Unusual Ferromagnetic Couplings in Single End-to-End Azide-Bridged Cobalt(II) and Nickel(II) Chain Systems

Abstract

Two new one-dimensional single azide-bridged metal(II) compounds [{M(5-methylpyrazole)₄(N₃)}_n](ClO₄)_n(H₂O)_n [M=Co (1 a), Ni (2 a)] were prepared by treating an M^II ion with stoichiometric amount of sodium azide in the presence of four equivalents of the 3(5)-methylpyrazole ligand. The isostructural compounds 1 a and 2 a 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 metal-azide-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 1 a and 2 a have unique magnetic properties of ferromagnetism, zero-field splitting, and spin canting. The MO calculations indicate that the quasi-orthogonality 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 T_C and coercive field of 2 K and 35 G for 1 b and 2.3 K and 20 G for 2 b, respectively.