Dilithiation of bis(benzene)molybdenum and subsequent isolation of a molybdenum-containing paracyclophane

Article abstract of DOI:10.1021/ja0692130

The homoleptic sandwich complex bis(benzene)molybdenum, [Mo(η⁶-C₆H₆)₂], was successfully dilithiated by employing an excess of BuLi in the presence of N,N,N,′,N′-tetramethylethylenediamine (up to 6 equiv each) at slightly elevated temperatures furnishing the highly reactive, ring metalated species [Mo-(η⁶-C₆H₅Li)₂] ·tmeda in high yields. Alternatively, this compound was synthesized upon prolonged sonication with 5 equiv of ^tBuLi/tmeda without heating. An X-ray crystal structure determination revealed a symmetrical, dimeric composition in the solid state, i.e., a formula of [Mo(η⁶-C ₆H₅Li)₂]₂·(thf)₆, where the six-membered rings are connected by two pairs of bridging lithium atoms. The synthesis of an elusive ansa-bridged complex failed in the case of a [1]bora and a [1]sila bridge due to the thermal lability of the resulting compounds. Instead, reverse addition of the dilithio precursor to an excess of the appropriate element dihalide facilitated the isolation of several unstrained, 1,1′-disubstituted derivatives, namely, [Mo{η⁶- C₆H₅(BN(SiMe₃)₂X)}₂] (X = Cl, Br) and [Mo{η⁶-C₆H₅(Si ⁱPr₂Cl)}₂], respectively. However, the incorporation of a less congesting [2]-sila bridge was accomplished. In addition to the formation of [Mo{ (η⁶-C₆H₅) ₂Si₂Me₄}], a molybdenum-containing paracylophane complex was isolated and characterized by means of crystal structure analysis. The ancillary formation of 1 equiv of bis(benzene)molybdenum strongly suggests that this species is generated by deprotonation of the ansa-bridged complex by the dilithiated precursor and subsequent reaction with a second equivalent of the disilane.

Full text of DOI:10.1021/ja0692130

Published on Web 03/22/2007
Dilithiation of Bis(benzene)molybdenum and Subsequent
Isolation of a Molybdenum-Containing Paracyclophane
Holger Braunschweig,*,† Nele Buggisch,† Ulli Englert,‡ Melanie Homberger,‡
Thomas Kupfer,† Dirk Leusser,§ Matthias Lutz,† and Krzysztof Radacki†
Contribution from the Institut fu¨r Anorganische Chemie, Julius-Maximilians UniVersita¨t
Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany, Institut fu¨r Anorganische Chemie der
Technischen Hochschule Aachen, Templergraben 55, D-52056 Aachen, Germany, and Institut fu¨r
Anorganische Chemie, UniVersita¨t Go¨ttingen, Tammannstrasse 4, D-37077 Go¨ttingen, Germany
Received December 22, 2006; E-mail: h.braunschweig@mail.uni-wuerzburg.de
Abstract: The homoleptic sandwich complex bis(benzene)molybdenum, [Mo(η6-C6H6)2], was successfully
dilithiated by employing an excess of BuLi in the presence of N,N,N′,N′-tetramethylethylenediamine (up to
6 equiv each) at slightly elevated temperatures furnishing the highly reactive, ring metalated species [Mo-
(η6-C6H5Li)2]‚tmeda in high yields. Alternatively, this compound was synthesized upon prolonged sonication
with 5 equiv of tBuLi/tmeda without heating. An X-ray crystal structure determination revealed a symmetrical,
dimeric composition in the solid state, i.e., a formula of [Mo(η6-C6H5Li)2]2‚(thf)6, where the six-membered
rings are connected by two pairs of bridging lithium atoms. The synthesis of an elusive ansa-bridged complex
failed in the case of a [1]bora and a [1]sila bridge due to the thermal lability of the resulting compounds.
Instead, reverse addition of the dilithio precursor to an excess of the appropriate element dihalide facilitated
the isolation of several unstrained, 1,1′-disubstituted derivatives, namely, [Mo{η6-C6H5(BN(SiMe3)2X)}2] (X
) Cl, Br) and [Mo{η6-C6H5(SiiPr2Cl)}2], respectively. However, the incorporation of a less congesting [2]-
sila bridge was accomplished. In addition to the formation of [Mo{(η6-C6H5)2Si2Me4}], a molybdenum-
containing paracylophane complex was isolated and characterized by means of crystal structure analysis.
The ancillary formation of 1 equiv of bis(benzene)molybdenum strongly suggests that this species is
generated by deprotonation of the ansa-bridged complex by the dilithiated precursor and subsequent reaction
with a second equivalent of the disilane.
Scheme 1. Metalation of Ferrocene Using BuLi/tmeda
Introduction
The discoveries and structural elucidation of the first sandwich
compounds, ferrocene, [Fe(η5-C5H5)2],1 and bis(benzene)chro-
mium, [Cr(η6-C6H6)2],2 are rightly regarded as landmark mo-
ments in organometallic chemistry. A host of interesting
chemical compounds and applications have stemmed from the
initial discoveries, which challenged traditional notions of
connectivity and enthroned the cyclopentadienyl ring as the
preeminent ligand in transition and f element metal chemistry.
Selective metalation of metallocenes and bisarene transition
metal complexes is a perennial area of related research, in which
1,1′-dilithiation of ferrocene by BuLi and N,N,N′,N′-tetrameth-
ylethylendiamine (tmeda) was an early and important achieve-
ment (Scheme 1).3 Such dimetalated derivatives have found
widespread application in the synthesis of ring-substituted
metallocenes, in particular for ansa-bridged type systems,4 which
have proven to be facile precursors for various metal-based
materials.5
Recently, the regioselective tetrametalation of ferrocene and
its higher homologues,6 the synergic monodeprotonation of bis-
(benzene)chromium,7 as well as the selective dimetalation of
ferrocene8 were reported by Mulvey and co-workers, ac-
complished by the enhanced basicity of mixed alkali metal-
magnesium amide bases. In addition, pentafluorophenylcopper
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Ed. 2005, 44, 5647.
† Julius-Maximilians Universita¨t Wu¨rzburg.
‡ Institut fu¨r Anorganische Chemie der Technischen Hochschule Aachen.
