Enhancing Proton Conductivity of a 3D Metal-Organic Framework by Attaching Guest NH3 Molecules.

By reaction of a newly designed organic ligand, [3-(naphthalene-1-carbonyl)-thioureido] acetic acid (C10H7C(O)NHC(S)NHCH2COOH; H3L), with Cu(OAc)2, a metal-organic framework [(CuI4CuII4L4)·3H2O] n (1) containing unique mixed-valence [CuI4Cu4IIL4] subunits has been successfully synthesized and structurally characterized. MOF 1 displays a three-dimensional open framework bearing one-dimensional channels. Consequently, a new derivative MOF [CuI4CuII4L4] n-NH3 (2) is procured upon exposure of 1 to NH3 vapors from 28 wt % aqueous NH3 solution, which bears 2 NH3 and 4 H2O molecules in accordance with the elemental and thermal analyses. Both 1 and 2 exhibit relatively high water stability, whose proton conduction properties under water vapor have been researched. Notably, 2 shows an ultrahigh proton conductivity of 1.13 × 10-2 S cm-1, which is 2 orders of magnitude larger than that of MOF 1 (4.90 × 10-4 S cm-1) under 100 °C and 98% RH. On the basis of the structural data, Ea values, H2O and ammonia vapor absorptions, and PXRD measurements, the proton transfer mechanisms were suggested. This is an efficient and convenient way to obtain suitable and highly proton-conducting materials by attaching NH3 molecules.