Thermal characteristics of thermosets formed by free radical photocuring

We have studied the compound MnAs with a giant magnetocaloric effect, which has a magneto-structural transition at 316 K. The magnetic phase diagram has been deduced from adiabatic heat-capacity measurements and from cooling and heating curves. The ferroto paramagnetic transition changes with field from 316 K at 0 T to 335.3 K at 6 T. There is a strong thermal hysteresis between 10 and 5 K, depending on field. Direct measurements of the adiabatic temperature change caused by the application of magnetic field were made and compared with the values deduced from heat-capacity data.Moreover, adiabatic field cycles were performed quasi-statically between 0 and 6 T around the phase coexistence region, showing the strength of the effect in each phase and on the coexistence lines.     

铜(Ⅱ)-二硫烯平面型混配配合物的合成、结构与性质

本文选取平面型配体2,2-联嘧啶(2,2-bipyrimidine,C8H6N4)和马来二睛基二硫烯二钠(Na2(mnt)),采用溶液合成法制备配合物[Cu(mnt)(C8H6N4)](1),并运用IR、XRD、X射线单晶衍射等方法进行了表征,还对其作了TG、EPR、磁性等研究。配合物1属于单斜晶系,空间群为P21/c,晶胞参数:a=0.71064(11)nm,b=1.6270(2)nm,c=1.27384(19)nm,β=98.926(2)°,V=1.4550(4)nm3;具有层状堆积的结构,层间距为0.3360 nm。磁性测试数据表明,配合物1具有顺磁性质。     

Nonoxovanadium(IV) and oxovanadium(V) complexes with mixed O, X, O-donor ligands (X = S, Se, P, or PO)

Ligating properties of four potentially tridentate bisphenol ligands containing [O, X, O] donor atoms (X = S 1, Se 2, P 3, or P=O 4) toward the vanadium ions in +IV or +V oxidation states have been studied. Each ligand with different heterodonor atoms yields as expected nonoxovanadium(IV) complexes, V(IV)L(2), whose structures have been determined by X-ray diffraction methods as having six-coordinate V(IV), VO(4)X(2), core. Compounds 1-4 have also been studied with electrochemical methods, variable-temperature (2-295 K) magnetic susceptibility measurements, X-band electron paramagnetic resonance (EPR) (2-60 K) spectroscopy, and magnetic circular dichroism (MCD) (5 K) measurements. Electrochemical results suggest metal-centered oxidations to V(V) (i.e., no formation of phenoxyl radicals from the coordinated phenolates). A combination of density functional theory calculations and experimental EPR investigations indicates a dramatic effect of the heteroatoms on the electronic structure of 1-4 with consequent reordering of the energy levels; 1 and 3 possess a trigonal ground state (d(z)()(2))(1), but 4 with the phosphoryl oxygen as the heterodonor atom in contrast exhibits a tetragonal ground state, (d(xy)())(1). On the basis of the intense electronic transitions in absorption spectra, all electronic transitions observed for 4 have been assigned to ligand-to-metal charge-transfer transitions, which have been confirmed by preliminary resonance Raman measurements and C/D ratios obtained from low-temperature MCD spectroscopy. Moreover, diamagnetic complexes 5 and 6 containing mononuclear and dinuclear oxovanadium(V) units have also been synthesized and structurally and spectroscopically ((51)V NMR) characterized.     

pH- and Dopant-Dependent CdMoO(4):Mn Nanocrystals: Luminescence and Magnetic Properties

CdMoO(4):Mn nanocrystals with a tetragonal crystal structure were prepared by aqueous coprecipitation method at a low temperature of 2 °C under different pH values. The size of the CdMoO(4):Mn nanocrystals of spherical morphology increases with the Mn dopant concentration from 35 to 55 nm for pH = 4. The morphology could be tuned from nanocrystals to microstructures consisting of smaller nanoparticles by the Mn concentration when the pH value of the precursor was increased to 8. The thermal stability of the luminescence and magnetic properties of the Mn-doped samples also depend on the pH and the doping level. The effects of the pH and dopant on the luminescence and magnetic properties, including magnetic susceptibility and electron paramagnetic resonance, were investigated. This approach contributes to better understanding of aqueous chemistry methods to control the growth of nanocrystals.     