§ Universita¨t Go¨ttingen.
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E. O.; Hafner, W. Z. Anorg. Allg. Chem. 1956, 286, 146. (c) Seyferth, D.
Organometallics 2002, 21, 1520. (d) Seyferth, D. Organometallics 2002,
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9
4840
J. AM. CHEM. SOC. 2007, 129, 4840-4846
10.1021/ja0692130 CCC: $37.00 © 2007 American Chemical Society

Dilithiation of Bis(benzene)molybdenum
A R T I C L E S
Scheme 2. Metalation of Group 6 Bis(benzene) Sandwich
Complexes
Here, the complete characterization of the dimetalated deriva-
tive [Mo(η6-C6H5Li)2], as its various tmeda (2) and thf (3)
solvates, is reported, along with structural data for [Mo(η6-C6H5-
Li)2]2‚(thf)6 (3). This highly reactive species is converted to
several structurally characterized 1,1′-disubstituted derivatives
via salt elimination reactions with appropriate element dihalides.
In addition, the isolation of an elusive ansa-bridged derivative
has been accomplished, as well as the full characterization of a
molybdenum-containing paracyclophane complex.
[C6F5Cu]4 has been successfully employed in the selective
synthesis of mono- and dimetalated ferrocenylcopper com-
plexes.9 While ferrocene and its derivatives, by virtue of the
precursor’s air stability and comparative ease of synthesis,
dominate this field of chemistry, the reactivity of the isoelec-
tronic group 6 bisarenes, of which [Cr(η6-C6H6)2] is the prime
example, is still the subject of considerable inquiry. [Cr(η6-
C6H6)2] can be 1,1′-dilithiated in a similar fashion to ferrocene,
viz., by treatment with BuLi and in the presence of a coordinat-
ing amine such as tmeda at slightly elevated temperatures
(Scheme 2).10 With regard to [Mo(η6-C6H6)2] (1) and [W(η6-
C6H6)2],11 only the former can be synthesized in reasonable
quantity and is susceptible to lithiation reactions following
Scheme 2 at elevated temperatures.12 To date, no unequivocally
characterized, dimetalated derivative of [Cr(η6-C6H6)2] or [Mo-
(η6-C6H6)2] (1) has been reported, though both dilithiates have
been employed in further syntheses. The corresponding chem-
istry of the isoelectronic, mixed ring metallocenes [M(η7-C7H7)-
(η5-C5H5)] (M ) Ti,13 V,13,14 Cr4i,15) is also pertinent to mention
in this regard, although a dilithiated example has again eluded
structural characterization. At the same time, while ansa-bridged
derivatives of [Cr(η6-C6H6)2]4c,16 and [Cr(η7-C7H7)(η5-C5H5)]4i,15
have been prepared and structurally characterized, for instance,
[Cr(η6-C6H5)2BN(SiMe3)2]16d and [Cr(η7-C7H6)(η5-C5H4)SiMe2],15
scant few higher homologues of Mo or W have been reported,
and none at all have been unequivocally characterized.
Results and Discussion
As shown in Scheme 2, treatment of bis(benzene)molybde-
num (1) with BuLi and tmeda in aliphatic solvents such as
heptane gives excellent yields of the dilithiated derivative 2.
The alkyl lithium reagent has to be present in excess (up to 6
equiv), and for complete conversion the temperature must be
maintained at 55-60 °C for a prolonged period (12-24 h).
Alternatively, prolonged sonication in an ultrasonic bath of the
t
starting material with BuLi/tmeda (5 equiv each) in hexanes
can access the dilithiated material without heating. The presence
of tmeda in concert with BuLi is known to be essential to effect
deprotonation of the arene rings; alternative bases such as MeLi,
tBuLi, or BuK formed in situ were ineffective without the
addition of the ancillary base. The pale brown, pyrophoric
product may be recovered on a Schlenk frit and washed clean
of the excess BuLi and tmeda with pentane. Compound 2 is
thermally stable under an inert atmosphere, and 1H NMR
spectroscopy in thf-d8 indicates the presence of three multiplets
due to the ortho, meta, and para protons of the benzene ring.
However, dissolving in thf causes the replacement of the tmeda
ligand by coordinating thf molecules, and free tmeda was
detected. The integration ratio of the signals suggests the
incorporation of one tmeda molecule per arene complex, i.e., a
formula of 2‚tmeda. 13C NMR spectroscopy after prolonged
acquisition times reveals the expected four carbon resonances
of the arene rings, with the ipso-C-Li resonances shifted
downfield to δ ) 106.4 ppm with respect to the CH carbon
atoms. In addition, this signal is significantly broadened by the
(5) (a) Foucher, D. A.; Tang, B.-Z.; Manners, I. J. Am. Chem. Soc. 1992, 114,
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Mulvey, R. E.; O’Hara, C. T.; Parkinson, J. A. J. Am. Chem. Soc. 2004,
126, 11612.
7
quadrupolar momentum conferred by the Li nucleus. While
2‚tmeda is totally insoluble in aliphatic and aromatic solvents,
recrystallization from thf/benzene/pentane mixtures gives deep
brown-orange crystals that were formulated as 2‚(thf)3 on the
1
basis of H NMR spectroscopy. To confirm the selectivity of
the dimetalation, the molecular structure of the dilithiated
precursor (3) was determined by crystal structure analysis
(Figure 1).
In the solid state, 3 exhibits a symmetrical dimeric structure,
in which both molecules are connected by two pairs of bridging
lithium atoms bound to the ipso carbons of each C6H5 moiety.
The unsaturated lithium centers are stabilized by the coordination
of the thf oxygen atoms, whereas two distinct lithium environ-
ments can be distinguished; Li1 and Li1_a are stabilized by
two thf molecules and Li2 and Li2_a are coordinated by one
(7) Hevia, E.; Honeyman, G. W.; Kennedy, A. R.; Mulvey, R. E.; Sherrington,
D. C. Angew. Chem. 2005, 117, 70; Angew. Chem., Int. Ed. 2005, 44, 68.
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R. B. Chem. Commun. 2001, 1678.
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F. Angew. Chem. 2006, 118, 6992; Angew. Chem., Int. Ed. 2006, 45, 6838.
(10) Elschenbroich, C. J. Organomet. Chem. 1968, 14, 157.
