Surprises in the Solvent-Induced Self-Ionization in the Uranium Tetrahalide UX4 (X = Cl, Br, I)/Ethyl Acetate System.

The reaction of the uranium(IV) halides UCl4, UBr4, or UI4 with ethyl acetate (EtOAc) leads to the formation of the complexes [UX3(EtOAc)4][UX5(EtOAc)] (X = Cl, Br) or [UI4(EtOAc)3]. Thus, both UCl4 and UBr4 show self-ionization in ethyl acetate to a distorted pentagonal bipyramidal [UX3(EtOAc)4]+ cation and a distorted octahedral [UX5(EtOAc)]- anion. Surprisingly, the chloride and bromide compounds are not isotypic. While [UCl3(EtOAc)4][UCl5(EtOAc)] crystallizes in the orthorhombic crystal system, space group P212121 at 250 K, the bromide compound crystallizes in the monoclinic crystal system, P121/n1 at 100 K. Unexpectedly, UI4 does not show self-ionization but forms [UI4(EtOAc)3] molecules, which crystallize in the monoclinic crystal system, P21/c, at 100 K. The compounds were characterized by single-crystal X-ray diffraction, IR, Raman, and NMR spectroscopy, as well as molecular quantum chemical calculations using solvent models.

Introduction

Self-ionization is a long known property of many inorganic compounds and has been proven to occur in the gas, liquid, and solid phases.[1−6] Water is certainly the best investigated example of a compound showing self-ionization. Neutral water contains only small concentrations of H3O+ and OH– ions under ambient conditions, that is, the autoprotolysis constant is quite small. Other compounds that show self-ionization are liquid NH3, BrF3, anhydrous HF, and other amphoteric molecules.[3,7−10] Cl2O6 and N2O5 show electrically neutral molecular structures in the gas phase, but in the solid-state ionic structures, [ClO2]+[ClO4]− and [NO2]+[NO3]− are present.[11,12] In the solid state of PCl5, tetrahedral [PCl4]+ cations and octahedral [PCl6]− anions occur.[13−17] Self-ionization can also be induced by ligands, and the compounds [TiF2([15]crown-5)][Ti4F18], [AuCl2py2][AuCl4]·2[AuCl3py], [NbF4(Me2S)4][NbF6], and [(LDipp)2SbF2][SbF4] or [Ta(N3)4(1,10-phen)2][M(N3)6], [Be2I2(dmf)4][Be2I6], and [BePh(12-crown-4)][BePh3] serve as examples.[18−29]

To the best of our knowledge, the first actinide complex resulting from self-ionization was reported in 1973.[30] The authors interpreted the UV–vis spectrum of the compound “UCl4(Me2SO)3” and deduced its composition as [UCl2(Me2SO)6][UCl6]. This was confirmed by its crystal structure in 1975.[30,31] Only a couple of other U-containing compounds which show self-ionization were reported.[31−39] In most of them, an octahedral [UX6]2– (X = Cl, Br, I) anion is present besides various cations. Examples are [UCl2(Me2SO)6][UCl6], [UBr2(1H-indene)(MeCN)4][UBr6], or the mixed-valent compound [U(MeCN)9][UI6]I.[30,31,34,37] A few compounds are known containing an anion of the form [UX5L]− (X = Cl, Br, I; L = organic ligand). Examples are [UCl3((EtC(O)N(Et)2))4], [UCl5(EtC(O)N(Et)2)], or [Et2OH][UX5(Et2O)]·Et2O (X = Br, I).[33,40,41]

While the chemistry of [UO2]2+-containing compounds in ethyl acetate is well examined,[42−50] surprisingly few ethyl acetate containing complexes of U(IV) seem to be known. Herein we describe the reaction of the uranium(IV) halides UX4 (X = Cl, Br, I) with ethyl acetate.

Results and Discussion

The reaction of uranium(IV) chloride or bromide leads to the formation of complexes resulting from self-ionization of [UX3(EtOAc)4][UX5(EtOAc)] (X = Cl, Br), while the iodide does—surprisingly—not dissociate and is obtained as [UI4(EtOAc)3]. Scheme gives an overview of the reaction conditions and products.

