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Transition-metal imidazolate polymers : a new family of molecule-based magnets Sánchez, Víctor

Abstract

One-, two- and three-dimensional transition-metal coordination polymers involving imidazolate-based ligands have been prepared and characterized structurally and magnetically. A 1-D material, [Fe(pz)2]x (pz = pyrazolate), which exhibits weak antiferromagnetic exchange (short-range), was found to possess a chain type structure in which metal ions are doubly bridged by pyrazolate ligands. In contrast, when imidazolate-type ligands were utilized in the synthesis of binary metal-azolate complexes, 3-D extended systems were produced as a consequence of the single-bridging of metal ions characteristic of imidazolate ligands. Hence, [Fe(4-abimid)2]x (4-abimid = 4-azabenzimidazolate), and its cobalt analogue, both of which have a novel 3-D single diamondoid structure, were prepared. Both of these materials exhibit long-range ferromagnetic ordering at low temperatures. [Co(imid)2]x, (imid = imidazolate); [Cu(2-meimid)2]x (2-meimid = 2-methylimidazolate); [Co(benzimid)2]x, [Ni(benzimid)2]x and [Cu(benzimid)2]x (benzimid = benzimidazolate); [Cu(4,5-dichloroimid)2]x (4,5-dichloroimid = 4i5-dichloroimidazolate); and [Co3(imid)6(imidH)2]x (imidH = imidazole), all exhibit magnetic behaviour that classifies them as molecule-based magnets. Indirect evidence suggests that these materials also have extended 3-D lattices. [Fe2(imid)4(bipy)]x (bipy = 2,2'-bipyridine), [Co2(imid)4(bipy)]x and [Fe4(imid)8(terpy)]x (terpy = 2,2':6',2"-terpyridine), have 2-D structures, a structural motif never before seen in polymetallic imidazolates. The 'pyridine' molecules act as chelating, capping, ligands which separate the extended sheets of imidazolate-bridged metal ions in these materials. [Fe2(imid)4(bipy)]x is unique in exhibiting two structural phase transitions. Both [Fe2(imid)4(bipy)]x and [Co2(imid)4(bipy)]x exhibit long-range ferromagnetic ordering at low temperatures while [Fe4(imid)8(terpy)]x shows more complex magnetization behaviour. All three of these materials can be considered molecule-based magnets. [Fe(l-Me-2-S-imid)20.5Cp2Fe]x (l-Me-2-S-imid = l-methyl-2-thioirnidazolate; Cp2Fe = ferrocene), was obtained as a rare example of a 1-D chain polymer that exhibits long-range magnetic ordering. Alternating FeN4 and FeS4 chromophores along the chains is a unique structural feature of this material. The single-bridging imidazolate ligands involved in most of the compounds studied here are efficient mediators of magnetic exchange interaction between metal centres. The observation of antiferromagnetic behaviour above a critical temperature, T[sub c] , and long-range ferromagnetic ordering below T[sub c] suggests canted spin structures for many of these compounds. Importantly, long-range tJhree-dimensional ordering of the residual spins, arising from the canting, leads to net magnetization at zero applied field. These magnetic properties classify these novel materials as molecule-based magnets.

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