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Feb 9, 2016 - Christopher L. Webster, Ryan R. Langeslay, Joseph W. Ziller, and William J. Evans*. Department of Chemistry, University of California, I...
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Synthetic Utility of Tetrabutylammonium Salts in Uranium Metallocene Chemistry Christopher L. Webster, Ryan R. Langeslay, Joseph W. Ziller, and William J. Evans* Department of Chemistry, University of California, Irvine, California 92697-2025, United States S Supporting Information *

ABSTRACT: Tetrabutylammonium chloride and nitrate salts react with (C5Me5)2UCl2 to expand the coordination sphere of the metallocene to form the formal 9- and 10-coordinate complexes, [NBu4][(C5Me5)2UCl2(NO3)], 1, and [NBu4][(C5Me5)2UCl2(NO3)], 2, respectively. Complex 2 displays modified reactivity compared to that of (C5Me5)2UCl2 in substitution reactions with Khpp [hpp =1,3,4,6,7,8-hexahydro-2H-pyrimido(1,2-a)pyrimidine] and K(NC4Me4) in the synthesis of (C5Me5)2U(hpp)Cl, (C5Me5)U(hpp)3, and (C5Me5)2U(NC4Me4)Cl. The U3+ complex, [NBu4][(C5Me5)2UCl2], can be formed by reduction of 2 with K(Hg), as well as with KC5Me5, K2C8H8, and Li3N.

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INTRODUCTION Recent studies of the tetraalkylammonium cyanide chemistry of uranium metallocenes by Ephritikhine and co-workers have led to spectacular results.1,2 It has been shown that cyanide can coordinate to uranium and open up the traditionally parallel rings in the linear bis(cyclooctatetraenyl)uranium structure of (C8H8)2U to form complexes with a bent (ring)−U−(ring) geometry, such as [NEt4][(C8H8)2U(CN)] (eq 1).1,2 These

earth salts since the alkyl groups provide enhanced solubility and the (NR4)+ cation is not as hard as the group I and II metal ions.4 In efforts to explore how tetraalkylammonium salt formation could lead to more crystalline or reactive organoactinide metallocenes, the reactivity of a common uranium metallocene precursor, (C5Me5)2UCl2, with tetrabutylammonium salts has been explored. Previous studies have shown that additional ligands can be added to the metal center of (C5Me5)2UCl2, and crystallographic information is available for (C5Me5)2UCl2(C3H4N2),5 (C5Me5)2UCl2(HNPPh3),6 and (C5Me5)2UCl2(NCMe).7 Expansion of the coordination sphere of uranium with tetrabutylammonium salts of nitrate and chloride is reported here as well as the modified reactivity observed with one of the resulting compounds, the metallocene trichloride, [NBu4][(C5Me5)2UCl3] compared to (C5Me5)2UCl2.

researchers have also used tetraalkylammonium cyanide to transform the traditionally bent bis(pentamethylcyclopentadienyl)uranium unit, [(C5Me5)2U]2+, into a complex with a linear (ring)−U−(ring) arrangement, for example, in [(C5Me5)2U(CN)5]3− (eq 2).2 In addition, tetraalkylammo-

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nium halide reagents have been used to transfer chloride to UCl4 to form [NBu4][UCl5].3 In all of these studies, the pseudohalide was delivered as the salt of a tetraalkylammonium cation, [NR4]+ (R = Et, Bu). Tetraalkylammonium salts often offer special chemistry compared to alkali metal and alkaline © 2016 American Chemical Society

EXPERIMENTAL SECTION

All syntheses and manipulations described below were conducted under argon with rigorous exclusion of air and water using a glovebox and vacuum line techniques. Solvents used were dried over columns containing Q-5 and molecular sieves. NMR solvents were dried over sodium−potassium alloy, degassed using three freeze−pump−thaw cycles, and vacuum-transferred before use. (C 5 Me 5 ) 2 UCl 2 , 8 (C5Me4H)2UCl2,9 (C5Me5)2UMe2,8 K(C4Me4N),10 Khpp (hpp = [1,3,4,6,7,8-hexahydro-2H-pyrimido(1,2-a)pyrimidine]), 11 and K2C8H812 were prepared according to the literature. Tetrabutylammonium chloride, tetrabutylammonium nitrate, and Li3N were purchased from Sigma-Aldrich and held under vacuum overnight before use in the glovebox. 1H NMR spectra were recorded with a Bruker DRX500 Received: November 12, 2015 Published: February 9, 2016 520

