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Thermal- and Pressure-Induced Cooperative Spin Transition in the 2D and 3D Coordination Polymers {Fe(5-Br-pmd)z[M(CN)x]y} (M = AgI, AuI, NiII, PdII, PtII)

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journal contribution
posted on 12.11.2007 by Gloria Agustí, Ana Belén Gaspar, M. Carmen Muñoz, José Antonio Real
A new family of cyanide-based spin-crossover polymers with the general formula {Fe(5-Br-pmd)z[M(CN)x]y} [M = AgI (1), AuI (2), NiII (3), PdII (4), PtII (5); 5-Br-pmd = 5-bromopyrimidine; z = 1 or 2, x = 2 or 4, and y = 2 or 1] have been synthesized and characterized using single-crystal X-ray diffraction (XRD), X-ray powder diffraction (XRPD), magnetic susceptibility measurements, and differential scanning calorimetry (DSC). At 293 K, compound 1 presents the monoclinic space group C2/c, whereas at 120 K, it changes to the monoclinic space group P21/c. At 293 K, the crystal structure of 1 displays an uninodal three-dimensional network whose nodes, constituted of FeII, lie at the inversion center of an elongated octahedron. The equatorial bond lengths are defined by the N atoms of four [AgI(CN)2]- groups belonging to two crystallographically nonequivalent AgI atoms, Ag(1) and Ag(2). They are shorter than those of the axial positions occupied by the N atoms of the 5-Br-pmd ligands. The Fe−N average bond length of 2.1657(7) Å is consistent with a high-spin (HS) state for the FeII ions. At 120 K, the crystal structure changes refer mainly to the FeII environment. There are two crystallographically independent FeII ions at this temperature, Fe(1) and Fe(2), which adopt the HS and low-spin (LS) states, respectively. The average Fe−N bond length for Fe(1) [2.174(5) Å] and Fe(2) [1.955(5) Å] agrees well with the reported magnetic data at this temperature. The spin transition of the FeII ions labeled as Fe(1) is found to be centered at Tc = 149 K and Tc = 167 K and accompanied by a drastic change of color from orange (HS) to red (LS). Magnetic susceptibility measurements under applied hydrostatic pressure performed on 1 have shown a linear displacement of the transition to higher temperatures while the hysteresis width remains unaltered in the interval of pressures of 105 Pa to 0.34 GPa. A further increase of the pressure induces the spin transition in the Fe(2) ions, which is completely accomplished at 1.12 GPa (T1/2 = 162 K). Compounds 1 and 2 are isostructural, but 2 does not exhibit spin-transition properties; the FeII centers remain in the HS state in the temperature range investigated, 5−300 K. Compounds 35 are not similar or isostructural with 1. A two-dimensional structure for 35 has been proposed on the basis of analytical data and the XRPD patterns. Compounds 35 undergo first-order spin transition where the critical temperatures for the cooling (Tc) and warming (Tc) modes are 170 and 180 K (3), 204 and 214 K (4), and 197 and 223 K (5), respectively. It is worth mentioning the color change from yellow to orange observed in 35 upon spin transition. The thermodynamic parameters associated with the spin transition estimated from DSC measurements are ΔH = 6 kJ mol-1 (1), 11 kJ mol-1 (3), 16 kJ mol-1 (4), and 16 kJ mol-1 (5) and ΔS = 38 J K-1 mol-1 (1), 62 J K-1 mol-1 (3), 76 J K-1 mol-1 (4), and 81 J K-1 mol-1 (5).