Co-crystallisation as a modular approach to the discovery of spin-crossover materials

Abstract

Herein we present co-crystallisation as a strategy for materials discovery in the field of switchable spin crossover (SCO) systems. Using [Fe(3-bpp)2]·2A (where 3-bpp = 2,6-bis(pyrazol-3-yl)pyridine, A = BF4- / PF6-)...

Supramolecular Tuning of Spin Crossover Properties in Isostructural Cocrystal Solvates

“…Furthermore, polarization-enhanced hydrogen bonding may also explain properties other than transition temperature such as cooperativity as described recently. 37 The effect of the anions on the transition temperature in this series of materials is opposite to what was observed in our previous work where the PF 6 30 This demonstrates that the effect of the anion is also dependent on its position and role within the crystal structure, i.e., whether it is participating in strong directional intermolecular interactions such as hydrogen bonds or whether it is participating in weaker interactions within a void. We are actively investigating the effects of the anion in cases where it does not participate in strong intermolecular interactions to further improve our understanding and provide us with additional design strategies.…”

“…However, in the crystallization vials from both solvents, there were also small, yellow crystals present that were found to be the 1b•dpds cocrystals, which we have discussed in our previous work. 30 Clearly then, it can be seen that using the PF 6 − counterion instead of BF 4 − pushes the limits of the supramolecular blueprint and affects the balance of structure directing interactions such that both the 1b•dpds and 1b•dpds•MeOH/1b•dpds•EtOH structures are formed, rather than just the cocrystal solvate as seen in the BF 4 − analogues. However, it is not clear whether this is due to the larger size or the weaker interactions of the PF 6…”

“…Cocrystallization typically involves the crystallization of a molecule of interest together with a coformer . It has been most widely used in the pharmaceutical industry where the molecule of interest is an active pharmaceutical ingredient (API), although it may also be an SCO complex. − A coformer is normally chosen primarily based on predictable intermolecular interactions (sometimes referred to as supramolecular synthons) with the molecule of interest, but there are also other reasons a specific coformer may be chosen, such as its size, conformation, functionality, etc . By introducing a coformer into an SCO system, the structure directing effects of the coformer can potentially act to “buffer” the effects of changing other components such as the anion or solvent.…”

“…24,32 We have recently reported on the use of cocrystallization to modify the structures and properties of SCO materials, namely, the BF 4 − and PF 6 − salts of the [Fe(3-bpp) 2 ] 2+ complex. 30 The work involved the use of ditopic dipyridyl coformers such as bipy (4,4′-dipyridine) to form supramolecular architectures such as 1D chains, 2D sheets, and 3D networks. In these cases, the N−H hydrogen bond donors on the 3-bpp ligands were all satisfied by the pyridine-based acceptors on the coformers.…”

Regulating Spin-State Switching by Integrating Polyoxometalate Anion into Spin Crossover System

“…The development of sensors and memory devices has motivated the study of bistable magnetic materials with spin-state switching capabilities. − Spin-crossover (SCO) complexes have been well studied as classic spin-state switchable molecules, wherein the 3d 4 –3d 7 transition metal ions can be reversibly tuned between high-spin (HS) and low-spin (LS) electronic states in response to external stimuli, such as temperature, pressure, light, magnetic field, etc. − As of today, numerous ligand-programmable, guest-tunable, and solvent-induced SCO complexes with characteristic behaviors have been investigated. − It is well-known that when the SCO-active fragment is positively charged, the associated counter-anions are imperative to isolate the complexes, though they play much wider roles beyond maintaining electric neutrality. − Particularly, the counter-anions may function as versatile templates in regulating structural packing modes and SCO behaviors. For example, the conformational change of N­(SO 2 CF 3 ) 2 – drives an abrupt SCO with wide hysteresis near room temperature .…”