DOI: 10.1021/acs.organomet.5b00942 Organometallics 2016, 35, 520−527

Article

Organometallics

hpp, 2H), −38.1 (s, hpp, 2H), −43.1 (s, hpp, 2H), −78.6 (s, hpp, 2H), −88.6 (s, hpp, 2H). IR: 2945s, 2934s, 2889s, 2844s, 2815s, 2723w, 1618w, 1533s, 1515s, 1468m, 1449s, 1379s, 1356w, 1320m, 1291s, 1258w, 1274m, 1193s, 1137s, 1111w, 1069m, 1023m, 729s, 718s, 688w cm−1. Anal. Calcd for C31H51N9U: C, 47.26; H, 6.53; N, 16.00. Found: C, 46.89; H, 6.50, N, 15.55. (C5Me5)2U(NC4Me4)Cl, 4. K(NC4Me4) (2 mg, 0.01 mmol) was added to a stirred red solution of 2 (10 mg, 0.012 mmol) in 2 mL of benzene. The stirred solution turned from orange to light red within 5 min. The mixture was centrifuged, and white solids were removed via filtration. Solvent was removed under reduced pressure, and the resulting solids were washed with n-hexane and dried under vacuum to yield (C5Me5)2U(NC4Me4)Cl, 4 (9 mg, 80%), as a red microcrystalline powder. 1H NMR (C6D6): δ 16.2 (s, C5Me5, 30H), −6.5 (s, NC4Me4, 3H), −10.2 (s, NC4Me4, 3H) −47.7 (s, NC4Me4, 3H), −103.3 (s, NC4Me4, 3H). IR: 2977s, 2905s, 2855s, 2726m, 1437w, 1378w, 1254s, 1142w, 1021w, 957m, 743s cm−1. Anal. Calcd for C28H42NClU: C, 50.49; H, 6.36; N, 2.10. Found: C, 50.77; H, 6.30, N, 1.88. (C5Me5)2U(κ2-O,O′-O2CNC4Me4)Cl, 5. A red 20 mL toluene solution of 4 (109 mg, 0.165 mmol) was degassed in a side-armsealable Schlenk flask via three freeze−pump−thaw cycles. An atmosphere of CO2 was introduced, and after the flask was sealed, the bottom part of the flask was placed in an 80 °C oil bath while the top half was cooled with a fan to protect the stopcock from the heat. Over 3 days, the solution turned from red to a bright red-orange. The solvent was removed under vacuum and yielded a microcrystalline redpink solid. After n-pentane extraction, centrifugation, filtration, and removal of solvent under reduced pressure, single crystals of 5 (96 mg, 83%) suitable for X-ray analysis were obtained from toluene solutions at −35 °C. 1H NMR (C6D6): δ 9.2 (s, C5Me5, 30H), −1.2 (s, NC4Me4, 6H). IR: 2979s, 2908s, 2859s, 1451w, 1378w, 1290w, 1170m, 863m, 747s, 479 cm−1. Anal. Calcd for C29H42ClNO2U: C, 49.05; H, 5.96; N, 1.97. Found: C, 48.61; H, 6.15; N, 2.30. [NBu4][(C5Me5)2UCl2], 6. An amalgam of Hg and 1% K by weight (15 mg K, 0.38 mmol) was added to a stirred 10 mL toluene solution of [NBu4][(C5Me5)2UCl3] (48 mg, 0.056 mmol). Over a period of 5 h, the solution slowly became dark green. Centrifugation and filtration removed insoluble solids as well as Hg, and the solvent was removed from the supernatant under reduced pressure to yield [NBu4][(C5Me5)2UCl2], 6, as a dark green microcrystalline solid (25 mg, 55% yield). 1H NMR (C6D6): δ −1.3 [s, N(CH2CH2CH2CH3)4, 8H], −2.9 [s, N(CH2CH2CH2CH3)4, 12H], −4.5 (s, C5Me5, 30H), −6.4 [s, N(CH2CH2CH2CH3)4, 8H], −8.2 [s, N(CH2CH2CH2CH3)4, 8H]. Anal. Calcd for C36H66NCl2U: C, 52.61; H, 8.09; N, 1.70. Found: C, 52.98; H, 7.51, N, 1.44. Flat crystalline squares suitable for X-ray crystallographic study were grown from toluene but gave data that indicated a disorder that could not be resolved. Complex 6 can also be made by several alternative methods. KC5Me5 (3 mg, 0.02 mmol) was quickly added to a stirred 2 mL solution of 2 (15 mg, 0.017 mmol) in C6H6. Over 5 min, the bright orange solution turned to brown, black, and then dark green and was stirred for a total of 2 h. Centrifugation and filtration removed white insoluble solids, and the supernatant was dried under reduced pressure to yield 6 (10 mg, 82%) as a microcrystalline green solid as confirmed by 1H NMR spectroscopy. (C5Me5)2 was also identified in the 1H NMR spectrum. K2C8H8 (2 mg, 0.01 mmol) was quickly added to a 2 mL solution of 2 (8 mg, 0.009 mmol) in C6H6 stirred in a scintillation vial. Over 5 min, the bright orange solution turned dark green and was stirred for a total of 1 h. Centrifugation and filtration removed white solids, and the supernatant was dried under reduced pressure to yield 6 (5 mg) as a microcrystalline green solid, as confirmed by 1H NMR spectroscopy in C6D6. Resonances for free C8H8 were also observed. A J-Young NMR tube containing 2 (15 mg, 0.017 mmol) and insoluble Li3N (1 mg, 0.03 mmol) in ca. 0.3 mL of C6D6 was sonicated in a water bath that reached 40 °C during operation. After 12 h, the solution was green and 1H NMR spectroscopy showed only 6 and no 2. The reaction could also be done with stirring at room temperature

