Solvent-Free Organometallic Reactivity: Synthesis of Hydride and

Note pubs.acs.org/Organometallics

Solvent-Free Organometallic Reactivity: Synthesis of Hydride and Carboxylate Complexes of Uranium and Yttrium from Gas/Solid Reactions 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: Gas/solid reactions involving H2 and CO2 with the metallocenes (C5Me5)2UMe2 and (C5Me5)2U(allyl)2 as solids in the absence of solvent provide an improved method to make organouranium hydride and carboxylate products. Decomposition products that can form in solution from the reactive hydrides can be avoided by this method, and this approach can also provide intermediates too reactive to isolate in some solution reactions. In contrast to the variable nature of the hydrogenolysis reaction of (C5Me5)2UMe2 in toluene that forms byproducts along with the mixture of [(C5Me5)2UH2]2 and [(C5Me5)2UH]2, a byproduct-free hydrogenolysis occurs when (C5Me5)2UMe2 in the solid state is treated with H2 gas to form predominantly [(C5Me5)2UH2]2. H2 reacts with solid (C5Me5)2U(C3H5)2 and (C5Me5)2U(C3H5) similarly. The reaction of CO2 (80 psi) with solid (C5Me5)2UMe2 forms the monocarboxylate (C5Me5)2U(O2CCH3-κ2O,O′)Me, in contrast to the solution reaction that forms the diacetate (C5Me5)2U(O2CCH3-κ2O,O′)2 in minutes. The reaction of H2 with solid (C5Me5)2Y(C3H5) provided (C5Me5)2Y(μ-H)YH(C5Me5)2 without the decomposition products that it forms in solution such that single crystals suitable for X-ray diffraction could be isolated for the first time.

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INTRODUCTION One of the classic reactions in organometallic actinide chemistry is the hydrogenolysis of (C5Me5)2UMe2 to form a mixture of the U4+ and U3+ hydrides [(C5Me5)2UH2]2, 1, and [(C5Me5)2UH]2, 2, respectively, eq 1.1,2 Since these hydride

complexes are excellent precursors for a wealth of other organouranium compounds,1−16 this reaction is crucial to many synthetic schemes. In our hands, eq 1 has not always given the desired hydrides in the yield and purity expected. This is not unexpected considering that the hydrogenolysis could proceed through intermediates with terminal hydride ligands such as (C5Me5)2UHMe that could attack solvent and metalate cyclopentadienyl rings. In efforts to find a more reliable way to form these important hydrides and because reactions carried out in more concentrated solutions appeared to give better results, the solvent-free reaction of gaseous H2 with solid (C5Me5)2UMe2 was examined. We report here that this is an improved method to these hydrides. The success of the solidstate hydrogenolysis encouraged us to try other organoactinide gas/solid reactions, and the results demonstrate that solventfree organometallic actinide and rare earth reactions should be more widely attempted as alternatives to solution syntheses. © 2013 American Chemical Society

RESULTS AND DISCUSSION

H2 reacts with solid (C5Me5)2UMe2 in the absence of solvent as shown in Scheme 1. After a 14 h reaction time, 1H NMR spectra performed on C6D6 samples of the reaction product show resonances only for [(C 5 Me 5 ) 2 UH 2 ] 2 , 1, and [(C5Me5)2UH]2, 2, without the multiple cyclopentadienyl resonances for unidentified byproducts that can form in minor amounts in the reaction of H2 with (C5Me5)2UMe2 in toluene. When the product of the gas/solid reaction is dissolved in C6D6 and quickly taken to the 1H NMR spectrometer, the measured ratio of intensities for 1 and 2 is roughly 9:1. When the hydrogenolysis is done in solution, the U4+ hydride is also the predominant product, but 1 is reported to lose 0.48 equiv of H2 per U within 3 h in solution according to the equilibrium in eq 1.17 In any case, the reaction of H2 with (C5Me5)2UMe2 as a solid is the preferred route to these hydrides since it avoids formation of byproducts and requires little workup. Analysis of the product of the solvent-free reaction by IR spectroscopy shows very broad peaks around 1330 and 1170 cm−1 assigned to U−H stretches as originally reported for 1.4 Assignment of these bands by deuterium labeling was reported4 to be impossible in solution since there is exchange between the hydride/deuteride position and the hydrogens in the [C5(CH3)5]− ligands in 1. However, the reaction of D2 with (C5Me5)2UMe2 in the solid state allowed a shift to be observed. The solid-state IR spectrum of the product of the gas/solid Received: November 19, 2013 Published: December 23, 2013 433

