Synthesis, Crystal Structure and Magnetic Properties of the Helical-Chain Compounds [M2(O2CC12H8CO2)2(H2O)8] [M = Cobalt(II), Nickel(II)]

New 1D complexes are obtained from the reaction of 1,1′-biphenyl-2,2′-dicarboxylic acid (diphenic acid) with KOH and [Co(NO3)2·6H2O] or [Ni(NO3)2·6H2O] under hydrothermal conditions. Compounds 1 and 2 are isostructural and their structure consists of stacked helical chains of octahedral metal(II) ions. Magnetic measurements show that the CoII atoms in compound 1 are quasi-isolated. In compound 2, the NiII chains exhibit a ferromagnetic behaviour. From a simple model derived from the scaling theory of phase transition, the exchange coupling between neighbouring NiII atoms is evaluated at J = +1.06 K. A zero-field splitting effect is observed below 16 K.