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complexes of chromium, molybdenum, and tungsten. In Science of
Synthesis; Imamoto, T., Noyori, R., Eds.; Georg Thieme Verlag: Stuttgart,
Germany, 2003; pp 153-228.
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Organomet. Chem. 1986, 306, 145.
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Chem., Int. Ed. 2004, 43, 5530. (c) Tamm, M.; Kunst, A.; Herdtweck, E.
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Nguyen, T.-I.; Radacki, K.; Seeler, F. Chem. Eur. J. 2006, 12, 1266.
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Wocadlo, S.; Pebler, J. Inorg. Chem. 1993, 32, 5421. (b) Hultzsch, K. C.;
Nelson, J. M.; Lough, A. J.; Manners, I. Organometallics 1995, 14, 5496.
(c) Elschenbroich, C.; Schmidt, E.; Gondrum, R.; Metz, B.; Burghaus, O.;
Massa, W.; Wocadlo, S. Organometallics 1997, 16, 4589. (d) Braunschweig,
H.; Homberger, M.; Hu, C.; Zheng, X.; Gullo, E.; Clentsmith, G. K. B.;
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9
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A R T I C L E S
Braunschweig et al.
Scheme 3. Syntheses of 4 and 5 via Salt Elimination Reaction
both six-membered rings [2.371(4) and 2.377(4) Å]. As
expected, the aromatic rings are arranged almost parallel with
an angle between the planes of the C6H5 moieties of 3.9(3)°,
and the angle δ, defined by the ring centroids and the metal
center, indicates an almost linear arrangement (178.9°).
Given the availability of gram quantities of 2‚tmeda the
obvious course was to treat 2 with a series of element dihalides
and attempt the isolation of the elusive ansa-bridged species.
In fact, treatment of 2 with 1 equiv of a range of aminoboranes,
Figure 1. Molecular structure of the [Mo(η6-C6H5Li)2)]2(thf)6 (3) solvate.
For clarity, only the oxygen atoms of the thf molecules are shown. Symmetry
related positions (-x, -y, -z) are labeled with _a. Selected distances
(angstroms) and angles (deg): Li1-Li2 2.355(10), Li2-Li2_a 3.156(15),
Li1-C11 2.191(9), Li1-C1_a 2.149(9), Li2-C11 2.190(9), Li2-C1_a
2.186(9), Li1-O30 1.964(10), Li1-O40 1.943(16), Li2-O20 1.959(8),
Mo1-Caryl 2.249(5)-2.377(5) [ave 2.282(5)], Mo1-XPh1 1.787, Mo1-
i
X2BNR2 (X ) Cl, Br; R ) Pr, SiMe3) gave only the
1,1′-disubstituted bis(benzene)molybdenum derivatives, [Mo-
{η6-C6H5(BNR2X)}2], in low yields (Scheme 3). Yields were
notably improved by reverse addition of a slurry of 2‚tmeda to
2.5 equiv of the aminoborane in heptane or pentane, and hence,
these experiments were sufficiently reproducible to allow the
isolation and structural characterization of [Mo{η6-C6H5(BN
(SiMe3)2Cl)}2] (4) and [Mo{η6-C6H5(BN(SiMe3)2Br)}2] (5).
While the aromatic protons of the C6H6 rings in bis(benzene)-
XPh2 1.794, C11-Li-C1_a 106.7(4), C11-Li2-C1_a 105.5(4), XPh1
Mo1-XPh2 178.9 (XPh1 ) centroid of the C6H5 ring C1 to C6, XPh2
centroid of the C6H5 ring C11 to C16, respectively).
-
)
thf molecule, respectively. The thf molecules show extensive
disorder, thus requiring a combination of constraints and
restraints in the refinement, and as a consequence, the structural
parameters within these moieties are excluded from the follow-
ing discussion. As expected for a tetra-coordination, the bridging
lithium atoms Li1 and Li1_a display a distorted tetrahedral
environment. In contrast, the coordination sphere of the unsatur-
ated, three-coordinate lithium atoms Li2 and Li2_a deviates
significantly from an expected trigonal planar geometry (Σ )
336°) and is best described as a trigonal pyramidal arrangement,
which is presumably adopted for steric reasons. The lithium
‚‚‚lithium separation distances within the two bridging units
[Li1-Li2 2.355(10) Å] differ perspicuously from that found
between these two moieties [Li2-Li2_a 3.156(15) Å]. The
former is comparable to that found in cyclohexyllithium [2.397
Å]17 and might suggest a stabilizing lithium-lithium interaction.
However, according to earlier investigations on alkyllithium
reagents18 or the related dilithiated ferrocene derivative [Fe(η5-
C5H4Li)2]‚pmdta,19 this type of stabilization is weak at best.
The Li-C bond distances [2.149(9)-2.191(8) Å],19,20 as well
as the Li-O bond lengths [1.943(16)-1.964(10) Å],21 lie within
previously reported ranges. The structural parameters of the bis-
(benzene)molybdenum units are in agreement with the antici-
pated attributes of an unstrained 1,1′-disubstituted sandwich
molecule. The molybdenum CH carbon bond distances are found
within a range of 2.249(5) and 2.283(5) Å, though the
molybdenum ipso carbon bonds are significantly elongated for
1
molybdenum give rise to only one resonance signal in the H
and 13C NMR spectra, the introduction of two substituents on
the arene rings in 4 and 5 results in the expected splitting pattern
for time-averaged Cs symmetric species in solution. Both
compounds feature two distinct pseudotriplets for the meta [δ
) 4.67 (4) and 4.67 ppm (5)] and para protons [δ ) 4.90 (4)
and 4.91 ppm (5)], as well as a pseudodoublet for the ortho
protons [δ ) 4.95 (4) and 4.95 ppm (5)] with a relative intensity
of 4:2:4. Accordingly, three carbon resonances are observed in
the 13C NMR spectra of 4 and 5, whereas the ipso carbons
bearing the boryl substituents are not observed at room
temperature, a fact that can be assigned to the quadrupolar
momentum of the boron nuclei.22 It should be noted here that
the isolated material from the synthesis of 5 always contained
small amounts of LiBr‚tmeda, which could not be removed even
after several recrystallization steps. However, the NMR spec-
troscopic data are in excellent agreement with the results
reported for the corresponding disubstituted ansa complexes of
bis(benzene)chromium.16d
To gain more detailed insight into the structural properties
of 4 and 5, the molecular structures were determined by single-
crystal X-ray diffraction analyses (Figures 2 and 3). Both
molecules crystallize in the triclinic space group P1h with two
independent molecules in the asymmetric unit, whereas, in each
case, the structural parameters of both moieties differ only
marginally. Hence, for simplicity reasons only one of each
molecular structure is discussed below. The structural parameters
are unremarkable and can be compared to those found for the
related silyl-substituted complex [Mo{η6-C6H5(SiMe2H)}2].12
As expected, the unstrained character of 4 and 5 is demonstrated
by the linear alignment of the metal centers and the ring
centroids [δ ) 180.0°], as well as the consummately parallel
(17) Zerger, R.; Rhine, W.; Stucky, G. D. J. Am. Chem. Soc. 1974, 96, 6048.