MHz spectrometer. Infrared spectra were recorded as KBr pellets on a Varian 1000 FT-IR Scimitar series spectrometer. Elemental analyses were performed on a PerkinElmer 2400 Series II CHNS analyzer. [NBu4][(C5Me5)2UCl2(NO3)], 1. [NBu4][NO3] (52 mg, 0.17 mmol) was added to a stirred red solution of (C5Me5)2UCl2 (95 mg, 0.16 mmol) in 10 mL of benzene. Over a period of 3 h, the solution slowly became bright orange. Centrifugation and filtration removed a small amount of green insoluble solids and yielded a bright orange supernatant. The solids obtained by removal of solvent under reduced pressure were washed three times with methylcyclohexane a n d d r i e d t o y i e ld o r a n g e m ic r o c r y s t a l l i n e [ N B u 4 ] [(C5Me5)2UCl2(NO3)], 1 (136 mg, 97% yield). 1H NMR (C6D6): δ 14.9 (s, C5Me5, 30H), −1.8 [s, N(CH2CH2CH2CH3)4, 8H], −4.0 [s, N(CH2CH2CH2CH3)4, 12H], −8.0 [s, N(CH2CH2CH2CH3)4, 8H], −10.7 [s, N(CH2CH2CH2CH3)4, 8H]. IR: 3458w, 2964s, 2897s, 2718w, 1512m, 1489s, 1474s, 1380w, 1299m, 1038w, 1022w, 890w, 748m, 732w cm−1. Anal. Calcd for C36H66Cl2N2O3U: C, 48.92; H, 7.53; N, 3.17. Found: C, 48.55; H, 7.36; N, 2.95. Orange rod-shaped crystals suitable for X-ray crystallographic study were grown from toluene at −30 °C. [NBu4][(C5Me5)2UCl3], 2. [NBu4][Cl] (145 mg, 0.523 mmol) was added to a stirred solution of (C5Me5)2UCl2 (304 mg, 0.525 mmol) in 20 mL of benzene. Over a period of 12 h, the solution slowly became dark orange. Centrifugation and filtration removed green insoluble material and yielded a bright orange supernatant. The solids obtained by removal of solvent under reduced pressure were washed three times with methylcyclohexane and dried to give [NBu4][(C5Me5)2UCl3], 2, as a bright orange microcrystalline solid (364 mg, 81% yield). 1H NMR (C6D6): δ 3.82 (s, C5Me5, 30H), 0.7 [s, N(CH2CH2CH2CH3)4, 8H], −1.2 [s, N(CH 2 CH 2 CH 2 CH 3 ) 4 , 12H], −1.8 [s, N(CH2CH2CH2CH3)4, 8H], −2.3 [s, N(CH2CH2CH2CH3)4, 8H]. IR: 3981w, 2964s, 2877s, 2719w, 1492m, 1470s, 1377m, 1168w, 1062w, 1022m, 891m, 733m, 696w cm−1. Anal. Calcd for C36H66Cl3NU: C, 50.44; H, 7.76; N, 1.63. Found: C, 50.01; H, 7.55; N, 1.88. Orange rod-shaped crystals suitable for X-ray crystallographic study were grown from toluene at −30 °C. (C5Me5)2U(hpp)Cl.13 Khpp (40 mg, 0.23 mmol) was added to a stirred solution of 2 (195 mg, 0.227 mmol) in 2 mL of toluene. Over 10 min, the solution changed from bright orange to yellow. After being stirred for 1 h, the solution was centrifuged and filtered to remove white solids. Removal of solvent from the supernatant under reduced pressure yielded yellow microcrystalline solids. 1H NMR spectroscopy of the solids showed the consumption of the starting material and the formation of only the known compound (C5Me5)2U(hpp)Cl13 (141 mg, 91%). (C5Me5)U(hpp)3, 3. Khpp (10 mg, 0.28 mmol) was added to a stirred solution of 2 (50 mg, 0.058 mmol) in 3 mL of toluene. Over 10 min, the solution turned from bright orange to yellow and then nearly colorless. After being stirred for 1 h, the solution was centrifuged and filtered to remove white solids. Removal of solvent from the supernatant via reduced pressure yielded gray microcrystalline solids, which in some experiments appeared to have a green or purple tint. The solids were extracted with n-hexane, the mixture was centrifuged and filtered, and the solvent was removed under reduced pressure. Green and purple crystals of (C5Me5)U(hpp)3 were grown from toluene at −35 °C and identified by X-ray crystallography. The green crystals, the purple crystals, and even crystals with a green core and a purple outer shell gave the same unit cell for 3. The 1H NMR spectrum at room temperature contains many broad features assignable to the methylene protons of the (hpp)− ligands and a sharp signal at 2.7 ppm assignable to (C5Me5)−. The signals sharpen and shift as the NMR sample is cooled, consistent with dynamic behavior of the (hpp)1− ligands. At 206 K with a window of 450 to −450 ppm, 16 resonances [of integrated area 2 compared to 15 for the (C5Me5)− ligand] attributable to methylene groups were observable [18 methylene resonance would be expected if all the (hpp)− ligands were unique]. 1H NMR (C6D6, 206 K): δ 55.3 (s, hpp, 2H), 25.2 (s, hpp, 2H), 8.4 (s, hpp, 2H), 6.4 (s, hpp, 2H), 6.0 (s, hpp, 2H), 3.6 (s, C5Me5, 15H), −1.0 (s, hpp, 2H), −4.6 (s, hpp, 2H), −6.2 (s, hpp, 2H), −6.8 (s, hpp, 2H), −7.4 (s, hpp, 2H), −23.1 (s, hpp, 2H), −29.0 (s, 521

