Uranium Chemistry in liquid Ammonia: Compounds obtained by adventitious Presence of Moisture or Air

UCl 4 , UBr 4 , UBr 5 , or UO 2 Cl 2 reacted with excess liquid ammonia – in adventitious presence of moisture and/or air – and formed some peculiar uranium compounds of which we present the crystal structures. [(NH 3 ) 7 (N 3 )U( μ -O)U(NH 3 ) 8 ]Cl 5 ·7NH 3 contains a dinuclear μ -O-bridged uranium(IV) cation, [{(NH 3 ) 4 UO 2 } 2 ( μ - O)]Cl 2 ·4NH 3 features a dinuclear μ -O-bridged uranyl(VI) cation, while the compounds [(U(VI)O 2 ) 2 (U(V)O 2 ) 2 ( μ 3 -O) 2 (NH 3 ) 12 ]Br 2 ·6NH 3 and [(U(VI)O 2 ) 4 (U(V)O 2 ) 4 ( μ 3 -O) 4 (NH 3 ) 22 ]Br 4 ·16NH 3 are mixed-valent containing uranyl(V)-uranyl(VI) units. For these tetra-and octanuclear complex cations we observed that the O atoms of the uranyl(V) units can be μ 2 - and even μ 3 -bridging to uranyl(VI) units, while the O atoms of the latter are acting as terminal ligands


Introduction
Knowledge of uranium species present in aqueous solutions, their composition and structure, as well as any solid products formed from such solutions is of enormous relevance for understanding how uranium compounds that are released in the event of an accident, for example, behave and spread in nature.The chemistry of uranium compounds in H 2 O or waterlike solvents such as NH 3 is more complex compared to organic solvents as in addition to solvolysis also protolysis reactions occur that depend on temperature, concentration, and pH value.3][4] While spectroscopic techniques allow to establish composition and structure of dissolved species to some extent, for the characterization of formed solids crystals are desired as they can be analyzed by single crystal X-ray diffraction which very reliably establishes composition and structure.However, crystal-lization of U compounds from aqueous solutions is often difficult when no auxiliary organic ligands are present as illdefined, amorphous precipitates are frequently obtained.Liquid anhydrous ammonia, aNH 3 , a water-like solvent, [5][6][7][8] proved to be valuable for U speciation studies as crystallization in the ammonia system seems to be favored. [9,10]he results for the ammonia system can be transferred to the aqueous system within certain limits, and vice versa.For example, hydrolysis reactions of uranium(IV) have been investigated to gain insight into the composition of dissolved species, [11,12] but their structures often remained undetermined.For a review on structurally characterized hydrolysis products see the literature. [13]The structural characterization of a homoleptic aquauranium(IV) cation, [U(H 2 O) 9 ] 4 + , which had been suspected based on EXAFS data to exist in solution, [14] was only recently successful for the solid state, [10] while for U(III) more than 200 crystalline compounds containing [U(H 2 O) 9 ] 3 + cations have been structurally characterized.][17] UCl 3 , UBr 3 , and UI 3 form mononuclear homoleptic [U(NH 3 ) 9 ] 3 + cations in the solid state [9] in analogy to the aqueous system.For the U(IV) halides UX 4 with X = FÀI the results differ to the U(III) case as mononuclear, heteroleptic complexes like [UF 4 (NH 3 ) 4 ], [UCl(NH 3 ) 8 ] 3 + were observed for the chemically harder halides, [9,17] while the softer bromide and iodide yielded mononuclear homoleptic [U-(NH 3 ) 10 ] 4 + cations. [10]So, the [U(NH 3 ) 9 ] 4 + complex is still unknown, while its aqueous relative exists.To the best of our knowledge, the structure of the [U(H 2 O) 10 ] 4 + cation is unknown, while its homologous NH 3 complex exists.This is likely due to the different Broenstedt acidities of the ligands.

