Isolation of a Uranyl [UO₂]⁺ Species: Crystallographic Comparison of the Dioxouranium(V) and (VI) Compounds [UO₂(OPPh₃)₄](OTf)_n (n=1, 2)

Abstract

The occurrence of the actinyl(v) species [AnO 2 ]+ (An = U, Np, Pu, Am) in the nuclear industry and in the natural environment is a particular reason for the current studies of their physicochemical properties.[ (n = 4, 5), [8] [UO 2 (H 2 O) 5 ] n+ (n = 1, 2) [9] and bare [UO 2 ] n+ ions (n = 1, 2) [8] were predicted from theoretical calculations using a variety of methods. In this context, the isolation of a pentavalent uranyl derivative, and the comparison of its crystal structure with that of its [UO 2 ] 2+ parent compound are highly desirable.In the course of our studies on the synthesis and structure of anhydrous uranyl triflate compounds, [10] Figure 1. The geometrical parameters are classical of hexavalent uranyl complexes; the linear {UO 2 } fragment is perpendicular to the equatorial plane defined by the uranium center and the four oxygen atoms of the OPPh 3 ligands. The mean U¼O bond length of 1.76(1) and the average equatorial UÀO bond length of 2.29(1) are identical to those found in [UO 2 {OP(NMe 2 ) 3 } 4 ](I 3 ) 2 . [12] In one attempt to crystallize complex 1, small red-orange, cube-shaped crystals were deposited in a small quantity along with the yellow crystals of the [UO 2 ] 2+ compound. X-ray diffraction analysis revealed that these crystals were of a new complex, the pentavalent uranyl derivative [UO 2 -(OPPh 3 ) 4 ](OTf) (2).[11] The unexpected formation of 2 requires elucidation; all attempts to produce it by reduction of 1 with UV light or various reducing agents are inconclusive at this time, and further work is in progress for controlling the synthesis of 2. Nevertheless, complex 2 was found to be kinetically stable under an inert atmosphere, which demonstrates that such [UO 2 ] + complexes can be isolated in the crystalline form, without suffering disproportionation.Crystals of 2 are composed of discrete cation-anion pairs; a view of one of the two independent [UO 2 (OPPh 3 ) 4 ]+ ions is represented in Figure 2. The cations of 1 and 2 adopt the same perfectly square bipyramidal configuration; in 2, the uranium center is located at a center of symmetry. The U ¼ O bond lengths of 1.817(6) and 1.821(6) are slightly longer than the Np¼O bond (1.797(2) ) in the isostructural [NpO 2 (OPPh 3 ) 4 ] + ion, [2] in agreement with the variation in

Synthesis, crystal structure, and spectra of the Np(V) terephthalate [C(NH2)3]3[NpO2(C6H4(COO)2)2]·5H2O and An(VI) terephthalates [AnO2(C6H4(COO)2)] (An = U, Np, Pu)

“…However, tetragonal bipyramids constituted exclusively by O atoms are untypical of the coordination surrounding of the An(VI) atoms. Such polyhedron usually occurs in compounds with bulky organic ligands, e.g., with O=PPh 3 [17][18][19][20][21], O=P(NMe 2 ) 3 [22], and (O=PPh 2 ) 2 N - [23], or with ligands containing long alkyl chains [24][25][26]. …”

Actinyl chemistry across the U, Np and Pu series

“…Many uranium phosphine oxide complexes have been structurally and spectroscopically characterised [14][15][16][17][18]. The triphenyl phosphine oxide ligand (TPPO) has been the subject of numerous investigations with both the uranyl and neptunyl moieties [16,[19][20][21][22]. At the CRR, we have also used TPPO as a co-ligand for uranyl(VI) and neptunyl(V) in the preparation of several novel complexes [23][24][25].…”

Anhydrous photochemical reduction of well-defined uranyl(VI) complexes

“…A trans-octahedral UX 2 L 4 geometry is observed for the uranium center, consisting of two axial methoxide groups and four neutral OPPh 3 ligands occupying the equatorial sites, with minimal deviation observed from idealized tetragonal geometry. Thus the solid-state structure of 3a closely resembles that of the precursor 1a except for the replacement of methoxide groups for the dioxo unit in the axial positions [16]. The U(1)-O(3) bond distance of 2.0575(5)Å is comparable to the terminal U-O bonds found in other structurally characterized uranium methoxide complexes [17], while the open U(1)-O(3)-C(13) bond angle of 180.000(1) • is typical of uranium alkoxide complexes and may also reflect enhanced -bonding analogous to the favorable bonding interactions that reinforce the trans-dioxo geometry.…”