Calorimetric Studies On Antiferromagnetic And Structural Phase Transitions Of The Metal-Assembled Complex Mncu(Obbz)Center Dot 5h(2)O

Heat capacities of the molecule-based antiferromagnet MnCu(obbz). 5H(2)O (obbz = oxamidobis(benzoato)) have been measured at temperatures from 0.1 to 300 K by means of adiabatic calorimetry. A phase transition due to three-dimensional antiferromagnetic ordering was observed at 2.18 K, above which a heat capacity hump appeared, arising from the short-range ordering characteristic of a low-dimensional magnet. The enthalpy and entropy gained at the magnetic phase transition were evaluated to be 48.6 J mol(-1) and 12.1 J K-1 mol(-1), respectively. This entropy gain is close to the value R ln 5 ( = 13.4 J K-1 mol(-1)) expected for the spin-manifold of the S = 2 ground state. The heat capacity hump above the magnetic transition temperature is explained well by the S = 2 one-dimensional ferromagnetic Heisenberg model with the exchange interaction J/k(B) = 0.75 K, where k(B) is the Boltzmann constant. These results confirm that the present complex behaves as a one-dimensional ferrimagnet, in which strong antiferromagnetic coupling exists between a pair of spins of Mn(II) (S = 5/2) and Cu(II) (S = 1/2) but the magnetic interaction is very weak between the pairs. A first-order phase transition was also found at 268.4 K. This transition may arise from orientational order-disorder mechanism of the crystalline water molecules in the complex.