Results and Discussion
The experiments leading to the compounds reported here could so far not be reproduced as all experiments with a deliberate addition of water led to formation of ill-defined precipitates, likely due to local excess of water.Leaving the reaction vessels deliberately open to air, to allow for moisture contact and oxidation, was also unsuccessful.
We sort the results of our studies according to the binding motifs of the complex cations, starting with a μ-O 2À bridged uranium(IV) compound and three uranyl compounds featuring μ-O 2À and μ 3 -O 2À bridging ligands.Finally, the UN 2 containing compound will be discussed.
In some cases, when no literature comparison is feasible due to the absence of similar compounds, atomic distances will be compared to results obtained from quantum-chemical calculations.
Scheme 1 summarizes the experiments, holds their conditions, as well as the formulas, space groups, and Pearson symbols of the compounds.100 K.For more details on the structure determination and selected crystallographic details see Table S6.
There are five symmetry-independent Cl atoms in the structure, which all reside on a general position (4e, 1).All Cl atoms are surrounded by N atoms in the shape of irregular coordination polyhedra.Hydrogen bonds are discussed in the Supporting Information.No global packing, neither by the  cations nor the anions, was observed, and therefore the crystal structure could not be traced back to a simple packing.
After removal of excess liquid NH 3 at room temperature in vacuo, crystalline NH 4 Cl was observed by powder X-ray diffraction, the pattern is shown in Figure S3.This observation agrees with the results from IR spectroscopy, Figure S4, where bands for NH 4 Cl were observed. [41]The observation of NH 4 Cl enables the overall reaction equations to be established, see below.
The UÀ(μ-O)ÀU bond shows an essentially ionic character, with mainly O atom contributions of approximately 80 %.The μ-O atom has a partial charge of À0.56 e À , while the U atoms show partial charges of + 1.26 and + 1.27 e À (partial charges based on Intrinsic Atomic Orbitals, IAO).Orbital contributions to the IBOs are handed in the Supporting Information, Table S18.
The contributions of the U atoms are dominated by the 6d orbitals.This observation coincides with previous IBO calculations on the cations [U(NH 3 ) 10 ] 4 + and [UCl(NH 3 ) 8 ] 3 + at the DFT-PBE0/TZVP level of theory. [43]Therefore, resonance structures for the complex cation can be drawn as shown in Scheme 2.
The quite ionic UÀ(μ-O)ÀU bond is best described with the resonance structure on the right side in Scheme 2. Nevertheless, also the other resonance structures occur to some extent.
There are two IBOs describing the U1ÀN 3 bond, of which one IBO is shown in Figure 4.The UÀN 3 bond has similar ionic character as the UÀ(μ-O)ÀU bond, with 72 % contribution of the N1 atom to the UÀN 3 bond.The partial charges of the N atoms within the N 3 À anion are À0.51 for N1, + 0.35 for N2 and À0.35 e À for N3.These charges show that the central N atom of the N 3 À anion is partially positively charged.Further discussion is available in the Supporting Information, together with the individual orbital contributions summarized in Table S18.
The presence of an oxygen atom is pointing to humidity in one of the starting materials, most likely contained in the used NH 4 N 3 .A possible formation sequence for the compound is shown in equations 1 to 4. An alternative way of its formation is given in equations 5 and 6.We note that there is no information on the species in solution and how and if they form.However, the equations serve as a plausible working hypothesis and are supported by our previous reports and the observed formation of NH 4 Cl.
Equation 1 describes the partial ammonolysis of UCl 4 and the formation of the complex compound [UCl(NH 3 ) 8 ]Cl 3 that had been previously observed by us as the ammoniate [UCl(NH 3 ) 8 ]Cl 3 • 3NH 3 from a bomb tube reaction at room temperature. [9]In a second step, equation 2, a ligand exchange, is postulated, where the remaining Cl À ligand in the [UCl(NH 3 ) 8 ] 3 + cation is replaced by an N 3 À ligand to form the [U(N 3 )(NH 3 ) 8 ] 3 + cation.Equation 3 describes the Cl À ligand exchange with a H 2 O molecule.In general, H 2 O is, with a reported pK A value of 18.9 in liquid NH 3 at À60 °C, not very acidic. [44]Due to the polarization of the UÀOH 2 bond, its acidity rises and a NH 3 molecule gets protonated and forms NH 4 Cl.In a last step, equation 4, an intermolecular H + transfer from the OH À ligand of the [U(OH)(NH 3 ) 8 ] 3 + cation to a NH 3 ligand of the [U(N 3 )(NH 3 ) 8 ] 3 + cation takes place, which of course could also be mediated by the solvent.Anyways, the two mononuclear cations condensate under "HCl" elimination to form the dinuclear complex compound [(NH 3 ) 7 (N 3 )U(μ-O)U-(NH 3 ) 8 ]Cl 5 • 7NH 3 .A driving force is certainly also the formation of NH 4 Cl.Another possibility for the formation of the dinuclear complex is via a dinuclear [{(NH 3 ) 8 U} 2 (μ-O)] 6 + cation, shown in equation 5, with a subsequent exchange of an NH 3 with an N 3 À ligand, given in equation 6.
To get more insights into the energetics of the proposed reaction sequences, we conducted molecular quantum-chem-ical calculations for the complex cations at the DFT-PBE0/TZVP level of theory.Conductor-like screening model (COSMO) was applied as a continuum solvation model to counter the charges of the involved species.More details on the calculations can be found in the quantum-chemical part of the experimental details.The calculated reaction energies at 0 K are given above the reaction arrows in equations 7 to 11.
The ligand exchange reaction of the chlorido to the azido complex in equation 7 is energetically favored by À59 kJ mol À1 .For the calculation, we assume a dissociative ligand exchange mechanism, which, of course, may be in competition with an associative mechanism.A detailed computational study on these two possibilities will be reported elsewhere.Another chlorido complex undergoes ligand exchange and deprotonation under formation of a hydroxido complex, [U(OH)(NH 3 ) 8 ] 3 + , in equation 8, which is favored by À93 kJ mol À1 .Lastly, the reaction of the hydroxido and azido complex to the dinuclear [(NH 3 ) 7 (N 3 )U(μ-O)U(NH 3 ) 8 ] 5 + complex in equation 9 is calculated to be essentially isoenergetic with 3 kJ mol À1 .Another possible way of formation of the dinuclear [(NH 3 ) 7 (N 3 )U(μ-O)U(NH 3 ) 8 ] 5 + complex is via the dinuclear [{(NH 3 ) 8 U} 2 (μ-O)] 6 + cation.Its formation from the mononuclear hydroxido and chlorido complexes is favored with À9 kJ mol À1 (Equation 10).Then, this dinuclear complex undergoes a ligand exchange from NH 3 to N 3 À which is favored by À47 kJ mol À1 (Equation 11).We note that the higher charge of the [(NH 3 ) 7 (N 3 )U(μ-O)U(NH 3 ) 8 ] 5 + cation has an essential influence on the ground state energy within the solvent field model and therefore might influence the resulting reaction energies.