(18) (a) Brown, T. L.; Seitz, L. M.; Kimura, B. Y. J. Am. Chem. Soc. 1968, 90,
3245. (b) Scovell, B. Y.; Kimura, B. Y.; Spiro, T. G. J. Coord. Chem.
1971, 1, 107.
(19) Walczak, M.; Walczak, K.; Mink, R.; Rausch, M. D.; Stucky, G. J. Am.
Chem. Soc. 1978, 100, 6382.
(20) Butler, I. R.; Cullen, W. R.; Ni, J.; Rettig, S. J. Organometallics 1985, 4,
2196.
(21) (a) Hoppe, I.; Marsch, M.; Harms, K.; Boche, G.; Hoppe, D. Angew. Chem.
1995, 107, 2328; Angew. Chem., Int. Ed. 1995, 34, 2158. (b) Wiberg, N.;
Hwang-Park, H.-S.; Mikulcik, P.; Mu¨ller, G. J. Organomet. Chem. 1996,
511, 239. (c) Nanjo, M.; Nanjo, E.; Mochida, K. Eur. J. Inorg. Chem. 2004,
2961.
(22) Herberhold, M.; Do¨rfler, U.; Wrackmeyer, B. J. Organomet. Chem. 1997,
530, 117.
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Dilithiation of Bis(benzene)molybdenum
A R T I C L E S
result, it appears plausible that the ansa-bridged complex has
been formed, but thermal instability prevented its isolation. The
reaction mixtures also routinely showed the presence of neutral
[Mo(η6-C6H6)2] (1), whose presence in a strictly anhydrous
medium argues that the dimetalated precursor is a potent
reductant and acted to reduce the added element dihalides to
unknown species. However, the yields of the 1,1′-disubstituted
derivative 6 have been essentially improved following a protocol
in analogy to the syntheses of 4 and 5, i.e., by reverse addition
of 2‚tmeda to an excess of the dichlorosilane (Scheme 4).
Scheme 4. Synthesis of 6 by Reaction of 2‚tmeda and SiiPr2Cl2
Figure 2. Molecular structure of 4. Symmetry related positions (-x, -y,
-z) are labeled with _a. Only one molecule of the asymmetric unit is shown
for clarity. Selected distances (angstroms) and angles (deg): Mo1-C1
2.294(5), Mo1-C2 2.265(5), Mo1-C3 2.293(5), Mo1-C4 2.273(5), Mo1-
C5 2.268(6), Mo1-C6 2.263(5), C1-B1 1.551(8), B1-Cl1 1.815(6), B1-
N1 1.420(8), N1-Si1 1.770(4), N1-Si2 1.768(4), Mo1-XPh 1.783, N1-
B1-C1 125.7(5), N1-B1-Cl1 118.6(4), C1-B1-Cl1 115.7(4), XPh
-
Mo1-XPh_a 180.0 (XPh ) centroid of the C6H5 ring).
The NMR spectroscopic data of 6 are in full agreement with
those of the 1,1′-diborylated complexes 4 and 5 and with the
presence of an unstrained sandwich derivative in general. The
1H NMR spectrum exhibits the expected splitting pattern for
the signal resonances of the ortho [δ ) 4.62 ppm], meta [δ )
4.71 ppm], and para protons [δ ) 4.84 ppm] with a relative
ratio of 4:4:2. In contrast to the aforementioned complexes 4
and 5, the 13C NMR spectrum shows four distinct resonances
for the chemically nonequivalent carbon atoms of the six-
membered rings; consequently, the ipso carbons bearing the silyl
substituents can easily be detected in this case. To confirm the
formation of an unstrained 1,1′-disubstituted derivative, a crystal
structure analysis of 6 was carried out (Figure 4).
Figure 3. Molecular structure of 5. Symmetry related positions (-x, -y,
-z) are labeled with _a. Only one molecule of the asymmetric unit is shown
for clarity. Selected distances (angstroms) and angles (deg): Mo1-C51
2.287(3), Mo1-C52 2.263(4), Mo1-C53 2.283(4), Mo1-C54 2.289(4),
Mo1-C55 2.284(4), Mo1-C56 2.269(4), C51-B1 1.550(5), B1-Br1
1.968(4), B1-N1 1.416(5), N1-Si1 1.773(3), N1-Si2 1.777(3), Mo1-
X
Ph 1.783, N1-B1-C51 125.6(2), N1-B1-Br1 119.1(3), C51-B1-Br1
115.4(3), XPh-Mo1-XPh_a 180.0 (XPh ) centroid of the C6H5 ring).
arrangement of the aromatic ring moieties; the angle between
the least-squared planes of the six-membered rings amounts to
exactly 0.0° in both 4 and 5. For steric reasons, the boryl
substituents are found in trans positions, and as a consequence,
the benzene rings adopt an eclipsed conformation. The boron-
nitrogen bond distances [4: 1.420(8), 5: 1.416(5) Å] are in
agreement with the presence of BdN double bonds, which is
further supported by the trigonal planar geometry of the boron
environments [4: Σ ) 360°, 5: Σ ) 360°].
