Two-step spin transition in a 1D Fe(II) 1,2,4-triazole chain compound.

A thermochromic 1D spin crossover coordination (SCO) polymer [Fe(ßAlatrz)3](BF4)2⋅2 H2O (1⋅2 H2O), whose precursor ßAlatrz, (1,2,4-triazol-4-yl-propionate) has been tailored from a ß-amino acid ester is investigated in detail by a set of superconducting quantum interference device (SQUID), (57)Fe Mössbauer, differential scanning calorimetry, infrared, and Raman measurements. An hysteretic abrupt two-step spin crossover (T1/2(↓) = 230 K and T1/2(↑) = 235 K, and T1/2(↓) = 172 K and T1/2(↑) = 188 K, respectively) is registered for the first time for a 1,2,4-triazole-based Fe(II) 1D coordination polymer. The two-step SCO configuration is observed in a 1:2 ratio of low-spin/high-spin in the intermediate phase for a 1D chain. The origin of the stepwise transition was attributed to a distribution of chains of different lengths in 1⋅2 H2O after First Order Reversal Curves (FORC) analyses. A detailed DFT analysis allowed us to propose the normal mode assignment of the Raman peaks in the low-spin and high-spin states of 1⋅2 H2O. Vibrational spectra of 1⋅2 H2O reveal that the BF4(-) anions and water molecules play no significant role on the vibrational properties of the [Fe(ßAlatrz)3](2+) polymeric chains, although non-coordinated water molecules have a dramatic influence on the emergence of a step in the spin transition curve. The dehydrated material [Fe(ßAlatrz)3](BF4)2 (1) reveals indeed a significantly different magnetic behavior with a one-step SCO which was also investigated.

Spin transition sensors based on ß-amino-acid 1,2,4-triazole derivative.

A ß-aminoacid ester was successfully derivatized to yield to 4H-1,2-4-triazol-4-yl-propionate (ßAlatrz) which served as a neutral bidentate ligand in the 1D coordination polymer [Fe(ßAlatrz)(3)](CF(3)SO(3))(2)·0.5H(2)O (1·0.5H(2)O). The temperature dependence of the high-spin molar fraction derived from (57)Fe Mossbauer spectroscopy recorded on cooling below room temperature reveals an exceptionally abrupt single step transition between high-spin and low-spin states with a hysteresis loop of width 4 K (T(c) (↑) = 232 K and T(c) (↓) = 228 K) in agreement with magnetic susceptibility measurements. The material presents striking reversible thermochromism from white, at room temperature, to pink on quench cooling to liquid nitrogen, and acts as an alert towards temperature variations. The phase transition is of first order, as determined by differential scanning calorimetry, with transition temperatures matching the ones determined by SQUID and Mössbauer spectroscopy. The freshly prepared sample of 1·0.5H(2)O, dried in air, was subjected to annealing at 390 K, and the obtained white compound [Fe(ßAlatrz)(3)](CF(3)SO(3))(2) (1) was found to exhibit a similar spin transition curve however much temperature was increased by (T(c) (↑) = 252 K and T(c) (↓) = 248 K). The removal of lattice water molecules from 1·0.5H(2)O is not accompanied by a change of the morphology and of the space group, and the chain character is preserved. However, an internal pressure effect stabilizing the low-spin state is evidenced.