A molecular silicon cluster with a "Naked" vertex atom

Article abstract of DOI:10.1002/anie.200462730

(Figure Presented) Reaction of a disilenide with SiCl₄ produces a silicon cluster with π-bonding topology reminiscent of a Moebius array, as determined by structural data and DFT calculations (see picture). Partially hydrogenated silicon cluste

Full text of DOI:10.1002/anie.200462730

Communications
Cluster Compounds
A Molecular Silicon Cluster with a “Naked”
Vertex Atom**
David Scheschkewitz*
Silicon clusters have been the subject of intense research
efforts[1] owing to the ubiquitous role of elemental silicon in a
variety of electronic and optical applications, such as inte-
grated circuits, photovoltaic cells, and optical sensors.[2] In
fact, the optical properties of many silicon-based materials
depend to a large extent on the presence of small- to medium-
sized, partially hydrogenated clusters within the surrounding
matrix of bulk silicon. Luminescence phenomena in porous
silicon, for instance, have been attributed to the photo-
excitation of such clusters.[3,4]
The difficulties in the structural characterization of these
compounds could be circumvented by the synthesis of
discrete molecular derivatives. Advances in the preparation
of molecular germanium and tin clusters with “naked”
(unsubstituted) vertices have been recently summarized.[5]
Indeed, in the case of siliconꢀs higher homologues a few
examples have been prepared.[6] Whereas a variety of alkyl-
terminated, medium-sized silicon clusters have been charac-
terized by electron microscopic techniques,[7] small discrete
molecular clusters are known only as either totally unsub-
stituted Zintl-type anions[8] or fully substituted deltahedral
derivatives of the type SinRn.[9] Theoretical studies predict
interesting structural motifs for their elusive partially sub-
stituted congeners.[10]
Reductive dehalogenation of a divalent species in the
presence of GeCl2 had proven successful for the preparation
of the germanium cluster Ge6R2 as reported by Power and co-
workers.[6c] Unfortunately, the instability of divalent silicon
compounds renders such an approach unsuitable.[11] On the
other hand, the recently reported disilenides, analogues of
vinyl anions, should be good precursors for multiply unsatu-
rated silicon p systems, which could subsequently rearrange to
yield the desired partially substituted clusters owing to the
=
low stability of Si Si double bonds. The pentadiene 2 was
chosen as the initial target compound, produced by treatment
of SiCl4 with 2 equivalents of the lithium salt of disilenide 1
(Scheme 1).[12a] Crystallization of the complex reaction mix-
ture yielded uniform red crystals in marginal yield (< 1%).
[*] Dr. D. Scheschkewitz
Institut fꢀr Anorganische Chemie
Bayerische Julius-Maximilians Universitꢁt
Am Hubland, 97074 Wꢀrzburg (Germany)
Fax: (+49)931-888-4623
E-mail: scheschkewitz@mail.uni-wuerzburg.de
[**] The author thanks Prof. H. Braunschweig for continuous support,
Dr. R. Bertermann for NMR spectra, Dr. M. Bꢀchner for mass
spectra, R. Schedl for combustion analysis and differential scanning
calorimetry, and Prof. M. Kaupp and Prof. A. Berndt for fruitful
discussions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200462730
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Angewandte
Chemie
Scheme 1. Synthesis of the partially substituted silicon cluster 3
(R=Tip=2,4,6-triisopropylphenyl).
However, a preliminary X-ray diffraction study on the crystals
of mediocre quality revealed the connectivity of the partially
substituted, five-vertex cluster 3 with one “naked” endohe-
dral silicon atom.
Figure 1. Molecular structure of 3 in the solid state (thermal ellipsoids
Formation of 3 from 1 and SiCl4 requires a further
2 equivalents of the disilenide 1 for reduction of the putative
intermediate 2. Indeed, the use of 4 equivalents of disilenide 1
essentially led to a 1:1 mixture of 3 and 4, the formal oxidation
product of 1 (Scheme 1). The tetrasilabutadiene 4 was
at 50% probability). Isopropyl groups and protons are omitted for
clarity. Selected bond lengths [ꢂ]: Si1–Si2 2.3651(9), Si1–Si5 2.3456(9),
Si2–Si3 2.3061(9), Si2–Si5 2.3430(9), Si3–Si5 2.3373(8), Si3–Si4
2.3596(9), Si4–Si5 2.3445(9), Si1–C1 1.929(2), Si1–C16 1.916(2), Si2–
C31 1.917(2), Si3–C46 1.913(2), Si4–C61 1.915(2), Si4–C76 1.925(2);
selected bond angles [8]: Si5-Si1-Si2 59.65(2), Si3-Si2-Si5 60.36(2), Si5-
Si2-Si1 59.76(3), Si2-Si3-Si5 60.60(2), Si5-Si3-Si4 59.89(3), Si4-Si5-Si1
161.02(3).
