Rb 2 [U(NH 2 ) 6 ], a Rubidium Hexaamidouranate(IV) obtained from the Reaction of UI 3 with RbNH 2 in Anhydrous Ammonia

. The pyrophoric compound Rb 2 [U(NH 2 ) 6 ] was obtained as a grey to black powder from the reaction of more than three equivalents of RbNH 2 with UI 3 in anhydrous liquid ammonia. During the process, U III is oxidized to U IV and ammonia is reduced under evolution of


Introduction
Our investigations on the reactions of uranium halides with anhydrous liquid ammonia and dissolved amides [1][2][3] were driven by the desire to obtain uranium nitrides such as UN under comparingly mild conditions.Similar research efforts had been undertaken in the past for the low-temperature synthesis of uranium as well as plutonium nitrides. [4,5]The peculiarity that UI 3 is able to act as a reducing agent for Rb + was recently briefly reported by us. [3]We were now able to identify one of the reaction products as rubidium hexaamidouranate(IV), Rb 2 [U(NH 2 ) 6 ].The existence of such a compound was not totally unexpected, as we were able to obtain K 2 [Zr(NH 2 ) 6 ] from comparable conditions in the past. [6]

Results and Discussion
Choosing UI 3 and RbNH 2 dissolved in anhydrous liquid ammonia as starting materials for the synthesis of uranium nitrides is reasonable.UI 3 and RbNH 2 , as well as the side product RbI are well soluble and therefore easily filtered off from the insoluble uranium nitrides.Previous reports that employed other alkali metal amides claimed that UN would directly precipitate. [4]When we carried out the reactions with up to three equivalents of RbNH 2 , we obtained a brown precipitate that was amorphous to X-rays.However, the brown precipitate could be annealed to obtain microcrystalline UN. [3] When more than three equivalents of RbNH 2 were used, the initially formed brown precipitate reacted to form a blue solution that was accompanied by a strong evolution of gas and the formation of a grey precipitate (Figure 1   The blue color of the solvated electrons, the gas evolution and the formation of a grey precipitate is clearly seen (left).The right side shows a green compound, probably [U(NH 3 ) 9 ]I 3 •13NH 3 or a higher ammoniate, that is converted to a brown compound (probably UN or U(NH 2 ) 3 ), and finally to the grey/black Rb 2 [U(NH 2 ) 6 ].
The observed blue color is absolutely typical for solvated electrons in anhydrous ammonia and the strong gas evolution is due to the formation of H 2 from the reduction of ammonia by the solvated electrons.16] We may conclude that U III acted as a reducing agent under these conditions and produced solvated electrons which obvi-

