High-spin Clusters Research Articles

Structural diversity of cyanide-bridged bimetallic clusters based on hexacyanometallate building blocks

Four new cyanide-bridged bimetallic complexes, with different geometries and nuclearities, have been synthesized and characterized by single crystal X-ray crystallography. The trinuclear compound {〚Zn(bpy) 2(OH 2)〛 2〚Fe(CN) 6〛}Cl ( 1)·2 MeOH·12 H 2O crystallizes in the triclinic space group P 1 , with a = 12.010(2) Å, b = 16.316(3) Å, c = 16.325(3) Å, α = 97.47(3)°, β = 92.14(3)°, γ = 92.43(3)°, V = 3166(1) Å 3 and Z = 4. The tetranuclear compound {〚Zn(phen) 2〛 2〚Fe(CN) 6〛 2} 2– ( 2) cocrystallizes with 〚Zn(bpy) 3〛 2+ and 32 H 2O molecules in the monoclinic space group C2/ c with a = 42.047(8) Å, b = 13.541(2) Å, c = 28.781(5) Å, β = 120.23(3)°, V = 14158(4) Å 3 and Z = 2. The pentanuclear compound {〚Zn(bpy) 2(H 2O)〛〚Zn(bpy) 2〛 2〚Fe(CN) 6〛 2}( 3)·23 H 2O crystallizes in the triclinic space group P 1 with a = 15.308(3) Å, b = 17.496(4) Å, c = 20.050(4) Å, a = 64.25(3)°, β = 72.96(3)°, γ = 88.50(3)°, V = 14158(4) Å 3 and Z = 5. The decanuclear compound {〚Zn(phen) 2〛〚Fe(CN) 6〛} 2{〚Zn(phen) 2〛〚Zn(phen) 2(OH 2)〛〚Fe(CN) 6〛} 2( 4)·4 MeOH·26 H 2O crystallizes in the monoclinic space group C2/ c, with a = 48.14(1) Å, b = 13.585(3) Å, c = 34.131(7) Å, β = 112.05(3)°, V = 20689(7) Å 3 and Z = 4. The unusual geometries discovered in this mixed Fe(III)/Zn(II) chemistry provide excellent models for the design of high-spin cyanide-bridged transition-metal clusters.

Fourth-order zero-field splitting parameters of [Mn(cyclam)Br2]Br determined by single-crystal W-band EPR

Single-crystal W-band (95 GHz) electron paramagnetic resonance (EPR) studies have been performed at 20 K and at room temperature on a tetragonal Mn(III) compound with potential application as a building block for high-spin clusters. The observed EPR spectra correspond to an anisotropic high-spinS = 2 ground state and have been attributed to equivalent centers related by four-fold symmetry. Accurate values for the spin Hamiltonian parameters were obtained from the analysis of the data at both temperatures. At 20 K the contribution of fourth-order zero-field splitting terms was shown to be significant, with parameter values B40 = 0.0009(3) cm−1, B42= 0.0006(2) cm−1 and B44 = 0.0017(3) cm−1, to be considered together with the second-order parametersD = −1.1677(7) cm−1 andE= −0.0135(6) cm−1.

Tuning the Magnetic Properties of the High-Spin Molecular Cluster Fe8

The synthesis, crystal structure, and magnetic characterization of a high-spin cluster comprising eight iron ions are presented in this contribution. The cluster has formula [(tacn)6Fe8O2(OH)12Br4.3(ClO4)3.7]⋅6 H2O (Fe8pcl), where tacn is the organic ligand 1,4,7-triazacyclononane. It can be considered a derivative of Fe8Br8, a cluster whose low-temperature magnetization dynamics has been extensively investigated, in which four of the bromide ions have been replaced by perchlorate anions. The structure of the central core of the two molecules, [Fe8O(OH)12(tacn)6]8+, is essentially the same, but Fe8pcl has a higher symmetry (the bromide derivative crystallizes in the acentric P1 space group while Fe8pcl crystallizes in the monoclinic P21/c space group). The magnetic properties of Fe8pcl suggest it is very similar to Fe8Br8 having a S=10 ground state as well. The zero-field splitting parameters were accurately determined by high field–high frequency EPR (HF-EPR) measurements. The two clusters have similar axial anisotropy D but Fe8pcl has a larger transverse anisotropy E: The value of E/D is 0.21 for the perchlorate derivative but 0.19 for Fe8Br8. AC susceptibility measurements revealed the cluster behaves like a superparamagnetic particle. However, due to the occurrence of large terms in the transverse anisotropy, the temperature dependence of the relaxation time cannot be reproduced by a simple Arrhenius law model. As observed in the bromide derivative, below 350 mK the relaxation time becomes temperature independent and indicating that a pure tunneling regime is attained. The comparison of the tunneling rate in the two clusters shows that in the perchlorate derivative the relaxation process is 35 times faster. The observed ratio of the tunneling rates is in reasonable agreement with that calculated from the tunneling splitting, namely the energy difference between the two almost-degenerate lowest levels Ms=±10, in the two clusters.

