Crystal structures of three uranyl-acetate-bipyridine complexes crystallized from hydraulic fracking fluid

Hydraulic fracking exposes shale plays to acidic hydraulic fracking fluid (HFF), releasing toxic uranium (U) along with the desired oil and gas. With no existing methods to ensure U remains sequestered in the shale, this study sought to add organic ligands to HFF to explore potential U retention in shale plays. To test this possibility, incubations were set up in which uranyl acetate and one organic bipyridine ligand (either 2,2'-, 2,3'-, 2,4'-, or 4,4'-bipyridine) were added to pristine HFF as the crystallization medium. After several months and complete evaporation of all volatiles, bulk yellow crystalline material was obtained from the incubations, three of which yielded crystals suitable for single-crystal analysis, resulting in two novel structures and a high-quality structure of a previously described compound. The UO₂^VI acetate complexes bis(acetato-κ²O,O')(2,2'-bipyridine-κ²N,N')dioxidouranium(VI), [U(C₂H₃O₂)₂O₂(C₁₀H₈N₂)₂] or [2,2'-bipyridine]U^VIO₂(CH₃CO₂)₂, (I), and bis(acetato-κ²O,O')(2,4'-bipyridine-κN^1')dioxidouranium(VI), [U(C₂H₃O₂)₂O₂(C₁₀H₈N₂)₂] or [2,4'-bipyridine]₂U^VIO₂(CH₃CO₂)₂, (III), contain eight-coordinate U^VI in a pseudo-hexagonal bipyramidal coordination geometry and are molecular, packing via weak C-H...O/N interactions, whereas catena-poly[bis(2,3'-bipyridinium) [di-μ-acetato-μ₃-hydroxido-μ-hydroxido-di-μ₃-oxido-hexaoxidotriuranium(VI)]-2,3'-bipyridine-water (1/1/1)], (C₁₀H₉N₂)₂[U₃(C₂H₃O₂)₂O₈(OH)₂]·C₁₀H₈N₂·H₂O or {[2,3'-bipyridinium]₂[2,3'-bipyridine][(U^VIO₂)₃(O)₂(OH)₂(CH₃CO₂)₂·H₂O]}_n, (II), forms an ionic one-dimensional polymer with seven-coordinate pentagonal bipyramidal U^VI centers and hydrogen-bonding interactions within each chain. The formation of these crystals could indicate the potential for bipyridine to bind with U in shale during fracking, which will be explored in a future study via ICP-MS (inductively coupled plasma mass spectrometry) analyses of U concentration in HFF/bipyridine/shale incubations. The variation seen here between the molecular structures may indicate variance in the ability of bipyridine isomers to form complexes with U, which could impact their ability to retain U within shale in the context of fracking.