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ously induced H 2 formation immediately.Therefore, U IV is formed.We would like to point out that such a reductive behavior is also known for solutions of Eu II or Yb II in ammonia.[19] A reduction of potassium amide by UBr 3 was suspected by Bergstrom. [19]The reaction of UI 3 with RbNH 2 in ammonia can be described by Equation (1), however, we will comment in more detail below.
After filtration and removal of excess of the solvent NH 3 , a solid, grey to black product is obtained which shows sharp reflections of RbI and a second crystalline phase in its powder X-ray diffraction pattern.Additionally, the reflection positions of the second phase are in very good agreement with those of K 2 [PtCl 6 ] (see Figure S1, Supporting Information), [20] to which the aforementioned K 2 [Zr(NH 2 ) 6 ] is structurally related as well. [6]So, the lattice parameters of the second crystalline phase besides RbI are very similar to those of K 2 [PtCl 6 ].The diffraction pattern could therefore be indexed in the cubic crystal system, a LeBail refinement (Figure S2, Supporting Information) lead to the lattice parameter a = 9.796(1) Å, V = 940.1(1)Å 3 at T = 293 K.For comparison, the lattice parameters of K 2 [PtCl 6 ] at room temperature were reported with a = 9.756 Å, V = 928.6Å 3 . [20]Due to the relative similarity of the reflection intensities, we assume that the indexed powder X-ray diffraction pattern of the second phase is due to the formation of the microcrystalline compound Rb 2 [U(NH 2 ) 6 ] which Applying the space group and the atomic coordinates of K 2 PtCl 6 and replacing the respective atoms by Rb, U, and N atoms only, a Rietveld refinement was carried out (Figure 2).The Rb, U, and N atoms could only be refined isotropic but hydrogen atoms could not be located, as may have been expected.The quantum chemical calculations (see below) indicated space group R3 ¯for the title compound, as otherwise the two f-electrons of the U atom could not have been localized.Eventually, spin-orbit coupling might enable calculations in the higher cubic symmetry.We attempted a Rietveld refinement in space group R3 ¯.However, it did not lead to a significant change of the structure model.Thus, the higher symmetry space group is chosen for Rb 2 [U(NH 2 ) 6 ].See Table 1 for selected crystallographic data and details of the structure determination and Table 2 for atomic coordinates and equivalent isotropic displacement parameters.The crystal structure of Rb 2 [U(NH 2 ) 6 ] is shown in Figure 3.
The Rietveld refinement leads to a U-N distance of 2.274(15) Å within the [U(NH 2 ) 6 ] 2-anion.In the amido complex compound [U(NH 2 )(NH 3 ) 8 ]Br 3 •3NH 3 , [3] the U-N amido distance was observed with 2.207(3) Å, while the U-N ammine distances ranged from circa 2.5 to 2.9 Å. Due to the lower coordination number of six in the [U(NH 2 ) 6 ] 2-complex, we would expect the U-N amido bond to be shorter than the one in the [U(NH 2 )(NH 3 ) 8 ] 3+ complex, where the coordination   number is nine.This expectation is fulfilled; however, such a comparison is not straight forward as bond lengths between central atom and ligands in an anionic complex are naturally longer than in a cationic complex of same coordination number and oxidation state of the central atom.In the complex [U(N 3 ) 4 (py) 4 ] with coordination number eight, a U azide -N distance of 2.314(3) Å has been observed. [21]This is slightly longer than the U-N distance in [U(NH 2 ) 6 ] 2-, as the coordination number is larger.A dimethylamido complex of U IV of the composition [U(NMe 2 ) 4 ] 3 has been structurally characterized as a trinuclear, centrosymmetric, linear complex where each U atom carries six NMe 2 ligands. [22]The terminally bound NMe 2 ligands show U-N distances of 2.25 Å, thus circa 0.02 Å shorter than in the title compound.As the steric demand of the NH 2 -ligand is less compared to that of the NMe 2 -ligand, a shorter distance would be expected for our case.However, the observed difference is likely due to the comparison of data obtained from powder and single-crystal X-ray diffraction and should therefore not be overstressed.[25] The U VI -N bonds are thus shorter than the U V -N bonds, and thus U IV -N bonds should be a little longer.This is fulfilled in our case with 2.274( 15 We expect, in analogy to oxo-and olation reactions in the aqueous system, that U(NH 2 ) 3 is easily converted, that is, it "ages", into uranium nitride, UN, see Equation (3):

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This ageing reaction would also be a plausible pathway for the formation of UN from the reaction of uranium tetraiodide, UI 4 , with Na or K dissolved in ammonia, as reported by Cleveland and co-workers. [4]If U IV is first reduced to a soluble U III species is unclear.The reaction according to Equation (3) seems to be favored over the formation of other complex amides, as besides the brown precipitate that could be annealed to UN only crystals of [U(NH 3 ) 9 ]I 3 •13NH 3 were obtained when up to three equivalents of RbNH 2 were used.So far, we have not observed the formation of complex amides of U III , such as [U(NH 2 ) 3+x ] x-with x = 1 to 3 or amidoammine com-

