Phase Transitions of Two Coordination Polymers [MCl2(beta-ala)](n), (M=Co, Ni; beta-ala=3-Aminopropionic Acid)

Two new isomorphic coordination polymers, [MCl2(beta-ala)](n) (M=Co2+ (1) or Ni2+ (2), beta-ala=3-aminopropionic acid), were constructed by assembling 3-aminopropionic acid and transition metal chlorides. Their reversible structural phase transitions were disclosed by differential scanning calorimetry (DSC), variable-temperature single-crystal X-ray diffraction and dielectric measurements. In 1 and 2, the adjacent M2+ ions are mix-bridged by two mu(2)-Cl- ions and a syn-syn carboxylate group from beta-ala ligand, resulting in one-dimensional polymeric coordination chains. Both 1 and 2 undergo reversible phase transitions, which occur at critical temperature (Tc-healing, Tc-cooling) of 236, 224 K and 309, 302 K, respectively, with a change of space group between Pnam at the high temperature phase (HTP) and Pna2(1) at the low-temperature phase (LTP). The phase transitions are mainly related to the dynamic change of ammonium ethyl group from two-fold disorder in HTP to static ordered state in LTP. Such order-disorder structural phase transitions lead to the coexistence of step-like dielectric anomaly and dielectric relaxation in the vicinity of the phase transition. CCDC: 1555337, 1_LTP; 1555336, 1_HTP; 1555339, 2_LTP; 1555338, 2_HTP.