Article pubs.acs.org/Organometallics
Reactivity of U3+ Metallocene Allyl Complexes Leads to a NanometerSized Uranium Carbonate, [(C5Me5)2U]6(μ‑κ1:κ2‑CO3)6 Christopher L. Webster, Joseph W. Ziller, and William J. Evans* Department of Chemistry, University of California, Irvine, California 92697-2025, United States S Supporting Information *
ABSTRACT: The U3+ allyl complexes (C5Me5)2U[CH2C(R)CH2] (R = H, Me) display three different types of reactivity, as exemplified by reactions with PhNNPh, cyclooctatetraene, and CO2. Two equivalents of (C5Me5)2U[CH2C(R)CH2] effect a four-electron reduction of PhN NPh to form the bis(imido) complex (C5Me5)2U(NPh)2 and the bis(allyl) species (C5Me5)2U[CH2C(R)CH2]2. Twoelectron reduction of C8H8 occurs to form (C5Me5)(C8H8)U[CH2C(R)CH2] products that contain only one cyclopentadienyl ring per metal. With CO2 at 80 psi, both reduction and insertion occur. A hexametallic uranium carbonate, [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, is isolated as well as the bis(carboxylate) complexes (C5Me5)2U[κ2-O,O′-O2CCH2C(R)CH2]2. The polymetallic carbonate complex can also be synthesized from [(C5Me5)2U]2(μ-η6:η6-C6H6) and [(C5Me5)2U][(μPh)2BPh2] and CO2.
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The U4+ allyl metallocenes (C5Me5)2U[CH2C(R)CH2]2 display η1-alkyl reactivity with formal insertion of CO2, as shown for R = H in eq 1.30 It was of interest to determine how this allyl reactivity would couple with the reduction chemistry of U3+.
INTRODUCTION Organometallic complexes of U3+ have proven to be excellent sources of new uranium chemistry and have led to an extensive collection of uranium-based transformations.1−30 The reducing nature of U3+, coupled with its Lewis acidity and the opportunity to access products with U4+, U5+, and U6+ oxidation states, has led to unique types of small-molecule activation2,3 with substrates such as N2,6−10 CO,9,11−15 CO2,9,11,16−27 and NO.28,29 Carbon dioxide, one of the substrates in the present study, provides a good example of U3+ reactivity. CO2 is readily converted into CO and uranium oxide products by many U3+ complexes,9,11,16−27 including early studies with (C5H4SiMe3)3U.26 Tris(aryloxide)-substituted triazacyclononane U3+ complexes formed oxide, carbonyl, and carbonate products,11,16,20−22,25,27 as well as the first crystal structure of a (CO2)− radical anion.19 The mixed sandwich U3+ complex (C5Me5)U[C8H6(SiiPr3-1,4)2] generates carbonate as well as a [(CO)4]2− squarate product from U3+ reduction of the CO byproduct.18 Recently, it has been shown that carbonate can also be generated from U3+ siloxide complexes.17 In each of these cases, the carbonate products are typically bimetallic uranium complexes with (μ-κ1:κ2-CO3)2− bridges, as exemplified in Figure 1A. One structure that is not bimetallic is the tetrauranium compound obtained by potassium graphite reduction of a bimetallic trisaryloxide-substituted triazacyclononane U4+ carbonate complex that contains four [U(ArO)3tacn]+ units located around a {[K(CO3)]4}4− network, Figure 1B.16 The recent synthesis of the U3+ allyl metallocene complexes (C5Me5)2U[CH2C(R)CH2] (R = H, 1; Me, 2)30 provided the opportunity to study U3+ reactivity with a ligand that can adopt π and σ metal−carbon bonding modes, namely, the allyl group. © XXXX American Chemical Society
We report here that 1 and 2 can display several types of reactivity depending on the substrate. This is illustrated in reactions with azobenzene, cyclooctatetraene, and carbon dioxide. The azobenzene reactivity is presented first since it demonstrates that 1 and 2 can engage in a previously observed pattern of U3+ metallocene reaction chemistry. The other two substrates involve more complicated reaction sequences including loss of a (C5Me5)− ring and the unexpected formation of a hexametallic uranium complex formed by bridging carbonate ligands, [(C5Me5)2U]6(μ-κ1:κ2-CO3)6. Interest in large polymetallic uranium compounds has been stimulated by the results of Burns and co-workers, who have reported an extensive series of 1.6−1.9 nm diameter polyactinyl compounds that are linked with peroxide.33−36 An extensive range of compounds containing up to 60 uranyl units with formulas such as (UO 2 ) 36 (O 2 ) 41 (OH) 26 36− , [(UO 2 )Received: June 10, 2013
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dx.doi.org/10.1021/om400526h | Organometallics XXXX, XXX, XXX−XXX
Organometallics
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Figure 1. (A) [U(C8H6{SiiPr3-1,4}2)(C5Me5)]2(μ-κ1:κ2-CO3).18 (B) The core structure of {[((Neop,MeArO)3-triazacyclononane)U(CO3)]K}4 with the triazacyclononane ligand system abbreviated as L.16 (C) The anion in the polyuranyl guanidinium salt [C(NH2)3]6[(UO2)(CO3)2]3.31,32.