Tuning the Magnetic Properties of the High-Spin Molecular Cluster Fe8

The synthesis, crystal structure, and magnetic characterization of a high-spin cluster comprising eight iron ions are presented in this contribution. The cluster has formula [(tacn)6Fe8O2(OH)12Br4.3(ClO4)3.7]·6H2O (Fe8pcl), where tacn is the organic ligand 1,4,7-triazacyclononane. It can be considered a derivative of Fe8 Br8 , a cluster whose low-temperature magnetization dynamics has been extensively investigated, in which four of the bromide ions have been replaced by perchlorate anions. The structure of the central core of the two molecules, [Fe8O(OH)12(tacn)6](8+), is essentially the same, but Fe8pcl has a higher symmetry (the bromide derivative crystallizes in the acentric P1 space group while Fe8pcl crystallizes in the monoclinic P2(1)/c space group). The magnetic properties of Fe8pcl suggest it is very similar to Fe8Br8 having a S=10 ground state as well. The zero-field splitting parameters were accurately determined by high field-high frequency EPR (HF-EPR) measurements. The two clusters have similar axial anisotropy D but Fe8pcl has a larger transverse anisotropy E: The value of E/D is 0.21 for the perchlorate derivative but 0.19 for Fe8Br8. AC susceptibility measurements revealed the cluster behaves like a superparamagnetic particle. However, due to the occurrence of large terms in the transverse anisotropy, the temperature dependence of the relaxation time cannot be reproduced by a simple Arrhenius law model. As observed in the bromide derivative, below 350 mK the relaxation time becomes temperature independent and indicating that a pure tunneling regime is attained. The comparison of the tunneling rate in the two clusters shows that in the perchlorate derivative the relaxation process is 35 times faster. The observed ratio of the tunneling rates is in reasonable agreement with that calculated from the tunneling splitting, namely the energy difference between the two almost-degenerate lowest levels Ms =±10, in the two clusters.

Properties of low-lying states in some high-nuclearity Mn, Fe, and V clusters: Exact studies of Heisenberg models

Using an efficient numerical scheme that exploits spatial symmetries and spin parity, we have obtained the exact low-lying eigenstates of exchange Hamiltonians for the high nuclearity spin clusters, Mn_{12}, Fe_8 and V_{15}. The largest calculation involves the Mn_{12} cluster which spans a Fock space of a hundred million. Our results show that the earlier estimates of the exchange constants need to be revised for the Mn_{12} cluster to explain the level ordering of low-lying eigenstates. In the case of the Fe_8 cluster, correct level ordering can be obtained which is consistent with the exchange constants for the already known clusters with butterfly structure. In the V_{15} cluster, we obtain an effective Hamiltonian that reproduces exactly, the eight low-lying eigenvalues of the full Hamiltonian.

A Prussian blue nanomolecule: crystal structure and low-temperature magnetism.

The single-crystal structure of a mixed-valence heptanuclear cyanide-bridged complex FeIIFeIII6 that may be considered as a model of Prussian blue has been determined. The low-temperature magnetic properties (T < 2 K) reveal a metamagnetic behavior below 220 mK. This is the result of the intermolecular antiferromagnetic interaction between the ferromagnetically coupled high-spin clusters.

MAGPACK1 A package to calculate the energy levels, bulk magnetic properties, and inelastic neutron scattering spectra of high nuclearity spin clusters

MAGPACK¹ A package to calculate the energy levels, bulk magnetic properties, and inelastic neutron scattering spectra of high nuclearity spin clusters

Structure and magnetic properties of a high-spin Mn6IICrIII cluster containing cyano bridges and Mn centres capped by pentadentate ligands

The formation of an elaborate H-bonding network between heptanuclear cations, consisting of a hexacyanochromate capped by six [Mn(dmptacn)]2+ moieties, and hexacyanochromate counterions results in the assembly of novel 1-D chains with short-range ferrimagnetic behaviour occurring between the S = 27/2 clusters and S = 3/2 ions.

