Polybis(pyrazolato)iron(II) and poly-2,2'-biopyridinetetrakis(imidazolato)-diiron(II) and -dicobalt(II): from short-range magnetic interactions in the pyrazolate to long-range ferromagnetic ordering in the imidazolates

摘要

Polybis(pyrazolato)iron(II), [Fe(pz)_2]_x (1) has an extendd chain 1D structure in which iron ions are doubly bridged by pyrazolate ligands. The compound exhibits weak antiferromagnetic interactions. The magnetic susceptibility data were fit to a Heisenberg model for chains of antiferromagnetically copled S = 2 metal centers, yielding the magnetic parameters -J = 0.59 cm~(-1) and g = 2.01. Poly-2,2'-biopyridinetetrakis(imidazolato)diiron(II). [Fe_2(imid)_4(biopy)]_x (2) and the isomorphous cobalt compound, 3, have 2D extended structures in which double layered sheets of alternating tetrahedrally and octahedrally coordinated metal ions are linked by single bridging imidazolates. The octahedral metal centers are additionally coordianted by 2,2'-bipyridine ligands, which occupy positions between the sheets, isolating the sheets from each other. The presence of the two different iron centers in 2 is confirmed by ambient temperature Mossbauer studies. The magnetic properties of these materials reveal a transition tolong-range ferromagnetic order below 11 K for 2 and below 13 K for 3. Both materials reveal magnetic hysteresis at 4.8 K. Analysis of the data yield, for 2 and 3, respectively, coercive fields of 15 and 125 G and remanent magnetizations of 200 and 1900 cm~3 G mol~(-1).