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The synthesis, structure and magnetic properties of [Fe(bpe)4(H2O)2](TCNQ)2 (1) are reported. 1 crystallizes in the triclinic P space group, a = 13.481(5), b = 14.887(3), c = 16.663(4) A, α = 101.048(18), β = 112.84(2), γ = 90.92(2)°, V = 3009.6(14) A3, Z = 2. The iron atom defines a compressed octahedron with the equatorial positions occupied by the bpe molecules which act as monodentate ligands and the two axial positions occupied by water molecules. The TCNQ− radical counterions are uncoordinated and interact by pairs defining (TCNQ)22− units strongly coupled antiferromagnetically. The iron(II) atoms are in the high spin state and its magnetic behaviour indicates the occurrence of zero-field splitting of the S = 2 ground state.

Pressure effect studies on the spin crossover behaviour of the mononuclear compounds {Fe[H2B(pz)2]2(bipy)}(1) and {Fe[H2B(pz)2]2(phen)}(2) have been performed in the range of 105 Pa–1.02 GPa at variable temperatures (100–310 K). Continuous spin transitions and displacement of its characteristic temperature has been observed for 1 with increasing pressure. Meanwhile the response of 2 under applied pressures is quite unexpected, and can only be understood in terms of a crystallographic phase transition or change in the bulk modulus of the compound. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

The synthesis and the crystal structures of the complexes [Cu(LI)2](ClO4) (1) and [Cu(LI)(CH3CN)2(ClO4)2] (2) are reported. 1 crystallizes in the monoclinic space group C2/c with the unit cell dimensions a = 13.169(4), b = 12.289(3), c = 14.732(3) A, β = 109.03(2)° and Z = 4. Copper(I) is coordinated to four N atoms of the two 1,10-Phenanthroline-5,6-dione (LI) ligands with a two-fold axis passing between the ligands. The copper(II) compound 2 crystallizes in the orthorhombic space group Pbn21 with unit cell dimensions of a = 7.498(5), b = 23.492(7), c = 12.363(4) A and Z = 4. Copper(II) coordination can be described as a distorted octahedron with the N donor atoms of one LI ligand and of two molecules of CH3CN occupying the equatorial positions completed by two oxygen atoms of the two perchlorate molecules in the axial positions.

[EN] Pressure effect investigations on the magnetic behaviour of the 3D SCO polymers {Fe(3CN-py)2[Ag(CN)2]2} · 2/3H2O (1) and {Fe(3CN-py)2[Au(CN)2]2} · 2/3H2O (2) have been carried out in the range of 105 Pa to 0.7 GPa. Despite both compounds are isostructural their magnetic behaviour under applied hydrostatic pressures is very different. Strong nonlinearity in the Tc(P) vs. P plot has been observed for compound 1 a fact which contrasts with the almost linear dependence observed for each spin transition in 2. However, both compounds are extremely sensitive to the application of pressure as well as the Tc(P) vs. P plots denote., Financial support is acknowledged from the Spanish Ministerio de Educación y Ciencia (MEC) (CTQ 2004 03456/BQU) and from the Generalitat Valenciana for the Research Grant ACOMP06/048. A.G. thanks the Universitat Politècnica de València for a predoctoral fellowship. A.B.G. thanks the Spanish MEC for a Research Contract (Programa Ramón y Cajal).

The crystal structure of [Fe(bt)2(NCS)2] (A) was determined by X-ray diffraction at 293 and at 150 K in order to analyze the structural changes associated with the spin transition. The space group is P1 with Z = 2 at both temperatures. Lattice constants are as follows: a = 8.5240(4), b = 11.0730(6), c = 12.5300(8) at 293 K and a = 8.1490(4), b = 11.4390(5), c = 12.1270(6) at 150 K. The iron(II) atom lies at the center of a distorted [FeN6] defined by two bt ligands arranged in a cis conformation. The two remaining coordination positions are occupied by two isothiocyanate anions. The average bond lengths of 2.159(4) A (293 K) and 1.951(2) A (150 K) clearly indicate the change in spin configuration. The trigonal distortion parameter phi has a value of 9.6 degrees and 5.5 degrees at 293 and 150 K, respectively. For A, DeltaV = DeltaV(SCO) = 28 A(3) per formula unit and is accompanied by a hysteresis of 10 K. chi(M)T vs T curves at atmospheric pressure for A show an abrupt spin transition with Tc downward arrow = 176 K and Tc upward arrow = 187 K. The thermodynamic parameters associated with the spin transition are DeltaH = 8.4 +/- 0.4 kJ mol(-1) and DeltaS = 46.5 +/- 3 J K mol(-1). The thermal dependence of the magnetic susceptibility at different pressures, 0.1-0.91 GPa, points out an unusual behavior, which can only be understood in terms of a crystallographic phase transition or a change in the bulk modulus of the complex. Polymorph B crystallizes in the C2/c space group with an average Fe-N bond length of 2.168(2) A and phi = 14.7 degrees at 293 K. B remains in the HS configuration even at pressures of 1.06 GPa.

