Uranium Cyanides from Reactions in Liquid Ammonia Solution

Reactions of uranium tri-and tetrahalides, UBr 3 , UI 3, UCl 4 , and UI 4 , with different cyanides M CN ( M = K, Ag) in liquid anhydrous ammonia led to three novel uranium(IV) cyanide compounds. The reaction of UCl 4 in the presence of KCN resulted in the compound [U(CN)(NH 3 ) 8 ]Cl 3 ·3NH 3 , while UBr 3 and UI 3 were oxidized in the presence of AgCN to form the compounds ∞ 1 ½ ( μ -CN){(H 3 N) 5 U( μ -NH 2 ) 3 U(NH 3 ) 5 }]Br 4 ·2NH 3 , and ∞ 1 ½ ( μ -CN)- {(H 3 N) 5 U( μ -NH 2 ) 3 U(NH 3 ) 5 }]I 4 ·2NH 3 . The reaction of UI 4 with KCN in aNH 3 also yielded the compound ∞ 1 ½ ( μ -CN){(H 3 N) 5 U( μ -NH


Introduction
Uranium cyanides are a little-known class of substances.Neither a pseudobinary U cyanide like U(CN) 4 nor a homoleptic cyanido complex such as for example [U(CN) 8 ] 4À are known to date.However, corresponding complex anions have been reported for the somewhat related elements Mo and W. [1] As oxidation state + IV is quite stable for U and the halides UX 4 (X = FÀI) are well characterized, we have set ourselves the goal of synthesizing U(IV) cyanides.In liquid anhydrous ammonia (aNH 3 ) as a solvent, a previous report on the reaction of UCl 4 with NaCN claimed the synthesis of [UCl 3 (CN)] • 4NH 3 which had been characterized by infrared spectroscopy. [2]Another UÀCN containing compound obtained from reactions in liquid aNH 3 is [U 2 (CN) 3 (NH 3 ) 14 ][KBr 6 ] • NH 3 , where the U atoms are bridged by CN À ligands to form a layer structure. [3]In an attempt to make a U cyanide, the compound [UCl 4 (HCN) 4 ] was obtained from the reaction of UCl 4 in anhydrous HCN. [4]Thermal decomposition led back to the starting materials.In a review article, [5] the compounds of [U(CN)(NH 3 ) 8 ]Cl 3 • 3NH 3 , ∞ 1 ½(μ-CN){(H 3 N) 5 U(μ-NH 2 ) 3 U(NH 3 ) 5 }]I 4 • 2NH 3 , and [U(CN) 3 (NH 3 ) 6 ]I were only briefly mentioned without providing structural data.

Results and Discussion
From reactions of uranium tri-and tetrahalides, UBr 3 , UI 3 , UCl 4 , and UI 4 , with the cyanides KCN or AgCN in liquid aNH 3 solution, we obtained compounds containing a monocyanidouranium(IV) complex cation or two structurally related (μ-NH 2 ) 3 À -and μ-CN Àbridged uranium(IV) compounds.Scheme 1 shows the carriedout reactions, their conditions as well as the products.See the Experimental Part for more details.