1
identified by comparison of the H and 29Si NMR spectro-
scopic shift values with those reported in the literature.[13]
Separation of the reaction mixture proved tedious, affording
yields of 3 that varied between 22 and 37% of theoretical
prediction.[14]
interactions, Si2 and Si3 are only slightly pyramidal (bond-
angle sums: Si2, 358.98; Si3, 359.08). The planes defined by
Si1-Si2-C31 and Si3-Si4-C46 are almost perpendicular (tor-
sion angle = 88.78). The corresponding angle was determined
The 29Si NMR spectrum shows three signals in a ratio of
2:2:1 at d = 7.4, À108.4, and À124.8 ppm. In the 29Si–1H-
correlated 2D NMR spectrum only the signals at d = 7.4 and
À108.4 ppm show cross-peaks to two and one aryl substitu-
ents, respectively. These findings, together with an apparent
C2 symmetry deduced from 13C and 1H NMR spectra, confirm
the structural integrity of the cluster 3 in solution. Two distinct
UV/Vis spectroscopic absorptions are observed at l = 365 and
540 nm, the latter being significantly less intense.
=
to be 54.48 for the highly twisted disilene (tBu2MeSi)2Si
Si(SiMetBu2)2.[17]
DFT calculations at the B3LYP/6-31G(d,p) level of theory
on the H-substituted parent compound 3u were carried out to
clarify the nature of the interactions that lead to the extreme
distortions of 3.[18] Interestingly, 3u was only found to be a
transition state of the ring inversion of the potential bis-
homoaromatic isomer in which both silicon bridges are
present on the same side of the central three-membered
ring moiety.[19] The bulky substituents apparently favor 3 to an
extent that the transition state becomes an energy minimum.
Nonetheless, the experimental structure is reasonably well-
reproduced by the calculations. Whereas the transannular
distances Si5–Si2 and Si5–Si3 in 3u are slightly longer (2.42 ꢁ;
3: 2.34 ꢁ), the Si2–Si3 bond is a bit shorter (2.26 ꢁ; 3: 2.30 ꢁ)
than the corresponding distances in 3. The dihedral angle H-
Si2-Si3-H of 138.28 is significantly wider (3: C31-Si2-Si3-
C46 = 112.88) and thus is even closer to an ideal trans
arrangement of the somewhat more pyramidal, formal sp2-
hybridized silicon centers (bond-angles sum: 354.38).
An especially notable analytical feature of 3 was found in
the EI MS data: a peak at m/z=1415.9 can be assigned to
Si7Tip6 by comparison with the calculated isotopic distribution
pattern. This cluster expansion apparently takes place upon
heating of the sample in the condensed phase (required for
evaporation of the compound under high vacuum preceding
ionization), given that no such peak was observed with milder
ionization techniques. The highest mass detected by FAB MS is
the molecular ion peak at m/z=1359.9. This interpretation is
supported by the detection of an exothermic process at 1448C
by differential scanning calorimetry of 3. It should be noted
that the hydrogenated congeners of 3 have been implied as
intermediates in the chemical vapor deposition of silanes for
the preparation of thin elemental silicon films.[10]
With a reproducible synthetic protocol in hand, good
quality single crystals of 3 were finally obtained. X-ray
crystallographic analysis (Figure 1) confirmed the structure of
the partially substituted silicon cluster.[15] The distance
To provide more insight into this curious bonding
situation, the molecular orbitals of 3u were calculated
(Figure 2). Whereas HOMO and HOMOÀ1 are only sepa-
rated by 0.14 eV, the difference to the next highest orbital
HOMOÀ2 (1.34 eV) is almost ten times as large. The
Heilbronner criterion for Mꢂbius aromaticity, namely that
4n mobile electrons must be available, is therefore formally
fulfilled.[20] Indeed, the appearance of the HOMOÀ1 does
resemble a Mꢂbius array, such as that calculated for trans-
between Si2 and Si3 (2.306(1) ꢁ) is significantly shorter
[16]
À
than a conventional Si Si single bond (2.34 ꢁ). Surpris-
ingly, the transannular distances Si2–Si5 (2.343(1) ꢁ) and Si3–
À
Si5 (2.337(1) ꢁ) precisely reflect the typical Si Si single bond
length. Nevertheless, if one ignores these transannular
Angew. Chem. Int. Ed. 2005, 44, 2954 –2956
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Communications
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[11] M. Kira, Pure Appl. Chem. 2000, 72, 2333.
