Supramolecular Co(II)-[2 x 2] grids: Metamagnetic behavior in a single   molecule
  The magnetic anisotropy of the supramolecular [2 x 2] grid [Co(II)4L4]8+, with a bis(bipyridyl)-pyrimidine based ligand L, was investigated by single-crystal magnetization measurements at low temperatures. The magnetization curves exhibit metamagnetic like behavior and are explained by the weak-exchange limit of a minimal spin Hamiltonian including Heisenberg exchange, easy-axis ligand fields, and the Zeeman term. It is also shown that the magnetic coupling strength can be varied by the substituent R1 in the 2-position on the central pyrimidine group of the ligand L. 
O. Waldmann, M. Ruben, U. Ziener, P. M\"uller, J. M. Lehn