, although challenging, suitable experiments are within the reach of today's instruments., 11 pages, 6 figures, REVTEX4, to appear in PRB

We feature our recent work in the field of single-molecule magnets (SMMs) using inelastic neutron scattering (INS). The term "pressure" in the title has a triple meaning. First, there is the expectation from research-funding agencies and the public to make some significant steps towards applications. Second, the synthesis of new compounds and the applications of physical techniques for their understanding are being pushed to their limits. And third, by applying hydrostatic pressure, valuable insight into the mechanisms behind the SMM phenomena can be gained. Examples from our research have been taken to illustrate these points. After a brief introduction to the technique, the strength of INS for the accurate determination of exchange and anisotropy interactions in SMMs is highlighted. We hope to demonstrate that, by pushing INS measurements to their limits, i.e., using the best available neutron sources and instrumentation, combined with high quality samples, new insights into the relevant physical processes in SMMs can be gained.

Nuclear and magnetic structures of sodium ruthenate (VI) have been studied by neutron powder diffraction in the temperature range 1.5–200 K. Na2RuO4 crystallizes in the monoclinic structure, with space group P 21/c. The structure consists of apical corner sharing RuO5 trigonal bipyramids forming infinite chains running along the b axis. These infinite [ RuO3O2/2] chains form a pseudo hexagonal close packing of rods with Ru–Ru distances of 3.51 A within the chains and 5.30–5.47 A between the chains. At TN=37.2 K a magnetic transition leads to an antiferromagnetic state. The Ru6+ magnetic moments are ordered antiferromagnetically along the chains (b-axis), while the inter-chain interaction is ferromagnetic. A classical infinite chain model was fitted to the magnetic susceptibility data in order to estimate the strength of the nearest-neighbor exchange interactions along and between the chains, resulting in an intrachain coupling parameter of 2J=-86 K, and an interchain parameter J⊥ with $\vert $ 2J⊥ $\vert $ = 3 K.

The synthesis and crystal structures of a family of decametallic Cr(III) "molecular wheels" are reported, namely [Cr10(OR)20(O2CR')10] [R' = Me, R = Me (1), Et (2); R' = Et, R = Me (3), Et (4); R' = CMe3, R = Me (5), Et (6)]. Magnetic studies on 1-6 reveal a remarkable dependence of the magnetic behaviour on the nature of R. In each pair of complexes with a common carboxylate (R') the nearest neighbour CrCr magnetic exchange coupling is more antiferromagnetic for the ethoxide-bridged (R = Et) cluster than for the methoxide analogue. In complexes 2, 4 and 6 the overall coupling is weakly antiferromagnetic resulting in diamagnetic (S = 0) ground states for the cluster, whilst in 1 and 5 it is weakly ferromagnetic thus resulting in very high-spin ground states. This ground state has been probed directly in the perdeuterated version of 1 ([D]1) by inelastic neutron scattering experiments, and these support the S = 15 ground state expected for ferromagnetic coupling of ten Cr(III) ions, and they also indicate that a single J-value model is inadequate. The ground state of 5 is large but not well defined. The trends in J on changing R are further supported by density functional calculations on 1-6, which are in excellent agreement with experiment. The very large changes in the nature of the ground state between 1 and 2, and 5 and 6 are the result of relatively small changes in J that happen to cross J = 0, hence changing the sign of J.

Energy splittings resulting from anisotropy and exchange interactions in the dimer of single-molecule magnets [Mn4O3Cl4(O2CEt)3(py)3]2.8MeCN are determined for both an undeuterated and a partially deuterated sample using inelastic neutron scattering. The antiferromagnetic (AF) exchange coupling between the two Mn4 subunits strongly depends on their separation. The Cl...Cl distance between the two subunits can be modified either by exchanging the solvent of crystallization or by deuteration of the C-H...Cl hydrogen bonds. The exchange of acetonitrile for n-hexane leads to a five times greater shortening of the Cl...Cl separation than does full deuteration of all the hydrogen bonds. As a result, the AF exchange coupling constants between the subunits are 0.0073(4) and 0.0103(9) meV in the samples with acetonitrile and n-hexane solvent molecules, respectively, in the crystal structure. On the other hand, the effect of C-H...Cl deuteration on the AF exchange coupling is not detectable within the experimental accuracy of INS.

The magnetic exchange interactions in a C0(3)(11) moiety encapsulated in Na(17) [(NaOH(2))Co(3)(H(2)O)(P(2)W(15)O(56))(2)] (NaCo(3)) were studied by a combination of magnetic measurements (magnetic susceptibility and low-temperature magnetization), with a detailed Inelastic Neutron Scattering (INS) investigation. The novel structure of the salt was determined by X-ray crystallography. The ferromagnetic Co(3)O(14) triangular cluster core consists of three octahedrally oxo-coordinated Co(II) ions sharing edges. According to the single-ion anisotropy and spin-orbit coupling usually assumed for octahedral Co(II) ions, the appropiate exchange Hamiltonian to describe the ground-state properties of the isosceles triangular Co(3) spin cluster is anisotropic and is expressed as H = - 2sigma(alpha)(=)(x,y,z)(J(alpha)(12)S(1alpha)S(2alpha) + J(alpha)(23)S(2alpha)S(3alpha) + J(alpha)(13)S(1alpha)S(3alpha)), where J(alpha) are the components of the exchange interactions between the Co(II) ions. To reproduce the INS data, nonparallel anisotropic exchange tensors needed to be introduced, which were directly connected to the molecular symmetry of the complex. The following range of parameters (value +/- 0.5 cm(-1)) was found to reproduce all experimental information while taking magnetostructural relations into account: J(x)(12) = J(y)(13) = 8.6 cm(-1); J(y)(12) = J(x)(13) = 1.4 cm(-1); J(z)(12) = J(z)(13) = 10.0 cm(-1); J(x)(23) = J(y)(23) = 6.5 cm(-1) and = 3.4 cm(-1).

