Heteroleptic FeII Complexes of 2,2′-Biimidazole and Its Alkylated Derivatives: Spin-Crossover and Photomagnetic Behavior

• Published in: Chemistry : a European journal 2012-12, Vol.18 (49), p.15805-15815
• Main Authors: Phan, Hoa V., Chakraborty, Pradip, Chen, Meimei, Calm, Yitzi M., Kovnir, Kirill, Keniley Jr, Lawrence K., Hoyt, Jordan M., Knowles, Elisabeth S., Besnard, Céline, Meisel, Mark W., Hauser, Andreas, Achim, Catalina, Shatruk, Michael
• Format: Article
• Language: English
• Subjects: Chemistry; iron complexes; LIESST effect; N ligands; photomagnetism; spin crossover
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201202045

Three iron(II) complexes, [Fe(TPMA)(BIM)](ClO4)2⋅0.5H2O (1), [Fe(TPMA)(XBIM)](ClO4)2 (2), and [Fe(TPMA)(XBBIM)](ClO4)2 ⋅0.75CH3OH (3), were prepared by reactions of FeII perchlorate and the corresponding ligands (TPMA=tris(2‐pyridylmethyl)amine, BIM=2,2′‐biimidazole, XBIM=1,1′‐(α,α′‐o‐xylyl)‐2,2′‐biimidazole, XBBIM=1,1′‐(α,α′‐o‐xylyl)‐2,2′‐bibenzimidazole). The compounds were investigated by a combination of X‐ray crystallography, magnetic and photomagnetic measurements, and Mössbauer and optical absorption spectroscopy. Complex 1 exhibits a gradual spin crossover (SCO) with T1/2=190 K, whereas 2 exhibits an abrupt SCO with approximately 7 K thermal hysteresis (T1/2=196 K on cooling and 203 K on heating). Complex 3 is in the high‐spin state in the 2–300 K range. The difference in the magnetic behavior was traced to differences between the inter‐ and intramolecular interactions in 1 and 2. The crystal packing of 2 features a hierarchy of intermolecular interactions that result in increased cooperativity and abruptness of the spin transition. In 3, steric repulsion between H atoms of one of the pyridyl substituents of TPMA and one of the benzene rings of XBBIM results in a strong distortion of the FeII coordination environment, which stabilizes the high‐spin state of the complex. Both 1 and 2 exhibit a photoinduced low‐spin to high‐spin transition (LIESST effect) at 5 K. The difference in the character of intermolecular interactions of 1 and 2 also manifests in the kinetics of the decay of the photoinduced high‐spin state. For 1, the decay rate constant follows the single‐exponential law, whereas for 2 it is a stretched exponential, reflecting the hierarchical nature of intermolecular contacts. The structural parameters of the photoinduced high‐spin state at 50 K are similar to those determined for the high‐spin state at 295 K. This study shows that N‐alkylation of BIM has a negligible effect on the ligand field strength. Therefore, the combination of TPMA and BIM offers a promising ligand platform for the design of functionalized SCO complexes. Persistent spin‐crossover: Heteroleptic spin‐crossover complexes of FeII were prepared by using a combination of tris(2‐pyridylmethyl)amine and either 2,2′‐biimidazole (BIM) or its alkylated derivatives. This set of ligands provides a promising platform for the preparation of spin‐crossover complexes because N‐alkylation of BIM has negligible effect on the ligand field strength. An abrupt spin transition with thermal