A large ionic water cluster H(H(2)O)(28)(+), consisting of a water shell (H(2)O)(26) and an encaged species H(H(2)O)(2)(+) as a center core, was trapped in the well-modulated cavity of a porous metal-organic framework (MOF) {[Co(4)(dpdo)(12)(PMo(12)O(40))(3)](-)}(infinity) and structurally characterized. Degeneration of the protonated water cluster H(H(2)O)(28)(+) into a smaller cluster H(H(2)O)(21)(+) and recovery of H(H(2)O)(28)(+) from the resulting H(H(2)O)(21)(+) cluster in a reversible way demonstrated the unusual stability of the protonated water clusters H(H(2)O)(28)(+) and H(H(2)O)(21)(+) in the robust crystal host. Proton transport and proton/potassium ion exchange through the channels of the crystal host have been investigated by a well-established fluorometry method. X-ray fluorescence experiments and X-ray structural analyses of the exchanged crystals confirmed the occurrence of the proton/potassium ion-exchange reaction and the transformation of the protonated water cluster H(H(2)O)(28)(+) to an ionic cluster K(H(2)O)(27)(+). Comparison of the H(+)/K(+) exchange of H(H(2)O)(28)(+) with that of its neighboring protonated water cluster H(H(2)O)(27)(+) suggested that the abundance of hydrogen bonds associated with the hydronium/water cluster in the H(H(2)O)(28)(+) cluster was essential for proton transport through the Grotthuss mechanism. On the basis of the results, our porous network could be described as a synthetic non-peptide ion channel, in terms of not only structural features but also the functions addressed. Direct observation of the structures of various large ionic water clusters trapped by porous MOFs, coupled with the proton/ion-exchange processes and the reversible dehydration/rehydration, provided valuable insights into the aqueous proton transfer and its mobility pertaining to the large protonated water clusters in the condensed phase.

A large protonated water cluster, H+(H2O)27, has been trapped and stabilized within the well-modulated cavity of a 3D metal-organic framework formed by cobalt(II) and 4,4'-bipyridine-N,N'-dioxide with a globular Keggin structure [PW12O4]3- anion as template. The structurally characterized protonated water cluster might comprise a (H2O)26 shell with Oh symmetry and a monowater core within the center which is suggested to be a hydronium ion with the Eigen model.

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%.