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Organometallics

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Scheme 1. Reaction of Solid (C5Me5)2UMe2 with H2 Gas

Scheme 2. Reaction of Solid (C5Me5)2U(C3H5)2 with H2 Gas

Scheme 3. Reaction of Solid (C5Me5)2U(C3H5) with H2 Gas

reaction with D2 lacked the 1330 and 1170 cm−1 bands, and a new absorption at 949 cm−1 was observed (see Supporting Information). This gives a νU−H/νU-D ratio for the higher frequency absorption of 1.40 that is close to the 1.414 expected for a pure uranium hydride vibration. A lower frequency U−D stretching vibration would be expected at 830 cm−1 based on the 1170 cm−1 U−H band, but due to the broad nature of these absorptions and other absorptions in this region, it could not be definitively identified in the solid-state spectrum. Once it was demonstrated that (C5Me5)2UMe2 would react in the solid state with H 2 , hydrogenolysis of the U 4+ metallocene allyl complex (C5Me5)2U(C3H5)218 was examined. H2 also reacts with this complex in the solid state, Scheme 2. 1H NMR spectroscopic analysis of the product of the gas/solid reaction again shows a similarly pure hydride spectrum with the U4+/U3+ intensity ratio of 9:1. In contrast, solution hydrogenolysis of (C5Me5)2U(C3H5)2 dissolved in toluene is more complicated: after work up, the 1H NMR resonances for the hydride complexes account for less than 50% of the intensities for all the resonances assignable to cyclopentadienyl-containing uranium products. Interestingly, the U3+ allyl metallocene, (C5Me5)2U(C3H5),18 also reacts with H2 in the solid state to make products that have a 1H NMR spectrum that integrates to the same 9:1 intensity ratio of U4+ and U3+ hydrides as seen in the other gas/solid reactions, Scheme 3. Formation of the U 4+ hydride, [(C5Me5)2UH2]2, from H2 and U3+ precursors is also known in solution from reactions starting with [(C5Me5)2UH]2, eq 1, and (C 5 Me 5 ) 2 U[CH(SiMe 3 ) 2 ]. 17 Like the bis(allyl) (C5Me5)2U(C3H5)2 solution phase hydrogenolysis, reaction of mono(allyl) complex (C5Me5)2U(C3H5) with H2 in toluene is more complicated and gives a yield of less than 50% hydride. Carbon dioxide also reacts with organoactinide complexes in the solid state and, like the hydrogenolysis reaction, provides some advantages over solution phase reactions. (C5Me5)2UMe2 and (C5Me5)2U(C3H5)2 each react with excess CO2 in C6D6 within minutes to form the bis(carboxylate) products,

(C5Me5)2U(O2CMe)219 and (C5Me5)2U(O2CC3H5)2,18 respectively. If a stoichiometric amount of CO2 is used in solution, the monocarboxylate (C5Me5)2U(O2CMe)Me can be isolated from (C5Me5)2UMe2,19 indicating a stepwise reaction, eq 2 (R = Me). On the other hand, the reaction of excess CO2

with solid (C5Me5)2UMe2 forms only the monocarboxylate, (C5Me5)2U(O2CMe)Me, exclusively in 6 h. Hence, the gas/ solid reaction allowed isolation of an intermediate without precise control of the stoichiometry of the gaseous reagent. The bis(carboxylate) product, (C5Me5)2U(O2CMe)2, can be formed by a gas/solid reaction if the (C5Me5)2UMe2/excess CO2 combination is allowed to proceed for 48 h. The bis(carboxylate) can also be synthesized by reaction of excess CO2 with isolated solid (C5Me5)2U(O2CMe)Me, Scheme 4. The gas/solid reaction of (C5Me5)2U(C3H5)Cl18 with CO2 was examined to check generality, and it yielded the known (C5Me5)2U(O2CC3H5)Cl18 in quantitative yield, eq 3. Scheme 4. Gas/Solid Reactions of (C5Me5)2UMe2 with CO2 Gas

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Complex 3 resembles the other asymmetric complexes 4 and 5 in that the M−(C5Me5 ring centroid) distances of the formally seven- and eight-coordinate metallocene units differ. In contrast, in the symmetric complex 6, there are only two crystallographically independent M−(C5Me5 ring centroid) distances, and they have very similar values. Likewise, 3−5 have four different dihedral (C5Me5 ring centroid)−M1−M2− (C5Me5 ring centroid) angles, and in 6 there are just two. The M1···M2 distances in the asymmetric complexes are also much larger than that for 6. This is consistent with the one bridge and one terminal H/Cl/Me ligand structures of 3−5 vs the two bridging hydride structure likely for 6. The large error limits on the Y−H distances in 3, as is typical for metal hydrides, do not allow a discussion of these parameters.

