Arene-bridged diuranium complexes: Inverted sandwiches supported by δ backbonding [2]

Full text of DOI:10.1021/ja994484e

6108
J. Am. Chem. Soc. 2000, 122, 6108-6109
Arene-Bridged Diuranium Complexes: Inverted
Sandwiches Supported by δ Backbonding
Paula L. Diaconescu, Polly L. Arnold, Thomas A. Baker,
Daniel J. Mindiola, and Christopher C. Cummins*
Department of Chemistry Room 2-227
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139-4307
ReceiVed December 23, 1999
1
Treatment of U(I)(N[R]Ar)3 (R ) C(CH3)3, Ar ) 3,5-C6H3-
Me2) with KC8 in toluene has been found to provide an inverted
sandwich compound in which a toluene molecule bridges two
uranium bis-amido fragments in a symmetrical η6,η6 fashion.
Compound 1, (µ-C7H8)[U(N[R]Ar)2]2, is obtained reproducibly
in ca. 40% isolated yield on scales of ca. 500 mg as a dark brown
crystalline substance. To facilitate assignment of the NMR
spectrum of paramagnetic 1, the deuterated variant 1-d8 was
prepared by carrying out the synthesis in toluene-d8. The four
resonances for the bound toluene were thereby identified at +18.7,
Figure 1. Structural drawing of complex 1b. Bulky peripheral substit-
uents are omitted for clarity. Selected bond distances (Å): U-C(av),
2.594(9); U-N(av), 2.334(6); C-C(µ-toluene, av), 1.438(13).
Shorter uranium-carbon distances are found in cases where a
CnHn ring interacting with uranium carries a formal charge. For
2
example, in uranocene the η8-C8H8 ligand exhibits an average
2-
-65.0, -83.6, and -88.8 ppm in the H NMR spectrum of the
dU-C of 2.647(10) Å,8 while in U(η5-C5H5)4 the η5-C5H5- ligands
evince an average dU-C value of 2.807(18) Å.9 In the latter two
cases, the ligating ring requires negative charge to achieve Hu¨ckel
aromaticity,10 explaining on electrostatic grounds the relatively
short U-C bond lengths.
compound, the high-field resonances being assigned to the aryl
deuterons and the downfield signal signifying the deuteriomethyl
group.
It was found also that benzene-bridged diuranium compounds
could be obtained by carrying out the KC8 reaction in benzene
or benzene-d6. A single peak was observed at -81.5 ppm in the
2H NMR spectrum of (µ-C6D6)[U(N[R]Ar)2]2, in accord with the
chemical shift assignments for 1. Furthermore, an N-1-adamantyl
derivative (µ-C7H8)[U(N[Ad]Ar)2]2 (1b) likewise was obtained
upon KC8 treatment in toluene of uranium(IV) precursor U(I)-
(N[Ad]Ar)3, or in low yield from the reaction of UI3(THF)42 with
Li(N[Ad]Ar)(OEt2).3,4
Structural data were obtained for derivative 1b by single-crystal
X-ray crystallography. Accordingly the average C-C distance
for the bridging toluene molecule was determined to be 1.438-
(13) Å. Thus the arene undergoes a slight (ca. 0.04 Å) increase
in dC-C upon complexation, relative to free toluene.5 Noteworthy
are the short uranium-carbon distances, averaging in the experi-
mentally determined structure to 2.593(9) Å, the shortest such
distance being U(2)-C(3) at 2.503(9) Å, and the longest being
2.660(8) Å for U(1)-C(3). The latter two outlying distances
reflect the fact that C(3) is displaced slightly from the mean plane
of the complexed toluene molecule.
