3074 Organometallics, Vol. 21, No. 15, 2002
Communications
thesis and characterization of the first actinide ketimido
complex, (C5Me5)2U(-NdCPh2)2.
Entry into this new class of organouranium complexes
is achieved by treatment of a diethyl ether solution of
(C5Me5)2UCl2 (1) with 2 equiv of a reducing species
(provided by KC8), followed by reaction with 1 equiv of
benzophenone azine, Ph2CdNNdCPh2. Filtration and
removal of solvent furnishes a brown powder, which
may be recrystallized from hot hexanes to provide
(C5Me5)2U(-NdCPh2)2 (2) in 68% yield (eq 1).7 Our
F igu r e 1. Molecular structure of 2 with thermal ellipsoids
at the 25% probability level. Selected bond distances (Å)
and angles (deg): U(1)-N(1), 2.179(6); U(1)-N(2), 2.185(5);
N(1)-C(1), 1.293(8); N(2)-C(14), 1.281(8); U(1)-N(1)-C(1),
173.4(6); U(1)-N(2)-C(14), 176.5(5); N(1)-U(1)-N(2),
107.2(2).
uranium(IV) bis(amido) complexes (e.g.: (C5Me5)2U-
{NH(2,6-Me2C6H3)}2, U-N ) 2.267(6) Å;4a (C5Me5)2U-
{NH(4-ClC6H4)}2, U-N ) 2.269(4) Å9).
convenient synthetic route into the bis(amide) ligand
system takes advantage of KC8 for the reduction of 1 to
generate the uranium(III) complex (C5Me5)2UCl(KCl),
which is known to function as an uranium(II) synthetic
equivalent.5c,d Reductive cleavage of the N-N bond in
benzophenone azine affords 2 in high yield.
The most striking aspects of the structure are the
nearly linear U-N-C bond angles (U(1)-N(1)-C(1) )
173.4(6)° and U(1)-N(2)-C(14) ) 176.5(5)°), which may
be consistent with the formulation of the ligands as
ketimido groups with the uranium metal center accept-
ing density from the nitrogen 2p lone pairs. Complex 2
is reminiscent of high-valent uranium(VI) bis(imido)
complexes which have significant uranium-imido multi-
ple bond character, as typified by the complex (C5Me5)2U-
(dN-Ph)2, which exhibits a U-N-C bond angle of
177.8(6)° and U-N ) 1.952(7) Å.10 The metal center in
complex 2 is not U(VI), which is evident upon compari-
son of the U-N bond distances in the two systems as
well as the NMR and UV-visible-near-IR spectra.
The modest isolated yield reflects the high solubility
of the complex rather than the formation of any byprod-
ucts. When the reaction is monitored by 1H NMR
spectroscopy, 2 is the only observable uranium-contain-
ing product and is formed in >95% yield. Also, the NMR
spectrum of 2 is sharp and paramagnetically shifted,
which suggests that the complex is a U(IV) species.
The identity of complex 2 as a uranium(IV) bis-
(ketimido) complex was unambiguously ascertained
by a single-crystal X-ray diffraction study (Figure 1).8
The molecular structure of 2 reveals a typical bent-
metallocene framework with a pseudotetrahedral coor-
dination environment about the uranium atom. Within
the metallocene wedge lie the two ketimido ligands,
which are terminally bound via nitrogen to the uranium
metal center (U(1)-N(1) ) 2.179(6) Å, U(1)-N(2) )
2.185(5) Å, and N-U-N ) 107.2(2)°). Importantly, the
uranium-nitrogen bond distances in 2 are significantly
shorter compared to other structurally characterized
In addition, the planes defined by N(1)dC(1)(Cipso 2
)
and N(2)dC(14)(Cipso 2 are essentially orthogonal
)
(111.1 and 85.1°, respectively) to the Cp(centroid)-U-
Cp(centroid) (Cp ) C5Me5) plane. The two NdC frag-
ments each have bond lengths (N(1)-C(1) ) 1.293(8)
Å, N(2)-C(14) ) 1.281(8) Å) typical for an sp2 carbon-
nitrogen double bond.11 The NdC bond distances
compare well with those found in other structurally
characterized ketimido complexes.6 All of the combined
structural features suggest that there is significant
ligand to metal π-bonding in both uranium ketimido
interactions.