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plexes of U III .When more than three equivalents of RbNH 2 are used, then the low solubility of Rb 2 [U(NH 2 ) 6 ] and its lattice energy could act as an additional driving force for the reduction of ammonia and the oxidation of U III , see Equation (4) and Equation (5): Why and how the reactions really proceed is beyond our capabilities.
Tschirne and Naumann suggested the formation of [U(NH 2 ) 4 ] from the reaction of UCl 4 and NaNH 2 .[U(NH 2 ) 4 ] was said to decompose already at low temperatures. [5]If [U(NH 2 ) 4 ] is really formed, then it is unclear as to why the formation of a hexaamidouranate(IV) complex, [U(NH 2 ) 6 ] 2-, was not reported.It is possible that either their reaction conditions were not suitable, or that an intermediate of trivalent uranium is essential for the formation of the U IV complex.Both we deem possible.The pyrophoric behavior and the tendency to detonate upon air contact make it plausible that we have obtained the complex amide Rb 2 [U(NH 2 ) 6 ] as Schmitz-Dumont has reported thorium amido complexes that were pyrophoric and exploded vigorously upon air contact, as well. [26]They reported thermal decomposition reactions according to the following Equation ( 6), Equation (7), Equation ( 8), and Equation (9): [26] K[Th(NH)(NH 2 ) 3 ] Ǟ 25 The obtained nitride K 3 Th 3 N 5 still reacted explosively with air.When the compound was heated even further, elemental potassium was released [Equation (10)]. [26]3 Th 3 N 5 Ǟ 270 °C 3 ThN + 3 K + N 2 (10)   It is known that the heavier alkali metal amides can be decomposed above their melting points into the elements.[15] The vigorous reaction of Rb 2 [U(NH 2 ) 6 ] in air could therefore be not only due to exothermic protolysis, hydrolysis and oxidation reactions, but may be also due to a thermal decomposition induced by reaction heat.The latter we assume due to an observation where Rb 2 [U(NH 2 ) 6 ] reacted explosively with an extremely small amount of air due to a capillary leak in a glass apparatus.Due to its high lattice energy UN would form as an additional product, among N 2 , Rb and H 2 . Thelatter two would also react strongly exothermic with (moist) air.In analogy to the report of Schmitz-Dumont, a hypothetical decomposition reaction can be formulated [Equation (11)].Our efforts to shed light on the thermal decomposition of Rb 2 [U(NH 2 ) 6 ] and its reactions with (moist) air, as well as attempts to synthesize other complex amidouranates(IV) are ongoing.
One may argue that instead of the U IV compound Rb 2 [U(NH 2 ) 6 ], the U VI amido imido complex Rb 2 [U(NH 2 ) 4 (NH) 2 ] could have been obtained.While the discrimination in the X-ray diffraction experiment would be difficult as hydrogen atoms could not be located and the molecular anions could be disordered showing an average U-N distance that would lie in between U-N amido and U-N imido distances, chemical reasoning excludes such U VI compounds.U IV is the most stable oxidation state in cold liquid ammonia and uranium cations in higher oxidation states are immediately reduced by ammonia to U IV or form only metastable compounds that are reduced over time.This of course is different for very high temperatures of several hundred °C, where ammonia can oxidize uranium to UN 2 .See for example the works on the ammonolyses of uranium halides, [27][28][29][30][31][32][33][34] the works on uranium(IV) amides, [5] and our own works on both topics. [3]In this light, it seems highly unlikely for uranium to be oxidized by ammonia to an oxidation state which is usually unstable towards reduction -especially in a solution of strongly reducing solvated electrons.
We also investigated the structure and electronic properties of Rb 2 [U(NH 2 ) 6 ] using quantum chemical calculations.We started the calculations using the subgroup Fm3 ¯(no.202) which enables an ordered structural model for the hydrogen atoms of the amide ligands.However, in this space group the calculations always converged into a metallic ground state regardless of the initial f-orbital occupation of the U IV atom.Such behavior suggests that the wavefunction breaks the crystallographic symmetry.Lowering the symmetry further to subgroup R3 ¯(no.148) resulted in an insulating ground state with two f-electrons localized at the U atom (α-spin bandgap 3.7 eV).
The optimized crystal structure of Rb 2 [U(NH 2 ) 6 ] is given in the Supporting information.Harmonic frequency calculations showed the structure to be a true local minimum.The lattice parameter differs from the experimental value by -1.4 %.The unit cell angles of the trigonal primitive cell are 60.26°, deviating by 0.4 % from the perfect 60°angle in the original primitive cell of the face-centered cubic unit cell.The U-N distance in the optimized structure is 2.36 Å, which is slightly longer in comparison to the U-N distance of 2.274(15) Å obtained from the Rietveld refinement.
We also investigated a hypothetical [U(NH 2 ) 6 ] 2-anion at the DFT-PBE0/def2-TZVP level of theory (the anionic charge was countered by a COSMO solvent field).Similar to Rb 2 [U(NH 2 ) 6 ], the symmetry of the molecular anion had to be reduced from T h to D 2h to obtain the correct electronic state with two f-electrons localized at the U atom.In line with the solid-state calculation, the structure is a true local minimum, the HOMO-LUMO gap is 3 eV, and the optimized U-N distance is 2.35 Å, which is close to the value of 2.274(15) Å.We structurally optimized also the U VI amido imido complex [U(NH 2 ) 4 (NH) 2 ] 2-, in order to get an impression on the U-N

