Insights into the Origin of Cooperative Effects in the Spin Transition of [Fe(NH2trz)3](NO3)2: the Role of Supramolecular Interactions Evidenced in the Crystal Structure of [Cu(NH2trz)3](NO3)2•H2O

The thermally induced hysteretic spin transition (ST) that occurs in the polymeric chain compound [Fe(NH(2)trz)(3)](NO3)(2) (1) above room temperature (T-c(sic) = 347 K, T-c(sic) = 314 K) has been tracked by Fe-57 Mossbauer spectroscopy, SQUID magnetometry, differential scanning calorimetry (DSC), and X-ray powder diffraction (XPRD) at variable temperatures. From the XRPD pattern indexation, an orthorhombic primitive cell was observed with the following cell parameters: a = 11.83(2) angstrom, b = 9.72(1) angstrom, c = 6.361(9) angstrom at 298 K (low-spin state) and a = 14.37(2) angstrom, b = 9.61(4) angstrom, c = 6.76(4) angstrom at 380 K (high-spin state). The enthalpy and entropy variation associated to the ST of 1, have been evaluated by DSC as Delta H = 23(1) kJ mol(-1) and Delta S = 69.6(1) J mol(-1) K-1. These thermodynamic data were used within a two-level Ising like model for the statistical analysis of First Order Reversal Curve (FORC) diagram that was recorded for 1, in the cooling mode. Strong intramolecular cooperative effects are witnessed by the derived interaction parameter of J = 496 K. The crystal structure of [Cu(NH(2)trz)(3)](NO3)(2) center dot H2O (2) was obtained thanks to high quality single crystals prepared by slow evaporation after hydrothermal pretreatment. The catena poly[mu-tris(4-amino-1,2,4-triazole-N1,N2) copper.(II)] dinitrate monohydrate (2) crystallizes in the monoclinic space group C2/c, with a = 16.635(6) angstrom, b = 13.223(4) angstrom, c = 7.805(3) angstrom, beta = 102.56(3)degrees, Z = 4. Complex 2 is.a 1D infinite chain containing triple N1,N2-1,2,4-triazole bridges with an intra-chain distance of Cu center dot center dot center dot Cu = 3.903(1) angstrom. A dense H-bonding network with the nitrate counteranion involved in intra-chain and inter-chain interactions is observed. Such a supramolecular network could be at the origin of the unusually large hysteresis loop displayed by 1 (Delta T similar to 33 K), as a result of an efficient propagation of elastic interactions through the network. This hypothesis is strengthened by the crystal structure of 2 and by the absence of crystallographic phase transition for 1 over the whole temperature range of investigation as shown by XRPD.