The 1A1 left arrow over right arrow 5T2 spin transition has been investigated in the solid solutions of Fe(x)M(1-x)(pyrazine)[Pt(CN)4] (M = Ni or Co, 0or = xor = 1) having a three-dimensional polynuclear structure. Both Ni and Co dilutions tend to decrease the hysteresis width and smooth the transition curves. The enthalpy (entropy) change associated with the spin transition was found to decrease from 26 kJ mol(-1) (84 J K(-1) mol(-1)) for x = 1 to 12 kJ mol(-1) (47 J K(-1) mol(-1)) for 47% Co dilution and to 15 kJ mol(-1) (54 J K(-1) mol(-1)) for 59% Ni dilution. Raman spectroscopy revealed a mixed one- and two-mode behavior in the solid solutions. For the first time, a correlation between vibrational frequencies exhibiting one-mode behavior and the entropy change, which drives the spin crossover, is established.

A novel one-dimensional coordination polymer made up of Fe", 3-cyanopyridine (3CNpy), and the singular in situ formed [Cu I (3CNpy)(CN) 2 ] - anionic bridge has been synthesized. This compound undergoes a spin-crossover behavior according to its magnetic and calorimetric properties. The crystal structure of the title compound has been studied in the high- and low-spin states and correlated with the character of the spin conversion. Evidence for intense spin-state-dependent Cu···Cu interactions between the chains is also reported.

A series of new complexes belonging to the [Co(4-terpyridone)2]X(p) x nS family (4-terpyridone = 2,6-bis(2-pyridyl)-4(1H)-pyridone) have been synthesized and characterized, using X-ray single crystal determination and magnetic susceptibility studies, to be X = [BF4]- (p = 2) and S = H2O for polymorphs 1 and 2, X = [BF4]- (p = 1) and [SiF6]2- (p = 0.5) and S = CH(3)OH for 3, X = [SiF6]2- (p = 1) and S = 3CH3OH and H2O for 4, X = [Co(NCS)4]2- (p = 1) and S = 0.5CH3OH for 5, X = I- (p = 2) and S = 5H2O for 6, X = [PF6]- (p = 1) for 7, and X = [NO3]- (p = 2) for 8. Compounds 1-7 can be grouped in three sets according to the space group in which they crystallize: (i) P1 triclinic (1, 3), (ii) P2(1) monoclinic (2), and (iii) P2(1)/c monoclinic (4-7). The tridentate 4-terpyridone ligands coordinate the Co(II) ions in a mer fashion defining essentially tetragonally compressed [CoN6] octahedrons. The Co-N axial bonds involving the pyridone rings are markedly shorter than the Co-N equatorial bonds collectively denoted as Co-N(central) and Co-N(distal), respectively. The differences in the average Co-N(central) or Co-N(distal) distances observed for 1-7 reflect the different spin states of Co(II). Complexes 7 and 4' are fully high spin (HS), while 5 and 6 are low spin (LS). However, the counterion [Co(NCS)4]2- in complex 5 is high spin. Complexes 1, 2, 3, and 8 exhibit spin-crossover behavior in the 400-100 K temperature region. Compounds 1 and 2 are polymorphs, and interestingly, 1 irreversibly transforms into 2 above 340 K because of a crystallographic phase transition which involves a drastic modification of the crystal packing. The relevant thermodynamic parameters associated with the spin transition of polymorph 2 have been estimated using the regular solution theory leading to DeltaH = 3.04 kJ mol(-1), DeltaS = 20 J K(-1) mol(-1), and Gamma = 0.95 kJ mol(-1).

