Chiral magnetic metal-organic frameworks of dimetal subunits: magnetism tuning by mixed-metal compositions of the solid solutions.

The isostructural, chiral molecular magnetic materials with the formula [MxM'(2-x)(ca)2(1,4-dimb)]n [H2ca = D-(+)-camphoric acid, 1,4-dimb = 1,4-di-(1-imidazolyl-methyl)-benzene, M = Ni(II), M' = CoII, 0 < or = x < or = 2] consist of ca-bridged (4,4) layers with [M2(O2CR)4] as secondary building units that are pillared by the 1,4-dimb ligands into a unique 3D framework. The high-spin octahedral symmetry and the proportions of the mixed-metal ions were characterized by UV-vis spectroscopy. The compounds exhibit the onset of antiferromagnetic ordering at 7.5 approximately 23 K, as well as weak ferromagnetism, spin-flop, and glassy behavior that result from the randomness of the mixed-metal pairs, magnetic anisotropy of the metallic cations, and antisymmetric exchange. The composites should be regarded as molecular alloys of the pure Ni(II) and Co(II) compounds. The magnetic behavior of the solid solutions shows unambiguously that the organic bridges, bond angles, and bond distances greatly influence the effective interactions and bring about cooperative magnetic behavior in the chiral 3D frameworks.