Synthesis and Characterization of [{(NH 3 ) 4 UO 2 } 2 (μ-O)]Cl 2 • 4NH 3
The solvation of UO 2 Cl 2 in not adequately pre-dried liquid NH 3 resulted in a yellowish solution with a residue of the same color.The solution must have had some contact with air.Within four months at À40 °C, yellow crystals of suitable size were obtained for an X-ray diffraction experiment.The structure analysis indicated a compound with the composition [ Tetraamminedioxidouranium(VI)-μ-oxidotetraamminedioxidouranium(VI) dichloride-ammonia(1/4) crystallizes in the triclinic crystal system in space group P � 1 (no.The U atoms are each coordinated by seven atoms to form a pentagonal bipyramidal-like coordination polyhedron, where the O atoms of the UO 2 2 + units form the tips.This motif has frequently been observed for UO 2 2 + compounds obtained from aNH 3 . [9,15,16,45]The coordination polyhedra are corner-connected via the μ-O 2À ligand.All U�O bond lengths are equal within their tripled standard uncertainties in a range from 1.793(3) to 1.811(3) Å.These distances are in agreement with known compounds containing [UO 2 (NH 3 ) 5 ] 2 + cations. [15,26]The O�U�O angles are with 175.25( 16)°and 175.33( 17)°also in agreement with previously reported ones.The UÀNH 3 distances are with 2.550(5)-2.630(4)Å for U1 and 2.549(4)-2.608(5 S9 and S10 of the Supporting Information.Hydrogen bonds and a packing analysis are also discussed there.
The formation of this reaction product clearly points towards the presence of H 2 O.A formation mechanism is postulated in the reaction sequence of equations 12 to 14.The given reaction energies at 0 K above the reaction arrows stem from molecular DFT-PBE0 calculations for the complex cations in COSMO continuum solvent field.
Equation 12 shows the known formation of the [UO 2 (NH 3 ) 5 ] 2 + cation, which was observed previously from the reaction of UO 2 Cl 2 with liquid aNH 3 , in the compound [UO 2 (NH 3 ) 5 ]Cl 2 • NH 3 . [15]To the best of our knowledge, there are no investigations about the different species of UO 2 2 + that may form in liquid ammonia as of today.It was shown theoretically and experimentally, using high-energy X-ray scattering, that in aqueous solution the [UO 2 (H 2 O) 5 ] 2 + cation seems to be the most energetically favorable species. [46,47]These findings shall serve as an argument for the proposed intermediates appearing in NH 3 solution, as liquid aNH 3 can be seen as a solvent with similar properties compared to H 2 O. [5][6][7][8]48,49] The exchange of an NH   S19.
Due to the high IBO contribution of more than 80 % of the μ-O atom, the O 2 UÀ(μ-O)ÀUO 2 bond has a quite ionic nature.
The U atoms both have a partial charge of + 1.04 e À and the μ-O atom of À0.55 e À .Compared to the [(NH 3 ) 7 (N 3 )U(μ-O)U-(NH 3 ) 8 ] 5 + cation above, the partial charges of the U atoms are smaller, + 1.26 e À versus + 1.04 e À , while the partial charge of the μ-O atom is equal.This smaller partial charge at the U atoms can be attributed to the UO 2 2 + units, as the O atoms show strong π-backbonding. [50]

