Francisco M. Romero

Pressure-Induced Magnetic Switching and Linkage Isomerism in K0.4Fe4(Cr(CN)6)2.8 ·16H2O: X-ray Absorption and Magnetic Circular Dichroism Studies

The effect of applied pressure on the magnetic properties of the Prussian blue analogue K0.4Fe4[Cr(CN)6]2.8 x 16 H2O (1) has been analyzed by dc and ac magnetic susceptibility measurements. Under ambient conditions, 1 orders ferromagnetically at a critical temperature (T(C)) of 18.5 K. Under application of pressure in the 0-1200 MPa range, the magnetization of the material decreases and its critical temperature shifts to lower temperatures, reaching T(C) = 7.5 K at 1200 MPa. Pressure-dependent Raman and Mossbauer spectroscopy measurements show that this striking behavior is due to the isomerization of some Cr(III)-C[triple bond]N-Fe(II) linkages to the Cr(III)-N[triple bond]C-Fe(II) form. As a result, the ligand field around the iron(II) centers increases, and the diamagnetic low-spin state is populated. As the number of diamagnetic centers in the cubic lattice increases, the net magnetization and critical temperature of the material decrease considerably. The phenomenon is reversible: releasing the pressure restores the magnetic properties of the original material. However, we have found that under more severe pressure conditions, a metastable sample containing 22% Cr(III)-N[triple bond]C-Fe(II) linkages can be obtained. X-ray absorption spectroscopy and magnetic circular dichroism of this metastable sample confirm the linkage isomerization process.

Single-molecule magnetic behavior in a neutral terbium(III) complex of a picolinate-based nitronyl nitroxide free radical

The terdentate anionic picolinate-based nitronyl nitroxide (picNN) free radical forms neutral and robust homoleptic complexes with rare earth-metal ions. The nonacoordinated Tb(3+) complex Tb(picNN)(3)·6H(2)O is a single-molecule magnet with an activation energy barrier Δ = 22.8 ± 0.5 K and preexponential factor τ(0) = (5.5 ± 1.1) × 10(-9) s. It shows magnetic hysteresis below 1 K.

Self-Assembly of a Copper(II)-Based Metallosupramolecular Hexagon

The self-assembly of a 1:1 mixture of copper(II) ions and a rigid heteroditopic ligand L containing phen and terpy binding units gives rise in the solid state to green crystals of a hexanuclear metallamacrocycle 1. X-ray crystallography reveals that 1 consists of molecular hexagons of the grid-type family in which each metal ion is bound to two different ligands through the phen and terpy units, plus a weakly coordinated PF6 (-) anion in a highly distorted octahedral geometry. ES-MS studies of acetonitrile solutions of L and copper(II) in a 1:1 ratio show mixtures of polynuclear complexes in which trinuclear L3Cu3 species are predominant.

Metal Complexes of a Picolinate-Based Nitronyl Nitroxide Free Radical

A nitronyl nitroxide free radical containing an appended picolinate moiety was synthesized. The resulting tridentate ligand picNN forms neutral mononuclear metal complexes of formula [M(picNN)(2)].3H(2)O (M = Mn, Co, Ni, Zn). These compounds are isostructural and crystallize in the orthorhombic Pnna space group. The metal complexes have a C(2) symmetric structure, with the metal centers lying on the binary axis and surrounded by two equivalent picNN radicals. The magnetic properties of this family of compounds indicate the presence of very strong metal-radical exchange interactions, ranging from J(Ni-rad) = -193 cm(-1) to J(Mn-rad) = -98 cm(-1). Relatively weak (J(rad-rad) = -15 cm(-1)) through-space magnetic interactions between free radicals coordinated to the same metal center were also evidenced in the study of the diamagnetic zinc(II) complex. Complexation with Cu(2+) affords the carboxylate-bridged tetranuclear copper(II) complex [Cu(4)(picNN)(4)(H(2)O)(4)](ClO(4))(4).4H(2)O having eight interacting S = 1/2 spins in a cyclic topology. The antiferromagnetic copper-radical exchange interaction (J(Cu-rad) = -268 cm(-1)) is one of the largest ever reported.

