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

Polybis(pyrazolato)iron(II), [Fe(pz)(2)](x) (1) has an extended chain ID st ructure in which iron ions are doubly bridged by pyrazolate ligands. The co mpound exhibits weak antiferromagnetic interactions. The magnetic susceptib ility data were fit to a Heisenberg model for chains of antiferromagnetical ly coupled S= 2 metal centers, yielding the magnetic parameters -J= 0.59 cm (-1) and g = 2.01. Poly-2,2'-bipyridinetetrakis(imidazolato)diiron(II), [Fe -2(imid)(3)(bipy)](x) (2) and the isomorphous cobalt compound, 3, have 2D e xtended structures in which double layered sheets of alternating tetrahedra lly and octahedrally coordinated metal ions are linked by single bridging i midazolates. The octahedral metal centers are additionally coordinated by 2 ,2'-bipyridine ligands, which occupy positions between the sheets, isolatin g the sheets from each other. The presence of the two different iron center s in 2 is confirmed by ambient temperature Mossbauer studies. The magnetic properties of these materials reveal a transition to long-range ferromagnet ic order below 11 K for 2 and below 13 K for 3. Both materials reveal magne tic hysteresis at 4.8 K. Analysis of the data yield, for 2 and 3, respectiv ely, coercive fields of 15 and 125 G and remanent magnetizations of 200 and 1900 cm(3) G mol(-1). (C) 2001 Elsevier Science Ltd. All rights reserved.