Diuranium inverted sandwiches involving naphthalene and cyclooctatetraene

Article abstract of DOI:10.1021/ja026200n

Treatment of IU(DME)(NC[^tBu]Mes)₃ (2-I-DME) with 4 equiv of KC₈ and 0.5 equiv of naphthalene in DME allowed the isolation of a naphthalene-bridged compound, K₂(μ-η⁶,η⁶-C₁₀H₈

Full text of DOI:10.1021/ja026200n

Published on Web 06/07/2002
Diuranium Inverted Sandwiches Involving Naphthalene and Cyclooctatetraene
Paula L. Diaconescu and Christopher C. Cummins*
Department of Chemistry, Room 2-227, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139-4307
Received March 14, 2002
Scheme 1
Discussions of bonding in the organometallic chemistry of
uranium have appeared with increasing frequency since the seminal
description of uranocene.1,2 Delta bonds have been proposed to play
a key role in stabilizing inverted sandwiches comprised of two
uranium atoms bridged by a cyclic aromatic hydrocarbon ligand.3
Examples to date of such inverted sandwiches involve benzene,
toluene,3 or cycloheptatrienyl4 as a planar symmetrical bridging
group. An ancillary ligand capable of supporting a broad range of
arene-bridged compounds was targeted to facilitate structural and
spectroscopic comparisons as a function of the bridging ligand. This
communication reports the chemistry of a uranium trisketimide
fragment that has allowed for the isolation of unique naphthalene-
and cyclooctatetraene-bridged diuranium complexes.
The previously described system (µ-C7H8)U2(N[tBu]Ar)4 (Ar )
3,5-C6H3Me2, 12-µ-C7H8)3 evinces a two-legged piano stool
coordination environment at uranium, one N-tert-butyl anilide ligand
having been stripped from each uranium center during conversion
from the uranium trisamide precursor, IU(N[tBu]Ar)3.5 Since the
6
synthesis involved treatment with excess KC8 in the neat arene
solvent, it is presumed that the stripped ligand was lost as its
potassium salt. The new ketimide ligand (NC[tBu]Mes, Mes )
2,4,6-C6H2Me3) employed in the present study allows for retention
of three supporting ligands per uranium, giving rise to a three-
legged piano stool geometry, and it allows also for incorporation
of potassium ions as tight ion pairs. A further advance accorded
by the implementation of ketimide ligands7 is the ability to use
dimethoxyethane (DME) solvent and stoichiometric amounts of a
particular desired hydrocarbon ligand, naphthalene, in the present
study.
8
Reaction of readily available UI3(DME)2 with KNC[tBu]Mes
in DME led to the isolation of dark green-brown IU(DME)(NC[t-
Bu]Mes)3 (2-I-DME), in 30% yield.9 A single-crystal X-ray
diffraction study of 2-I-DME revealed that a molecule of DME
coordinates to the uranium center in the pocket formed by the
mesityl groups. The observed near-linear UNC angles (average
168.2(8)°)10 are suggestive of significant π bonding between
nitrogen and uranium, as are the UN distances, which are shorter
on average by ca. 0.1 Å than those observed for related uranium
amides.3,5
Treatment of 2-I-DME with 4 equiv of KC8 and 0.5 equiv of
naphthalene in DME allowed the isolation of a naphthalene-bridged
compound, K2(µ-η6,η6-C10H8)[U(NC[tBu]Mes)3]2 (K2-22-µ-C10H8,
Scheme 1) in 60% yield as a dark brown powder.
The most interesting structural feature of this compound is the
coordination mode, µ-η6,η6, of the bridging naphthalene to the
uranium centers11 (Figure 1), reminiscent of the coordination mode
of toluene in compound 12-µ-C7H8. The twelve U-C distances
are quite short, varying from 2.565(11) Å to 2.749(10) Å. The
longer bonds are registered to the two carbon atoms fusing the two
six-membered rings, a fact understandable inasmuch as the LUMO
of naphthalene lacks any orbital contributions from these atoms.12
The C-C distances in the bound ring are regular and not
alternating (average of 1.443(6) Å), consistent with the aromatic
character expected for that ring, while in the pendant ring a diene-
like character is suggested by bond alternation (1.470(16), 1.319-
(17), 1.467(18) 1.381(15), and 1.395(15) Å). Each potassium ion
is clasped by a complement of two mesityl rings, the pendant portion
of the naphthalene ligand, and two ketimide nitrogen atoms in a
side-on fashion. Complexation of the potassium ions in this way
provides them with a near-spherical shroud of electron density,
revealing [22-µ-C10H8]2- to be an excellent alkali-metal cation
receptor. Furthermore, internalizing the positive ions permits
* Corresponding author. E-mail: ccummins@mit.edu.
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J. AM. CHEM. SOC. 2002, 124, 7660-7661
10.1021/ja026200n CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. For both complexes [(µ-C10H8)U2(NCH2)6]2- and [(µ-C8H8)U2-
(NCH2)6], electrons 5-8 are involved in covalent δ interactions between
the metals and the ring, the electrons being numbered in sequence of