Here we report the synthesis and characterisation of a polymer made up of a system of parallel 2-D grids of Fe(II) ions linked by [Au(CN)2]– bridges and its transformation into a new system of three interpenetrated 3-D coordination open frameworks with the NbO topology. Reversibility of this crystal-to-crystal transformation is evidenced by X-ray crystallographic data and from their spin crossover properties. Real Cabezos, Jose Antonio, Jose.A.Real@uv.es

Two coordination polymers formulated [Fe(pmd)2[Ag(CN)2]2] (pmd = pyrimidine) have been synthesized and characterized. Both polymers, considered to be architectural isomers, display different crystal structures and magnetic properties. Isomer 1 crystallizes in the monoclinic C2/c space group with a = 6.9750(8) angstroms, b = 16.1700(9) angstroms, c = 14.2020(8) angstroms, beta = 97.954(2) degrees, V = 1586.37(14) angstroms3, and Z = 4. The crystal structure of isomer 2 has been studied at 250 and 150 K. At both temperatures, 2 displays the orthorhombic Pccn space group with a = 15.7700(2) [14.8950(2)] angstroms, b = 8.2980(4) [8.1580(4)] angstroms, c = 13.4180(6) [13.3480(5)] angstroms, V = 1755.87(14) [1621.96(10)] angstroms3, and Z = 4 for 250 [150] K. The iron(II) ions define distorted octahedral [FeN6] chromophores in both isomers. The equatorial positions are occupied by four [Ag(CN)2]- bridging ligands, which connect the defining layers of two iron(II) ions. Isomer 1 has two crystallographically distinct [Ag(CN)2]- groups; one is essentially linear, while the other is severely distorted [C(5)-Ag(2)-C(5i)] = 138.8(5) degrees. This fact facilitates the parallel interpenetration of two layers, which in addition show short Ag(1)....Ag(2) interactions (distance Ag(1)....Ag(2) = 2.9972(10) angstroms). Isomer 2 shows only one type of Ag atom, which is slightly bent [C-Ag-C = 161.54(12) degrees], and as a consequence, the layers defined are not interpenetrated. In both cases, the axial positions are occupied by the pmd ligands which interact with the Ag atoms of adjacent layers defining a 3D coordination polymer. Compound 1 is high spin in the whole range of temperatures, while 2 undergoes a cooperative high-spin--low-spin effect centered at ca. 184 K with a hysteresis loop ca. 5 K wide. The experimental enthalpy and entropy variations were 11.5 +/- 0.4 kJ mol(-1) and 64 +/- 3 J K(-1) mol(-1). Consistency between the experimental thermodynamic data and the magnetic data was checked in the frame of regular solution theory.

The compound {Fe(pmd)[Ag(CN)2][Ag2(CN)3]} (pmd=pyrimidine) was synthesized and characterized. Magnetic, calorimetric and single crystal visible spectroscopic studies demonstrate the occurrence of a two-step high-spin (HS) right arrow over left arrow low-spin (LS) transition. The critical temperatures are T(c1)=185 and T(c2)=148 K. Each step involves approximately 50 % of the iron centers, with the low-temperature step showing a hysteresis of 2.5 K. The enthalpy and entropy variations associated with the two steps are DeltaH(1)=3.6+/-0.4 kJ mol(-1) and DeltaS(1)=19.5+/-3 J K(-1) mol(-1); DeltaH(2)=4.8+/-0.4 kJ mol(-1) and DeltaS(2)=33.5+/-3 J K(-1) mol(-1). Photomagnetic and visible spectroscopy experiments show that below 50 K, where the LS state is the thermodynamically stable state, the compound can be switched quantitatively to the HS state using green-red light (550-650 nm). HS-to-LS relaxation experiments in the dark at temperatures between 15 and 55 K show that the relaxation takes place via a two-step cooperative process, which was analyzed in the context of the mean field theory. The crystal structure has been studied at 290, 220, 170, 90 and 30 K together with 30 K after irradiation. The compound adopts monoclinic symmetry (P2(1)/c, Z=16) at all temperatures. There are five [FeN6] pseudo-octahedral sites linked by pmd, [Ag(CN)2]- and [Ag2(CN)3]- bridging ligands to form an unprecedented three-dimensional (6,6) topology. The structural analysis allows for an understanding of the microscopic mechanism of the two-step behavior of the thermally induced spin transition as well as the corresponding relaxation of the photoexcited compound based on the individual changes of the five sites. Synergy between metallophilic interactions and the spin transition is also shown by the variation of the AgAg distances. Correlations between the variation of the unit-cell volume and the change of AgAg interactions within each step with the asymmetric change of the anomalous heat capacity have also been inferred.

