Coupled Crystallographic Order-Disorder and Spin State in a Bistable Molecule: Multiple Transition Dynamics

• Published in: Chemistry : a European journal Vol. 17; no. 11; pp. 3120 - 3127
• Main Authors: Craig, Gavin A., Sánchez Costa, José, Roubeau, Olivier, Teat, Simon J., Aromí, Guillem
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 07-03-2011; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: bistability; Chemistry; Coupling (molecular); Crystallography; Dynamics; Hysteresis; iron; Joining; magnetic properties; Magnetization; Order disorder; order-disorder transitions; spin crossover; Trapping
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201003197

A novel bispyrazolylpyridine ligand incorporating lateral phenol groups, H4L, has led to an FeII spin‐crossover (SCO) complex, [Fe(H4L)2][ClO4]2⋅H2O⋅2 (CH3)2CO (1), with an intricate network of intermolecular interactions. It exhibits a 40 K wide hysteresis of magnetization as a result of the spin transition (with T0.5 of 133 and 173 K) and features an unsymmetrical and very rich structure. The latter is a consequence of the coupling between the SCO and the crystallographic transformations. The high‐spin state may also be thermally trapped, exhibiting a very large TTIESST (≈104 K). The structure of 1 has been determined at various temperatures after submitting the crystal to different processes to recreate the key points of the hysteresis cycle and thermal trapping; 200 K, cooled to 150 K and trapped at 100 K (high spin, HS), slowly cooled to 100 K and warmed to 150 K (low spin, LS). In the HS state, the system always exhibits disorder for some components (one ClO4− and two acetone molecules) whereas the LS phases show a relative ≈9 % reduction in the FeN bond lengths and anisotropic contraction of the unit cell. Most importantly, in the LS state all the species are always found to be ordered. Therefore, the bistability of crystallographic order–disorder coupled to SCO is demonstrated here experimentally for the first time. The variation in the cell parameters in 1 also exhibits hysteresis. The structural and magnetic thermal variations in this compound are paralleled by changes in the heat capacity as measured by differential scanning calorimetry. Attempts to simulate the asymmetric SCO behaviour of 1 by using an Ising‐like model underscore the paramount role of dynamics in the coupling between the SCO and the crystallographic transitions.
All is in order when the spin is low: A spin‐crossover FeII complex with a bispyrazolylpyridine ligand incorporating lateral phenol groups exhibits a large, unsymmetrical bistable domain that correlates with crystallographic ordering (see figure) and propagates through an intricate network of intermolecular interactions.