Known uranium complexes of benzene or its derivatives tend
to exhibit significantly longer U-C bond lengths. An example
of this is U(η6-C6Me6)(BH4)3,6 a uranium(III) complex exhibiting
a mean dU-C value of 2.93(2) Å. The latter complex can be
prepared from its benzene analogue U(η6-C6H6)(BH4)3 by treat-
ment with hexamethylbenzene, indicating that the more electron
rich arene is the better ligand for uranium(III). The related
tetrachloroaluminate derivative U(η6-C6H5Me)(AlCl4)3 displays
similarly long bonds involving its toluene ligand, the mean U-C
bond length in this case being 2.94(1) Å.7
Transition metal systems in which benzene or toluene bridges
two metal centers in a symmetrical η6,η6 fashion are rare,11 an
example being (µ-C6H6)[V(η5-C5H5)]2.12
Formulation of compound 1 as (µ-C7H8)[U(N[R]Ar)2]2 suggests
various possibilities for the uranium valency. One extreme requires
a formally divalent uranium center13,14 with a neutral arene bridge,
while another invokes trivalent uranium with an arene dianion.15
The chemical reactivity of compound 1 is consistent with the
formality of divalent uranium, inasmuch as 1 behaves as a 4e
reductant, giving rise to uranium(IV) derivatives and extruding
neutral toluene upon reaction with appropriate substrates.
Treatment of 1 with Ph2S2 (2 equiv) in cold pentane led to a
rapid color change to yellow. The yellow compound was isolated
in 74% yield and was determined via X-ray crystallography to
be the dimeric thiolate-bridged uranium(IV) derivative [U(µ-SPh)-
(SPh)(N[R]Ar)2]2 (2). No gases were evolved in the reaction of
1 with Ph2S2, according to a Toepler pump experiment.16
Similarly, treatment of 1 with azobenzene (1 equiv) in cold
n-hexane led to a reaction that was complete in less than 15 min.
A red crystalline compound was thereby obtained in 67% isolated
(8) Avdeef, A.; Raymond, K. N.; Hodgson, K. O.; Zalkin, A. Inorg. Chem.
1972, 11, 1083.
(9) Burns, C. J. Organomet. Chem. 1974, 69, 225.
(10) Berson, J. A. Angew. Chem., Int. Ed. Engl. 1996, 35, 2750.
(11) Neithamer, D. R.; Prknyi, L.; Mitchell, J. F.; Wolczanski, P. T. J.
Am. Chem. Soc. 1988, 110, 4421.
(12) Duff, A. W.; Jonas, K.; Goddard, R.; Kraus, H.-H.; Kru¨ger, C. J. Am.
Chem. Soc. 1983, 105, 5479.
(13) Warner, B. P.; Scott, B. L.; Burns, C. J. Angew. Chem., Int. Ed. 1998,
37, 959.
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120, 5836.
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H.; Day, V. W. Organometallics 1982, 1, 170.
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(3) Ruppa, K. B. P.; Desmangles, N.; Gambarotta, S.; Yap, G.; Rheingold,
A. L. Inorg. Chem. 1997, 36, 1194.
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(15) Further studies employing techniques such as XANES will be of
interest for determining the actual degree of oxidation of the uranium centers
in complex 1. See, for example: DenAuwer, C.; Madic, J.; Berthet, J. C.;
Ephritikhine, M.; Rehr, J. J. Radiochim. Acta 1997, 76, 211.
(16) Although the formulation of compound 1 as indicated in the text and
in Figure 2 is entirely consistent with the data in hand, it is difficult to rule
out the possibility that compound 1 might contain unobserved hydride ligands.
If such were the case, however, the reaction of 1 with Ph2S2 would be expected
to occur with production of H2. This is not the case, as evidenced by the
Toepler pump experiment showing that no gases are evolved in the course of
said reaction.