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distances.The point group symmetry of this molecular anion is C 2h .The U-N amido distances are 2.34 Å and those of U-N imido are 1.93 Å, respectively.In quantum chemical solidstate calculations of Rb 2 [U(NH 2 ) 4 (NH) 2 ], the symmetry had to be lowered to the monoclinic crystal system (C2/m) in order to obtain a true local minimum structure.Then, however, all Rb•••Rb and U•••U distances completely disagree with the ones observed in Rb 2 [U(NH 2 ) 6 ].So, also from this purely theoretical point of view, the presence of such a U VI species is unlikely.

Conclusions
When more than three equivalents of RbNH 2 were reacted with UI 3 dissolved in anhydrous liquid ammonia, the typical color of solvated electrons was observed visually.The solvated electrons triggered ammonia reduction under evolution of hydrogen.U III was oxidized to U IV and precipitated as a grey to black powder of highly pyrophoric rubidium hexaamidouranate(IV), Rb 2 [U(NH 2 ) 6 ].As evidenced from its powder X-ray diffraction pattern, the compound crystallizes in the K 2 PtCl 6 structure type.Within the [U(NH 2 ) 6 ] 2-anion, a U-N distance of 2.274(15) Å was determined from Rietveld refinement.Quantum chemical calculations for the solid-state compound as well as for the isolated [U(NH 2 ) 6 ] 2-anion, where the U-N distance is calculated to 2.35-2.36Å, agree well.

