Advances in Spin Crossover: Synthesis, Mechanosynthesis and Switchable Multifunctional Hybrids

Spin crossover (SCO) complexes are molecular materials with d⁴–d⁷ metal ions that possess a set of properties relevant for practical applications, since they are able to display a spin transition in response to external perturbation, such as a change of temperature, pressure, light irradiation or pulsed magnetic field.  To date, many coordination SCO complexes have been studied with different metallic SCO active centres. Among the diverse variety of metal ions, Fe(II) has been the most studied one as, in some cases, the spin transition occurs abruptly, with hysteresis, close to room temperature and is stable over successive cycles. These features make Fe(II) SCO complexes suitable for relevant practical applications in molecular electronics, data storage, display devices, non-linear optics, and photomagnetism. 

In the present doctoral thesis, several approaches in the field of SCO have been performed. Firstly, a neutral SCO polymeric chain has been obtained via coprecipitation of mixed-ligand, obtaining the compound with formula [Fe(L)(NH₂–trz)₂] (L = 4−(1,2,4−triazol−4−yl)ethanedisulfonate and NH2–trz = 4NH2−1,2,4−triazole). Secondly, a new compound has been synthesised via mechanosynthesis of L and Fe(ClO₄)₂ with completely different magnetic properties and structure than the solution-synthesised SCO compound from the same precursors. Thirdly, the post-synthetic grinding of the well-studied [Fe(trz)(Htrz)₂]_n(BF₄)_n and [Fe(NH₂−trz)₃]_n(SO₄)_n compounds has been used to fine tune their magnetic properties, downshifting the transition temperatures in both of the cases due to a mechanical recrystallization process. Finally, the last mentioned SCO polymers have been successfully inserted into a conductive organic conductive matrix via mechanical processing, obtaining highly conductive hybrid composites with memory effect close to room temperature. 

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