Recently, research on metal-organic frameworks (MOFs) serving as a new type of proton conductive material has resulted in many exciting achievements. However, direct observation of a well-established proton-transfer mechanism still remains challenging in MOFs and other crystalline compounds, let alone other conductive materials. Herein we report the solvothermal synthesis of a new proton-conducting MOF, (MeNH)[Eu(L)] (HL = 5-(phosphonomethyl)isophthalic acid). The compound consists of a layered anionic framework [Eu(L)] and interlayer-embedded counter cations (MeNH), which interact with adjacent uncoordinated O atoms of phosphonate groups to form strongly (N-H···O) hydrogen-bonded chains aligned parallel to the c-axis. Facile proton transfer along these chains endows the compound with single-crystal anhydrous conductivity of 1.25 × 10 S·cm at 150 °C, and water-assisted proton conductivity for a compacted pellet of microcrystalline crystals attains 3.76 × 10 S·cm at 100 °C and 98% relative humidity (RH). Proton dynamics (vibrating and transfer) within N-H···O chains of the compound are directly observed using a combination of anisotropic conductivity measurements and control experiments using large single-crystals and pelletized samples, in situ variable-temperature characterization techniques including powder X-ray diffraction (PXRD), single-crystal X-ray diffraction (SCXRD), diffuse reflectance infrared Fourier transform spectrum (DRIFTS), and variable-temperature photoluminescence. In particular, a scarce single-crystal to single-crystal (SCSC) transformation accompanied by proton transfer between an anionic structure (MeNH)[Eu(L)] and an identical neutral framework [Eu(HL)] has been identified.

Three new alkaline earth metal based metal-organic frameworks (MOFs), namely M-BPTC (M = Mg, Sr, Ba), have been synthesized by using BPTC (2,2',6,6'-tetracarboxybiphenyl) as ligand under hydrothermal conditions. These MOFs exhibit interesting structural diversity, variable chemical and thermal stability, as well as proton conductivity. Mg-BPTC with the formula {[Mg(BPTC)0.5(H2O)3]·5H2O}n consists of BPTC(4-) extended metal layers, and novel highly ordered infinite tape-like structures of cyclic water octamers reside interlayer. Three-dimensional porous {[Sr2(BPTC)(H2O)6]·H2O}n (Sr-BPTC) features inorganic Sr-O chains (I(1)O(2)) and open hydrophilic channels where water heptamers and carboxyl oxygen atoms conspire to form H-bond networks, whereas 3D {[Ba6(BPTC)3(H2O)6]·11H2O}n (Ba-BPTC) shows Ba-O inorganic layer (I(2)O(1)) and 1D channels incorporating large water 14-mers and 18-mers. M-BPTC (M = Mg, Sr) species exhibit excellent water stability and proton conductivity due to their respective appropriate pathways for proton transporting. M-BPTC (M = Sr, Ba) structures are highly thermally stable due to the presence of the inorganic connectivity. The present results suggest that M-BPTC (M = Mg, Sr) are promising materials for proton conduction and provide insight into the hydrogen bonding motif.

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