Synthesis and Characterization of [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ]Br 2 • 6NH 3
The reaction of UBr 4 with an excess of liquid aNH 3 resulted in a yellowish solution with an immediate formation of a black precipitate.The reaction solution was allowed to crystallize at À40 °C for 36 months with contact to the atmosphere before yellow crystals of suitable size for an X-Ray diffraction experiment were observed.The single crystal structure analysis showed the composition [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ]Br 2 • 6NH 3 .This compound contains two UO 2 2 + and two UO 2 + moieties, and therefore the formula is better given as dibromide-ammonia(1/6) crystallizes in the monoclinic crystal system, space group P2 1 /c (no.S6. The [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ] 2 + cation, shown in Figure 7, has an inversion center in-between the U1 atoms.There are two symmetry-independent U atoms, U1 and U2, which reside on a general position (4 e, 1).
The U1 atom is coordinated by seven atoms, two ammine ligands and five O atoms.The U1ÀNH 3 distances are 2.581 (7)    6) Å, which is significantly elongated compared to the U�O bonds and in agreement with previously observed lengths for the related bonding motif in the mixed-valent uranyl(V,VI) compound "HNU-46" with a U•••O�U distance of 2.46 Å. [51] The results from DFT also match with 2.404 Å.The U1À(μ 3 -O) bond lengths are 2.229(5) and 2.290(5) Å and agree with distances between 2.22 to 2.29 Å observed in the structure of "HNU-46". [51]DFT calculations agree with 2.215 and 2.303 Å.
The U2 atom is also surrounded by seven atoms, four NH 3 ligands, two O atoms of its UO 2 unit, and one μ 3 -O 2À ligand.The U2ÀNH 3 distances range from 2.575(7) to 2.615(7) Å, which agree with those of U1.The U2�O bond lengths are 1.832(6) and 1.872(6) Å.These distances are significantly elongated compared to those for U1 and point towards a uranyl(V) cation, U(V)O 2 + .U(V)�O bond lengths have been reported before in the [UO 2 (NH 3 ) 5 ] + cation with 1.861(3) to 1.867(3) Å, [26] and 1.817(6) to 1.821(6) Å in the [UO 2 (OPPh 3 ) 4 ] + cation. [52]The difference between the two U2�O bond lengths is due to the one O atom of the uranyl(V) ion acting μ-O-bridging towards the U1 center.This results in a U�O bond elongation to 1.872(6) Å and agrees with the higher coordination number of this O atom.The U2À(μ 3 -O) distance is with 2.242(5) Å slightly elongated compared to the one for U1, but matches the results from DFT with 2.233 Å. Selected experimentally determined bond lengths and angles for the [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ] 2 + cation are compared to the calculated ones at the DFT-PBE0/TZVP level of theory in Tables S11 and S12 of the Supporting Information.
There is one symmetry-independent Br atom (4e, 1), which is surrounded by eleven N atoms, from which eight are NH 3 ligands and three are NH 3 molecules of crystallization.Hydrogen bonds are discussed in the Supporting Information.Each [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ] 2 + cation is surrounded by twelve others in the shape of a quite distorted cuboctahedron meaning that hexagonally close packed layers in ABC stacking according to the cubic close-packed Cu type are present.The pseudo-cubic unit cell is shown in Figure S1 in the Supporting Information.However, the Br atoms are neither residing close to the octahedral nor the tetrahedral voids of the packing.
As only a few examples of UO 2 + -containing compounds are known from reactions in liquid aNH 3 , [26] we were interested in its formation.We therefore calculated the reaction energies for the following equations 15 to 17 using the DFT-PBE0 method for the molecular cations at 0 K (including COSMO continuum solvent field).
Equation 15 describes the comproportionation of U(VI) and U(IV) to U(V) in aNH 3 .With liquid ammonia, UO 2 2 + cations can react forming [UO 2 (NH 3 ) 5 ] 2 + cations and UBr 4 dissolves under the formation of [U(NH 3 ) 10 ] 4 + cations in the solid state. [9,10,15,43]As of today, no information on the composition of dissolved species is present.It was shown for the aqueous system that the U 4 + ion has coordination number nine within the [U-(H 2 O) 9 ] 4 + cation, but theoretical works indicate only a small energetic difference to the [U(H 2 O) 10 ] 4 + cation. [53]The existence of the [UO 2 (NH 3 ) 5 ] + cation has also been observed in the compound [UO 2 (NH 3 ) 5 ]NO 3 • NH 3 . [26]Nevertheless, for the uranyl(V) cation, UO 2 + , no investigations on the composition of potential species in solution have been conducted so far, neither for the aqueous, nor for the ammonia system.The reaction product of equation 15, [UO 2 (NH 3 ) 5 ] + , is energetically favored by 229 kJ mol À1 .The second step, equation 16, is equal to the previously discussed equation 13.Equation 17 describes the reaction of the [UO 2 (NH 3 ) 5 ] + cation and the [UO 2 (OH)(NH 3 ) 4 ] + cation under deprotonation of OH À ligands under formation of the μ 3 -O 2À ligands.This reaction is energetically favored by 38 kJ mol À1 .
We did not investigate reaction mechanisms that describe the oxidation of U(IV) by pure O 2 as previous experiments to oxidize U(IV) in aNH 3 with pure O 2 gas failed.This was attempted with excesses of O 2 as well as with stoichiometric amounts of O 2 . [9]In addition, the oxidation of U(IV) with O 2 to UO 2 2 + would also require the oxidation of NH 3 to N 2 , for which we have no evidence.Therefore, oxidation of U(IV) with pure O 2 is very unlikely.However, in the presence of moisture an oxidation reaction of U(IV) with O 2 can be formulated according to equation 18.
The calculated reaction energy at the DFT-PBE0/TZVP level of theory at 0 K in the gas phase favors the reaction products with 652 kJ mol À1 and suggests how the uranyl cations formed.
The structural observation that the U2 atom of the [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ] 2 + cation has likely an oxidation state of + V is supported by quantum-chemical calculations using the DFT-PBE0 method.These strongly suggest that the unpaired electron resides in an f-type orbital of the U2 atom, as the IBO analysis yielded a matching IBO located at the U2(V) atom with 100 % 5 f contribution.
The energetic difference between the [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ] 2 + cations with spin S = 1 and S = 0 is 0.1 kJ mol À1 , what essentially excludes any intramolecular U•••U interactions.In addition, the yellow color of the compound is not pointing to an interaction of U(V) and U(VI) atoms, as then usually deeper colors are observed for mixed-valent compounds.
As the first example of a mixed-valent ammine uranyl(V,VI) complex featuring μ 3 -O bridges, we were interested in the electronic situation of the U atoms, as well as in the UÀ( The IBOs describing the σ-type UÀ(μ 3 -O) bond have a quite ionic character, with a contribution dominated by the O atom of 79 %.The U1(VI) atoms participate with either 8 or 4 %, while the U2(V) atom contributes with 9 %.The π-bond IBO of the UÀ(μ 3 O) bonds, shown on the right side of Figure 8, shows an even more ionic character.As described above for the σ-type IBO, the orbital contributions do not differ for the different oxidation states, which leads to the conclusion that the bonding situation is very similar.The U(VI)•••O�U(V) bond is of quite ionic character.The contributions of the U2(V) and U1(VI) are slightly different due to their different oxidation states and bonding situations and also different compared to the [{(NH 3 ) 4 UO 2 } 2 (μ-O)] 2 + cation where both U atoms contribute similarly.There are only 6d and 5 f orbitals involved in the IBO from the U2(V) atom, one can interpret their involvement as a π-acceptor motif.The orbital contributions to the IBOs are handed in the Supporting Information in Table S20.
Table 1 summarizes the structural arguments for the absence of μ 3 -N 3À ligands as the direct comparison of calculated and observed bond lengths allows for a clear discrimination of N-versus O-bridging.The energetic perspective also supports absence of μ 3 -N 3À ligands at least at the DFT level.The reaction equation for the formation of the hypothetical [(U(VI)O 2 ) 4 (μ 3 -N) 2 (NH 3 ) 12 ] 2 + cation is shown in equation 19 (reaction energy at 0 K from molecular calculations using COSMO continuum solvent field).
There are four symmetry-independent U atoms residing on general positions (2i, 1).
The U1 atom is coordinated by seven atoms to form a pentagonal bipyramidal-like coordination polyhedron, where the tips are formed by the UO 2 unit.The observed U�O bond lengths are equal with values of 1.803( 5    The U2 atom is also surrounded by seven atoms in the shape of a pentagonal bipyramidal-like coordination polyhedron.The U2�O bond lengths are 1.809(6) and 1.831(5) Å. Contrary to expectation, the O atom of the shorter bond is bridging to the U3 atom and the O atom of the longer one is the terminal O atom.The four U2ÀNH 3 distances range between 2.579(6) and 2.619(9) Å, which agree with those for the U(V)O 2 units in the [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ] 2 + cation with 2.575(7) to 2.615(7) Å, but clearly do not deviate from those for the U1 atom, which is an U(VI)O 2 unit.The U2À(μ 3 -O) bond length is 2.225(4) Å and agrees with those described above.
