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Article

Expanding the Chemistry of Actinide Metallocene Bromides. Synthesis, Properties and Molecular Structures of the Tetravalent and Trivalent Uranium Bromide Complexes: (C5Me4R)2UBr2, (C5Me4R)2U(O-2,6-iPr2C6H3)(Br), and [K(THF)][(C5Me4R)2UBr2] (R = Me, Et)

by
Alejandro G. Lichtscheidl
1,
Justin K. Pagano
1,
Brian L. Scott
2,
Andrew T. Nelson
3,* and
Jaqueline L. Kiplinger
1,*
1
Chemistry Division, Los Alamos National Laboratory, Mail Stop J-514, Los Alamos, NM 87545, USA
2
Materials Physics & Applications Division, Los Alamos National Laboratory, Mail Stop J-514, Los Alamos, NM 87545, USA
3
Materials Science & Technology Division, Los Alamos National Laboratory, Mail Stop E-549, Los Alamos, NM 87545, USA
*
Authors to whom correspondence should be addressed.
Inorganics 2016, 4(1), 1; https://doi.org/10.3390/inorganics4010001
Submission received: 23 November 2015 / Revised: 9 December 2015 / Accepted: 11 December 2015 / Published: 6 January 2016
(This article belongs to the Special Issue Rare Earth and Actinide Complexes)
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Abstract

:
The organometallic uranium species (C5Me4R)2UBr2 (R = Me, Et) were obtained by treating their chloride analogues (C5Me4R)2UCl2 (R = Me, Et) with Me3SiBr. Treatment of (C5Me4R)2UCl2 and (C5Me4R)2UBr2 (R = Me, Et) with K(O-2,6-iPr2C6H3) afforded the halide aryloxide mixed-ligand complexes (C5Me4R)2U(O-2,6-iPr2C6H3)(X) (R = Me, Et; X = Cl, Br). Complexes (C5Me4R)2U(O-2,6-iPr2C6H3)(Br) (R = Me, Et) can also be synthesized by treating (C5Me4R)2U(O-2,6-iPr2C6H3)(Cl) (R = Me, Et) with Me3SiBr, respectively. Reduction of (C5Me4R)2UCl2 and (C5Me4R)2UBr2 (R = Me, Et) with KC8 led to isolation of uranium(III) “ate” species [K(THF)][(C5Me5)2UX2] (X = Cl, Br) and [K(THF)0.5][(C5Me4Et)2UX2] (X = Cl, Br), which can be converted to the neutral complexes (C5Me4R)2U[N(SiMe3)2] (R = Me, Et). Analyses by nuclear magnetic resonance spectroscopy, X-ray crystallography, and elemental analysis are also presented.

Graphical Abstract

1. Introduction

The bis(cyclopentadienyl) complexes of uranium (C5Me4R)2UX2 (R = Me or Et; X = Cl or I) [1,2,3], (1,3-R′2C5H3)2UX2 and (1,2,4-R′3C5H3)2UX2 (R′ = tBu, SiMe3; X = Cl, I) [4,5,6,7] have been known for years, owing to the ease of preparation of reliable chloride and iodide starting materials. With the vast amount of literature attention devoted to (C5Me4R)UX2 compounds, it is curious that the corresponding bis(cyclopentadienyl) uranium bromide systems have not been investigated to a similar extent. Considering that the structure and reactivity of these uranium compounds are strongly influenced by the nature of the halide [8], it would be useful to have the (C5Me4R)UBr2 congeners available for synthetic actinide chemistry.
Uranium bromide complexes are rare compared to their chloride and iodide counterparts. This is succinctly illustrated in Figure 1, which presents the organometallic tetravalent and trivalent uranium complexes that have been reported to date. The vast majority of uranium(IV) bromide complexes have been prepared by salt metathesis with either UBr4 or UBr4(NCCH3)4. These compounds include the dicarbollide complex [Li(THF)4]2[(C2H11B9)2UBr2] (1) [9]; the cyclopentadienyl complexes (C5H5)3UBr (2), (C5H5)UBr3(L)2 (L = Me2NC(O)tBu (3), OPPh3 (4), THF (5)), (C5H5)2UBr2(L) (L = Me2NC(O)tBu (6), OPPh3 (7)), {(C5H5)UBr2X[OP(Ph)2C2H4P(O)(Ph2)]}2 (X = C5H5 (8), Br (9)); and the indenyl complexes (C9H7)3UBr (10), (C9H7)UBr3(OPPh3)(THF) (11), (C9H7)UBr3(THF)2 (12), (C9H7)UBr3(OPPh3) (13), [(C9H7)UBr2(NCCH3)4][UBr6] (14) and {[(C9H7)UBr(NCCH3)4]2O}[UBr6]2 (15) [10,11,12,13,14,15,16]. An alternative method that has been employed to prepare uranium(IV) bromide complexes is halide exchange. For example, (1,2,4-tBu3C5H2)2UBr2 (16) [5] and (1,3-R2C5H3)2UBr2 (R = SiMe3 (17), tBu (18)) [4] were prepared by reacting the chloride analogues with excess Me3SiBr, and (17) from its chloride analogue and BBr3 [7].
In recent years, oxidation chemistry has been developed to access a variety of uranium(IV) bromide complexes such as (C5Me5)2U(X)(Br) (X = N(SiMe3)2 (19) [17], (iPrN)2C(Me) (20) [18], MeNC(Me)NAd (21) [19]), which were synthesized by treating (C5Me5)2U(X) with CuBr or AgBr. In a similar fashion, (C5Me5)3U was shown to react with benzyl bromide to afford (C5Me5)3UBr (22) [20]. The η2-suldenamido derivative (C5Me5)2U(η2-tBuNSPh)(Br) (23) was obtained after long reaction times of chloride complex with MeMgBr [21]. Finally, protonolysis of (η3-C3H5)4U with HBr provided (η3-C3H5)3UBr (24) [22].
Reduction chemistry has been the primary method for accessing the handful of known bromide complexes. For example, treatment of [Li(THF)4]2[(C2H11B9)2UBr2] (1) [9] and (1,3-R2C5H3)2UBr2 (R = SiMe3 (17), tBu (18)) [4] with tert-butyl lithium or sodium amalgam gave [Li(THF)4]2[(C2H11B9)2UBr(THF)] (25) and [(1,3-R2C5H3)2UBr]2 (R = SiMe3 (26), tBu (27)), respectively [7,23,24,25]. Complex (C5Me5)3UBr (22) is thermally unstable and leads to isolation of (C5Me5)2UBr (28), from which (C5Me5)2UBr(THF) (29) can be formed [20]. Finally, [1,3-(SiMe3)2C5H3]2UBr(CNtBu)2 (30) was obtained by treating complex 26 with excess CNtBu [26].
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Figure 1. Structures of known organometallic uranium(IV) and uranium(III) bromide compounds.
Figure 1. Structures of known organometallic uranium(IV) and uranium(III) bromide compounds.
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Herein, we report the preparation of the bromide complexes (C5Me4R)2UBr2 (R = Me (33), Et (34)) and show that they and (C5Me4R)2UCl2 (R = Me (31), Et (32)) can be used as precursors to mixed-ligand species (C5Me4R)2U(O-2,6-iPr2C6H3)(X) (R = Me, Et; X = Cl and Br) and uranium(III) bromide and chloride “ate” species [K(THF)n][(C5Me4R)2UX2] (R = Me, Et; X = Cl, Br; n = 0.5 or 1). In addition, we show that (C5Me4R)2U[N(SiMe3)2] can be synthesized from such “ate” species, demonstrating that the [K(THF)n][(C5Me4R)2UX2] complexes are useful precursors for the synthesis of neutral uranium(III) compounds. The syntheses of all complexes are discussed and the characterization by 1H NMR spectroscopy, melting point, elemental analysis and single crystal X-ray diffraction is provided.

