Structural Transition in the Perovskite-like Bimetallic Azido Coordination Polymers: (NMe4)2[B′·B″(N3)6] (B′ = Cr3+, Fe3+; B″ = Na+, K+)

Through in situ variable-temperature single-crystal X-ray diffraction analysis, a solid–solid structural phase transition induced by the successive displacements of the [NMe4]+ guest and a subsequent abrupt order–disorder transform for both the cationic guest and the cage-like framework was tracked in four perovskite-like bimetallic azido coordination polymers: (NMe4)2­[B′·B″(N3)6] (B′ = Cr3+, Fe3+; B″ = Na+, K+). Such structural transition was also confirmed by differential scanning calorimeter measurement, variable-temperature powder X-ray diffraction analysis, and variable-temperature dielectric permittivity measurement, as well as molecular dynamics simulation. Conclusively, these compounds provide a good host–guest model for understanding and modulating the thermal motion behavior of the [NMe4]+ guest in various confined spaces constructed by the perovskite-like azido frameworks.