2012   2003   2000 
  • Heteroleptic FeII Complexes of 2,2′-Biimidazole and Its Alkylated Derivatives: Spin-Crossover and Photomagnetic Behavior
    H.V. Phan, P. Chakraborty, M. Chen, Y.M. Calm, K. Kovnir, L.K. Keniley, J.M. Hoyt, E.S. Knowles, C. Besnard, M.W. Meisel, A. Hauser, C. Achim and M. Shatruk
    Chemistry - A European Journal, 18 (49) (2012), p15805-15815
    Keywords: iron complexes;LIESST effect;N ligands;photomagnetism;spin crossover
    DOI:10.1002/chem.201202045 | unige:24449 | Abstract | Article PDF
 
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 2features 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.
  
  • Bond-Stretch Isomerism: Trapped Isomeric Structures of Hexacoordinate Copper(II) Bispidine Chromophores along a Jahn-Teller Active Vibrational Coordinate
    P. Comba, A. Hauser, M. Kerscher and H. Pritzkow
    Angewandte Chemie International Edition, 42 (37) (2003), p4536-4540
    Keywords: coordination chemistry; copper; Jahn-Teller effects; N ligands
    DOI:10.1002/anie.200351900 | unige:3511 | Abstract | Article HTML | Article PDF
Copper(II) complexes of the pentadentate bispidine ligands exist in two isomeric forms (see structure) with bond-length differences up to 0.5 Å. The stabilization of either isomer may be achieved by a variation of the substituent at N7.
Incorporation of [Co(bpy)3]2+ into the cavities of the three-dimensional oxalate network structure in [Co(bpy)3][LiCr(ox)3] produces chemical pressure that destabilises the normal high-spin ground state 4T1 to such an extent that the [Co(bpy)3]2+ complex becomes a spin-crossover complex. It shows a temperature-dependent equilibrium between the 2E low-spin and the 4T1 high-spin states.

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