Megagauss magnetization measurements

Techniques of magnetization measurements in ultrahigh magnetic fields are discussed. A high rate of magnetic field growth and electromagnetic noises impose severe restriction on an experimental set-up. It is shown that a magnetization step (at a metamagnetic transition) and the conductivity can be estimated from magnetization data. Magnetic oscillations are also observable in ultrahigh magnetic fields. Experimental results on FeSi, high-temperature superconductors, rare-earth intermetallic compounds, and high-spin molecular clusters are briefly reviewed.

Giant and time-dependent magnetocaloric effect in high-spin molecular magnets

We have measured and calculated the magnetocaloric effect in macroscopic samples of oriented high-spin molecular clusters like Mn12 and Fe8 as a function of the temperature and both the intensity and the sweeping rate of the applied magnetic field. We have observed a high magnetic entropy variation around the blocking temperature of the magnetic moment of molecules and calculated the shift of the entropy variation and cooling temperature, with the sweeping rate of the magnetic field.

“No-Pair Bonding” in High-Spin Lithium Clusters: n+1Lin (n = 2−6)

High-spin lithium clusters (n+1Lin, n = 2−6) have been studied using several density functional methods. Although these high-spin clusters have no bonding electron pairs, they are stable with respect to isolated lithium atoms. Full geometry optimizations have been performed with alternatives under a variety of symmetry constraints which led to local minima. In general, the most stable structure is the one with the maximum coordination number for the lithium atoms. The agreement between B3P86/cc-pVDZ and B3PW91/cc-pVDZ density functional methods with UQCISD(T)/6-31G* and UCCSD(T)/cc-pVDZ calculations is excellent. Trends of bond dissociation energies are discussed as a function of total number of “bonds” and number of atoms.

Molecular-Based Magnetism in High-Spin Molecular Clusters and Three-Dimensional Networks Based on Cyanometalate Building Blocks

The field of molecule-based magnets is a relatively new branch of chemistry, which involves the design and study of molecular compounds that exhibit a spontaneous magnetic ordering below a critical temperature, Tc. One major goal involves the design of materials with tuneable Tc's for specific applications in memory storage devices. Molecule-based magnets with high magnetic ordering temperatures have recently been obtained from bimetallic and mixed-valence transition metal ?-cyanide complexes of the Prussian blue family. Since the µ-cyanide linkages permit an interaction between paramagnetic metal ions, cyanometalate building blocks have found useful applications in the field of molecule-based magnets. Our work involves the use of octacyanometalate building blocks for the self-assembly of two new classes of magnetic materials namely, high-spin molecular clusters which exhibit both ferromagnetic intra- and intercluster coupling, and specific extended network topologies which show long-range ferromagnetic ordering.

EPR study of Ni 2FeVO 6− δ

The temperature dependence of the EPR spectra for the oxygen deficient Ni 2FeVO 6− δ compound has been investigated in the range of 50–300 K. An intense EPR line associated with high-spin iron centers or ferromagnetic clusters, is observed. The strong temperature dependence of the EPR intensity, resonance field and line width is consistent with the stepwise magnetization variation and supports the presence of successive magnetic anomalies at lower temperatures.

Quantum transformation of spin structure of a Fe8 nanocluster in ultrastrong magnetic fields

The transformation of the spin structure of a high-spin Fe8 cluster in a strong magnetic field has been investigated. The magnetization and magnetic susceptibility of the material are calculated at different external magnetic fields and temperatures. It is shown that the magnetic field induces transformation of the spin structure of a Fe8 cluster from the quasi-ferrimagnetic structure with an average magnetic moment of 20 µB per molecule to the quasi-ferromagnetic structure with a magnetic moment of 40 µB. Unlike a similar transformation of a Neel ferrimagnet, which is continuous and occurs through an intermediate angular phase, this process in Fe8 at low temperatures manifests itself as a cascade of discrete quantum jumps, each being the transition accompanied by an increase in the spin number of the complex. At high temperatures, the behavior of the magnetic cluster approaches the cluster behavior described by the classical theory. The nature of quantum jumps is discussed in terms of the magnetic-field-induced energy level crossing in the ground state of a magnetic cluster.

New high-spin clusters featuring transition metals

Three possible routes to polynuclear transition metal complexes are discussed. The first route, oligomerization induced by desolvation of small cages, is exemplified by synthesis of a dodecanuclear cobalt cage, and by reactions which give octa– and dodecanuclear chromium cages. The second route involves linking cages through organic spacers, and is illustrated by use of phthalate to link together cobalt and nickel cages. For the nickel case a complex consisting of four cubanes and a sodium octahedron is found. The third route involves the use of water to introduce hydroxide bridges into cages. One method of introducing water is to use hydrated metal salts; the transformation of a Cu6Na cage into a Cu12La8 illustrates this approach. Alternatively, adventitious water within solvents can be used as a source, and this approach has led to a Co24 cage. The structures and magnetic properties of these various cages are discussed.