Assembly of FeII, 3-cyanopyridine and [Au(CN)2]– affords, in one-pot reaction, three coordination polymers that represent a genuine example of supramolecular isomerism with strong influence in the spin crossover regime of the FeII ions. Real Cabezos, Jose Antonio, Jose.A.Real@uv.es

The spin crossover complexes {Fe[H2B(pz)2]2L} ([H2B(pz)2]2 = dihydrobis(pyrazolyl)borate, L = 2,2'-bipyridine (1), bipy and 1,10-phenanthroline, phen (2)) undergo both thermal and light induced spin crossover, but the structure of the low spin and light induced high spin states for 2 are different from that of the thermally induced high spin state and from those of 1. Real Cabezos, Jose Antonio, Jose.A.Real@uv.es

A net change: A fully reversible ligand substitution involving coordination/ uncoordination of gaseous water and pyrimidine induces the repetitive allosteric transformation of three interpenetrated nets into a single three-dimensional net. The transformation does not affect the crystallinity of the sample but alters significantly the spin-crossover transition; the compound shows magnetic and chromatic bistability (see picture).

[EN] Self-assembly of [Ag(CN)2]-, 3-cyanopyridine, and iron(II) gives triple interpenetration of three-dimensional spin crossover (SCO) networks with the NbO topology, which interact each other via argentophilic interactions. Superposition of the networks along the c-axis defines an infinite mosaic of trigonal and hexagonal cross-sectional tubes where water molecules are installed. The structural changes associated with the SCO tune the homoatomic interactions and induce an uncommon change of the lattice volume., We are grateful for the financial assistance from the Ministerio Español de Ciencia y Tecnología (project BQU 2001-2928). A.G. thanks the Universitat Politécnica de València for a predoctoral fellowship. We are grateful to Dr. A. E. Goeta for helpful discussions

{Fe(pmd)2[Cu(CN)2]2} (pmd = pyrimidine) displays a rigid two-dimensional structure and undergoes thermal- and optical-driven spin crossover behaviour; cooperative elastic coupling between iron(II) ions in the framework induces thermal hysteresis in the HS↔LS conversion and sigmoidal HS→LS relaxation of the photo-induced HS state at low temperatures. Niel, Virginie, Virginie.Niel@uv.es ; Galet Domingo, Ana Guadalupe, Ana.Galet@uv.es ; Gaspar Pedros, Ana Belen, Ana.B.Gaspar@uv.es ; Real Cabezos, Jose Antonio, Jose.A.Real@uv.es

[EN] Low-spin, high-spin and spin-transition behaviours have been observed for the doubly interpenetrating three-dimensional bimetallic compounds {Fe-II(pz)[Ag(CN)(2)](2)}.pz (pz= pyrazine), {Fe-II(4,4'-bipy)(2)[Ag(CN)(2)](2)} (4,4'-bipy-4,4'-bipyridine), and {Fe-II-(bpe)(2)[Ag(CN)(2)](2)} (bpe = bispyridylethylene), respectively. The single crystals of the bpe derivative undergo a spin transition with a large hysteresis loop at about 95 K. After several warming and cooling cycles, the single crystals become a microcrystalline powder with 50% spin transition. Influence of pressure- as well as light-induced excited spin-state trapping (LIESST) on the thermal 50% spin transition of the microcrystalline sample has also been investigated. Thermal spin-transition behaviour has also been induced at pressures higher than 1 bar for the 4,4'-bipy derivative. Both the 4,4'-bipy and bpe derivatives show strong pressure dependence of the spin state at 300 K., We are grateful for financial support from the European Commission, granting the TMR-Network Thermal and Optical Switching of Spin States (TOSS) , Contract No. ERB-FMRX-CT98-0199EEC/TMR. We thank the Spanish DGICYT for financial assistance through Project PB97-1397.