The reaction of [Mn(3)O(O(2)CMe)(6)(py)(3)] with the tripodal ligand H(3)thme (1,1,1-tris(hydroxymethyl)ethane) affords the enneanuclear complex [Mn(9)O(7)(O(2)CCH(3))(11)(thme)(py)(3)(H(2)O)(2)] 1.1MeCN.1Et(2)O. The metallic skeleton of complex 1 comprises a series of 10 edge-sharing triangles that describes part of an idealized icosahedron. Variable temperature direct current (dc) magnetic susceptibility data collected in the 1.8-300 K temperature range and in fields up to 5.5 T were fitted to give a spin ground state of S = (17)/(2) with an axial zero-field splitting parameter D = -0.29 cm(-)(1). Ac susceptibility studies indicate frequency-dependent out-of-phase signals below 4 K and an effective barrier for the relaxation of the magnetization of U(eff) = 27 K. Magnetic measurements of single crystals of 1 at low temperature show time- and temperature-dependent hysteresis loops which contain steps at regular intervals of field. Inelastic neutron scattering (INS) studies on complex 1 confirm the S = (17)/(2) ground state and analysis of the INS transitions within the zero-field split ground state leads to determination of the axial anisotropy, D = -0.249 cm(-)(1), and the crystal field parameter, B(4)(0) = 7(4) x 10(-)(6) cm(-)(1). Frequency domain magnetic resonance spectroscopy (FDMRS) determined the same parameters as D = -0.247 cm(-)(1) and B(4)(0) = 4.6 x 10(-)(6) cm(-)(1). DFT calculations are fully consistent with the experimental findings of two Mn(II) and four Mn(III) ions "spin up" and three Mn(IV) ions "spin down" resulting in the S = (17)/(2) spin ground state of the molecule, with D = -0.23 cm(-)(1) and U = 26.2 K.

The anisotropy splitting in the ground state of the single-molecule magnet [Mn4O3Br(OAc)3(dbm)3] is studied by inelastic neutron scattering as a function of hydrostatic pressure. This allows a tuning of the anisotropy and thus the energy barrier for slow magnetisation relaxation at low temperatures. The value of the negative axial anisotropy parameter $D_{\rm cluster}$ changes from -0.0627(1) meV at ambient to -0.0603(3) meV at 12 kbar pressure, and in the same pressure range the height of the energy barrier between up and down spins is reduced from 1.260(5) meV to 1.213(9) meV. Since the $\rm Mn-Br$ bond is significantly softer and thus more compressible than the $\rm Mn-O$ bonds, pressure induces a tilt of the single ion Mn$^{3+}$ anisotropy axes, resulting in the net reduction of the axial cluster anisotropy., 4 pages, 3 figures

A new family of tetranuclear Mn complexes [Mn4X4L4] (H2L = salicylidene-2-ethanolamine; X = Cl (1) or Br (2)) and [Mn4Cl4(L')4] (H2L' = 4-tert-butyl-salicylidene-2-ethanolamine, (3)) has been synthesized and studied. Complexes 1-3 possess a square-shaped core with ferromagnetic exchange interactions between the four Mn(III) centers resulting in an S = 8 spin ground state. Magnetochemical studies and high-frequency EPR spectroscopy reveal an axial magnetoanisotropy with D values in the range -0.10 to -0.20 cm(-1) for complexes 2 and 3 and for differently solvated forms of 1. As a result, these species possess an anisotropy-induced energy barrier to magnetization reversal and display slow relaxation of the magnetization, which is observed as hysteresis for 1 and 3 and frequency-dependent peaks in out-of-phase AC susceptibility measurements for 3. The effective energy barrier was determined to be 7.7 and 7.9 K for 1 and 3, respectively, and evidence for quantum tunneling of the magnetization was observed. Detailed magnetochemical studies, including measurements at ultralow temperatures, have revealed that complexes 1 and 2 possess solvation-dependent antiferromagnetic intermolecular interactions. Complex 3 displays ferromagnetic intermolecular interactions and approaches a ferromagnetic phase transition with a critical temperature of approximately 1 K, which is coincident with the onset of slow relaxation of the magnetization due to the molecular anisotropy barrier to magnetization reversal. It was found that the intermolecular interactions have a significant effect on the manifestation of slow relaxation of the magnetization, and thereby, these complexes represent a new family of "exchange-biased single-molecule magnets", where the exchange bias is controlled by chemical and structural modifications.

A 3D metal-organic framework, [Na{Mn(3)(Hbtc)(2)(btc)}.5H(2)O](n) () (H(3)btc = 1,3,5-benzene tricarboxylic acid), was synthesized under hydrothermal conditions. The structure of was established by single crystal X-ray diffraction analysis; crystallizes in the monoclinic space group P2/c, a = 9.753(3) A, b = 12.751(2) A, c = 14.174(4) A, beta = 109.41(1) degrees . The complex is isostructural to previously reported MIL-45 and consists of one dimensional wave like chains of carboxylate bridged hexa-coordinated Mn(ii) ions. Variable temperature magnetic susceptibility measurements revealed dominant antiferromagnetic exchange interactions and the intra-chain exchange constants J(1) = -2.4 cm(-1) and J(2) = -0.6 cm(-1) compare well with literature values for similar materials. Inter-chain interactions are expected to be very small in this compound and there is no indication of magnetic ordering phenomena in the temperature range from 300-2 K.