Scheme 1

Overview of the Reactions of the Uranium Halides UX4 (X = Ck, Br, I) with EtOAc and the Respective Reaction Conditions

The starting materials were dissolved in an excess of EtOAc at room temperature. Then, a vacuum was applied to remove the solvent until crystallization started. These solutions were subsequently stored at 2 °C for a couple of days to allow for the growth of single crystals, which were filtered off. Details are available from the Experimental Section.

Crystal Structure of the Chloride

The blueish black compound [UCl3(EtOAc)4][UCl5(EtOAc)] crystallizes in the space group P212121, No. 19, with a = 15.1255(4), b = 15.4634(7), c = 17.4324(7) Å, V = 4077.3(3) Å3, and Z = 4 at T = 250 K. Unfortunately, the crystals broke apart upon cooling to 100 K, so the single-crystal X-ray structure determination had to be conducted at 250 K to prevent the shattering of the crystals. See Table for selected crystallographic data and details of the structure determinations.

Table 1

Selected Crystallographic Data and Details of the Structure Determinations of the Uranium(IV) Halide Ethyl Acetate Complexes

formula	[UCl3(EtOAc)4][UCl5(EtOAc)]	[UBr3(EtOAc)4][UBr5(EtOAc)]	[UI4(EtOAc)3]
molar mass/g·mol–1	1200.18	1555.86	1009.94
space group (no.)	P212121 (19)	P121/n1 (14)	P21/c (14)
a/Å	15.1255(4)	15.7380(10)	15.085(3)
b/Å	15.4634(7)	17.5732(9)	11.753(2)
c/Å	17.4324(7)	16.2532(11)	14.176(3)
β/°	90	113.853(5)	93.93(3)
V/Å3	4077.3(3)	4111.2(2)	2507.4(9)
Z	4	4	4
Pearson symbol	oP320 w. H atoms	mP320 w. H atoms	mP188 w. H atoms
ρcalcd/g·cm–3	1.955	2.514	2.675
μ/mm–1	8.496	15.682	11.417
color	dark blue	dark green	dark red
crystal morphology	block	block	block
crystal size/mm3	0.15 × 0.10 × 0.08	0.20 × 0.10 × 0.07	0.10 × 0.08 × 0.06
T/K	250(2)	100(2)	100(2)
λ/Å	0.71073 (Mo Kα)	0.71073 (Mo Kα)	0.71073 k
no. of reflections	42194	57562	36981
θ range/°	2.634–25.028	2.606–27.984	2.577–26.833
range of Miller indices	–18 ≤ h ≤ 15	–20 ≤ h ≤ 20	–19 ≤ h ≤ 19
	–18 ≤ k ≤ 18	–23 ≤ k ≤ 23	–14 ≤ k ≤ 14
	–20 ≤ l ≤ 20	–21 ≤ l ≤ 19	–17 ≤ l ≤ 17
absorption correction	numerical	numerical	numerical
Tmax, Tmin	0.2722, 0.3859	0.0738, 0.3021	0.1784, 0.5198
Rint, Rσ	0.0620, 0.0390	0.0415, 0.0866	0.0506, 0.1142
completeness of the data set	1.000	0.973	0.992
no. of unique reflections	7193	9651	5333
no. of parameters	429	371	214
no. of restrains	446	0	0
no. of constrains	0	0	0
S (all data)	1.002	1.036	1.013
R(F) (I ≥ 2σ(I), all data)	0.0312, 0.0631	0.0360, 0.0525	0.0313, 0.0450
wR(F2) (I ≥ 2σ(I), all data)	0.0522, 0.0591	0.0776, 0.0839	0.0690, 0.0730
Δρmax, Δρmin/e·Å–3	0.607, −0.650	1.523, −1.818	1.587, −0.732
flack x	0.393(10)	-	-

The uranium atom of the [UCl3(EtOAc)4]+ cation is coordinated by seven ligands, three chloride atoms, and four ethyl acetate molecules, in the shape of a pentagonal bipyramid considering only the ligating atoms. The coordination polyhedron of the [UCl5(EtOAc)]− anion is a slightly distorted octahedron where the central uranium atom is coordinated by five chlorido ligands and one ethyl acetate molecule. See Figure for an impression.