Synthesis and structural characterization of [U(CN)(NH 3 ) 8 ]Cl 3 • 3NH 3
From the reaction of UCl 4 with one equivalent of KCN in liquid aNH 3 at room temperature, we observed the formation of green plate-shaped crystals after six months of crystallization time.A single-crystal X-ray structure analysis showed the composition [U(CN)(NH 3 ) 8 ]Cl 3 • 3NH 3 , octaamminemonocyanidouranium(IV) chloride-ammonia(1/3).Its formation can be described by equation 1.
The formation of KCl was evidenced by its powder X-ray diffraction pattern recorded on the dried residue obtained after removal of the liquid aNH 3 from the reaction mixture.The diffraction pattern is shown in Figure S5 of the Supporting Information.
In the absence of cyanide ions, the ammonolysis product of UCl 4 was shown to be the composition-wise related [UCl(NH 3 ) 8 ]Cl 3 • 3NH 3 . [5]Nonetheless, species that are formed in solution as intermediates are unknown as of yet.It is therefore possible that in the presence of cyanide ions the [UCl(NH 3 ) 8 ] 3 + and the [U(CN)(NH 3 ) 8 ] 3 + cations are in chemical equilibrium with each other.A putative ligand exchange reaction can be formulated as follows in equations 2 and 3.Both linkageisomers of the CN À ligands were considered.[U(CN)(NH 3 ) 8 ]Cl 3 • 3NH 3 crystallizes in the orthorhombic crystal system, space group Pna2 1 (no.33), with the lattice parameters a = 8.2426(2), b = 14.2004(4), c = 16.6927(6)Å, V = 1953.85(10)Å 3 , and Z = 4, at T = 100 K.The diffraction pattern of the crystal appeared to be non-merohedrally twinned, but only the main domain was used during integration of the dataset and for the solution and refinement of the crystal structure model, see the Experimental Section and Table 1 for details.The Supporting Information also contains an explanation for the selection of the space group Pna2 1 over Pnma and less likely alternatives.
The crystal structure contains one symmetry-independent U atom (4c, .m.) which is surrounded by eight NH 3 ligands and one disordered NC À /CN À ligand to form a tricapped trigonal prism-like coordination sphere.The C and N atoms in the CN À unit are statistically disordered, with a 50 : 50 mixed site occupancy and in the following the NC/CN À unit will be written as CN À .See the Supporting Information for more crystallographic details.
The molecular structure of the [U(CN)(NH 3 ) 8 ] 3 + cation in its salt is shown in Figure 1.
The UÀNH 3 distances are with 2.531(16) to 2.595(5) Å in the expected range and agree to those within the [U(NH 3 ) 10 ] 4 + cation with 2.5390(15) to 2.621(2) Å where however a bicappedsquare antiprismatic coordination sphere is present. [6]The two longer UÀN bonds are 2.654(2) and 2.881(4) Å. [6] The UÀC/N bond has a length of 2.585(6) Å. Due to the lack of comparable structures of mononuclear uranium ammine complexes containing terminally bound CN À ligands, we calculated both linkage-isomers, kC and kN, of the [U-(CN)(NH 3 ) 8 ] 3 + cation at the DFT-PBE0/TZVP level of theory.The calculated UÀCN and UÀNC bond lengths are 2.511 and 2.395 Å, respectively.These distances are shortened for both linkage-isomers, in comparison to the experimentally observed Scheme 1.An overview of the compounds reported here in boxes and the reaction conditions for their formation.Chlorides in the green, bromides in the brown, and iodides in the purple boxes.Space groups and Pearson symbols, without H atoms, are given for easier comparison of the crystal structures.value.Selected experimentally determined bond lengths for the [U(CN)(NH 3 ) 8 ] 3 + cation are compared to the calculated ones at the DFT-PBE0/TZVP level of theory in Table S6 of the Supporting Information.
A description of the coordination sphere of the anions, hydrogen bonding, and a packing analysis of the crystal structure is available in the Supporting Information.
A closer look at the UÀC/N bonding situation in the [U(CN)(NH 3 ) 8 ] 3 + cation was taken with the help of calculations at the DFT-PBE0/TZVP level of theory using the Intrinsic Bond Orbital (IBO) analysis. [8]One σ-type IBO describing the UÀC/N bond, while no IBO related to a π-type backbond was not observed.The IBOs of both linkage-isomers, kN and kC, are compared with each other in Figure 2, with the respective contributions of the U and C or N atom.A detailed discussion on the orbital contributions is handed in the IBO section of the Supporting Information.
Absorption correction multi-scan and numerical multi-scan and numerical multi-scan and numerical