(O2)1.5]4444−, and [UO2(O2)(OH)]6060− are readily prepared from uranyl nitrate and hydrogen peroxide in alkaline solution in the presence of various alkali metal cations. In some cases, these can self-assemble within 15 min to micrometer-sized crystals.35 The reports on these peroxides have pointed out the value of such polyuranium species for nuclear waste stream remediation using size-selective processes.36 A recent review36 on large polymetallic uranium molecules details how nanometer-sized uranium species readily form with oxide or peroxide linkages, but can also include oxalate, sulfate, pyrophosphate, and selenate bridges.34,35,37−39 However, the largest crystallographically characterized complex in which multiple uranium atoms are connected to each other by carbonate ligands is the trimetallic uranyl species in Figure 1C.31,32 This is surprising given the numerous examples of carbonate bridges in bimetallic uranium complexes.16−18,20,21,24,25
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solution of 1 (100 mg, 0.182 mmol) in hexane (10 mL). The green solution quickly turned dark brown and was stirred for 1 h. The solvent was removed via reduced pressure, and the resulting solid was washed with cold hexane to leave 5 as a brown, microcrystalline solid (74 mg, 79%). Removal of solvent from the unfiltered washings left a solid that contained (C5Me5)2 by 1H NMR spectroscopy in C6D6. 1H NMR (C6D6, 298 K): δ 2.8 (s, C5Me5, 15 H), −32.2, (s, C8H8, 8H), −33.9 (s, CH2CHCH2, 1H). IR: 2964m, 2901s, 2856s, 2725w, 1630m, 1436m, 1377s, 1253w, 1107w, 1020w, 900w, 722s cm−1. Anal. Calcd for C21H28U: C, 48.65; H, 5.44. Found: C, 48.20; H, 5.78. (C5Me5)(C8H8)U[CH2C(Me)CH2], 6. As described for 5, C8H8 (21 μL, 0.19 mmol) reacts with 2 (100 mg, 0.177 mmol) to form 6 as a brown, microcrystalline solid. Concentrated samples in hexane at −35 °C yielded single crystals suitable for X-ray diffraction (81 mg, 85%). 1 H NMR (C6D6, 298 K): δ 4.5 (s, C5Me5, 15 H), −4.1 [s, CH2C(Me)CH2, 3H], −30.4, (s, C8H8, 8H). IR: 2964m, 2900s, 2856s, 2724w, 1633m, 1435m, 1377s, 1253w, 1106w, 1020w, 900w, 721s cm−1. Anal. Calcd for C22H30U: C, 49.62; H, 5.68. Found: C, 49.90; H, 5.80. [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, from (C5Me5)2U(CH2CHCH2), 1. A solution of (C5Me5)2U(CH2CHCH2) (100 mg, 0.182 mmol) in benzene (8 mL) in a Fischer−Porter high-pressure apparatus in an argon-containing glovebox was sealed and attached to a high-pressure line. The vessel was evacuated to the solvent vapor pressure and charged with CO2 (80 psi). The solution turned from dark green to orange over 1 h, and orange crystals grew over 3 days. The CO2 pressure was reduced to 15 psi on the vacuum line, and the residual CO2 was removed under vacuum in the glovebox. The mother liquor was decanted, and a small sample of single crystals was taken for analysis by X-ray diffraction. The rest were washed three times with benzene and dried under reduced pressure (51 mg, 96% yield based on ligand redistribution). IR: 2979w, 2902m, 2857w, 1534w, 1475s, 1433s, 1400s, 1080m, 1021w, 831m, 766w, 675w, 652w cm−1. Anal. Calcd for C126H180O18U6·C6H6: C, 45.44; H, 5.37. Found: C, 45.01; H, 5.80. The benzene washings were combined with the mother liquor, and the solvent was evaporated. The 1H NMR spectrum of the solids in C6D6 indicated (C5Me5)2U(κ2-O,O′-O2CCH2CHCH2)2,30 8, was the exclusive byproduct. [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, from (C5Me5)2U[CH2C(Me)CH2], 2. Following the procedure above, 2 was reacted with CO2 (80 psi). Complex 7 was identified by elemental analysis, IR spectroscopic analysis, and X-ray unit cell analysis of a single crystal matching that for 7·C6H6. No significant difference in yield or rate of crystal formation was observed. (C5Me5)2U(κ2-O,O′-O2CCH2C(Me)CH2)2,30 9, was identified via 1H NMR spectroscopy as the only other product. [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, from [(C5Me5)2U]2(μ-η6:η6-C6H6). A solution of [(C5Me5)2U]2(μ-η6:η6-C6H6) (100 mg, 0.091 mmol) in benzene (10 mL) was reacted with CO2 as described for 1, but with a reaction time of 12 h to produce orange single crystals of 7 (83 mg, 78%). Elemental analysis, infrared spectroscopic analysis, and X-ray unit cell analysis of the single crystals formed matched that for 7·C6H6. Anal. Calcd for C126H180O18U6·C6H6: C, 45.44; H, 5.37. Found: C, 45.08; H, 5.15. [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, from [(C5Me5)2U][(μ-Ph)2BPh2]. A solution of [(C5Me5)2U][(μ-Ph)2BPh2] (60 mg, 0.072 mmol) in
EXPERIMENTAL INFORMATION
All syntheses and manipulations described below were conducted under argon with rigorous exclusion of air and water using glovebox, Schlenk, and vacuum line techniques. Solvents were sparged with UHP argon and dried over columns containing Q-5 alumina. NMR solvents were dried over sodium−potassium alloy, degassed using three freeze− pump−thaw cycles, and vacuum transferred before use. PhNNPh (99%) was purchased from Sigma-Aldrich and placed under vacuum (10−3 Torr) for 12 h prior to use. 1,3,5,7-Cyclooctatetraene (98%) was purchased from SigmaAldrich and distilled under nitrogen before use. 13 CO2 gas (99%) was purchased from SigmaAldrich and used as received. (C 5 Me 5 ) 2 U[CH2 C(R)CH 2] (R = H, 1; Me, 2),30 (C5 Me 5 ) 2 U[κ2 -O,O′-O2 CCH2 C(R)CH2 ]Cl (R = H, Me),30 [(C5Me5)2U][(μ-Ph)2BPh2],40 and [(C5Me5)2U]2(μ-η6:η6-C6H6)41 were synthesized as previously described. 1H NMR spectra were recorded with a Bruker DRX500 MHz spectrometer. Infrared spectra were recorded as KBr pellets on a Varian 1000 FT-IR spectrometer. Elemental analysis was performed on a Perkin-Elmer 2400 Series II CHNS analyzer. (C5Me5)2U(NPh)2 from (C5Me5)2U(CH2CHCH2), 1. In a glovebox, PhNNPh (4 mg, 0.02 mmol) was added quickly to a vigorously stirred solution of (C5Me5)2U(CH2CHCH2), 1 (12 mg, 0.022 mmol), in hexane (5 mL). The green solution quickly turned black and was stirred for 1 h. The solvent was removed via reduced pressure to yield a tacky black solid. (C5Me5)2U(NPh)242 and (C5Me5)2U(CH2CHCH2)2, 3,30 were identified by 1H NMR spectroscopy in a 1:1 ratio and were the only products observed. (C5Me5)2U(NPh)2 from (C5Me5)2U[CH2C(Me)CH2], 2. In an analogous reaction, PhNNPh reacted with (C5Me5)2U[CH2C(Me)CH2], 2, to form (C5Me5)2U(NPh)242 and (C5Me5)2U[CH2C(Me)CH2]2, 4,30 which were identified by 1H NMR spectroscopy in a 1:1 ratio. (C5Me5)(C8H8)U(CH2CHCH2), 5. C8H8 (21 μL, 0.19 mmol) was added dropwise over 15 s using a 25 μL syringe to a vigorously stirred B
dx.doi.org/10.1021/om400526h | Organometallics XXXX, XXX, XXX−XXX
Organometallics
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benzene (10 mL) was reacted with CO2 as described for 1. Crystals of 7 were identified by X-ray crystallography, and although crystals were seen to form after 24 h, only 3 mg (7%) was isolated even after 7 days. No starting material was observed by 1H NMR spectroscopy. The NMR spectrum contained resonances for BPh3. {[(C5Me5)2U](μ-κ1:κ1-O,O′-O2CCH2CHCH2)}2, 10. K(Hg) (1 wt %) (44 mg K, 1.1 mmol) was poured into a vigorously stirred solution of (C5Me5)2U(O2CCH2CHCH2)Cl30 (100 mg, 0.159 mmol) in hexane (10 mL). Over 10 min, the orange solution turned green and was stirred for 1 h. After removing gray insoluble solids via centrifugation and filtration, the solvent was removed via reduced pressure to yield a dark green, microcrystalline solid. Concentrated samples in hexane at −35 °C yielded single crystals suitable for X-ray diffraction (88 mg, 93%). 1H NMR (C6D6, 298 K): δ −5.6 (s, C5Me5, 30 H). No resonance for the allylcarboxylate moiety was observed between 200 and −200 ppm. IR: 2977m, 2916s, 2723m, 1556s, 1434s, 1397m, 1277m, 1242m, 895w, 840m, 746w, 618w cm−1. Anal. Calcd for C48H70O4U2: C, 48.56; H, 5.94. Found: C, 48.70; H, 5.50. {[(C5Me5)2U][μ-κ1:κ1-O,O′-O2CCH2C(Me)CH2]}2, 11. As described for 10, (C5Me5)2U(O2CCH2C(Me)CH2)Cl30 (100 mg, 0.156 mmol) was converted to 11, which was isolated as a dark green, microcrystalline solid (92 mg, 97%). 