Light-induced spin change by photodissociable external ligands: a new principle for magnetic switching of molecules

Magnetic bistability in spin-crossover materials generally is a collective phenomenon that arises from the cooperative interaction of a large number of microscopic magnetic moments within the crystal lattice in the solid state. We now report on individual molecules in homogeneous solution that are switched between the diamagnetic and paramagnetic states at room temperature by light-driven coordination-induced spin-state switching (LD-CISSS). Switching of the coordination number (and concurrently of the spin state) was achieved by using Ni-porphyrin as a square-planar platform and azopyridines as photodissociable axial ligands. The square-planar Ni-porphyrin is diamagnetic (low-spin, S = 0), and all complexes with axial ligands are paramagnetic (high-spin, S = 1). Association constants were determined for all conceivable 1:1 and 1:2 porphyrin/azopyridine complexes. The binding constants of the trans azopyridines are larger than those of the corresponding cis isomers. Thus, upon irradiation with UV light (365 nm, trans → cis) and visible light (455 nm, cis → trans), switching of the magnetic properties was achieved. Upon substitution of the azopyridines at the 4- and 4'-positions with larger substituents, the difference in trans and cis association constants, and thus the switching efficiency, was increased. A photoinduced, reversible switching between 20 and 68% paramagnetic Ni species in solution was achieved with isopropyl substituents at room temperature.     

Crystal structure, spin polarization, solid-state electrochemistry, and high n-type carrier mobility of a paramagnetic semiconductor: vanadyl tetrakis(thiadiazole)porphyrazine

We report the synthesis, crystal structure, and magnetic, electrochemical, and carrier-transport properties of vanadyl tetrakis(thiadiazole)porphyrazine (abbreviated as VOTTDPz) with S = ?. X-ray crystal analysis reveals two polymorphs, the α and β forms; the former consists of a 1D regular π stacking, while the latter forms a 2D π network. Molecular orbital calculations suggest a V(4+)(d(1)) ground state and a characteristic spin polarization on the whole molecular skeleton. The temperature dependence of the paramagnetic susceptibility of the α form clearly indicates a ferromagnetic interaction with a positive Weiss constant of θ = 2.4 K, which is well-explained by McConnell's type I mechanism. VOTTDPz forms amorphous thin films with a flat and smooth surface, and their cyclic voltammogram curves indicate a one-electron reduction process, which is highly electrochromic, because of a reduction of the porphyrazine π ring. Thin-film field-effect transistors of VOTTDPz with ionic-liquid gate dielectrics exhibit n-type performance, with a high mobility of μ = 2.8 × 10(-2) cm(2) V(-1) s(-1) and an on/off ratio of 10(4), even though the thin films are amorphous.     

Synthesis and Magnetic Properties of a Copper Cube: [Cu4(OH)4(C16H18N2)4]4+ (ClO4)4 C3H6O [C16H18N2=(E)-1,6-[Di(pyridin-4-yl)hex-3-ene]

The synthesis of a Cu₄(OH)₄ cube which is coordinated by four molecules of the dipyridyl ligand 1,6-[di(pyridin-4-yl)hex-3-ene] is reported. This compound has a trans double bond which restricts the conformational freedom of the ligand and favours coordination within a unique copper cube. The structure was solved by an X-Ray single crystal structure determination and low temperature magnetic susceptibility measurements examined its magnetic properties. The cube classification corresponds to the type I classification of Mergehenn and Haase and the short/long distribution of Cu ⋅⋅⋅ Cu separations in the cube as defined by Ruiz. The magnetic susceptibility measurements show paramagnetic behaviour down to 50 K but below this the copper cube shows weak ferromagnetic exchange interactions. The low temperature magnetic susceptibility characteristics are examined in detail then modelled and compared to other similar Cu₄O₄ copper cubes.     

Magnetic resonance investigation of vanadia and vanadium-molybdenum gels synthesized with peroxovanadate precursors

Vanadia gels and vanadium-molybdenum oxide gels were investigated using the magnetic resonance techniques, EPR spectroscopy and (51)V MAS NMR spectroscopy. The vanadium oxide gels were derived from the reaction of H(2)O(2) and V(2)O(5), and the vanadium-molybdenum oxide (VMoO) gels were derived from the reaction of peroxovanadates with an ammonium molybdate solution. EPR spectroscopy was utilized to determine quantitative information about the concentration of V(4+) paramagnetic species present in the samples and additional structural information about the V(4+) coordination environment. (51)V MAS NMR spectroscopy was used to elucidate the V(5+) electronic environment and how it changes as a function of molybdenum content. The observed line broadening of the (51)V NMR signal with increasing molybdenum content was correlated with an increase in the concentration of paramagnetic species as monitored by EPR spectroscopy. The evolution of various vanadium sites during thermal treatment was also investigated. This work provides further support for the hypothesis that the selectivity of VMoO catalysts in the oxidation of 1,3-butadiene to maleic anhydride is due to the presence of paramagnetic V(4+) sites.     

