Azide and oxo bridged ferromagnetic clusters: Three face-shared tetracubane Ni(II)/Co(II) hexamers and a wheel-shaped SMM-like Co(II) heptamer

Four azide and oxo bridged polynuclear Ni(II) and Co(II) complexes have been constructed by the simultaneous use of Hhdeo (Hhdeo = 2-hydroxy-[1,2-di(pyridin-2-yl)] ethane-1-one) and azide as ligands: [Ni-6(Hhdeo)(2)(hdeo)(4)(N-3)(6)] center dot (ClO4)(2) center dot 2DMF center dot 2H(2)O (1), [Co-6(Hhdeo)(2)(hdeo)(4)(N-3)(6)] center dot (ClO4)(2) center dot 2DMF center dot 2H(2)O (2), [Ni-6(Hhdeo)(2)(hdeo)(4)(N-3)(6)] center dot (ClO4)(2) center dot 6H(2)O (3) and [Co-7(hdeo)(6)(N-3)(6)] center dot (ClO4)(2) center dot 2H(2)O (4). Compounds 1-3 all crystallize in monoclinic space group P2(1)/c, and consist of face-shared tetracubane hexanuclear cores, which are bridged by mu(2), mu(3)-O and mu(2)-1,1, mu(3)-1,1,1-N-3. Compound 4 crystallizes in triclinic space group P (1) over bar, and is made up of wheel-like heptanuclear cores [Co-7(hdeo)(6)(N-3)(6)](2+) bridged by mu O2- and mu(3)-1,1,1-N3. For Co(II), the reaction solvents are found to play important roles in directing the final products with hexanuclear or heptanuclear topologies. Magnetic studies demonstrate overall intra-molecular ferromagnetic (F) couplings in these four compounds, which result from, according to our theoretical analysis, the competition between different kinds of exchange interactions within the metallic triangle units. No single-molecule magnet behavior was observed above 1.9 K in complex 1-3. However, compound 4 shows significant slow relaxation at static zero and 600 Oe fields below 4 K, towards single molecule magnet (SMM) behavior. (C) 2008 Elsevier B. V. All rights reserved.