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

Two new one-dimensional single azide-bridged metal(II) compounds [{M(5-meth ylpyrazole)(4)(N-3)}(n)]- (ClO4)(n)(H2O)(n) [M=Co (1a), Ni (2a)] were prepa red by treating an M-11 ion with stoichiometric amount of sodium azide in t he presence of four equivalents of the 3(5)-methylpyrazole ligand. The isos tructural compounds la and 2a crystallize in the monoclinic space group P2( 1)/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 n ickel(ii) chains. The two neighboring metal atoms at inversion centers adop t octahedral environments with four equatorial 3(5)-methylpyrazole ligands and two axial azide bridges. Two adjacent equatorial least-squares planes f orm dihedral angles of 60.5 degrees and 60.6 degrees for Cc and Ni. respect ively. In addition, the metalazide-metal units form large M-N-3-M torsion a ngles, which are magnetically important geometrical parameters, of 71.6 deg rees for M = Co and 75.7 degrees for M = Ni. It should also be noted that t he M-N-N angles associated with end-to-end azide group, another magneticall y important structural parameter, fall into the experimentally observed ran ge of 120- 140 degrees as 128.3(3) and 147.8(3)degrees for cobalt species a nd 128.4(2) and 146.1(3)degrees for nickel species, these values deviate fr om the theoretical value of around 164 degrees at which the incidental orth ogonality is achieved under the torsion angle of 0 degrees. The compounds l a and 2a have unique magnetic properties of ferromagnetism, zero-field spli tting, and spin canting. The MO calculations indicate that the quasiorthogo nality between the magnetic orbitals of metal ions and the p atomic orbital s of the bridging azide is possible in the observed structures and leads to the ferromagnetism. The spin canting related to the perturbation of ferrom agnetism arises from the magnetic anisotropy and antisymmetric interactions judged by the structural parameters of the zero-field splitting and the ti lted MN4 planes in a chain. The enhancement of magnetic interactions was ac complished by dehydrating the chain compounds to afford two soft magnets wi th critical temperature Tc. and coercive field of 2 K and 35 G for 1 b and 2.3 K and 20 G for 2b. respectively.