(7) A flask was charged with 1 (0.952 g, 1.64 mmol), KC8 (0.493 g,
3.65 mmol), and diethyl ether (60 mL) and the mixture was stirred at
room temperature for 6 h. To this green slurry was added Ph2CdNNd
CPh2 (0.542 g, 1.50 mmol) in diethyl ether (40 mL). The resultant
mixture was stirred at room temperature for 12 h and then filtered
through a Celite-padded frit to remove the charcoal and KCl byprod-
ucts, and the volatiles were removed under reduced pressure to afford
a brown solid. This solid was taken up in toluene (30 mL) and filtered
through a Celite-padded frit to remove any residual KCl. The filtrate
was collected, and the volatiles were removed under reduced pressure
to give crude 2 as a dark yellow crystalline solid. Analytically pure
samples of 2 were obtained by recrystallization from hot hexanes (0.888
g, 1.02 mmol, 68%). 1H NMR (toluene-d8, 300 MHz, -75 °C): δ 88.88
(br s, 4H, Ar H), 26.32 (br s, 2H, Ar H), 24.49 (br s, 4H, Ar H), 9.18 (br
s, 2H, Ar H), -4.34 (br s, 4H, Ar H), -4.70 (s, 30H, Cp*-CH3), -10.89
(br s, 4H, Ar H). UV-vis-near-IR (ꢀ, M-1 cm-1, toluene): 323 (15 860),
465 (2210), 615 (680), 700 (210), 940 (35), 985 (61), 1111 (249), 1185
(295), 1245 (426), 1355 (150), 1381 (160), 1529 (196). MS (EI, 70 eV):
m/z 868 (M+), 733 (M+ - Cp*), 688 (M+ - NdCPh2). Anal. Calcd
for C46H50N2U (mol wt 868.91): C, 63.58; H, 5.80; N, 3.23. Found: C,
63.51; H, 5.85; N, 3.27.
The bis(ketimido) complex 2 is rather interesting in
that it is an f 2 system and yet it does not behave
chemically or electronically like known U(IV) amido
complexes.12 Perhaps the most distinguishing chemical
property of complex 2 is that it displays no reaction
chemistry with substrates such as benzophenone, phenyl-
acetylene, diphenylacetylene, carbon monoxide, pyridine
N-oxide, or dihydrogen. This is in marked contrast with
observations reported by Marks and Eisen, who have
shown that uranium(IV) amide complexes of the type
(9) Peters, R. G.; Scott, B. L.; Burns, C. J . Unpublished results.
(10) Arney, D. S. J .; Burns, C. J .; Smith, D. C. J . Am. Chem. Soc.
1992, 114, 10068-10069.
(11) Allen, F. H.; Kennard, P.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J . Chem. Soc., Perkin Trans. 2 1987, S1-S19.
(12) Complex 2 also exhibits an unusually high melting point
(201-203 °C). The mass spectrometry fragmentation pattern reveals
loss of C5Me5 rather than loss of NdCPh2, providing further evidence
for the robust character of the U(-NdCPh2)2 core.
(8) Data for brown crystal of 2: triclinic, P1h, Z ) 4, a ) 10.544(1)
Å, b ) 20.260(3) Å, c ) 20.501(3) Å, R ) 64.548(2)°, â ) 83.120(3)°,
γ ) 82.388(3)°, V ) 3909.9(9) Å3, T ) 203 K, R(I > 2σ) ) 0.0376,
and GOF ) 1.242.