A spatially separated [KBr₆]⁵⁻ anion in the cyanido-bridged uranium(IV) compound [U₂(CN)₃(NH₃)₁₄]⁵⁺[KBr₆]⁵⁻·NH₃

1 Introduction

While a multitude of cyanide complexes of the transition metals are known [1], [2], only a few cyanido complexes of uranium(IV) have been reported [3], [4], [5], [6], [7], [8], [9], [10]. The first actinoid compound containing cyanide anions was K₂[UO₂(CN)₄] described by Aloy, followed by K₂[UO₂(CN)₂(NO₃)₂] [11], [12]. However, the existence of both compounds was doubted and literature suggests to regard them as non-existent [13], [14]. Besides uranium compounds containing hexacyanido complexes of group eight elements, such as {[U(H₂O)₁₀][M(CN)₆]} (M^II=Fe, Ru, Os), and some tetracyanidoplatinate(II) complexes like {U₂(H₂O)₁₀(O)[Pt(CN)₄]₃}·4H₂O, also some CN⁻-containing compounds with organic ligands are described [15], [16]. These are, for example [UN*₃CN] (N*=N(SiMe₃)₂) or [M][(UN*₃)₂(μ-CN)] (M=K(18-crown-6), N*=N(SiMe₃)₂), of which the crystal structures have been determined [8], [9]. Also, some CN⁻-bridged uranium complexes with sterically demanding ligands are known [6], [7], [9], [17], [18], [19]. To the best of our knowledge a pseudo-binary uranium cyanide, such as U(CN)₄, is still unknown. Our attempts and investigations on the synthesis of uranium cyanides in liquid ammonia led to the discovery of the light-green compound [UCl₃(CN)·4NH₃], obtained from UCl₄ and NaCN in liquid NH₃ [3], [20]. The reaction of UCl₄ with one equivalent of KCN in liquid ammonia at room temperature gave green crystals of [U(CN)(NH₃)₈]Cl₃·NH₃ [20]. Uranium tetraiodide, UI₄, reacts with one equivalent of KCN leading to a compound with infinite chains of cyanido- and amido-bridged uranium atoms in the crystal structure [20]. Hexaamminetricyanidouranium(IV) iodide [U(CN)₃(NH₃)₆]I can be obtained by increasing the concentration of KCN and heating the reaction mixture [20]. Here we present the synthesis and crystal structure of the cyanido bridged uranium(IV) compound [U₂(CN)₃(NH₃)₁₄]⁵⁺[KBr₆]⁵⁻·NH₃ that contains, to the best of our knowledge, an unprecedented spatially separated [KBr₆]⁵⁻ anion.

2 Results and discussion

The reaction of UBr₄ with KCN dissolved in liquid ammonia was carried out in a sealed “bomb tube” at room temperature. Over 6 months of storage and crystallization time some dark brown crystals grew of which the composition was evidenced by X-ray diffraction as tetradecaamminetricyanido-κC,κN-diuranium(IV) hexabromidopotassiate(I) ammonia(1/1) [U₂(CN)₃(NH₃)₁₄]⁵⁺[KBr₆]⁵⁻·NH₃. The formation of this compound can be described by eq. (1).

The compound crystallizes in the trigonal space group P3̅m1 (No. 164) with a=10.3246(13), c=8.4255(17) Å, V=777.8(3) ų and Z=1 at T=100 K. Selected crystallographic data and details of the structure determination are available from Table 1. Table 2 contains Wyckoff positions and atom coordinates. The crystal structure of this compound contains the [KBr₆]⁵⁻ anion and the [U₂(CN)₃(NH₃)₁₄]⁵⁺ cation, as well as an ammonia molecule of crystallization. The [KBr₆]⁵⁻ anion and the coordination sphere of the U(IV) cation are shown in Fig. 1.

