1
Synthesis and crystal structure of [UO2(BH4)2(hmpa)2], a novel
uranyl complex and the first metal oxoborohydride
Claude Villiers a,*, Pierre Thuéry a, Michel Ephritikhine a,*
a
Service de Chimie Moléculaire, DSM, DRECAM, CNRS URA 331, Laboratoire Claude
Fréjacques, CEA Saclay, Bâtiment 125, 91191 Gif-sur-Yvette, France.
Abstract
Reaction of [UO2Cl2(THF)2]2 with NaBH4 in THF led to the formation of
[UO2Cl2–x(BH4)x(THF)n],
and
addition
of
hmpa
induced
the
crystallization
of
[UO2(BH4)2(hmpa)2], whose structure exhibits the bidentate ligation mode of the borohydride
ligand; this uranyl borohydride, which is thermally unstable with respect to elimination of
borane, is a catalyst in the hydroboration of substituted alkenes by NaBH4.
Keywords: Uranyl; Borohydride ligands; Hydroboration; Crystal structure
* Corresponding authors. Tel.: 33 1 6908 6436; fax: 33 1 6908 6640.
E-mail addresses: claude.villiers@cea.fr, michel.ephritikhine@cea.fr.
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Metal borohydride (tetrahydroborate) complexes attract much attention for the
diversity of their structures, related to the various ligation modes and fluxional character of
the BH4 ligand, and their reactions which give rise to a number of applications from synthesis
of borides or hydrides to catalysis, in particular hydrogenation, polymerization and
hydroboration of unsaturated molecules [1]. The most general route to these complexes is the
simple substitution of a borohydride ligand for a halide ion and in many cases, this
substitution is accompanied by the reduction of the metal centre. The reducing ability of BH4–
seems to be anathema to the presence of sensitive auxiliary ligands like terminal oxo groups.
Indeed, reactions of oxochloride compounds with NaBH4 gave borohydride or hydride
derivatives, as illustrated by the formation of [Na(dme)][V(BH4)4] [2] and [ReH7(PPh3)2] [3]
from [VOCl3] and [ReOCl3(PPh3)2], respectively, while treatment of the metalates [MO4]2–
(M = Cr, Mo, W, V) with borohydrides represents an efficient route to reduced transition
metal oxides MO2 [4]. Here we report on the preparation and X-ray crystal structure of
[UO2(BH4)2(hmpa)2], the first metal oxoborohydride complex and the first borohydride
compound of uranium in its highest +6 oxidation state. This new member in the family of
uranyl derivatives UO2X2 is an efficient catalyst in the hydroboration of substituted alkenes
by NaBH4.
The synthesis of the uranyl borohydride [UO2(BH4)2(THF)n] was attempted by
reacting [UO2Cl2(THF)2]2 or [UO2Cl2(THF)3] [5] with the stoichiometric amount or an excess
of NaBH4 in THF [6]. The colour of the solution turned from pale to bright yellow and the 1H
NMR spectrum showed a broad quartet (J = 85 Hz) centred at  7.8, attributed to the
borohydride ligand in [UO2Cl2–x(BH4)x(THF)n]; the intensity of this signal was maximum (x =
1.8) after 4 h at 20 °C. Attempts to grow crystals suitable for X-ray diffraction were
unsuccessful. The solution progressively deposited a not yet identified red-brown solid,
resulting from decomposition of [UO2Cl2–x(BH4)x(THF)n], indicated by the eventual decrease
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of the intensity of the BH4 resonance and liberation of free borane. As expected, uranyl
borohydride is much less stable than U(BH4)4 and its Lewis base adducts which require higher
temperature to be thermally reduced into the corresponding uranium(III) borohydrides [7].
Thus, the isolation of the pure uranyl borohydride was impeded by both its incomplete
formation and competing decomposition.
