Disclosing the Ligand- and Solvent-Induced Changes on the Spin Transition and Optical Properties of Fe(II)-Indazolylpyridine Complexes

The family of the spin crossover (SCO) compounds based on the 1-bpp unit has furnished striking examples of how subtle changes in the crystal packing have important consequences in their spin transition. Small modifications of the 1-bpp unit itself have been recently reported, obtaining the indazolyl and pirazolyl derivatives [Fell(1-bip)](2+) (1, 1-bip = 2,6-bis(indazol-1-yl)pyridine), [Fell(1,2-bip)]2+ (2, 1,2-bip = 2-(indazol-1-yl)-6-(indazol-2-yl)pyridine), [Fell(2-bip)]2+ (3, 2-bip = 2,6-bis(indazol-2-yl)pyridine), [Fell(1-ipp)]2+ (4, 1-ipp = 2-(indazol-1-yl)-6-(pyrazol-1-yl)pyridine) and [Fell(2-ipp)]2+ (5, 2-ipp = 2-(indazol-2-yl)-6-(pyrazol-1-yl)pyridine). In this work we study the consequences of a change in the ligand structure and solvent on the SCO of 1-5. More specifically, we demonstrate that their different behavior is not due to an intraligand (HH)-H-center dot center dot center dot contact, as suggested experimentally, but to an unfavorable arrangement of the FeN6 core that some of the ligands might create, which destabilizes their Low Spin (LS) state structure and, thus, alters the transition temperature. Further, by means of solid state calculations, we disclose the effect of the solvent on the structure and crystal cohesion of the crystals. Finally, we analyze the emission and adsorption properties of 1-5, with special interest in the evolution of the absorption spectroscopy of the ligands upon complexation, and its relation with the spin multiplicity of the iron ion.