Figure 1

Sections of the crystal structures, the pentagonal bipyramidal [UX3(EtOAc)4]+ cation (on the left), and the octahedron like [UX5(EtOAc)]− anion (on the right) of [UX3(EtOAc)4][UX5(EtOAc)] with X = Cl and Br. The displacement ellipsoids are shown at the 70% probability level at 100 K. Hydrogen atoms are omitted, and carbon atoms are shown as a wire frame for clarity.

The bond lengths between the uranium atom and the chlorido ligands in the cation range from 2.566(3) to 2.624(2) Å and from 2.556(4) to 2.588(3) Å in the anion. These distances are in good agreement with the literature.[32,35] The atomic distances between the uranium atom and the coordinating oxygen atoms of the ethyl acetate ligands range from 2.346(8) to 2.374(9) Å in the cation. For the disordered EtOAc ligands of the anion, the distances are 2.30(2) and 2.36(2) Å. The U–O distances are in agreement with the literature.[51−54] The C–H···Cl hydrogen bonds between ethyl acetate molecules and the chlorido ligands are comparable with those in compounds such as [UCl3(N,N-diethylpropionamide)4][UCl5(N,N-diethylpropionamide)].[33] Selected L–U–L (L = coordinating ligand atom) angles in the [UCl5(EtOAc)]− anion range from ca. 81 to 98° and show a significant distortion of the octahedron-like coordination polyhedron, as expected. The pentagonal bipyramidal [UCl3(EtOAc)4]+ cation is only slightly distorted with L–U–L angles between 69.4(3) and 74.6(2)° (ideal 72°) for the equatorial and between 88.8(2) and 91.51(11)° (ideal 90°) for the axial ligands. All distances and angles are however additionally biased as the compound shows a distinct disorder of the coordinated ethyl acetate molecule in the [UCl5(EtOAc)]− anion. The closest U···U distance is between neighboring cations and anions with ca. 7.59 Å. The cations are surrounded by six anions in the shape of very distorted octahedra and vice versa with the U···U distances in a range from ca. 7.59 to 9.98 Å. However, the distortion is so severe that we could not identify any reasonable packing of cations and/or anions, such as in the NaCl structure type for example.

Crystal Structure of the Bromide

The reaction of uranium(IV) bromide with ethyl acetate leads to the formation of dark green [UBr3(EtOAc)4][UBr5(EtOAc)]. Although the compound has a similar composition and the same structural constitution as the chloride described above, it is not isotypic. It crystallizes in the monoclinic crystal system, space group P121/n1, No. 14, with a = 15.7380(10), b = 17.5732(9), c = 16.2532(11) Å, V = 4111.2(5) Å3, and Z = 4 at T = 100 K (Table ). The uranium atom of the cation is again coordinated by seven ligands, three bromide atoms, and four ethyl acetate molecules, in the shape of a pentagonal bipyramid. The coordination polyhedron of the anion is a distorted octahedron with a central uranium atom with five bromido ligands and one ethyl acetate molecule coordinating to it. A section of the crystal structure illustrating both ions is shown in Figure . The atomic distances between the uranium atom and the bromido ligands range from 2.7373(7) to 2.8223(7) Å in the cation and from 2.7460(7) to 2.7711(8) Å in the anion. This is comparable to distances in compounds like [NEt4]2[UBr4(t-butylimido)2].[34,55] The C–H···Br hydrogen bonds between the ethyl acetate and the bromido ligands range from 3.546(9) to 4.092(9) Å and are in good agreement with the literature.[40,55,56] Selected L–U–L (L = coordinating ligand atom) angles in the octahedral [UBr5(EtOAc)]− anion range from 81.93(13) to 99.41(3)° and show a significant distortion of the coordination polyhedron. The pentagonal bipyramidal [UBr3(EtOAc)4]+ cation is only slightly distorted with angles between 69.64(16) and 74.30(12)° (ideal 72°) for the equatorial ligands and between 87.19(13) and 92.95(2)° (ideal 90°) for the axial ligands. The closest U···U distances are between neighboring cations and anions with ca. 7.72 Å. This is slightly larger compared to the chloride above. As in the chloride, the cations are surrounded by six anions in the shape of quite distorted octahedra and vice versa with U···U distances in the range from ca. 7.72 to 9.97 Å. However, the distortions are less pronounced compared to the chloride. Therefore, the cations of the compound form a distorted cubic close packing with the anions residing in the vicinity of its octahedral voids. That is, the bromide compound is related to the NaCl structure type. For the chloride, too much distortion is present, preventing this structure relation.