Addition of hmpa to the THF solution of [UO2Cl2–x(BH4)x(THF)n] readily afforded
light yellow crystals of [UO2Cl2–x(BH4)x(hmpa)2] which were characterized by X-ray
diffraction analysis [8]. The degree of BH4 substitution was dependent on the duration and
stoichiometry of the reaction of [UO2Cl2(THF)3] and NaBH4. When hmpa was added 4 h after
mixing these compounds in the 1 : 4 molar ratio, a mixture of isomorphous crystals which
analyzes as [UO2Cl(BH4)(hmpa)2] was deposited; crystals of [UO2(BH4)2(hmpa)2] (1) were
picked up from this mixture. The borohydride 1 is a new representative of the molecular
uranyl complexes UO2X2Ln which have witnessed a systematic development during the last
past years with the synthesis and structural characterization of the triflate [11a], iodide
[11b,11c], isocyanate [11d], perrhenate [11e] and pertechnetate [11f] derivatives.
In the IR spectrum of [UO2Cl2–x(BH4)x(hmpa)2], the U=O and P=O stretching
frequencies of 921 and 1089 cm–1 are virtually identical to those in the parent halides
[UO2X2(hmpa)2] (917 and 1086 cm–1 for X = Cl, 919 and 1084 cm–1 for X = Br) [11c,12]; the
two strong absorptions at 2360 and 2163 cm–1 are characteristic of bidentate borohydride
ligands [1]. These structural features have been confirmed by X-ray crystallography.
A view of 1 is shown in Figure 1 together with selected distances and angles. The
crystals are isomorphous with those of [UO2X2(hmpa)2] (X = Cl [12], Br [11c], I [11c], NO3
[13]); the line passing through the U and B atoms is a C2 axis of symmetry. The U=O(1) and
U–O(2) distances of 1.760(4) and 2.296(4) Å are identical to those measured in the halide and
nitrate congeners. The borohydride ligands are bidentate, the bridging hydrogen atoms H(1)
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and H(3) (found on a Fourier-difference map) being respectively at 0.091 and 0.005 Å from
the equatorial plane perpendicular to the linear UO2 fragment which contains the U, B(1),
B(2) and O(2) atoms. This mode of attachment of the tetrahedral BH4 ligand should be
preferred over the tridentate ligation pattern in which the coordinating hydrogen atoms would
be displaced from the equatorial plane [14]. In agreement with the variation in the radii of the
U4+ and U6+ ions [15], the U···B(1) and U···B(2) distances of 2.713(9) and 2.833(12) Å are
smaller than those of bidentate BH4 ligands in uranium(IV) complexes, which vary from
2.84(5) Å in [U(BH4)4(OPPh3)2] [16] to 2.92(1) Å in [U(BH4)5(-O)U(BH4)(18-crown-6)]
[17]; these U···B distances are larger than those of tridentate borohydrides of uranium(IV)
which average 2.6(1) Å from the Cambridge Structural Database [18].
We previously demonstrated that LiBH4 or NaBH4, in the presence of a catalytic
amount of BH3 (or metal halides like SnCl4 or TiCl4 which promote the decomposition of the
alkali metal borohydride), are able to hydroborate alkenes by following the unusual order of
decreasing reactivity: tetramethylethylene > 1-methylcyclohexene > cyclohexene > 1-hexene
[19]. The thermal instability of 1 with respect to elimination of borane could thus be exploited
in the borane-catalyzed hydroboration of substituted alkenes by NaBH4 [20]. Reaction of
Me2C=CMe2 with 1 molar equivalent of NaBH4 in the presence of 0.75 mol% of
[UO2Cl2(THF)2]2 gave Na(BH3CMe2CMe2H) which, after usual oxidative work-up, was
transformed into 2,3-dimethylbutan-2-ol in 60% yield; under the same conditions, 1-hexene
was converted into B(n-hexyl)3 in 2% yield, and no catalytic activity was observed [19].
5
Caption to Figure 1
Fig. 1. View of complex 1. The carbon-bound hydrogen atoms have been omitted.
Displacement ellipsoids are drawn at the 10% probability level. Symmetry code: ' = y, x, –z.