Experimental Section
Caution! Rb 2 [U(NH 2 ) 6 ] turned out to be very sensitive towards air and several explosions occurred (really a dirty bomb…).
All work was carried out trying to exclude moisture and air in an atmosphere of dried and purified argon (5.0, Praxair) using high vacuum glass lines and a glovebox (MBraun).Liquid ammonia was dried by storage over Na.The glass vessels were flame-dried under fine vacuum several times before utilization.
Synthesis of UI 3 : UI 3 was prepared according to the literature. [35]nthesis of RbNH 2 : Rb was reacted with an excess of ammonia in the presence of a catalyst (trace of rust) at -78 °C and slowly warming to the boiling point. [15]After removal of the excess of NH 3 , pure RbNH 2 was obtained as evidenced by powder X-ray diffraction.
Reaction of UI 3 with 5 Equivalents of RbNH 2 : 31.1 mg (0.050 mmol, 1 equiv.) of UI 3 and 27.8 mg (0.274 mmol, 5 equiv.) of RbNH 2 were reacted in a flame-dried Schlenk tube with approximately 10 mL of liquid ammonia.The reaction mixture became brown and turned then blue due to solvated electrons.The solution evolved gas, became colorless and a brown precipitate formed.After three months of storage at -36 °C we looked for single crystals inside the precipitate.However, only a pale-brown powder and aggregates of a dark-brown powder were present.A powder X-ray diffraction pattern has shown the presence of RbI and the title compound reported here.
Reaction of UI 3 with 6 Equivalents of RbNH 2 : 400.0 mg (0.647 mmol, l eq.) of UI 3 and 411.8 mg (4.06 mmol, 6 equiv.) of RbNH 2 were placed together with glass coated stirring bars into the separate tubes of an H-shaped reaction vessel. [36]Both sides were filled with circa 10 mL of ammonia each.The UI 3 side reacted with ammonia under an enormous increase in volume and looked orange-

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brown to pink under the light of a luminescent tube, while under sunlight it appeared green (Figure 4).In 10 mL of ammonia, the given amount of UI 3 only dissolved partially, forming a dark green solution.When the RbNH 2 solution was poured via the fritt onto the UI 3 side of the H-tube, the solution turned immediately blue and a strong evolution of gas set in which lasted more than 1 h.During this time a grey precipitate formed.The undissolved green residue reacted slowly from top to bottom.Directly on the green solid a brown powder was formed, which slowly was converted into the grey powder that was stirred up by the evolving gas (Figure 4).30 minutes after the gas evolution had stopped a grey-black precipitate was filtered off and washed twice with ammonia.On the other side of the H-tube a white powder was obtained after the removal of NH 3 .Samples for powder X-ray diffraction were obtained from both sides of the H-tube.Residual grey-black powder exploded with a sharp bang when brought from the glovebox to air.The white powder was inert, and the powder X-ray pattern showed it to be RbI, while the grey-black powder showed the reflections of  To obtain single crystals and purify the grey-black powder of Rb 2 [U(NH 2 ) 6 ] further by thorougly removing RbI, a Soxhlet-like extractor for liquid ammonia was charged. [36,37]When evacuating the extractor for flame-sealing, the grey-black powder detonated with an orange flash destroying the extractor as soon as the powder on the fritt was swirled by the pumped-off argon (that due to a pinhole in the glassware contained some air, presumably).
Reaction of UI 3 with 8 Equivalents of RbNH 2 : 224 mg (0.362 mmol, 1 equiv.) of UI 3 and 307.4 mg (3.029 mmol, 8 equiv.) of RbNH 2 were placed together with glass coated stirring bars into the separate tubes of an H-shaped reaction vessel.Both sides were filled with circa 10 mL of ammonia each.The RbNH 2 solution was poured via the fritt to the UI 3 -side of the H-tube.The mixture became immediately blue and a black precipitate formed during a strong evolution of gas.After filtration through the fritt and washing three times with ammonia, a colorless filtrate was obtained from which RbI crystallized after removal of NH 3 .On the other side of the H-tube 110 mg of an almost black powder -which appeared darker than in the synthesis described with six equivalents of RbNH 2 -were obtained.Unfortunately, the black powder appeared amorphous to X-rays but contained still a trace of RbI.

Powder X-ray Diffraction:
The sample was filled into a borosilicate glass capillary with a diameter of 0.3 mm.The powder X-ray pattern was recorded with a StadiMP diffractometer (Stoe & Cie) in Debye-Scherrer setup.The diffractometer was operated with Cu-K α1 radiation (1.5406 Å, germanium monochromator) and equipped with a MYTHEN 1 K detector.The diffraction pattern was indexed using the WinXPOW suite. [38]For the extraction of integrated intensities a Le Bail decomposition was performed with the JANA2006 software. [39]n order to solve the structure the analogy to the K 2 PtCl 6 structure

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type was used.Finally, a Rietveld refinement of the obtained structure model was performed with the JANA2006 software.In the course of this refinement, the reflection profiles were fitted with a pseudo-Voigt function.In addition to the profile parameters, a zero-shift parameter and a manual background were refined.The peak asymmetry was refined using a four-term Berar-Baldinozzi correction.A cylindrical absorption correction was applied.All atoms, except for the hydrogen atoms, which could not be located, were refined with isotropic displacement parameters.
Crystallographic data (excluding structure factors) for the structure in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK.