The U3 atom is exclusively surrounded by seven O atoms in the shape of a pentagonal bipyramidal-like coordination polyhedron described as a [UO 7 ] 8À unit, which has been reported before. [51,54]The U3�O bond lengths are with 1.801( 5) and 1.809(5) Å in line with known ones for U(VI)O 2 2 + units. [26,52]he O atom of the U3•••O�U2 bridging unit shows a U3•••O distance of 2.444(5) Å which is in line with 2.424(6) Å observed for the one in the [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ] 2 + cation discussed above.There are two U3Àμ 3 -O bonds with distances of 2.319(5) and 2.334(5) Å which are larger compared to those of the U1 atom.A special feature of the U3 atom is its bond towards a (μ 3 -O)�U4 unit, where an O atom of a U(V)O 2 unit is μ 3 -bridging.The U3•••(μ 3 -O)�U4 distances are with 2.494(6) and 2.536(5) Å significantly elongated compared to the other μ 3 -O 2À ligand with distances of 2.319(5) and 2.334(5) Å.These might result from a different bonding situation, which will be discussed in the respective part below.
The U4 atom is similar to the U2 atom and coordinated in the shape of a pentagonal bipyramidal-like coordination polyhedron.The U4�O distances are 1.812(5) Å for the apical O atom and 1.843(6) Å for the U4�(μ 3 -O) atom.The first distance matches the length of U(V)O 2 units, while the latter is significantly elongated, as expected due to the higher coordination number of the O atom.The U4ÀNH 3 distances range from 2.569(6) to 2.595(6) Å.The U4�(μ 3 -O) bond length is with 2.204(4) Å in line with the previously reported ones.Selected experimentally determined bond lengths and angles for the [(UO 2 ) 8 (μ 3 -O) 4 (NH 3 ) 22 ] 4 + cation are compared to the calculated ones at the DFT-PBE0/TZVP level of theory in Tables S13 and  S14 of the Supporting Information.Hydrogen bonds are also discussed in the Supporting Information.
The calculated reaction energy at the DFT-PBE0/TZVP level of theory is essentially isoenergetic with 2 kJ mol À1 .However, the COSMO solvent field used in the calculations to counter the charge of the complexes has an influence on the energetics and might affect the resulting value in a way that the formation is more favored.
However, this overall reaction is energetically more unfavorable with 77 kJ mol À1 at 0 K.
The structural observation that the atoms U2 and U4 have an oxidation state of + V, can be supported by quantumchemical calculations using the DFT-PBE0 method, combined with the IBO localization procedure.For the U(V) atoms, matching IBOs were found with 100 % 5 f contribution, which coincides with the findings for the [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ] 2 + cation above.The energetic difference between the structure with S = 2 and S = 0 is 0.5 kJ mol À1 , what excludes an interaction between the U atoms.
The IBOs of the UÀ(μ 3 -O) bonds and the U(VI)•••O�U(V) bonds are similar to those of the [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ] 2 + cation with comparable atomic contributions.The IBOs are given in the Supporting Information in Figures S6 and S7.Therefore, only the (μ 3 -O)�U4(V) bond will be discussed in detail, as this seems to be a novel bonding motif in uranyl(V,VI) chemistry.The IBO is shown in Figure 11.
The partial charges of the U3 and U3* atoms are with + 1.05 e À equal compared to the partial charge of U4 with + 1.06 e À , despite that the U3 atom belongs to an U(VI)O 2 2 + unit, while the U4 atom is part of an U(V)O 2 + unit.The partial charge of the μ 3 -O atom is with À0.51 e À , equal to the μ-O 2À atom of the U(VI)•••O�U(V) bond with À0.50 e À .
Mainly the μ 3 -O atom contributes with 85 % to the IBO rendering the bond even more ionic than those discussed for the above examples.The orbital contributions to the IBOs are handed in the Supporting Information in Table S21.
As discussed above, also here the substitution of the μ 3 -O 2À ligands with μ 3 -N 3À ligands in the [(UO 2 ) 8 (μ 3 -O) 4 (NH 3 ) 22 ] 4 + cation was carried out in silico and the respective reaction is shown in equation 22.
With these structural data the oxidation numbers of the U atoms in the [(NH 3 ) 8 U(μ-N)U(NH 3 ) 5 (μ-N)UO 2 (NH 3 ) 4 ] 6 + cation can be assigned.The oxidation state of the U atom of the (NH 3 ) 8 U unit is + IV, what has been confirmed for the UN 2 complexes, [18] and also for other octaammine U(IV) complexes containing an additional hetero ligand. [9,43]The central U atom has an oxidation state of + VI, as the UN 2 (NH 3 ) 5 unit is isoelectronic to the [UO 2 (NH 3 ) 5 ] 2 + cation. [18]The UO 2 (NH 3 ) 4 unit, with the U atom in oxidation state + VI, obviously stems from a [UO 2 (NH 3 ) 5 ] 2 + cation. [16]here are six symmetry-independent Br atoms in the crystal structure, all residing on a general position (4e, 1).The Br1 and Br 2 atoms are each surrounded by twelve N atoms, from which six are NH 3 molecules of crystallization and six are NH Assuming that the here used UBr 5 contained UO 2 Br 2 as an impurity, the formation of the [(NH 3 ) 8 U(μ-N)U(NH 3 ) 5 (μ-N)UO 2 (NH 3 ) 4 ] 6 + cation can be described with equation 23.
A comparison of the calculated atomic distances and angles at the DFT-PBE0/TZVP level of theory of the two cations versus the experimentally observed ones is given in Table 2.
A very clear difference between the two species is the U1�XÀU3 angle, which is observed with 165.4(3)°and calculated with 160.1°for the nitrido-bridged cation, whereas for the hypothetical oxido-bridged complex, 144.8°is obtained.Addi- tionally, the observed U3�O distances agree with those calculated for the U(VI)O 2 unit in the nitrido-complex but not to those of the U(V)O 2 unit of the oxido-bridged cation.The latter calculated distances of 1.808 and 1.838 Å would however agree with those expected for uranyl(V) UÀO bond lengths. [26]The observed U1�XÀU2 and U1�XÀU3 bond lengths are in agreement with X = N and not with X = O.
With these results it is obvious that N�U�N units are present in the compound [(NH 3 ) 8 U(μ-N)U(NH 3 ) 5 (μ-N)UO 2 (NH 3 ) 4 ]Br 6 • 18NH 3 .This makes it the first example of N�U�N units bridging to uranyl(VI) units as previously only bridging to [U(NH 3 ) 8 ] 4 + units had been observed. [18]e conducted an IBO analysis of the bonding situation of the UN 2 unit.Only two selected IBOs, one for the U�N σ-and one for the π-bond, are depicted in Figure 14.The other IBOs are shown in Figures S8 and S9 of the Supporting Information.
The partial charge of the U1 atom is with + 0.77 e À the smallest, compared to the partial charges of the U2 and U3 atoms with + 1.29 e À and + 1.05 e À , respectively.These values agree for the central U1 atom with the previously obtained one of + 0.78 e À for the [(NH 3 ) 8 U(μ-N)U(NH 3 ) 5 (μ-N)U(NH 3 ) 8 ] 8 + cation. [18]The partial charge for the U2 atom agrees with the findings for the [UCl(NH 3 ) 8 ] 3 + cation with a calculated charge of + 1.3 e À . [43]Finally, the partial charge of the U3 atom matches the findings for the [{(NH 3 ) 4 UO 2 } 2 (μ-O)] 2 + cation with + 1.04 e À .The partial charges of the μ-N atoms are with À0.45 e À and À0.40 e À to each other.The N atom contributes with 59 % to the σ-type IBO of the U1�NÀU2 bond while the U1 atom participates with 37 and the U2 atom only with 3 %.[57][58][59] A comprehensive study on the PFB effect and its importance for actinide ammine complexes will be given elsewhere.For the πtype IBO of the U1�NÀU3 bond, shown on the right in Figure 14, similar contributions of the N atom with 66 % were observed.The U atoms contribute with 30 and 4 % for U1 and U3, respectively.Orbital contributions and a discussion on the IBOs are handed in the Supporting Information in Table S22.
In the ammonia system, we observed the dinuclear complex [(UO 2 ) 2 (μ-O)(NH 3 ) 8 ] 2 + (Figure 15) which has only one bridging ligand and not two as is the case for the hydroxido-bridged dinuclear species.Currently, there are no uranyl complexes known in the ammonia system containing bridging amido or imido ligands.Amido ligands, NH 2 À , would correspond to hydroxido ligands, OH À , and imido ligands, NH 2À , to oxido ligands, O 2À .No trinuclear uranyl complex has been obtained from NH 3 yet.The center of the tetranuclear complex [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 10 ] 2 + (Figure 15) shows similarity to the center of the [(UO 2 ) 4 (μ 3 -O) 2 (μ-Cl) 2 (μ-OH) 2 (OH 2 ) 6 ] complex, but in the "ammonia case" no bridging Cl À and OH À ligands are present.Instead, Figure 15.Structural formulas of [UO 2 ] 2 + complexes obtained from the aqueous system from the literature on the left and the structurally somewhat related uranyl(VI) and uranyl(V) ammine complexes obtained from the ammonia system.some of the yl-O-ligands of the uranyl(V) ions engage in the bridging, which is currently unique for the ammonia system.To the best of our knowledge, this has also never been observed for the aqueous system.In addition, also no mixed-valent uranyl(V)-uranyl(VI) complexes are known for the aqueous system.Finally, the octanuclear complex [(UO 2 ) 8 (μ 3 -O) 4 (NH 3 ) 22 ] 4 + (Figure 15) is also unmet in the aqueous system.However, it may act as an indicator that a related complex may be also present in aqueous solution or obtained from it in a solid compound.