1H NMR (C6D6, 298 K): δ −6.1 (s, C5Me5, 30 H), −10.1 [s, CH2C(CH3)CH2, 1 H], −15.0 [s, CH2C(CH3)CH2, 1 H], −19.2 [s, CH2C(CH3)CH2, 3 H], −40.1 [s, CH2C(CH3)CH2, 2 H]. IR: 2977m, 2914s, 2856m, 2723m, 1559s, 1438w, 1397m, 1277m, 1242m, 986w, 896w, 840m, 746w, 686w cm−1. Anal. Calcd for C48H70O4U2: C, 49.42; H, 6.14. Found: C, 49.80; H, 5.91. [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, from {[(C5Me5)2U](μ-κ1:κ1-O,O′O2CCH2CHCH2)}2, 10. A solution of 10 (100 mg, 0.084 mmol) in benzene (10 mL) was reacted with CO2 as described for 1. Elemental analysis, infrared spectroscopic analysis, and X-ray unit cell analysis of the single crystals formed (48 mg, 98% based on ligand redistribution) matched those for 7·C6H6. Anal. Calcd for C126H180O18U6·C6H6: C, 45.44; H, 5.37. Found: C, 45.30; H, 5.20. Detection of the CO Byproduct. A solution of 1 (15 mg, 0.027 mmol) in C6D6 (ca. 0.3 mL) was placed into a J-Young NMR tube. This solution was degassed using three freeze−pump−thaw cycles on a high-vacuum line (10−5 Torr). 13CO2 at 5 psi was added to the JYoung tube, and it was sealed. After 1 h, the solution turned orange, and after 5 h the 13C NMR spectrum contained a resonance at 181.4 ppm consistent with 13CO. The 1H NMR spectrum contained the resonances for (C5Me5)2U(κ2-O,O′-O2CCH2CHCH2)2,30 8. X-ray Data Collection, Structure Determination, and Refinement. Crystallographic information for complexes 6, 7, and 10 (CCDC 928812−928814) is summarized in Table 1. Full details are given in the Supporting Information.
Table 1. X-ray Data and Collection Parameters on (C5Me5)(C8H8)U[CH2C(Me)CH2], 6, {[(C5Me5)2U](μκ1:κ2-CO3)}6, 7, and {[(C5Me5)2U](μ-κ1:κ1-O,O′O2CCH2CHCH2)}2, 10 empirical formula temperature (K) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z ρcalcd (Mg/m3) μ (mm−1) R1 (I > 2.0σ(I)) wR2 (all data)
6
7
10
C22H30U 83(2) triclinic P1̅ 8.8131(4) 14.2385(6) 16.0147(6) 106.6569(5) 98.8404(5) 90.4651(5) 1899.52(14) 4 1.862 8.544 0.0178 0.0396
C126H180O18U6·C6H6 143(2) trigonal R3̅ 31.339(2) 31.339(2) 10.9920(8) 90 90 120 9349.4(15) 3 1.859 7.830 0.0292 0.0818
C48H70O4U2 83(2) monoclinic P21/n 9.7800(5) 15.6266(9) 15.0373(8) 90 102.8544(6) 90 2240.5(2) 2 1.760 7.260 0.0145 0.0332
Definitions: R1 = ∑∥Fo| − |Fc∥/∑|Fo|, wR2 = [∑w(Fo2 − Fc2)2/ ∑w(Fo2)2]1/2.
a
These U3+ reactions are analogous to the reaction of the “(C5Me5)2UCl” units in [(C5Me5)2UCl]3 that engage in reductions to form (C5Me5)2UCl2 as a byproduct.46 The halfreaction for the U3+ metallocene chloride is shown in eq 4,
where n = 2 for U4+ products and n = 4 for a mixture of U6+ and U4+ products. This method has been used to reduce PhN NPh to two (NPh)2− units with (C5Me5)2UCl2Na,44 and we have found as part of this study that the neutral chloride (C5Me5)2UCl(Et2O)46 reacts similarly with azobenzene to form (C5Me5)2U(NPh)2. C8H8. In contrast to the four-electron reactivity with azobenzene, 1 and 2 react with 1,3,5,7-cyclooctatetraene as two-electron reductants. In this case, a (C5Me5)− ligand is lost and the mixed ligand U4+ complexes (C5Me5)(C8H8)U[CH2C(R)CH2] (R = H, 5; Me, 6) are formed, eq 5. No bis(allyl)
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RESULTS PhNNPh. The allyl complexes (C5Me5)2U[CH2C(R)CH2] (R = H, 1; Me, 2) react rapidly with PhNNPh in a transformation that demonstrates their capacity to effect fourelectron reduction. As shown in eq 2, both 1 and 2 reduce
azobenzene by four electrons to form the bis(imido) U6+ complex (C5Me5)2U(NPh)2, which was fully characterized in the past by Burns and co-workers.42−45 Both reactions also form the bis(allyl) U4+ metallocenes (C5Me5)2U[CH2C(R)CH2]230 as byproducts. The formal half-reaction is shown in eq 3, where n = 4 and Cp* = C5Me5.
