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[EN] Assembly of the tetranuclear oxomanganese(III) acetato cluster [Mn4O2(O2CMe)(7)(phen)(2)](BF4) from the dinuclear oxo-di(acetato)bridged manganese(III) species [Mn2O(O2CMe)(2)(H2O)(2)(phen)(2)](BF4)(2) . 3H(2)O in aqueous/acetic acid MeOH solution occurs via the new 'dimer of dimers' Mn-III complex [Mn2O(O2CMe)(3)(H2O)(phen)(2)](BF4) . MeOH possesing an unprecedent [Mn-4(mu-O)(2)(mu-O2Me)(4) (mu-(OH2O2CMe)-O-...)(2)] core., This work was supported by the DGICYT, Ministerio de Educación y Ciencia (Spain) through project PB97-1397. R. Ruiz thanks the 3MD-EU Network (contract no. ERB 4061, PL97-0197) for a grant.

[EN] The new heterometallic sodium(I)¿copper(II) compound Na4Cu2 (2) · 10.5 H2O (3), where H8[2] stands for N,N',N'',N'''-methanetetrayltetrakismethylenetetrakis(oxamic acid), has been synthesized and its crystal structure determined by single-crystal X-ray diffraction. The structure of 3 consists of cationic [Cu2(¿4:¿4-2)]4- dinuclear units, coordinated sodium cations, and water molecules. In the crystal, the dinuclear copper entities are joined through discrete aggregates of eight sodium atoms linked by oxamato and water bridges, leading to a three-dimensional polymeric network., This work was supported by the CNRS (France) and the DGICYT (Spain) through project PB94-1002. B. C. thanks the Conselleria de Educació i Ciència de la Generalitat Valenciana (Spain) for a grant. We would also like to express our gratitude to Profs. M. Julve and T. Mallah for fruitful discussions during the preparation of this manuscript.

[EN] Novel two-dimensional polymers, [Fe(L-1)(H2O)(2)(NCX)(2)]. L-1 (L-1 =4.4'-bipyridine (bipy)) (1, 2) and [Fe(L-2)(CH3OH)(2)-(NCX)(2)]. L-2 (L-2 =4,4'-azopyridine (azpy)) (3) and X = S (1, 3), Se (2), have been synthesized and characterized by X-ray crystallography. The structures reveal the formation of tranzs-L-bridged [Fe(NCX)(2)(Y)(2)] where Y=H2O, CH3OH linear chains assembled into two-dimensional networks by hydrogen bonds between the uncoordinated ligand L and the coordinated solvent molecules., We are grateful for financial assistance from the Direction General de Investigation Cientifica y Tecnica (DGICYT) (Spain) through Project PB97-1397. We acknowledge the European Commission for granting the TMR-Network Thermal and Optical Switching of Molecular Spin States (TOSS) , Contract No. ERB-FMRX-CT9&0199EEC/ TMR.

[EN] The compounds [Fe(abpt)(2)(NCS)(2)] (1) and [Fe(abpt)(2)(NCSe)(2)] (2) with abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole have been synthesized. The X-ray structures have been determined at 293 K. 1 and 2 are isostructural and crystallize in the monoclinic space group P2(1)/n with Z = 2, a = 8.538(8), b = 10.246(8), c = 16.45(2) Angstrom, beta = 93.98(9)degrees for 1 and a = 8.623(2), b = 10.243(3), c = 16.585(3) Angstrom, beta = 93.19(2)degrees for 2. In both complexes, the coordination core has a similar pseudo-octahedral geometry with the NCS- (1) and NCSe- (2) groups in the trans-position. Variable-temperature magnetic susceptibility data give evidence for a low-spin (LS)high-spin (HS) conversion centered at T-1/2 around 180 and 224 K for 1 and 2, respectively. The spin conversion takes place gradually, without hysteresis. The enthalpy and entropy changes associated with the spin conversion are evaluated from the DSC measurements: Delta H = 5.8 +/- 0.5 (1) and 8.6 +/- 0.8 kJ mol(-1) (2); BS = 32.5 +/- 3 (1) and 38 +/- 4 J mol(-1) K-1 (2). At 10 K the light-induced excited spin state trapping (LIESST) effect has been observed within the SQUID magnetometer cavity, by irradiating powder samples with a Kr+ laser coupled to an optical fiber. The critical LIESST temperatures T-liesst are around 40 and 32 K for 1 and 2, respectively. The magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed a light-induced thermal hysteresis (LITH) effect. The HS-->LS relaxation kinetics have been investigated in the temperature range 6-40 K. A thermally activated mechanism at elevated temperatures and a nearly temperature independent relaxation behavior at low temperatures can be observed for 1. The very fast relaxation precludes the estimation of the kinetic parameters for 2 at temperatures higher than 10 K. (C) 1999 Elsevier Science S.A. All rights reserved., We thank the financial assistance from the Spanish DGICYT and CICYT through Projects PB97-1397 and MAT97-0987. We acknowledge the European Commission for granting the TMR-Network Thermal and Optical Switching of Molecular Spin States (TOSS) , contract no. ERB-FMRX-CT98-0199EEC/TMR.