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[14] Experimental details are given in the Supporting Information.
[15] Crystal structure determination of 3: red blocks from hexane;
C90H138Si5, monoclinic, space group P21/c; a = 1426.8(3), b =
1361.4(3), c = 4453.1(10) pm, a = 908, b = 95.296(4)8, g = 908,
V= 8613(3) ꢃ 10À30 m3; Z = 4, 1calc = 1.049 gcmÀ3; crystal dimen-
sions: 0.28 ꢃ 0.25 ꢃ 0.12 mm3; diffractometer: Bruker SMART
APEX CCD; MoKa radiation, 173 K; 2qmax = 52.74; 68217
reflections, 17602 independent (Rint = 0.0408), direct methods;
absorption correction SADABS (m = 1.24 cmÀ1); refinement
(against F2o) with SHELXTL (version 5.1) and SHELXL-97,
971 parameters, 0 restraints, R1 = 0.0562 (I > 2s) and wR2 (all
data) = 0.1429, GooF = 1.038, max/min residual electron den-
sity: 0.426/À0.184 ꢃ 1030 emÀ3. CCDC-256745 contains the sup-
plementary 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.
Figure 2. Molecular orbitals of 3u as calculated at the B3LYP/6-31G-
(d,p) level of theory: a) HOMO and HOMOÀ1, b) schematic represen-
tation of the atomic orbitals involved.
benzene.[21] The unsuccessful search for corresponding 3c-2e
bonds in 3u by NBO analysis, however, draws a picture that
seemingly contradicts this interpretation of the canonical
MOs. The analysis yields two degenerate orbitals with high p-
character (95.26%) which are each strictly localized between
two silicon atoms: Si5 and Si2, and Si5 and Si3. Therefore, an
alternative description of 3 as a classical Lewis structure
exclusively with 2c-2e bonds must be considered as well.
In conclusion, the first discrete partially substituted silicon
cluster has been isolated and structurally characterized. Its
chemical and physical features suggest that it might serve as a
model for its partially hydrogenated congeners in bulk silicon
materials.
[16] M. Weidenbruch, The Chemistry of Organic Silicon Compounds,
Vol. 3 (Eds.: Z. Rappoport, Y. Apeloig), Wiley, Chichester, UK,
2001, chap. 5.
[17] A. Sekiguchi, S. Inoue, M. Ichinohe, Y. Arai, J. Am. Chem. Soc.
2004, 126, 9626.
[18] Gaussian03 (RevisionB.04), M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A.
Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M.
Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M.
Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M.
Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida,
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Knox, H. P. Hratchian, J. B. Cross, C. Adamo, J. Jaramillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A.
Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S.
Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q.
Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G.
Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J.
Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M.
Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong,
C. Gonzalez, J. A. Pople, Gaussian, Inc., Pittsburgh, PA, 2003.
[19] The Cs symmetric isomer with both sp3-hybridized silicon atoms
on the same side of the ring is a minimum at the B3LYP/6-
31G(d,p) level of theory, 14.8 kcalmolÀ1 lower in energy than 3u.
The internal reaction coordinate that connects it to 3u features
an intermediate local minimum, 9.0 kcalmolÀ1 lower than 3u. It
shows one significantly elongated (2.80 ꢁ) and one slightly
shortened (2.31 ꢁ) transannular interaction. Details on all
stationary points are given in the Supporting Information.
[20] E. Heilbronner, Tetrahedron Lett. 1964, 1923.
Received: November 26, 2004
Revised: January 14, 2005
Published online: April 13, 2005
Keywords: cluster compounds · density functional calculations ·
.
silicon · structure elucidation · subvalent compounds
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2956
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