Fig. 1:

Section of the crystal structure of [U₂(CN)₃(NH₃)₁₄]⁵⁺[KBr₆]⁵⁻·NH₃, showing the octahedron-like [KBr₆]⁵⁻ anion (left) and the coordination of the U(IV) cation (right). The displacement ellipsoids are shown at the 70% probability level at T=100 K. As some of the H atoms show site disorder, only one set of positions is shown.

The spatially separated [KBr₆]⁵⁻ anion is, to the best of our knowledge, not described in the literature to date. However, a inversely comparable species has been observed in the compound [K₁₇(Sb₈)₂(NH₂)]·17.5NH₃, where [(H₂N)K₆] octahedra are present that however form straight, infinite chains through trans-corner connections. Thus, a similarity to the crystal structure of KNH₂ was noted [21]. In our case, the [KBr₆]⁵⁻ anion is spatially separated and contains a potassium atom (1a, 3̅m.) as the central atom, which is surrounded by six symmetry-equivalent bromide anions (6i, .m.) in the shape of a distorted octahedron. The K–Br distances of such a moiety are therefore all equal with 3.2684(19) Å. This distance is in good agreement with K–Br distances observed for pure KBr (3.289(6) Å, T=293 K) [22]. Interatomic distances are available from Table 3. In the compound K₂ZnBr₄, where the coordination polyhedron around K⁺ is a trigonal prism, the K−Br distances agree with 3.240–3.420 Å (T=125 K) [23], [24]. An octahedral coordination polyhedron for potassium cations and bromide anions is also present in the compound KInBr₃ [25]. There, the K–Br distances of 3.182(9) and 3.284(9) Å are also comparable with those in the spatially separated anion presented here. The Br–K–Br angles in the distorted coordination octahedron of the [KBr₆]⁵⁻ anion are 102.52(4) and 77.48(4)°; in good agreement with reported data (KInBr₃: 105.09 and 74.01°) [25]. Overall, the shape of the spatially separated [KBr₆]⁵⁻ anion corresponds well to the literature.

The uranium atom occupies the Wyckoff position 2d (3m.) and is surrounded by seven ammine ligands and three cyanido ligands. The coordination polyhedron can be described as a slightly distorted sphenocorona (Johnson solid J₈₆). This sphenocorona, with a mild distortion of its top quadrangle is shown in Fig. 2. The three disordered cyanido ligands are κC,κN-bridging two symmetry-equivalent uranium atoms. The C≡N bond length of the disordered cyanido ligand is 1.11(3) Å and despite the disorder it agrees well with C≡N bond lengths of cyanido ligands described in the literature [26], [27], [28], [29]. Because of space group symmetry, the C and N atoms of the cyanide anion are indistinguishable and a statistical distribution with a 50:50 mixed site occupancy of the atom position 6i (.m.) was applied in the refinement. The cyanide anion resides symmetrically in between the bridged U atoms. The U–N distances for the seven ammine ligands range from 2.47(2) to 2.659(13) Å and are also in good agreement with distances observed for ammine ligands in the uranium-ammonia system [20]. Because of the different distances between the uranium atom and the ten ligands, the sphenocorona is slightly distorted. As the U atoms are bridged by the cyanido ligands, the complex cation forms corrugated infinite layers parallel to the ab plane. This connection can be described by the Niggli formula {[U(CN)32(NH3)71]}∞2. A section of one layer is shown in Fig. 3 left. As every sphenocorona is connected to three others, a motif of packed sphenocoronas is obtained that reminds of the arrangement of As atoms in the crystal structure of grey arsenic (Fig. 3 right), which is best seen when viewed parallel to the c axis. Overall, the sphenocoronas show AB stacking, that is, the AB layers are stacked directly above each other and channels parallel to the c axis result (Fig. 3 right).

Fig. 2:

Section of the crystal structure of [U₂(CN)₃(NH₃)₁₄]⁵⁺[KBr₆]⁵⁻·NH₃, showing the coordination sphere of the uranium atom, which is surrounded by ten ligands in the shape of a slightly distorted sphenocorona (Johnson solid J₈₆). A view along the trigonal prism of the sphenocorona is selected. The displacement ellipsoids are shown at the 70% probability level at T=100 K.