Synthesis and structural characterization of
From the reaction of UBr 3 with AgCN in liquid aNH 3 at + 40 °C in bomb tubes, we obtained green crystals after 10 months of crystallization time for which single-crystal structure analysis led to the composition ∞ 1 ½(μ-CN){(H 3 N) 5 U(μ-NH 2 ) 3 U(NH 3 ) 5 }]Br 4 • 2NH 3 .The formation of the compound can be described by equation 4.
The side products, Ag and NH 4 Br, have been identified by powder X-ray diffraction of the dried left-overs of ∞ 1 ½(μ-CN){(H 3 N) 5 U(μ-NH 2 ) 3 U(NH 3 ) 5 }]Br 4 • 2NH 3 after evaporation of the liquid aNH 3 .The diffraction pattern is shown in Figure S6 of the Supporting Information.For the balance of the reaction equation, another product is needed to account for the missing CN À unit.A recorded infrared spectrum of the dried residue shows no CN mode, which is consistent with the assumption of the formation of NH 4 CN, which sublimes in vacuo at room temperature and would be removed during the removal of liquid aNH 3 . [10]We have therefore put "NH 4 CN" in quotation marks.
Due to the bridging of the CN À unit, an infinite chain is formed, shown in Figure 3.The strings run parallel to the a axis and can be described with the Niggli formula . The UÀCNÀU angle is 174.38(11)°, which results in a slightly corrugated chain.
A description of the coordination sphere of the anions, hydrogen bonding, and a packing analysis of the crystal structure is available in the Supporting Information.
Figure 4 shows a Raman spectrum recorded on the green crystals of the compound.It is compared with results of a solidstate DFT calculation at the DFT-PBE0/TZVP level of theory.See computational details for specifics.The calculated wavenum-  bers show an overall agreement with the observed ones.Table S6 of the Supporting Information summarizes the band assignments.
The characteristic ν s -stretching C�N mode was observed at 2100 cm À1 , which is red-shifted compared to 2158 cm À1 in KCN. [17]The observed red-shift is likely due to the coordination of the cyanide unit to the U atoms.This red-shift has been previously observed in several CN-bridged metal complex compounds. [18]The calculated wavenumber from DFT is with 2264 cm À1 significantly shifted, but this disagreement can be attributed to the harmonic approximation within the DFT formalism.The modes at 3277 and 3235 cm À1 correspond to the ν as and ν s stretch modes of the NH 3 groups, respectively.The band at 3163 cm À1 can be assigned to the ν s NÀH mode of the bridging NH 2 À ligands.The low-energy modes at 1594 and 1220 cm À1 belong to the δ-wagging of the NH 3 groups, while the modes at 620 cm À1 are the δ-twisting of the NH 3 and μ-NH 2 groups.Lastly, the mode at 470 cm À1 can be described as the wagging of the UÀ(μ-NH 2 ) 3 ÀU group.The lower modes describe mostly rotations of NH 3 molecules and lattice vibrations.

Synthesis and structural characterization of
was obtained in two different ways.One was the reaction of UI 3 and AgCN in 1 : 1 ratio at 40 °C in liquid aNH 3 in a bomb tube, where after six days, green blocks of suitable size for an X-ray diffraction experiment were observed.The results of the structure determination on crystals from this route will be presented in the following.In the second synthetic approach, UI 4 was reacted with KCN in a 1 : 1 ratio in liquid aNH 3 at room temperature in a bomb tube.Green crystals of suitable size for an X-ray diffraction experiment were obtained after two weeks of crystallization time.Unfortunately, the crystal and the recorded data set were of very low quality and only the determined lattice parameters are given in the Supporting Information.
For the first approach, reaction equation 5 can be formulated.The second synthesis can be described by reaction equation 6.
We note that the compounds both crystallize in the orthorhombic crystal system, but with different centering, oP versus oI, respectively.Both structures are strongly related and may be isotypic.However, our X-ray diffraction data are not good enough to unambiguously resolve this issue.A detailed discussion on the crystallographic details, choice of centering and space groups, is given in the Supporting Information.
A description of the coordination sphere of the anions, hydrogen bonding, and a packing analysis of the crystal structure is available in the Supporting Information.
The kN-, kN-isomer shown on the left of Figure 5, and the kN-, kC-linkage-isomer in the middle are isoenergetic at 0 K.In contrast, the kC-, kC-isomer, shown on the right in Figure 5, is 33 kJ • mol À1 higher in energy compared to the other two structures.These findings are in line with those for the [U(CN)(NH 3 ) 8 ] 3 + cation, where the kN-is favored over the kClinkage-isomer by 23 kJ • mol À1 at 0 K.
Due to the quite short U•••U distance of 3.68490(18) Å in the {(H 3 N) 5 U(μ-NH 2 ) 3 U(NH 3 ) 5 } unit, we calculated the energy difference between the quintet state, S = 2, and the singlet state, S = 0, which appeared to be 0 kJ • mol À1 .This excludes an interaction between the U atoms.
The N atom contributes with 80 % to the UÀ(μ-NH 2 ) IBO, while the U atoms participate either with 16 % to the σ-type IBO or with 3 % in a π-type interaction.The IBOs describing the UÀN/C interaction are with contributions of the C/N atom of 86 % for UÀN and 82 % for UÀC, respectively, slightly more ionic compared to the UÀ(μ-NH 2 ) bond.These findings coincide with the one for the [U(CN)(NH 3 ) 8 ] 3 + cation.In conclusion, the UÀ(μ-NH 2 ) and UÀC/N interactions seem to be quite ionic σ-type bonds, while in case of the UÀ(μ-NH 2 ) bond a π-type interaction towards the second U atom is present.A detailed discussion on the orbital contributions is handed in the IBO section of the Supporting Information.