We first thought that the orthorhombic chloride compound could represent a high-temperature polymorph structurally related to the low-temperature monoclinic bromide compound. As there is no direct group-subgroup relation between the respective space groups, a displacive phase transition can be ruled out, but a reconstructive phase transition could still be possible. Usually, higher symmetry is present in high-temperature phases, which is not the case here. The crystal structure of the bromide, monoclinic with space group P121/n1 at 100 K, shows a recognizable relation to the NaCl structure type, whereas the crystal structure of the chloride, orthorhombic with space group P212121 at 250 K, does not.

Crystal Structure of the Iodide

The reaction of uranium(IV) iodide in ethyl acetate at room temperature leads to the formation of dark red crystals of [UI4(EtOAc)3]. The compound does not show self-ionization and crystallizes in the monoclinic crystal system P21/c, No. 14, with a = 15.085(3), b = 11.753(2), c = 14.176(3) Å, β = 93.93(3)°, V = 2507.4(9) Å3, and Z = 4 at T = 100 K. Although the anions [UI5L]− and [UI6]2– are known, a formation of one of these was not observed by us at various temperatures (−35 to 70 °C).[37,41] The uranium atom is surrounded by the shape of a slightly distorted pentagonal bipyramid by four iodine and three oxygen atoms (Figure ). The atomic distances between the U atom and the iodido ligands range from 2.9938(6) to 3.11525(8) Å and for the coordinating O atoms of the ethyl acetate ligands from 2.316(4) to 2.349(4) Å. These distances are in agreement with the literature.[57−59] The large iodido ligands next to the small O ligand atoms of the ethyl acetate molecules induce a distortion of the pentagonal bipyramidal coordination sphere, with L–U–L (L = coordinating ligand atom) angles between 68.36(14) and 74.84(10)° (ideal 72°) for equatorial and between 87.95(11) and 94.49(10)° (ideal 90°) for axial ligands (Figure ).

Figure 2

Left: Section of the crystal structure of [UI4(EtOAc)3]. The displacement ellipsoids are shown at the 70% probability level at 100 K. Hydrogen atoms are omitted, and carbon atoms are shown as a wire frame for clarity. Right: Polyhedron showing the distortion of the pentagonal bipyramid. Atoms are shown isotropic with arbitrary radii.

The packing of the molecules of the iodide, that is their U atoms, corresponds to the Mg structure type with the hexagonal close-packed layers parallel to the bc plane. The closest U···U distances are ca. 8.24 Å.

IR and Raman Spectroscopic Investigations

The ATR IR spectra of the three compounds (Figures S1–S3) were recorded at room temperature and are dominated by the vibration bands of the ethyl acetate ligands. The bands of the C–H stretching modes are observed around 2936 and 2983 cm–1 and for the C–O–C stretching modes at 1315 and 1034 cm–1, which is in good agreement with noncoordinating ethyl acetate.[60] The C=O stretching modes of the chloride, the bromide, and the iodide reported here are observed at 1610, 1603, and 1601 cm–1, respectively. The bathochromic shift of 131–140 cm–1, compared to the free ester, is attributed to a significant weakening of the C=O bond. This is indicative for a strong interaction of the uranium atoms with the coordinating carbonyl oxygen atoms and is typical for carboxylic acid ester complexes of strongly Lewis acidic metals.[61] The uranium halide U–X (X = Cl, Br, I) stretching frequencies do not show intensities in the region accessible to our instrument from 4000 to 400 cm–1.[62−64] Also, there is no hint toward characteristic bands of commonly observed impurities such as the uranyl cation UO22+ that would give rise to a band in the region from ∼911 to 960 cm–1.[65]

The Raman spectra of the compounds (Figure S4) were also recorded at room temperature and are comparable to the Raman spectrum of free ethyl acetate.[66] An overview of the band assignment is given in Table , and other bands could not be assigned unambiguously.