DOI: 10.1021/acs.organomet.5b00942 Organometallics 2016, 35, 520−527

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Organometallics

Table 1. X-ray Data and Collection Parameters for [NBu4][(C5Me5)2UCl2(NO3)], 1, [NBu4][(C5Me5)2UCl3], 2, (C5Me5)U(hpp)3, 3, (C5Me5)2U(κ2-O,O′-O2CNC4Me4)Cl, 5, [NBu4][(C5Me4H)2UCl2], 8, and [NBu4]2{[(C5Me5)UCl2]2(μCl)2(μ-O)}, 9 compound

1

2

empirical formula temperature (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z ρcalcd (Mg/m3) μ (mm−1) R1a (I > 2.0σ(I)) wR2 (all data)

C36H66Cl2N2O3U·1/2(C7H8)

C36H66Cl3NU·1/2(C7H8)

88(2)

a

3

5

8

9

C31H51N9U

C29H42ClNO2U

C34H62Cl2NU

C52H102Cl6N2OU2

143(2)

143(2)

88(2)

143(2)

143(2)

triclinic P1̅ 11.2214(5) 11.7908(5) 17.7903(8) 77.9253(5) 88.4904(5) 65.5842(5) 2091.12(16) 2 1.477 4.045 0.0202

triclinic P1̅ 11.3610(7) 11.5361(7) 17.3158(10) 79.0585(6) 89.4727(6) 66.6659(6) 2040.5(2) 2 1.470 4.200 0.0167

orthorhombic Pbca 18.9445(10) 16.9108(9) 19.7306(10) 90 90 90 6321.0(6) 8 1.656 5.172 0.0187

triclinic P1̅ 8.4285(13) 10.3929(16) 17.363(3) 94.9062(18) 95.2116(18) 111.1593(17) 1401.0(4) 2 1.683 5.913 0.0331

monoclinic P21/n 22.7263(18) 12.3259(10) 25.706(2) 90 97.0365(10) 90 7146.6(10) 8 1.475 4.713 0.0244

monoclinic C2/c 21.0303(12) 16.6868(10) 17.6872(10) 90 103.9622(7) 90 6023.6(6) 4 1.610 5.671 0.0284

0.0486

0.0410

0.0259

0.0836

0.0552

0.0695

Definitions: R1 = ∑∥F0| − |Fc∥/Σ|F0|, wR2 = [∑w(F02 − Fc2)2/∑w(F02)2]1/2.

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without sonication, but it took over 4 days to reach completion by 1H NMR spectroscopy. [NBu4][(C5Me4H)2UCl2], 8. [NBu4][Cl] (114 mg, 0.410 mmol) was added to a stirred solution of (C5Me4H)2UCl2 (226 mg, 0.407 mmol) in 20 mL of C6H6. After 2 h, the solution turned bright orange. After centrifugation, filtration, and removal of solvent under reduced pressure, a product with spectroscopic features consistent with “[NBu4][(C5Me4H)2UCl3]” was isolated as a bright orange semicrystalline solid (302 mg). 1H NMR (C6D6): δ 8.2 (s, C5Me4H, 12H), 3.42 (s, C5Me4H, 12H), −0.5 [s, N(CH2CH2CH2CH3)4, 8H], −1.0 [s, N(CH2CH2CH2CH3)4, 12H], −1.5 [s, N(CH2CH2CH2CH3)4, 8H], −2.0 [s, N(CH2CH2CH2CH3)4, 8H], −52.5 (s, C5Me4H, 2H). An amalgam of Hg and 1% K by weight (15 mg K, 0.38 mmol) was added to a stirred 10 mL toluene solution of “[NBu4][(C5Me4H)2UCl3]” (45 mg, 0.054 mmol). Over a period of 5 h, the solution slowly became dark green. Centrifugation and filtration removed insoluble solids as well as Hg, and the solvent was removed from the supernatant under reduced pressure to yield [NBu4][(C5Me4H)2UCl2], 8 (33 mg, 76%), as a dark green, microcrystalline solid. Crystals of 8 suitable for X-ray crystallographic study were grown from toluene. 1H NMR (C6D6): δ 12.3 (s, C5Me4H, 12H), −0.1 [s, N(CH2CH2CH2CH3)4, 12H], −0.5 [s, N(CH2CH2CH2CH3)4, 8H], −3.2 [s, N(CH2CH2CH2CH3)4, 8H], −3.9 [s, N(CH2CH2CH2CH3)4, 8H], −21.4 (s, C5Me4H, 12H), −55.9 (s, C5Me4H, 2H). IR: 3062w, 2963s, 2934s, 2908s, 2877s, 2723w, 1472s, 1461m, 1445m, 1381m, 1319w, 1169w, 1067w, 1031w, 887, 766s, 739m cm−1. Anal. Calcd for C34H62Cl2NU: C, 51.44; H, 7.87; N, 1.76. Found: C, 51.84; H 8.23, N 1.83. [NBu4]2{[(C5Me5)UCl2]2(μ-Cl)2(μ-O)}, 9. Complex 9 was identified crystallographically as a decomposition product formed from solutions of 1 and 2. The insolubility of 9 in alkane, arene, and ethereal solvents precluded further characterization other than elemental analysis. Solutions of 1 or 2 left standing at either room temperature or at −35 °C quantitatively form crystalline 9 as green crystalline blocks. 1H NMR spectroscopy of the solutions shows the presence of both C5Me5H and (C5Me5)2. Anal. Calcd for C52H102Cl6N2OU2: C, 42.77; H, 7.04; N, 1.92. Found: C, 42.74; H 6.95, N, 1.69. Single crystals that deposited from solutions of 2 overnight were of sufficient quality for X-ray diffraction analysis. X-ray Crystallographic Data. Crystallographic information for complexes 1, 2, 3, 5, 8, and 9 is summarized in Table 1 and in the Supporting Information.

