Uranium alkoxide chemistryV. Synthesis, characterization and interconversion of uranium(IV) tert-butoxide complexes

Polyhedron Vol. 9, No. 15/16, pp. 1843-1848, 1990 Printed in Great Britain

0277-5387/90 $3.00+.00 0 1990 Pergamon Press plc

URANIUM ALKOXIDE CHEMISTRY-V.* SYNTHESIS, CHARACTERIZATION AND INTERCONVERSION OF URANIUM(W) TERT-BUTOXIDE COMPLEXES? WILLIAM

G. VAN DER SLUYS and ALFRED P. SATTELBERGERS

Inorganic and Structural Chemistry Group (INC-4), Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. and MICHAEL W. McELFRESH IBM T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, U.S.A. Abstract-The reaction of [(Me,Si),Nj2UN(SiMe,)SiMe2CH2 with 2 4.5 equivalents of t-butanol in toluene provides a green crystalline solid formulated as U2(0BuQB (HOBut) on the basis of elemental analysis, ‘H NMR, IR and UV-vis spectroscopy. U,(OBu’),(HOBu’) reacts with KOBu’ (or KH) to give the potassium salt KU2(0Bt&, with dry dioxygen to give U2(OB~t)9, and with H20 to give U30(0Bu’)l,,. Temperaturedependent magnetic susceptibility measurements (2-250 K) on solid U30(0Bu’)i,, show Cur&Weiss behaviour from 90 to 250 K. Below 90 K the susceptibility becomes temperature independent with no evidence of magnetic ordering.

that they were unable to reproduce the synthesis of U(OBu’),. Following Gilman’s procedure, they isolated a grey-brown crystalline uranium(V) product, which was formulated as UO(OBt&(Bu’OH) on the basis of a uranium analysis and a valency determination. 3 In 1979, Perego and co-workers claimed that they were able to prepare Gilman’s U(OBu’), by a different route. They isolated a green solid by treatment of tetra(n-allyl)uranium(IV), U(C3H5)4, with excess t-butanol in diethyl ether.4 Characterization of the product as U(OBU~)~was based on acceptable uranium analyses. Characteristic IR bands were UC14+4KNH2% also reported for U(OBU’)~, but no ‘H NMR data “U(NH2)4” - excessBu’oHU(OBU’)~. (1) were presented. In view of subsequent work, however, it would appear that Perego’s U(OBtQ4 After removal of NH3, a green solid was extracted is distinct from the compound reported by Gilman. into hexane and isolated by evaporation of the solIn 1984, Cotton and co-workers once again vent. The identity of the final product as U(OBu’), repeated Gilman’s original procedure [eq. (l)] and, was established solely on the basis of a uranium unlike Bradley et al., they were able to isolate a analysis. Six years later, Bradley et al. reported green, hexane-soluble product. The latter was shown to be KU,(OBt& by X-ray crystallography.’ The Cotton group indicated that rig* For part IV, see ref. 1. t This paper is dedicated to Professor Dick Fenske, orous exclusion of oxygen and water, and the mainfriend and teacher, on the occasion of his retirement from tenance of low temperatures (< - 1OC) during work-up, were required to obtain this product pure the University of Wisconsin. $ Author to whom correspondence should be addressed. and in reasonable yield. They also reported that, in

Early in the development of uranium(IV) chemistry, Gilman and co-workers reported the preparation of uranium(IV) alkoxides, U(OR),. These were obtained by alcoholysis of p(NEt2)4]2, or via the reaction of UC14 with LiOR in alcohol solution (R = Me, Et).2 In the case of U(OBU’)~, a third procedure was advocated. Uranium tetrachloride and four equivalents of potassium amide were allowed to react in liquid ammonia, and the product, formulated as U(NH2)4 (not isolated), was treated with excess t-butanol [eq. (l)] :

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W. G. VAN DER SLUYS et al.