Spin crossover complexes as building units of hydrogen-bonded nanoporous structures

The paper reports on the synthesis, crystal structure, differential scanning calorimetry (DSC) and magnetic properties of a spin crossover salt of formula [Fe(bpp)2](cis,cis-1,3,5-Hchtc)·5.5 H2O (1) (bpp = 2,6-bis(pyrazol-3yl)pyridine; cis,cis-1,3,5-Hchtc = monoprotonated cyclohexanetricarboxylate dianion). The salt shows a porous structure with water molecules sitting in the channels. The electronic configuration of the Fe2+ ions is high-spin (HS). Desolvation of 1 yields a material exhibiting an abrupt spin crossover with a thermal hysteresis loop located near room temperature (T1/2↑ = 319 K and T1/2↓ = 309 K). Rehydration of this desolvated salt yields an essentially low-spin (LS) material. This is the first example of a series of hydrogen-bonded porous structures that combine a Fe(bpp)2 spin crossover unit acting as a hydrogen-bond donor and polycarboxylate anions acting as hydrogen-bond acceptors with different connectivities.

Synthesis, structure and properties of benzoic acids bearing para- or meta-imino nitroxides or ortho-nitronyl nitroxide radical centres

2-(4-Carboxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-yloxyl ( p-IMBAH), 2-(3-carboxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-yloxyl (m-IMBAH) and 2-(2-carboxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-3-oxido-1H-imidazol-3-ium-1-yloxyl (o-NITBAH) have been prepared. The m-IMBAH and p-IMBAH radicals crystallised in monoclinic space group P2 1 /n while the o-NITBAH radical occupied the monoclinic space group Cc. In p-IMBAH, a 12-membered ring is formed via strong hydrogen bonds between two water molecules and two radicals, this unit being propagated along the b axis via additional hydrogen bonds. In m-IMBAH, the radicals are arranged in dimers held together by strong hydrogen bonds between the acid and the imine functions, each dimer being connected with its neighbours via hydrogen bonds forming alternating chains along the c axis. No cyclic arrangements are evident in o-NITBAH, the radicals being organized into infinite chains. In each case, the product of paramagnetic susceptibility with T lies in the 0.362 to 0.374 emu K mol –1 range at room temperature while the reciprocal susceptibility follows a Curie-Weiss law above 20 K with θ=+1.38, +5.24 and –0.81 K, respectively, for p-IMBAH, m-IMBAH and o-NITBAH. The isotropic 14 N hyperfine coupling constants are found from solution experiments to be 9.20 (a N1 ) and 4.28 (a N2 ) G for the imino radicals and 7.59 G for the nitronyl nitroxide radical. The principal values of the g-tensor measured on single crystals afforded an average of 2.0066/7 while in solution g iso =2.0060 for p/m-IMBAH and 2.0065 for o-NITBAH.

Magneto‐Optical Properties of Electrodeposited Thin Films of the Molecule‐Based Magnet Cr5.5(CN)12·11.5H2O

Prof. E. Coronado , M. Makarewicz , J. P. Prieto-Ruiz , Dr. H. Prima-Garcia , Dr. F. M. Romero Instituto de Ciencia Molecular (ICMol) Universitat de Valencia C/Catedratico Jose Beltran, 2, 46980-Paterna, Spain E-mail: eugenio.coronado@uv.es; helena.prima@uv.es; fmrm@uv.es

Design of chiral magnets: cyanide-bridged bimetallic assemblies based on cyclohexane-1,2-diamine

Abstract Four magnetic compounds based on chiral ligands trans-(1S,2S)-chxn and trans-(1R,2R)-chxn (chxn: cyclohexane-1,2-diamine), [Ni(trans-(1S,2S)-chxn)2]3[Fe(CN)6]2·2H2O (1), [Ni(trans-(1R,2R)-chxn)2]3[Fe(CN)6]2·2H2O (2), [Cu(trans-(1S,2S)-chxn)2]3[Fe(CN)6]2·4.5H2O (3) and [Cu(trans-(1R,2R)-chxn)2]3[Fe(CN)6]2·4.5H2O (4), are reported. The four compounds are chiral, as confirmed by X-ray analyses and circular dichroism measurements. From the magnetic point of view, 1 and 2 behave as ferromagnets, whereas 3 and 4 show a paramagnetic behavior.

Multifunctionality in hybrid molecular materials: design of ferromagnetic molecular metals and hybrid magnets

We report on the synthesis and physical properties of novel hybrid organic–inorganic molecular materials combining ferromagnetic bimetallic oxalato-based networks and functional organic molecules as the donor bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) or cationic nitroxide free radicals. # 2002 Elsevier Science B.V. All rights reserved.

One-dimensional organization of free radicals via halogen bonding

Halogen bonds have been applied for the supramolecular organization of organic free radicals in the solid state and their role in the propagation of the magnetic exchange has been studied.