While an ansa-bridge in respect of boron might be considered
unlikely, given the short B-C bond length, no such restriction
should apply to a silicon ansa-bridge. In fact, however, after
numerous attempts, no such silicon-bridged ansa complex could
be isolated. This result was also observed in ethereal solvents,
i.e., thf and Et2O, the use of which was prevented in the case
of the aforementioned boron halide reagents due to the likelihood
of ether cleavage. While a 1,1′-disubstituted product could be
obtained in poor yields for SiiPr2Cl2, i.e., [Mo{η6-C6H5(SiiPr2-
Cl)}2] (6), no tractable material could be obtained with SiMe2-
Cl2, SiPh2Cl2, SiiPrMeCl2 or SiMe2(O3SCF3)2, despite numerous
attempts under different conditions. In line with the reaction of
2‚tmeda and SiiPrMeCl2, however, analysis of the reaction
mixture by GC/MS gave a peak at m/z ) 240, which is
attributable to the free ligand SiiPrMePh2. On the basis of this
Figure 4. Molecular structure of 6. Symmetry related positions (-x + 2,
-x + y + 1, -z + 1/3) are labeled with _a. Selected distances (angstroms)
and angles (deg): Mo1-C1 2.263(4), Mo1-C2 2.278(4), Mo1-C3 2.291-
(4), Mo1-C4 2.278(4), Mo1-C5 2.270(4), Mo1-C6 2.284(5), C3-Si1
1.861(4), C13-Si1 1.877(4), C16-Si1 1.868(4), Si1-Cl1 2.077(1), Mo1-
XPh1 1.780, Mo1-XPh2 1.780, C3-Si1-C13 113.3(2), C3-Si1-C16 111.0-
(2), C13-Si1-C16 110.4(2), C3-Si1-Cl1 107.6(1), C13-Si1-Cl1 107.8-
(1), C16-Si1-Cl1 106.5(1), XPh1-Mo1-XPh2 176.8 (XPh1 ) centroid of
the C6H5 ring C1 to C6_a, XPh2 ) centroid of the C6H5 ring C1_a to C6,
respectively).
The structural parameters of 6 are unremarkable with respect
to the crystal structures of 4, 5, and [Mo{η6-C6H5(SiMe2H)}2],12
save that the silyl substituents do not adopt an anti arrangement
due to crystal packing effects; the angle between them amounts
to 123.5°. However, both rings are virtually planar and can be
considered as η6 coordinated. The separation distances between
the molybdenum atom and the centroids [1.780 Å] lie within
the expected range and are almost identical to those found in 4
and 5 [both 1.783 Å], respectively. As anticipated, the silicon
9
J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007 4843

A R T I C L E S
Braunschweig et al.
Figure 5. 1H NMR spectrum of the isolated solid comprising 7, 8, and 1 in a ratio of 8:1:1 in toluene-d8 at -60 °C.
centers display a distorted tetrahedral environment that differs
only marginally from the ideal geometry.
The utilization of dichlorotetramethyldisilane Si2Me4Cl2,
which undoubtedly should confer less molecular strain upon
the anticipated [2]sila derivative of bis(benzene)molybdenum
(7) in comparison to its hypothetical [1]bora and [1]sila
congeners, and therefore should make its isolation more likely,
has led to a surprising result. Reaction of the dilithio precursor
2‚tmeda with Si2Me4Cl2 afforded after workup an orange powder
Figure 6. Molecular structure of 8. Symmetry related positions (-x, -y
+ 2, -z) are labeled with _a. Selected distances (angstroms) and angles
(deg): Mo1-C1 2.264(1), Mo1-C2 2.268(1), Mo1-C5 2.267(1), Mo1-
C4 2.264(1), Mo1-C5 2.270(1), Mo1-C6 2.269(1), Si1-Si2 2.381(1), C1-
Si1 1.895(1), C4_a-Si2 1.897(1), Si1-C7 1.876(2), Si1-C8 1.874(2), Si2-
C9 1.871(2), Si2-C10 1.876(2), Mo1-XPh 1.761, C1-Si1-Si2 106.6(1),
C4_a-Si2-Si1 106.4(1), C1-Si1-Si2-C4_a -0.5(1), XPh-Mo1-XPh_a
180.0 (XPh ) centroid of the C6H5 ring).
that turned out to be a mixture of three different compounds
1
(Scheme 5). The H NMR spectrum exhibits signals for the
Scheme 5. Synthesis of the ansa-Bridged Derivative 7
One of the additional compounds (1H, 4.65 ppm; 13C, 75.2
ppm) formed in the course of this reaction can be unambiguously
identified as bis(benzene)molybdenum (1) by comparison of the
chemical shifts with a corresponding pure sample. However,
the identity of the other remained unclear until its composition
has been revealed by a single-crystal X-ray diffraction study,
due to its preeminent crystallization properties. The molecular
structure of the isolated paracyclophane 8 is depicted in Figure
6 and represents along with its lighter congeners of chromium,
namely, [Cr(η6-C6H4Si2Me4)2]23 and [Cr(η6-C6H4C2H4)2],24 the
only known paracyclophane complex derived from the benzene
perimeter.
The structural parameters of 8 show several differences in
comparison to the unstrained 1,1′-disubstituted derivatives 4,
5, and 6, which are related to the rigid ligand framework. Hence,
the distance between the centroids of the six-membered rings
is slightly contracted to a value of 3.522 Å [cf., 4 and 5, 3.566
Å; 6, 3.560 Å] and the molybdenum carbon bond lengths
[between 2.264(1) and 2.270(1) Å] are found in a closer interval
than in 4, 5, and 6 [between 2.261(4) and 2.294(5) Å]. However,
these alterations are much less pronounced than expected for
such a paracyclophane species. The silicon-silicon bond
carbocyclic hydrogen atoms which can be unambiguously
assigned to the desired ansa-bridged complex 7 [δ ) 4.89 ppm
(m, 4H); 4.97 ppm (m, 4H); 5.01 ppm (m, 2H)] as the main
product but shows additional singlets at δ ) 4.65 ppm and δ )
5.31 ppm in a relative ratio of 4:3 (Figure 5). In addition, the
aromatic carbon atoms of 7 give rise to four resonance signals
in the 13C NMR spectrum [between 77.3 and 80.3 ppm], whereas
three ancillary signals were observed at δ ) 75.2, 81.1, and
84.6 ppm. According to two-dimensional 1H,13C-HMQC NMR
experiments, the former resonance is associated with the singlet
1
at 4.65 ppm in the H NMR spectrum and the latter one with
that at 5.31 ppm. The remaining resonance proved to be a
quaternary aromatic carbon atom, evidenced by 13C DEPT 135
NMR spectroscopy, that display a cross-peak with the singlet
at 5.31 ppm in the two-dimensional long-range 1H,13C-HMBC
NMR spectrum. Accessorily, the 29Si NMR spectrum comprises
two distinct resonances with chemical shifts of δ ) -14.8 and
-17.3 ppm, respectively, indicating similar chemical environ-
ments of the silicon nuclei. Even the [2]sila derivative 7 is
thermally unstable and decomposes within 10 h in benzene
solutions to yield the free ligand Ph(Me)2Si-Si(Me)2Ph and
black molybdenum metal. However, in the solid state, 7 can be
stored for several months at room temperature under an inert
atmosphere.