Table 2

Selected Raman Bands and Assignment of the Three Title Compounds[66,67]

[UCl3(EtOAc)4][UCl5(EtOAc)]	[UBr3(EtOAc)4][UBr5(EtOAc)]	[UI4(EtOAc)3]	EtOAc
390	391	389	381	δ(C–C), m
637	641	703	697	γ(CH3COO), w
852	858	847	849	v(C–C), w
1120	1125	1115	1117	v(skeletal), m
1666	1659	1743	1738	v(C=O), m
2881	2886	2878	2879	v(CH), m
2936	2932	2937	2943	v(CH), vs
2978	2977	2974	2974	v(CH), s

NMR Spectroscopic Investigations

1H and 13C NMR spectroscopy was used to investigate which species is present in the solutions of the title compounds. Therefore, crystals of the compounds were dissolved in CD2Cl2. The applicability of NMR spectroscopy for the analysis of uranium(IV) compounds is limited due to its paramagnetism. Due to coupling between nuclear and electron spins, the chemical shift becomes unpredictable, and the signals are broadened, sometimes to an extent that they are not observable.[68] The 1H and 13C NMR spectra of the title compounds show discrete signals for the two methyl groups, the methylene group, and the carbon nucleus of the carbonyl group, respectively. However, all signals are broadened and show significant paramagnetic shifts (Figure ).

Figure 3

1H NMR spectra of (a) [UCl3(EtOAc)4][UCl5(EtOAc)], (b) [UBr3(EtOAc)4][UBr5(EtOAc)], and (c) [UI4(EtOAc)3] in CD2Cl2.

All 1H NMR signals are shifted upfield in comparison to noncoordinated ethyl acetate.[69] In the 13C NMR spectra, only the signals of the methyl carbon nuclei are shifted upfield, while the signal of the methylene carbon nucleus is shifted upfield in the case of the chloride and bromide compounds and downfield in the iodide. Only in the chloride complex the signal of the carbonyl carbon nucleus is shifted upfield, whereas in the bromide and iodide these signals are shifted downfield (Table ). The coordination shift increases from the iodide via the bromide to the chloride compound, which indicates increasing interaction between the uranium atoms and the ester ligands. This is in line with expectations due to the increasing electronegativity of the halides. This results in lower electron density at the metal center and increasing U–O bond strength from [UI4(EtOAc)3] via [UBr3(EtOAc)4][UBr5(EtOAc)] to [UCl3(EtOAc)4][UCl5(EtOAc)] as is expected also from the Pearson concept of hard and soft Lewis acids and bases.

Table 3

NMR Chemical Shifts of [UCl3(EtOAc)4][UCl5(EtOAc)], [UBr3(EtOAc)4][UBr5(EtOAc)], and [UI4(EtOAc)3] and Respective Coordination Shifts of Ethyl Acetate in CD2Cl2

	CH2CH3				C(O)CH3				CH2CH3				C(O)
	1H/ppm		13C/ppm		1H/ppm		13C/ppm		1H/ppm		13C/ppm		13C/ppm
	δ	Δδ	δ	Δδ	δ	Δδ	δ	Δδ	δ	Δδ	δ	Δδ	δ	Δδ
Cl	–4.75	–5.98	5.7	–8.7	–10.55	–12.55	5.2	–16.0	–10.17	–14.25	51.3	–9.3	154.9	–16.3
Br	–2.56	–3.79	8.4	–6.0	–7.07	–9.07	12.5	–8.7	–5.56	–9.64	58.2	–2.4	174.3	3.1
I	–1.17	–2.40	10.5	–3.9	–4.28	–6.28	20.0	–1.2	–2.03	–6.11	62.6	2.0	188.7	17.5

Even though some uncoordinated ethyl acetate was present in all NMR samples, only one signal set was observed. This indicates fast exchange between free and coordinated ester ligands on the NMR time scale, which results in an average signal. Therefore, also fast exchange between the EtOAc ligands of the [UX3(EtOAc)4]+ and [UX5(EtOAc)]− ions is expected, which would also lead to the observation of only one signal set. Since also neutral [UX4(EtOAc)3] would give only one signal set in 1H and 13C NMR spectra and the paramagnetism of U(IV) leads to unpredictable chemical shifts, signals of [UX3(EtOAc)4][UX5(EtOAc)] and [UX4(EtOAc)3] are indistinguishable in the NMR spectra. Thus, no conclusions may be drawn, whether self-ionization also occurs in solution.

