Influence of Substituents of Bisindazolylpyridine Iron(II) Complexes on the Metal Ion Spin State
Suhad Omar, Gerald Hörner and Andreas Grohmann
Institut für Chemie, Bioanorganische Chemie, Technische Universität Berlin, Straße des 17. Juni, 10623, Berlin, GER
The use of metal ion-based spin crossover (SCO) materials as switchable components in macroscopic and nanoscale devices is well established: A suitable 3d metal ion (e. g., iron(II)) can undergo a reversible electronic transition from a high-spin (HS) to a low-spin state (LS) when subjected to an external stimulus, such as the variation of temperature or pressure, light irradiation or application of a magnetic field 1. The relationship between ligand design and the metal ion spin state has been widely studied. A sterically crowded ligand sphere favours expanded metal-ligand bonds associated with the HS state 2. Hence, gradually adding or removing or even changing the type of substituents on the ligand, could enhance spin state changes and “poise” the system to show SCO. In this work, the isomers of 4-amino-3,5-dichloro-bisindazolylpyridine and their Fe(II) complexes have been synthesised and characterised. Isomer separation was unusually successful. The Fe(II) complexes of unsubstituted bisindazolylpyridine native ligands 3 show that [Fe(1-bindpy)2]2+ and [Fe(1,2-bindpy)2]2+ are LS, whereas [Fe(2-bindpy)2]2+ is HS in solution but exhibits thermal SCO in the solid state. In case of our ligand system, all complexes have more sterically crowded ligand spheres; as expected, and shown by single-crystal X-ray diffraction data, all the Fe(II) complexes are HS. We aim in this work to prepare SCO systems by gradual ligand dehalogenation, which would affect complex behaviour, as reducing the sterically crowded ligand sphere the LS state should be made more favourable. The completed dehalogenation of such isomers will open the door for the preparation of more complicated/diverse ligands of the 4-substituted bisindazolylpyridine ligand type; many variants of this ligand type will be accessible by applying the Sandmeyer reaction (amine-halogen substitution reaction), followed by C-C coupling reactions at that position.
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