ligand redistribution products were observed as found in eq 2. Hence, the two electrons added to C8H8 formally arise from a single equivalent of the U3+ allyl complex according to a new half-reaction for these complexes, eq 6. One electron formally comes from a U3+/U4+ redox couple. The observation of (C5Me5)2 as a byproduct by 1H NMR spectroscopy is C
dx.doi.org/10.1021/om400526h | Organometallics XXXX, XXX, XXX−XXX
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consistent with a second electron coming from a (C5Me5)−/ (C5Me5) redox couple, which is common in sterically crowded rare earth and actinide organometallic complexes.47 The combination of the cyclopentadienyl, allyl, and cyclooctatetraenyl ligands in eq 5 could form a sterically crowded intermediate with sufficient steric bulk for sterically induced reduction, since the composition “(C5Me5)2U(μ-C8H8)” appears to be too sterically crowded to isolate.48 The solid-state structure of 6 was determined by X-ray crystallography, Figure 2, and reveals that the allyl ligand binds
those in the closely related amidinate (C5Me5)(C8H8)U[iPrNC(Me)NiPr],48 which has 2.516 Å and 136.4° values. The 1.975 and 1.973 Å U−(C8H8 ring centroid) distances are not as close to the 2.008 Å distance in the amidinate complex, but they fall in the 1.894−2.008 Å range typical for [(C5Me5)(C8H8)U]+ complexes.49 The 2.627(3)−2.697(3) Å U−C distances to the end carbon atoms of the allyl group are longer than the 2.469(3) Å U−C bond in the sigma-bound alkyl complex (C5Me5)(C8H8)U[CH(SiMe3)2],48 as is typical for allyl complexes.30 The σ-bonded (C5Me5)(C8H8)UR′ complexes, where R′ = Me, Et, Np, and Ph, readily engage in C−H bond activation to form the tuck-in complex (η5:η1-C5Me4CH2)(C8H8)U.50 This has not been observed with 5 and 6, which are stable at 100 °C for 14 h in toluene. CO2. The reaction of 1 and 2 with CO2 (80 psi) showed yet another mode of reactivity for these U3+ allyl species. Two products are isolated from this reaction: an unprecedented hexametallic U4+ carbonate, [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, and a U4+ bis(allylcarboxylate) complex, (C5Me5)2U[κ2-O,O′O2CCH2C(R)CH2]2 (R = H, 8; Me, 9), eq 7. Complex 7 was separated by crystallization and identified by X-ray crystallography, Figure 3. The IR spectrum of 7 shows a characteristic strong carbonate absorption at 1400 cm−1.51 The
Figure 2. Thermal ellipsoid plot drawn at the 50% probability level of one of the two crystallographically independent molecules of (C5Me5)(C8H8)U[CH2C(Me)CH2], 6, in the unit cell. Hydrogen atoms are omitted for clarity, and only one of the two molecules in the asymmetric unit is shown.
in a trihapto manner. The 2.520 and 2.521 Å U−(C5Me5 ring centroid) distances and the 136.3° and 136.5° (C5Me5 ring centroid)−U−(C8H8 ring centroid) angles are very close to
Figure 3. (A) Thermal ellipsoid plot at the 50% probability level of [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, with hydrogen atoms omitted for clarity. (B) Side view of the uranium carbonate core of 7 with (C5Me5)− ligands omitted. D
dx.doi.org/10.1021/om400526h | Organometallics XXXX, XXX, XXX−XXX
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previously reported 8 and 930 were identified by 1H NMR spectroscopy. The formation of uranium carbonates from CO2 and U3+ complexes has been well documented in recent years16−18,20,21,23−25 and is thought to occur through reductive disproportionation of two CO2 to CO32− and CO using two equivalents of U3+, eq 8. 2U3 + 2CO2 → 2U 4 + + CO32 − + CO
carboxyl binding mode, oxidation state, and coordination number. Compounds 10 and 11 also produce 7 under CO2 (80 psi) with a half-reaction analogous to eq 3 with n = 2 and carboxylate instead of allyl. Scheme 1 shows both possible pathways for formation of 7. Complex 7 crystallizes as the hexameric cyclic structure [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, Figure 3, with one crystallographically unique repeat unit. The crystallographic parameters of the [(C5Me5)2U]2+ metallocene moiety are conventional. For example, the 2.445 and 2.472 Å U−(C5Me5 ring centroid) distances and the 137.3° (C5Me5 ring centroid)−U−(C5Me5 ring centroid) are similar to the 2.478 Å average distance and 137.4° angle in (C5Me5)2U(κ2-O,O′-O2CCH2CHCH2)2, 8, which has two carboxylates coordinated to the metallocene unit.30 The latter complex, which has a higher formal coordination number, has longer U−O bonds, 2.42(2)− 2.465(2) Å, compared to 2.360(4) and 2.372(3) Å U−O distances for the κ2 part of the carbonate ligand. The U−O distance for the κ1 part of the carbonate, 2.229(3) Å, is much shorter, although the C−O distances within the carbonate fall in the narrow range 1.280(6)−1.290(6) Å. In comparison, the U−O distance in [(C5Me5)2FU]2(μ-O) is 2.118(3) Å.54 The two most distant (C5Me5)− methyl carbon atoms in 7 span a length of 1.97 nm. The complex adopts a “chair” conformation generated by the bridging (μ-κ1:κ2-CO3)2− carbonates, which are oriented with 60° angles between one carbonate plane and the next. This twists each metallocene by 60° around the ring. Since the yield of 7 from 1 and 2 or from 10 and 11 can be no higher than 50%, other syntheses of 7 from U3+ metallocene precursors were examined, Scheme 2. Two U3+ metallocenes with an extensive reduction chemistry were examined: [(C5Me5)2U][(μ-Ph)2BPh2]40 and [(C5Me5)2U]2(μ-η6:η6C6H6).41 Both compounds were found to react with CO2 to generate polymetallic 7, with the latter compound giving the best yield: 78%. The formation of the carbonate from U3+ sources thus appears to be general.
