A Spin-Crossover Molecular Material Describing Four Distinct Thermal Pathways

Abstract

Spin-crossover (SCO) molecular solids are valued switchable materials for their common abrupt and reversible thermal transitions, large thermal hysteresis, or guest-dependent effects. These properties usually involve crystallographic transitions coupled to the SCO events. These phenomena are of great value for the understanding of solid-state transformations and also for exploiting them. We present here a lattice of the complex [FeL(bbp)](ClO) (1; L and bbp are tris-imine ligands) featuring an unprecedented rich succession of SCO and crystallographic phase transformations. Magnetometry measurements unveil a thermally irreversible sequence of spin conversions that delineate four different thermal pathways. All of these are single-crystal-to-single-crystal processes and can thus be monitored by single crystal X-ray diffraction using one unique specimen. Fresh crystals of 1 contain one molecule of acetone per Fe center (1·ac) that abandons the lattice upon warming at the same time that a SCO from an ordered mixed spin state (1:1 high spin/low spin; HS/LS) to a fully HS state, 1, occurs. This crystallographic phase, accessed through a template effect by the solvent, converts into another one, 1, upon cooling, as triggered by a HS to LS SCO. Warming of 1 induces a new SCO (LS to ordered HS/LS) coupled to another crystallographic phase transition, 1 → 1. The fully HS state of 1 can not be reached before decomposition of the compound. Instead, this phase cycles between the HS/LS and the LS states through superimposable pathways, different from that of the prerequired 1 → 1 phase change. Analysis of the thermal variation of the free energy, G, through density functional theory methods provides trends in agreement with the observation of these transformations and clarifies the possible metastable nature of the various phases identified. This unique behavior allows the access to four different magnetic responses depending on the thermal history of the sample, within a given range of temperatures near the ambient conditions.

Thermal spin crossover in Fe(ii) and Fe(iii). Accurate spin state energetics at the solid state

“…18,19 Consequently, it is rather common to find polymorphs or solvatomorphs of the same SCO complex displaying completely different magnetic transitions. [20][21][22][23] Moreover, other solid state phenomena like phase transitions, solvent absorption/desorption or order disorder processes might occur concomitantly to the spin transition, occasionally leading to multiple SCO pathways, 24 reverse SCO, 25 or a spin state blocking along the entire range of temperatures. 26,27 As a result, the evaluation of ∆ needs to account for the energetic impact of such processes on the SCO.…”

Heteroleptic Iron(II) Spin-Crossover Complexes Based on a 2,6-Bis(pyrazol-1-yl)pyridine-type Ligand Functionalized with a Carboxylic Acid

“…This quasi-exclusive formation of heteroleptic adducts has been proven as a prolific source of new compounds with very diverse properties. 17,18 In previous reports, we showed that the solvent-free perchlorate salt of the homoleptic iron(II) complex of the ligand 1bppCOOH (2,6-bis(1H-pyrazol-1-yl)isonicotinic acid, Scheme 1) exhibits a hysteretic (3 K) abrupt thermal spin transition at high temperatures (380 K) affected by the presence of a linear network of hydrogen-bonded complexes. 19 Moreover, it has been proved that the introduction of a carboxylic acid group on the 4-pyridyl position promotes the formation of a polynuclear metal complex 20 and could facilitate the anchoring to metal-oxide surfaces.…”

Bond length of perchlorate at different temperatures: X-ray and neutron comparison

“…Results for this variable-temperature study are: 1.441 (1) at 18 K, 1.439 (1) at 78 K, and 1.436 (1) Å at 158 K. A search of the Cambridge Structural Database (Groom et al, 2016; Conquest, Version 2.0.1) for X-ray diffraction studies of perchlorate distances in the temperature range 283-303 K having no disorder or errors, powder data excluded, and with an R 1 value < 5%, yielded 3138 hits and a mean distance of 1.405 Å ; in the range 160-180 K, gave 845 entries with a mean value of 1.426 Å ; and in the range 80-100 K, gave 1217 entries with a mean value of 1.436 Å . An extensive synchrotron X-ray investigation into a spin-crossover solid produced data at temperatures ranging between 30 and 360 K with four perchlorate anions (Bartual-Murgui et al, 2018). The two iron complexes in the crystal have H-atom donors and it is apparent that the O-atom acceptors have slightly longer Cl-O bonds.…”