Researchers Prepare and Characterize First Transuranic Crown Ether Complex

Crown ether ligands have the ability to selectively bind metal ions of a characteristic size; thus, it's not surprising that crown ether ligands have been considered as potential extractants for actinide ions. They are a special type of polydentate ligand in which the ligating atoms lie approximately in a plane about the central metal atom, and the remainder of the molecule lies in a "crown" arrangement. All of the oxygen atoms of the polyether "point" inward towards the metal atom, and these macrocylic ligands have the unusual property of forming stable complexes with alkali metal ions. This exceptional stability is related to the cavity size of the ligand, with different cavity sizes favoring specific alkali metal ions. For example, 12-crown-4 (I) selectively forms a complex with Li⁺, 15-crown-5 (II) forms a complex best with Na⁺, and 18-crown-6 (III) favors K⁺.

Crown ether ligands have been shown to afford actinide extraction in hydrocarbon-water systems, and many of these studies invoke the standard cavity-size argument to explain the observed results. However, in spite of the observed extraction behavior, we note that there are relatively few examples of actinide crown ether complexes wherein the actinide ion is actually coordinated to the crown ether ligand by one or more oxygen donor atoms of the crown ligand itself. Examples of inclusion compounds of actinide trans dioxo cations AnO₂ⁿ⁺ completely encapsulated by a crown ether ligand are limited to only two uranium examples, [UO₂(18-crown-6)]²⁺ (IV) and [UO₂(dicyclohexano-18-crown-6)] ²⁺, with non-coordinating perchlorate or triflate counter ions in the crystal lattice. It has been argued that the synthesis of these rare uranyl crown ether inclusion complexes could be achieved only by the use of weakly coordinating anions, nonaqueous conditions, and proper choice of cavity size in the crown. Indeed, the majority of reports on actinide crown ether complexes reveal that the crown ether ligand prefers to reside in the outer sphere of the actinide ion and engage in second-sphere hydrogen bonding between the crown ether oxygen atoms and water molecules coordinated to the actinide metal center (as illustrated in V). Well-known examples of second sphere crown ether complexes would include [UO₂(H₂O)₅· 2(18-crown-6)·H₂O· 2CH₃CN][ClO₄] ₂ (V) and [UO₂(H₂O)₅·(18-crown-6)][CF_{3SO3] 2. There are no known examples of crown ether inclusion complexes (such as IV) containing a transuranic ion in any oxidation state.}

In an attempt to employ this well-known ability of crown ether ligands to form second-sphere hydrogen-bonded complexes of trans dioxo ions (as in V), we reacted the NpO₂²⁺ ion with 18-crown-6 in order to isolate single crystals of the Np(VI) aquo ion. This effort is part of our "Actinide Molecular Science" LDRD (Laboratory-Direct Research and Development) project with the Seaborg Institute. We were quite surprised to observe the unexpected and complete encapsulation of the NpO₂⁺ ion in acid solution by the 18-crown-6 ligand. Addition of one equivalent of 18-crown-6 to a stirring solution of NpO₂²⁺ (aq) in 1M perchloric or triflic acid results in reduction of Np^VI to Np^V, and deposition of large turquoise crystals of [NpO₂(18-crown-6)][X], [X = ClO₄, CF₃SO₃], after 12 to 24 hours. Near infrared (NIR) electronic absorption spectra of the crystalline solids dissolved in 1M HClO₄ confirms that reduction of Np^VI to Np^V has occurred based on the presence of the strong electronic absorption feature at 980 nm (e = 395 M^-1cm^-1) characteristic of the NpO₂⁺ ion. Even in the presence of ozone (O₃, a strong oxidant) Np^VI reduction and deposition of turquoise crystals of the Np^V crown complex was observed when 18-crown-6 was added. Ultimately, we find that 1M HX solutions of Np^V in the form NpO₂⁺ (aq) react smoothly with 18-crown-6 to give the [NpO₂(18-crown-6)][X], product in nearly quantitative crystalline yield.

