Arno F. Stassen

Synthesis, magnetic properties, and crystal structure of dinuclear antiferromagnetic [Cu2(5-aminomethyl-3-methylpyrazole)2Cl4]

Abstract The synthesis, optical and magnetic properties and the X-ray crystal structure of [Cu 2 (amp) 2 Cl 4 ] (amp=5-aminomethyl-3-methylpyrazole), a new dinuclear chloro-bridged copper(II) compound, are described. Both copper ions are in a distorted square pyramidal N 2 Cl 3 environment, with the apical position of the copper being occupied by the bridging chloride anion, which is equatorial to the other copper. An anti-ferromagnetic coupling of J =−9.4 cm −1 is found, with a rather large zJ ′ value of −2.1 cm −1 , which is probably due to the different pathways of interaction that exist between the coppers in the crystal lattice. The coppers within each dinuclear unit are connected by the chloride bridges (intra pathway), whereas the coppers from different dinuclear units in the lattice are connected by hydrogen bonds (inter pathway).

Physical properties of the spin-crossover compound hexakis(1-methyltetrazole-N4)iron(II) triflate, steady state and relaxation studies, X-ray structure of the isomorphic Ni(II) compound

Abstract The synthesis and characterization of the spin transition compound hexakis(1-methyltetrazole-N4)iron(II) trifluoromethanesulfonate obtained from the reaction of 1-methyltetrazole and iron(II) triflate are described. The spin transition of [Fe(mtz)6](CF3SO3)2 (1) presents a hysteresis loop in magnetism with T↓=157.0 and T↑=179.5 K. Only one-third of the Fe(II) ions undergo this transition, the other two-thirds of the ions remain in the high-spin state. Mossbauer spectroscopy and relaxation studies have shown that the actual width of the hysteresis is only 12 K, with T↓=167.5 and T↑=179.5 K. Thermal spin-state trapping appears to be possible. The trapped high-spin state is stable up to 103 K. The relaxation is not first-order. After irradiation with red light, reverse light-induced excited spin state trapping (LIESST) is observed. The new low-spin state is stable up to 42 K. Also, its relaxation to HS is non-first-order. A reverse light-induced thermal hysteresis loop of approximately 7 K width is present too. No LIESST effect could be observed. Finally, a ferromagnetic type of coupling has been recognized, which is temperature and field dependent.

Spin crossover in hexakis(1-(2-chloroethyl)-tetrazole)iron(II) complexes; synthesis and magnetic properties

Abstract The spin-crossover complexes [Fe(teec)6](X)2 (with teec=1-(2-chloroethyl)-tetrazole and X=BF4, ClO4, PF6) possess two iron(II) sites each having a different spin-crossover behaviour. The magnetic behaviour of the complexes obtained by slow crystallisation differs strongly from those obtained by precipitation. This difference results in either a spin crossover of 50% (X=ClO4, crystalline), two gradual steps (X=BF4, PF6), or two steps, of which one is gradual and the other having a thermal hysteresis (X=ClO4 precipitated). Both 57Fe Mossbauer spectroscopy and magnetic susceptibility measurements have been recorded as a function of T. The crystal structure of [Fe(teec)6](BF4)2 has been determined using X-ray synchrotron powder diffraction. Of [Fe(teec)6](ClO4)2 only the cell parameters could be determined. The ligand field spectra of the low-spin and high-spin states clearly indicate a 1A1→5T2 spin transition with 10 DqLS/10 DqHS=1.72.

New dinuclear Co(II) and Mn(II) complexes of the phenol-based compartmental ligand containing formyl and amine functions: structural, spectroscopic and magnetic properties

The phenol-based compartmental ligand Hpy2ald contains a tridentate amino arm and a weak donor aldehyde group at the 2 0 and at the 6 0 positions of the phenol ring, respectively. This ligand reacts with cobalt(II) perchlorate, cobalt(II) tetrafluoroborate and manganese(II) perchlorate, yielding dinuclear complexes, where two metal ions are doubly bridged by two deprotonated cresolate moieties. The coordination environment around the metal ions is then completed to a very distorted octahedron by three nitrogen donor atoms from the pendant amino arm and the oxygen atom of the aldehyde group. The crystal structures of the complexes, their spectroscopic and magnetic properties are reported. 2003 Elsevier B.V. All rights reserved.

Trapping of metastable high-spin species and weak cooperativity in a mononuclear Fe(II) spin-crossover system

The magnetic and structural properties of three gradual spin transition monomeric compounds based on the cation [Fe(Hpt)3] 2þ (Hpt ¼ 3-(pyrid-2-yl)-1,2,4-triazole) are presented. The non-cooperative character of the spin-crossover in [Fe(Hpt)3](BF4)2·2H2 O( I) is evaluated in light of its calorimetric properties, which yielded the thermodynamic values DtrsH ¼ 5.81 kJ mol 21 and DtrsS ¼ 39.5 J mol 21 K 21 . The light-induced excited spin-state trapping effect is performed on [Fe(Hpt)3](BF4)2·2H2 O( I) and [Fe(Hpt)3](SO4)0.4(BF4)1.2·3H2 O( II), and the subsequent HS ! LS relaxations are studied. Their merely first-order kinetics are affected by disorder in the structure of both complexes, which appears in the presence of a distribution of activation energies. HS species can also be frozen-in in I by rapid cooling. Continuous irradiation is shown to induce only apparent light-induced thermal hysteresis effect in I and II, stemming from slow kinetics of relaxation with respect to the kinetics of measurement. q 2003 Elsevier Science Ltd. All rights reserved.

