A crystallizable f-element tuck-in complex: The tuck-in tuck-over uranium metallocene [(C5Me5)U{μ-η5: η1:η1-C5Me3(CH 2)2}(μ-H)2U-(C5Me5)2]

Article abstract of DOI:10.1002/anie.200801062

(Chemical Equation Presented) Long sought structural data on an f-element tuck-in complex have been obtained for the title compound 1 that contains the first example of both tuck-in and tuck-over bonding in a ligand derived from C₅Me₅^- by metalation (see scheme).

Full text of DOI:10.1002/anie.200801062

Angewandte
Chemie
DOI: 10.1002/anie.200801062
Diuranium Complexes
A Crystallizable f-Element Tuck-In Complex: The Tuck-in Tuck-over
Uranium Metallocene [(C5Me5)U{m-h5:h1:h1-C5Me3(CH2)2}(m-H)2U-
(C5Me5)2]**
William J. Evans,* Kevin A. Miller, Antonio G. DiPasquale, Arnold L. Rheingold,
TimothyJ. Stewart, and Robert Bau
One of the features of the C5Me5À group that makes it such a
desirable ligand in organometallic chemistry is the fact that it
À
is relatively inert to the C H activation that often complicates
the chemistry of C5H5À metallocene complexes.[1–4] Although
À
À
C5Me5 is more resistant to attack on the C H bond, its
methyl groups can be metalated with some highly reactive
metal species.[5–10] Since these C H activated ligands arise
À
from extremes in reactivity, they have been involved in
À
unprecedented reactions. For example, the homogeneous C
H
activation of methane was first discovered with
[{(C5Me5)2LuMe}n][5] based on a mechanism involving the
“tucked-in”[6] complex “[(C5Me5){C5Me4(CH2)}Lu]”. Tuck-
in[6] intermediates have also played prominent roles in
À
explaining the C H activation in complexes such as
[(C5Me5)2ScMe][6] and [(C5Me5)2Th(CH2CMe3)2].[11] Despite Lu[17]) in which the methylene group formed by C H
the repeated use of tuck-in complexes in mechanistic schemes activation is attached to a second metal atom. In some
À
À
involving f elements, no spectroscopic or crystallographic cases, double C H activation at two methyl groups resulted in
evidence has ever been presented to support the existence of C5Me3(CH2)23À ligands, for example, in [{(C5Me5)3Ln2[C5Me3-
such an intermediate in a lanthanide or actinide complex.
Tuck-in complexes have been crystallographically char-
(CH2)2]}2] (3; Ln = Ce[18] and Sm[19]).[13,20]
)
À
The only crystallographic evidence for this type of C H
acterized with transition metals.[7,9,10,12] For example, in the activation of a C5Me5À ligand in an actinide complex involves
{(C5Me5)2TiH} system, [(C5Me5)Ti{h5:h1-C5Me4(CH2)}] (1) a methylene group attached to nitrogen not the metal.
could be isolated.[7] However, in these transition-metal tuck- Thermolysis of the U6+–imido complex [(C5Me5)2U(
=
in compounds, low-oxidation-state tetramethylfulvalene res- NAd)2] (Ad = 1-adamantyl) formed [(C5Me5)U{h1:h5-NAd-
onance structures can contribute to the stability of the (CH2C5Me4)}(NHAd)] (4,[21])—a reaction that could involve a
complexes.[7,9,10,12,13] With the limited oxidation states avail- tuck-in (or tuck-over) intermediate. The only other sugges-
able for f elements, this is less likely.
tion of C5Me5À metalation in actinide chemistry is kinetic data
Crystallographic evidence for an alternative type of on a multiple-pathway transformation involving formation of
À
C5Me5 metalation has been obtained with lanthanides in [(C5Me5)2Th(m-CH2)2EMe2] from [(C5Me5)2ThR2] (R =
the form of “tuck-over” complexes [(C5Me5)2Ln(m-H)(m- CH2EMe3; E = C, Si).[11]
h1:h5-CH2C5Me4)Ln(C5Me5)] (2; Ln = Y,[14] La,[15] Sm,[16] and
We report here the first crystallographically characterized
tuck-in complex of an f element and the first crystallographic
data on uranium tuck-in and tuck-over structures. Both are
found in the same structure (Figure 1). This bonding mode
has not been observed previously in any metal complex of
ligands derived from C5Me5À to our knowledge.
