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 [(Cu^I₄Cu^II₄L₄)·3H₂O] _n (1) containing unique mixed-valence [Cu^I₄Cu₄^IIL₄] 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 [Cu^I₄Cu^II₄L₄] _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, E_a 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.