Two-Step Spin Conversion for the Three-Dimensional Compound Tris(4,4‘-bis-1,2,4-triazole)iron(II) Diperchlorate

Abstract

The title compound, [Fe(btr)(3)](ClO(4))(2), has been synthesized. The investigation of its magnetic properties has revealed a low-spin <--> high-spin conversion occurring in two steps, each step involving 50% of the Fe(2+) ions. The low-temperature step is very abrupt and occurs with a thermal hysteresis whose width is about 3 K around T(1) = 184 K. The high-temperature step, centered around T(2) = 222 K, is rather gradual. Differential scanning calorimetric measurements have confirmed the occurrence of a two-step spin conversion. The enthalpy and entropy variations associated with the two steps have been found as DeltaH(1) = 5.7 kJ mol(-)(1) and DeltaS(1) = 30.1 J mol(-)(1) K(-)(1), and DeltaH(2) = 6.5 kJ mol(-)(1) and DeltaS(2) = 28.6 J mol(-)(1) K(-)(1), respectively. The crystal structure of [Fe(btr)(3)](ClO(4))(2) has been solved at three temperatures, namely, above the high-temperature step (260 K), between the two steps (190 K), and below the low-temperature step (150 K). The compound crystallizes in the trigonal system, space group R&thremacr;, at the three temperatures. The structure is three-dimensional. There are two Fe(2+) sites, denoted Fe1 and Fe2. Each of them is located on a 3-fold symmetry axis and an inversion center and is surrounded by six btr ligands through the nitrogen atoms occupying the 1- or 1'-positions. Each btr ligand bridges an Fe1 and an Fe2 site, with an Fe1-Fe2 separation of 8.67 Å at 260 K. The perchlorate anions are located in the voids of the three-dimensional architecture and are hydrogen bonded to the triazole rings of the btr ligands. These anions do not interact with the Fe1 and Fe2 sites exactly in the same way. At 260 K, both the Fe1 and Fe2 sites are high-spin (HS) with Fe-N bond lengths of 2.161(3) and 2.164(3) Å, respectively. At 190 K, the Fe1 site remains HS while the Fe2 site is low-spin (LS) with Fe-N bond lengths of 2.007(3) Å. Finally, at 150 K, both the Fe1 and Fe2 sites are LS with Fe-N bond lengths of 1.987(5) and 1.994(5) Å, respectively. It turns out that the two-step spin conversion is associated with the presence of two slightly different Fe(2+) sites. The spin conversion regime has also been followed by Mössbauer spectroscopy. These findings have been discussed and compared to the previously reported cases of two-step spin conversions.

Thermal‐ and Photoinduced Spin‐State Switching in an Unprecedented Three‐Dimensional Bimetallic Coordination Polymer

“…In this particular case, the reason for the observation of the two steps is the occurrence of two crystallographically inequivalent iron(ii) sites with slightly different ligand field strengths and, consequently, different values of DH and DS and critical temperatures. [28] So, in these cases a variety of behavior has been seen, but these examples demonstrate that there is precedence for ordering and the examples like those discussed by Bürgi and co-workers show how an inability to find evidence of ordering may be due to limitations of the experiment rather than an absence of order.…”

Coordination Polymers and Metal Organic Frameworks Derived from 1,2,4-Triazole Amino Acid Linkers

“…In 2, the carboxylic group is kept away from the coordination sphere unlike in 3 thus demonstrating role of synthetic method in directing network construction through self-assembly. This is a crucial point for the design of 1D coordination polymer involving switchable Fe II spin crossover (SCO) centers with a MN 6 core [15][16][17][18]. A similar situation is also noticed in 1.…”

“…The family of thermochromic 1D iron(II) 1,2,4-triazole spin crossover (SCO) coordination polymers have attracted our attention in recent years due to their promising prospects in the thriving field of molecular electronics [15][16][17][18]. Such triazole based precursors not only provide appropriate ligand field strength but also favors the formation of CPs due to its dominant N1,N2 bridging mode ending up in high dimensional networks which are recognized for their ability to efficiently propagate elastic cooperative effects associated to the SCO event [116].…”

“…Among all, particularly interesting are hydrogen storage applications in energy conservative technologies and clean energy. Recent scenario with subsequent generation of MOFs have rationalized with applications extending to technologically relevant domains like non linear optics (NLO) materials [14], spin crossover materials [15][16][17][18], porous magnets [12,[19][20][21][22], and for medical applications [23][24][25] like drug deliveries, implantable devices, diagnostic sensors, and imaging sensors.…”

Pressure-induced two-step spin transition with structural symmetry breaking: X-ray diffraction, magnetic, and Raman studies

“…Intriguing two-step thermal high-spin → low-spin transitions in which the temperature dependence of the order parameter, n HS (T)-the high-spin fraction-displays a plateau at values of temperature for which 0 < n HS < 1 have been reported for a number of mononuclear 1 and multinuclear spin crossover (SCO) molecular complexes. [2][3][4][5][6][7][8] Mono-or multinuclear systems that have more than one crystallographically distinct SCO site prior to a stepped transition may be merely interpreted in terms of the individual sites having different chemical environments and/or intermolecular interactions and hence different ligand field and/or interaction parameters.…”