A new heterotrinuclear complex of formula [Cu(2)Zn(Hdmg)(2)(dmg)(2)(H(2)O)].0.5H(2)dmg.H(2)O (C(18)H(34)Cu(2)N(9)O(11)Zn, 1) (H(2)dmg = dimethylglyoxime) has been synthesized and its crystal structure determined by single-crystal X-ray diffraction. It crystallizes in the triclinic system, space group Ponemacr;, with a = 11.414(1) Å, b = 11.992(3) Å, c =12.567(10) Å, alpha = 91.27(6) degrees, beta = 111.46(2) degrees, gamma = 112.24(2) degrees, and Z = 2. The structure consists of a chain of neutral [Cu(2)Zn(Hdmg)(2)(dmg)(2)(H(2)O)] trinuclear units and noncoordinated H(2)dmg and water molecules. The configuration around the zinc atom is distorted trigonal bipyramidal with four oximate oxygens from two [Cu(Hdmg)(dmg)](-) fragments (each one acting as a bidentate ligand through its deprotonated oximate oxygens in cis positions) occupying one axial and the three equatorial positions and an additional oximate oxygen from a symmetry-related [Cu(Hdmg)(dmg)](-) fragment filling the remaining axial position. The environment around Cu(1) and Cu(2) is distorted square pyramidal with four oximate nitrogen atoms building the equatorial plane. An oxygen atom [O(9)] from a water molecule and an oximate oxygen from a symmetry-related [Cu(Hdmg)(dmg)](-) fragment occupy the apical position of the square pyramids around Cu(1) and Cu(2), respectively. The trinuclear units are repeated through inversion centers standing at the middle of the Zn(1).Zn(1)(i) and Cu(2).Cu(2)(ii) vectors leading to a chain which runs parallel to the diagonal of the ac-plane. The Cu(1).Cu(2), Zn(1).Zn(1)(i), and Cu(2).Cu(2)(ii) separations are 5.506(2), 3.390(2), and 3.930(2) Å, respectively. 1 exhibits a characteristic ferromagnetic behavior with a continuous increase of the chi(M)T product as the temperature is lowered from 300 to 2.0 K. The field dependence of the magnetization at 2.0 K is consistent with a low-lying quintet state. The only efficient exchange pathways responsible for the overall ferromagnetic coupling in 1 are the diamagnetic zinc(II) ion and the out-of-plane double-oximato bridge, the magnitude of the magnetic coupling between the copper(II) ions through these bridging units being 3.9 and 5.1 cm(-)(1), respectively.

