Formation of a 1-zircona-2,5-disilacyclopent-3-yne: Coordination of 1,4-disilabutatriene to zirconocene?

Article abstract of DOI:10.1002/anie.201000021

(Figure Presented) Alkyne under stress: A novel metallacycle containing one Zr atom, two Si atoms, and a C≡C bond has been prepared and its structure elucidated (Zr green, Si blue, C gray). According to X-ray data, spectral properties, and DFT calculations, the bonding situation in this compound is characterized as a l-metalla-2,5-disilacyclopent-3-yne with a weak metal-triplebond interaction.

Full text of DOI:10.1002/anie.201000021

Angewandte
Chemie
DOI: 10.1002/anie.201000021
Metallacycles
Formation of a 1-Zircona-2,5-disilacyclopent-3-yne: Coordination of
1,4-Disilabutatriene to Zirconocene?**
ˇ
Martin Lamac,* Anke Spannenberg, Haijun Jiao, Sven Hansen, Wolfgang Baumann,
Perdita Arndt, and Uwe Rosenthal*
Dedicated to Professor Gꢀnther Wilke on the occasion of his 85th birthday
The inherent instability of small cyclic alkynes has restricted
attempts to isolate these strained structures. Nevertheless,
their use as reactive in situ generated intermediates has been
demonstrated in many synthetic applications.[1] Significant
advances in the synthesis of stable cycloalkynes have been
achieved by Suzuki et al., who reported the first preparation
of 1-zirconacyclopent-3-ynes in 2002.[2] The success of this
approach was based on the formation of metallacycles, in
which a specific type of interaction with the metal center
stabilizes the strained geometry of the cycloalkyne. Subse-
quently, analogous complexes containing titanium and haf-
nium were reported.[3] A discussion about the structure and
bonding nature of this class of compounds[4–6] led to a better
understanding of the uncommon bonding interactions of
transition metals with unsaturated bonds.
ones), which may exist in the target compound, have not been
reported to date.
The synthesis of an unsubstituted 1-titanacyclopent-3-yne
reported by our research group utilized the reaction of 1,4-
dichlorobut-2-yne with two equivalents of the titanocene
2
alkyne complex [Cp2Ti(h -Me3Si C C SiMe3)] (Cp = h5-
À ꢀ À
cyclopentadienyl), which served simultaneously as a source
of the “Cp2Ti” fragment and as an acceptor of chlorine atoms
(producing [Cp2TiCl2] as a by-product).[3b] We investigated
À ꢀ
the application of this procedure to the alkyne ClMe2Si C
À
C SiMe2Cl (1). Surprisingly, the formation of [Cp2TiCl2] was
not observed and the isolated material was identified as
complex 2, which is a product of an alkyne-exchange reaction
(Scheme 1). Similarly, by utilizing the zirconocene source,
Silicon-containing cycloalkynes have also been success-
fully isolated; the smallest structurally characterized species
was the six-membered ring of a substituted tetrasilacyclohex-
yne.[7] In addition, some metallacyclosilanes of Group 4
metals are also known.[8] Considering these facts, we con-
cluded that it should be possible to form a five-membered Si-
containing metallacycloalkyne that is analogous to the known
metallacyclopentynes. It should be noted that Marschner
et al. recently described an unsuccessful attempt to synthesize
an analogous compound from a dipotassium bis(silyl)alkyne
reagent.[8b] Nevertheless, we were prompted to investigate
pathways to the title compound, which was expected to
exhibit interesting bonding properties. Furthermore, stable
silicon-containing analogues of butatriene (even coordinated
Scheme 1. Formation of complexes 2 and 3 from alkyne-exchange
reactions that occurred during attempts to prepare compound 4 from
alkyne complexes.