The electronic ground state of [V(urea)6]3+ probed by NIR luminescence, electronic Raman, and high-field EPR spectroscopies

The electronic structure of a trigonally distorted vanadium(III) complex, [V(urea)6](ClO4)3, and its deuterated analogue, [V(urea-d4)6](ClO4)3 has been investigated with Raman, luminescence, and high-frequency high-field electron paramagnetic resonance spectroscopies and with magnetic measurements. A broad electronic Raman transition is observed at around 1400 cm(-1) and attributed to a transition to the (3)E (D3) component of the (3)T1g (O(h)) ground state. The same splitting is seen in the near-infrared luminescence spectrum in the form of a similarly broad peak at 8450 cm(-1), 1400 cm(-1) lower in energy than the (1)E --> (3)A2 spin-flip transition. Powder high-frequency and high-field electron paramagnetic resonance spectra, magnetic susceptibilities, and magnetization studies give a precise measurement of the zero-field splitting and of the g values in this complex (D = 6.00(2) cm(-1), E = 0.573(6) cm(-1), g(x) = 1.848(2), g(y) = 1.832(4), and g(z) = 1.946(7)). A set of angular overlap model parameters is proposed that reproduces all spectroscopic observations, and an analysis of the influence of the bonding of urea on the trigonal distortion of the complex is given.     

Field-induced ferrimagnetic state in a molecule-based magnet consisting of a Co(II) ion and a chiral triplet Bis(nitroxide) radical

We present the synthesis, crystal structure, and temperature and field dependence of the magnetic properties of a new molecule-based magnet, [Co(hfac)2].BNO* (1), where hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato and BNO* is the chiral triplet bis(nitroxide), 1,3-bis(N-tert-butyl-N-oxylamino)-5-{1'-methyl-1'-[2' '-(S)-methylbutoxy]ethyl}benzene. The presence of enantiomer-pure BNO induces the formation of chiral one-dimensional chains that are packed parallel to each other in the noncentrosymmetric P1 space group. 1 exhibits four magnetic ground states: paramagnetic; antiferromagnetic; forced ferrimagnetic; field-induced metastable ferrimagnetic. In the paramagnetic state (T > 20 K), it presents short-range antiferromagnetic interaction between Co ion and nitroxide radical and has a minimum of chimT value at 220 K. The Weiss temperature estimated in the temperature range 220-300 K is found to be -89.9 K. At 20 K (TN), an antiferromagnetic long-range ordering is established. In the temperature range 4 K < T < 20 K, the isothermal magnetization curve show a spin-flip transition to the forced ferrimagnetic state at around 850 Oe. Below 4 K, this compound enters into a field-induced ferrimagnetic state, which is metastable and stabilized by the Ising character of the Co ion. In the low-temperature phase, the material becomes a very hard magnet with wide hysteresis loop whose coercive field reaches 25 kOe at 2 K. The magnetic phase diagram based on these magnetic data is presented.     

Synthesis and EPR studies of soluble vanadyl octakis(4-tert-butylbenzylthio) porphyrazine

Magnesium porphyrazinate substituted with eight 4-tert-butylphenylthio-groups on peripheral positions has been synthesized by cyclotetramerization of 1,2-bis(4-t-butylphenylthio)maleonitrile in the presence of magnesium butanolate. The metal-free derivative was obtained by treatment with trifluoroacetic acid and further reaction of this product with vanadyl(IV) sulfate led to the vanadyl porphyrazinate. These new compounds have been characterized by elemental analysis, together with FTIR, and UV–vis spectral data. The magnetic properties of the complex have been investigated by electron paramagnetic resonance spectroscopy.     

Single-molecule magnets: preparation and properties of low symmetry [Mn4O3(O2CPh-R)4(dbm)3] complexes with S = 9/2

The preparation and properties of [Mn(4)O(3)(O(2)CPh-R)(4)(dbm)(3)] (R = H, p-Me, p-OMe, and o-Cl; dbm(-) is the anion of dibenzoylmethane) single-molecule magnets (SMMs) with virtual C(S) symmetry are reported. They were prepared by controlled potential electrolysis in 26-80% yields. The structures comprise a distorted-cubane core of virtual C(S) symmetry, in contrast to the other, more common complexes of this type with virtual C(3)(V) symmetry. Solid-state magnetic susceptibility data establish the complexes have S = 9/2 ground-state spins, and ac susceptibility studies indicate they are single-molecule magnets (SMMs). Magnetization vs dc field sweeps below 1.00 K reveal hysteresis loops confirming a SMM, with a very large step at zero applied field diagnostic of fast quantum tunneling of magnetization (QTM) through the anisotropy barrier. The fast QTM rate suggested a significant rhombic ZFS parameter E, as expected from the low (virtual C(S)) symmetry. This was confirmed by high-frequency electron paramagnetic resonance spectroscopy on polycrystalline and single-crystal studies. The results confirm the importance of symmetry on the QTM rates.     