Recent results on the chemistry of actinide hydride
complexes[22,23] made it desirable to determine if the equilib-
rium between tetravalent [{(C5Me5)2UH2}2] (5)[24] and triva-
lent [{(C5Me5)2UH}2] (6)[24] could be shifted to form pure 6 by
heating. When the mixture of 5 and 6 was heated to 1108C in
toluene, complex 7 was generated [Equation (1)].
[*] Prof. W. J. Evans, K. A. Miller
Department of Chemistry
University of California, Irvine, CA 92697-2025 (USA)
Fax: (+1)949-824-2210
E-mail: wevans@uci.edu
Dr. A. G. DiPasquale, Prof. A. L. Rheingold
Department of Chemistry and Biochemistry
University of California, San Diego, CA 92093-0358 (USA)
T. J. Stewart, Prof. R. Bau
Department of Chemistry
University of Southern California, Los Angeles, CA 90089 (USA)
X-ray crystallography revealed the structure of complex 7
(Figure 1). A doubly-metalated [m-h5:h1:h1-C5Me3(CH2)2]3À
ligand is engaged in both tuck-in and tuck-over binding to
uranium. The large thermal ellipsoids in the C1–C5 ring are
likely the result of multiple ring orientations. However,
[**] We thank the ChemicalSciences, Geosciences, and Biosciences
Division of the Office of Basic Energy Sciences of the Department of
Energy for support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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The reactions of 7 with phenol and C6H5OD gave further
support for the presence of hydrides. Reaction of 7 with
phenol yields H2 and the bis(phenoxide) complex
[(C5Me5)2U(OPh)2] (8) in 92% yield [Eq. (3)]. The identity
of 8 was confirmed by X-ray crystallography (Figure 2).
Reaction of 7 with C6H5OD gave HD[28] and a product that
had 2D NMR resonances at 3.1 ppm consistent with the
presence of deuterium in place of hydrogen in C5Me5À rings of
8.
Figure 1. Molecular structure of 7 (thermal ellipsoid drawn at the 50%
probability level).
resolution limits in the X-ray data were not adequate to
propose a disordered model. Consequently, the disorder was
treated as high thermal activity. Although the bridging
hydride ligands were not located in the X-ray crystal
structure, their presence was established by other means.
Attempts to prepare a diamagnetic thorium analog according
to Equation (1) were unsuccessful.
The infrared spectrum of 7 displayed a broad band
centered at 1164cm À1 in the region typical for U-H-U
stretching modes. This absorption band is similar to those
observed for both 5 and 6,[22,24] which give rise to broad bands
centered at 1188 and 1176 cmÀ1, respectively. Attempts to
synthesize a deuterium analog of 7 were thwarted by the fact
that neither [{(C5Me5)2UD2}2] nor [{(C5Me5)2UD}2] were
Figure 2. Molecular structure of 8 (thermal ellipsoid drawn at the 50%
probability level). The structure is similar to those of [(C5Me5)2U(EPh)2]
(E=S, Se)[26] except the U-O1-C21 and U-O2-C27 angles (174(1) and
172(1)8) are larger.[27]
accessible due to hydride exchange with the C5Me5 rings.[24]
The structure of 7 contains two uranium atoms separated
by 3.7917(5) Š, a distance intermediate between the U···U
distances in tetravalent 5 (3.606(6) Š) and trivalent 6
(3.8530(7) and 3.8651(7) Š).[22] The larger distance in 7 vs 5
is consistent with the more extensive bridging structure that
includes the tuck-over unit. The presence of the two “tuck”
À
The infrared spectrum of the product made according to
Equation (1), but from precursors synthesized from
[(C5Me5)2UMe2] and D2, was identical to that of 7.
The presence of hydrides in 7 was probed by measuring
the gas evolution during the synthesis of 7 from 6[22] by means
of a Toepler pump. Only one equivalent of dihydrogen per
two uranium atoms was obtained as shown in Equation (1).
Two equivalents of dihydrogen would be expected, if a U3+
complex without hydride ligands was formed instead of 7.
