Spin-crossover in an iron(III) complex showing a broad thermal hysteresis.

A pentadentate Schiff-base ligand 3,5Cl-L2- and NCSe- form an iron(iii) mononuclear complex, namely [Fe(3,5Cl-L)(NCSe)], which shows a thermally induced spin crossover with a broad hysteresis width of 24 K between 123 K (warming) and 99 K (cooling). Analogous complexes of the [Fe(3,5X-L)(Y)] type, where X = Cl or Br and Y = Cl-, N3-, NCS-, and NCSe-, are high-spin over the whole temperature interval.

Slow magnetic relaxation in Ni-Ln (Ln = Ce, Gd, Dy) dinuclear complexes.

Three new isomorphous complexes [Ni(o-van-en)LnCl3(H2O)] [H2(o-van-en) = N,N'-ethylene-bis(3-methoxysalicylaldiminate; Ln = Ce (1), Gd (2), Dy (3)] were prepared by a stepwise reaction using mild conditions and were structurally characterised as dinuclear molecules in which Ni and Ln are coordinated by the compartmental Schiff base ligand (o-van-en)= and doubly bridged by O atoms. While the nickel(ii) centre is diamagnetic within the N2O2 square-planar coordination of the Schiff base ligand, the lanthanide atoms are octa-coordinated to give an {LnCl3O5} chromophore with a fac-arrangement of the chlorido ligands. AC magnetic measurements revealed that all three complexes, including the nominally isotropic Gd(iii) system, show field induced slow magnetic relaxation with two or three relaxation channels: at T = 1.9 K the low-frequency relaxation time is τLF(1) = 0.060 s at BDC = 0.5 T, τLF(2) = 0.37 s at BDC = 0.3 T, and τLF(3) = 1.29 s at BDC = 0.15 T.

Slow magnetic relaxation in a µ1,1'-azido cobalt(ii) methylquinoline chain complex.

A [Co(N3)2(mqu)] complex with a 1D chain architecture (mqu - 4-methylquinoline) displays the paramagnetic behaviour with a slight exchange interaction of antiferromagnetic nature among Co(ii) centres. It shows slow magnetic relaxation with three distinct modes. The relaxation time strongly depends upon the applied external field. The slowest relaxation time refers to the low-frequency mode τLF = 1.6(2) s at BDC = 0.8 T and T = 1.9 K.

Exceptionally slow magnetic relaxation in cobalt(ii) benzoate trihydrate.

Cobalt(ii) benzoate trihydrate prepared by the reaction of CoCO3 with benzoic acid (HBz) in boiling water followed by crystallization has been structurally characterized as a chain-like system with the formula unit [Co(Bz)(H2O)2]Bz·H2O where the Co(ii) atoms are triply linked by one bridging syn-syn benzoato (Bz) and two aqua ligands; additional benzoate counter ions and solvate water molecules are present in the crystal structure. DC magnetic measurements reveal a sizable exchange coupling of a ferromagnetic nature between the Co(ii) atoms. At TN = 5.5 K the paramagnetic phase switches to the antiferromagnetic phase. Though the remnant magnetization is zero, the magnetization curve shows two lobes of a hysteresis loop and the DC relaxation experiments confirm a long relaxation time at T = 2.0 K. AC susceptibility data confirm a slow relaxation of magnetization even in the antiferromagnetic phase. In the absence of the magnetic field, two relaxation channels exist. The relaxation time for the low frequency channel is as slow as τLF > 1.6 s and data fitting yields τLF (2.1 K) = 14 s. The high-frequency relaxation time obeys the Orbach process at a higher temperature whereas the Raman process dominates the low-temperature region. Three slow relaxation channels are evidenced at the applied magnetic field BDC = 0.1 T.

Self-assembly synthesis, structure, topology, and magnetic properties of a mononuclear Fe(iii)-violurate derivative: a combined experimental and theoretical study.

A new iron(iii) complex [Fe(H2Vi)3]·py·4H2O (1) {H3Vi = violuric acid, py = pyridine} was self-assembled and characterized. In the crystal structure, adjacent [Fe(H2Vi)3] blocks are H-bonded into a complex 3D network, which was topologically classified as a pcu [alpha-Po primitive cubic] network. Importance of π-stacking interactions was also highlighted and analyzed computationally. Magnetic studies confirmed antiferromagnetic exchange coupling, while ZFCM/FCM experiments indicated an ordering temperature Tc = 50 K. Finally, magnetic hysteresis at 5 K possesses the coercive field of Bc = 69 mT.

Spacer-armed copper(II) complexes with benzenecarboxylic acids and trifluoroacetylacetone aroylhydrazones.

New bi- and trinuclear copper(ii) complexes of aroylhydrazones of TFA and benzenecarboxylic acids were synthesized and characterized. Despite long range intramolecular CuCu distances of about 10 Å, exchange couplings between paramagnetic centres are observed with J values of 0.33 and 0.37 cm(-1) in binuclear complexes, and -0.33 cm(-1) in trinuclear complexes. Seven hyperfine peaks with a value of aCu = 37.0 × 10(-4) cm(-1) are observed in EPR spectra of binuclear complexes. The possible mechanisms responsible for the observed weak long range coupling via aromatic spacers are discussed.

The interplay of iron(II) spin transition and polymorphism.

The mononuclear compound (1) [Fe(II)(L)(2)](BF(4))(2) (L = 4-ethynyl-2,6-bis(pyrazol-1-yl)pyridine) was prepared and structurally as well as magnetically characterised. The crystallisation revealed the formation of two polymorphs--the orthorhombic 1A and the tetragonal form 1B. A third, intermediate phase 1C was found exhibiting a different orthorhombic space group. Reversibility of the phase transition between 1A and 1C was studied by variable-temperature single-crystal and powder X-ray diffraction studies, while an irreversible phase transition was observed for the transition of 1B→1C. The magnetic studies show that the 1A↔1C transition is accompanied by a very abrupt spin transition (ST) with 8 K hysteresis width (T(1/2)(↓) = 337 K, T(1/2)(↑) = 345 K). The ST was confirmed by Mössbauer spectroscopy as well as by DSC studies. In contrast, the 1B polymorph remained low-spin up to 420 K. In conclusion, a full cycle of intertwined phase- and spin-conversions of three polymorphs could be proven following the general scheme 1B→1C↔1A.