(23) Elschenbroich, C.; Hurley, J.; Massa, W.; Baum, G. Angew. Chem. 1988,
100, 727; Angew. Chem., Int. Ed. 1988, 27, 684.
(24) Elschenbroich, C.; Mo¨ckel, R.; Zenneck, U. Angew. Chem. 1978, 90, 560;
Angew. Chem., Int. Ed. 1978, 17, 531.
9
4844 J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007

Dilithiation of Bis(benzene)molybdenum
A R T I C L E S
Scheme 6. Proposed Mechanism for the Formation of the
Paracyclophane Derivative 8
lective dilithiation of 8 is in agreement with previous results
obtained for the metalation of substituted ferrocenes, which
yields 1,2-disubstituted complexes for donor-substituted fer-
rocenes such as [Fe(η5-C5H4CH2NMe2)(η5-C5H5)]27 or 1,3-
disubstituted derivatives for different functionalized ferrocenes
t
such as [Fe(η5-C5H4X)2] (X ) SiMe3, Bu).25,28 It should be
noted that all attempts to prepare 8 selectively by tetrametalation
of bis(benzene)molybdenum and subsequent reaction with 2
equiv of Si2Me4Cl2, or by deprotonation of the ansa-bridged
species 7 with BuLi/tmeda followed by treatment with the
disilane, were unsuccessful to date.
Summary and Conclusions
In this paper, the derivatization of metallocenes via metalation
and subsequent salt elimination reaction with element dihalides
has been extended to bis(benzene)molybdenum. The use of
conventional bases such as BuLi and tmeda has allowed for
the isolation and full characterization of [Mo(η6-C6H5Li)2]‚
tmeda, including a crystal structure analysis of its thf solvate,
i.e., [Mo(η6-C6H5Li)2]2‚(thf)6 (3). Structural characterization of
metalated sandwich complexes is still a challenging area due
to the high reactivity of these compounds, and hence, the
solution of the molecular structure of 3 contributes to the
understanding of the fundamentals that determine the conforma-
tion of this class of organometallic compounds. The surprising
feature of the molybdenum chemistry was the thermal stability
of the dilithiated derivative on the one hand and the difficulties
of converting this reactive material to the appropriate ansa-
bridged bis(benzene)molybdenum derivatives on the other hand.
The introduction of a [1]bora and a [1]sila bridge was prevented
by the thermal lability of the resulting products. Instead, the
1,1′-disubstituted derivatives were exclusively formed, whereas
the 1,1′-diboryl- and the 1,1′-disilyl-substituted complexes 4,
5, and 6 were isolated and characterized by means of X-ray
diffraction. The utilization of the larger disilane bridge led to
the isolation of the first ansa-bridged bis(benzene)molybdenum
derivative, [Mo{(η6-C6H5)2Si2Me4}] (7), as well as the structural
characterization of the unexpected paracyclophane complex 8,
[Mo(η6-C6H4Si2Me4)2]. The formation of 8 was explained by
the enhanced reactivity of 7 toward deprotonation by dimetalated
bis(benzene)molybdenum and subsequent reaction with a second
equivalent of the disilane.
distance [2.381(1) Å], as well as the torsion angle C1-Si1-
Si2-C4_a [-0.5(1)°], are comparable to the values found in
the corresponding chromium derivative [2.361 Å and 0.22°,
respectively].23 In addition, the silicon centers display an almost
ideal tetrahedral environment, whereas the largest discrepancy
is found for the ipso carbon-Si-Si angles [C1-Si1-Si2 106.6-
(1)° and C4_a-Si2-Si1 106.4(1)°]; consequently, these data
suggest a minor molecular ring strain present in 8 than in the
chromium congener [102.7 and 102.9°].23 As anticipated for
such a structural motif, the planes of the arene rings are disposed
parallel to each other, and a perfect linear arrangement of the
metal center and the ring centroids is observed. With the
structural composition of 8 in mind, all observed signal
resonances of the NMR spectroscopic experiments can be
completely assigned to the ansa-bridged derivative 7, the
paracyclophane species 8, and bis(benzene)molybdenum (1).
Additionally, the integration of the resonances for 8 (8 protons)
and 1 (12 protons) in the 1H NMR spectrum, which was found
to be 3:4, results in a relative ratio of 1:1 of these compounds
in the reaction mixture.