RESULTS [NBu4][(C5Me5)2UCl2(NO3)], 1. Initial attempts to add a tetrabutylammonium salt to (C5Me5)2UCl2 used nitrate as the anion to take advantage of the oxophilicity of uranium. [NBu4][NO3] reacts with (C5Me5)2UCl2 in benzene to form [NBu4][(C5Me5)2UCl2(NO3)], 1, which was identified by Xray crystallography (eq 3 and Figure 1). The 1H NMR

Figure 1. Thermal ellipsoid plot of [NBu4][(C5Me5)2UCl2(NO3)], 1, drawn at the 50% probability level. Hydrogen atoms, cocrystallized lattice solvent, and the [NBu4]+ cation are omitted for clarity.

spectrum of 1 displays a single resonance for the [(C5Me5)2U]2+ component and a characteristic set of four resonances for the [NBu4]+ cation (Table 2). The [NBu4]+ resonances are shifted from those of [NBu4][Cl] at 3.4, 1.7, 1.5, and 1.0 ppm due to the paramagnetism of the U4+ ion. The solid-state structure of 1 (Figure 1 and Table 3) retains its bent metallocene character with a (C5Me5 ring centroid)− 522

DOI: 10.1021/acs.organomet.5b00942 Organometallics 2016, 35, 520−527

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Organometallics Table 2. Resonances (ppm) in the 1H NMR Spectra of [NBu4][(C5Me5)2UCl2(NO3)], 1, [NBu4][(C5Me5)2UCl3], 2, [NBu4][(C5Me5)2UCl2], 6, [NBu4][(C5Me4H)2UCl3], 7, and [NBu4][(C5Me4H)2UCl2], 8 compound

[(C5Me4R)2U]2+

{N[(CH2)3CH3]4}+

{N[(CH2)3CH3]4}+

1 2 1+2 6 7 8

14.9 3.8 14.9/3.8 −4.5 8.2/3.42/−52.5 12.3/−21.4/−55.9

−1.8/−4.0/−8.0 0.7/−1.2/−1.8 −3.5/−6.4/−6.8 −1.3/−2.9/−6.4 −0.5/−1.5/−2.0 −0.5/−3.2/−3.9

−10.7 −2.3 −1.9 −8.2 −1.0 −0.1

Table 3. Selected Bond Distances (Å) and Angles (deg) for [NBu4][(C5Me5)2UCl2(NO3)], 1, [NBu4][(C5Me5)2UCl3], 2, (C5Me5)2UCl2,8 (C5Me5)2UCl2(C3H4N2),5 (C5Me5)2UCl2(HNPPh3),6 [NBu4][(C5Me4H)2UCl2], 8, and (C5Me4H)2UCl29 compound

U−(C5Me5 centroid)

[NBu4][(C5Me5)2UCl2(NO3)], 1

2.503/2.510

[NBu4][(C5Me5)2UCl3], 2

2.492/2.501

(C5Me5)2UCl2 (C5Me5)2UCl2(C3H4N2) (C5Me5)2UCl2(HNPPh3) [NBu4][(C5Me4H)2UCl2], 8 (C5Me4H)2UCl2

2.47 2.548 2.49(4) 2.505/2.497 2.418

U−O/N

U−Cl

(C5Me5 ring centroid)−U−(C5Me5 ring centroid)

Cl−U−Cl

2.510(2)/ 2.540(2)

2.6993(6)/2.6996(6)

130

145.72

133.4

153.85

2.607(8) 2.43(1)

2.6850(5)/2.6912(5)/ 2.6915(5) 2.583(8) 2.696(2) 2.730(4)/2.658(4) 2.7168(7)/2.6966(7) 2.5909(7)

132 137.1 133.8 127.0 133.1

97.4(9) 148.29(8) 151.0(1) 100.3 99.79(3)

U−(C5Me5 ring centroid) angle of 130.0° compared to 132° in (C5Me5)2UCl2.15 The nitrate ion adds into the coordination sphere between the two chloride ligands with the plane of the UONO ring forming an 89° angle with the plane of uranium and the two chloride ligands. A similar structural motif is found in (C5Me5)2UCl2(C3H4N2)5 and (C5Me5)2UCl2(HNPPh3).6 Addition of nitrate enlarges the Cl−U−Cl angle of 97.9(4)° in (C5Me5)2UCl2 to 145.72(2)° in 1, which is similar to the 148.29(8) and 151.0(1)° angles in (C5Me5)2UCl2(C3H4N2) and (C5Me5)2UCl2(HNPPh3), respectively. Complex 1 also has longer U−Cl bonds, 2.6993(6) and 2.6996(6) Å, compared to 2.583(6) Å in (C5Me5)2UCl2.15 The pyrazole and HNPPh3 adducts also have long 2.658(4) to 2.730(4) Å U−Cl distances. [NBu4][(C5Me5)2UCl3], 2. To determine if chloride or nitrate could be selectively removed from 1 in subsequent reactions, reaction of 1 with NaBPh4 was examined. Since this gave mixtures of dichloride and chloride nitrate products, reactions of (C5Me5)2UCl2 with [NBu4][Cl] were next examined to obtain a complex with the same three inorganic ligands. This combination produces an analogue of 1, [NBu4][(C5Me5)2UCl3], 2, which was also identified by X-ray crystallography (eq 4 and Figure 2).