The first example of spin crossover iron(II) complexes based on dihydrobis(1-pyrazolyl)borate are presented here. The complexes {Fe[H(2)B(Pz)(2)](2)(phen)} (phen = 1,10-phenanthroline), 1, and {Fe[H(2)B(Pz)(2)](2)(bipy)} (bipy = 2,2'-bipyridine), 2, have been synthesized and their structures determined by X-ray diffraction methods. Crystals 1 and 2 are monoclinic, space group C2/c, Z = 4 with a = 17.448(4) Å, b = 16.101(4) Å, c = 10.611(2) Å, and beta = 112.47(2) degrees for 1 and a = 16.307(2) Å, b = 15.075(4) Å, c = 11.024(4) Å, and beta = 114.95(5) degrees for 2 at 293 K. The crystal structure of 2 was also determined at 139 K in order to detect the structural changes associated with the S = 0--S = 2 spin conversion. 2 retains the same space group upon spin conversion with a = 16.086(6) Å, b = 14.855(6) Å, c = 10.812(2) Å, and beta = 114.18(3) degrees. The structures of 1 and 2 are made up of mononuclear neutral species where the positive charge of iron(II) is neutralized through the coordination of two chelate bidentate dihydrobis(pyrazolyl)borate anions, and phen or bipy neutral ligands are used to fill the iron(II) coordination sphere. The molecular structures for both compounds are very similar, with Fe-N bond lengths in the 2.212-2.158 Å range for the high-spin phase. The structural modifications associated with the spin change in 2 mainly consist of a large reorganization of the metal environment: the Fe-N decreases by 0.15 Å (mean value) when the temperature is lowered from 290 to 139 K and a more regular shape of the [FeN(6)] octahedron is achieved through a slight modification of the trigonal deformation angle from 5.3 degrees to 3.2 degrees along with remarkable variations of the N-Fe-N angles. The thermodynamic model of Slichter and Drickamer was applied to account for the magnetic data. The intermolecular interaction parameter and the enthapy and entropy changes associated with the spin transition were estimated as Gamma = 3.3 kJ mol(-)(1), DeltaH = 13.4 kJ mol(-)(1), and DeltaS = 81.9 J mol(-)(1) K(-)(1) for 1 and Gamma = 1.7 kJ mol(-)(1), DeltaH = 13.4 kJ mol(-)(1), and DeltaS = 83.9 J mol(-)(1) K(-)(1) for 2, respectively.

A new iron(III)–carbonato monomeric complex of orthophenylenebis( oxamato) (opba) 1 is synthesized, and spectroscopically and structurally characterized; it is a moderately efficient non-heme catalyst for the aerobic epoxidation of alkenes with co-oxidation of pivalaldehyde. Ruiz Garcia, Rafael, Rafael.Ruiz@uv.es ; Fernandez Picot, Isabel, Isabel.Fernandez@uv.es ; Pedro Llinares, Jose Ramon, Jose.R.Pedro@uv.es

[EN] Self-assembly of 1,4-phenylenebis(oxamate) and 4,4`-biphenylenebis(oxamate) ligands and Cu2+ ions gives two new dinuclear copper(II) metallacyclophanes where the two metal centers are connected by double para-substituted aromatic diamide bridges. Despite the relatively large intramolecular Cu¿Cu distance of ca. 8 and 12 Å for each complex, magnetic susceptibility measurements evidence strong to moderately strong intramolecular antiferromagnetic couplings (¿J values of the order of 100 and 10 cm-1, respectively). Density functional theory calculations on these compounds and their homologues with linear oligo-p-phenylenediamide bridges predict a rather slow exponential decay of magnetic coupling with increasing intermetal distance (r values up to 25 Å) along this novel series of dicopper metalla-amidocyclophanes., This work was supported by the Ministerio de Ciencia y Tecnología (Spain) through the Plan Nacional de Investigación Científica, Desarrollo e Innovación Tecnológica (Projects BQU2001-2928 and BQU2001-3017) and the Ramón y Cajal program. Further support from the TMR program of the European Union (Contract ERBFM-RXCT980181) is acknowledged. E.P. thanks the Ministerio de Educación, Cultura y Deporte (Spain) for a grant.

Inclusion of thiourea guest molecules in the tridimensional spin crossover porous coordination polymers {[Fe(pyrazine)[M(CN)(4)]} (M = Pd, Pt) leads to novel clathrates exhibiting unprecedented large thermal hysteresis loops of ca. 60 K wide centered near room temperature.