2. Results and Discussion

Synthesis: Our study takes advantage of the halide exchange reagent, trimethylsilyl bromide, Me3SiBr. The compounds (C5Me4R)2UBr2 (R = Me (33), Et (34)) were obtained by treating (C5Me4R)2UCl2 (R = Me (31), Et (32)) with excess Me3SiBr (Scheme 1) similar to the method reported by Andersen for the preparation of the bent cyclopentadienyl species (1,3-R′2C5H3)2UBr2 [4] and (1,2,4-R′3C5H2)2UBr2 [5] (R′ = tBu, SiMe3). Complexes 33 and 34 can be obtained in 96% and 90% yield, respectively, using this procedure, but it is important to add fresh Me3SiBr three times to the reaction mixture and to let the mixture react for at least 12 h each time. Lower time intervals and/or lesser number of treatments lead to incomplete reactions. The uranium(IV) bromide complex (C5Me5)2UBr2 (33) has been mentioned before, but no experimental data for this compound were provided [20,27]. To the best of our knowledge, this is the first time that compounds 33 and 34 have been fully characterized. The 1H Nuclear Magnetic Resonance (NMR) spectrum of 33 in benzene-d6 shows a singlet at 15.70 ppm, which is intermediate in value compared to (C5Me5)2UCl2 (13.5 ppm) [3] and (C5Me5)2UI2 (17.9 ppm) [28,29,30]; while that of complex 34 shows four singlets corresponding to the three inequivalent methyl groups (25.1, 15.6 and 13.1 ppm) and the methylene group (0.31 ppm) of the C5Me4Et ligand. The 1H NMR shifts of 34 are also downfield compared to the analogous (C5Me4Et)2UCl2 (22.9, 13.8 and −2.8 ppm, respectively) and (C5Me4Et)2U(CH3)2 (14.1, 5.6, 5.1 and −4.3 ppm, respectively) [31].
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Scheme 1. Synthesis of the uranium(IV) bromide complexes (C5Me4R)2UBr2 (R = Me (33), Et (34)) by halide exchange with trimethylsilyl bromide.
Scheme 1. Synthesis of the uranium(IV) bromide complexes (C5Me4R)2UBr2 (R = Me (33), Et (34)) by halide exchange with trimethylsilyl bromide.
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The X-ray structure of complexes 33 and 34 are shown in Figure 2. The U(1)–Br(1) bond lengths of 33 (2.7578(5) Å) and 34 (2.7607(4) and 2.7609(4) Å) fall within the range of known neutral U(IV) terminal bromides (2.734(1)–2.831(7) Å) [6,7,9,16,32,33,34,35,36,37,38,39,40,41,42,43,44,45] and the Br(1)–U(1)–Br(2) bond angles of 33 (97.64(3)°) and 34 (96.224(14)°) are similar to those of (C5Me5)2UX2 (X = Cl (97.9(4)°), I (95.96(2)°)) [1,2] and [1,3-(SiMe3)2C5H3]2UBr2 (94.60(4)°) [4], but larger than most non-cyclopentadienyl-based complexes containing cis-bromide ligands (75.48(8)–103.83(12)°) [6,7,9,16,32,33,34,35,36,37,38,39,40,41,42,43,44,45]. The U(1)–CCent distances for 33 (2.438(4) Å) and 34 (2.444(2) Å) fall within the typical distances for C5Me5-based complexes (2.393–2.562 Å) [1,2,31,46,47,48,49,50]. Similarly, the CCent(1)–U(1)–CCent(2) bond angles of 33 (137.57(15)°) and 34 (138.50(8)°) fall within the known range for bis(pentamethylcyclopentadienyl)-based complexes (126.2–142.6°) [1,2,31,46,47,48,49,50].
As outlined in Scheme 2, the halide complexes 31–34 react with bulky aryloxides such as K[O-2,6-iPr2C6H3] to yield the corresponding halide aryloxide mixed-ligand complexes in excellent yields, (C5Me4R)2U(O-2,6-iPr2C6H3)(Cl) (35, R = Me, 92%; 36, R = Et, 90%) and (C5Me4R)2U(O-2,6-iPr2C6H3)(Br) (37, R = Me, 99%; 38, R = Et, 96%). Complex 35 has been previously synthesized by oxidation of (C5Me4R)2U(O-2,6-iPr2C6H3)(THF) with CuCl [8]. Synthesis of the halide aryloxide mixed-ligand complexes 35–38 by salt metathesis not only complements the known oxidative synthetic route to 35, but also helps to introduce complexes 36–38 here for the first time. Finally, similar to the synthesis of complexes 33 and 34, treatment of 35 and 36 with excess Me3SiBr in THF also yields 37 and 38 in 93% and 91% yields, respectively (Scheme 3).
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Figure 2. Molecular structure of (C5Me5)2UBr2 (33) (A) and (C5Me4Et)2UBr2 (34) (B) with thermal ellipsoids projected at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (°) for 33: U(1)–Br(1), 2.7578(5); U(1)–CCent, 2.438(4); Br(1)–U(1)–Br(2), 97.64(3); CCent(1)–U(1)–CCent(2), 137.57(15). For 34: U(1)–Br(1), 2.7609(4); U(1)–Br(2), 2.7607(4); U(1)–CCent, 2.444(2); Br(1)–U(1)–Br(2), 96.224(14); CCent(1)–U(1)–CCent(2), 138.50(8).
Figure 2. Molecular structure of (C5Me5)2UBr2 (33) (A) and (C5Me4Et)2UBr2 (34) (B) with thermal ellipsoids projected at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (°) for 33: U(1)–Br(1), 2.7578(5); U(1)–CCent, 2.438(4); Br(1)–U(1)–Br(2), 97.64(3); CCent(1)–U(1)–CCent(2), 137.57(15). For 34: U(1)–Br(1), 2.7609(4); U(1)–Br(2), 2.7607(4); U(1)–CCent, 2.444(2); Br(1)–U(1)–Br(2), 96.224(14); CCent(1)–U(1)–CCent(2), 138.50(8).
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Inorganics 04 00001 g006 1024
Scheme 2. Synthesis of the halide aryloxide mixed-ligand uranium(IV) complexes (C5Me4R)2U(O-2,6-iPr2C6H3)(Cl) (R = Me (35), Et (36)) and (C5Me4R)2U(O-2,6-iPr2C6H3)(Br) (R = Me (37), Et (38)) by salt metathesis.
Scheme 2. Synthesis of the halide aryloxide mixed-ligand uranium(IV) complexes (C5Me4R)2U(O-2,6-iPr2C6H3)(Cl) (R = Me (35), Et (36)) and (C5Me4R)2U(O-2,6-iPr2C6H3)(Br) (R = Me (37), Et (38)) by salt metathesis.
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Inorganics 04 00001 g007 1024
Scheme 3. Synthesis of the bromide aryloxide mixed-ligand uranium(IV) complexes (C5Me4R)2U(O-2,6-iPr2C6H3)(Br) (R = Me (37), Et (38)) by halide exchange with trimethylsilyl bromide.
Scheme 3. Synthesis of the bromide aryloxide mixed-ligand uranium(IV) complexes (C5Me4R)2U(O-2,6-iPr2C6H3)(Br) (R = Me (37), Et (38)) by halide exchange with trimethylsilyl bromide.
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It is worth mentioning that in the family of the halide aryloxide series of compounds (C5Me5)2U(O-2,6-iPr2C6H3)(X) (X = F, Cl, Br, I), the bromide complex is the only member that has not been previously reported [2,8,48]. Comparison of the methyl proton resonance of C5Me5 by 1H NMR spectroscopy reveals an upfield shift in the order I (9.85 ppm) < Br (8.76 ppm) < Cl (7.85 ppm) << F (3.19 ppm). This trend has been observed before in other cyclopentadienyl complexes and it is directly correlated with the π-donating ability of the halide ligand [51]. Similarly, comparison of the methyl peaks of the C5Me4Et ligand of 36 (11.9, 8.5, 7.8, 7.8 and 6.6 ppm) and 38 (11.9, 9.11, 9.07, 8.3 and 7.8 ppm) shows the same pattern, where the peak positions of (C5Me4Et)2U(O-2,6-iPr2C6H3)(Br) appear downfield from those of (C5Me4Et)2U(O-2,6-iPr2C6H3)(Cl).