One final example of the benefit of gas/solid reactions involves the reaction of (C5Me5)2Y(C3H5) with H2, eq 4. The

advantage in this case is that the solvent-free reaction avoids time in solution when the hydride product can engage in C−H bond activation involving the methyl groups of the (C5Me5)− ligands. The hydride product, [(C5Me5)2YH]2, 3,20 is highly reactive in solution and readily forms the tuck-over13,16,21−25 complex (C5Me5)2Y(μ-H)[μ-η1:η5-C5Me4(CH2)]Y(C5Me5).22 Performing the hydrogenolysis in the solid state allows for immediate transfer of the complex from a H2 atmosphere to a glovebox freezer without removal of solvent or other workup. Samples transferred directly to a minimum amount of hexane/ toluene produced single crystals of 3 that for the first time were suitable for X-ray crystallography. As shown in Figure 1, X-ray

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CONCLUSION

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EXPERIMENTAL SECTION

The results presented here demonstrate the utility of performing solvent-free reactions between gases and uranium and rare earth organometallic complexes in the solid state. With both H2 and CO2, different product mixtures are observed in reactions with the organoactinide complexes in the solid state compared to solution phase reactions. In the case of both the uranium and yttrium hydride reactions, the gas/solid method gives reactions with fewer byproducts and is the new preferred method to make these hydrides in our laboratory. While the products in these reactions have been known for decades and gas/solid reactions are widely used, particularly in industrial catalysis, this technique has received little attention in the molecular organo-f-element field. The results reported here will hopefully encourage more exploration of this method in the future.

The syntheses and manipulations described below were conducted with rigorous exclusion of air and water using Schlenk, vacuum line, and glovebox techniques. Solvents were sparged with UHP argon and dried by passage through columns containing Q-5 and molecular sieves. Benzene-d6 (Cambridge Isotope Laboratories) was dried over NaK alloy, degassed by three freeze−pump−thaw cycles, and vacuum transferred before use. (C5Me5)2UMe2,1 (C5Me5)2U(C3H5)2,18 (C5Me5)2U(C3H5),18 and (C5Me5)2U(C3H5)Cl18 were prepared as previously reported. Ultra-high-purity H2 and CO2 were purchased from Airgas and used as received. NMR experiments were conducted with a Bruker DRX 500 MHz spectrometer. Infrared spectra were collected as KBr pellets on a Varian 1000 FT-IR Scimitar Series spectrometer. Elemental analyses were performed with a Perkin-Elmer 2400 CHN elemental analyzer. Reactions. H2 with (C5Me5)2UMe2. In an argon-containing glovebox, (C5Me5)2UMe2 (100 mg, 0.186 mmol) was placed into a Fischer−Porter high-pressure apparatus. This vessel was sealed and attached to a high-pressure gas line. The pressure in the vessel was reduced to half an atmosphere and then charged with H2 (80 psi) before it was sealed. Over 2 h, the solid sample changed from orange to dark green. After 14 h, the pressure was reduced to 20 psi, and the vessel was transferred to an Ar glovebox. The residual hydrogen was removed under vacuum, and a sample of the green solid (94 mg, 99%) was completely dissolved in C6D6 and quickly taken to the 1H NMR spectrometer. The resonances for [(C 5 Me 5 ) 2 UH 2 ] 2 , 1, and [(C5Me5)2UH]2, 2,1 integrated to a 9:1 intensity ratio. The IR spectrum of the initially isolated solid product, the solution product, and reactions run under D2 are presented in the Supporting Information.

Figure 1. Thermal ellipsoid plot drawn at the 50% probability level of (C5Me5)2Y(μ-H)YH(C5Me5)2, 3,20 with hydrogen atoms removed for clarity except for the hydride ligands.

diffraction revealed that the complex has an asymmetric structure in the solid state with one bridging and one terminal hydride. The complex was originally assigned an asymmetric dimeric hydride structure by 1H NMR studies,20,26 but subsequent low-temperature22 studies showed it to be a symmetric doubly bridged species at low temperature in solution. The only other [(C5Me5)2LnH]x (x = 1, 2) complex characterized by X-ray crystallography is [(C5Me5)2SmH]2,27 and the hydride ligands were not located in that case. Structural data on (C5Me5)2Y(μ-H)YH(C5Me5)2 are compared in Table 1 with those of the asymmetric bimetallic chloride and methyl analogues (C 5 Me 5 ) 2 Y(μ-Cl)YCl(C5Me5)2,28 4, and (C5Me5)2Lu(μ-Me)LuMe(C5Me5)2,29 5, which also have one bridging and one terminal ligand per metal, and with symmetric [(C5Me5)2SmH]2,27 6, which is thought to have two bridging hydrides. 435

