Two homometallic antiferromagnets based on oxalato-bridged honeycomb assemblies: (A)2[M(II)2(C2O4)3] (A = ammonium salt derived from diethylenetriamine; M(II) = Fe2+, Co2+).

Oxalato-bridged divalent homometallic compounds A(2)[M(II)(2)(C(2)O(4))(3)] (M(II) = Fe(2+) (1), Co(2+) (2)) were obtained by a solvothermal method. They consist of honeycomb anions and cations, that is, the 5-oxo-1,4,7-triazabicyclo[4.3.0]non-6-en-7-yl ammonium ion (hereafter abbreviated as A(1)) in 1 and the 2-(2,3-dioxo-1-piperazinyl)eth-1-yl ammonium ion (hereafter abbreviated as A(2)) in 2, which were generated from in situ reactions of diethylenetriamine (DETA) with oxalic acid catalyzed by the metal ions and yielded two compounds with different cell parameters: a = 17.2224(4)A, b = 9.3151(2)A, c = 15.1518(4)A, beta = 95.767(1) degrees , V = 2418.5(1)A(3), and Z = 4, C2/c for 1 and a = 9.6924(2)A, b = 15.8325(4)A, c = 17.2995(4)A, beta = 95.144(1) degrees , V = 2644.0(1)A(3), Z = 4, and P2(1)/n for 2. A(1) points its carbonyl group to the pocket of the honeycomb network. A(2) forms a helical chain around the anion layers through hydrogen bonds along the 2(1) axis, and the crystal remains achiral due to the existence of the inversion symmetry. The methanol molecules occupy the holes situated between A(2) and the oxalate network in 2. The distance between two anion layers in 1 was shorter than in 2 due to the template effect of the ammonium salts. In the anion layers, the hexagonal rings are elongated along the a axis in 1 and 2. There are interactions as hydrogen bonds between the cation and anion and between cations. A broad maximum observed in the temperature-dependent susceptibility curve shows antiferromagnetic interactions between paramagnetic ions. The antiferromagnetic ordering at 28 K in 1 and 21 K in 2 was confirmed by ac suspceptibility and specific heat measurements. Hysteresis loops with a coercive field of 17 Oe in 1 and 2300 Oe in 2 were observed at 2 K.