Compound 37 crystallizes with two asymmetric units in the crystal lattice and the structures are shown in Figure 3. The U–Br bond distances (2.7951(12) and 2.8023(12) Å) are somewhat longer than those of 33 (2.7578(5) Å) and 34 (U(1)–Br(1) = 2.7609(4) and U(1)–Br(2) = 2.7607(4) Å), but still fall within the range of the known neutral U–Br complexes (2.734(1)–2.831(7) Å) [6,7,9,16,33,34,35,36,37,38,39,40,41,42,43,44,45]. The U–O bond distances of 37 (2.110(7) and 2.134(7) Å) are similar to those of the known halide and pseudohalide derivatives (C5Me5)2U(O-2,6-iPr2C6H3)(X) (X = F (2.124(6) Å), Cl (2.110(5) Å), I (2.114(6) Å), N3 (2.117(5) Å) and CH3 (2.126(4) Å)) [2,48,50]. Likewise, the U–O–Cipso and O–U–Br bond angles of 37 (U–O–Cipso = 166.9(6) and 161.7(6)°; O–U–Br = 104.64(18)° and 104.04(17)°) compare favorably with the known complexes (C5Me5)2U(O-2,6-iPr2C6H3)(X) (X = F: U–O–Cipso = 165.4(6)°, O–U–F = 104.1(2)°; X = Cl: U–O–Cipso = 164.0(4)°, O–U–Cl = 102.06(14)°; X = I: U–O–Cipso = 166.6(6)°, O–U–I = 104.87(15)°; X = N3: U–O–Cipso = 165.2(4)°, O–U–N = 100.7(2)° and X = CH3: U–O–Cipso = 163.2(4)°, O–U–CH3 = 98.80(19)°). Finally the U–CCent bond distances and the CCent–U–CCent bond angles of 37 (U–CCent = 2.47(1), 2.46(1) and 2.45(1) Å; CCent–U–CCent = 134.7(3)° and 133.2(2)°) are comparable with those of other bis(pentamethylcyclopentadienyl) complexes (U–CCent = 2.420–2.562 Å; CCent–U–CCent = 126.2–142.6°) [1,2,31,46,47,48,49,50] and to the other (C5Me5)2U(O-2,6-iPr2C6H3)(X) derivatives (X = F: U–CCent = 2.444(2)–2.451(1) Å, CCent–U–CCent = 135.8–135.9°; X = Cl: U–CCent = 2.444(3)–2.457(3) Å; CCent–U–CCent = 133.1–134.5°; X = I: U–CCent = 2.447(3)–2.456(4) Å; CCent–U–CCent = 133.5–133.6°; X = N3: U–CCent = 2.505(3)–2.532(3) Å; CCent–U–CCent = 133.2°; X = CH3: U–CCent = 2.466(2)–2.471(3) Å; CCent–U–CCent = 134.0°) [2,48,50].
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Figure 3. Molecular structures of the two crystallographically-independent (C5Me5)2U(O-2,6-iPr2C6H3)(Br) (37) complexes (A and B) with thermal ellipsoids projected at the 30% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (°): U(1)–O(1), 2.110(7); U(2)–O(2), 2.134(7); U(1)–Br(1), 2.7951(12); U(2)–Br(2), 2.8023(12); U(1)–CCent(1), 2.47(1); U(1)–CCent(2), 2.46(1); U(2)–CCent(3), 2.45(1); U(2)–CCent(4), 2.46(1); Br(1)–U(1)–O(1), 104.64(18); Br(2)–U(2)–O(2), 104.04(17); U(1)–O(1)–C(21), 166.9(6); U(2)–O(2)–C(53), 161.7(6); CCent(1)–U(1)–CCent(2), 134.7(3); CCent(3)–U(2)–CCent(4), 133.2(2).
Figure 3. Molecular structures of the two crystallographically-independent (C5Me5)2U(O-2,6-iPr2C6H3)(Br) (37) complexes (A and B) with thermal ellipsoids projected at the 30% probability level. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (°): U(1)–O(1), 2.110(7); U(2)–O(2), 2.134(7); U(1)–Br(1), 2.7951(12); U(2)–Br(2), 2.8023(12); U(1)–CCent(1), 2.47(1); U(1)–CCent(2), 2.46(1); U(2)–CCent(3), 2.45(1); U(2)–CCent(4), 2.46(1); Br(1)–U(1)–O(1), 104.64(18); Br(2)–U(2)–O(2), 104.04(17); U(1)–O(1)–C(21), 166.9(6); U(2)–O(2)–C(53), 161.7(6); CCent(1)–U(1)–CCent(2), 134.7(3); CCent(3)–U(2)–CCent(4), 133.2(2).
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Treatment of the uranium(IV) dihalide complexes 31–34 with one equivalent of KC8 in toluene at room temperature affords the corresponding uranium(III) chloride and bromide “ate” species [K(THF)n][(C5Me4R)2UX2] (39, X = Cl, R = Me, n = 1; 40, X = Cl, R = Et, n = 0.5; 41, X = Br, R = Me, n = 1; 42, X = Br, R = Et, n = 0.5) as illustrated in Scheme 4. During the course of the reaction, the vivid red-maroon color of complexes 31–34 changes to an insoluble green solid. After workup, all complexes were isolated in good to excellent yields (39 (79%), 40 (93%), 41 (90%) and 42 (93%)).
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Scheme 4. Synthesis of uranium(III) complexes [K(THF)][(C5Me5)2UX2] (X = Cl (39), Br (41)) and [K(THF)0.5][(C5Me4Et)2UX2] (X = Cl (40), Br (42)) by KC8 reduction of the corresponding uranium(IV) dihalides.
Scheme 4. Synthesis of uranium(III) complexes [K(THF)][(C5Me5)2UX2] (X = Cl (39), Br (41)) and [K(THF)0.5][(C5Me4Et)2UX2] (X = Cl (40), Br (42)) by KC8 reduction of the corresponding uranium(IV) dihalides.
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Complexes 41 and 42 are rare examples of trivalent uranium bromides. A truncated form of the solid-state structure of compound 41 is shown in Figure 4. The complex is best described as a polymer with potassium ions linking the uranium centers with potassium–bromide bridges. The geometry at each uranium center is pseudo tetrahedral, while that at each potassium is distorted trigonal pyramidal. The K–Br bond lengths of 41 (3.1684(14), 3.1671(14), 3.2577(15) and 3.2477(15) Å) are comparable to those reported for other compounds with K–Br interactions (2.988(11)–3.844(3) Å) [52,53,54,55,56,57,58,59,60,61,62,63,64,65,66]. The U–Br bond lengths of 41 (U–Br = 2.9126(6) and 2.8953(6) Å) are comparable to the few known U(III) bromides in the literature: {[1,3-(SiMe3)2C5H3]2UBr}2 (26, U–Br = 2.93(1) and 2.94(1) Å) [24], [U(Br)2(H2O)5(NCMe)2][Br] (U–Br = 3.074(4) Å) [37], [Li(THF)4]2[(C2H20B9)2U(Br)(THF)] (25, U–Br = 2.883(2) Å) [25], [1,3-(SiMe3)2C5H3]2U(Br)(CNtBu)2 (30, U–Br = 2.8761(10) Å) [26] and [UBr3(DME)2]2 (U–Br = 2.898(2), 2.887(2), 3.016(2) and 3.098(2) Å) [40] and are longer than those of 33 (U–Br = 2.7578(5) Å) and 34 (U–Br = 2.7609(4) and 2.7607(4) Å).
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Figure 4. Solid state structure (30% probability ellipsoids) of a tetrameric unit of [K(THF)][(C5Me5)2UBr2] (41) Hydrogen atoms and the carbon atoms of THF molecules are omitted for clarity. Selected bond distances (Å) and bond angles (°): U(1)–Br(1), 2.9126(6); U(1)–Br(2), 2.8953(6); K(1)–Br(1), 3.1681(14), 3.2577(15); K(1)–Br(2), 3.1671(14), 3.2477(15); K(1)–O(1), 2.641(6); U(1)–CCent(1), 2.495(7); U(1)–CCent(2), 2.485(7); Br(1)–U(1)–Br(2), 88.841(19); Br(1)–K(1)–Br(2), 114.32(15), 87.50(4), 87.35(3), 77.35(3); Br(1)–K(1)–O(1), 85.26(14), 93.41(16); Br(2)–K(1)–O(1), 100.91(18), 132.49(19); CCent(1)–U(1)–CCent(2), 136.6(3).
Figure 4. Solid state structure (30% probability ellipsoids) of a tetrameric unit of [K(THF)][(C5Me5)2UBr2] (41) Hydrogen atoms and the carbon atoms of THF molecules are omitted for clarity. Selected bond distances (Å) and bond angles (°): U(1)–Br(1), 2.9126(6); U(1)–Br(2), 2.8953(6); K(1)–Br(1), 3.1681(14), 3.2577(15); K(1)–Br(2), 3.1671(14), 3.2477(15); K(1)–O(1), 2.641(6); U(1)–CCent(1), 2.495(7); U(1)–CCent(2), 2.485(7); Br(1)–U(1)–Br(2), 88.841(19); Br(1)–K(1)–Br(2), 114.32(15), 87.50(4), 87.35(3), 77.35(3); Br(1)–K(1)–O(1), 85.26(14), 93.41(16); Br(2)–K(1)–O(1), 100.91(18), 132.49(19); CCent(1)–U(1)–CCent(2), 136.6(3).
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In addition, the U–CCent bond distances of 41 (2.495(7) and 2.485(7) Å) are comparable to those of 33 (2.438(4) Å), 34 (2.444(2) Å), and 37 (2.45(1), 2.46(1) and 2.47(1) Å). Finally, the CCent–U–CCent bond angle of 41 (136.6(3)°) is similar to those of compounds 33 (137.57(15)°), 34 (138.50(8)°) and 37 (137.7(3)°), as well as other (C5Me5)2U(X)2 complexes (121.1–146.1°) [29,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81].
Few other similar types of complexes have been reported in the past. For instance, Marks and co-workers reported that Na/Hg reduction of (C5Me5)2UCl2 formed [Na(THF)2][(C5Me5)2UCl2] [72], but no structure was presented and two other examples reported by Lappert and co-workers show monomeric motifs for [M(THF)2][{1,3-(Me3Si)2C5H3}2UCl2] (M = Li or Na) [7]. The polymeric structure of 41 can be explained by the presence of the considerably larger potassium ions, versus lithium and sodium adducts, which yield monomeric complexes.
As shown in Scheme 5, the synthetic utility of the “ate” complexes [K(THF)][(C5Me4R)2UX2] (39–42) was demonstrated by their reaction with one equivalent of Na[N(SiMe3)2] in toluene to afford (C5Me4R)2U[N(SiMe3)2] (R = Me, (43), Et (44)) in good yields (76%–86%).
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Scheme 5. Preparation of neutral uranium(III) complexes (C5Me4R)2U[N(SiMe3)2] (R = Me, (43), Et (44)).
Scheme 5. Preparation of neutral uranium(III) complexes (C5Me4R)2U[N(SiMe3)2] (R = Me, (43), Et (44)).
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3. Experimental Section

