Crystal Structures and Magnetic Properties of Cobalt (II) Pyrazinecarboxylate and Pyrazinedicarboxylate Complexes

The synthesis and the structural and magnetic characterization of three cobalt(II) complexes of 2-pyrazinecarboxylic acid and 2,3-pyrazinedicarboxylic acid are reported. In each case the cobalt is in a distorted octahedral environment, coordinated to two nitrogen atoms and two carboxylate oxygens from different ligands and two water molecules. Pairs of like ligands are always trans to each other. The pyrazine ligands do not link adjacent cobalt atoms directly. The complexes all consist of three-dimensionally hydrogen-bonded polymers with potential magnetic-exchange pathways. In [Co(pz(COO)-COOH)2(H2O)2], the carboxylate group at the 2-position is deprotonated, while the other carboxylate forms intramolecular hydrogen bonds with it and intermolecular hydrogen bonds with the coordinated water. In [Co(pzCOO)2(H2O)2], the coordinated carboxylate oxygen is hydrogen bonded to the coordinated water of adjacent molecules. This causes lengthening of the Co-O bond to the carboxylate and forms infinite chains along the a axis. It is the most direct linkage observed in the three complexes. The remainder of the three-dimensional network is formed by hydrogen bonding between the uncoordinated carboxylate oxygen and the coordinated water molecule. In [Co(pzCOO)2(H2O)2]·2H2O, the uncoordinated carboxylate groups form infinite chains along the a axis by hydrogen bonding to the coordinated water ligands of adjacent molecules. The remainder of the three-dimensional network is less direct, being completed by uncoordinated water molecules hydrogen bonding to both the uncoordinated carboxylate oxygens and coordinated water molecules of adjacent molecules. Powder magnetic susceptibilities were measured on each of the cobalt pyrazinecarboxylate complexes in the region 4-100 K. The low-temperature behavior of the Kramers doublet of cobalt(II) may be analyzed by the Curie-Weiss law. This procedure gives a Weiss constant of about θ ≈ -1.0 K for each of the complexes. The small negative value of J is indicative of weak antiferromagnetic interactions, with critical regions below the temperature range of measurement. Crystal data for [Co(pz(COO)COOH)2(H2O)2]: space group P21/n, Z = 4, a = 9.396 (3) Å, b = 7.692 (2) Å, c = 10.136 (2) Å, β = 93.32 (4)°, V = 731 Å3, R = 3.2% for 1225 reflections. Crystal data for [Co(pzCOO)2(H2O)2]·2H2O: space group P21/n, Z = 4, a = 7.259 (3) Å, b = 13.028 (3) Å, c = 8.271 (5) Å, β = 103.09 (3)°, V = 765 Å3, R = 3.1% for 1195 reflections. Crystal data for [Co(pzCOO)(H2O)2]: space group P21/n, Z = 4, a = 5.2496 (7) Å, b = 11.140 (1) Å, c = 10.299 (2) Å, β = 99.55 (1)°, V = 594 Å3, R = 2.2% for 997 reflections. © 1981 American Chemical Society