Quantum Chemical Calculations

To clarify the self-ionization behavior, we studied the thermodynamics of reaction for X = Cl, Br, and I with quantum chemical methods (DFT-PBE0/def2-TZVP, see the Experimental Section for the computational details).The conductor-like screening model (COSMO) was used to simulate the influence of the EtOAc solvent (relative dielectric constant of 6).[70] We evaluated the reaction energies at 0 K and the reaction Gibbs free energies at 298 K (Table ). The dissociation reaction shows a positive reaction energy at 0 K for all halides with the iodide showing the least unfavorable one with +80 kJ/mol. Consideration of the Gibbs free energies does not change the picture. Overall, the reactions become thermodynamically more favorable but still show a positive ΔG, when moving from X = Cl to I, which is in contrast to the self-ionization observed within the crystal structures of the chloride and the bromide.

Table 4

Comparison of Reaction Energies at 0 K (ΔE) and Gibbs Free Energies at 298 K (ΔG298) for the [UX4(EtOAc)3] Speciesa

X	quantity	energy/kJ mol–1
Cl	ΔE	105
	ΔG298	84
Br	ΔE	111
	ΔG298	64
I	ΔE	80
	ΔG298	28

Values obtained at the DFT-PBE0/def2-TZVP level of theory with COSMO solvent model (relative dielectric constant of EtOAc = 6).

We also tested the effect of an electrostatically ideal solvent with COSMO (dielectric constant of infinity). In this case, the reaction becomes thermodynamically slightly more favorable, but the trend remains contrary to the trend observed experimentally in the solid state: self-ionization is then thermodynamically favored for X = I and unfavored for X = Cl (Table S1 in Supporting Information).

In summary, the [UX4(EtOAc)3] species are not expected to dissociate in EtOAc solution based on our DFT calculations. Of course, differently composed molecular species compared to [UX4(EtOAc)3] could be present and involved in various equilibria upon crystallization of the compounds. However, we can confirm neither the existence nor the absence of such species from the NMR spectra.

Conclusion

The solid-state compounds [UCl3(EtOAc)4][UCl5(EtOAc)] and [UBr3(EtOAc)4][UBr5(EtOAc)] were obtained by self-ionization reactions of the respective uranium tetrahalide in the presence of the solvent and O-donor ligand ethyl acetate (EtOAc). UI4 does not show the same reactivity under similar conditions, as a compound with the composition [UI4(EtOAc)3] is obtained. The crystal structures of the chloride and bromide salts are not isotypic, despite both featuring isostructural complex ions with U atom coordination spheres of distorted pentagonal bipyramids for the cations and slightly distorted octahedra. As the chloride crystallizes orthorhombic, space group P212121, and the bromide monoclinic, space group P121/n1, no direct group–subgroup relation is present excluding a displacive phase transition of an orthorhombic high-temperature to a monoclinic low-temperature phase. However, a reconstructive phase transition may of course be possible, or there simply is no structural relation between the two compounds. While the crystal structure of the bromide can be related to the NaCl structure type, the distortions within the chloride are so profound that no such relation became obvious. The packing of the molecules of the iodide corresponds to the Mg structure type.

In solution, only one signal set of coordinated ethyl acetate ligands can be observed with NMR spectroscopy. The spectra show that exchange on the NMR time scale occurs between the ethyl acetate ligands in solution. Thus, it remains unclear if self-ionization is also present in solution or if there is an effect of different solubilities and shifted equilibria of various dissolved species due to the inset of crystallization of the compounds. IR and NMR spectra as well as quantum chemical calculations show the expected trend of decreasing bond strength and electronegativity from U–Cl to U–I.

Based on the DFT results, the [UX4(EtOAc)3] species are not expected to dissociate in EtOAc solution under the studied conditions. So, the question why the chloride and bromide show autodissociation and the iodide does not remains currently unanswered.

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). Aluminum chloride and bromide (Merck, 98%/Alfa Aesar, 98%) were purified by sublimation in vacuo before use. Elemental iodine was sublimed in vacuo several times, the last time from phosphorus pentoxide. Aluminum powder (Fluka, purum >99%) was dried in vacuo at 250 °C. UO2(NO3)2·6H2O (Merck, p.a.) was used without further purification. Ethyl acetate was predried with P4O10 and directly distilled onto the uranium halides.

Uranium compounds with natural U isotope distribution are radioactive, and appropriate measures for safe handling need to be taken.