2
À ꢀ À
[Cp2Zr(h -Me3Si C C SiMe3)(C5H5N)] (C5H5N = pyridine),
ˇ
[*] Dr. M. Lamac
we obtained the pyridine-stabilized alkyne complex 3. Solid-
state structures of both complexes were determined (see the
Supporting Information), while the obtained structural
parameters together with observed spectroscopic properties
confirmed the expected metallacyclopropene structures and
were comparable with known complexes of this type.[9]
In contrast, an approach that parallels the reported
procedure for the preparation of metallacyclopentynes utiliz-
ing magnesium reduction[3a] led to the desired product:
Stoichiometric amounts of [Cp2ZrCl2] and 1 were treated
with two equivalents of magnesium in tetrahydrofuran at
room temperature. This procedure afforded the desired 1-
zircona-2,5-disilacyclopent-3-yne derivative 4 (Scheme 2) as a
crystalline solid in a rather low yield of 37%.[10] The
´
J. Heyrovsky Institute of Physical Chemistry
Academy of Sciences of the Czech Republic
Dolejskova 3, 182 23 Prague 8 (Czech Republic)
E-mail: martin.lamac@jh-inst.cas.cz
ˇ
Dr. A. Spannenberg, Dr. H. Jiao, S. Hansen, W. Baumann,
Dr. P. Arndt, Prof. Dr. U. Rosenthal
Leibniz-Institut fꢀr Katalyse e. V. an der Universitꢁt Rostock
Albert-Einstein-Straße 29a, 18059 Rostock (Germany)
Fax: (+49)381-1281-51176
E-mail: uwe.rosenthal@catalysis.de
Homepage: http://www.catalysis.de
[**] We thank our technical staff, in particular Regina Jesse, for
assistance. Financial support by the DFG (GRK 1213) is acknowl-
edged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000021.
1
occurrence of two singlet signals in the H NMR spectrum
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Suzuki et al., range from 151 to 1568).[2,3a] The average
ꢀ
distance from the Zr center to the C C carbon atoms (2.54 ꢀ)
is longer than that in 1-zirconacyclopent-3-ynes (2.29–
2.32 ꢀ); this value suggests only weak interaction of the
unsaturated bond with the metal. Substitution of two carbon
atoms with Si thus results in the triple bond being located
further from the Zr center, thus reducing their mutual
interaction.
Scheme 2. Synthesis of complex 4.
and three signals in the 13C NMR spectrum confirm a
Calculations performed by Lam and Lin[4] for 1-zircona-
ꢀ
À ꢀ À
symmetrical structure; the C C resonance appears at d =
cyclopent-3-yne [Cp2Zr(H2C C C CH2)] suggested that
both the metallacyclopentyne (analogous to structure A in
110.9 ppm, which is comparable to that of
1
(d =
111.1 ppm), and is shifted downfield compared to that of
Scheme 3) and a coordinated cumulene (analogous to struc-
[3a]
À ꢀ À
[Cp2Zr(H2C C C CH2)] (d = 102.45 ppm). The signal at
d = 58.2 ppm in the 29Si NMR spectrum lies in the range of Si
=
C unsaturated bonds,[11] or Si-containing aromatic systems
coordinated to transition metals,[12] and the value is also
comparable with that observed in zirconocene complexes of
alkynylsilanes with an Si–H–M agostic interaction.[13] Such a
downfield shift can be explained by partial delocalization of
the electron density along the ring.[12c] The IR spectrum shows
Scheme 3. 1-Metalla-2,5-disilacyclopent-3-ynes (A) and their possible
resonance forms (B, C).
À1
ꢀ
~
a band attributable to C C bond stretching at n = 1899 cm ,
which is indicative of a weakening of the corresponding bond
~
(cf. the value for the analogous 1-zirconacyclopent-3-yne, n =
2018 cmÀ1).[14]
ture B) resonance forms contribute to the bonding in this
molecule. On the basis of an experimental electron density
study, Hashizume et al. claimed that the cumulene structure is
the dominant resonance form.[5] On the other hand, Jemmis,
Jiao, et al.[6] concluded the existence of metal–p interaction of
the metallocene core with the unsaturated bond (analogous to
structure C). Such a coordination mode agrees better with the
observed structural and spectral parameters and explains the
enhanced stability.
