Synthesis, Crystal Structures, and Proton Conductivity of Two Linear-Chain Uranyl Phenylphosphonates
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Two uranyl phenylphosphonates, [UO2(HO3PC6H5)2(H 2O)]2·8H2O (1) and UO2(HO3PC6H5) 2·2CH3CH2OH (2), have been synthesized and their structures solved by single-crystal methods. Both compounds crystallize in the triclinic space group P1̄. Unit cell parameters are a = 11.724(4) Å, b = 16.676(8) Å, c = 11.375(2) Å, α = 101.61(5)°, β= 106.76(3)°, γ = 102.57(4)°, and Z = 2 for compound 1 and a = 9.332(6) Å, b = 11.48(1) Å, c = 5.672(2) Å, α = 98.98(6)°, β= 92.78(5)°, γ = 108.54(5)°, and Z = 1 for compound 2. In both compounds the metal to phosphonate ratio is 1:2, and they both form linear chains. Each metal atom in these structures is surrounded by four oxygens of four different phosphonate groups which take up the metal's equatorial positions. However, in compound 1 an additional oxygen of a water molecule binds to the uranium atom in the equatorial plane. This leads to a distorted pentagonal bipyramidal geometry of uranium in compound 1 and to a distorted octahedral geometry of uranium in compound 2. Adjacent uranium atoms are bridged by two phosphonates, and every phosphonate group uses only two of its oxygens for this purpose. The third oxygen is not involved in metal coordination, and it is protonated. In compound 1, all phenyl rings of each uranyl phosphonate chain point into one general direction, perpendicularly to the chain, and the adjacent chains orient their "phenyl sides" almost toward each other to form planes of alternating hydrophobic and hydrophilic regions. The hydrophilic regions are filled with solvent water molecules. In compound 2, the phenyl rings also point perpendicularly to the chains, but in opposite directions, and the chains are stacked in a "staircase" fashion without forming regions of different natures as in compound 1. Compound 1 also exhibits a reasonably high proton conductivity σ(25 °C) = 3.25 × 10-3 Ω-1 cm-1 at 85% humidity due to the Bronsted acidity of its phosphonate OH groups. Both compounds are unstable in air mainly due to the loss of their solvent molecules.
author list (cited authors)
Grohol, D., Subramanian, M. A., Poojary, D. M., & Clearfield, A.