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Organometallics

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Table 1. Comparison of Relevant Bond Distances (Å) and Angles (deg) for (C5Me5)2Y(μ-H)YH(C5Me5)2, 3, (C5Me5)2Y(μCl)YCl(C5Me5)2,28 4, (C5Me5)2Lu(μ-Me)LuMe(C5Me5)2,29 5, and [(C5Me5)2SmH]2,27 6 M = Y, Lu, Sm; X = H, Me, Cl

[(C5Me5)2YH]2 3

[(C5Me5)2YCl]2 4

[(C5Me5)2LuMe]2 5

(Cnt)−M1 (Cnt)−M2 M1−M2 M1−X M1−(μ-X) (μ-X)−M2 X−M1−(μ-X) M1−(μ-X)−M2 (Cnt)−M−(Cnt) (Cnt1)−M1−M2−(Cnt2)a (Cnt1)−M1−M2−(Cnt4) (Cnt3)−M1−M2−(Cnt2) (Cnt3)−M1−M2−(Cnt4)

2.392/2.382 2.329/2.349 4.330 1.89(3) 2.22(2) 2.18(2) 100(1) 159.1 138.6/135.1 98.9 −69.9 −108.3 83.0

2.384/2.395 2.303/2.334 5.354 2.579(6) 2.776(5) 2.640(5) 93.4(2) 162.8(2) 135.8/139.3 117.4 −73.8 −94.9 74.0

2.367/2.348 2.278, 2.289 5.158 2.423(3) 2.737(3) 2.442(3) 91.3 169.5(2) 135.9/138.8 94.9 −77.3 −116.3 71.5

[(C5Me5)2SmH]2 6 2.478 2.470 3.905

130.4/134.5 93.3 −86.7 −86.7 93.3

a

Cnt1 and Cnt3 denote the centroids of the C5Me5 rings attached to the formally eight-coordinate metal center that is labeled M1. Cnt2 and Cnt4 denote the centroids of the C5Me5 rings attached to the formally seven-coordinate metal center that is labeled M2.

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(15) Evans, W. J.; Miller, K. A.; Ziller, J. W. Angew. Chem., Int. Ed. 2008, 47, 589. (16) Montalvo, E.; Miller, K. A.; Ziller, J. W.; Evans, W. J. Organometallics 2010, 29, 4159. (17) Fagan, P. J.; Manriquez, J. M.; Marks, T. J.; Day, C. S.; Vollmer, S. H.; Day, V. W. Organometallics 1982, 1, 170. (18) Webster, C. L.; Ziller, J. W.; Evans, W. J. Organometallics 2012, 31, 7191. (19) Moloy, K. G.; Marks, T. J. Inorg. Chim. Acta 1985, 110, 127. (20) Den Haan, K. H.; Wielstra, Y.; Teuben, J. H. Organometallics 1987, 6, 2053. (21) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. Organometallics 1991, 10, 134. (22) Booij, M.; Deelman, B. J.; Duchateau, R.; Postma, D. S.; Meetsma, A.; Teuben, J. H. Organometallics 1993, 12, 3531. (23) Evans, W. J.; Champagne, T. M.; Ziller, J. W. J. Am. Chem. Soc. 2006, 128, 14270. (24) Evans, W. J.; Perotti, J. M.; Ziller, J. W. Inorg. Chem. 2005, 44, 5820. (25) Johnson, K. R. D.; Hayes, P. G. Chem. Soc. Rev. 2013, 42, 1947. (26) den Haan, K. H.; Teuben, J. H. J. Chem. Soc., Chem. Commun. 1986, 682. (27) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1983, 105, 1401. (28) Evans, W. J.; Peterson, T. T.; Rausch, M. D.; Hunter, W. E.; Zhang, H.; Atwood, J. L. Organometallics 1985, 4, 554. (29) Evans, W. J.; Perotti, J. M.; Ziller, J. W. J. Am. Chem. Soc. 2005, 127, 3894.

ASSOCIATED CONTENT

S Supporting Information *

X-ray data collection, structure solution and refinement (PDF), and X-ray diffraction details of compounds (CIF, CCDC #956578), experimental details and infrared spectra of deuterium experiments 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.

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REFERENCES

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