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

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• 作者：Gloria Agustí ; Ana Bel&eacute ; n Gaspar ; M. Carmen Muñ ; oz ; Jos&eacute ; Antonio Real
• 刊名：Inorganic Chemistry
• 出版年：2007
• 出版时间：November 12, 2007
• 年：2007
• 卷：46
• 期：23
• 页码：9646 - 9654
• 全文大小：408K
• 年卷期：v.46,no.23(November 12, 2007)
• ISSN：1520-510X

文摘

A new family of cyanide-based spin-crossover polymers with the general formula {Fe(5-Br-pmd)_z[M(CN)_x]_y} [M =Ag^I (1), Au^I (2), Ni^II (3), Pd^II (4), Pt^II (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, compound1 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 ofFe^II, lie at the inversion center of an elongated octahedron. The equatorial bond lengths are defined by the Natoms of four [Ag^I(CN)₂]^- groups belonging to two crystallographically nonequivalent Ag^I 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-Naverage bond length of 2.1657(7) Å is consistent with a high-spin (HS) state for the Fe^II ions. At 120 K, the crystalstructure changes refer mainly to the Fe^II environment. There are two crystallographically independent Fe^II ions atthis temperature, Fe(1) and Fe(2), which adopt the HS and low-spin (LS) states, respectively. The average Fe-Nbond length for Fe(1) [2.174(5) Å] and Fe(2) [1.955(5) Å] agrees well with the reported magnetic data at thistemperature. The spin transition of the Fe^II ions labeled as Fe(1) is found to be centered at T_c = 149 K and T_c= 167 K and accompanied by a drastic change of color from orange (HS) to red (LS). Magnetic susceptibilitymeasurements under applied hydrostatic pressure performed on 1 have shown a linear displacement of the transitionto higher temperatures while the hysteresis width remains unaltered in the interval of pressures of 10⁵ Pa to 0.34GPa. A further increase of the pressure induces the spin transition in the Fe(2) ions, which is completely accomplishedat 1.12 GPa (T_1/2 = 162 K). Compounds 1 and 2 are isostructural, but 2 does not exhibit spin-transition properties;the Fe^II centers remain in the HS state in the temperature range investigated, 5-300 K. Compounds 3-5 are notsimilar or isostructural with 1. A two-dimensional structure for 3-5 has been proposed on the basis of analyticaldata and the XRPD patterns. Compounds 3-5 undergo first-order spin transition where the critical temperaturesfor the cooling (T_c) and warming (T_c) 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 3-5 upon spin transition.The thermodynamic parameters associated with the spin transition estimated from DSC measurements are H =6 kJ mol^-1 (1), 11 kJ mol^-1 (3), 16 kJ mol^-1 (4), and 16 kJ mol^-1 (5) and S = 38 J K^-1 mol^-1 (1), 62 J K^-1mol^-1 (3), 76 J K^-1 mol^-1 (4), and 81 J K^-1 mol^-1 (5).