Thermal- and pressure-induced cooperative spin transition in the 2D and 3D coordination polymers {Fe(5-Br-pmd)z]M(CN)x]y} (M = Ag1, Au1, NiII, PdII, PtII)

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, compound 1 presents the monoclinic space group C2/c, whereas at 120 K, it changes to the monoclinic space group P2(1)/c. At 293 K, the crystal structure of 1 displays an uninodal three-dimensional network whose nodes, constituted of Fe-II, lie at the inversion center of an elongated octahedron. The equatorial bond lengths are defined by the N atoms of four [Ag-I(CN)(2)](-) groups belonging to two crystallographically nonequivalent Ag-I atoms, Ag(l) 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) angstrom is consistent with a high-spin (HS) state for the Fell ions. At 120 K, the crystal structure changes refer mainly to the Fell environment. There are two crystallographically independent Fell 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) angstrom] and Fe(2) [1.955(5) angstrom] agrees well with the reported magnetic data at this temperature. The spin transition of the Fe-II ions labeled as Fe(1) is found to be centered at T-c down arrow= 149 K and T-c up arrow = 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 10(5) 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 (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 not similar or isostructural with 1. A two-dimensional structure for 3-5 has been proposed on the basis of analytical data and the XRPD patterns. Compounds 3-5 undergo first-order spin transition where the critical temperatures for the cooling (T-c down arrow) and warming (T-c up arrow) 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 Delta H = 6 kJ mol(-1) (1), 11 kJ mol-1 (3), 16 kJ mol(-1) (4), and 16 kJ mol(-1) (5) and Delta S = 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).