For a better understanding of the bonding interaction in
the novel complex 4, we carried out DFT computations with
the B3LYP hybrid functional (see the Supporting Information
for details). Compound 4 has C2 symmetry and the computed
structural parameters agree very well with those of the
experimentally determined molecular structure. Natural
localized molecular orbital (NLMO) analysis reveals a metal-
The X-ray diffraction analysis of 4 revealed the expected
structure (Figure 1) of the five-membered planar zircona-
ꢀ
cycle, with two Si atoms and a C C bond in the ring. The C1–
C2 distance (1.240(3) ꢀ) is longer, while the Si–C distances
(1.83 ꢀ on average) are only slightly shorter than those of free
À ꢀ À
silyl-group-substituted alkynes (cf. the values in Me3Si C C
SiMe3,[15] 1.208(3) ꢀ for C C and 1.844(1) ꢀ for Si C,
ꢀ
À
À
respectively). Zr Si distances (average 2.74 ꢀ) are compara-
ble with those of other zirconocene silyl complexes.[8b,16] The
strained geometry of the ring is demonstrated by acute Zr-Si-
À ꢀ
C angles (average 64.08) and by average Si C C angles of
151.58 (cf. values in 1-zirconacyclopent-3-ynes prepared by
ꢀ
lacyclopentyne complex with a C C bond (structure A in
Scheme 3), rather than a 1,4-disilabutatriene complex (struc-
ꢀ
ture B). This C C bond (1.246 ꢀ) is longer than the
À ꢀ À
corresponding bond in Me3Si C C SiMe3 (1.216 ꢀ), and
À
the Si C bond of complex 4 (1.832 ꢀ) is very slightly shorter
À ꢀ À
than that in Me3Si C C SiMe3 (1.848 ꢀ), thus indicating a
À
ꢀ
Si C single bond. The Wiberg bond orders for the C C and
À
Si C bonds are 2.431 and 0.793 for complex 4, and 2.888 and
À ꢀ À
0.787 for Me3Si C C SiMe3, respectively. For comparison,
=
=
the computed Si C and C C bond lengths for 1,4-disilabuta-
= = =
triene Me2Si C C SiMe2 (D2h symmetry) are 1.716 and
1.263 ꢀ, and their corresponding Wiberg bond orders are
1.362 and 2.286, respectively.
In addition, NLMO analysis also shows the interaction
Figure 1. A view of the molecular structure of 4 perpendicular to the
plane Zr-Si1-C1-C2-Si2. Thermal ellipsoids correspond to the 30%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths [ꢂ] and angles [8]: Zr–Si1 2.7395(7), Zr–Si2 2.7352(6),
Zr–C1 2.538(2), Zr–C2 2.539(2), C1–Si1 1.827(2), C2–Si2 1.834(2), C1–
C2 1.240(3); Si1-Zr-Si2 109.03(2), Zr-Si1-C1 63.91(7), Zr-Si2-C2
64.01(7), Si1-C1-C2 151.7(2), Si2-C2-C1 151.3(2).
ꢀ
between the Zr center and the C C bond in 4 (corresponding
to structure C), with a Zr–C distance of 2.580 ꢀ; the
À
computed Zr C Wiberg bond order is 0.379, which might
ꢀ
explain the slightly elongated C C bond. However, the
ꢀ
contribution of the Zr center to the C C bond is about half
the value of the contribution in [Cp2Zr(H2C C C CH2)];
[6]
À ꢀ À
2938
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Chemie
the combustion process). NMR (297 K, [D6]benzene): d(1H) = 0.57 (s,
À
this difference can be ascribed to the long Zr C distance
12H, SiMe2), 4.88 ppm (s, 10H, C5H5); d(13C{1H}) = À0.2 (SiMe2),
À ꢀ À
compared to that in [Cp2Zr(H2C C C CH2)]. On one hand,
29
1
ꢀ
96.8 (C5H5), 110.9 ppm (C C); d( Si{ H}) = 58.2 ppm; IR (Nujol):
the metal–ligand interaction is shown by the negative value of
the nucleus-independent chemical shifts (NICS). On the
other hand, the lower NICS value at the center (NICS(0) =
À40.6 ppm) of 4, compared to the NICS value (À49.4 ppm) of
À1
+
ꢀ
~
n = 1899 cm (w, C C); MS (CI, isobutane): m/z 360 [M] .
Received: January 4, 2010
Published online: March 16, 2010
[6]
À ꢀ À
complex [Cp2Zr(H2C C C CH2)],
indicates a weaker
ꢀ
interaction between the Zr center and the C C bond.
Keywords: cycloalkynes · density functional calculations ·
.
In conclusion, we have reported the preparation and
characterization of a novel Zr complex. According to the
observed structural and spectral parameters, as well as DFT
calculations, the bonding situation in this compound is
plausibly described by the 1-zircona-2,5-disilacyclopent-3-
yne structure with a weak metal–triple-bond interaction (C),
rather than a zirconocene-stabilized 1,4-disilabutatriene com-
plex (B).
metallacycles · silicon · zirconium
[1] For selected reviews, see: a) A. Krebs, J. Wilke, Top. Curr. Chem.
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Experimental Section
All manipulations were carried out under an argon atmosphere using
standard Schlenk or glove-box techniques. Prior to use, solvents were
pre-dried, freshly distilled from sodium tetraethylaluminate, and
stored under argon. [Cp2ZrCl2] was purchased from Sigma–Aldrich.
