Temperature-induced-to-configuration-regulated reversible isostructural phase transition in bis(triethylbenzylammonium) tetrachlorocobaltate(II)

Bis(triethylbenzylammonium) tetrachlorocobaltate(II), (C13H22N+)2·CoCl42– was synthesized and separated as blue block crystals. DSC measurement detected that this compound undergoes a reversible phase transition at ca. 336 K with a hysteresis of 10 K. Dielectric measurements confirm the transition at ca. 336 K. The crystal structures determined at 291(2) K (a = 16.72 Å, b = 9.32 Å, c = 20.36 Å, β = 106.90°, V = 3035.1 Å3, Z = 4) and 353(2) K (a = 20.16 Å, b = 9.35 Å, c = 34.29 Å, β = 108.61°, V = 6126.7 Å3, Z = 8) show that the phase transition is a type of isostructural change with the same space group P21/c (No. 14). The most distinct difference between the two structures in high-temperature phase (HTP) and room-temperature phase (RTP) is the different molar ratios of configurations for the cations. The rotation of ethyl groups bonding with the N atom between the propeller-like configuration and butterfly-like configuration is probably the driving force to the phase transition. Non-classic hydrogen bonding effect on the transition was shown to be negligible.