The formation of 8 in concert with the appearance of bis-
(benzene)molybdenum during the synthesis of 7 can be ex-
plained by the enhanced reactivity of the ansa-bridged complex
7 toward deprotonation, which can be accomplished by the
reactive precursor 2‚tmeda; i.e., 2 can also act as a strong base
in this reaction (Scheme 6). Thus, once the [2]silametal-
loarenophane 7 has formed, the arene rings are regioselectively
dimetalated in the para position to yield a highly reactive,
dilithiated intermediate in small concentrations, which, on its
part, is converted to the paracyclophane 8 by salt elimination
reaction with another equivalent of Si2Me4Cl2. Given the fact
that the reaction generates 8 and bis(benzene)molybdenum (1)
in a ratio of 1:1, the proposed mechanism appears plausible,
although, the reactive intermediate has been neither isolated nor
observed spectroscopically in any case. However, the proposed
mechanism is supported by the following aspects: (1) substituted
ferrocenes, for instance, [Fe(η5-C5H4SiMe3)2],25 [Fe(η5-C5H4-
Cl)2],26 or [Fe(η5-C5H4CH2NMe2)(η5-C5H5)],27 are prone to
deprotonation under conditions similar to, or even milder than,
those for the metalation of the parent ferrocene; (2) a regiose-
Experimental Section
General remarks: All manipulations were performed under an inert
atmosphere of dry argon using standard Schlenk techniques or in a
glovebox. Solvents were dried according to standard procedures and
stored under argon over molecular sieves. Deuterated solvents were
distilled from potassium over a glass bridge and subjected to several
freeze-pump-thaw cycles. [Mo(η6-C6H6)2],29 Cl2BN(SiMe3)2,30 and
31
Br2BN(SiMe3)2 were prepared according to known methods. All
dichlorosilanes were obtained from Aldrich and distilled from mag-
nesium turnings before use. BuLi and tBuLi were purchased from Acros
as 1.6 mol L-1 and 1.5 mol L-1 solutions in hexanes or pentane,
respectively. The tmeda was dried over K and distilled under argon
prior to use. The NMR spectra were recorded on a Bruker Avance 200
(1H, 200.13 MHz), a Bruker DRX 300 (7Li, 116.64 MHz), and on a
(25) Brown, R. A.; Houlton, A.; Roberts, R. M. G.; Silver, J. Polyhedron 1992,
(28) Masson, G.; Beyer, P.; Cyr, P. W.; Lough, A. J.; Manners, I. Macromol-
ecules 2006, 39, 3720.
11, 2611.
(26) Blake, A. J.; Mayers, F. R.; Osborne, A. G.; Rosseinsky, D. R. J. Chem.
Soc., Dalton Trans. 1982, 2379.
(27) Rausch, M. D.; Moser, G. A.; Meade, C. F. J. Organomet. Chem. 1973,
51, 1.
(29) Silverthorn, W. E. Inorg. Synth. 1977, 17, 54.
(30) Geymayer, P.; Rochow, E. G.; Wannagat, U. Angew. Chem. 1964, 76, 499;
Angew. Chem., Int. Ed. 1964, 9, 633.
(31) Haubold, W.; Kraatz, U. Z. Anorg. Allg. Chem. 1976, 421, 105.
9
J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007 4845

A R T I C L E S
Braunschweig et al.
Bruker AV 500 (1H, 500.13 MHz; 11B, 160.46 MHz, 13C, 125.76 MHz,
29Si, 99.36 MHz) FT-NMR spectrometer. 1H and 13C{1H} NMR spectra
were referenced to external TMS via the residual protio of the solvent
(1H) or the solvent itself (13C). 7Li{1H} NMR spectra were referenced
to external LiCl, 11B{1H} NMR spectra to BF3‚OEt2, and 29Si{1H} NMR
spectra to external TMS. Microanalyses for C, H, and N were performed
on a Leco CHNS-932 elemental analyzer.
(s, 36H, Si(CH3)3), 4.67 (m, 4H, m-C6H5), 4.91 (m, 2H, p-C6H5), 4.95
(m, 4H, o-C6H5). 11B{1H} NMR (160 MHz, C6D6, 297 K): δ ) 46.6.
13C{1H} NMR (126 MHz, C6D6, 297 K): δ ) 2.87 (Si(CH3)3), 76.2,
81.3, 83.4 (C6H5). Correct elemental analysis could not be obtained
due to LiBr‚tmeda impurities.
[Mo{η6-C6H5(SiiPr2Cl)}2], 6. A slurry of 2‚tmeda (0.49 g, 1.28
mmol) in heptane (20 mL) was added over a period of 10 min to a
well-stirred solution of Cl2SiiPr2 (0.71 g, 3.84 mmol, 3.0 equiv) in
heptane (20 mL) at -78 °C. The brown mixture was allowed to warm
to room temperature and was stirred for an additional 3 h. After the
solid had settled, the solution was filtered into another flask by a filter
canula and the brown filtrate was concentrated to about 10 mL in
volume to yield orange-brown crystals of 6 (0.26 g, 0.47 mmol, 37%),
which were subsequently washed with cold pentane (3 × 5 mL, -100
°C) and dried in vacuo. 1H NMR (500 MHz, C6D6, 297 K): δ ) 1.10
(m, 24H, CH(CH3)2), 1.19 (m, 4H, CH(CH3)2), 4.62 (m, 4H, o-C6H5),
4.71 (m, 4H, m-C6H5), 4.84 (m, 2H, p-C6H5). 13C{1H} NMR (126 MHz,
C6D6, 297 K): δ ) 15.2 (CH(CH3)2), 17.8 (CH(CH3)2), 74.6 (ipso-
C6H5), 75.9, 78.9, 81.1 (C6H5). Anal. Calcd for C24H38Cl2MoSi2
(549.57): C, 52.45; H, 6.96. Found: C, 52.62; H, 6.61.
[Mo(η6-C6H5Li)2]‚tmeda, 2: Method A. BuLi (26.17 mL, 65.43
mmol, 5.5 equiv) was added to a mixture of [Mo(η6-C6H6)2] (3.00 g,
11.90 mmol) (1), tmeda (7.60 g, 65.43 mmol, 5.5 equiv), and heptane
(75 mL) via syringe, and the resulting suspension was heated to 55 °C
over a period of 20 h, while the color of the suspension changed from
green to dark brown. The resulting precipitate was collected by filtration,
washed several times with pentane (3 × 10 mL), and dried in vacuo to
yield 2‚tmeda as a pyrophoric pale brown powder (3.79 g, 9.97 mmol,
84%).
Method B. To a suspension of [Mo(η6-C6H6)2] (1.00 g, 3.96 mmol)
(1) and tmeda (2.53 g, 21.81 mmol, 5.5 equiv) in pentane (40 mL)
t
was added BuLi (21.81 mmol, 14.54 mL) via syringe at 0 °C. After
warming to room temperature the green suspension was sonicated over
a period of 12 h accompanied by the formation of a brown precipitate
and a color change to dark brown. The solid product was collected by
filtration, washed with pentane (3 × 10 mL), and dried in vacuo. 2‚
tmeda was isolated as a pale brown powder (1.32 g, 3.47 mmol, 87%).