Alkyne 1,[17] [Cp2Ti(h2-Me3SiC2SiMe3)],[9a] and [Cp2Zr(h2-
Me3SiC2SiMe3)(C5H5N)][9c] were synthesized according to published
procedures. The following spectrometers were used: NMR: Bruker
AV 400, AV 300; chemical shifts (1H, 13C, 29Si) given relative to SiMe4
and referenced to residual solvent signals ([D6]benzene: dH =
7.16 ppm, dC = 128.0 ppm). IR: Bruker Alpha-P. Elemental analyses:
Leco CHNS-932 analyzer. Melting points: Bꢁchi 535 apparatus,
capillaries sealed under Ar.
[3] a) N. Suzuki, N. Aihara, H. Takahara, T. Watanabe, M. Iwasaki,
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Hirose, T. Chihara, Chem. Lett. 2004, 33, 1488 – 1489; d) U.
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[4] K. C. Lam, Z. Lin, Organometallics 2003, 22, 3466 – 3470.
[5] D. Hashizume, N. Suzuki, T. Chihara, Chem. Commun. 2006,
1233 – 1235.
[6] E. D. Jemmis, A. K. Phukan, H. Jiao, U. Rosenthal, Organo-
metallics 2003, 22, 4958 – 4965.
X-ray structure determination of 4: Crystals suitable for mea-
surement were obtained from n-hexane/toluene at À788C. Diffrac-
tion data were collected on a STOE IPDS II diffractometer using
graphite-monochromated MoKa radiation. The structure was solved
by direct methods (SHELXS-97)[18] and refined by full-matrix least-
squares techniques on F2 (SHELXL-97).[18] The crystal lattice
contained disordered solvent molecules, which are not shown in the
figures. The DIAMOND program was used for graphical representa-
tions.[19] C16H22Si2Zr, Mr = 361.74, space group P21/n, monoclinic, a =
7.9232(2), b = 10.4110(2), c = 23.9891(7) ꢀ, b = 96.180(2)8, V=
1967.3(1) ꢀ3, Z = 4, 1calcd = 1.377 gcmÀ3, 32926 reflections measured,
4700 were independent of symmetry, 4052 of which were observed
(I > 2s(I)), R1 = 0.028, wR2 (all data) = 0.077, 212 parameters.
Synthetic procedures, X-ray structure determination details,
molecular structures of compounds 2 and 3, an alternative view of
structure 4, and all computational details are given in the Supporting
Information. CCDC 756947 (2), 756949 (3), and 756948 (4) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of 4: Alkyne 1 (107 mg, 0.51 mmol) was added to a
stirred suspension of [Cp2ZrCl2] (150 mg, 0.51 mmol) and magnesium
turnings (25 mg, 1.07 mmol) in THF (10 mL) at room temperature.
The color of the mixture turned to dark amber over a period of 30 min
and the stirring was continued for further 18 h. All volatiles were
subsequently removed under vacuum and the residue was extracted
with n-hexane (2 ꢂ 10 mL) at 508C and filtered. The solution was
concentrated to ca. 5 mL, and upon standing at À788C overnight,
brown crystals of 4 were formed. Yield: 68 mg (37%); m.p. 180–
1858C dec. without melting; anal. calcd. for C16H22Si2Zr
(361.74 gmolÀ1) C 53.13, H 6.13; found: C 49.91, H 6.45 (better
values could not be obtained probably because of a partial formation
of SiC even in the presence of an additional chemical oxidant V2O5 in
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[10] It is noted that an analogous reaction, which was performed with
[Cp2TiCl2], led only to an intractable mixture of products. The
desired product could not be formed, possibly because of steric
reasons. In addition, for titanium, the related complex
À ꢀ À
[Cp2Ti(Me2C C C CMe2)] could not be isolated, since an
À
additional C H bond activation and the formation of
a
titanacyclopentene occurred, see: A. Maercker, A. Groos,
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[13] N. Peulecke, A. Ohff, P. Kosse, A. Tillack, A. Spannenberg, R.
[15] J. Bruckmann, C. Krꢁger, Acta Crystallogr. Sect. C 1997, 53,
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from 2.72 to 2.89 ꢀ.
[17] B. Wrackmeyer, G. Kehr, J. Sꢁß, E. Molla, J. Organomet. Chem.
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[14] Since the molecule has C2 symmetry (the two Cp rings are not
perfectly eclipsed; the molecule would adopt C2v symmetry for
two Cp rings in perfect eclipsed position) and a permanent
dipole moment, the symmetric stretching of the triple bond
causes a change of its dipole moment, and therefore the
molecule is IR-active (although the intensity is weak).
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