Conclusions
The reaction of KCN with UCl  investigations on the bonding situation of the μ-NH 2 À and μ-CN À ligands towards the U atoms showed quite ionic interactions.In addition to the σ-type UÀ(μ-NH 2 ) bonds additional π-type interactions towards the second U atom are present.

Experimental Details
All work was carried under argon atmosphere (5.0, Praxair) using a fine-vacuum line and a glovebox (MBraun).Liquid ammonia was dried by storage over Na.The self-made borosilicate glass bomb tubes for the reactions with liquid ammonia were flame-dried at least three times before use.The uranium containing starting materials UBr 3 , UI 3 , UCl 4 , and UI 4 were prepared as described previously. [19,20]All compounds obtained here are unstable at room temperature due to loss of NH 3 .Therefore, no further analyses, such as elemental analyses, could be carried out.For the same reason, yields could not be determined.

Preparation of [U(CN)(NH 3 ) 8 ]Cl 3 • 3NH 3
A borosilicate bomb tube was charged with 58.3 mg UCl 4 (0.154 mmol) and 10 mg KCN (0.154 mmol), filled with approximately 2 mL liquid NH 3 , cooled to liquid nitrogen temperature, flame-sealed, and stored at room temperature.A few green crystals were obtained which had grown large enough for X-ray diffraction experiments after one month.After removal of the residual liquid NH 3 in vacuo at room temperature, a greenish powder remained.Its diffraction pattern is shown in the Supporting Information in Figure S5.

Preparation of
A borosilicate bomb tube was charged with 20 mg UBr 3 (0.04 mmol) and 6 mg AgCN (0.04 mmol), filled with approximately 2 mL liquid NH 3 , cooled to liquid nitrogen temperature, flamesealed, and stored at 40 °C.After 10 months, a few green crystals of suitable size for single-crystal X-ray diffraction were obtained.After removal of residual liquid NH 3 in vacuo at room temperature, a greyish powder remained.The diffraction pattern is shown in the Supporting Information in Figure S6.

Preparation of
The compound has been obtained in two different ways:  S8.

Raman spectroscopy
The Raman spectrum was recorded through the glass wall of the bomb tube at room temperature with a Monovista CRS + confocal Raman microscope (Spectroscopy & Imaging GmbH) using a solidstate laser with λ = 532 nm and a 300 grooves/mm (low-resolution mode, FWHM: < 4.62 cm À1 ) grating.