Figure 2. Thermal ellipsoid plot of [NBu4][(C5Me5)2UCl3], 2, drawn at the 50% probability level. Hydrogen atoms, cocrystallized lattice solvent, and the [NBu4]+ cation are omitted for clarity.

[NBu4][Cl] as a reversible coordinating agent. Like 1, complex 2 displayed a characteristic set of four resonances for the [NBu4]+ cation (Table 2), again shifted by the paramagnetism of the U4+ ion but different from those of 1. Since a 1:1 mixture of 1 and 2 gave four resonances with values intermediate between those of 1 and 2, some ion pairing and cation exchange appears to occur in solution. The solid-state structure of 2 is similar to that of 1, as shown in Table 3. Both the (C5Me5 ring centroid)−U−(C5Me5 ring centroid) and Cl−U− Cl angles in the chloride complex 2 are larger than those in the nitrate complex 1. (C5Me5)2U(hpp)Cl and (C5Me5)U(hpp)3, 3. Reactivity d ifferen ces bet ween (C 5 Me 5 ) 2 UCl 2 and [NBu 4 ][(C5Me5)2UCl3], 2, were examined with a variety of reagents. The most definitive differences were identified in reactions with Khpp [hpp =1,3,4,6,7,8-hexahydro-2H-pyrimido(1,2-a)pyrimidine]. (C5Me5)2UCl2 reacts with Khpp to generate (C5Me5)2U(hpp)Cl,13 but the reaction requires 12 h. In contrast, 2 reacts with 1 equiv of Khpp to form (C5Me5)2U(hpp)Cl in just 5 min. Furthermore, reaction of 2 with 3 equiv of Khpp generates the monocyclopentadienyl complex, (C5Me5)U(hpp)3, 3 (eq 6), a complex that had not been

One equivalent of [NBu4][Cl] was easily removed from 2 using NaBPh4 (eq 5), demonstrating the facile use of

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to the C5Me5 ring centroid, with a (C5Me5 ring centroid)−U− N2 angle of 164.7°. This arrangement is similar to that found in the mono(pentamethylcyclopentadienyl)uranium complex, (C5Me5)U[η3-CH2C(Me)CH2]3,16 but differs from that in (C5Me5)U(η2-CH2Ph)317 (Figure 4). In (C5Me5)U[η3-CH2C(Me)CH2]3, the trans-carbon has a 2.71(2) Å U−C distance at the long end of the range of other U−C(allyl) distances in the molecule, 2.61(1)−2.70(2) Å. This contrasts with 3, where the 2.380(2) Å trans U−N distance is much shorter than all of the other U−N distances, 2.439(2)−2.472(2) Å. This difference does not translate into the hpp C−N distances involving the nitrogen bound to uranium: these are all the same within experimental error. For comparison, the U−N(hpp) distances in (C5Me5)2U(hpp)X (X = Cl, I, N3, CCPh, Ph),11,13 which are all nine-coordinate U4+ species like 3, have a range of 2.330(2)−2.412(2) Å. [(C5Me5)2(hpp)U]2(μ-Se)11 has longer U−N(hpp) lengths, 2.465(2)/2.472(2) Å, but this may be due to steric crowding. (C5Me5)2U(NC4Me4)Cl, 4. Modified reactivity of 2 versus (C5Me5)2UCl2 was also observed in K(NC4Me4) substitution reactions. While the reaction of (C5Me5)2UCl2 and K(NC4Me4) to form (C5Me5)2U(NC4Me4)Cl, 4, requires 4−5 equiv of K(NC4Me4) and a reaction time of several days, complex 2 reacts with 1 equiv of K(NC4Me4) to form 4 in 10 min (eq 7). Although 4 was characterized by NMR spectroscopy and elemental analysis, crystals suitable for X-ray crystallography were elusive.

observed in Khpp reactions with (C5Me5)2UCl2.11 Attempts to prepare 3 from (C5Me5)2UCl2 with 3 equiv of Khpp were not successful. (C5Me5)2U(hpp)Cl was initially observed by 1H NMR spectroscopy in these 3 equiv reactions as well as other unidentified products, but no 3 was observed. Complex 3 was identified by X-ray crystallography (Figure 3). The 1H NMR spectrum of 3 contained a complex array of

Figure 3. Thermal ellipsoid plot of (C5Me5)U(hpp)3, 3, drawn at the 50% probability level, with hydrogen atoms omitted for clarity. Relevant bond distances (Å) and angles (deg) are as follows: U− (C5Me5 centroid), 2.491; U−N1/N2, 2.472(2)/2.380(2); U−N4/N5, 2.469(2)/2.439(2); U−N7/N8, 2.448(2)/2.443(2); Cnt−U1−N1 110.0; Cnt−U1−N2 164.7; Cnt−U1−N4 99.8; Cnt−U1−N5 103.0; Cnt−U1−N7 100.1; Cnt−U1−N8 103.3; U out of N4/5/7/8 plane distance, 0.44.

