Solvothermal synthesis, multi-temperature crystal structures and physical properties of isostructural coordination polymers, 2C4H12N+[M3(C8H4O4)4]2-.3C5H11NO, M = Co, Zn.

Two isostructural metal organic framework (MOF) structures have been synthesized by solvothermal methods and examined by single-crystal X-ray diffraction. A microcrystal of 2C4H12N+[Co3(C8H4O4)4]2-.3C5H11NO (1) was investigated at T = 120 K using synchrotron radiation. 2C4H12N+[Zn3(C8H4O4)4]2-.3C5H11NO (2) was investigated at multiple temperatures (T = 30, 100, 200 and 300 K) on a conventional diffractometer. The thermal expansion of the structure of (2) is anisotropic and along the a axis, which corresponds to the metal chain direction. The structures contain anionic frameworks with cations and solvent molecules trapped in the voids. The magnetic susceptibility (chi) and heat capacity (C(p)) have been measured from 1.8 to 350 K. Compound (1) orders ferromagnetically with a broad phase transition observed in C(p) at approximately 6 K. The magnetic moment reaches a value of 3 micro(B) per Co at 2 K in a magnetic field of 9 T, and a Curie-Weiss fit to chi(T) gives an effective moment (mu(eff)) of 4.2 mu(B) and a Weiss temperature (theta) of 23 K. The exchange mechanism for the magnetic coupling is suggested to involve the Co-O-Co bridges in the individual three-metal-atom subchains. The three-dimensional magnetism presumably is due to super-exchange through two out of the three unique C8H4O4 linker molecules, which have the carboxylate and benzene pi systems well aligned.