Fig. 3:

Section of the crystal structure showing a corrugated {[U(CN)32(NH3)71]}∞2 layer (left) and a view along the c axis (right).

The [KBr₆]⁵⁻ anions fill the octahedral voids, with the potassium atom K(1) occupying the Wyckoff position 1a (3̅m.). The complex anion can be described by the Niggli formula {[KBr61]}∞0. The motif of the arrangement of sphenocoronas and [KBr₆]⁵⁻ anions corresponds overall to an ABC-type stacking. Leaving the Br, C, and N atoms aside and looking only on the arrangement of the K and U atoms, one recognizes that the K atoms are surrounded octahedron-like by U atoms. These octahedra reside in the ab planes and share common edges corresponding to the Niggli formula {[KU63]}∞2. The arrangement of the potassium and uranium atoms reminds of the Ag₂F structure type [30]. The ammonia molecules of crystallization, with the N(5) atom on the 1b (3̅m.) position, are thus also surrounded octahedron-like by six uranium atoms, however these octahedra are strongly compressed along the threefold symmetry axis parallel to the crystallographic c axis. These polyhedra form {[NU63]}∞2 layers which reside at z=1/2 parallel to the ab plane. Thus, the layers of {[NU63]}∞2 octahedra are sharing common faces with the {[KU63]}∞2 octahedra. The crystal structure of the compound is shown in Fig. 4.

Fig. 4:

Section of the crystal structure of [U₂(CN)₃(NH₃)₁₄]⁵⁺[KBr₆]⁵⁻·NH₃. The [KBr₆]⁵⁻ anions are shown as dark green polyhedra. Displacement ellipsoids are shown at the 70% probability level at T=100 K, H atoms isotropic with arbitrary radii. H atoms of the ammonia molecules of crystallization could not be located.

Between the layers of the interconnected uranium atoms and the spatially separated [KBr₆]⁵⁻ anions the only attractive interactions are N–H···Br hydrogen bonds. The hydrogen atoms of the ammine ligands (N(2), N(3) and N(4)) form hydrogen bonds to the bromine atoms Br(1). The donor···acceptor distances of these hydrogen bonds range from 3.377(10) to 3.642(8) Å. This is in good agreement with known N–H···Br hydrogen bonds for example in [Co(NH₃)₆]Br(S₂O₃)·H₂O (3.509(4) to 3.539(3) Å, T=295 K) or in [CoCO₃(NH₃)₅]Br·H₂O (3.41–3.63 Å) [31], [32].

3 Conclusions

Single crystals of [U₂(CN)₃(NH₃)₁₄]⁵⁺[KBr₆]⁵⁻·NH₃ (P3̅m1, No. 164) were obtained by the reaction of UBr₄ and KCN dissolved in liquid ammonia at room temperature over several months of crystallization time. The crystal structure of this compound has been determined and found to contain two peculiar building units: (i) The uranium atoms are coordinated by ten ligands in total of which seven are ammine and three are cyanido ligands. The coordination sphere of the U atom is described best as a sphenocorona. The connection of the κC,κN-cyanido bridged uranium atoms leads to the formation of infinite corrugated layers of sphenocoronas. (ii) The unprecedented spatially separated, octahedron-like [KBr₆]⁵⁻ anion could be observed for the first time.

4 Experimental section

All work was carried out excluding moisture and air in an atmosphere of dried and purified argon (5.0, Praxair) using high vacuum glass lines and a glovebox (MBraun). The glass vessels were flame dried several times under vacuum before being used. Aluminum bromide (Alfa Aesar, 98%) was purified by sublimation in vacuo. Aluminum (Fluka, purum >99%), as well as uranyl nitrate (Riedel de Haën, zur Analyse), was used as supplied.