The presence of hydrides was also shown by the reactions
in Equation (2) and (3). Complex 7 reacts with (Et3NH)BPh4
to form the cation [(C5Me5)2U]BPh4[25] cleanly with evolution
of hydrogen as monitored by 1H NMR spectroscopy and
Toepler pump [Eq. (2)]. The yield of H2 was only 53% of that
expected, but if no hydrides were present in 7, only simple
protonation of the methylene groups would occur.
À
units in the C11–C15 ring led to significant variation of the U
C bond lengths to that ring, although the ring planarity is not
À
affected and the C C distances are equivalent within the
error limits. The ring carbon atoms attached to the methylene
À
groups, C11 and C15, have the shortest U1 Cring distances
(2.422(6) and 2.436(7) Š), for C12 and C14 these distances are
2.709(6) and 2.783(8) Š, and the ring carbon most distant
À
from the methylene groups has a U1 C13 bond length of
2.935(7) Š.
À
The U2 C16 tuck-over linkage (2.640(1) Š) is longer than
À
typical U Calkyl single bonds (for example, 2.414(7) and
2.424(7) Š for U CMe in [(C5Me5)2UMe2][29]). This is consis-
À
À
tent with the long Ln C bonds of the CH2 tuck-over groups
observed in lanthanide complexes.[14–19] The U1 C16 distance
À
À
(2.722(8) Š) is in the broad range of U CC Me distances, and
5
5
hence C16 may also transfer electron density to U1. A similar
situation is seen in the lanthanide tuck-over complexes.[14–19]
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Angewandte
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À
8: In a glovebox, a solution of PhOH (56 mg, 0.59 mmol) in
The U1 C20 distance, the first structurally characterized
toluene (3 mL) was added to a stirred solution of dark green 7
(152 mg, 0.150 mmol) in toluene (5 mL). The solution immediately
turned dark orange. After the mixture was stirred overnight, the
solution was evaporated to dryness to yield 8 as a dark orange
crystalline powder (192 mg, 92%). Crystals of 8 suitable for X-ray
diffraction studies were grown at À358C from a concentrated toluene
bond length between an f element and the carbon atom of a
CH2 tuck-in group, is 2.564(1) Š. Hence, this bond is longer
than a terminal alkyl bond, as expected for bridging ligands,
but it is not as long as for a methylene bridge attached to a
À
second metallocene. In comparison, the Ti CH2 distance in 1
is very similar to a Ti3+ Calkyl bond.[7]
solution. 1H NMR (500 MHz, C6D6): d = 3.19 (s, 30H, C5Me5, Dn1/2
=
À
5 Hz), 3.9 (t, 2H, JH,H = 8 Hz, p-H), 1.8 (t, 4H, 3JH,H = 8 Hz, m-H),
À13.6 ppm (d, 4H, 3JH,H = 8 Hz, p-H). 13C NMR (125 MHz, C6D6):
d = À32.8 (C5Me5), 141.1 (C5Me5), 125.1 (m-Ph), 104.1 (o-Ph),
108.1 ppm (p-Ph). IR (KBr): n˜ = 2972 (m), 2907 (m), 2857 (m), 1588
(vs), 1489 (vs), 1475 (vs), 1377 (w), 1252 (vs), 1276 (vs), 1160 (s), 1065
(m), 998 (m), 873 (vs), 863 (vs), 754(vs), 691, 604(s) cm À1. C,H
analysis calcd for C32H40O2U: C 55.33, H 5.80; found: C 55.62, H 5.50.
In a similar experiment, 7 (12 mg, 0.012 mmol) in C6D6 was added
to a J-Young tube containing a frozen slurry of (Et3NH)BPh4 (10 mg,
0.024mmol) in C 6D6. The J-Young tube was capped immediately and
3
The isolation of 7 raises several basic questions about the
À
À
reactivity of U H groups. Although it is reasonable that a U
H group in 6 can metalate a methyl group in C5Me5À, as has
[14,16]
À
previously been observed with Ln H bonds,
a sigma-
bond metathesis with elimination of H2 would lead to a
trivalent [(C5Me5)U(C5Me4CH2)] moiety from one of the
{(C5Me5)2UH} units in dimeric 6. It is not clear how or why a
2À
second metalation would occur at the C5Me4CH2 ligand to
form the observed tuck-in and tuck-over structure. It is
possible that the hydride ligands in 7 are formed by reduction
of H2 with U3+ ions in the same way that hydride ligands in 5
are formed from H2 and 6 in the reverse of the equilibrium in
Equation (1). Hence, as the C5Me5À rings are being metalated,
the H2 produced in this process may react with the U3+ centers
before it leaves the metal coordination sphere.