The porous coordination polymers (PCPs) of general formula {Fe(bpac)[M(CN)4]}·guest (M = Pt, Pd) exhibit larger channels than previously synthesised 3D-Hofmann-like PCP. The channels are partially occupied by uncoordinated guest bpac ligands and labile H2O molecules. These PCPs exhibit very scarce cooperative spin crossover behaviour around room temperature with a large hysteresis loop (up to 49 K) and also display sensitivity to humidity and guest molecules. The inclusion of bpac molecules in the 3D network can be avoided by adding competitive volatile molecules during the crystallization process, affording the guest-free material. The spin crossover behavior of different guest and guest-free materials is also presented.

Three salts built up from (E)-4′-(dimethylamino)-stilbazolium (DMAS)H+, (E)-4′-(diethylamino)-stilbazolium (DEAS)H+, (E)-4′-{2-(methoxymethyl) pyrrolidinyl}-stilbazolium (MPS)H+, and gold cyanide as a counter-ion, are reported. The crystal structures have been solved for (DEAS)H+ Au(CN)2− (Cc space group), and for (MPS)H+ Au(CN)2− (P1 space group). The semi-empirical (ZINDO) calculated static hyperpolarizability (β0) of (MPS)H+ is equal to 147 × 10−30 cm5esu−1, in solid state, which is 25% higher than that of the cation of the well known (E)-4′-(dimethylamino)-methylstilbazolium tosylate (DAST). (MPS)H+ Au(CN)2− exhibits a unique crystal structure in which the cations are perfectly aligned. This combination of large hyperpolarizability and strict 1-dimensional character leads to a giant dzzz quadratic susceptibility (χ(2)) tensor component estimated between ∼4500 pm V−1 (∼11 × 10−6 esu) and 12 400 pm V−1 (∼30 × 10−6 esu) at 1.064 μm, by a confocal μ-SHG technique. The use of such material in ultra-thin devices is critically evaluated.

The synthesis and crystal structure of the interpenetrated metal–organic framework material Fe(bpac)2[Ag(CN)2]2 (bpac = 4,4′-bis(pyridyl)acetylene) are reported along with the characterization of its spin crossover properties by variable temperature magnetometry and Mossbauer spectroscopy. The complex presents an incomplete stepped spin transition as a function of temperature that is modified upon successive thermal cycling. The pressure-induced transition has also been investigated by means of high pressure Raman spectroscopy using a diamond anvil cell. The results show that it is possible to reach the thermally-inaccessible fully low spin state at room temperature by applying hydrostatic pressure to the sample.