3.1. General Synthetic Considerations

Unless otherwise noted, all reactions and manipulations were performed at ambient temperatures in a recirculating Vacuum Atmospheres NEXUS model inert atmosphere (N2) drybox equipped with a 40 CFM Dual Purifier NI–Train. Glassware was dried overnight at 150 °C before use. All NMR spectra were obtained using a Bruker Avance 400 MHz spectrometer at room temperature. Chemical shifts for 1H NMR spectra were referenced to solvent impurities [82]. Melting points were determined with a Mel-Temp II capillary melting point apparatus equipped with a Fluke 50 S K/J thermocouple using capillary tubes flame-sealed under N2; values are uncorrected. Elemental Analyses were performed by ALS Environmental (Tucson, AZ, USA) or Atlantic Microlab, Inc. (Norcross, GA, USA).

3.2. Materials

Unless otherwise noted, reagents were purchased from commercial suppliers and used without further purification. Celite (Aldrich, Saint Louis, MO, USA), neutral alumina (Aldrich), and 3 Å molecular sieves (Aldrich) were dried under dynamic vacuum at 220 °C for 48 h prior to use. All solvents (Aldrich), benzene-d6 and THF-d8 (Cambridge Isotopes, Tewksbury, MA, USA) were purchased anhydrous and dried over KH for 48 h, passed through a column of activated alumina, and stored over activated 3 Å molecular sieves prior to use. HO-2,6-iPr2C6H3 (Aldrich) was dried with activated 3 Å molecular sieves prior to use. Potassium metal (Aldrich) was rinsed with hexane, dried and used immediately. Me3SiBr (Aldrich), graphite (Aldrich) and Na[N(SiMe3)2] (Aldrich) were used as received. (C5Me5)2UCl2 (31) [3], (C5Me4Et)2UCl2 (32) [31], and K[O-2,6-iPr2C6H3] [30] were prepared according to literature procedures.

3.3. Caution

Depleted uranium (primary isotope 238U) is a weak α-emitter (4.197 MeV) with a half-life of 4.47 × 109 years; manipulations and reactions should be carried out in monitored fume-hoods or in an inert atmosphere drybox in a radiation laboratory equipped with α- and β-counting equipment.

