Synthesis and Structure of the First Transuranium Crown Ether Inclusion Complex: [NpO2([18]Crown-6)]ClO4

Abstract

Crown ether ligands [1,2] have been studied as potential actinide extractants for many years, and they have been shown to influence the partitioning of actinides in two-phase aqueous systems. [3±5] Although many examples of crown ether ligands that extract actinides in hydrocarbon/water systems are known, [6,7] relatively few actinide crown ether complexes in which the actinide ion is coordinated by one or more donor atoms of the crown ether have been reported. [8] There are only two inclusion compounds of trans-dioxoactinide cations AnO n 2 in which the actinide ion is completely encapsulated by a crown ether ligand and the counterions (perchlorate [9,10] or triflate [11] ) are uncoordinated in the crystal lattice: [UO 2 ([18]crown-6)] 2 (I) and [UO 2 (dicyclohexano[18]crown-6)] 2 . It has been claimed that synthesis of inclusion complexes from uranyl ions UO 2 2 and crown ether ligands requires the use of weakly coordinating anions, nonaqueous conditions, and proper choice of cavity size. [8] Indeed, the majority of actinide crown ether complexes exhibit secondsphere hydrogen bonding between the oxygen atoms of the crown ether and water molecules coordinated to the actinide center. [12±19] Well-known examples include [UO 2 (H 2 O) 5 ([18]crown-6) 2 (H 2 O)(CH 3 CN) 2 ][ClO 4 ] 2 (central unit: II) [15] and [UO 2 (H 2 O) 5 ([18]crown-6)][CF 3 SO 3 ] 2 . [17] Crown ether inclusion complexes of a transuranic ion in any oxidation state are unknown.On attempting to employ this well-known ability of crown ether ligands to form second-sphere hydrogen-bonded complexes of trans-dioxometal ions (as in II), we were surprised to observe the complete encapsulation of the NpO 2 ion by the [18]crown-6 ligand. Addition of one equivalent of [18]crown-6 to a stirred solution of NpO 2 2 in 1m HX (X ClO À 4 , CF 3 SO À 3 ) resulted in reduction of Np VI to Np V and formation of large turquoise crystals of [NpO 2 ([18]crown-6)][X] (1[X]) over 12 ± 24 h. The presence of an absorption band at 980 nm (e 395 m À1 cm À1 ), characteristic of the NpO 2 ion, [20] in the NIR spectra of the crystalline solids dissolved in 1m HClO 4 confirms that reduction of Np VI to Np V has occurred. Even in the presence of O 3 , the Np VI was reduced to 1[X] on addition of [18]crown-6. Finally, solutions of Np V compounds in 1m HX, which are present as hydrated NpO 2 ions, react smoothly with [18]crown-6 to give 1[X] in almost quantitative yield.A single-crystal X-ray diffraction study of 1[ClO 4 ] revealed an NpO 2 ion completely encapsulated by a disordered [18]crown-6 ligand. [21] The Np center of the [NpO 2 ([18] crown-6)] ion is coordinated by two trans oxo ligands and six approximately coplanar crown ether O atoms to give an approximate hexagonal bipyramidal coordination environ-

Computational Study of Actinyl Ion Complexation with Dipyriamethyrin Macrocyclic Ligands

“…As shown in Table , the calculated bond lengths are slightly overestimating both axial (for UO by about 0.022 Å) and equatorial (U–N by about 0.020–0.055 and 0.050–0.061 Å for pyrrolic and pyridine nitrogen, respectively) bonds when compared to the experimentally predicted values. A similar agreement with experiment including a slight overestimation of bond lengths has been seen before for GGA functionals such as PBE. − , The calculated OUO bond angle is about 180.0°, which agrees well with the experimental value of 179.4°. It is worth mentioning that the crystal structure of the U VI O 2 L complex is characterized by a distorted hexagonal bipyramidal geometry, in agreement with the calculated gas phase structure.…”

“…A similar agreement with experiment including a slight overestimation of bond lengths has been seen before for GGA functionals such as PBE. [29][30][31][32][33]35 The calculated OUO bond angle is about 180.0°, which agrees well with the experimental value of 179.4°. It is worth mentioning that the crystal structure of the U VI O 2 L complex is characterized by a distorted hexagonal bipyramidal geometry, in agreement with the calculated gas phase structure.…”

“…During the past decades, Sessler and co-workers have reported a large number of expanded porphyrin actinyl complexes. ,− Expanded porphyrins were reported to form stable complexes with AnO 2 n + ions, including superphthalocyanine, , sapphyrin and oxasapphyrin, pentaphyrin, alasakaphyrin, amethyrin, , isoamethyrin, the Schiff-base Pacman calixpyrrole, and cyclo[6]­pyrrole . Other important families of macrocyclic ligands are calixarenes − and crown ethers, − the capacity of which to form stable complexes with AnO 2 n + ions is known as well. In addition to the experimental studies, a large number of quantum chemical investigations on actinide complexes of various macrocycles have been reported. − An important theoretical work relevant to the current study is Shamov and Schreckenbach’s theoretical investigation of uranyl isoamethyrin complexes, in which they reported that the inclusion of peripheral substituents was essential for reproducing the experimental geometry.…”

Synthesis and Characterizations of a Plutonium(III) Crown Ether Inclusion Complex

“…That is, Sessler et al demonstrated increased covalency trends on the ligand–cation orbital interactions through structurally characterizing a series of Th­(IV)-, U­(IV)-, and Np­(IV)-containing porphyrin complexes . Clark et al pioneered the first transuranic macrocyclic complex of Np V O 2 + -18-crown-6 inclusion complex . Furthermore, Np VI O 2 2+ was proved to be stabilized within a functionalized crown ether .…”

Complex Uranyl Dichromates Templated by Aza-Crowns

“…It is of interest to explore a template effect when employing organic species with restricted adaptability and fixed mutual positioning of the donor atoms. Macrocycles including crown ethers and particularly their aza derivatives seem to be the most proper candidates due to their applications in actinide partitioning [28,29], given their high selectivity for uranium [30] and neptunium [31,32]. Recently, crown ether complexes with alkali metals [33] and aza-crown ether complexes with Ni [34] were used as assemblers and linkers in uranyl-organic coordination polymers.…”