IR spectroscopy
The IR spectra of the dried residues were recorded on a Bruker alpha FT-IR spectrometer using the ATR Diamond module with a resolution of 4 cm À1 .The spectrometer was located inside a glovebox (MBraun) under argon atmosphere.The spectra were processed with the OPUS software package. [21]wder X-ray diffraction The samples of the dried residues were filled into a flame-dried borosilicate glass capillaries with a diameter of 0.3 mm.The powder X-ray diffraction patterns were recorded with a StadiMP diffractometer (Stoe & Cie) in Debye-Scherrer geometry.The diffractometer was operated with CuÀKα 1 radiation (1.5406 Å, germanium monochromator) and equipped with a MYTHEN 1 K detector.The diffraction patterns were processed using the WinXPOW suite. [22]ngle-crystal X-ray diffraction ∞ 1 ½(μ-CN){(H 3 N) 5 U(μ-NH 2 ) 3 U(NH 3 ) 5 }]Br 4 • 2NH 3 and ∞ 1 ½(μ-CN){(H 3 N) 5 U-(μ-NH 2 ) 3 U(NH 3 ) 5 }]I 4 • 2NH 3 crystals were selected under nitrogencooled, pre-dried perfluorinated oil (Galden HT270, PFPE, Solvey Solexis) and under the absence of air.The crystals were mounted with a MiTeGen loop.Intensity data of suitable crystals 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 III C14 detector.Evaluation, integration, and reduction of the diffraction data was carried out with the APEX4 software suite. [23]The diffraction data were corrected for absorption utilizing the multiscan method of SADABS within the APEX4 software suite.The structures were solved with dualspace methods (SHELXT) and refined against F 2 (SHELXL). [24,25]All atoms were refined with anisotropic displacement parameters, H atoms were either located from the Difference Fourier map and refined isotropic, or with a riding model, or fixed, in some structures some H atoms could not be positioned.Representations of the crystal structures were created with the Diamond software. [26](CN)(NH 3 ) 8 ]Cl 3 • 3NH 3 crystals were selected under nitrogen-cooled, pre-dried perfluorinated oil (Galden HT270, PFPE, Solvey Solexis) and under the absence of air.The crystals were mounted with a MiTeGen loop.Intensity data of suitable crystals were recorded with an IPDS 2T diffractometer (Stoe & Cie).The diffractometer was operated with Mo-K α radiation (0.71073 Å, graphite monochromator) and equipped with an image plate detector.Evaluation, integration, and reduction of the diffraction data were carried out using the X-Area software suite. [27]Numerical absorption corrections were applied with the modules X-Shape and X-Red32 of the X-Area software suite.The structures were solved with dual space methods (SHELXT) and refined against F 2 (SHELXL).All atoms were refined values of À19 kJ • mol À1 and À43 kJ • mol À1 on the reaction arrows are the calculated reaction energies at the DFT-PBE0/TZVP level of theory for the complex cations at 0 K (in COSMO continuum solvent field).See the computational details for more details.The exchange reaction slightly favors the formation of the [U(CN)(NH 3 ) 8 ] 3 + cation energetically with 19 kJ • mol À1 .In comparison, the [U(NC)(NH 3 ) 8 ] 3 + cation is more favored, with 43 kJ • mol À1 .When considering Gibbs Free Energies at 298 K, the left side of equation 2 is favored by 34 kJ • mol À1 .For equation 3, the left side is favored with 11 kJ • mol À1 .In conclusion, this suggests a strong thermal dependency of the exchange reaction, valid for both linkageisomers.The energy difference between both linkage-isomers [U(CN)(NH 3 ) 8 ] 3 + and [U(NC)(NH 3 ) 8 ] 3 + is 23 kJ • mol À1 , favoring the latter.

Figure 2 .
Figure 2. Two IBOs for the [U(CN)(NH 3 ) 8 ] 3 + cation (isosurfaces drawn in green and red).Left: IBO showing one the σ-type bond between UÀNC.Right: IBO showing one the σ-type bond between UÀCN.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 3 .
Figure 3. Representation of the ∞ 1 ½U(NH 3 Þ5 1 (NH 2 Þ3 2 (CNÞ1 2 ] 2 + chain of the bromide running parallel to the a axis.Anisotropic displacement ellipsoids are shown at the 70 % probability level at 100 K.The H atoms are shown isotropic with arbitrary radii.

( 5 )
According to equation 4, U 3 + is oxidized while Ag + is reduced.The side product NH 4 I was identified by powder X-ray diffraction on the dried left-overs of ∞1 ½(μ-CN){(H 3 N) 5 U(μ-NH 2 ) 3 U-(NH 3 ) 5 }]I 4 • 2NH3 after the evaporation of the liquid aNH 3 .The diffraction pattern is shown in Figure S7 of the Supporting Information.Ag metal was observed as it formed as a mirror on the inner wall of the reaction vessel during the reaction.A photograph is shown in the Supporting Information in Figure S9.For the balance of the reaction equation, the formation of NH 4 CN is assumed, as discussed above.

Figure 6 .
Figure 6.Two sets of IBOs for the kN-, kC-[({(NH 3 ) 5 U} 2 (μ-NH 2 ) 3 )(CN) 2 ] 3 + cation (isosurfaces drawn in green and red).Left: σ-type IBO between UÀC/N.Right: One of the σ-type IBOs between UÀ(μ-NH 2 ), with the π-interaction towards the second U atom.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, C atoms in grey, and H atoms in white color.

Table 1 .
Selected crystallographic data and details of the structure determinations of [U(CN)(NH 3 ) 8

3
Formula UCl 3 N 12 CH 33 U 2 Br 4 N 16 CH 39 U 2 I 4 N 16 CH 39 , 2.4239(19), and 2.440(3) Å, elongated compared to the UÀNH 2 bond length of 2.274(15) Å in the compound Rb 2 [U(NH 2 ) 6 ] containing only terminally bound NH 2 Cie).Only a few crystals of the compound have been yielded.After removal of the residual liquid NH 3 in vacuo at room temperature, a greenish powder remained.Its diffraction pattern is shown in the Supporting Information in Figure