Proton conducting materials have garnered immense attention for their role as electrolytes in fuel cells. Metal Organic Frameworks (MOFs) and coordination polymers have recently been investigated as possible candidates for proton-conducting applications. Their crystallinity, chemically functionalizable pores and options for systematic structural variation are some of the factors that allow for the targeted design of better proton conductors operating over a wide variety of temperatures and/or humidity conditions. This review will examine selected examples from this nascent field, and will focus on the design and synthesis of proton conducting MOFs, their properties and conditions under which they operate.

Two complementary design strategies, isomorphous ligand replacement and heterocycle doping, have been applied to iteratively enhance the proton conductivity of a metal-organic framework, β-PCMOF2. The resulting materials, PCMOF2/(Pz) and PCMOF2/(Tz) (Pz = 1H-pyrazole, Tz = 1H-1,2,4-triazole), have their proton conduction raised almost 2 orders of magnitude compared to β-PCMOF2. The bulk conductivities of these materials are over 10 S cm at 85 °C and 90% relative humidity (RH), while maintaining the parent MOF structure. A solid state synthetic route for doping 1-D channels is also presented.

From the outset of the study of MOFs as proton conductors, both conductivity and hydrolytic robustness of the materials have needed to be improved. Here, we report a layered magnesium carboxyphosphonate framework, PCMOF10, that shows an extremely high proton conductivity value of 3.55 × 10(-2) S·cm(-1) at 70 °C and 95% RH. Moreover, PCMOF10 is water stable owing to strong Mg phosphonate bonding. The 2,5-dicarboxy-1,4-benzenediphosphonic acid (H6L) linker anchors a robust backbone and has hydrogen phosphonate groups that interact with the lattice water to form an efficient proton transfer pathway.

Seven isomorphous lanthanide metal-organic frameworks in the PCMOF-5 family, [Ln(HL)(HO)n](HO) (L = 1,2,4,5-tetrakis(phosphonomethyl)benzene, Ln = La, Ce, Pr, Nd, Sm, Eu, Gd) have been synthesized and characterized. This family contains 1-D water-filled channels lined with free hydrogen phosphonate groups and gives a very low activation energy pathway for proton transfer. The lanthanide contraction was employed to systematically vary the unit cell dimensions and tune the proton conducting pathways. LeBail fitting of the crystalline series shows that the crystallographic a-axis, along the channel, can be varied in increments less than 0.02 Å correspondingly shortening the proton transfer pathway. The proton conductivities for the La and Pr complexes were roughly an order of magnitude higher than other members of the series (10 S cm versus 10 S cm). Single crystal structures of the high and low conducting members of the series (La, Pr for high and Ce for low) affirm the structural similarities extend beyond the unit cell parameters to positions of free acid groups and included water molecules. Scanning electron microscopy reveals marked differences in particle size of the different members of the Ln series owing to lattice strain effects induced by changing the lanthanide. Notably, the high conducting La and Pr complexes have the largest particle sizes. This result contradicts any notion that degradation of the MOF at grain boundaries is enabling the observed conductivity as proton conduction dominated by extrinsic pathways would be enabled by small particles (i.e., the La and Pr complexes would be the worst conductors). Proton conductivity measurements of a ball milled sample of the La complex corroborate this result.

Experimental details and spectroscopic and gas sorption data (PDF) Crystallographic information file (CIF)

Combined experimental and computational efforts have demonstrated the utility of divergent photocyclization/1,4-sigmatropic rearrangement reactions for developing a general strategy toward the synthesis of cubebane-, spiroaxane-, and guaiane-type sesquiterpenes and related analogues. The configuration of the bridgehead substituent, the choice of solvent, and the wavelength of irradiation all impact diastereoselectivity in this tandem reaction process.

We report a cesium sulfonate metal organic framework (Cs 3 (L)(H 2 O) 3.3 , 1, L = 1,3,5-trisulfonato-2,4,6-trihydroxybenzene) with hydrated channels that conduct protons at 1.1 × 10 −5 S cm −1 at 50% relative humidity and 70°C. Water was crystallographically observed in the structure, and the likely proton-transfer pathway was studied. The material was not robust, and appropriate parameters were employed to ensure meaningful data were extracted from the study.Résumé : Nous présentons le composé 1 de formule Cs 3 (L)(H 2 O) 3.3 (L = 1,3,5-trisulfonato-2,4,6-trihydroxybenzène), un réseau métallo-organique de sulfonate de césium, dont les canaux hydratés permettent une conduction protonique à 1,1 × 10 −5 S cm −1 à un taux d'humidité relative de 50 % à 70°C. Nous avons observé l'eau dans la structure par cristallographie et nous avons étudié la voie probable de transfert de proton. Le composé n'est pas robuste, et nous avons employé des paramètres appropriés pour nous assurer de tirer de l'étude des données significatives. [Traduit par la Rédaction]Mots-clés : réseaux métallo-organiques, conduction protonique, césium, sulfonate.

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