(8) 13
The generation of CO in eq 7 was confirmed using CO2 at 5 psi followed by 13C NMR spectroscopy. NMR data were not obtainable for 7 because of its insolubility in common solvents. Complexes 1 and 2 could affect analogous U3+ reduction of CO2 with the formal half-reaction in eq 3. This would form the bis(allyl) complexes, (C5Me5)2U[CH2C(R)CH2]2, 3 and 4, as byproducts. If these are formed, they would not be observable since they react quickly with CO 2 to form the U 4+ bis(carboxylate) byproducts 8 and 9,30 which are observed. However, as described in the next paragraph, another pathway to 7 is possible in which CO2 insertion occurs to form allylcarboxylate products before the formation of 7. 1 H NMR studies of 1 and 2 under only 1 atm of CO2 suggest that there is rapid formation of the U3+ carboxylates {[(C5Me5)2U][μ-κ1:κ1-O,O′-O2CCH2C(R)CH2]}2 (R = H, 10; Me, 11). Complexes 10 and 11 were identified after independent synthesis by K(Hg) reduction of (C5Me5)2U[κ2O,O′-O2CCH2C(R)CH2]Cl,30 eq 9. The structure of 10 was determined by X-ray crystallography, Figure 4. It is
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DISCUSSION The reactions of (C5Me5)2U[CH2C(R)CH2] (R = H, 1; Me, 2) with azobenzene, eq 2, demonstrate that these U3+ metallocenes can participate in the traditional U3+ metallocene (C5Me5)2UX reduction chemistry shown in eq 4, in which two equivalents of (C5Me5)2UX deliver two electrons and form (C5Me5)2UX2 as a byproduct. The X ligand historically has been limited to chloride,45,46 but eq 2 shows that X can be extended to allyl. The CO2 reactions with 1 and 2 may also have a component of this reaction pattern, and the reduction of CO2 by {[(C5Me5)2U](μ-κ1:κ1-O,O′-O2CCH2CHCH2)}2, 10, Scheme 1, shows that X in eq 4 can also be carboxylate.
Figure 4. Thermal ellipsoid plot drawn at the 50% probability level of {[(C5Me5)2U](μ-κ1:κ1-O,O′-O2CCH2CHCH2)}2, 10. Hydrogen atoms are omitted for clarity.
isomorphous with [(C5Me5)2Sm(μ-O2CCH2CHCH2)]2.52 The metrical parameters for 10 given in Table 2 are roughly similar to those in the samarium complex considering that U3+ has a 0.067 Å larger radial size.53 Comparison of the 2.356(1) and 2.375(1) Å U−O(μ-O2CR) distances with the 2.42(2)− 2.449(2) Å U−O(κ2-O2CR) distances in (C5Me5)2U(κ2O2CCH2CHCH2)2 is not direct due to the difference in E
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Table 2. Selected Bond Distances (Å) and Angles (deg) for (C5Me5)(C8H8)U[CH2C(Me)CH2], 6, [(C5Me5)2U]6(μ-κ1:κ2CO3)6, 7, and {[(C5Me5)2U](μ-κ1:κ1-O,O′-O2CCH2CHCH2)}2, 10 6 (C5Me5 centroid)−U (C5Me5 centroid)−U−(C5Me5 centroid) (C5Me5 centroid)−U−(C8H8 centroid) (C8H8 centroid)−U C19/20/21−U1 C41/42/43−U2 C19−C20−C21 C41−C42−C43 U−O(κ1-CO3 and RCO2)
7
2.520/2.521
10
2.445/2.472 137.3
2.509/2.493 133.8
2.229(3)
2.356(1) 2.375(1)
136.3/136.5 1.975/1.973 2.627(3)/2.797(3)/2.697(3) 2.670(3)/2.790(3)/2.645(3) 122.1(3) 122.0(3)
U−O(κ2-CO3) C21−O1/O2/O3
2.360(4)/2.372(3) 1.280(6)/1.286(6)/1.290(6)
1.259(2)/ 1.257(2)/−
Scheme 1. Synthesis of the Hexauranium Carbonate 7; (a) −6 (C5Me5)2U[κ2-O,O′-O2CCH2C(R)CH2]2, −6 CO
Scheme 2. Additional Syntheses of 7; (a) −6 CO, −3 Ph2, −6 BPh3; (b) −6 CO, −3 C6H6
hapticity over sigma-bonded ligands in U3+ metallocene complexes such as (C5Me5)2U(η1-alkyl). However, only one such example exists for comparison, (C 5 Me 5 ) 2 U[CH(SiMe3)].46 The formation of the monometallic monocyclopentadienyl complex, 6, rather than a bimetallic bis(cyclopentadienyl) C 8 H 8 reduction product such as (C5Me5)2U[CH2C(R)CH2](μ-C8H8)U(C5Me5)2[CH2C(R)CH2] can similarly be rationalized by the likely steric strain in the latter species. The synthesis of [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7, from 1, 2, 10, [(C5Me5)2U][(μ-Ph)2BPh2], and [(C5Me5)2U]2(μ-η6:η6C6H6) reveals yet another aspect of U3+ metallocene reactivity. This hexametallic carbonate ring system is unique to carbonate bridging, although there are many polymetallic molecules