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

Abstract

One-dimensional (1D) coordination polymers of formula [Fe(NH(2)trz)(3)]A.nH(2)O, {A = TiF(6)(2-), n = 0.5 (1) and n = 1 (2); A = ZrF(6)(2-), n = 0.5 (3) and n = 0 (4); A = SnF(6)(2-), n = 0.5 (5) and n = 1 (6); A = TaF(7)(2-), n = 3 (7) and n = 2.5 (8); A = GeF(6)(2-), 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 (57)Fe 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 ZrF(6)(2-) counteranion and the 1D chains, an organization which is also revealed in [Cu(NH(2)trz)(3)]ZrF(6).H(2)O (11). Trinuclear spin crossover compounds of formula [Fe(3)(NH(2)trz)(10)(H(2)O)(2)](SbF(6))(6).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 <or= V (nm(3)) <or= 0.09) and for divalent anions (above V >or= 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.

Control of Spin-Crossover Phenomena in One-Dimensional Triazole-Coordinated Iron(II) Complexes by Means of Functional Counter Ions

“…To our knowledge, this is the first system with systematic control of the Fe-Fe interaction in the 1D triazole-coordinated iron(II) SCO complexes. Moreover, there is a noticeable difference between this and other studies that investigated the dependence of SCO behavior by spherical counter anions, as described in numerous previous reports and our compounds [24,25,[37][38][39][41][42][43]45,[52][53][54][55][56]. The relationship between ∆T 1/2 and the alkyl chain length is shown in Figure 18b.…”

“…Moreover, Garcia et al reported on the correlation between T 1/2 and the volume of the counter anion in the series of fluorinated inorganic divalent anions (TiF 6 2− , ZrF 6 2− , SnF 6 2− , GeF 6 2− , and TaF 7 2− ) salts of [Fe(II)(NH 2 -trz) 3 ] complexes, in addition to the related monovalent complexes [43].…”

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.…”