3.4. Synthetic Procedures

(C5Me5)2UBr2 (33): A 20 mL scintillation vial was charged with a stir bar, (C5Me5)2UCl2 (31) (0.301 g, 0.518 mmol) and Et2O (5 mL). To this solution, Me3SiBr (0.397 g, 2.59 mmol) was added dropwise. The reaction mixture was stirred at ambient temperature for 12 h and the volatiles were removed under reduced pressure. Et2O (5 mL) and Me3SiBr (0.397 g, 2.59 mmol) were added again and the solution was left stirring at ambient temperature for another 12 h and the volatiles were removed under reduced pressure. Et2O (5 mL) and Me3SiBr (0.397 g, 2.59 mmol) were added for the third and final time and the reaction mixture was stirred at ambient temperature for 12 h and the volatiles were removed under reduced pressure. The resulting residue was dissolved in toluene (15 mL) and filtered through a Celite-padded coarse-porosity fritted filter. The red colored filtrate was collected and the volatiles were removed under reduced pressure to give (C5Me5)2UBr2 (33) as a red solid (0.333 g, 0.498 mmol, 96%). Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a toluene solution at ambient temperature. 1H NMR (benzene-d6, 298 K): δ 15.70 (s, 30H, v1/2 = 111.8 Hz, C5Me5). m.p. 276–278 °C. Anal. calcd. for C20H30Br2U (mol. wt. 668.29): C, 35.94; H, 4.52. Found: C, 35.20, H, 4.42.
(C5Me4Et)2UBr2 (34): A 20 mL scintillation vial was charged with a stir bar, (C5Me4Et)2UCl2 (32) (0.500 g, 0.823 mmol) and Et2O (5 mL). To this solution Me3SiBr (0.630 g, 4.12 mmol) was added dropwise. The reaction mixture was stirred at ambient temperature for 12 h and the volatiles were removed under reduced pressure. Et2O (5 mL) and Me3SiBr (0.630 g, 4.12 mmol) were added again and the solution was left stirring at ambient temperature for another 12 h and the volatiles were removed under reduced pressure. Et2O (5 mL) and Me3SiBr (0.630 g, 4.12 mmol) were added for the third and final time and the reaction mixture was stirred at ambient temperature for 12 h and the volatiles were removed under reduced pressure. The resulting residue was dissolved in toluene (15 mL) and filtered through a Celite-padded coarse-porosity fritted filter. The red colored filtrate was collected and the volatiles were removed under reduced pressure to give (C5Me4Et)2UBr2 (34) as a red solid (0.515 g, 0.740 mmol, 90%). Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a toluene solution at ambient temperature. 1H NMR (benzene-d6, 298 K): δ 25.06 (s, 6H, v1/2 = 43.3 Hz, CH2CH3), 15.57 (s, 12H, v1/2 = 95.7 Hz, CH3), 13.06 (s, 12H, v1/2 = 93.8 Hz, CH3), 0.31 (s, 4H, v1/2 = 112.5 Hz CH2). m.p. 160–162 °C. Anal. calcd. for C22H34Br2U: C, 37.95; H, 4.92. Found: C, 37.59; H, 5.05.
(C5Me5)2U(O-2,6-iPr2C6H3)(Cl) (35): A 20 mL scintillation vial was charged with a stir bar, (C5Me5)2UCl2 (29) (0.149 g, 0.257 mmol), K[O-2,6-iPr2C6H3] (0.056 g, 0.257 mmol) and THF (5 mL). The reaction mixture was stirred at ambient temperature for 35 min and the volatiles were removed under reduced pressure. The resulting residue was dissolved in toluene (10 mL) and filtered through a Celite-padded coarse-porosity fritted filter. The red filtrate was collected and the volatiles were removed under reduced pressure to give (C5Me5)2U(O-2,6-iPr2C6H3)(Cl) (35) as a red solid (0.170 g, 0.236 mmol, 92%). The 1H NMR spectrum collected in benzene-d6 was consistent with data previously reported for complex 35 [8]. 1H NMR (C6D6, 298 K): δ 8.37 (d, 1H, m-Ar-H), 8.35 (d, 1H, m-Ar-H), 7.85 (s, 30H, C5(CH3)5), 7.06 (t, 1H, p-Ar-H), −6.01 (s, 6H, CH(CH3)2), −12.37 (s, 6H, CH(CH3)2), −35.74 (m, 1H, CH(CH3)2), −44.65 (m, 1H, CH(CH3)2).
(C5Me4Et)2U(O-2,6-iPr2C6H3)(Cl) (36): A 20 mL scintillation vial was charged with a stir bar, (C5Me4Et)2UCl2 (32) (0.300 g, 0.494 mmol), K[O-2,6-iPr2C6H3] (0.107 g, 0.494 mmol) and THF (5 mL). The reaction mixture was stirred at ambient temperature for 35 min and the volatiles were removed under reduced pressure. The resulting residue was dissolved in toluene (5 mL) and filtered through a Celite-padded coarse-porosity fritted filter. The red colored filtrate was collected and the volatiles were removed under reduced pressure. The resulting red oil was dissolved in hexane (5 mL) and passed through a Celite-padded coarse-porosity fritted filter. The red filtrate was collected, the volatiles were removed under reduced pressure and the oily product triturated three times with acetonitrile (5 mL) to give (C5Me4Et)2U(O-2,6-iPr2C6H3)(Cl) (36) as a red solid (0.332 g, 0.443 mmol, 90%). 1H NMR (benzene-d6, 298 K): δ 11.87 (s, 6H, v1/2 = 13.6 Hz, CH3), 8.49 (s, 6H, v1/2 = 5.9 Hz, CH3), 8.26 (d, 1H, JHH = 8.5 Hz, m-Ar-CH), 8.15 (d, 1H, JHH = 8.5 Hz, m-Ar-CH), 7.82 (s, 6H, v1/2 = 13.6 Hz, CH3), 7.78 (s, 6H, v1/2 = 13.6 Hz, CH3), 6.91 (t, 1H, JHH = 8.5 Hz, p-Ar-H), 6.58 (s, 6H, v1/2 = 4.7 Hz, CH3), −0.55 (s, 2H, v1/2 = 29 Hz, CH2), −1.37 (s, 2H, v1/2 = 29 Hz, CH2), −5.85 (d, 6H, JHH = 3.6 Hz, CH(CH3)2), −12.75 (s, 6H, v1/2 = 6.9 Hz, CH(CH3)2), −35.67 (m, 1H, CH(CH3)2), −46.81 (s, 1H, v1/2 = 28.6 Hz, CH(CH3)2). m.p. 92–95 °C. Anal. calcd. for C34H51ClOU (mol. wt. 749.25): C, 54.50; H, 6.86. Found: C, 54.08; H, 7.16.
(C5Me5)2U(O-2,6-iPr2C6H3)(Br) (37): Method A: From (C5Me5)2UBr2. A 20 mL scintillation vial was charged with a stir bar, (C5Me5)2UBr2 (33) (0.100 g, 0.150 mmol), K[O-2,6-iPr2C6H3] (0.0324 g, 0.150 mmol) and THF (5 mL). The reaction mixture was stirred at ambient temperature for 1 h and the volatiles were removed under reduced pressure. The resulting residue was dissolved in toluene (5 mL) and filtered through a Celite-padded coarse-porosity fritted filter. The red filtrate was collected and the volatiles were removed under reduced pressure to give (C5Me5)2U(O-2,6-iPr2C6H3)(Br) (37) as a red solid (0.113 g, 0.148 mmol, 99%).