bridged with carbonate in the literature of transition metals,58 alkaline earths,59 and lanthanides.60 The isolation of 7 suggests that new options in shape and size are available not only to polyuranium species but also to polymetallic carbonate compounds in general. Complex 7 also indicates carbonate can be added to the list of bridging ligands that make large polymetallic species, i.e., oxide, peroxide, oxalate, sulfate, pyrophosphate, and selenate bridges.33−39 The shape and nuclearity of 7 can be traced to the angular requirements of the (μ-κ1:κ2-CO3)2− binding mode coupled with the presence of the metallocene moieties. Burns has pointed out the importance of the bent U−(O2)−U angles in peroxide-bridged compounds in forming large polymetallic species.36 In the carbonate {[(UO2)(CO3)2]3}6−, Figure 1,31,32 the combination of a linear uranyl unit and three (μ-κ1:κ2CO3)2− ligands leads to a planar array with 120° angles between the monomeric units that closes with only three
Given the diversity of X = Cl, allyl, and carboxylate, it is likely that other (C5Me5)2UX complexes will behave similarly. The reactions of (C5Me5)2U[CH2C(R)CH2] (R = H, 1; Me, 2) with cyclooctatetraene, eq 5, show that allyl U3+ metallocenes can engage in other types of reduction in which the (C5Me5)− ligands become active reductants. This reactivity is similar to (C5Me5)2UX reactivity where X = C5Me5.47,55−57 In the (C5Me5)3U case, this “sterically induced reduction” arises because the three (C5Me5)− groups cannot get close enough to the metal center to achieve their normal stabilization. Although 1 and 2 do not have the steric crowding found in (C5Me5)3U, the presence of the X = allyl ligand evidently leads to sterically crowded intermediates in which (C5Me5)− can function as a reductant. This may be possible with allyl due to its enhanced F
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(9) Mansell, S. M.; Kaltsoyannis, N.; Arnold, P. L. J. Am. Chem. Soc. 2011, 133, 9036. (10) Gambarotta, S.; Korobkov, I.; Yap, G. P. A. Angew. Chem., Int. Ed. 2002, 41, 3433. (11) Castro-Rodriguez, I.; Meyer, K. J. Am. Chem. Soc. 2005, 127, 11242. (12) Summerscales, O. T.; Cloke, F. G. N.; Hitchcock, P. B.; Green, J. C.; Hazari, N. Science 2006, 311, 829. (13) Frey, A. S.; Cloke, F. G. N.; Hitchcock, P. B.; Day, I. J.; Green, J. C.; Aitken, G. J. Am. Chem. Soc. 2008, 130, 13816. (14) Frey, A. S. P.; Cloke, F. G. N.; Coles, M. P.; Maron, L.; Davin, T. Angew. Chem., Int. Ed. 2011, 50, 6881. (15) Gardner, B. M.; Stewart, J. C.; Davis, A. L.; McMaster, J.; Lewis, W.; Blake, A. J.; Liddle, S. T. Proc. Natl. Acad. Sci. 2012, 109, 9265. (16) Meyer, K.; Schmidt, A. C.; Nizovtsev, A. V.; Scheurer, A.; Heinemann, F. W. Chem. Commun. 2012, 8634. (17) Mougel, V.; Camp, C.; Pécaut, J.; Copéret, C.; Maron, L.; Kefalidis, C. E.; Mazzanti, M. Angew. Chem., Int. Ed. 2012, 51, 12280. (18) Summerscales, O. T.; Frey, A. S. P.; Cloke, F. G. N.; Hitchcock, P. B. Chem. Commun. 2009, 198. (19) Meyer, K.; Castro-Rodriguez, I.; Nakai, H.; Zakharov, L. N.; Rheingold, A. L. Science 2004, 305, 1757. (20) Bart, S. C.; Anthon, C.; Heinemann, F. W.; Bill, E.; Edelstein, N. M.; Meyer, K. J. Am. Chem. Soc. 2008, 130, 12536. (21) Lam, O. P.; Bart, S. C.; Kameo, H.; Heinemann, F. W.; Meyer, K. Chem. Commun. 2010, 46, 3137. (22) Lam, O. P.; Anthon, C.; Meyer, K. Dalton Trans. 2009, 9677. (23) Lam, O. P.; Meyer, K. Polyhedron 2012, 32, 1. (24) Matson, E. M.; Forrest, W. P.; Fanwick, P. E.; Bart, S. C. J. Am. Chem. Soc. 2011, 133, 4948. (25) Castro, L.; Lam, O. P.; Bart, S. C.; Meyer, K.; Maron, L. Organometallics 2010, 29, 5504. (26) Berthet, J.-C.; Le Maréchal, J.