The borosilicate glass vessels were flame-dried several times under a dynamic vacuum (10–3 mbar) before use. For the syntheses, glass ampules were used with a length of 16 cm, an outer diameter of 18 mm, and a wall thickness of 1.5 mm as described previously.[64,71] The top of the ampule carries an NS14.5 inner ground joint for filling of the educts and a constriction for easier flame sealing. The second constriction at one-third length of the ampule is used to allow for easier opening of the ampule after the reaction.

Synthesis of UO2

Amounts of 12.8 g of UO2(NO3)2·6H2O (25.4 mmol) were decomposed to 7.13 g of U3O8 (8.47 mmol) by heating to 700 °C in air for 12 h inside an open silica test tube. The black product was powdered in air and reduced in a stream of hydrogen at 800 °C for 8 h to obtain 6.86 g (24.9 mmol, 98%) of phase-pure UO2.

Synthesis of UCl4

UCl4 was synthesized according to the literature.[64] An ampule was charged with 1084 mg of UO2 (4 mmol) and 1067 + 109 mg of AlCl3 (8 mmol + transport agent) and flame-sealed under a vacuum (1 × 10–3 mbar). The starting materials were reacted at 250 °C for 12 h before the transport reaction was conducted with a source temperature of 350 °C and a sink temperature of 250 °C. An amount of 1428 mg (4.3 mmol, 94%) of green plate-shaped crystals of UCl4 was obtained after 4 days.

Synthesis of UBr4

UBr4 was synthesized according to the literature.[64] An ampule was charged with 1088 mg of UO2 (4 mmol) and 2150 + 65 mg of AlBr3 (8 mmol + transport agent) and flame-sealed under a vacuum (1 × 10–3 mbar). The starting materials were reacted at 250 °C for 12 h before the transport reaction was conducted with a source temperature of 350 °C and a sink temperature of 230 °C. An amount of 1978 mg (4.3 mmol, 86%) of brown plate-shaped crystals of UBr4 was obtained after 6 days.

Synthesis of UI4

UI4 was synthesized according to the literature.[64] An ampule was charged with 1003 mg of UO2 (4 mmol), 3261 + 11 mg of AlI3 (8 mmol + transport agent), and 238 mg of I2 (2 bar I2 at 350 °C) and flame-sealed under a vacuum (1 × 10–3 mbar). The starting materials were reacted at 250 °C for 12 h before the transport reaction was conducted with a source temperature of 350 °C and a sink temperature of 250 °C. An amount of 2714 mg (3.64 mmol, 91%) of needle-like black crystals of UI4 was obtained after 5 days.

Synthesis of Trichlorido Tetra(ethyl acetate) Uranium(IV) Pentachloride (Ethyl Acetate) Uranate(IV) [UCl3(EtOAc)4][UCl5(EtOAc)]

An amount of 25 mg (0.07 mmol) of UCl4 was reacted with an excess of ethyl acetate (ca. 5 mL) in a Schlenk tube at room temperature. After the UCl4 was dissolved completely, the excess of ethyl acetate was slowly removed under a vacuum to ca. 1 mL until crystallization was observed. Then the Schlenk tube was stored at 2 °C. Dark blueish crystals could be obtained after 7 days of storage. Yield was essentially quantitative.

1H NMR (500 MHz, CD2Cl2): δ = −10.55 (bs, 3H, C(O)CH3), −10.17 (bs, 2H, CH2CH3), −4.75 (bs, 3H, CH2CH3). 13C NMR (126 MHz, CD2Cl2): δ = 5.2 (C(O)CH3), 5.7 (CH2CH3), 51.3 (CH2CH3), 154.9 (C(O)).

Synthesis of Tribromido Tetra(ethyl acetate) Uranium(IV) Pentabromido (Ethyl Acetate) Uranate(IV) [UBr3(EtOAc)4][UBr5(EtOAc)]

An amount of 25 mg (0.04 mmol) of UBr4 was reacted with an excess of ethyl acetate (ca. 5 mL) in a Schlenk tube at room temperature. After the UBr4 was dissolved completely, the excess of ethyl acetate was slowly removed under a vacuum to ca. 1 mL until crystallization was observed. Then the Schlenk tube was stored at 2 °C. Dark brown crystals could be obtained after 7 days of storage. Yield was essentially quantitative.