[EN] The structures of the series of N,N¿-1,n-phenylenebis(oxamic acid ethyl ester) molecules with n = 2 (H2Et2opba, 1), 3 (H2Et2mpba, 2), and 4 (H2Et2ppba, 3) have been determined by single-crystal X-ray diffraction (XRD) methods. Density functional (DF) calculations have been performed on the simplest model system N-phenyloxamic acid methyl ester (HMepma). Compounds 1¿3 have either folded (H2Et2opba), bent (H2Et2mpba), or linear (H2Et2ppba) almost planar (periplanar) molecular configurations with the two oxalamide moieties being slightly tilted up and down, respectively, with respect to the benzene ring. The energy calculations as a function of the torsion angle (¿) around the N(amide)¿C(benzene) bond for HMepma reveal that the minimum energy syn and anti periplanar conformations of the carboxamide functions are more stable than the corresponding syn and anti planar ones (¿ = 0 and 180°) by 0.18 and 0.13 kcal mol¿1, respectively. The calculated ¿ values for the syn and anti periplanar minimized conformers of HMepma are 16.0 and 200.0°, respectively, in reasonable agreement with the experimental values for 1¿3 [¿ = 39.0(4) and 225.0(3) (H2Et2opba), 32.6(5) (H2Et2mpba), and 34.7(2)° (H2Et2ppba)]. This situation likely minimizes the forced repulsive interactions between the amide hydrogen and the nearest benzene hydrogen atoms while it maximizes the attractive interactions between the carbonyl amide oxygen and the nearest benzene hydrogen atoms, which are then implicated in a relatively weak, intramolecular C¿H(benzene)¿O[double bond, length as m-dash]C(amide) hydrogen bond [d(H¿O) = 2.45(2)¿2.57(2) Å]. A supramolecular aggregation of molecules into either a duplex (H2Et2opba) or a brick-wall sheet (H2Et2ppba) occurs for 1 and 3, respectively, through moderately strong, intermolecular N¿H(amide)¿O[double bond, length as m-dash]C(amide) hydrogen bonds [d¿(H¿O) = 2.17(2)¿2.37(2) Å]. By contrast, moderately weak, intermolecular N¿H(amide)¿O[double bond, length as m-dash]C(ester) hydrogen bonds between the H2Et2mpba molecules are involved in 2 to give a meso-helical chain with a unique hydrogen-bonded oxalamide acid ester dimeric unit. The energy calculations as a function of the intermolecular N¿H(amide)¿O[double bond, length as m-dash]C(ester) hydrogen bond distance (d¿) for the {HMepma}2 dimer show an energy minimum at 2.37 Å, in excellent agreement with the experimental value of 2 [d¿(H¿O) = 2.42(4) Å]. The calculated value of the hydrogen bond energy for {HMepma}2 (EHB = 4.83 kcal mol¿1) is consistent with a partially covalent nature of the interaction between the amide hydrogen and the carbonyl ester oxygen atoms, as confirmed by the existence of a significant electron density delocalization within the resulting four-center H2O2 diamond core., This work was supported by the Ministerio de Educacion y Ciencia (Spain) (projects CTQ2006-14199 and CTQ2007-61690) and the Generalitat Valenciana (Spain) (project PROMETEO/2009/108). We thank Prof. Miguel Julve and Jose Antonio Real for continuous interest in this work and fruitful discussions during the preparation of the manuscript.

Bidirectional chemo-switching of magnetism occurs in a microporous coordination polymer containing spin-crossover subunits, as described by M. Ohba, J. A. Real, S. Kitagawa, and co-workers in their Communication on page 4767 ff. In situ magnetic measurements reveal that most guest molecules transform the framework spin state from diamagnetic low spin (red) to paramagnetic high spin (yellow), whereas the guest CS2 stabilizes the low-spin state. These induced spin states are retained as a memory effect after the release of the guest.

Das chemische Schalten des Magnetismus in zwei Richtungen wurde in einem mikroporosen Koordinationspolymer mit Spin-Crossover-Einheiten beobachtet. M. Ohba, J. A. Real, S. Kitagawa und Mitarbeiter stellten in ihrer Zuschrift auf S. 4861 ff. magnetische Messungen vor, die belegen, dass die meisten Gastmolekule einen Ubergang des Netzwerks vom diamagnetischen Low-Spin- (rot) in den paramagnetischen High-Spin-Zustand (gelb) bewirken. Allein CS2 stabilisiert den Low-Spin-Zustand. Die induzierten Spinzustande werden auch nach Freisetzung der Gastspezies beibehalten.

[EN] A mandelamide ligand, derived from (S)-mandelic acid and (S)-phenylethanamine, catalyzes the addition of aryl-, alkyl-and silyl-alkynylzinc reagents to aromatic and heteroaromatic aldehydes with good yields and good to high enantioselectivities., Financial support from the Ministerio de Ciencia e Innovacion and FEDER (CTQ 2006-14199/BQU) is gratefully acknowledged.

The suitability of the stilbazole push–pull ligands, 4′-dimethylaminostilbazole (DMAS) and 4′-diethylaminostilbazole (DEAS), for the construction of bimetallic FeII–AgI/AuI cyanide-based coordination polymers that exhibit spin crossover properties is investigated. The structural and physical characterization of four novel two-dimensional FeII polymers formulated as {Fe(DMAS)2[Ag(DMAS)(CN)2]2} (1) and {Fe(L)2[M(CN)2]2} (L = DMAS, M = Au (2); DEAS, Ag (3); DEAS, Au (4)) is reported. Polymers 1 and 4 are paramagnetic over the whole range of temperatures studied (5–300 K), whereas 2 and 3 exhibit spin crossover properties.

