Magnetic and structural studies on 1,3-diazolate complexes of cobalt(II) -: The characterization of three new cobalt(II) molecule-based magnets

The cobalt(II) compounds ([Co(imid)(2)](x) (1), [Co(2-meimid)(2)](x) (2), [Co(4-meimid)(2)](x) (3), [Co(benzimid)(2)](x) (4), and [Co-3(imid)(6)(imidH)(2)](x) (5) (imid = imidazolate, 2-meimid = 2-methylimidazolate, 4-meimid = 4-methylimi dazolate, benzimid = benzimidazolate)), have been synthesized and structurally and magnetically characterized. Electronic and vibrational spectroscopy and thermogravimetric studies on all five compounds support structures involving tetrahedrally coordinated cobalt centers with single-bridging 1,3-diazolate ligands forming extended polymeric lattices. Moreover, X-ray powder diffraction studies on 1 and 5 established, through isomorphism with published structures, that the molecular connectivity in these materials is 3-D. Variable temperature and applied field magnetization studies revealed antiferromagnetism as the primary magnetic exchange process in all five compounds. In addition, 1, 4, and 5 show magnetic phase transitions to ferromagnetically ordered states below critical temperatures of 16, 13, and 15 K, respectively. Magnetization measurements at 4.8 K as the applied field was cycled between +55 000 and -55 000 G revealed typical hysteresis behavior and gave remnant magnetizations of 334, 257, and 175 cm(3) G mol(-1) and coersive fields of 6620, 5280, and 4140 G for 1, 4, and 5, respectively. No evidence for long-range magnetic order was obtained for either 2 or 3. A comparison of the magnetic properties of three pairs of isostructural iron(II) and cobalt(II) molecule-based magnets shows that while the coersive fields are in general larger for cobalt over iron, the magnitude of the difference varies significantly suggesting that one cannot conclude that cobalt analogues will always be harder magnets.