Topological polymorphism and temperature-driven topotactic transitions of metal–organic coordination polymers

Abstract

Two metal–organic frameworks (MOFs), [Mg₃(btdc)₃(dmf)₄]·DMF (1) and [Mg₃(btdc)₃(dmf)₄] (2) (H₂btdc = 2,2′-bithiophene-5,5′-dicarboxylic acid; dmf = N,N-dimethylformamide), were synthesized from a similar reaction mixture at 80 °C or 130 °C, respectively. Both compounds are based on {Mg₃(RCOO)₆} trinuclear carboxylate units, serving as 6-connected nodes of the corresponding 3D frameworks. The low-temperature phase (1) adopts the frequent primitive-cubic (pcu) topology, while the high-temperature phase (2) crystallizes in the very rare sxb topology. The heating of 1 results in a facile solid-state phase transition into 2 both in DMF solvent and under solvent-free conditions, involving a significant rearrangement of bulky bithiophenedicarboxylate anions in a regular fashion. The detailed analysis of the crystal structures of both polymorphs reveals possible reaction sites and provides valuable insights into such an unprecedented topotactic process.