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Three polymorphic modifications A-C of [Fe(II)(DPPA)(NCS)(2)], where DPPA = (3-aminopropyl)bis(2-pyridylmethyl)amine is a new tetradentate ligand, have been synthesized, and their structures, magnetic properties, and Mössbauer spectra have been investigated. For polymorph A, variable-temperature magnetic susceptibility measurements as well as Mössbauer spectroscopy have revealed the occurrence of a rather gradual HS if LS transition without hysteresis, centered at about 176 K. The same methods have shown that polymorph B is paramagnetic over the temperature range 4.5-295 K, whereas polymorph C exhibits a very abrupt S = 2 if S = 0 transition with a hysteresis. The hysteresis width is 8 K, the transitions being centered at T(c) downward arrow = 112 K for decreasing and T(c) upward arrow = 120 K for increasing temperatures. The crystal structures of the three polymorphs have been solved by X-ray diffraction at 298 K. Polymorph A is triclinic, space group P&onemacr; with Z = 2, a = 8.710(2) Å, b = 15.645(2) Å, c = 7.985(1) Å, alpha = 101.57(1) degrees, beta = 112.59(2) degrees, and gamma = 82.68(2) degrees. Polymorph B is monoclinic, space group P2(1)/c with Z = 4, a = 8.936(2) Å, b = 16.855(4) Å, c = 13.645(3) Å, and beta = 97.78(2) degrees. Polymorph C is orthorhombic, space group Pbca with Z = 8, a = 8.449(2) Å, b = 14.239(2) Å, and c = 33.463(5) Å. In the three polymorphs, the asymmetric units are almost identical and consist of one chiral complex molecule with the same configuration and conformation. The distorted [FeN(6)] octahedron is formed by four nitrogen atoms belonging to DPPA and two provided by the cis thiocyanate groups. The two pyridine rings of DPPA are in fac positions. The main differences between the structures of the three polymorphs are found in their crystal packing. The stabilization of the high-spin ground state of polymorph B is tentatively explained by the presence of two centers of steric strain in the crystal lattice resulting in the elongation of the Fe-N(aromatic) distance. The observed hysteresis in polymorph C seems to be due to the existence of an array of intermolecular contacts in the crystal lattice making the spin transition more cooperative than in polymorph A.

The synthesis and detailed characterization of the new spin crossover mononuclear complex [Fe(II)(DAPP)(abpt)](ClO(4))(2), where DAPP = [bis(3-aminopropyl)(2-pyridylmethyl)amine] and abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, are reported. Variable-temperature magnetic susceptibility measurements and Mössbauer spectroscopy have revealed the occurrence of an abrupt spin transition with a hysteresis loop. The hysteresis width derived from magnetic susceptibility measurements is 10 K, the transition being centered at T(c) downward arrow = 171 K for decreasing and T(c) upward arrow = 181 K for increasing temperatures. The crystal structure was resolved in the high-spin (293 and 183 K) and low-spin (123 K) states. Both spin-state structures belong to the monoclinic space group P2(1)/n (Z = 4). The thermal spin transition is accompanied by the shortening of the mean Fe-N distances by 0.177 A. The two main structural characteristics of [Fe(DAPP)(abpt)](ClO(4))(2) are a branched network of intermolecular links in the crystal lattice and the occurrence of two types of order-disorder transitions (in the DAPP ligand and in the perchlorate anions) accompanying the thermal spin change. These features are discussed relative to the magnetic properties of the complex. The electronic structure calculations show that the structural disorder in the DAPP ligand modulates the energy gap between the HS and LS states. In line with previous studies, the order-disorder phenomena and the spin transition in [Fe(DAPP)(abpt)](ClO(4))(2) are found to be interrelated.

