Spin Spin Interactions in Polynuclear Nickel(II) Complexes: A Low-Temperature Susceptibility and Magnetization Study of the Cubane Tetranuclear Complex Ni4II(chta)4(μ-OH)4(NO3)4.7H2O

The bulk magnetic susceptibility of the complex Ni4(chta)4(µ-OH)4(NO3)4.7H2O ( chta = cyclohexane-r-1,c-3,c-5-triamine) has been measured over the temperature range from 1.9 to 300 K at a field of 1 T. The observed data were fitted by a Heisenberg exchange model for the susceptibility of a cubane [Ni4II(µ-OH)4]4+ cluster with a single exchange interaction J between each pair of NiII ions for a single ion g value 2.20 and an antiferromagnetic spin-spin coupling parameter 2J = -1.14 cm-1. The bulk magnetization measured at fields of 1 to 5 T in the temperature range from 5 to 1.9 K differed from that calculated from the simple Heisenberg model above. Several possibilities were examined for this deviation including intercluster interactions, single ion zero-field splitting and differing exchange coupling constants between nickel(II) ions in the cluster. This last model was successful in describing the complete set of magnetic data. A comparison with other known oxygen-bridged nickel(II) cubane clusters shows a change from antiferromagnetic to ferromagnetic coupling as the Ni-O-Ni bond angle decreases. It appears that the cubane cluster studied here is in a region very close to the crossover from antiferromagnetic to ferromagnetic coupling between the nickel ions.