solution, KU2(0B& was slowly and spontaneously transformed into the mixed-valent diuranium(IV,V) compound U2(0But)+ The same authors also found that another green uranium(IV) complex, viz. trinuclear UjO(OBut),,,, could, in at least one instance, be isolated from the same reaction mixture.6a In a subsequent paper on U30(OBu’), 0,6b the authors mentioned that their procedure for isolating the trimer differed from later preparations of KU,(OBt& in that the green hexane extract was kept at room temperature for an extended period of time. Dormond et al7 have found that alcoholysis of the mononuclear uranium(IV) metallocycle complex, [(Me,Si),Nj2UN(SiMe,)SiMe,CH,,* with either HO-2,6-Me2C6H3 or ButOH proceeds via initial rapid protonation of the U-C bond to form [(Me,Si)&UOR complexes. It was also noted that, in the presence of an excess of either alcohol, substitution of amide ligands occurred. These multiply substituted products were not characterized further. We have recently shown that Dormond’s procedure can be used to prepare U(O-2,6BL&,H~)~, which is the first structurally characterized example of a monomeric U(OAr)4 complex. ’ In this report, we describe the product obtained from the reaction of excess t-butanol with [(Me3Si)2N]ZUN(SiMe,)SiMe2CH2, some of its reaction chemistry, and its relationship to the compounds reported by Perego and Cotton.5*6 RESULTS AND DISCUSSION

The room temperature ‘H NMR spectrum of U2(0Bu’)8(HOBu’) in benzene-d, shows only one broad Bu’ signal (6 1.5) and a broad signal at 6 13.0, which we assign as the OH proton. The intensity ratio of these two signals is ca 80 : 1. We were unable to “freeze out” a static structure in toluene-d, in the temperature range +25 to -90°C. A v(O-H) stretch in the IR spectrum of U2(0Bu’)8(HOBu’) was not readily apparent. We tentatively assign it as the very weak band located at 3537 cm- ‘. Other IR features are similar, but not identical, to those reported by Perego and co-workers4 for their sample of U(OBu’),. U2(0Bu’)8(HOBut) is stable in toluene at room temperature. It reacts readily with potassium tbutoxide or potassium hydride in toluene or hexane to generate KU,(OBu’), [eqs (4X5)] : U2(0Bu’)8(HOBu’) + KOBu’= KUz(OBu’)g + HOBu’

(4)

U,(OBU’)~(HOBU’) + KH = KU2(OBut)g+0.5H2.

(5)

Upon exposure to dry air, green solutions of U2(0Bu’)8(HOBu’) rapidly turn orange-red. The UV-vis spectrum of the latter is essentially identical to that reported by Cotton and co-workers for a sample of the crystallographically characterized mixed-valent compound U2(OBtQg (Fig. l).’ These results, coupled with the fact that the UV-vis spectra of U2(0But),(HOBu’) and KU2(0Bu’)g are very

Synthesis and characterization

Reaction of [(Me3Si)2Nj2UN(SiMe,)SiMe,CHZ with > 4.5 equivalents of Bu’OH in toluene [eq. (2)], results in complete substitution of amides for alkoxides, but the resulting complex does not have the empirical formula of U(OBtQ4. We formulate the green, hydrocarbon-soluble product as U,(OBU~)~(HOBU~)on the basis of elemental analyses, ‘H NMR, IR and W-vis spectroscopies, and chemical studies. The same compound can be prepared in an analogous fashion from [U(NEt,),], [eq. (3)J :*T’

I

1

[(Me3Si)2Nj2Ul(SiMe3)SiMe2CH,+ > 4.5Bu’OH 3 0.5 U2(OBut),(HOBut)+

0

3(Me,Si),NH

(2)

[U(NEt2)4]2 + 2 9.0Bu’OH s U2(OBut),(HOBut) + 8Et2NH.

(3)

Ii”‘“” 500

600 nm

700

Fig. 1. Electronic absorption spectra of a 5.5 x low3 M and hexane solution of U,(OBut),(HOBu’) before (-) after (---) exposure to dry air.

Uranium alkoxide chemistry-V

0.21 -

A_ 0.07 -

400

500

nm

600

700

(4

Fig. 2. Electronic absorption spectra of U,(OBu’), (HOBu’) (---) and KUz(OBut)9 (), 3.1 x 1O-3 M hexane solutions.

similar (Fig. 2), suggests that the compounds have similar structures. Chem3D representations of KUz(OBu’)9 and the proposed structure of U2(0Bu’)8(HOBu’) are shown in Fig. 3. We have also found that when toluene solutions of Uz(OBu’)8(HOBu’) are treated with oxygen-free H20, another green compound is reproducibly isolated in good yield [eq. (6)] :

(b)

3U,(OBu’),(HOBu’) + 2H,O= 2U,O(OBu’) 1,,+ 7HOBu’.

