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

Authors

  • Hoa V. Phan,

    1. Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, FL 32306 (USA), Fax: (+1) 850-644-8281
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  • Pradip Chakraborty,

    1. Departement de Chimie Physique, Université de Genève, 30, quai Ernest-Ansermet, 1211 Geneve 4 (Switzerland), Fax: (+41) 22-379-6103
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  • Meimei Chen,

    1. Department of Chemistry, Carnegie Mellon University, 4400 Fifth Ave, Pittsburgh, PA 15213 (USA), Fax: (+1) 412-268-1061
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  • Yitzi M. Calm,

    1. Department of Physics and NHMFL, University of Florida, Gainesville, FL 32611-8440 (USA), Fax: (+1) 325-392-9147
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  • Dr. Kirill Kovnir,

    1. Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, FL 32306 (USA), Fax: (+1) 850-644-8281
    2. Present address: Department of Chemistry, University of California at Davis, Davis, CA 95616 (USA)
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  • Lawrence K. Keniley Jr.,

    1. Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, FL 32306 (USA), Fax: (+1) 850-644-8281
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  • Jordan M. Hoyt,

    1. Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, FL 32306 (USA), Fax: (+1) 850-644-8281
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  • Elisabeth S. Knowles,

    1. Department of Physics and NHMFL, University of Florida, Gainesville, FL 32611-8440 (USA), Fax: (+1) 325-392-9147
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  • Dr. Céline Besnard,

    1. Laboratoire de Cristallographie, Université de Genève, 24, quai Ernest-Anserment, 1211 Genève 4 (Switzerland), Fax: (+41) 22-397-6103
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  • Prof. Mark W. Meisel,

    Corresponding author
    1. Department of Physics and NHMFL, University of Florida, Gainesville, FL 32611-8440 (USA), Fax: (+1) 325-392-9147
    • Department of Physics and NHMFL, University of Florida, Gainesville, FL 32611-8440 (USA), Fax: (+1) 325-392-9147
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  • Prof. Andreas Hauser,

    Corresponding author
    1. Departement de Chimie Physique, Université de Genève, 30, quai Ernest-Ansermet, 1211 Geneve 4 (Switzerland), Fax: (+41) 22-379-6103
    • Departement de Chimie Physique, Université de Genève, 30, quai Ernest-Ansermet, 1211 Geneve 4 (Switzerland), Fax: (+41) 22-379-6103
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  • Prof. Catalina Achim,

    Corresponding author
    1. Department of Chemistry, Carnegie Mellon University, 4400 Fifth Ave, Pittsburgh, PA 15213 (USA), Fax: (+1) 412-268-1061
    • Department of Chemistry, Carnegie Mellon University, 4400 Fifth Ave, Pittsburgh, PA 15213 (USA), Fax: (+1) 412-268-1061
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  • Prof. Michael Shatruk

    Corresponding author
    1. Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, FL 32306 (USA), Fax: (+1) 850-644-8281
    • Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, FL 32306 (USA), Fax: (+1) 850-644-8281
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Abstract

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.

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