Versatility in the binding of 2-pyrazinecarboxylate with iron. Synthesis, structure and magnetic properties of iron(II) and iron(III) complexes

The synthesis and characterization of two new iron(II) complexes, [Fe(pca)(2)(py)(2)]center dot py (1) and {[Fe(pca)(2)(H2O))] center dot H2O}(n) (2) and one new iron(m) complex, Na-2{[Fe(pca)(3))](2)O}center dot 2H(2)O center dot 2CH(3)CN (3) (pca(-) stands for 2-pyrazinecarboxylate), are reported. Complex 1 is obtained from the reaction of iron powder with 2-pyrazinecarboxylic acid. The reaction of Fe(ClO4)(3)center dot 10H(2)O with Hpca in the presence of 3 equiv. of Bu4NOH yields 2, whereas the presence of NaOH yields 3. The molecular structure of 1 contains ail iron(II) ion with a pseudo-octahedral environment resulting from the coordination of two pca- ligands in a bidentate chelating fashion and two pyridine molecules; pi-pi stacking interactions between pyridine and pyrazine rings lead to a one-dimensional chain. Complex 2 is ail iron(II) coordination polymer with an infinite zig-zag motif and an Fe center dot center dot center dot Fe separation of 7. 1 angstrom. In 2, the pi-pi stacking interactions involving the pyrazine rings and the strong hydrogen bonds between the coordinated water molecule and the carboxylate oxygens of two pca(-) ligands result in a three-dimensional network structure. Complex 3 consists of an anionic mu-oxo-bridged diiron(III) core with two crystallographically distinct iron(III) ions; the negative charge is compensated by two sodium cations. Complex 3 is assembled in a three dimensional network structure through coordination of Na(I) and hydrogen bond interactions. Temperature dependent magnetic susceptibility and Mossbauer spectroscopic studies indicate that 1 and 2 have similar magnetic properties. Both complexes are paramagnetic above 12 K, whereas antiferromagnetic ordering is observed below 12 K. The magnetic properties of 3 reveal strong intramolecular antiferromagnetic interactions between the two iron(III) ions with a J value of -221 cm(-1); no long range intermolecular magnetic coupling is observed between 295 and 4.2 K