Figure 1. Thermal ellipsoid plot showing the molecular structure of the [NpO₂(18-crown-6)]⁺ cationic unit of 1[ClO₄] (ellipsoids drawn at the 50% probability level).

A single-crystal x-ray diffraction study revealed that [NpO₂(18-crown-6)][X], contains an NpO₂⁺ ion completely encapsulated by a 18-crown-6 ligand. A thermal ellipsoid diagram of the [NpO₂(18-crown-6)]⁺ ion (Figure 1) shows two trans oxo ligands and six approximately coplanar crown ether O atoms coordinated to the NpO₂⁺ unit, forming an approximate hexagonal bipyramidal coordination environment about the Np center. The Np=O distance of 1.800(5) Å is unusually short for an NpO₂⁺ ion, (generally ca. 1.85 Å) while the average equatorial Np-O distance of 2.594(10) Å is unusually long for a neutral O-donor ligand. Coordination of s-donor ligands about the equatorial plane of an actinide trans dioxo ion generally results in a slight lengthening of the Np=O bond and a concomitant decrease in the equatorial Np-O bonds. In [NpO₂(18-crown-6)][X] however, the coordination of 18-crown-6 about the equatorial plane results in the opposite effect, namely a shortening, and presumably strengthening of the Np=O bond.

For trans dioxo ions, the Raman-active nu₁ vibrational mode of the O=An=O unit is a much better indication of bond strength than bond length determined by an x-ray diffraction analysis. For the [NpO₂(H₂O)₅⁺] ion, with Np=O = 1.85 Å, the nu₁ vibrational mode is 767 cm^-1. Raman spectra of [NpO₂(18-crown-6)][X], showed a strong, higher-frequency band at 778 cm^-1 consistent with a stronger Np=O bond than in the aquo ion. To confirm this spectroscopic assignment, an electrochemical labeling method was used to incorporate ¹⁸O into the Np=O unit.

¹⁸O-enriched H₂O (98% ¹⁸O) was added to 0.56 M NpO₂⁺ (aq) in 1M HClO₄ and reduced electrochemically at -0.2 V to a mixture of aquo Np³⁺/Np⁴⁺, thereby removing the oxo ligands). The potential was reversed, and the aquo Np³⁺/Np⁴⁺ reoxidized to Np18O22+ aq), where the oxo ligands were then enriched in ¹⁸O. Oxidation state purity was confirmed by NIR electronic absorption spectroscopy using the 1223 nm absorption feature (e = 45 M^-1cm^-1) of NpO₂²⁺ (aq). 18-crown-6 was added, and aquamarine crystals of [NpO₂(18-crown-6)][ClO₄] formed after the mixture cooled to 5°C overnight. The Raman spectra of ¹⁸O-enriched [NpO₂(18-crown-6)][ClO₄] crystals show nu₁ stretching frequencies for all three isotopic compositions: ¹⁸O =Np=¹⁸O, ¹⁸O =Np=¹⁶O, and ¹⁶O =Np=¹⁶O at 734, 751, and 780 cm^-1, respectively, confirming the original assignment.

The complete encapsulation of the NpO₂⁺ ion by a crown ether ligand under any conditions is unprecedented and represents the first structural and spectroscopic characterization of any transuranic crown ether complex. The relative ease of encapsulation in aqueous solutions contrasts with the necessary use of anhydrous conditions and poorly coordinating counter-ions to insure bonding of crown ether O atoms to the inner sphere of the related UO₂²⁺ ion. This difference attests to the ability of crown ether ligands to separate ions of nearly identical chemical behavior, and the apparent preference for Np^V over Np^VI may be related to the differing ionic radii of these ions. The complete encapsulation illustrates the additional point that the preferential encapsulation of a pentavalent actinide ion would never have been observed by studying uranium only, since uranium does not form a stable pentavalent state in aqueous solution. The potential ability of crown ether ligands to extract actinide ions selectively from radioactive process streams or nuclear waste solutions is particularly intriguing because of its economical and environmental clean-up implications.

Researchers on this project are David L. Clark, D. Webster Keogh, Phillip D. Palmer, Brian L. Scott, and C. Drew Tait.

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