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Abstract of [Hong01]

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Unusual ferromagnetic couplings in single end-to-end azide-bridged cobalt(II) and nickel(II) chain systems

Chang Seop Hong, Ja-eung Koo, Sang-Kil Son, Yoon Sup Lee, Yaung-Soo Kim, and Youngkyu Do

Chem. Eur. J. 7, 4243–4252 (2001)


Two new one-dimensional single azide-bridged metal(II) compounds [M(5-methylpyrazole)4(N3)n]-(ClO4)n(H2O)n [M=Co (1a), Ni (2a)] were prepared by treating an MII ion with stoichiometric amount of sodium azide in the presence of four equivalents of the 3(5)-methylpyrazole ligand. The isostructural compounds 1a and 2a crystallize in the monoclinic space group P21/n. The azide bridging ligands have a unique end-to-end coordination mode that brings two neighboring metal centers into a cis-position with respect to the azide unit to form single end-to-end azide-bridged cobalt(II) and nickel(II) chains. The two neighboring metal atoms at inversion centers adopt octahedral environments with four equatorial 3(5)-methylpyrazole ligands and two axial azide bridges. Two adjacent equatorial least-squares planes form dihedral angles of 60.5 and 60.6 for Co and Ni, respectively. In addition, the metalazide-metal units form large M-N3-M torsion angles, which are magnetically important geometrical parameters, of 71.6 for M=Co and 75.7 for M=Ni. It should also be noted that the M-N-N angles associated with end-to-end azide group, another magnetically important structural parameter, fall into the experimentally observed range of 120-140 as 128.3(3) and 147.8(3) for cobalt species and 128.4(2) and 146.1(3) for nickel species; these values deviate from the theoretical value of around 164 at which the incidental orthogonality is achieved under the torsion angle of 0. The compounds 1a and 2a have unique magnetic properties of ferromagnetism, zero-field splitting, and spin canting. The MO calculations indicate that the quasiorthogonality between the magnetic orbitals of metal ions and the p atomic orbitals of the bridging azide is possible in the observed structures and leads to the ferromagnetism. The spin canting related to the perturbation of ferromagnetism arises from the magnetic anisotropy and antisymmetric interactions judged by the structural parameters of the zero-field splitting and the tilted MN4 planes in a chain. The enhancement of magnetic interactions was accomplished by dehydrating the chain compounds to afford two soft magnets with critical temperature TC and coercive field of 2 K and 35 G for 1b and 2.3 K and 20 G for 2b, respectively.

Tags: molecular magnet, experiment, KAIST

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