Method B: From (C5Me5)2U(O-2,6-iPr2C6H3)(Cl). A 20 mL scintillation vial was charged with a stir bar, (C5Me5)2U(O-2,6-iPr2C6H3)(Cl) (35) (0.102 g, 0.141 mmol), Et2O (5 mL) and Me3SiBr (0.108 g, 0.707 mmol). The reaction mixture was stirred at ambient temperature for 12 h and the volatiles were removed under reduced pressure. The resulting residue was dissolved in toluene (5 mL) and filtered through a Celite-padded coarse-porosity fritted filter. The red filtrate was collected and the volatiles were removed under reduced pressure to give (C5Me5)2U(O-2,6-iPr2C6H3)(Br) (37) as a red solid (0.100 g, 0.131 mmol, 93%). Single crystals suitable for X-ray diffraction were obtained from a 50 mM toluene solution at −35 °C. 1H NMR (benzene-d6, 298 K): δ 9.41 (d, 1H, JHH = 8.6 Hz, m-Ar-H), 9.07 (d, 1H, JHH = 8.6 Hz, m-Ar-H), 8.76 (s, 30H, C5(CH3)5), 7.40 (t, 1H, JHH = 8.7 Hz, p-Ar-H), −5.89 (d, 6H, JHH = 3.8 Hz, CH(CH3)2), −11.68 (s, 6H, CH(CH3)2), −35.67 (m, 1H, CH(CH3)2), −42.99 (m, 1H, CH(CH3)2). m.p. 214–216 °C. Anal. calcd. for C32H47BrOU (mol. wt. 765.65): C, 50.20; H, 6.19. Found: C, 49.46, H, 5.98.
(C5Me4Et)2U(O-2,6-iPr2C6H3)(Br) (38): Method A: From (C5Me4Et)2UBr2. A 20 mL scintillation vial was charged with a stir bar, (C5Me4Et)2UBr2 (34) (0.100 g, 0.144 mmol), K[O-2,6-iPr2C6H3] (0.0311 g, 0.144 mmol) and THF (5 mL). The reaction mixture was stirred at ambient temperature for 1 h and the volatiles were removed under reduced pressure. The resulting residue was dissolved in toluene (5 mL) and filtered through a Celite-padded coarse-porosity fritted filter. The red filtrate was collected and the volatiles were removed under reduced pressure. The resulting red oil was dissolved in hexane (5 mL) and passed through a Celite-padded coarse-porosity fritted filter. The red filtrate was collected, the volatiles removed under reduced pressure and the oily product triturated three times with acetonitrile (5 mL) to give (C5Me4Et)2U(O-2,6-iPr2C6H3)(Br) (38) as a red solid (0.110 g, 0.139 mmol, 96%).
Method B: From (C5Me4Et)2U(O-2,6-iPr2C6H3)(Cl). A 20 mL scintillation vial was charged with a stir bar, (C5Me4Et)2U(O-2,6-iPr2C6H3)(Cl) (36) (0.100 g, 0.133 mmol), Et2O (5 mL) and Me3SiBr (0.102 g, 0.667 mmol). The reaction mixture was stirred at ambient temperature for 12 h and the volatiles were removed under reduced pressure. The resulting residue was dissolved in toluene (5 mL) and filtered through a Celite-padded coarse-porosity fritted filter. The red filtrate was collected and the volatiles were removed under reduced pressure. The resulting red oil was triturated with acetonitrile (3 mL) to give (C5Me4Et)2U(O-2,6-iPr2C6H3)(Br) (38) as a red solid (0.096 g, 0.121 mmol, 91%). 1H NMR (benzene-d6, 298 K): δ 11.92 (s, 6H, v1/2 = 14.0 Hz, CH3), 9.11 (s, 6H, v1/2 = 5.3 Hz, CH3), 9.07 (s, 6H, v1/2 = 5.3 Hz, CH3), 8.96 (d, 1H, JHH = 8.3 Hz, m-Ar-CH), 8.33 (s, 6H, v1/2 = 13.6 Hz, CH3), 7.77 (s, 6H, v1/2 = 5.4 Hz, CH3), 7.23 (t, 1H, JHH = 8.7 Hz, p-Ar-H), 0.35 (s, 2H, v1/2 = 27 Hz, CH2), −0.52 (s, 2H, v1/2 = 31 Hz, CH2), −5.72 (d, 6H, JHH = 3.2 Hz, CH(CH3)2), −12.15 (s, 6H, v1/2 = 7.0 Hz, CH(CH3)2), −35.62 (m, 1H, CH(CH3)2), −45.56 (m, 1H, CH(CH3)2). m.p. 69–72 °C. Anal. calcd. for C34H51BrOU (mol. wt. 793.70): C, 51.45; H, 6.48. Found: C, 51.51; H, 6.65.
KC8: This is a modification of literature procedures [83,84]: A 20 mL scintillation vial was charged with potassium metal (0.040 g, 1.03 mmol) and graphite (0.099 g, 8.26 mmol) in an nitrogen atmosphere glove box. The reaction mixture was stirred with a spatula at 100 °C until the solid was completely bronze in color (~5 min), yielding quantitative solid KC8 (0.140 g, 1.03 mmol, 100%).
[K(THF)][(C5Me5)2UCl2] (39): A 20 mL scintillation vial was charged with a stir bar, (C5Me5)2UCl2 (31) (0.600 g, 1.03 mmol), KC8 (0.140 g, 1.03 mmol), and toluene (10 mL). The reaction mixture was stirred at ambient temperature for 4 h, yielding a green precipitate. The reaction mixture was filtered through a Celite-padded coarse-porosity fritted filter. The toluene-insoluble green solid, which was collected on the Celite-padded fritted filter was dissolved by adding THF (15 mL). The green filtrate was collected and the volatiles were removed under reduced pressure to give [K(THF)][(C5Me5)2UCl2] (39) as a green solid (0.560 g, 0.405 mmol, 79%). Single crystals suitable for X-ray diffraction were grown from a saturated THF solution at −35 °C. 1H-NMR (THF-d8, 298 K): δ 3.62 (8H, m, THF-CH2), 1.78 (8H, m, THF-CH2), −5.49 (60H, v1/2 = 52 Hz, C5(CH3)5). m.p. 230.0 °C (Decomp). Anal. calcd. for C48H76Cl4K2O2U2 (mol. wt. 1381.18): C, 41.74; H, 5.55. Found: C, 41.41; H, 5.30.
[K(THF)0.5][(C5Me4Et)2UCl2] (40): A 20 mL scintillation vial was charged with a stir bar, (C5Me4Et)2UCl2 (32) (0.500 g, 0.823 mmol), KC8 (0.111 g, 0.823 mmol), and toluene (10 mL). The reaction mixture was stirred at ambient temperature for 2 h, yielding a green precipitate. The reaction mixture was filtered through a Celite-padded coarse-porosity fritted filter. The insoluble green solid on the Celite-padded coarse-porosity fritted filter. The toluene-insoluble green solid, which was collected on the Celite-padded frit was dissolved by adding THF (15 mL). The green filtrate was collected and the volatiles were removed under reduced pressure to give [K(THF)0.5][(C5Me4Et)2UCl2] (40) as a green solid (0.549 g, 0.764 mmol, 93%). 1H NMR (THF-d8, 298 K): d 11.39 (s, 6H, v1/2 = 29.0 Hz, CH2CH3), 3.62 (m, 4H, THF-CH2), 1.79 (m, 4H, THF-CH2), −4.61 (s, 12H, v1/2 = 58.6 Hz, CH3), −5.79 (s, 4H, v1/2 = 87.3 Hz, CH2), −6.43 (s, 12H, v1/2 = 60.0 Hz, CH3). m.p. 189–192 °C (Decomp). Anal. calcd. for C48H76Cl4K2OU2 (mol. wt. 1365.18): C, 42.43; H, 5.61. Found: C, 42.54; H, 6.04.
