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Alkali Metal Cation (K+, Cs+) Induced Dissolution/Reorganization of Porous Metal Carboxylate Coordination Networks in Water



Reversible structural transformations of porous metal–organic coordination networks based on Co or Ni and the benzene-1,2,4,5-tetracarboxylate tetraanion among 3D (4,8)-connected nets and a 1D zigzag chain structure occur by an unexpected dissolution/reorganization process induced by alkali metal cations (K+, Cs+) in aqueous solution (see scheme).

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Porous metal–organic coordination networks (MOCNs) {A2[M3(btec)2(H2O)4]}n (1, A=K, M=Co; 2, A=K, M=Ni; 3, A=Cs, M=Co; and 4, A=Cs, M=Ni; btec=benzene-1,2,4,5-tetracarboxylate) with nearly identical structural features were hydrothermally prepared. These compounds adopt (4,8)-connected scu nets but exhibit subtle differences in the topology of the final three-dimensional architectures. Compound 1 has a regular net with the largest solvent void (21.1 %), while the nets of the other three (24) are slightly distorted from a regular shape and have malformed pores with smaller solvent voids (5.4–11.4 %). Likely, the different supramolecular environments among 14 subtly depend on the eight-connected binodal cubical vertices/four-connected square-planar connectivity between the trimetallic clusters and the btec ligands. Cobalt species 3 dissolved in an aqueous solution of KCl, and then reorganized to form 1 at ambient temperature. Interestingly, under similar conditions, 1 dissolved and then was regenerated to give the same structure. Nickel species 2 and 4 also underwent a dissolution/reorganization process in an aqueous solution of KCl to afford new metal–carboxylate product {K2[Ni3(btec)2(H2O)4]}n (2′). This compound forms a (4,8)-connected scu net with regular pores, which is isostructural and isomorphous with 1, and is a supramolecular isomer of 2. Similarly, in an aqueous solution of CsCl, 14 were converted to 1D zigzag chain structures {Cs2[M(btec)(H2O)4]}n (5, M=Co; 6, M=Ni) that enlarged to hydrogen-bonded 3D porous supramolecular networks. Remarkable, reversible alkali metal cation induced structural transformations between 1 and 5 occurred via dissolution/reorganization processes. Thermogravimetric analyses showed that these metal–carboxylate species have high thermal stability (T>300 °C).

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