Selected distances (Å) and angles (°): U–O(1) 1.760(4), U–O(2) 2.296(4), U···B(1) 2.713(9),
U···B(2) 2.833(12), U–H(1) 2.32, U–H(3) 2.28, B(1)–H(1) 1.11, B(1)–H(2) 1.08, B(2)–H(3)
1.13, B(2)–H(4) 0.99, P–O(2) 1.491(4), O(1)–U–O(1’) 177.5(3), O(1)–U–O(2) 90.2(2), O(1)–
U···B(1) 91.24(15), O(1)–U···B(2) 88.76(15), O(2)–U···B(1) 88.87(12), O(2)–U···B(2)
91.13(12), B(1)···U···B(2) 180, U–O(2)–P 178.7(4).
6
References
[1]
(a) T.J. Marks, J.R. Kolb, Chem. Rev. 77 (1977) 263;
(b) M. Ephritikhine, Chem. Rev. 97 (1997) 2193.
[2]
J.A. Jensen, G.S. Girolami, Inorg. Chem. 28 (1989) 2114.
[3]
D. Baudry, M. Ephritikhine, H. Felkin, J. Organomet. Chem. 224 (1982) 363.
[4]
(a) J. Kim, A. Manthiram, J. Electrochem. Soc. 144 (1997) 3077;
(b) C. Tsang, A. Dananjay, J. Kim, A. Manthiram, Inorg. Chem. 35 (1996) 504;
(c) C. Tsang, S.Y. Lai, A. Manthiram, Inorg. Chem. 36 (1997) 2206;
(d) C. Tsang, A. Manthiram, J. Electrochem. Soc. 144 (1997) 520;
(e) A. Manthiram, A. Dananjay, Y.T. Zhu, Chem. Mater. 6 (1994) 1601.
[5]
M.P. Wilkerson, C.J. Burns, R.T. Paine, B.L. Scott, Inorg. Chem. 38 (1999) 4156.
[6]
Synthesis and characterization of uranyl borohydride: All reactions were carried out
under argon with the rigorous exclusion of air and water (< 5 ppm oxygen or water)
using standard Schlenk-vessel and vacuum line techniques or in a glove box. Solvents
were dried by standard methods and distilled immediately before use. A mixture of
[UO2Cl2(THF)2]2 (19.4 mg, 0.020 mmol) and NaBH4 (3.0 mg, 0.080 mmol) in THF-d8
(0.4 mL) was stirred for 4 h at 20 °C. The 1H NMR spectrum of the bright yellow
solution showed the formation of [UO2Cl0.2(BH4)1.8(THF)n]. 1H NMR (200 MHz, THFd8, 22 °C): 7.8 (br. q, J = 85 Hz, 7.2 H, BH4). After elimination of the pale brown
precipitate, hmpa (13.9 L, 0.080 mmol) was added into the solution and a crop of
yellow crystals which analyzes as [UO2Cl(BH4)(hmpa)2] was deposited (18 mg, 66%).
Anal. Found: C, 22.28; H, 6.25; B, 1.60; Cl, 5.30. Calc For C12H40BClN6O4P2U: C,
21.22; H, 5.84; B, 1.62; Cl, 5.23. 1H NMR (200 MHz, THF-d8, 22 °C): 7.30 (q, J = 81
Hz, 4 H, BH4), 2.79 (d, J = 9.5 Hz, 36 H, hmpa). No exchange was found to occur
7
between free and coordinated hmpa molecules. IR (cm–1): 2360 and 2163 (BH4), 1089
(P=O), 921 (U=O).
[7]
(a) H.I. Schlesinger, H.C. Brown, J. Am. Chem. Soc. 75 (1953) 219;
(b) J. Brennan, R. Shinomoto, A. Zalkin and N. Edelstein, Inorg. Chem. 23 (1984) 4143.