1H NMR (500 MHz, CD2Cl2): δ = −7.07 (bs, 3H, C(O)CH3), −5.56 (bs, 2H, CH2CH3), −2.56 (bs, 3H, CH2CH3). 13C NMR (126 MHz, CD2Cl2): δ = 8.4 (CH2CH3), 12.5 (C(O)CH3), 58.2 (CH2CH3), 174.3 (C(O)).

Synthesis of Tri(ethyl acetate) Tetraiodido Uranium(IV) [UI4(EtOAc)3]

An amount of 40 mg (0.05 mmol) of UI4 was reacted with an excess of ethyl acetate (ca. 5 mL) in a Schlenk tube at room temperature. After the UI4 was dissolved completely, the excess of ethyl acetate was slowly removed under a vacuum to ca. 1 mL until crystallization was observed. Then the Schlenk tube was stored at 2 °C. Dark red crystals could be obtained after 5 days of storage. Yield was essentially quantitative.

1H NMR (500 MHz, CD2Cl2): δ = −4.28 (bs, 3H, C(O)CH3), −2.03 (bs, 2H, CH2CH3), −1.17 (bs, 3H, CH2CH3). 13C NMR (126 MHz, CD2Cl2): δ = 10.5 (CH2CH3), 20.0 (C(O)CH3), 62.6 (CH2CH3), 188.7 (C(O)).

Raman Spectroscopy

Raman spectra were recorded with a confocal raman microscope S+I MonoVista CRS+, using the 532 nm excitation line of an integrated diode laser (resolution <1 cm–1; range 50–9000 cm–1). A sample of the compound was located inside a 10 mm borosilicate glass Schlenk tube, which was flame-dried several times under vacuum before use. The spectra were processed with the OriginPro 2017 software package.[72]

IR Spectroscopy

The IR spectra were recorded on a Bruker alpha FT-IR spectrometer using the ATR Diamond module with a resolution of 4 cm–1. The spectrometer was located inside a glovebox (MBraun) under an argon atmosphere. The spectra were processed with the OPUS software package.[73]

NMR Spectroscopy

1H and 13C spectra were recorded on a Bruker Avance III 500 spectrometer equipped with a Prodigy Cryo-Probe. 1H NMR (500 MHz) and 13C NMR (126 MHz) chemical shifts are given relative to the solvent signal for CD2Cl2 (5.32 and 54.0 ppm). NMR spectra were processed with the MestReNova software.[74]

Single-Crystal X-ray Diffraction

Single crystals of the compounds described above were selected at room temperature under predried perfluorinated oil and mounted using a MiTeGen loop. Intensity data of a suitable crystal were recorded with an IPDS 2T diffractometer (Stoe & Cie). The diffractometer was operated with Mo–Kα 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.[75] 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-2018/2) and refined against F2 (SHELXL-2018/3).[76,77] All atoms were refined with anisotropic displacement parameters, and H atom isotropic and riding models were adequate. Disorder was modeled using DSR.[78] Cif files were deposited with the CCDC (https://www.ccdc.cam.ac.uk/), depository numbers: 2124219–2124221 (X = I, Cl, Br).

Quantum chemical calculations

All calculations were carried out with the TURBOMOLE[79,80] program suite using the PBE0[81,82] hybrid density functional method (DFT-PBE0). Karlsruhe def2-TZVP[83] basis sets were applied for hydrogen, carbon, oxygen, chlorine, and bromine. For uranium and iodine, scalar relativistic effects were taken into account by using 60-electron effective core potentials,[84−86] together with respective def-TZVP and def2-TZVP valence basis sets.[87] Multipole-accelerated resolution-of-the-identity approximation (MA-RIJ) was used to speed up the DFT calculations,[88−90] and the m4 integration grid was used for the numerical integration of the exchange-correlation part. The conductor-like screening model (COSMO) was applied in all calculations to describe an ethyl acetate solvent field.[91] The geometries of the complexes were fully optimized within the constraints of their point group symmetries. Numerical harmonic frequency calculations were performed to check if the optimized structures are true local minima on the potential energy surface. The Cartesian coordinates of the optimized structures are available in the Supporting Information. The thermal contributions to the free enthalpy were obtained within the harmonic oscillator rigid rotor model at room temperature, using the freeh module. The harmonic frequencies were not scaled when evaluating the thermal contributions.