[K(THF)][(C5Me5)2UBr2] (41): A 20 mL scintillation vial was charged with a stir bar, (C5Me5)2UBr2 (33) (0.263 g, 0.393 mmol), KC8 (0.0530 g, 0.393 mmol), and toluene (10 mL). The reaction mixture was stirred at ambient temperature for 3 h, yielding a green precipitate. The reaction mixture was filtered through a Celite-padded coarse-porosity fritted filter. The toluene-insoluble green solid, which was collected on the Celite-padded fritted filter was dissolved by adding THF (15 mL). The green filtrate was collected and the volatiles were removed under reduced pressure to give [K(THF)][(C5Me5)2UBr2] (41) as a green solid (0.275 g, 0.353 mmol, 90%). Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a THF solution at ambient temperature. 1H NMR (THF-d8, 298 K): δ 3.62 (m, 4H, THF-CH2), 1.78 (m, 4H, THF-CH2), −3.42 (s, 30H, v1/2 = 84.5 Hz, C5(CH3)5). m.p. 190–192 °C (Decomp). Anal. calcd. for C48H76Br4K2O2U2 (mol. wt. 1558.99): C, 36.98; H, 4.91. Found: C, 37.00; H, 4.98.
[K(THF)0.5][(C5Me4Et)2UBr2] (42): A 20 mL scintillation vial was charged with a stir bar, (C5Me4Et)2UBr2 (34) (0.300 g, 0.431 mmol), KC8 (0.058 g, 0.431 mmol), and toluene (10 mL). The reaction mixture was stirred at ambient temperature for 2 h, yielding a green precipitate. The reaction mixture was filtered through a Celite-padded coarse-porosity fritted filter. The toluene-insoluble green solid, which was collected on the Celite-padded fritted filter was dissolved by adding in THF (10 mL). The green colored filtrate was collected and the volatiles were removed under reduced pressure. The resulting solid was triturated twice with hexane (3 mL) to give [K(THF)0.5][(C5Me4Et)2UBr2] (42) as a green solid (0.323 g, 0.400 mmol, 93%). 1H NMR (THF-d8, 298 K): δ 13.61 (s, 6H, v1/2 = 76.5 Hz, CH2CH3), 3.62 (m, 4H, THF-CH2), 1.79 (m, 4H, THF-CH2), −3.10 (s, 12H, v1/2 = 131.9 Hz, CH3), −4.23 (s, 12H, v1/2 = 135.8 Hz, CH3), −4.47 (s, 4H, CH2). m.p. 189–191 °C (Decomp). Anal. calcd. for C48H76Br4K2OU2 (mol. wt. 1542.99): C, 37.36; H, 4.96. Found: C, 37.61; H, 5.23.
(C5Me5)2U[N(SiMe3)2] (43): Method A: From [K(THF)][(C5Me5)2UCl2]. A 20 mL scintillation vial was charged with a stir bar, [K(THF)][(C5Me5)2UCl2] (39) (0.100 g, 0.148 mmol), Na[N(SiMe3)2] (0.027 g, 0.148 mmol), and toluene (5 mL). The reaction mixture was stirred at ambient temperature for 1 h, and the volatiles were removed under reduced pressure. The resulting residue was dissolved in hexane (5 mL) and filtered through a Celite-padded coarse-porosity fritted filter. The gray filtrate was collected and the volatiles were removed under reduced pressure to give (C5Me5)2U[N(SiMe3)2] (43) as a gray solid (0.085 g, 0.127 mmoles, 86%).
Method B: From [K(THF)][(C5Me5)2UBr2]. A 20 mL scintillation vial was charged with a stir bar, [K(THF)][(C5Me5)2UBr2] (41) (0.100 g, 0.128 mmol), Na[N(SiMe3)2] (0.024 g, 0.128 mmol), and toluene (5 mL). The reaction mixture was stirred at ambient temperature for 1 h and the volatiles were removed under reduced pressure. The resulting residue was dissolved in hexane (5 mL) and filtered through a Celite-padded coarse-porosity fritted filter. The gray filtrate was collected and the volatiles were removed under reduced pressure to give (C5Me5)2U[N(SiMe3)2] (43) as a gray solid (0.065 g, 0.097 mmol, 76%). The 1H NMR spectrum collected in benzene-d6 was consistent with data previously reported for complex 43 [85]. 1H NMR (benzene-d6, 298 K): δ −5.69 (30H, s, C5(CH3)5), −25.57 (18H, s, Si(CH3)3).
(C5Me4Et)2U[N(SiMe3)2] (44): Method A: From [K(THF)0.5][(C5Me4Et)2UCl2]. A 20 mL scintillation vial was charged with a stir bar, [K(THF)0.5][(C5Me4Et)2UCl2] (40) (0.100 g, 0.139 mmol), Na[N(SiMe3)2] (0.026 g, 0.139 mmol), and toluene (5 mL). The reaction mixture was stirred at ambient temperature for 1 h, and the volatiles were removed under reduced pressure. The resulting residue was dissolved in hexane (5 mL) and filtered through a Celite-padded coarse-porosity fritted filter. The gray filtrate was collected and the volatiles were removed under reduced pressure to give (C5Me4Et)2U[N(SiMe3)2] (44) as a gray solid (0.079 g, 0.113 mmol, 81%).
Method B: From [K(THF)0.5][(C5Me4Et)2UBr2]. A 20 mL scintillation vial was charged with a stir bar, [K(THF)0.5][(C5Me4Et)2UBr2] (42) (0.100 g, 0.124 mmol), Na[N(SiMe3)2] (0.023 g, 0.124 mmol), and toluene (5 mL). The reaction mixture was stirred at ambient temperature for 1 h, and the volatiles were removed under reduced pressure. The resulting residue was dissolved in hexane (5 mL) and filtered through a Celite-padded coarse-porosity fritted filter. The gray filtrate was collected and the volatiles were removed under reduced pressure to give (C5Me4Et)2U[N(SiMe3)2] (44) as a gray solid (0.073 g, 0.104 mmol, 84%). The 1H NMR spectrum collected in benzene-d6 was consistent with data previously reported for complex 44 [85]. 1H NMR (benzene-d6, 298 K): δ 10.28 (6H, s, CH2CH3), 4.03 (4H, s, CH2CH3), −5.62 (12H, s, CH3), −6.52 (12H, s, CH3), −25.48 (18H, s, Si(CH3)3).
X-ray Crystallography: Data for 33, 34, and 41 were collected on a Bruker D8 Quest diffractometer, with a CMOS detector in shutterless mode. The crystals were cooled to 100 K employing an Oxford Cryostream liquid nitrogen cryostat. Data for 37 were collected on a Bruker D8 diffractometer, with an APEX II CCD detector. The crystal was cooled to 140 K using a Bruker Kryoflex liquid nitrogen cryostat. Both data collections employed graphite monochromatized MoKα (λ = 0.71073 Å) radiation. Cell indexing, data collection, integration, structure solution, and refinement were performed using Bruker and SHELXTL software [86,87,88,89]. CIF files representing the X-ray crystal structures of 33, 34, 37 and 41 (Supplementary files) have been submitted to the Cambridge Crystallographic Database as submission numbers CCDC 1428938–1428941.