1H NMR (500 MHz, thf-d8, 297 K): δ ) 2.16 (s, 12H, N(CH3)2), 2.30
[Mo{(η6-C6H5)2Si2Me4}], 7 and [Mo(η6-C6H4Si2Me4)2], 8. A
solution of Si2Me4Cl2 (0.29 g, 1.33 mmol) in heptane (10 mL) was
added dropwise over a period of 1 h to a slurry of 2‚tmeda (0.51 g,
1.33 mmol) in heptane (30 mL) at -78 °C. The reaction mixture was
kept for additional 3 h at -78 °C and then slowly warmed to room
temperature, while the color of the suspension changed from dark brown
to deep red and a white precipitate deposited. Insoluble materials were
removed by filtration, and the red filtrate was concentrated to about
10 mL. Storage at -30 °C yielded an orange solid (0.23 g, 0.63 mmol,
47%), which was isolated as a powder after washing with cold pentane
7
(s, 4H, N(CH2)2N), 4.10 (m, 4H, C6H5), 4.41 (m, 6H, C6H5). Li{1H}
NMR (117 MHz, thf-d8, 297 K): δ ) 1.6. 13C{1H} NMR (126 MHz,
thf-d8, 297 K): δ ) 46.7 (N(CH3)2), 59.3 (N(CH2)2N), 69.1, 77.1, 84.9
(C6H5), 106.4 (ipso-C6H5). Anal. Calcd for C18H26Li2N2Mo (380.23):
C, 56.86; H, 6.89; N, 7.37. Found: C, 56.48; H, 7.12; N, 7.23.
[Mo(η6-C6H5Li)2]2‚(thf)6, 3. The recrystallization of 2‚tmeda in thf/
benzene/pentane at -35 °C yielded orange-brown crystals of the 3 that
1
(3 × 5 mL, -100 °C) and drying in vacuo. According to H NMR
spectroscopy the product contained a mixture of the ansa complex 7,
the paracyclophane species 8, and bis(benzene)molybdenum (1) in a
ratio of about 8:1:1. 7: 1H NMR (500 MHz, C7D8, 213 K): δ ) 0.27
(s, 12H, CH3), 4.89 (m, 4H, m-C6H5), 4.97 (m, 4H, o-C6H5), 5.01 (m,
2H, p-C6H5). 13C{1H} NMR (126 MHz, C7D8, 213 K): δ ) -2.0 (CH3),
77.3 (ipso-C6H5), 78.1, 79.9, 80.3 (C6H5). 29Si{1H} NMR (99 MHz,
C7D8, 213 K): δ ) -14.8. 8: 1H NMR (500 MHz, C7D8, 213 K): δ
) 0.31 (s, 24H, CH3), 5.31 (s, 8H, C6H4). 13C{1H} NMR (126 MHz,
C7D8, 213 K): δ ) -1.4 (CH3), 81.1 (ipso-C6H4), 84.6 (C6H4). 29Si-
{1H} NMR (99 MHz, C7D8, 213 K): δ ) -17.3. 1: 1H NMR (500
MHz, C7D8, 213 K): δ ) 4.65 (s, C6H6). 13C{1H} NMR (126 MHz,
C7D8, 213 K): δ ) 75.2 (C6H6).
1
were suitable for crystal structure analysis. H NMR (200 MHz, thf-
d8, 297 K): δ ) 1.70 (m, 12H, (CH2)2), 3.60 (s, 12H, O(CH2)2), 4.06
(m, 4H, C6H5), 4.39 (m, 6H, C6H5).
[Mo{η6-C6H5(BN(SiMe3)2Cl}2], 4. A suspension of 2‚tmeda (0.75
g, 1.97 mmol) in pentane (30 mL) was added rapidly to a well-stirred
solution of Cl2BN(SiMe3)2 (1.19 g, 4.93 mmol, 2.5 equiv) in pentane
(20 mL) at room temperature. Upon stirring for 18 h the color of the
slurry changed from dark brown to deep red. Insoluble materials were
removed by filtration, and the red filtrate was concentrated to about
10 mL in volume. Cooling to -60 °C afforded 4 as a dark orange,
microcrystalline solid, which was obtained analytically pure after
washing with cold pentane (3 × 3 mL, -100 °C) and drying in vacuo
(0.45 g, 0.68 mmol, 34%). X-ray quality crystals were grown from
saturated heptane solutions at -25 °C. 1H NMR (500 MHz, C6D6, 297
K): δ ) 0.32 (s, 36H, Si(CH3)3), 4.67 (m, 4H, m-C6H5), 4.90 (m, 2H,
p-C6H5), 4.95 (m, 4H, o-C6H5). 11B{1H} NMR (160 MHz, C6D6, 297
K): δ ) 47.1. 13C{1H} NMR (126 MHz, C6D6, 297 K): δ ) 4.29
(Si(CH3)3), 77.4, 81.3, 83.9 (C6H5). Anal. Calcd for C24H46B2Cl2N2-
MoSi4 (663.44): C, 43.45; H, 6.99; N, 4.22. Found: C, 43.62; H, 6.61;
N, 4.33.
Crystallographic data have been deposited with the Cambridge
Crystallographic Data Centre. Copies of the data [2, 4-6, 8: CCDC
631647-631651] can be obtained free of charge via www.ccdc.ca-
m.ac.uk/data_request/cif, by e-mailing data_request@ccdc.cam.ac.uk,
or by contacting the Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge CB 1EZ, UK; fax +44 1223 336033.
Acknowledgment. This work was supported by the DFG.
T.K. thanks the FCI for a Ph.D. stipend. We are grateful to the
BASF AG for a generous donation of chemicals.
[Mo{η6-C6H5(BN(SiMe3)2Br}2], 5. In a procedure analogous to the
preparation of 4, employing 2‚tmeda (0.27 g, 0.71 mmol) in pentane
(20 mL) and Br2BN(SiMe3)2 (0.59 g, 1.78 mmol, 2.5 equiv) in pentane
(10 mL), afforded 5 (0.23 g, 0.31 mmol, 44%) as dark red crystals
after crystallization and drying in vacuo. The product always contained
traces of LiBr‚tmeda, which could not be removed even after several
Supporting Information Available: Crystallographic data in
CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
recrystallization steps. H NMR (500 MHz, C6D6, 297 K): δ ) 0.32
JA0692130
9
4846 J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007