Crystal structures and proton conductivities of a MOF and two POM-MOF composites based on Cu(II) ions and 2,2'-bipyridyl-3,3'-dicarboxylic acid.

We have succeeded in constructing a metal-organic framework (MOF), [Cu(bpdc)(H(2)O)(2)](n) (H(2) bpdc=2,2'-bipyridyl-3,3'-dicarboxylic acid, 1), and two poly-POM-MOFs (POM=polyoxometalate), {H[Cu(Hbpdc)(H(2)O)(2)](2) [PM(12)O(40)]·nH(2)O}(n) (M=Mo for 2, W for 3), by the controllable self-assembly of H(2) bpdc, Keggin-anions, and Cu(2+) ions based on electrostatic and coordination interactions. Notably, these three compounds all crystallized in the monoclinic space group P2(1)/n, and the Hbpdc(-) and bpdc(2-) 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%.