10.1021/ja994484e CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/09/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 25, 2000 6109
Figure 3. Two near-degenerate δ symmetry back-bonding orbitals, from
an ADF geometry-optimization calculation on (µ-C6H6)[U(NH2)2]2 in
idealized D2 symmetry.20 Selected calculated bond distances (Å): U-C,
2.569; U-N, 2.216; C-C, 1.461.
of the benzene LUMO. Stabilization of four electrons in this
manner is consonant with two δ symmetry backbonds from the
formal uranium(II) centers to the formally neutral benzene
molecule. The δ backbonding molecular orbitals are of a and b1
symmetry types. In the limit of complete transfer of electrons
5-8 from the two U centers to the bridging arene, uranium would
attain the +4 oxidation state and the bridging ligand would be
[arene]4-. That the δ bonding interactions in question are expected
to be quite covalent is indicated by the appearance of the contour
plots in Figure 3. In related work, Bursten has implicated a metal-
ring δ interaction in the bonding scheme for [U(η7-C7H7)2]-,21
while this type of interaction is seen also to be important for
metal-ring binding in Ti(η8-C8H8)(NtBu).22 Similar electronic
structure considerations presumably also apply to the inverse
sandwich compounds reported by Ephritikhine and co-workers,
compounds featuring two uranium centers bridged by a [η7-C7H7]
ring.23,24
Because of their high nodality,25 f orbitals can be construed as
ill-suited for π-back-bonding, such a construct being in accord
with the paucity of N21,26-28 and CO29-31 complexes of the early
actinide elements. On the other hand, the results reported herein
suggest that δ symmetry backbonding represents a vehicle for
gaining access to a divalent uranium synthon in the context of
arene binding.
Figure 2. Reactions of compound 1 with azobenzene and diphenyl
disulfide.
Acknowledgment. For support of this work the authors are grateful
to the National Science Foundation (CAREER Award CHE-9501992),
the National Science Board (1998 Alan T. Waterman award to C.C.C.),
and the Packard Foundation (Fellowship to C.C.C.). For technical
assistance with the Toepler experiment the authors are grateful to Alan
F. Heyduk and Daniel G. Nocera. For stimulating and inspirational
discussions concerning uranium chemistry C.C.C. thanks Carol J. Burns.
We also thank reviewer David L. Clark for insightful comments and
helpful discussions.
yield, formulated as the uranium(IV) phenylimido-bridged dimer
[U(µ-NPh)(N[R]Ar)2]2 (3) by virtue of a single-crystal X-ray
diffraction study. When said reaction was carried out with
azobenzene-d10, NMR spectral assignment was facilitated, three
signals being observed in the 2H NMR spectrum of 3-d10 at -6.45,
-7.86, and -43.06 ppm. Toluene extrusion in the azobenzene
reductive cleavage reaction was ascertained by treating 1-d8 with
[N(C6H5)]2 in hexamethyldisiloxane solvent with octane-d18
2
present as an internal integration standard. Integration of the H
Supporting Information Available: Full synthetic details and
spectroscopic data for all new compounds, full X-ray structural details
for compounds 1b, 2, and 3, and details of the computational study (PDF).
This material is available free of charge via the Internet at http://pubs.acs.org.
NMR spectrum of the crude mixture indicated production of ca.
0.75 equiv of C7D8. Azobenzene reductive cleavage to form bis-
phenylimido derivatives represents an intriguing N-N bond
cleavage process, few examples of which have been reported
previously.13,17-19
JA994484E
The calculated structure of (µ-C6H6)[U(NH2)2]2 (constrained
to D2 symmetry, see caption to Figure 3) reproduced the key
features of the structure of 1b quite closely.20 Electrons 1-4, the
four most energetic electrons in the system, were found to be
uranium-based and nonbonding in character. Electrons 5-8,
however, were found to be stabilized via bonding interactions
involving uranium 6d and 5f orbital participation, and participation
(21) Li, J.; Bursten, B. E. J. Am. Chem. Soc. 1997, 119, 9021.
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Mountford, P. Chem. Cummun. 1998, 1235.
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Chem. Soc., Chem. Commun. 1994, 847.
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Chem. Commun. 1997, 2501.
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(20) See the Supporting Information for details and references on the density
functional theory calculations, which were carried out using the Amsterdam
Density Functional (ADF) suite of programs.
(31) Conejo, M. D.; Parry, J. S.; Carmona, E.; Schultz, M.; Brennann, J.
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