We explored the proton conductivities of two 3D Co metal-organic frameworks (MOFs), {[Co(m-ClPhIDC)(HO)]·2HO} [1; m-ClPhHIDC = 2-(m-chlorophenyl)imidazole-4,5-dicarboxylic acid] and {[Co(p-ClPhHIDC)(HO)]·6HO} (2; p-ClPhHIDC = 2-(p-chlorophenyl)imidazole-4,5-dicarboxylic acid), under water and aqua-ammonia vapors, respectively. The experimental results revealed that the proton conductivities of 1 and 2 at aqua-ammonia vapor were 2.89 × 10 and 4.25 × 10 S/cm, respectively, and approximately 2 orders of magnitude greater than those at water vapor. On the basis of the activation energy, water and ammonia vapor absorption, and powder X-ray diffraction patterns, their proton-conduction mechanisms have been discussed. We believe that this is a novel approach to drastically improving the proton conductivity of MOFs.

Our investigation on the proton conductivities of two water-stable isostructural 3D Co(II) MOFs, {[Co(DMPhIDC)(HO)]·2HO} (1) [DMPhHIDC = 2-(3,4-dimethylphenyl)-imidazole-4,5-dicarboxylic acid] and {[Co(m-BrPhIDC)(HO)]·2HO} (2) [m-BrPhHIDC = 2-(m-bromophenyl)-imidazole-4,5-dicarboxylic acid], under water or aqua-ammonia vapor shows that the optimized proton conductivities of both 1 and 2 under aqua-ammonia vapor are 4.41 × 10 S·cm and 5.07 × 10 S·cm (at aqua-ammonia vapor from 1.5 M NH·HO solution and 100 °C), respectively, which are approximately 1 order of magnitude greater than those maximum values (8.91 × 10 S·cm and 7.64 × 10 S·cm) under water vapor (at 98% RH and 100 °C). The plausible proton pathways and mechanisms of the MOFs have been proposed in terms of the structural analyses, activation energy calculations, water and NH vapor absorptions, and PXRD determinations.

By reaction of a newly designed organic ligand, [3-(naphthalene-1-carbonyl)-thioureido] acetic acid (CHC(O)NHC(S)NHCHCOOH; HL), with Cu(OAc), a metal-organic framework [(CuCuL)·3HO] (1) containing unique mixed-valence [CuCuL] 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 [CuCuL] -NH (2) is procured upon exposure of 1 to NH vapors from 28 wt % aqueous NH solution, which bears 2 NH and 4 HO 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 S cm, which is 2 orders of magnitude larger than that of MOF 1 (4.90 × 10 S cm) under 100 °C and 98% RH. On the basis of the structural data, E values, HO 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 NH molecules.

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