A magnetic route to measure the average oxidation state of mixed-valent manganese in manganese oxide octahedral molecular sieves (OMS)

A magnetic route has been applied for measurement of the average oxidation state (AOS) of mixed-valent manganese in manganese oxide octahedral molecular sieves (OMS). The method gives AOS measurement results in good agreement with titration methods. A maximum analysis deviation error of +/-7% is obtained from 10 sample measurements. The magnetic method is able to (1) confirm the presence of mixed-valent manganese and (2) evaluate AOS and the spin states of d electrons of both single oxidation state and mixed-valent state Mn in manganese oxides. In addition, the magnetic method may be extended to (1) determine AOS of Mn in manganese oxide OMS with dopant "diamagnetic" ions, such as reducible V5+ (3d0) ions, which is inappropriate for the titration method due to interference of redox reactions between these dopant ions and titration reagents, such as KMnO4, (2) evaluate the dopant "paramagnetic" ions that are present as clusters or in the OMS framework, and (3) determine AOS of other mixed-valent/single oxidation state ion systems, such as Mo3+(3d3)-Mo4+(3d2) systems and Fe3+ in FeCl3.     

Synthesis, structural, and magnetic studies on a redox family of tetrametallic vanadium clusters: {VIV4}, {VIII2VIV2}, and {VIII4} butterfly complexes

The synthesis, crystal structures, and magnetic properties are reported for a redox family of butterfly-type tetrametallic vanadium alkoxide clusters, namely [V2(VO)2(acac)4(RC{CH2O}3)2] (R=Me 1, Et 2, CH2OH 3), [V2(VO)2(acac)2(O2CPh)2(MeC{CH2O}3)2] (5), [(VO)4(MeOH)2(O2CPh)2({HOCH2}C{CH2O}3)2] (6), [V4Cl2(dbm)4(RC{CH2OH}3)2] (R=Me 7, Et 8, CH2OH 9), and [V4Cl2(dbm)4(MeO)6] (10). The cluster cores are {VIV4} (6), {VIII2VIV2} (1-5), and {VIII4} (7-10), with examples of both isomeric forms of the of the mixed-valence cores (either VIII or VIV ions forming the butterfly body). Magnetic studies reveal the clusters to be dominated by antiferromagnetic exchange interactions in each case. The magnetic exchange parameters are determined for representative examples of each core type. {VIV4} and {VIII4} have diamagnetic ground states. The two isomeric {VIII2VIV2} types are found to give rise to either an S=0 ground state with a number of low-lying excited states due to competing antiferromagnetic exchange interactions (VIII2 butterfly body) or to a well-isolated S=1 ground state (VIV2 butterfly body).     

Structural and thermochemical chemisorption of CO2 on Li(4+x)(Si(1-x)Al(x))O4 and Li(4-x)(Si(1-x)V(x))O4 solid solutions

Different Li(4)SiO(4) solid solutions containing aluminum (Li(4+x)(Si(1-x)Al(x))O(4)) or vanadium (Li(4-x)(Si(1-x)V(x))O(4)) were prepared by solid state reactions. Samples were characterized by X-ray diffraction and solid state nuclear magnetic resonance. Then, samples were tested as CO(2) captors. Characterization results show that both, aluminum and vanadium ions, occupy silicon sites into the Li(4)SiO(4) lattice. Thus, the dissolution of aluminum is compensated by Li(1+) interstitials, while the dissolution of vanadium leads to lithium vacancies formation. Finally, the CO(2) capture evaluation shows that the aluminum presence into the Li(4)SiO(4) structure highly improves the CO(2) chemisorption, and on the contrary, vanadium addition inhibits it. The differences observed between the CO(2) chemisorption processes are mainly correlated to the different lithium secondary phases produced in each case and their corresponding diffusion properties.     