(6)

The identity of the latter as U,O(OBu’) , o was confirmed by comparison of the unit cell dimensions of a single crystal with those reported in the literature.6 The ‘H NMR spectrum of U,O(OBu’), o, which has not been reported previously is shown in Fig. 4. The 3 : 3 : 3 : 1 pattern is consistent with the solid state CSO structure.6 A Chem3D representation of UjO(OBu’),,, is also shown in Fig. 3. The KU,(OBu’), used to record the spectrum shown in Fig. 2 was prepared in good yield directly from UC14 by the addition of 4.5 equiv. of KOBu’ in THF [eq. (7)]. This method is more convenient than the original liquid ammonia synthesis. KU,(OBuf), is thermally stable at room temperature under rigorously dry and anaerobic conditions. UC& + 4Seq. KOBu’B 0.5 KUz(OBu’)9+4KC1.

(7)

When exposed to dry air, solutions of KU2(0Bu’)9 turn orange-red much more slowly (days) than do solutions of U2(0But),(HOBut). This is consistent

(c) Fig. 3. Chem3D ‘* representations of (a) KU2(OB~‘)9,5 (b) U,(OBu’),(HOBut) (proposed) and (c) U,O(OBu’), ,,.6 Uranium, oxygen, carbon and potassium atoms are indicated with black, speckled, open and hashed circles, respectively; the t-butyl methyl groups have been omitted from the drawings for clarity.

with Gilman’s observations on the air-sensitivity of “U(OBU’);‘.~ KU2(0Bt&, like U2(OBu’),(HOBut), also reacts with H20 to produce UXO(OBu’),O.* Thus, the reaction properties of both KUz(OBu’)9 and U2(0Bu’)8(HOBu’) are complementary. A summary of all of this chemistry is given in Scheme 1. Magnetic properties of U,O(OBut), o

* The presence or absence of small amounts of adventitious water in the synthesis of KU,(OBu’), might account for the irreproducibility of the trimer synthesis.6b

As noted by Cotton,6b U30(0But)10 is structurally similar, in a qualitative sense, to the

Uranium

1847

alkoxide chemistry-V

one doublet state and two triplet states. I5 In the trimer, the local symmetry about each uranium(IV) centre is only C,. Lowering the symmetry to this level lifts all of the octahedral degeneracies. We propose that the empirical magnetic susceptibility is the sum of the susceptibilities of the three isolated uranium(IV) ions. The inverse susceptibility is nearly temperature independent in the region below 90 K and the measured results are consistent with a singlet ground state with a second singlet state (or states) located approximately 60 cm-’ above the former.*

cate that there is no magnetic ordering of its urani&IV) centres down to 2 K. It seems clear to us that U(OBU’)~ has not yet been reported in the literature. Since uranium(IV) compounds with sterically non-demanding ligands favour high coordination numbers (2 6), U(OBu’),, assuming it can be isolated, will surely adopt an oligomeric structure, [U(OBU’)~]~.Efforts to prepare this compound and related analogues are continuing in our laboratory. ”

EXPERIMENTAL SUMMARY

The history of “U(OBt&” dates back over 30 years. Early reports on the synthesis of this compound failed to provide conclusive proof for its existence.*-4 In our opinion, Cotton and co-workers have established beyond reasonable doubt that the compound originally prepared in Gilman’s laboratory was actually the potassium salt, KU2(0But)+ 5We have shown here that the alcohol adduct, U2(0But)8(HOBut), can be synthesized from well characterized starting materials, and have obtained spectroscopic data which support its formulation. We suggest that this is the same compound prepared by Perego and co-workers from U(C3H5)4 and excess t-butanol.4 We have also shown that Cotton’s KUz(OBt& can be prepared from U2(0Bu’),(HOBut), or directly from UCl, and potassium t-butoxide. KU2(0Bi_& and U2(0B& (HOBut) exhibit similar reactivity toward oxygen and water, and their UV-vis spectroscopic signatures indicate that they have similar structures.t Finally, we have developed a reproducible route to the previously reported trinuclear uranium(IV) complex, U,O(OBu’), ,,, via hydrolysis of either UZ(OBu’)8(HOBut) or KUz(OBut).+ U30(0Bu’),o is a promising starting material for further explorations of uranium(IV) cluster chemistry. Magnetic susceptibility data obtained on U,O(OBu’) , o indi-

* The inverse molar susceptibilitiesof [U(NEt,),],’ and U(S04),*4H20’6 also exhibit temperature independent behaviour at low temperature. Similar interpretations have been advanced to explain these observations. t The latter point remains to be confirmed by Xray crystallography. Structural characterization of U2(0But),(HOBu’) is in progress. $ The ‘H NMR spectra of the uranium complexes described in this paper are temperature dependent. The chemical shifts reported are for spectra collected at approximately

22°C.