-F.; Nierlich, M.; Lance, M.; Vigner, J.; Ephritikhine, M. J. Organomet. Chem. 1991, 408, 335. (27) Zuend, S. J.; Lam, O. P.; Heinemann, F. W.; Meyer, K. Angew. Chem., Int. Ed. 2011, 50, 10626. (28) Frey, A. S. P.; Cloke, F. G. N.; Coles, M. P.; Hitchcock, P. B. Chem. Eur. J. 2010, 16, 9446. (29) Siladke, N. A.; Meihaus, K. R.; Ziller, J. W.; Fang, M.; Furche, F.; Long, J. R.; Evans, W. J. J. Am. Chem. Soc. 2011, 134, 1243. (30) Webster, C. L.; Ziller, J. W.; Evans, W. J. Organometallics 2012, 31, 7191. (31) Allen, P. G.; Bucher, J. J.; Clark, D. L.; Edelstein, N. M.; Ekberg, S. A.; Gohdes, J. W.; Hudson, E. A.; Kaltsoyannis, N.; Lukens, W. W. Inorg. Chem. 1995, 34, 4797. (32) Clark, D. L.; Hobart, D. E.; Neu, M. P. Chem. Rev. 1995, 95, 25. (33) Burns, P. C.; Kubatko, K.-A.; Sigmon, G.; Fryer, B. J.; Gagnon, J. E.; Antonio, M. R.; Soderholm, L. Angew. Chem., Int. Ed. 2005, 44, 2135. (34) Qiu, J.; Ling, J.; Sui, A.; Szymanowski, J. E. S.; Simonetti, A.; Burns, P. C. J. Am. Chem. Soc. 2011, 134, 1810. (35) Sigmon, G. E.; Burns, P. C. J. Am. Chem. Soc. 2011, 133, 9137. (36) Qiu, J.; Burns, P. C. Chem. Rev. 2012, 1097. (37) Albrecht-Schmitt, T. E. Angew. Chem., Int. Ed. 2005, 44, 4836. (38) Krivovichev, S. V.; Kahlenberg, V.; Tananaev, I. G.; Kaindl, R.; Mersdorf, E.; Myasoedov, B. F. J. Am. Chem. Soc. 2005, 127, 1072. (39) Alekseev, E. V.; Krivovichev, S. V.; Depmeier, W. Angew. Chem., Int. Ed. 2008, 47, 549. (40) Evans, W. J.; Nyce, G. W.; Forrestal, K. J.; Ziller, J. W. Organometallics 2002, 21, 1050. (41) Evans, W. J.; Kozimor, S. A.; Ziller, J. W.; Kaltsoyannis, N. J. Am. Chem. Soc. 2004, 126, 14533. (42) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1995, 117, 9448. (43) Arney, D. S. J.; Burns, C. J.; Smith, D. C. J. Am. Chem. Soc. 1992, 114, 10068. (44) Warner, B. P.; Scott, B. L.; Burns, C. J. Angew. Chem., Int. Ed. 1998, 37, 959. (45) Peters, R. G.; Warner, B. P.; Burns, C. J. J. Am. Chem. Soc. 1999, 121, 5585.
metals. In 7, the metallocene moiety requires bridging ligands in the plane that bisects the (C5Me5 centroid)−U−(C5Me5 centroid) angle. The space in this wedge is evidently optimum for binding one κ1 carbonate and one κ2 carbonate at a 60° angle. This leads to the twisting in the observed structure. Since the size of cyclopentadienyl rings is easily modified, other options for nuclearity and shape may be accessible with bridging carbonates and related species. Although cyclopentadienyl ligands are not viable for environmental applications, these results show that carbonate, an important ligand in the environmental chemistry of uranium,31,32,51 can facilitate the formation of nanometer-sized polymetallic molecules given the right coordination environment.
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CONCLUSION The (C5Me5)2UX U3+ metallocenes with X = allyl, i.e., (C5Me5)2U[CH2C(R)CH2] (R = H, 1; Me, 2), have the capacity for several types of reductive reactivity. They can engage in four-electron reduction as found for X = Cl with a (C5Me5)2UX2 byproduct, eq 4, but they can also effect twoelectron reductions involving loss of a cyclopentadienyl ring and formation of products containing {(C5Me5)U[CH2C(R)CH2]}2+ moieties. The U3+ allyl metallocenes can also lead to unusual types of polymetallic uranium complexes, as demonstrated by the synthesis of the hexauranium carbonate [(C5Me5)2U]6(μ-κ1:κ2-CO3)6, 7. The reaction with CO2 of 1 and 2 also led to another variant of (C5Me5)2UX complexes, namely, those with X = carboxylate, {[(C5Me5)2U][μ-κ1:κ1O,O′-O2CCH2C(R)CH2]}2 (R = H, 10; Me, 11), which can also engage in U3+ metallocene reaction chemistry including the reduction of CO2 to the hexauranium carbonate compound.
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ASSOCIATED CONTENT
S Supporting Information *
X-ray data collection, structure solution and refinement (PDF), and X-ray diffraction details of compounds 6, 7, and 10 (CIF, CCDC 928812−928814) are available free of charge via the Internet at http://pubs.acs.org
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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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REFERENCES
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H
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