To obtain structural data on this system, synthesis of a more crystalline derivative of 4 was pursued. Complex 4 reacts only slowly with CO2 (1 atm) at room temperature, but at 80 °C, the carbamate complex (C5Me5)2U(κ2-O,O′-O2CNC4Me4)Cl, 5, forms over 3 days by formal CO2 insertion into the U−N bond (eq 8). Although insertion of CO2 into U−C linkages has been frequently reported,14,18−20 CO2 insertion into U−N bonds is not as common.21,22 Crystallographic data on 5 (Figure 5) revealed a κ2-bound carbamate with nearly equal O(1)−C(21) and O(2)−C(21) distances of 1.274(5) and 1.262(5) Å, respectively, and O(1)− C(21)−O(2) and O(1)−C(21)−N(1) angles of 120.3(4) and

resonances attributable to the many methylene environments possible for the three (hpp)− ligands. Upon cooling, these resonances are resolved, as described in the Experimental Section to give 16 of the expected 18 methylene resonances for three unique hpp environments (see Supporting Information, Figure S1). In the solid-state structure of 3, the three (hpp)− ligands are structurally similar, and each attaches to uranium through two nitrogen atoms. However, the central (hpp)− has a (U−N1−C11−N2) ring that is roughly perpendicular to the UNCN rings of the other (hpp)− ligands (74° to U−N4− C18−N5; 80° to U−N7−C25−N8). This puts N2 nearly trans

Figure 4. Thermal ellipsoid plot of (C5Me5)U(X) {X = [η3-CH2C(Me)CH2]3, (η2-CH2Ph)3}, drawn at the 50% probability level, with hydrogen atoms omitted for clarity. 524

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Figure 6. Thermal ellipsoid plot drawn at the 50% probability level of the anion in [NBu4][(C5Me4H)2UCl2], 8. Hydrogen atoms have been removed for clarity. Only one of two molecules in the unit cell is shown.

U3+ versus U4+ according to Shannon, 0.135 Å.25 The U−Cl bonds of 8 are very similar to the 2.6850(5)/2.6912(5)/ 2.6915(5) Å values of 2, a result that is likely due to the opposing effects of a decreased oxidation state and smaller coordination number in 8. Interestingly, the 127.0° (C5Me5 ring centroid)−U−(C5Me5 ring centroid) angle of 8 is smaller than those of 1, 2, (C5Me4H)2UCl2, and (C5Me5)2UCl2: 130.0, 133.4, 133.1, and 132.0°, respectively. A Complex of a Bimetallic Oxide Bridged Dianion, [NBu4]2{[(C5Me5)UCl2]2(μ-Cl)2(μ-O)}, 9. Consistent with the higher reactivity of 1 and 2 described above, they are also observed to decompose in the absence of an external substrate in solution at room temperature over 2 days. The singlecrystalline product that is isolated in these reactions is also a tetrabutylammonium salt, [NBu4]2{[(C5Me5)UCl2]2(μ-Cl)2(μO)}, 9 (Figure 7). This is described here since oxide

Figure 5. Thermal ellipsoid diagram of (C5Me5)2U(κ2-O,O′O2CNC4Me4)Cl, 5, drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity.

119.2(4)°, respectively. Fully characterized CO2 insertions into U−N bonds were previously reported with the polydentate amides of Meyer et al.: (L)UIV−N(H)Ph [L = 1,4,7-tris(3,5-ditert-butyl-2-hydroxybenzylate)-1,4,7-triazacyclononane and its adamantyl analogue].22 The N−C(CO2) distance of 1.383(6) Å in 5 is nearly the same as that reported by Meyer (1.38 Å), and the pyrrolyl ring in 5 has localized C−C bonds: the C22− C23 and C24−C25 distances are 1.355(7)/1.366(7) Å compared to 1.444(6) Å for C23−C24. [NBu4][(C5Me5)2UCl2], 6. Given the modified reactivity of 2 versus (C5Me5)2UCl2 with Khpp and K(NC4Me4), a reaction with KC5Me5 was examined to see if 2 would provide a better route to the sterically crowded complex, (C5Me5)3UCl,23 which traditionally requires four synthetic steps beyond (C5Me5)2UCl2. Instead of substitution, however, reduction to form the U3+ salt, [NBu4][(C5Me5)2UCl2], 6, shown in eq 9,

Figure 7. Thermal ellipsoid diagram plotted at the 50% probability level of [NBu4]2{[(C5Me5)UCl2]2(μ-Cl)2(μ-O)}, 9. Hydrogen atoms and the [NBu4]+ cations are omitted for clarity. Selected bond distances (Å) and angles (deg): U−Cl1, 2.812(1); U−Cl1b, 2.902(1); U−Cl2, 2.674(1); U−Cl3, 2.678(2); U−O1, 2.087(2); U···U, 3.603; U−(C5Me5 centroid), 2.481 Å.

