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

The temperature and magnetic field dependent magnetization of the polymeric solids $[{\mathrm{Cu}}_{\mathrm{3}}{\mathrm{(TMA)}}_{\mathrm{2}}{\mathrm{L}}_{\mathrm{3}}\mathrm{],}$ where TMA=benzene-1,3-5-tricarboxylate and $\mathrm{L}={\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ or pyridine have been measured up to 5 T. These polymers contain dimeric $[{\mathrm{Cu}}_{\mathrm{2}}{\mathrm{(O}}_{\mathrm{2}}{\mathrm{CR)}}_{\mathrm{4}}]$ 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 $[{\mathrm{Cu}}_{\mathrm{3}}{\mathrm{(TMA)}}_{\mathrm{2}}{\mathrm{L}}_{\mathrm{3}}\mathrm{]}$ polymers resembles that of individual the $[{\mathrm{Cu}}_{\mathrm{2}}]$ 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 $\mathrm{\chi =C/(T- \Theta ).}$ 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 $[{\mathrm{Cu}}_{\mathrm{2}}]$ units.