Conclusions
The uranium complex compounds [(NH 3 ) 7 (N 3 )U(μ-O)U-(NH 3 ) 8 ]Cl  are peculiar, containing mixed-valent uranyl(V)-uranyl(VI) units and we observed that the O atoms of such uranyl units can be μ 2 -and even μ 3 -bridging to other U atoms leading to tetra-and octanuclear complex cations which is unprecedented in the aqueous system.In addition to crystallographic considerations, quantum-chemical calculations excluded unambiguously that bridging N instead of O atoms might be present.The chemical bonding in the complex cations was studied using intrinsic bond orbitals, and the analysis revealed a new bonding motif in the structure of the [(U-(VI)O 2 ) 4 (U(V)O 2 ) 4 (μ 3 -O) 4 (NH 3 ) 22 ] 4 + cation, where a O atom of a U(V)O 2 unit is μ 3 -bridging.

Experimental Details
All work was carried out under argon atmosphere (5.0, Praxair) using a fine-vacuum line and a glovebox (MBraun).Liquid ammonia was usually dried by storage over Na.The self-made borosilicate glass bomb tubes for reactions in liquid ammonia were usually flame-dried at least three times under vacuum.In some of the reactions presented here, moisture must have been present in the ammonia or the reaction vessels were not tight and the reaction mixtures had contacts to moisture and oxygen from air.In none of these reactions we determined the yields as the resulting ammine complexes and ammoniates decompose under loss of NH 3 when being warmed to room temperature.Uranium compounds are radioactive, and appropriate/required measures for their handling need to be taken.UCl 4 , UBr 4 , and UO 2 Cl 2 were synthesized as reported previously. [15,61]nthesis of [(NH 3 ) 7 (N 3 )U(μ-O)U(NH 3 ) 8 ]Cl 5 • 7NH 3 A Schlenk tube was charged with 31 mg UCl 4 (0.08 mmol) and 9 mg NH 4 N 3 (0.15 mmol, 2 eq.) that likely contained moisture, filled with approximately 3 mL liquid NH 3 at À78 °C and stored at À36 °C.After 12 months, green crystals had grown large enough for X-ray diffraction experiments.A few crystals of the compound were obtained.After removal of residual liquid NH 3 in vacuo at room temperature, a greenish powder remained.The diffraction pattern is included in the Supporting Information in Figure S3.A Schlenk tube was charged with 71 mg UBr 4 (0.18 mmol) and filled with approximately 10 mL liquid NH 3 at À78 °C and stored at À36 °C with contact towards atmosphere so that O 2 may have been the oxidant leading to [UO 2 ] 2 + formation.After 36 months, yellow crystals had grown large enough for X-ray diffraction experiments.A few crystals of the compound were obtained.A Schlenk tube was charged with 50.1 mg UBr 5 (0.08 mmol) that likely contained some UO 2 Br 2 as impurity and filled with approximately 10 mL liquid NH 3 at À78 °C and stored at À36 °C.After 6 months, yellow-green crystals had grown large enough for X-ray diffraction experiments.A few crystals of the compound were obtained.