Structure and magnetic properties of the weak ferromagnet bis(1-(2-chloroethyl)-tetrazole)bis(nitrato)copper(II)

Abstract The novel mononuclear complex [Cu(teec)2(NO3)2] with teec=1-(2-chloroethyl)tetrazole, has been prepared from 1 equiv. copper(II) nitrate with 2 equiv. of teec. The X-ray structure shows the formation of a mononuclear complex with hexa-coordinated copper(II) ions. The teec ligands are positioned in trans orientation with a distorted axial geometry. Two of the CuO bonds of the chelating nitrate anions are significantly longer than the other two CuO bonds (1.99 vs. 2.48 A). Linear-chain packing of the mononuclear units is reflected in very weak, linear ferromagnetic chains with θ=0.30 K and C=0.44 cm3 K mol−1. The spectral properties are in accord with the structural and magnetic data.

Structure determination of the [Fe(teec)6](BF4)2 metal complex from laboratory and synchrotron X-­ray powder diffraction data with grid-search techniques

The structure of the coordination compound [Fe(teec)(6)](BF(4))(2), hexa[1-(2-chloroethyl)tetrazole]iron(II) di(borotetrafluoride), has been determined using the grid-search techniques of the program suite MRIA. A Guinier-camera data set was used to determine the unit cell, the space group and to position the initial model. A high-resolution synchrotron powder data set was used to position a more detailed model using torsion-angle variation and to refine the structure leading to Rp = 0.0689, Rw = 0.0805 and GoF = 1.38. The crystal structure at room temperature shows the existence of two symmetry-equivalent iron(II) ions in the high-spin state.

Structures of Fe(II) spin-crossover complexes from synchrotron powder-diffraction data.

Crystal structure determination and analysis have been carried out for the two spin-crossover compounds [Fe(teeX)(6)](BF(4))(2) (teeX is haloethyltetrazole; X = I: teei; X = Br: teeb), in both their high-spin (near 300 K) and their low-spin states (T = 90 K), using high-resolution powder-diffraction data collected at the ESRF (Grenoble, France) and SPring8 (Japan) synchrotron radiation facilities. The structures of teei have been solved using various direct-space structure determination techniques (grid search, genetic algorithm and parallel tempering) and refined with the Rietveld method using geometrical restraints. In the case of teeb, a structural model was found but a full refinement was not successful because of the presence of a significant amount of an amorphous component. Analysis of the structures (space group P2(1)/c, Z = 2) and diffraction data, and the absence of phase transitions, show the overall structural similarity of these compounds and lead to the conclusion that the gradual spin-crossovers are likely to be accompanied by small structural changes only.

A New Layered Compound Containing [PMo12O40]3− and Both 5- and 6-Coordinated Homoleptic (1-(2-Chloroethyl)tetrazole)Copper(II) Cations

Summary. The synthesis, crystal structure and physical properties of the complex obtained from the reaction between the polyoxometalate anion [PMo12O40]3−, copper(II) and the ligand 1-(2-chloroethyl)tetrazole (teec) are described. This compound has been synthesized as a model for designing materials containing both magnetic polyoxometalate anions and iron(II) spin-crossover cations.The compound, with formula [Cu(teec)5]2[Cu(teec)6][PMo12O40]2·2H2O, consists of alternating layers of polyoxometalates and cationic complexes. Both, five and six coordinated Cu(II) ions are present, each positioned in different layers. Despite these layers having a similar width, the layer of pentacoordinated Cu(II) ions contains twice as many cationic complexes as the layer of hexacoordinated Cu(II) ions. This unusual coexistence of complexes with different coordination number is most likely caused by the steric hindrance induced by the bulky polyoxometalates in the layer of pentacoordinated Cu(II) which prevents the presence of a sixth teec ligand.

Linkage isomers of dibromobis(1-(2-methoxyethyl)tetrazole)copper(II) containing either a bromide or a unique tetrazole bridge

The ligand 1-(2-methoxyethyl)tetrazole (teeOMe) reacts with copper(II) bromide to form two different isomers with the formula [Cu(teeOMe)2Br2]n. The coordination details on the crystal packing are determined by the solvent from which the compound crystallises. In methanol, green crystals of α-[Cu(teeOMe)2Br2] (1) are formed, and these have been characterised by X-ray crystallography (at three temperatures) and magnetic susceptibility measurements. The crystal structure of 1 is built up from square-planar trans-[Cu(teeOMe)2Br2] units. Additional weak axial interactions between the copper centers and the N(2) tetrazole atoms of neighbouring complexes link the [Cu(teeOMe)2Br2] units into two-dimensional layers. Compound 1 displays paramagnetic behaviour (C = 0.41 cm3 K mol−1). In ethanol, yellow–brown crystals of β-[Cu(teeOMe)2Br2] (2) are formed, and these have been characterised by magnetic susceptibility measurements, in addition to IR, ligand field and EPR spectroscopy. The observed magnetic behaviour (C = 0.41 cm3 K mol−1, θ = 30.5 K and J/kB = 16.4 K) is consistent with a structure containing two-dimensional bromide-bridged copper(II) grids. It is proposed that in 2 the square-planar [Cu(teeOMe)2Br2] units are connected via axial Cu–Br interactions, as deduced from comparison of the magnetic properties.