General procedures

All reactions were carried out using standard techniques for the manipulation of air-sensitive materials. [(Me3Si)2N12UN(SiMe,)SiMe,CH, was prepared and purified as described in the literature.* ‘H NMR spectra were recorded on an IBM AF250 spectrometer at 250 MHz. Chemical shifts are reported in ppm relative to the solvent ‘H impurity of benzene-d, (7.15 ppm).$ IR spectra were recorded as Nujol mulls between KBr salt plates using a Digilab FTS40 spectrophotometer. Electronic absorption spectra were recorded as hexane solutions using 1.O and 0.10 cm matched quartz cells and a Perkin-Elmer Lambda 9 spectrophotometer. The X-ray powder pattern of KU2(0Bt& was collected using a Siemens D-500 diffractometer with a 0-0 configuration. Low temperature (- 9OC) single crystal X-ray data on U,O(OBu’), ,, were collected using a CAD-4 diffractometer. Elemental analyses were performed at Los Alamos using a Perkin-Elmer 2400 CHN Elemental Analyser and standard air-sensitive techniques. Samples were dried under high vacuum ( 10e4 torr) for 12 h. Hexane, toluene and THF were distilled under argon from sodium-potassium alloy and stored in a helium-filled dry-box operating at < 1 ppm oxygen. Bu’OH was dried by refluxing a benzene solution over magnesium, distilling the azeotrope and storing the solution in the dry-box. KOBu’ was purchased from Aldrich and resublimed before use to remove non-volatile impurities. Magnetic susceptibility data on U,O(OBu’), ,, were collected at the IBM T. J. Watson Research Center on a Quantum Design Superconducting Quantum Interference Device (SQUID) Magnetometer for temperatures ranging from 2 to 250 K at fields of 1 kOe and 10 kOe. Plots of magnetization vs field were linear at all temperatures. Data were background corrected for the sample tube (a 5 mm Pyrex NMR tube) and the ligands. The weight of the sample was 90 mg.

W. G. VAN DER SLUYS er al.

1848 U2(0But)s(HOBut)

Excess Bu’OH in benzene was slowly added to [(Me3Si)2Nj2UN(SiMe,)SiMe,CHz (1.00 g, 1.39 mmol) in 20 cm3 of toluene. The orange solution immediately turned green. The latter was stirred for 12 h at room temperature. The solution was evaporated to dryness in V~CUOand the resulting green solid was recrystallized from hexane at -40°C and dried under high vacuum. Yield : 0.67 g, 85%. ‘H NMR (benzene-d,, 22°C): 6 1.4 (br s, C(CH,),) ; 6 13.0 (br s, OH). IR (cm- ‘) : 3537 (VW, OH); 1600 (w); 1352 (s), 1217 (ms), 1185 (s); 959 (vs) ; 903 (ms) ; 761 (m) ; 747 (mw) ; 726 (mw) ; 498 (ms); 474 (m). Found: C, 36.37, 36.26; H, 6.89, 6.84. Calc. for C36Hs20&JZ: C, 38.09; H, 7.29%. KU,(OBu’), UC14 (1 .OOg, 2.63 mmol) was dissolved in 20 cm3 of THF to give a green solution. 1.33 g (11.84 mmol) of KOBu’ was added, in one portion, resulting in a colour change to purple-red and the formation of a white precipitate (KCl). Within a few minutes, the colour of the solution changed to green. After 12 h of stirring the solution was evaporated to dryness in um.m. The green product was extracted into hexane and filtered through Celite. The volume was reduced and the solution cooled to - 40°C yielding 1.00 g of large green crystals, 65%. ‘H NMR (benzene-d,, 22°C) : 6 3.1 (s, 36H), 2.7 (s, 18H), -0.5 (s, 9H), -1.1 (s, 18H). IR (cm-‘): 1360 (s); 1350 (s); 1220 (s); 1197 (s); 1177 (vs); 1050 (mw); 1022 (w) ; 980 (s) ; 950 (vs) ; 921 (ms) ; 904 (s) ; 765 (ms) ; 755 (m) ; 665 (w) ; 523 (m) ; 494 (vs) ; 475 (s). Found : C, 36.61 ; H, 6.52. Calc. forC36Hs109KU2 : C; 36.85; H, 6.97%. An X-ray powder pattern was in good agreement with the pattern calculated based on the single crystal data of Cotton et al.’

