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

The temperature and magnetic field dependent magnetization of the polymeric solids [Cu₃(TMA)₂L₃], where TMA=benzene-1,3-5-tricarboxylate and L=H₂O or pyridine have been measured up to 5 T. These polymers contain dimeric [Cu₂(O₂CR)₄] units. Discrete dimers in molecules such as cupric acetate have strong antiferromagnetic coupling between their two Cu d⁹ centers and at low T their susceptibility diminishes to nearly zero. While at higher T (300-100 K) the magnetism of the [Cu₃(TMA)₂L₃] polymers resembles that of individual the [Cu₂] dimer molecules, at low T a new cooperative behavior is found, with a sharp increase in χ seen at temperature below 75 K. In the case of L=pyridine a plot of 1/χ vs T shows linearity from 65 to 20 K and fits the Curie-Weiss law χ=C/(T - Θ). Θ 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 [Cu₂] units.