One‐Dimensional Magnetism in New, Layered Structures: Piperazine‐Linked Copper and Nickel Oxalate Chains

Two new coordination network materials with the composition M(pip)(ox) ( pip = piperazine; ox = oxalate; M = Ni, Cu) have been synthesised under ambient and hydrothermal conditions. These compounds adopt related structures, both consisting of intersecting [M(pip)]∞ and [M(ox)]∞ chains, which result in layered structures. The nickel compound crystallises in P1 . The metal ions are octahedrally coordinated and are linked by symmetric chelating bridging oxalate ions. In contrast, the copper compound crystallises in the chiral space group P212121. Here the copper atom displays square pyramidal coordination geometry and the oxalate ions bridge metals in an unsymmetric fashion resulting in a polar metal‐oxalate chain structure. The magnetic behaviour is determined by the nature of the exchange‐coupled network. Only the antiferromagnetic interaction mediated by the bridging oxalate group is significant and the compounds are well modelled as 1‐D antiferromagnetically coupled chains. For the copper compound (S = 1/2) we determined J/kB to be −25.9 K, while the nickel compound (S = 1) shows a larger exchange coupling (J/kB = −42.2 K). In the nickel compound we see a significant deviation between the observed and calculated magnetic susceptibilities at the lowest experimental temperatures. This may be due either to the formation of a Haldane quantum antiferromagnetic ground state or to single‐ion zero‐field splitting effects. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)