[EN] Compound [Co(4-terpyridone)(2)](CF3SO3)(2)center dot 1H(2)O, where 4-terpyridone is 2,6-bis(2-pyridyl)-4(1H)-pyridone, forms two polymorphs. Polymorph 1 displays a continuous spin conversion in the temperature region 300-120 K while polymorph 2 shows, on cooling, the onset of a continuous high-spin (HS) to low-spin (LS) conversion interrupted by an abrupt "reverse'' spin transition in the temperature region 217-203 K. The formed unstable HS intermediate phase (IP) undergoes a strong cooperative "normal'' spin transition characterised by a hysteresis loop 33 K wide. The structural data give support for a crystallographic phase transition, which takes place concomitantly with the "reverse'' spin state transition., Financial support from the Spanish Ministerio de Educacion y Ciencia (MEC) (CTQ 2007-64727) is acknowledged. A.B.G. thanks the Spanish MEC for a research contract (Programa Ramón y Cajal). We also acknowledge the European Project MAGMANET.

This article reviews the most relevant chemical and structural aspects that influence the spin-crossover phenomenon (SCO). Special attention is focussed on the recent development of SCO coordination polymers. The different approaches currently being explored in order to achieve multifunctionality in SCO materials are discussed.

[EN] The intermediate-spin five-co-ordinate complex [Fe(4,4'-bipy)(2)(NCS)(3)].(CH3)(2)CO has been prepared, its structure determined and magnetic properties characterized. The structure is made up of mononuclear neutral species linked by van der Waals interactions. The co-ordination polyhedron is defined by live nitrogen atoms around the iron(III) ion in a distorted trigonal bipyramid. The three vertices of the equatorial plane are occupied by the thiocyanate ligands while the 4,4'-bipyridine ligands fill the axial positions. The Fe-N(CS) distances are 2.025(7), 2.04(2) and 2.02(2) Angstrom and the axial Fe-N bond distances are 2.28(2) and 2.21(2) Angstrom. The magnetic properties are consistent with an S = 3/2 ground state. The zero-field splitting in the ground state was found to be ca. 6 cm(-1). Variable temperature EPR spectra are briefly reported., We are grateful for financial support from the Dirección General de Investigación Científica y Técnica (DGICYT) (Spain) through Project PB94-1002 and the Training and Mobility Research Program from the European Community (TMR Contract ERBFMRXCT-980181). We also acknowledge the Servicio de Difracción de Rayos X de Monocristal de la Universidad Politécnica de Valencia for diffraction facilities.

The synthesis, crystal structure, magnetic and calorimetric studies of a new clathrate compound of the Hofmann-type spin crossover (SCO) metal-organic framework (MOF) {Fe(bpb)[MII(CN)4]}·xGuest (bpb = bis(4-pyridyl)butadiyne, and MII= Ni, Pt) with characteristicfsctopology is reported. The framework {Fe(bpb)[MII(CN)4]} can host up to 1.5 guest molecules of (trifluoromethyl)benzene and display complete one-step cooperative SCO behavior. Our systematic study on {Fe(bpb)[Pt(CN)4]}·xGuest shows a general reciprocal correlation between the SCO temperature with the volume of the guest molecules.

An organic–metalloorganic analogue of the inorganic clay minerals has been obtained from the cyclization of N,N′-bis(ethoxalyl)phenylenediamine in basic medium in the presence of iron(III) ions leading to formation of a cyclic oxamide ligand (L). The combination of intramolecular (covalent) interactions between metal and ligand and intermolecular (noncovalent) ionic interactions creates a novel layered compound with an intriguing crystal structure (shown on the right).