Single Crystal X-Ray Diffraction
The crystals of the compound [(NH 3 ) 7 (N 3 )U(μ-O)U(NH 3 ) 8 ]Cl 5 • 7NH 3 were selected under nitrogen-cooled, pre-dried perfluorinated oil (Galden HT270, PFPE, Solvey Solexis) under the exclusion of air.The crystals were mounted with a MiTeGen loop.Intensity data of a suitable crystal were recorded with a D8 Quest diffractometer (Bruker).The diffractometer was operated with monochromatized Mo-K α radiation (0.71073 Å, multi layered optics) and equipped with a PHOTON 100 CMOS detector.Evaluation, integration, and reduction of the diffraction data was carried out with the APEX3 software suite. [62]The diffraction data were corrected for absorption utilizing the multi-scan method of SADABS within the APEX3 software suite.The structure was solved with dualspace methods (SHELXT) and refined against F 2 (SHELXL). [63,64]All atoms were refined with anisotropic displacement parameters, H atoms were refined isotropic with a riding model.Representations of the crystal structure were created with the Diamond software. [65] Scheme 1.An overview of the compounds presented here and the reaction conditions of their formation.Chlorides in the green, bromides in the brown boxes.Space groups and Pearson symbols, without H atoms, are given in green for easier comparison of these and other crystal structures.The asterisk indicates that the used UBr 5 contained very likely some O-containing impurities.

Figure 3 .Scheme 2 .
Figure 3. Two sets of IBOs for the [(NH 3 ) 7 (N 3 )U(μ-O)U(NH 3 ) 8 ] 5 + cation (isosurfaces drawn in green and red).Left: One of two σ-type IBOs between UÀ(μ-O)ÀU.Right: One of two π-type IBOs between UÀ(μ-O)ÀU.The listed percentages show the contribution of each atom in the IBO.In a purely covalent 2c-bond, each atom would contribute 50 %.Atomic contributions smaller than 2 % to any IBO are not listed.The isovalue for IBO isosurface plots is 0.03 a.u.U atoms in cyan, N atoms in blue, O atoms in red, and H atoms in white color.

Figure 4 .
Figure 4. IBO showing the U1ÀN 3 bond in the [(NH 3 ) 7 (N 3 )U(μ-O)U(NH 3 ) 8 ] 5 + cation (isosurfaces drawn in green and red).The listed percentages show the contribution of each atom in the IBO.In a purely covalent 2c-bond, each atom would contribute 50 %.Atomic contributions smaller than 2 % to any IBO are not listed.The isovalue for IBO isosurface plots is 0.03 a.u.U atoms in cyan, N atoms in blue, O atoms in red, and H atoms in white color.
) Å for U2 in agreement with each other.In comparison to compounds known from the literature, they are elongated, e. g., in [UO 2 (NH 3 ) 5 ]Cl 2 • NH 3 the UÀNH 3 distances range between 2.506(1) to 2.551(3) Å.The elongation is due to the additional electronegative μ-O 2À ligand.Selected experimentally determined bond lengths and angles for the [{(NH 3 ) 4 UO 2 } 2 (μ-O)] 2 + cation are compared to the calculated ones at the DFT-PBE0/ TZVP level of theory in Tables