U,WBu’)

Io

10 cm3 of distilled Hz0 were added to 500 cm3 of toluene, and the resulting mixture was degassed by bubbling with HP argon for several hours. Addition of 50 cm3 of this wet toluene to 1.0 g of either KUz(OBut)g or U,(OBu’),(HOBu’) produced a slight colour change and the precipitation of a small amount of a solid. The solvent was immediately removed in UQCUO.The green solid was extracted with hexane, the extracts were filtered and the green solution was stripped to dryness. At this point, a small amount of an unidentified impurity could be detected by ‘H NMR spectroscopy(647.3,46.0, 15.5,8.8, -59.8, -60.8, -81.0; 1 : 2: 1 : 2: 1 : 2: 1). The compound was recrys-

tallized several times from hexane by slow cooling of a concentrated solution. The yields of U,O(OBu’) , o from these reactions range from 50 to 80%. ‘H NMR (benzene-de, 22°C) : 6 46.2 (s, 27H), 12.3 (s, 27H), -60.1 (s, 27H), -79.2 (s, 9H). Found : C, 32.63 ; H, 5.98. Calc. for C40H9001 ,U3 : C, 32.87 ; H, 6.22%. Acknowledgements-We wish to thank Drs C. J. Bums, J. Weinrach and E. Garcia for helpful discussion, and Dr D. Bish for obtaining the X-ray powder diffraction data on KU,(OBt&. Generous financial support for this work was provided by the Office of Energy Research, Dvision of Chemical Sciences, U.S. Department of Energy. REFERENCES 1. W. G. Van Der Sluys, A. P. Sattelberger, W. E. Streib and J. C. Huffman, Polyhedron 1989,8, 1247. 2. R. G. Jones, G. A. Martin and H. Gilman, J. Am. Chem. Sot. 1956,78,4285. 3. D. C. Bradley, R. N. Kapoor and B. D. Smith, J. Znorg. Nucl. Chem. 1962, 24, 863. 4. M. Brunelli, G. Perego, G. Lugli and A. Mazzei, J. Chem. Sot., Dalton Trans. 1979, 861. 5. F. A. Cotton, D. 0. Marler and W. Schwotzer, Znorg. Chem. 1984,23,4211. 6. (a) F. A. Cotton, D. 0. Marler and W. Schwotzer, Znorg. Chim. Acta 1984, 85, L31; (b) F. A. Cotton, D. 0. Marler and W. Schwotzer, Znorg. Chim. Acta 1984,95,207. 7. A. Dormond, A. A. E. Bouadili and C. Moise, J. Chem. Sot., Chem. Commun. 1985,914. 8. (a) S. J. Simpson, H. W. Turner and R. A. Andersen, Znorg. Chem. 1981, 20, 2991; (b) S. J. Simpson and R. A. Andersen, J. Am. Chem. Sot. 1981,103,4063. 9. J. G. Reynolds, A. Zalkin, D. H. Templeton, N. M. Edelstein and 1. K. Templeton, Znorg. Chem. 1976, 15, 2498. 10. (a) M. H. Chisholm, K. Folting, J. C. Huffman and E. M. Kober, Znorg. Chem. 1985, 24, 241; (b) M. H. Chisholm, K. Folting, J. C. Huffman and C. C. Kirkpatrick, Znorg. Chem. 1984,23, 1021. 11. J. G. Reynolds, A. Zalkin, D. H. Templeton and N. M. Edelstein, Znorg. Chem. 1977, 16, 599. 12. C. Miyake, Y. Hinatsu and S. Imoto, Chem. Phys. Lett. 1979, 63, 529. 13. E. Sinn, Coordination Chem. Rev. 1970, 5, 313. 14. C. J. O’Connor, Prog. Znorg. Chem. 1982,29,203. 15. C. A. Hutchinson Jr and G. A. Candela, J. Chem. Phys. 1957,27, 707. 16. J. Maluk, J. Solid State Chem. 1978,25, 355. 17. A dinuclear siloxide complex of uranium(IV), [U(OSiMe2Bu1)&, has recently been isolated and structurally characterized. W. G. Van Der Sluys, J. C. Huffman, W. E. Strieb and A. P. Sattelberger, in preparation. 18. Chem3D is a registered trademark of Cambridge Scientific Computing, Inc., Cambridge, MA 02139, U.S.A.