Cooperative magnetic behavior in the coordination polymers [Cu3(TMA)2L3] (L=H2O, pyridine)

The temperature and magnetic field dependent magnetization of the polymeric solids [Cu-3(TMA)(2)L-3], where TMA=benzene-1,3-5-tricarboxylate and L=H2O or pyridine have been measured up to 5 T. These polymers contain dimeric [Cu-2(O2CR)(4)] units. Discrete dimers in molecules such as cupric acetate have strong antiferromagnetic coupling between their two Cu d(9) centers and at low T their susceptibility diminishes to nearly zero. While at higher T (300-100 K) the magnetism of the [Cu-3(TMA)(2)L-3] polymers resembles that of individual the [Cu-2] dimer molecules, at low T a new cooperative behavior is found, with a sharp increase in chi seen at temperature below 75 K. In the case of L=pyridine a plot of 1/chi vs T shows linearity from 65 to 20 K and fits the Curie-Weiss law chi=C/(T - Theta ). Theta is found to be +4.7 K, indicative of a weak ferromagnetic coupling between dimers. This interaction may be facilitated by the planar aromatic bridges between the [Cu-2] units. (C) 2000 American Institute of Physics. [S0021-8979(00)17708-4].