decreasing energy. Calculations were spin unrestricted such that the orbital
containing electron 5 is pictured at the left for [(µ-C10H8)U2(NCH2)6]2-
and at the right for [(µ-C8H8)U2(NCH2)6].
Figure 1. Structural drawings of K2-22-µ-C10H8 (left) and 22-µ-COT (right)
with thermal ellipsoids at the 35% probability level. Methyl groups have
been omitted for clarity.
the system to present to its exterior solely lipophilic residues,
accounting for the observed high solubility in hydrocarbon sol-
vents.13 It is worth mentioning that neither the uranium nor the
potassium centers retain DME as a coordinated solvent molecule.
The U-N distances are elongated by about 0.1 Å with respect to
those in precursor 2-I-DME, consistent with an increase in formal
negative charge (decrease in oxidation state) at uranium.
character) exist for samarium, europium, and ytterbium.14 Com-
pounds Na-2-COT and 22-µ-COT (see the Supporting Information
for pictures and details) have been crystallographically character-
ized. The U-Carene distance in compound 22-µ-COT is longer on
average than that in its naphthalene counterpart K2-22-µ-C10H8
(2.822 vs 2.634 Å), in accord with bonding considerations (Figure
2) that indicate poorer covalent overlap in the former.
The corresponding sodium derivative, Na2(µ-η6,η6-C10H8)[U(NC[t-
Bu]Mes)3]2 Na2-22-µ-C10H8, was obtained as dark green-brown
crystals in 40% yield by reducing 2-I-DME over a sodium mirror
in tetrahydrofuran (THF) in the presence of 0.6 equiv of naphtha-
lene. A preliminary X-ray crystal structure indicated that Na2-22-
µ-C10H8 crystallizes with two THF molecules coordinated to each
Acknowledgment. For support of this work, the authors are
grateful to the National Science Foundation (CHE-9988806), the
Packard Foundation (Fellowship to C.C.C., 1995-2000), and the
National Science Board (Alan T. Waterman award to C.C.C., 1998).
The authors want to thank Theodor Agapie for performing the DFT
calculations for [(µ-C8H8)U2(NCH2)6] and V´ıctor Dura` Vila` for help
with the calculations for [(µ-C10H8)U2(NCH2)6]2-. P.L.D. thanks
Arjun Mendiratta for helpful suggestions and Jeff Simpson for help
with some NMR experiments.
1
sodium center, while H NMR spectroscopic data are consistent
with desolvation after vacuum drying for several hours.
Investigation of the naphthalene-bridged systems by H NMR
1
spectroscopy revealed fluxional behavior, a single ketimide ligand
environment being observed. For both M2-22-µ-C10H8 (M ) Na,
K) the corresponding monodeuterated and fully deuterated naph-
thalene-bridged compounds were prepared with R-naphthalene-d1
and naphthalene-d8. The solution structure is consistent with that
observed in the solid state with respect to the naphthalene
Supporting Information Available: Details of the X-ray crystal-
lographic studies, DFT calculations, synthetic procedures and charac-
terization data for the reported compounds (PDF and CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
2
coordination, since four H NMR signals are observed for the d8
derivative. Combining the results of 2D NMR correlation experi-
ments with the line width of the signals in the 1H NMR spectrum,
and with the signals found in the 2H NMR spectrum of Na2-22-µ-
C10H7D(R), the peaks at 79.2 (â-H) and -128.9 (R-H) ppm were
assigned to the deuterons of the ring bridging the two uranium
centers, while the peaks at -28.9 (R-H) and -36.5 (â-H) ppm were
assigned to the deuterons of the dangling ring (see the Supporting
Information for details and assignments for the potassium salt).
Treatment of M2-22-µ-C10H8 (M ) Na, K) with 2 equiv of
1,3,5,7-cyclooctatetraene afforded a mixture of two products
(Scheme 1). Compounds K[(COT)U(NC[tBu]Mes)3] (K-2-COT)
and [Na(S)][(COT)U(NC[tBu]Mes)3] (Na-2-COT, S ) Et2O) are
insoluble in pentane, facilitating their separation from the neutral
coproduct 22-µ-COT, (µ-η8,η8-COT)U2(NC[tBu]Mes)6 (Figure 1).
The ratio in which the two compounds are formed seems indepen-
dent of the solvent employed. If Na2-22-µ-C10H8 is used the two
compounds form in a 1:1 ratio and 22-µ-COT may be isolated in
35% yield. When K2-22-µ-C10H8 is used as a starting material for
the reaction with 1,3,5,7-cyclooctatetraene, almost only K-2-COT
is formed. Thus, the [2-COT]- anion is easiest to isolate as its
potassium salt (diethyl ether, 60% yield). Interestingly, compound
22-µ-COT can be assembled independently in 90% yield by salt
elimination upon reaction of M-2-COT with iodide 2-I-DME.
Reactions forming 22-µ-COT we refer to as “inverting ura-
nocene” because they result in a C8H8 ring being sandwiched
symmetrically between two uranium atoms instead of the reverse.
Structurally related systems (with presumably far greater ionic
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