The new spin transition compound [Fe(II)(DPEA)(NCS)(2)], where DPEA [(2-aminoethyl)bis(2-pyridylmethyl)amine] is a new tetradentate ligand, has been synthesized, and its structure, magnetic properties, and Mössbauer spectra have been investigated. The crystal structure has been determined by X-ray diffraction at 298 K. The compound crystallizes in the monoclinic system, space group is P2(1)/c, with Z = 4,a = 9.358(1) Å, b = 11.812(2) Å, c = 17.135(2) Å, and beta = 94.5(4) degrees. The distorted [FeN(6)] octahedron is formed from four nitrogen atoms belonging to DPEA and two provided by the cis thiocyanate groups. The two pyridine rings of DPEA are in mer positions. Each molecule is linked to its neighbors by hydrogen-bonding interactions as well as by numerous van der Waals contacts supposed to be responsible for the cooperativity of the system. Variable-temperature magnetic susceptibility measurements (20-290 K) have evidenced a relatively abrupt S = 2 right harpoon over left harpoon S = 0 transition centered at T(1/2) = 138 K. The thermal variation of the high spin state fraction observed by Mössbauer spectroscopy is in agreement with that obtained from magnetic susceptibility measurements. The fitting of Mössbauer and magnetic data with the Ising-like model allowed us to determine the energy gap between the high-spin and low-spin states (Delta(eff) = 835 K) and to estimate the variation of the thermodynamic parameters upon spin transition. The calculated variations of enthalpy (DeltaH = 6.76 kJ mol(-)(1)) and entropy (DeltaS = 49 J mol(-)(1) K(-)(1)) associated with the spin transition are in agreement with those previously observed for iron(II) spin-crossover compounds. The spin conversion is found to be close to a first-order phenomenon.

Keywords: Iron / N ligands / Spin crossover / Mössbauer spectroscopy / PhotomagnetismThe synthesis and characterization of the new spin crossover mononuclear complex [Fe II (DPEA)(bim)](ClO 4 ) 2 ·0.5 H 2 O, where DPEA = (2-aminoethyl)bis(2-pyridylmethyl)amine and bim = 2,2-bisimidazole, are reported. Variable-temperature magnetic susceptibility measurements (77−295 K) reveal the occurrence of a two-step spin transition. Two steps on the magnetic curve are separated by an inflection point at 200 K, corresponding to about 50% of the complexes that have undergone a thermal spin transition. The first step is centered at 171 K and the second one at 218 K. Mössbauer spectroscopy and X-ray analysis show that the profile of the magnetic curve is a consequence of the presence of two inequivalent iron(II) molecules in the crystal lattice. The crystal structure was resolved at 293 K (high-spin form) and at 123 K (low-spin form). Both spin-state isomers belong to the mono-

Abstract

We report on the synthesis and characterization of a first iron(II) spin-crossover coordination polymer of formula [Fe(pyim) 2 (bpy)](ClO 4 ) 2 ‚2C 2 H 5 OH, where pyim ) 2-(2-pyridyl)imidazole and bpy ) 4,4′-bipyridine, with zigzag chain structure. The chains are linked by π stacking into a two-dimensional network. Variable-temperature magnetic susceptibility measurements have revealed the occurrence of the HS T LS transition without hysteresis. The spin conversion temperature T 1/2 (temperature for which the HS fraction is equal to 0.5) is found equal to 205 K. The crystal structure was resolved at 293 K (high-spin form) and at 173 K (low-spin form). Both spin-state structures belong to the monoclinic space group C2/c (Z ) 4). The complex assumes one-dimensional zigzag chain structure, in which the iron(II) active sites are linked to each other by a chemical bridge as the rigid rodlike 4,4′-bipyridine molecule. The Fe-Fe separation via the bpy ligand equals 11.523(2) and 11.201(2) Å in the high-spin and low-spin structures, respectively. The polymeric chains extended along the c axis are stacked in the b direction forming two-dimensional sheets of bound molecules. The sheets are linked together by several intermolecular H-bonds in the a direction. The ensemble of structural features of the complex, potentially leading to the cooperativity, provides a basis for strong interactions. However, they are not strong enough in comparison with the HS-LS energy gap and result in a relatively gradual spin transition of the complex. The structural features and magnetic properties of the complex have been discussed and compared with those for earlier reported iron(II) spin-crossover coordination polymers.