Resonance Raman spectroscopy of matrix-isolated mass-selected Fe3 and Ag3

Raman spectroscopy of matrix-isolated, mass-selected Fe3 and Ag3 reveal that both are Jahn–Teller distorted triangular molecules. The observed spectrum of Ag3 can be accounted for adequately using an approximate Jahn–Teller potential truncated at the quadratic term [Wedum et al., J. Chem. Phys. 100, 6312 (1994)] with parameters ωa=119 cm−1, ωe=99 cm−1, k=1.92, and g=0.29. For Fe3, the spectrum is more complex, most likely due to the fact that spin–orbit coupling plays an important role in this high-spin cluster in addition to the quadratic Jahn–Teller terms. The overall pattern of the spectrum suggests that in Fe3 the Jahn–Teller distortion is likely small, and a peak at 249 cm−1 is tentatively assigned to its symmetric stretch.

Electronic States and Spin Characterization of Polyanionic High-Spin Molecules and Clusters Derived from 1,3-Dibenzoylbenzene Derivatives as Studied by ESR/Electron Spin Transient Nutation Spectroscopy

The systems under study were 1,3-dibenzoyl benzene (1), its methyl derivatives (2,3), and 2,6-dibenzoyl pyridine (4), all of which have topologically pseudogenerate π-LUMO's. The corresponding dianions are expected to form triplet ground states and the anionic molecular assemblages expected to form polyanionic high-spin clusters. The polyanionic molecules and clusters were generated by chemical reduction and investigated by cw/FT pulsed ESR spectroscopy. Alkali metal-bridged high-spin clusters were identified directly by FT pulsed ESR-based 2D Electron Spin Transient Nutation (2D-ESTN) spectroscopy. We found ground-state quintet, quartet, and triplet dimers originating in the oligoketone with pluri-electron doping. We determined all the zero-field splitting parameters, g-values, and the concentration ratio of the high-spin systems with the help of spectral simulation.

No-Pair Bonding in the High-Spin 3 State of Li2. A Valence Bond Study of Its Origins

Lithium forms high-spin clusters, n+1Lin which are bonded even though there are no electron-pairs! This no-pair bonding is weak for the 3 state of Li2 but becomes very significant for larger clusters reaching up to 1.8 eV for 7Li6. To understand the nature of “no-pair bonding”, we performed valence bond (VB) calculations on the states of Li2, benchmarked them against high-level MO-based calculations which account for static as well as dynamic electron correlation, and derived bonding mechanisms for the no-pair triplet state vis a vis the singlet ground state. It is shown that both the singlet-pair and no-pair bonds are bonded by covalency but differ in the mechanism of VB mixing. The singlet-pair bond is sustained by covalency augmented by Coulomb correlation of the electron pair, while the no-pair bond originates solely in the resonance energy between the repulsive fundamental triplet VB structure with the secondary VB structures. Understanding of the fundamental no-pair bond in 3Li2 enables one to derive insight into the bonding and geometric features of no-pair n+1Lin high-spin clusters. Experimental characterization of such clusters will broaden the current conception of bonding beyond the traditional spin-pairing paradigm.

Local spin moment of LaCoO 3 probed by a core hole

The electronic structures of the ground state and the 1s −1 core-hole state of high-spin cobalt oxide clusters are calculated. The calculated results indicate that the high-spin Co 3+ oxide becomes low spin in the core-hole state. To check the validity of the present calculation, we have measured the high-resolution X-ray fluorescence spectra of the Co K–L 3 (Kα 1) line shape for various cobalt compounds using a double-crystal X-ray fluorescence spectrometer: Co(acac) 2, Co(acac) 3, CoF 3, CoCl 2, CoBr 2, K 3[Co(CN) 6], CoO, Co 3O 4, LaCoO 3, and Co (metal), where acac denotes CH 3COCHCOCH 3 −. We conclude from the comparison of the calculations with the experimental results that the cobalt ion in LaCoO 3 at room temperature is high-spin state in spite that the reported spin state of this perovskite is different among the probing methods. The implications of the spin-flip shake-down process for the interpretation of the core-level X-ray photoelectron spectra, X-ray absorption spectra, and X-ray fluorescence spectra are discussed.

Features of the magnetocaloric effect of Mn12−Ac in ultra-strong magnetic fields

A theoretical analysis is made of the behavior of the magnetocaloric effect of the high-spin organic cluster Mn12−Ac in ultra-strong magnetic fields. Features of the effect which are attributable to intersection of the lower energy levels of the cluster in strong magnetic fields are examined.