was observed. Marks and co-workers showed that (C5Me5)2UCl2 can be reduced with Na(Hg) to [Na(THF)x][(C5Me5)2UCl2],24 so the reduction product is not unusual. However, KC5Me5 does not react with (C5Me5)2UCl2. Reduction of 2 to form 6 was also observed with K2C8H8 and Li3N. Once 6 was identified, its synthesis with a more conventional reducing agent, K(Hg), was performed (eq 9). [NBu4][(C5Me4H)2UCl3], 7, and [NBu4][(C5Me4H)2UCl2], 8. Since several batches of crystals of 6 gave X-ray data that showed disordered structures, the synthesis of the tetramethylcyclopentadienyl analogue was examined. As shown in eq 9, [NBu4][(C5Me4H)2UCl2], 8 (Figure 6), can be made by K(Hg) reduction of [NBu4][(C5Me4H)2UCl3], 7, which was synthesized in analogy to 2. The structure of 8 is compared to that of 2 and (C5Me4H)2UCl2 in Table 3. The U−Cl bond lengths in 8, 2.7168(7)/2.6966(7) Å, are larger than those in (C5Me4H)2UCl2, 2.5909(7) Å, by the difference in ionic radii of

decomposition products are often difficult to define while still in a molecular state even though oxide formation is common.26−44 The mother liquor contains (C5Me5)2 as well as HC5Me5 identified by 1H NMR spectroscopy. The (X2U)2(μ-X)3 framework found in 9 has been observed with X = Cl in [(dmpe)Cl3U(μ-Cl)3U(dmpe)Cl(μ-Cl)]2,45 {[(C 6 Me 6 )Cl 2 U] 2 (μ-Cl) 3 }(AlCl 4 ), 4 6 (C 6 Me 6 )Cl 2 U(μCl)3UCl2(μ-Cl)3UCl2(C6Me6),47 and [C(PPh2NSiMe3)2]U(Cl)(μ-Cl)3(Cl)U[HC(PPh2NSiMe3)2].48 In 9, one of the seven X ligands is an (O)2− ion, and the overall charge of the anion is −2. The 2.087(2) Å U−(μ-O1) bridging distance is much shorter than the 2.812(1) Å U−(μ-Cl1) and 2.9017(1) Å 525

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U−(μ-Cl1b) distances, leading to a much more acute U−O−U angle, 78.18(3)°, versus 119.4(2)° for U−Cl1−U. The 3.6033(4) Å U···U distance in the dianion is shorter than the 3.934(1) Å distance in the cation in {[(C6Me6)Cl2U]2(μCl)3}(AlCl4)46 and the 4.031(1) and 4.035(1) Å distances in (C6Me6)Cl2U(μ-Cl)3UCl2(μ-Cl)3UCl2(C6Me6).47 The 2.481 Å U−(C5Me5 ring centroid) distance is in the range of values in Table 3.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences of the Department of Energy (DE-SC0004739) for support, and Jordan F. Corbey for crystallographic assistance.

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DISCUSSION Tetrabutylammonium nitrate and chloride salts readily add to the metallocene dichloride, (C5Me5)2UCl2, to make the anionic adducts [NBu4][(C5Me5)2UCl2(NO3)], 1 (eq 3), and [NBu4][(C5Me5)2UCl3], 2 (eq 4), respectively. This converts the neutral metallocene to salts with different solubility and may be a generally useful technique to make crystalline derivatives of neutral complexes that are difficult to structurally characterize. Another benefit from making these salts is that addition of another anion to the metallocene coordination sphere modifies the reactivity, as evidenced by reactions of 2 versus (C5Me5)2UCl2. In reactions with 1 equiv of Khpp and 1 equiv of K(NC4Me4), complex 2 undergoes substitution faster than (C 5 Me 5 ) 2 UCl 2 to form (C 5 Me 5 ) 2 U(hpp)Cl and (C5Me5)2U(NC4Me4)Cl, 4, respectively. The reasons for this change in reactivity may be complicated. Expanding the coordination sphere by adding chloride to (C5Me5)2UCl2 causes all the U−Cl bonds to increase in length. These longer bonds could be more labile, but adding the extra ligand could also inhibit reactivity by filling up the coordination sphere of the metal. Reactions of 2 with 3 equiv of Khpp produce a new uranium hpp complex, (C5Me5)U(hpp)3, 3 (eq 6), that is not observed in reactions with (C5Me5)2UCl2. This difference is even more difficult to rationalize since it involves the unusual loss of a (C5Me5)1− ligand. The facile reduction of 2 to [NBu4][(C5Me5)2UCl2] by KC5Me5 compared to the absence of reaction between (C5Me5)2UCl2 and KC5Me5 is also unusual since addition of another anionic ligand to (C5Me5)2UCl2 might be expected to diminish its reducibility.

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REFERENCES

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CONCLUSION The simple addition of [NBu4][Cl] to uranium metallocenes can lead to new opportunities to manipulate reactivity and should be more widely examined. In a field where achieving the targeted chemistry often requires the laborious process of investigating new ancillary ligands and increasing yields by careful adjustment of detailed reaction conditions, the ability to modify the reactivity and solubility of a product in such a facile manner is worth broader investigation.

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Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00942. X-ray data collection, structure solution and refinement (PDF) X-ray data for 1, CCDC 1435666 (CIF) X-ray data for 2, CCDC 1435667 (CIF) X-ray data for 3, CCDC 1435671 (CIF) X-ray data for 5, CCDC 1435668 (CIF) X-ray data for 8, CCDC 1435670 (CIF) X-ray data for 9, CCDC 1435669 (CIF) 526

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