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Keywords:

  • conducting materials;
  • metal–organic frameworks;
  • polyoxometalates;
  • proton transport;
  • structure elucidation

Abstract

We have succeeded in constructing a metal–organic framework (MOF), [Cu(bpdc)(H2O)2]n (H2bpdc=2,2′-bipyridyl-3,3′-dicarboxylic acid, 1), and two poly-POM–MOFs (POM=polyoxometalate), {H[Cu(Hbpdc)(H2O)2]2[PM12O40]n H2O}n (M=Mo for 2, W for 3), by the controllable self-assembly of H2bpdc, Keggin-anions, and Cu2+ ions based on electrostatic and coordination interactions. Notably, these three compounds all crystallized in the monoclinic space group P21/n, and the Hbpdc and bpdc2− ions have the same coordination mode. Interestingly, in compounds 2 and 3, Hbpdc and the Keggin-anion are covalently linked to the transition metal copper at the same time as polydentate organic ligand and as polydentate inorganic ligand, respectively. Complexes 2 and 3 represent new and rare examples of introducing the metal N-heterocyclic multi-carboxylic acid frameworks into POMs, thereby, opening a pathway for the design and the synthesis of multifunctional hybrid materials based on two building units. The Keggin-anions being immobilized as part of the metal N-heterocyclic multi-carboxylic acid frameworks not only enhance the thermal stability of compounds 2 and 3, but also introduce functionality inside their structures, thereby, realizing four approaches in the 1D hydrophilic channel used to engender proton conductivity in MOFs for the first time. Complexes 2 and 3 exhibit good proton conductivity (10−4 to ca. 10−3 S cm−1) at 100 °C in the relative humidity range 35 to about 98 %.