Clearly, the chemistry of uranium metallocene hydrides
has some extra dimensions that have not yet been fully
explored. New uranium hydrides are accessible that combine
1
a color change from brown-green to brown was observed. H NMR
spectroscopy showed quantitative conversion of starting material to
[25]
the previously characterized [(C5Me5)2U]BPh4
exhibited a H NMR resonance at 4.46 ppm.
and H2, which
1
¯
Compound 7 crystallizes in the space group P1 with a =
10.5198(17), b = 11.0156(17), c = 16.281(3) Š, a = 89.529(3), b =
81.943(3),
g = 80.842(3)8,
V= 1844.0(5) Š3,
Z = 2,
1calcd =
1.828 MgmÀ3, R1 = 0.0436 [I > 2s(I)], wR2 = 0.1114, GOF = 1.047.
Compound 8 crystallizes in the space group P1 with a = 9.409(2), b =
¯
9.667(2), c = 17.020(4) Š, a = 99.753(4), b = 96.714(4), g =
108.070(4)8, V= 1426.6(6) Š3, Z = 1, 1calcd = 1.617 MgmÀ3
0.078 [I > 2s(I)], wR2 = 0.184.
, R1 =
À
hydride and alkyl functionality. In addition, double C H
activation is possible in this class of compounds to form tuck-
in and tuck-over structures in a single complex. In any case,
the long sought crystallographic evidence for the postulated
f element tuck-in intermediates has been obtained and the
existence of both tuck-in and tuck-over structures for
actinides has been established.
Received: March 5, 2008
Published online: May 27, 2008
Keywords: actinides · coordination modes ·
.
cyclopentadienyl ligands · hydrides · uranium
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Experimental Section
7: In a glovebox, a green-brown solution of a 1:1 mixture of 5 and 6
(248 mg, 0.244 mmol) in toluene (10 mL) was heated to 1108C for 3
minutes with frequent venting. The mixture was allowed to cool
slowly to room temperature, and solvent was removed under reduced
pressure to yield a green oil. A concentrated solution in toluene
(3 mL) produced dark green crystals of 7 (188 mg, 0.185 mmol, 75%)
after 2 days at À358C. 1H NMR (C6D6, 298 K): d = À23.9 (br s, 30H,
C5Me5, Dn1/2 = 600 Hz), À2.6 ppm (s, 15H, C5Me5, Dn1/2 = 8 Hz), [À0.3
(s, 2H), 1.5 (s, 3H), 1.1 (s, 2H), C5Me5(CH2)2]. 1H NMR (C6D6,
343 K): d = À19.3 (s, 30H, C5Me5, Dn1/2 = 70 Hz), À7.7 (s), À1.5 (s,
15H, C5Me5, Dn1/2 = 11 Hz), 3.4(s), 1.2(s), 0.4(s). 13C NMR (C6D6,
343 K): d = À47.5 (C5Me5), À58.6 (C5Me5), 126.0 (C5Me5), 129.7 ppm
(C5Me5). IR (KBr): n˜ = 2966 (m), 2903 (vs), 2854(vs), 2722 (w), 1434
(m), 1377 (m), 1164(s), 1020 (s), 903 (m), 799 (m), 586 (m) cm
À1
.
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freeze-pump-thaw cycles, the solution was heated to 1108C for
20 minutes. The reaction mixture was frozen in liquid nitrogen and
evacuated using a Toepler pump equipped with a U-trap cooled in
liquid nitrogen. The non-condensable gas was collected (9.8 mmol,
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0.93 equiv) and subsequently analyzed by H NMR spectroscopy in
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crystalline material (0.469 g, 94%).
Angew. Chem. Int. Ed. 2008, 47, 5075 –5078
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