A study was conducted to form a unified hypothesis regarding the gonadotropin-related mechanisms that underlie alterations in the male reproductive system in individuals with diabetes. Streptozotocin-induced diabetes resulted in reduced fertility, prolificacy, and libido. Testes showed a marked decrease in the number and function of Leydig cells, the latter manifested as changes in the expression of biochemical markers, including the GLUT-3 hexose transporter, c-kit, insulin-like growth factor I (IGF-I), androgen receptors, and overall tyrosine phosphorylation, as assessed by Western blot and immunocytochemical analyses. The expression of c-kit, IGF-I, insulin, and follicle-stimulating hormone (FSH) receptors in the seminiferous tubules was also affected. Serum levels of luteinizing hormone (LH), FSH, and testosterone significantly decreased. There was a significant (P

[EN] A new manganese(III)-1,10-phenanthroline-5,6-dione (phendione) complex possessing a putative Mn-2(mu-O) (mu-O2CMe)(2) core has been found to undergo a Ligand-based oxidative cleavage of the C(5)-C(6) bond in weak acid aqueous MeOH under aerobic conditions at room temperature to yield 2,2'-bipyridyl-3,3'-dicarboxylate with co-reduction to the corresponding Mn-II-phendione species., This work was supported by the DGICYT, Ministerio de Educación y Ciencia (Spain) through projects PB97-1397 and PB97-1411. R.R. thanks the 3MD-EU Network (contract no. ERB 4061, PL 97-0197) for a grant. We would also like to express our gratitude to Prof. A. Dei for fruitful discussions and continuous interest in this work.

[EN] Compounds [Fe(tzpy)3](BF4)2 (1), [Fe(tzpy)2(NCS)2]·S (S = 2CHCl3 (2), H2O (3)), and [Fe(tzpy)2(NCSe)2] (4) (tzpy is 3-(2-pyridyl)[1,2,3]triazolo[1,5-a]pyridine) have been synthesized and characterized. 1 crystallizes in the monoclinic noncentrosymmetric system, Cc space group, Z = 4, with a = 11.4680(6) Å, b = 27.449(2) Å, c = 12.4510(8) Å, ß = 108.860(5)°, V = 3709.0(4) Å3, and T = 293(2) K. The structure consists of mononuclear [Fe(tzpy)3]2+ diamagnetic species, which stack via ¿-interactions. Disordered BF4- anions fill the voids generated by complex cations. 2 crystallizes in the triclinic system, P1¿ space group, Z = 1, with a = 8.3340(4) Å, b = 8.6520(4) Å, c = 11.6890(6) Å, ¿ = 89.113(2)°, ß = 81.612(2)°, ¿ = 77.803(2)°, V = 814.90(7) Å3, and T = 293(2) K. The structure consists of mononuclear [Fe(tzpy)2(NCS)2] neutral species, which interact each other via ¿-staking defining layers separated by two-dimensional arrays of CHCl3. The average Fe¿N bond distance, 2.176(3) Å, corresponds to what is expected for an iron(II) ion in the high-spin state. Compounds 2¿4 undergo thermal-driven spin conversion. The regular solution model was applied to account for the corresponding to thermodynamic parameters. The intermolecular interaction parameter, the characteristic temperature, and the enthalpy and entropy changes associated with the spin conversion were estimated as ¿ = 0.86 (2), 0.89 (3), and 3.79 (4) kJ mol-1, T1/2 = 75 (2), 118 (3), and 251 K (4), ¿H = 3.67 (2) and 4.08 (3) kJ mol-1, and ¿S = 34 (2) and 34.5 (3) J K-1 mol-1. ¿H = 8.75 kJ mol-1 and ¿S = 34.8 J K-1 mol-1 were estimated from calorimetric measurements and used as fixed parameters for 4. A quantitative light-induced excited spin state trapping (LIESST) effect was observed for 3, and the high-spin to low-spin relaxation was studied in the temperature region 20¿63 K., This work was funded by the European Commission through the TMR-Network Thermal and Optical Switching of Spin States (TOSS) , Contract No. ERB-FMRX-CT98-0199EEC/TMR, and the Ministerio Español de Ciencia y Tecnología (Projects BQU 2001-2928 and PB98-1422).