Tuning Intrinsic and Extrinsic Proton Conduction in Metal-Organic Frameworks by the Lanthanide Contraction.

Seven isomorphous lanthanide metal-organic frameworks in the PCMOF-5 family, [Ln(H5L)(H2O)n](H2O) (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 (10-3 S cm-1 versus 10-4 S cm-1). 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.

A Water Stable Magnesium MOF That Conducts Protons over 10(-2) S cm(-1).

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.

MOFs as proton conductors--challenges and opportunities.

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.

Highly proton conductive nanoporous coordination polymers with sulfonic acid groups on the pore surface.

Three new sulfonated porous coordination polymers (PCPs)/metal-organic frameworks (MOFs) have been synthesized using solvothermal methods. These PCPs possess porous structures with non-coordinating sulfonic acid groups or sulfonate with dimethyl ammonium cations and exhibit high proton conductivity at a low humidity of 60% RH (relative humidity) at ambient temperature.

Synthesis, structure, and solid-state transformation studies of phosphonoacetate based hybrid compounds of uranium and thorium.

Three new phosphonoacetate hybrid frameworks based on the actinide elements uranium and thorium have been synthesized. The compounds [C(4)N(2)H(14)][(UO(2))(2)(O(3)PCH(2)COO)(2)] x H(2)O, I, [C(4)N(2)H(14)][(UO(2))(2)(C(2)O(4))(O(3)PCH(2)COOH)(2)], II, and Th(H(2)O)(2)(O(3)PCH(2)COO)(C(2)O(4))(0.5) x H(2)O, III, are built up from the connectivity between the metal polyhedra and the phosphonoacetate/oxalate units. Compound II has been prepared using a solvent-free approach, by a solid state reaction at 150 degrees C. It has been shown that II can also be prepared through a room temperature mechanochemical (grinding) route. The layer arrangement in III closely resembles to that observed in I. The compounds have been characterized by powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and fluorescence studies.

Synthesis, structure, and transformation studies in a family of inorganic-organic hybrid framework structures based on indium.

Eight new open-framework inorganic-organic hybrid compounds based on indium have been synthesized employing hydrothermal methods. All of the compounds have InO(6), C(2)O(4), and HPO(3)/HPO(4)/SO(4) units connected to form structures of different dimensionality. Thus, the compounds have zero- (I), two- (II, III, IV, V, VII, and VIII), and three-dimensionally (VI) extended networks. The formation of the first zero-dimensional hybrid compound is noteworthy. In addition, concomitant polymorphic structures have been observed in the present study. The molecular compound, I, was found to be reactive, and the transformation studies in the presence of a base (pyridine) give rise to the polymorphic structures of II and III, while the addition of an acid (H(3)PO(3)) gives rise to a new indium phosphite with a pillared layer structure (T1). Preliminary density functional theory calculations suggest that the stabilities of the polymorphs are different, with one of the forms (II) being preferred over the other, which is consistent with the observed experimental behavior. The oxalate units perform more than one role in the present structures. Thus, the oxalate units connect two In centers to satisfy the coordination requirements as well as to achieve charge balance in compounds II, IV, and VI. The terminal oxalate units observed in compounds I, IV, and V suggest the possibility of intermediate structures. Both in-plane and out-of-plane connectivity of the oxalate units were observed in compound VI. The compounds have been characterized by powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and (31)P NMR studies.