Cooperativity and Metal–Linker Dynamics in Spin Crossover Framework Fe(1,2,3-triazolate)₂

Abstract

Cooperative interactions are responsible for the useful properties of spin crossover (SCO) materialslarge hysteresis windows, critical temperatures near room temperature, and abrupt transitionswith hybrid framework materials exhibiting the greatest cooperativity and hysteresis of all SCO systems. However, little is known about the chemical origin of cooperativity in frameworks. Here, we present a combined experimental−computational approach for identifying the origin of cooperativity in the metal−organic framework (MOF) Fe(1,2,3-triazolate) 2 (Fe(TA) 2 ), which exhibits the largest known hysteresis window of all SCO materials and unusually high transition temperatures, as a roadmap for understanding the manipulation of SCO behavior in general. Variable-temperature vibrational spectroscopy provides evidence that "soft modes" associated with dynamic metal−linker bonding trigger the cooperative SCO transition. Thermodynamic analysis also confirms a cooperativity magnitude much larger than those of other SCO systems, while electron density calculations of Fe(TA) 2 support previous theoretical predictions that large cooperativity arises in materials where SCO produces considerable differences in metal−ligand bond polarities between different spin states. Taken together, this combined experimental−computational study provides a microscopic basis for understanding cooperative magnetism and highlights the important role of dynamic bonding in the functional behavior of framework materials.

Guest-dependent bond flexibility in UiO-66, a “stable” MOF

“…Consistent with UiO-66 being chemically robust, it possesses larger flexibility constants and larger endothermic enthalpy barriers than reported previously for CuBTC or M(1,2,3-triazolate) 2 MOFs. 7,8 And yet, a ln X of 1.84 is lower than stability constants reported for coordination polymers, including metal–carbene and metal–pyridyl materials known to exhibit self-healing behavior. 20,21 As reference, a table of stability constants for these materials is provided in the ESI,† Table S4).…”

Giant Redox Entropy in the Intercalation Chemistry of MOF Nanocrystals with Confined Pores

“…S16), suggesting that higher temperatures suppress the intercalation of BF4anions into the Fe(TA)2 nanopores. A possible contributing factor could be that enhanced metal-ligand dynamic bonding at elevated temperatures 46,47 might induce additional phonon scattering that interferes with BF4intercalation.…”

105 K Wide Room Temperature Spin Transition Memory Due to a Supramolecular Latch Mechanism

“… 17 A detailed investigation of this unique compound attributed the origin of the behavior to the combination of a rigid lattice and ionic polarizable bonds due to the bridging triazolate ligands. 22 For less symmetric discrete Fe II compounds with meridional tridentate ligands exhibiting reproducible hysteresis exceeding 100 K 16 , 23 , 24 ( Scheme 1 ), the underlying structural mechanisms of the bistability remain unknown. Namely, the exceptional chemical variability of the tridentate ligands offers attractive prospects for tuning the transition temperature to the desired operating range.…”

“…The only fully structurally characterized compound with the hysteresis >100 K is a 3D polymeric complex [Fe II (1,2,3-triazolate) 2 ] 0 (Δ T h = 110 K), which undergoes ST with an outstandingly large and highly symmetric variation of the lattice (cubic → cubic) . A detailed investigation of this unique compound attributed the origin of the behavior to the combination of a rigid lattice and ionic polarizable bonds due to the bridging triazolate ligands . For less symmetric discrete Fe II compounds with meridional tridentate ligands exhibiting reproducible hysteresis exceeding 100 K ,, (Scheme ), the underlying structural mechanisms of the bistability remain unknown.…”