Figure 6 .
Figure 6.Two sets of IBOs for the [{(NH 3 ) 4 UO 2 } 2 (μ-O)] 2 + cation (isosurfaces drawn in green and red).Left: One of two σ-type IBOs between O 2 UÀ(μ-O)ÀUO 2 .Right: One of two π-type IBOs between O 2 UÀ(μ-O)ÀUO 2 .The listed percentages show the contribution of each atom in the IBO.In a purely covalent 2c-bond, each atom would contribute 50 %.Atomic contributions smaller than 2 % to any IBO are not listed.The isovalue for IBO isosurface plots is 0.03 a.u.U atoms in cyan, N atoms in blue, O atoms in red, and H atoms in white color.
μ 3 -O) bonds.Due to technical limitations of the IBO localization procedure in Turbomole, the localization of the wavefunction has been conducted in point group C 1 , resulting in slightly different contributions of the respective symmetry-equivalent U atoms.There are six σ-type IBOs and two π-type IBOs for the μ 3 -O 2À ligands.One of each type is shown in Figure 8.The orbital contributions to the IBOs are handed in the Supporting Information, Table S20.The U1(VI) atoms have a similar partial charge compared to the U2(V) atoms with + 1.03 and + 1.01 e À .These values match the above findings for the [{(NH 3 ) 4 UO 2 } 2 (μ-O)] 2 + cation.The μ 3 -O 2À ligands have a partial charge of À0.60 e À , which is, as expected, more negative than the μ 2 -O 2À ligand in the above [{(NH 3 ) 4 UO 2 } 2 (μ-O)] 2 + cation with À0.55 e À .The O atom of the U(V)O 2 unit, which is bridging to the U(VI)O 2 unit, has a more negative partial charge with À0.50 e À compared to the terminally bound O atoms of the U(VI)O 2 unit with À0.38 e À , but is equal to the terminal O atom of the U(V)O 2 unit with also À0.50 e À .

Figure 8 .
Figure 8. Two sets of IBOs for the [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ] 2 + cation (isosurfaces drawn in green and red).Left: One of six σ-type IBOs between O 2 UÀ(μ 3 -O)ÀUO 2 .Right: One of two π-type IBOs between O 2 UÀ(μ 3 -O)ÀUO 2 .The listed percentages show the contribution of each atom in the IBO.In a purely covalent 2c-bond, each atom would contribute 50 %.Atomic contributions smaller than 2 % to any IBO are not listed.The isovalue for IBO isosurface plots is 0.03 a.u.The asterisk marks the U atoms that are identical by means of the crystallographic symmetry but not by means of the wavefunction in point group C 1 .U atoms in cyan, N atoms in blue, O atoms in red, and H atoms in white color.

Figure 9 .
Figure 9.One IBO for the [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ] 2 + cation (isosurfaces drawn in green and red).One of the four IBOs for the bond between U(VI)•••O�U(V).The listed percentages show the contribution of each atom in the IBO.In a purely covalent 2c-bond, each atom would contribute 50 %.Atomic contributions smaller than 2 % to any IBO are not listed.The isovalue for IBO isosurface plots is 0.03 a.u.U atoms in cyan, N atoms in blue, O atoms in red, and H atoms in white color.

Figure 11 .
Figure 11.One IBO for the [(UO 2 ) 8 (μ 3 -O) 4 (NH 3 ) 22 ] 4 + cation (isosurfaces drawn in green and red).One of the four IBOs for the (μ 3 -O)�U(V) bond.The listed percentages show the contribution of each atom in the IBO.In a purely covalent 2c-bond, each atom would contribute 50 %.The asterisk marks the U atom, which is identical by means of the molecular symmetry but not by means of the wavefunction in point group C 1 .Atomic contributions smaller than 2 % to any IBO are not listed.The isovalue for IBO isosurface plots is 0.03 a.u.U atoms in cyan, N atoms in blue, O atoms in red, and H atoms in white color.

igure 14 .
Two sets of IBOs of the [(NH 3 ) 8 U(μ-N)U(NH 3 ) 5 (μ-N)UO 2 (NH 3 ) 4 ] 6 + cation (isosurfaces drawn in green and red).Left: One of four σ-type IBOs between U1�NÀU2, the π-type IBO is shown in the Supporting Information.Right: One of four π-type IBOs between U1�NÀU3, the σ-type IBO is shown in the Supporting Information.The listed percentages show the contribution of each atom in the IBO.In a purely covalent 2c-bond, each atom would contribute 50 %.Atomic contributions smaller than 2 % to any IBO are not listed.The isovalue for IBO isosurface plots is 0.03 a.u.U atoms in cyan, N atoms in blue, O atoms in red, and H atoms in white color.

3 A
Schlenk tube was charged with 250 mg UO 2 Cl 2 (0.73 mmol) and filled with approximately 10 mL moist liquid NH 3 at À78 °C and stored at À36 °C.After four months, yellow crystals had grown large enough for X-ray diffraction experiments.A few crystals of the compound were obtained.Synthesis of [(UO2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ]Br 2 • 6NH 3 From the reaction mixture of [(UO 2 ) 4 (μ 3 -O) 2 (NH 3 ) 12 ]Br 2 • 6NH 3 another type of yellow crystals had grown large enough for X-ray diffraction experiments after 40 months.A few crystals of the compound were obtained.Synthesis of [(NH3 ) 8 U(μ-N)U(NH 3 ) 5 (μ-N)UO 2 (NH 3 ) 4 ]Br 6 • 18NH 3 3 ligand with a H 2 O molecule combined with a consecutive deprotonation of the H 2 O ligand and protonation of the NH 3 molecule to form an NH 4 + cation, is postulated in equation 13.This reaction is energetically favored by 31 kJ mol À1 .The H 2 O ligand is acidified due to the polarized UÀO bond and is therefore easier to deprotonate.In a last step, shown in equation 14, another [UO 2 (NH 3 ) 5 ] 2 + cation reacts with the [UO 2 (OH)(NH 3