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A New Family of Trinuclear Nickel(II) Complexes as Single-Molecule Magnets

Authors

  • Rituparna Biswas,

    1. Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009 (India)
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  • Yumi Ida,

    1. Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo, 182-8585 (Japan)
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  • Dr. Michael L. Baker,

    1. Institute of Material Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577 (Japan)
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  • Saptarshi Biswas,

    1. Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009 (India)
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  • Paramita Kar,

    1. Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009 (India)
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  • Prof. Dr. Hiroyuki Nojiri,

    1. Institute of Material Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577 (Japan)
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  • Prof. Dr. Takayuki Ishida,

    Corresponding author
    1. Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo, 182-8585 (Japan)
    • Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo, 182-8585 (Japan)
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  • Prof. Dr. Ashutosh Ghosh

    Corresponding author
    1. Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009 (India)
    • Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009 (India)
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Abstract

Three new trinuclear nickel (II) complexes with the general composition [Ni3L3(OH)(X)](ClO4) have been prepared in which X=Cl (1), OCN (2), or N3 (3) and HL is the tridentate N,N,O donor Schiff base ligand 2-[(3-dimethylaminopropylimino)methyl]phenol. Single-crystal structural analyses revealed that all three complexes have a similar Ni3 core motif with three different types of bridging, namely phenoxido (μ2 and μ3), hydroxido (μ3), and μ2-Cl (1), μ1,1-NCO (2), or μ1,1-N3 (3). The nickel(II) ions adopt a compressed octahedron geometry. Single-crystal magnetization measurements on complex 1 revealed that the pseudo-three-fold axis of Ni3 corresponds to a magnetic easy axis, being consistent with the magnetic anisotropy expected from the coordination structure of each nickel ion. Temperature-dependent magnetic measurements indicated ferromagnetic coupling leading to an S=3 ground state with 2J/k=17, 17, and 28 K for 1, 2, and 3, respectively, with the nickel atoms in an approximate equilateral triangle. The high-frequency EPR spectra in combination with spin Hamiltonian simulations that include zero-field splitting parameters DNi/k=−5, −4, and −4 K for 1, 2, and 3, respectively, reproduced the EPR spectra well after a anisotropic exchange term was introduced. Anisotropic exchange was identified as Di,j/k=−0.9, −0.8, and −0.8 K for 1, 2, and 3, respectively, whereas no evidence of single-ion rhombic anisotropy was observed spectroscopically. Slow relaxation of the magnetization at low temperatures is evident from the frequency-dependence of the out-of-phase ac susceptibilities. Pulsed-field magnetization recorded at 0.5 K shows clear steps in the hysteresis loop at 0.5–1 T, which has been assigned to quantum tunneling, and is characteristic of single-molecule magnets.

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