Coupled crystallographic order-disorder and spin state in a bistable molecule: multiple transition dynamics

Gavin A. Craig, José Sánchez Costa, Olivier Roubeau, Simon J. Teat, Guillem Aromí

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Abstract A novel bispyrazolylpyridine ligand incorporating lateral phenol groups, H4L, has led to an FeII spin-crossover (SCO) complex, [Fe(H4L)2][ClO4]2H2O2(CH3)2CO (1), with an intricate network of intermolecular interactions. It exhibits a 40K wide hysteresis of magnetization as a result of the spin transition (with T0.5 of 133 and 173K) 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 (≈104K). 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; 200K, cooled to 150K and trapped at 100K (high spin, HS), slowly cooled to 100K and warmed to 150K (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 FeN 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.
Original languageEnglish
Pages (from-to)3120-3127
Number of pages8
JournalChemistry - A European Journal
Issue number11
Early online date15 Feb 2011
Publication statusPublished - 7 Mar 2011



  • bistability
  • hysteresis
  • iron
  • magnetic properties
  • order–disorder transitions
  • spin crossover

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