Quantum Chemical Calculations:
The solid-state quantum chemical calculations were carried out with the CRYSTAL17 program package. [40]PBE0 hybrid density functional method and Gaussian-type basis sets of triple-zeta-valence + polarization quality were used (splitvalence + polarization for Rb). [41][44] The basis set for U was a slightly modified version of the basis set used previously in the literature (details and full basis set listing in the Supporting Information). [45]The reciprocal space was sampled using a 4 ϫ 4 ϫ 4 Monkhorst-Pack-type k-point grid. [46]For the evaluation of the Coulomb and exchange integrals (TOLINTEG), tight tolerance factors of 8, 8, 8, 8, and 16 were used.We carried out spin-polarized calculations as the U IV species possesses two unpaired f-electrons.Both the atomic positions and lattice constants were fully optimized within the constraints imposed by the space group symmetry.The harmonic vibrational frequencies were obtained by using the computational schemes implemented in CRYSTAL. [47,48]Molecular calculations on [U(NH 2 ) 6 ] 2-anion were carried out with TUR-BOMOLE program package at the DFT-PBE0/def2-TZVP level of theory (def-TZVP for U). [41,49,50]The anionic charge was countered with COSMO solvent field. [51]The structure was fully optimized within D 2h point group (the optimized structure is available as Supporting Information).

Supporting Information (see footnote on the first page of this article):
The supporting information contains powder X-ray patterns, the uranium basis set in CRYSTAL17 input format, and the quantumchemically optimized structures. left).

Figure 1 .
Figure 1.A photograph of a reaction of UI 3 with six equivalents of RbNH 2 in anhydrous ammonia.The reaction takes place at -78 °C.The blue color of the solvated electrons, the gas evolution and the formation of a grey precipitate is clearly seen (left).The right side shows a green compound, probably [U(NH 3 ) 9 ]I 3 •13NH 3 or a higher ammoniate, that is converted to a brown compound (probably UN or U(NH 2 ) 3 ), and finally to the grey/black Rb 2 [U(NH 2 ) 6 ].

Figure 2 .
Figure 2. Observed (black) and calculated powder X-ray pattern of Rb 2 [U(NH 2 ) 6 ] after Rietveld refinement.The calculated reflection positions are indicated by the vertical bars below the patterns.Upper trace (Phase #2) presents RbI and the lower (Phase #1) Rb 2 [U(NH 2 ) 6 ].The curve at the bottom represents the difference between the observed and the calculated intensities.R p = 0.0205, R wp = 0.0267, S = 2.03.
) Å.In comparison with quantum chemical calculations for the solid-state compound as well as for the isolated [U(NH 2 ) 6 ] 2-anion (both see below), where the U-N distance is calculated to 2.35-2.36Å, the observed bond length agrees well.The next nearest Rb•••U distance in Rb 2 [U(NH 2 ) 6 ] is 4.2379(6) Å, which is a plausible value as the metal cations are separated far enough, as expected.For the formation of the title compound we assume a formal or maybe even an intermediate formation of uranium triamide U(NH 2 ) 3 for the reactions of UI 3 with up to three equivalents of RbNH 2 [Equation (2)].UI 3 + 3 RbNH 2 Ǟ NH 3 U(NH 3 ) 2 + 3 RbI (2)

Figure 4 .
Figure 4. Photographs of the reaction.