[8]
Crystal structure analysis: Data were collected on a Nonius Kappa-CCD area-detector
diffractometer with Mo-K radiation and processed with HKL2000 [9]. The data were
recorded at 270(2) K since cooling at lower temperatures resulted in breakage of the
crystal, likely as a result of phase transition. The structure was solved by direct methods
and refined by full-matrix least-squares on F2 with SHELXTL [10]. All non-hydrogen
atoms were refined with anisotropic displacement parameters. The hydrogen atoms of
the borohydride groups have been found on Fourier-difference maps and refined as
riding atoms with an isotropic displacement parameter equal to 1.2 times that of the
parent boron atom. Crystal data for 1: C12H44B2N6O4P2U, M = 658.12, tetragonal, space
group P43212, a = b = 10.5519(4), c = 24.5691(8) Å, V = 2735.59(17) Å3, Z = 4, Dc =
1.598 g cm3,  6.075
mm1, F(000) = 1288, 18970 measured reflections, 2594
independent (Rint = 0.083), 2253 with I > 2(I), 131 parameters, R1 = 0.033, wR2 =
0.067, S = 1.069, min = –0.43, max = 0.42 e Å–3. CCDC-625237 contains the
supplementary crystallographic data for this paper. These data can be obtained free of
charge
from
the
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
Crystals of the partially substituted, isomorphous compound [UO2Cl2–x(BH4)x(hmpa)2]
were also obtained. Attempts at refinement of both chlorine and boron positions led to a
value of x close to 1.5, but they were unsatisfying due to the many restraints on bond
lengths and displacement parameters which were necessary. It is to be noted that these
crystals, like those of [UO2Cl2(hmpa)2], are stable down to 100 K.
8
[9]
Z. Otwinowski, W. Minor, Methods Enzymol. 276 (1997) 307.
[10]
G.M. Sheldrick, SHELXTL, Version 5.1, Bruker AXS Inc., Madison, WI, USA, 1999.
[11] (a) J.C. Berthet, M. Lance, M. Nierlich, M. Ephritikhine, Eur. J. Inorg. Chem. (2000)
1969;
(b) J.C. Berthet, M. Nierlich, M. Ephritikhine, Chem. Commun. (2004) 870;
(c) M.J. Crawford, A. Ellern, K. Karaghiosoff, P. Mayer, H. Nöth, M. Suter, Inorg.
Chem. 43 (2004) 7120;
(d) M.J. Crawford, P. Mayer, H. Nöth, M. Suter, Inorg. Chem. 43 (2004) 6860;
(e) G.J. John, I. May, M.J. Sarsfield, H.M. Steele, D. Collison, M. Helliwell, J.D.
McKinney, Dalton Trans. (2004) 734;
(f) M.J. Sarsfield, A.D. Sutton, I. May, G.H. John, C. Sharrad, M. Helliwell, Chem.
Commun. (2004) 2320.
[12] J.C. Russell, M.P. Du Plessis, L.R. Nassimbeni, J.G.H. Du Preez, B.J. Gellatly, Acta
Crystallogr., Sect. B 33 (1977) 2062.
[13] P. Charpin, M. Lance, E. Soulié, D. Vigner, Acta Crystallogr., Sect. C 41 (1985) 884.
[14] J.C. Berthet, M. Nierlich, M. Ephritikhine, Dalton Trans. (2004) 2814.
[15] D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751.
[16] P. Charpin, M. Lance, G. Chevrier, D. Vigner, M. Lance, D. Baudry, Acta Crystallogr.,
Sect. C 43 (1987) 1255.
[17] C. Villiers, P. Thuéry, M. Ephritikhine, Acta Crystallogr., Sect. C 62 (2006) m243.
[18] F.H. Allen, Acta Crystallogr., Sect. B 58 (2002) 380.
[19] C. Villiers, M. Ephritikhine, Tetrahedron Lett. 44 (2003) 8077.
[20] Catalytic hydroboration of tetramethylethylene: A flask was charged with NaBH4 (61.2
mg, 1.61 mmol) and [UO2Cl2(THF)2]2 (12.2 mg, 0.012 mmol) and THF (4 mL) was
condensed in. After addition of Me2C=CMe2 (200 L, 1.61 mmol), the reaction mixture
9
was stirred for 24 h at 24 °C. Oxidative work-up with alkaline hydrogen peroxide
afforded 2,3-dimethylbutan-2-ol (60% by glc).
10
Figure 1