4. Conclusions

In summary, we have expanded the family of organouranium bromides with the preparation of the tetravalent and trivalent uranium bromide complexes (C5Me4R)2UBr2, (C5Me4R)2U(O-2,6-iPr2C6H3)(Br), and [K(THF)][(C5Me4R)2UBr2] (R = Me, Et). The uranium(IV) compounds were easily accessed by treating the corresponding chloride analogues with excess Me3SiBr. The uranium(IV) chloride complexes were all easily obtained from UCl4 and their halide substitution chemistry with Me3SiBr constitutes a practical and efficient route to new bromide derivatives, even in the presence of alkoxide ligands.
The reduction of (C5Me4R)2UX2 complexes with one equivalent of KC8 allowed for the isolation of the uranium(III) species [K(THF)][(C5Me5)2UX2] and [K(THF)0.5][(C5Me4Et)2UX2], which can be converted to the neutral uranium(III) complexes (C5Me4R)2U[N(SiMe3)2] upon treatment with Na[N(SiMe3)2]. Halide and anion compatibility can be very important in the synthesis of organouranium complexes, and this work provides new options for the actinide synthetic chemistry toolbox.

Supplementary Materials

Supplementary materials can be accessed at www.mdpi.com/2304-6740/4/1/1/s1.

Acknowledgments

For financial support of this work, we acknowledge the U.S. Department of Energy through the Los Alamos National Laboratory (LANL) Laboratory Directed Research and Development (LDRD) Program and the Los Alamos National Laboratory Glenn Theodore Seaborg Institute for Transactinium Science (PD Fellowship to Alejandro G. Lichtscheidl), the Advanced Fuels Campaign Fuel Cycle Research & Development Program (Alejandro G. Lichtscheidl and Andrew T. Nelson), the Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program (Fellowship to Justin K. Pagano), and the Office of Basic Energy Sciences, Heavy Element Chemistry program (Jaqueline L. Kiplinger, Brian L. Scott, materials and supplies). The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the DOE (contract DE-AC05-06OR23100). Los Alamos National Laboratory is operated by Los Alamos National Security, Limited Liability Corporation, for the National Nuclear Security Administration of U.S. Department of Energy (contract DE-AC52-06NA25396).

Author Contributions

Alejandro G. Lichtscheidl performed the experiments and drafted the manuscript. Justin K. Pagano solved the crystal structure of compound 37. Brian L. Scott solved the crystal structures of compounds 33, 34 and 41 and drafted the X-ray crystallography experimental section of the manuscript. Andrew T. Nelson and Jaqueline L. Kiplinger conceived the project and supervised the work. All authors discussed the results, interpreted the data, and contributed to the preparation of the final manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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MDPI and ACS Style

Lichtscheidl, A.G.; Pagano, J.K.; Scott, B.L.; Nelson, A.T.; Kiplinger, J.L. Expanding the Chemistry of Actinide Metallocene Bromides. Synthesis, Properties and Molecular Structures of the Tetravalent and Trivalent Uranium Bromide Complexes: (C5Me4R)2UBr2, (C5Me4R)2U(O-2,6-iPr2C6H3)(Br), and [K(THF)][(C5Me4R)2UBr2] (R = Me, Et). Inorganics 2016, 4, 1. https://doi.org/10.3390/inorganics4010001

AMA Style

Lichtscheidl AG, Pagano JK, Scott BL, Nelson AT, Kiplinger JL. Expanding the Chemistry of Actinide Metallocene Bromides. Synthesis, Properties and Molecular Structures of the Tetravalent and Trivalent Uranium Bromide Complexes: (C5Me4R)2UBr2, (C5Me4R)2U(O-2,6-iPr2C6H3)(Br), and [K(THF)][(C5Me4R)2UBr2] (R = Me, Et). Inorganics. 2016; 4(1):1. https://doi.org/10.3390/inorganics4010001

Chicago/Turabian Style

Lichtscheidl, Alejandro G., Justin K. Pagano, Brian L. Scott, Andrew T. Nelson, and Jaqueline L. Kiplinger. 2016. "Expanding the Chemistry of Actinide Metallocene Bromides. Synthesis, Properties and Molecular Structures of the Tetravalent and Trivalent Uranium Bromide Complexes: (C5Me4R)2UBr2, (C5Me4R)2U(O-2,6-iPr2C6H3)(Br), and [K(THF)][(C5Me4R)2UBr2] (R = Me, Et)" Inorganics 4, no. 1: 1. https://doi.org/10.3390/inorganics4010001

APA Style

Lichtscheidl, A. G., Pagano, J. K., Scott, B. L., Nelson, A. T., & Kiplinger, J. L. (2016). Expanding the Chemistry of Actinide Metallocene Bromides. Synthesis, Properties and Molecular Structures of the Tetravalent and Trivalent Uranium Bromide Complexes: (C5Me4R)2UBr2, (C5Me4R)2U(O-2,6-iPr2C6H3)(Br), and [K(THF)][(C5Me4R)2UBr2] (R = Me, Et). Inorganics, 4(1), 1. https://doi.org/10.3390/inorganics4010001

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