Rapid Cooling Experiments and Use of an Anionic Nuclear Probe to Sense the Spin Transition of the 1D Coordination Polymers [Fe(NH₂trz)₃]SnF₆⋅n H₂O (NH₂trz=4‐amino‐1,2,4‐triazole)

Abstract

[Fe(NH2trz)3]SnF6n x H2O (NH(2)trz=4-amino-1,2,4-triazole; n=1 (1), n=0.5 (2)) are new 1D spin-crossover coordination polymers. Compound 2 exhibits an incomplete spin transition centred at around 210 K with a thermal hysteresis loop approximately 16 K wide. The spin transition of 2 was detected by the Mössbauer resonance of the 119Sn atom in the SnF6 (2-) anion primarily on the basis of the evolution of its local distortion. Rapid-cooling 57Fe Mössbauer and superconducting quantum interference device experiments allow dramatic widening of the hysteresis width of 2 from 16 K up to 82 K and also shift the spin-transition curve into the room temperature region. This unusual behaviour of quenched samples on warming is attributed to activation of the molecular motion of the anions from a frozen distorted form towards a regular form at temperatures well above approximately 210 K. Potential applications of this new family of materials are discussed.

Complete and incomplete spin transitions in 1D chain iron(ii) compounds

“…In the case of rigid linkers the ST behaviour depends on the intermolecular interactions (hydrogen bonds, p-stacking, van der Waals interactions) that are discussed to be suitable for transmitting cooperative interactions. This was recently demonstrated for 4,4 0 -bipyridine linked SCO complexes, where either gradual 7 or abrupt ST with 18 K wide thermal hysteresis loops 8 were obtained. A similar observation was made for triply 1,2,4-triazole bridged iron(II) complexes.…”

“…4 The origin of hysteresis loops in ST materials and their thermal width as well as the reason for stepwise or incomplete spin transitions are not yet fully understood. In the case of 1D chain SCO compounds, bridges with flexible linkers (triply bis-tetrazole bridges with flexible spacers 5 or flexible single bridges as 1,2-bis(4-pyridyl)ethane 6 ) so far resulted in gradual ST.…”

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.…”