Hybrid organic-inorganic conductor with a magnetic chain anion: kappa-BETS2[Fe(III)(C2O4)Cl2] [BETS = bis(ethylenedithio)tetraselenafulvalene

The synthesis, crystal structure, and electrical, optical, and magnetic properties of kappa-BETS2[Fe(III)(C2O4)Cl2], where BETS is bis(ethylenedithio)tetraselenafulvalene, are reported. The black plate crystals consist of parallel donor layers, two per unit cell, displaying a kappa-type packing of BETS(0.5+) within the bc plane and anionic magnetic chains, [Fe(C2O4)Cl2-]n, running along the c axis. It displays metallic behavior down to 4.2 K, and analysis of the optical reflectivity data gives unscreened plasma energies of 0.69 eV (E parallel c) and 0.40 eV (E perpendicular c). The optical anisotropy is larger than that seen for other kappa phases and is described well by transfer integrals obtained from extended Hückel calculations. However, the transfer integrals need to be scaled down uniformly by a factor of 1.21 to reproduce the absolute experimental plasma frequencies. The band structure consists of a one-dimensional (1D) band and a hole pocket, characteristics of kappa phases. The magnetic properties were modeled by the sum of a 1D antiferromagnetic chain contribution from the d spins of Fe3+, a temperature-independent paramagnetic contribution, and a Curie impurity term. At 4.5 K, there is a signature of long-range magnetic ordering to a canted-antiferromagnetic state in the zero-field-cooled-field-cooled magnetizations, and at 2 K, a small hysteresis loop is observed.     

Ab initio study on the electronic, magnetic, and mechanical properties of CaCu3V4O12

The electronic, magnetic, and mechanical properties of CaCu3V4O12 are investigated by use of the density functional theory method. The calculated results indicate that CaCu3V4O12 is a half-metallic and ferrimagnetic compound. The magnetic coupling for Cu-V is antiferromagnetic, while those for Cu-Cu and V-V are ferromagnetic. The obtained elastic constants suggest that the compound is mechanically stable. The calculated oxidation states and density of states reveal the existence of a mixed valence for Cu and V. This supports the experimental observation of the mixed valence in Ca(2+)Cu(2+)Cu2(+)(V2(5+)V2(4+))O12.     

Hybrid molecular magnets obtained by insertion of decamethyl-metallocenium cations into layered, bimetallic oxalate complexes:

A new series of hybrid organometallic - inorganic layered magnets with the formula [Z(III)Cp*2][M(II)M(III)(ox)3] (Z(III) = Co, Fe; M(III) = Cr, Fe; M(II) = Mn, Fe, Co, Cu, Zn; ox = oxalate; Cp* = pentamethylcyclopentadienyl) has been prepared. All of these compounds are isostructural and crystallize in the monoclinic space group C2/m, as found by X-ray structure analysis. Their structure consists of an eclipsed stacking of the bimetallic oxalate-based extended layers separated by layers of organometallic cations. These salts show spontaneous magnetization below To, which corresponds to the presence of ferro-, ferri-, or canted antiferromagnetism. Compounds in which the paramagnetic deca-methylferrocenium is used instead of the diamagnetic decamethylcobaltocenium are good examples of chemically constructed magnetic multilayers with alternating ferromagnetic and paramagnetic layers. The physical properties of this series have been thoroughly studied by means of magnetic measurements and ESR and Mossbauer spectroscopy. We have found that the two layers are electronically quasiindependent. As a consequence, the bulk properties of these magnets have not been significantly affected by the insertion of a paramagnetic layer of S = 1/2 spins in between the extended layers. In fact, the critical temperatures remain unchanged even when comparing [MCp*2]+ derivatives with [XR4]+ compounds (X = N, P; R = Ph, nPr, nBu). Nevertheless, the presence of the paramagnetic layer has been shown to have some influence on the hysteresis loops of these compounds. In the same context, the spin polarization of the paramagnetic units (which arises from the internal magnetic field created by the bimetallic layers in the ordered state) has been observed by Mossbauer and ESR spectroscopy.     

Cationic Mn4 single-molecule magnet with a sterically isolated core

The synthesis, structure, and magnetic properties of a ligand-modified Mn(4) dicubane single-molecule magnet (SMM), [Mn(4)(Bet)(4)(mdea)(2)(mdeaH)(2)](BPh(4))(4), are presented, where the cationic SMM units are significantly separated from neighboring molecules in the crystal lattice. There are no cocrystallized solvate molecules, making it an ideal candidate for single-crystal magnetization hysteresis and high-frequency electron paramagnetic resonance studies. Increased control over intermolecular interactions in such materials is a crucial factor in the future application of SMMs.