Two‐Step Spin Transition in a 1D Fe^II 1,2,4‐Triazole Chain Compound

Abstract

A thermochromic 1D spin crossover coordination (SCO) polymer [Fe(βAlatrz)3](BF4)2⋅2 H2O (1⋅2 H2O), 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), (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(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.

Elastic Frustration in 1D Spin-Crossover Chains: Evidence of Multi-Step Transitions and Self-Organizations of the Spin States

“…Moreover, quite recently, some materials have been found to exhibit several steps 21,[24][25] at the transition, among them the case of the "Devil's Staircase" 22 is a very instructive example of the complexity generated by elastic interactions inside the SCO materials. Last but not least, several two-step transitions have also reported in 1D SCO materials [26][27][28] REF, which confirms the general character of elastic frustration in these molecular systems.…”

Elastic Modeling of Two-Step Transitions in Sterically Frustrated 1D Binuclear Spin-Crossover Chains

“…Among these was the famous "Devil's Staircase" [50], in which the number of steps was also enhanced by the presence of inequivalent iron centers. Interestingly, similar behaviors have been also observed in one-dimensional SCO solids [54][55][56], which means that the elastic frustration from which these phenomena originate presents a common trend.…”

Spin state switching of metal complexes by visible light or hard X-rays

“…103 Interestingly, chain length distributions were also taken to be responsible for the two step character of the thermally induced spin transition of [Fe(βAlatrz) 3 ](BF 4 ) 2 ·2H 2 O (βAlatrz = 1,2,4-triazol-4-yl-propionate), the first of its kind for a 1D iron(II) chain with 1,2,4-triazole ligands. 104 The topic was recently reviewed with the focus centred on the LIESST effect on 1D iron(II) 1,2,4-triazole SCO chains. 105 We highlight below some examples of multinuclear SCO systems.…”