Two-step spin transition in a 1D FeII 1,2,4-triazole chain compound

A thermochromic 1D spin crossover coordination (SCO) polymer [Fe(βAlatrz)3](BF4)2·2H2O (1·2H2O), whose precursor βAlatrz, (1,2,4-triazol-4-yl-propionate) has been tailored from a β-amino acid ester is investigated in detail by a set of superconducting quantum interference device (SQUID), 57Fe 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 FeII 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 1·2H2O 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 1·2H2O. Vibrational spectra of 1·2H2O reveal that the BF4 - anions and water molecules play no significant role on the vibrational properties of the [Fe(βAlatrz)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(βAlatrz)3](BF4)2 (1) reveals indeed a significantly different magnetic behavior with a one-step SCO which was also investigated. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim