Prediction of the Spin Transition Temperature in FeII One-Dimensional Coordination Polymers: an Anion Based Database

One-dimensional (1D) coordination polymers of formula [Fe(NH(2)trz)(3)]A center dot nH(2)O, {A = TiF62-, n = 0.5 (1) and n = 1 (2); A=ZrF62-, n = 0.5(3) and n = 0 (4); A = SnF62-, n = 0.5 (5) and n = 1 (6); A = TaF72-, n = 3 (7) and n = 2.5 (8); A = GeF62-, n = 1 (9) and n = 0.5 (10), NH(2)trz = 4-amino-1,2,4-triazole} have been synthesized, fully characterized, and their spin crossover behavior carefully studied by SQUID magnetometry, Mossbauer spectroscopy, and differential scanning calorimetry. These materials display an abrupt and hysteretic spin transition around 200 K on cooling, as well as a reversible thermochromic effect. Accurate spin transition curves were derived by Fe-57 Mossbauer spectroscopy considering the corrected f factors for the high-spin and low-spin states determined employing the Debye model. The unusual hysteresis width of 3 (28 K), was attributed to a dense hydrogen bonding network involving the ZrF62- counteranion and the 1D chains, an organization which is also revealed in [Cu(NH(2)trz)(3)]ZrF6 center dot H2O (11). Trinuclear spin crossover compounds of formula [Fe-3(NH(2)trz)(10)(H2O)(2)](SbF6)(6)center dot S {S = 1.5CH(3)OH (12), 0.5C(2)H(5)OH (13)} were also obtained. A structural property relationship was derived between the volume of the inserted counteranion and the transition temperature T-1/2 of the 1D chains. Two linear size regimes were identified for monovalent anions (0.04 <= V (nm(3)) <= 0.09) and for divalent anions (above V >= 0.11 nm(3)) with saturation around T-1/2 = 200 K. These characteristics allowed us to derive an anion based database that is of interest for the prediction of the transition temperature of such functional switchable materials. Diffuse reflectivity measurements under hydrostatic pressure for 3,4 combined with calorimetric data allow an estimation of the electrostatic pressure between cationic chains and counteranions in the crystal lattice of these materials. The chain length distribution that ranges between 1 and 4 nm was also derived.