A stable derivative of the global minimum on the Si6H 6 potential energy surface

Article abstract of DOI:10.1002/anie.201102623

Silicon shuffle minimizes energy: Isomerization of the dismutational isomer of hexasilabenzene (see structure; R=2,4,6-iPr₃C₆H ₂) produces the Si-bridged propellane, a stable representative of the global minimum on the Si<

Full text of DOI:10.1002/anie.201102623

Communications
DOI: 10.1002/anie.201102623
Silicon Clusters
A Stable Derivative of the Global Minimum on the Si6H6 Potential
Energy Surface**
Kai Abersfelder, Andrew J. P. White, Raphael J. F. Berger, Henry S. Rzepa, and
David Scheschkewitz*
The aromatic benzene molecule is by far the lowest-energy
structure with the empirical formula C6H6.[1] The high
aromatic stabilization that is conferred by the cyclic delocal-
ization of six p electrons distinguishes benzene from its
approximately 200 theoretically known isomers and accounts
for the ubiquitous occurrence of the benzene motif in many
consequently it does not exhibit p aromaticity in the classical
sense.[14] Recently, we reported the tricyclic hexasilabenzene
isomer 2 (Tip = 2,4,6-iPr3C6H2), which we identified as being
aromatic on the basis of its unusually high thermal stability
and various calculations on model compounds.[15] We sug-
gested the term “dismutational aromaticity” for this new type
of cyclic delocalization in isomers of Hꢀckel aromatics that
involve permutation of p-type, s-type, and nonbonding
electrons as shown in Scheme 1. Computational analysis of
the magnetically induced ring current across the central Si4
moiety of 2 revealed it to be strongly diamagnetic and akin in
nature to the cluster current of three-dimensional aromatic
structures.[16]
We now report the isolation and characterization of an
aryl-substituted version of the likely global minimum on the
Si6H6 potential energy surface.[12f] Thus, the thermal or
photolytic rearrangement of the silicon scaffold of 2 affords
the valence isomeric 3 as orange crystals in 58% yield after
crystallization from hexane (Scheme 2).[17] The electron
areas of chemistry. Despite impressive progress regarding the
[2]
À
synthesis of stable compounds with Si Si p bonds, homo-
nuclear aromatic entities based on silicon are still rare and
restricted to systems with less than five ring atoms.[2a,3–6] While
stable derivatives of monosilabenzene,[7] 1,2-,[8] and 1,4-
disilabenzene[9] have been reported, knowledge on all-silicon
aromatic six-membered rings stems exclusively from numer-
ous computational studies.[10] In addition, the parent hexasi-
labenzene has been repeatedly used as a benchmark for
computational methods designed to evaluate aromaticity.[11]
In marked contrast to the C6H6 potential energy surface,
however, the lower thermodynamic stability of silicon
p bonding[2] accounts for an energetic preference for cluster-
like structures in the case of the Si6H6 manifold.[12] As a
consequence, the only known stable substituted derivative of
an Si6H6 isomer was until recently the hexasilaprismane 1
(Scheme 1).[13] All silicon atoms of 1 are tetracoordinate and
Scheme 2. Rearrangement of 2 to 3 and addition of halogens to 3
(R=Tip=2,4,6-iPr3C6H2).
Scheme 1. Stable Si6R6 isomers (1: R=Dip=2,6-iPr2C6H3; 2:
R=Tip=2,4,6-iPr3C6H2).
impact mass spectrum with the most intense peak corre-
sponding to the molecular ion at m/z 1388 is in agreement
with the expected formula Si6C90H138 . Apparently, 3 is readily
transferred into the gas phase despite its high molecular
weight; indeed short-path distillation of the compound is
possible at 10À2 mbar and approximately 3508C with only
slight decomposition. The kinetic stability of 3, however, is
significantly less pronounced than in the isomeric 2, as 3
decomposes within minutes upon exposure to air in solution
and the crystalline state.
Single crystals of 3 suitable for X-ray diffraction analysis
were obtained as the hexane solvate 3·(C6H14)1.75 , but disorder
of some of the peripheral groups and the included solvent
molecules required extensive modeling and reduced the
quality of the resulting data (see the Supporting Information).
A better data set was acquired by using crystals obtained from
a thf solution of 3. Both structural models unequivocally
confirm the constitution of a persilapropellane in which two
[*] K. Abersfelder,[+] Dr. A. J. P. White, Prof. Dr. H. S. Rzepa,
Prof. Dr. D. Scheschkewitz[+]
Department of Chemistry, Imperial College London
London SW7 2AZ (UK)
Dr. R. J. F. Berger
FB Materialwissenschaften und Physik, Abteilung Materialchemie
Paris-Lodron Universitꢀt Salzburg, 5020 Salzburg (Austria)
[+] New address:
Lehrstuhl fꢁr Allgemeine und Anorganische Chemie
Universitꢀt des Saarlandes, 66125 Saarbrꢁcken (Germany)
E-mail: scheschkewitz@mx.uni-saarland.de
[**] The financial support by the EPSRC (EP/H048804/1) and the
Aventis Foundation (Karl Winnacker-fellowship) is gratefully
acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102623.
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Angew. Chem. Int. Ed. 2011, 50, 7936 –7939

Figure 1. Solid-state structure of 3·thf3 (thermal ellipsoids at 30%;
hydrogen atoms and thf molecules are omitted for clarity). Selected
À
À
bond lengths [ꢂ] and angles [8]: Si1 Si1’ 2.7076(8), Si1 Si2 2.3536(6),
À
À
À
Si1 Si2’ 2.3819(5), Si1 Si6 2.3806(6), Si2 Si3 2.3782(6); Si2-Si1-Si6
97.465(19), Si2-Si1-Si2’ 75.64(2), Si6-Si1-Si2’ 96.693(18), Si1-Si2-Si3
93.649(18), Si1-Si2-Si1’ 69.74(2), Si2’-Si3-Si2 75.25(2), Si1-Si6-Si1’
69.32(2).
Figure 2. Molecular orbitals at isovalue 0.05 calculated for 3Dip at the
B3LYP/6-31G(d) level of theory.
of the “propeller blades” are connected by an additional
SiTip2 unit (Figure 1).[17]
reduction in idealized symmetry (C2 for 3 vs. D3h for Si5Mes6).
The value for the lowest-energy vertical triplet excitation of 3
The following discussion, however, is based on the thf
solvate 3·thf3 exclusively (which has a crystallographic C2 axis
coincident with the Si6···Si3 vector). The distance between the
À
is calculated as 50.5 kcalmolÀ1, which corresponds to lmax
=
566.5 nm. The fact that the corresponding absorption is not
observed for 3 may be due to the more rigid structure and thus
less vibronic coupling or possibly the tailing of the intense
band at 473 nm.
unsubstituted
bridgehead
silicon
atoms
Si1 Si1’
(2.7076(8) ꢁ) is significantly longer than the corresponding
distance in the pentasilapropellane Si5Mes6 (2.636 ꢁ, Mes =
2,4,6-Me3C6H2), which was recently prepared by Breher
The 29Si NMR spectrum of 3 in [D6]benzene showed four
signals at d = 174.6, 14.8, 7.5, and À274.2 ppm in a 1:1:2:2 ratio
in accordance with the change in symmetry of 3 (C2 vs. Ci for
2). The dispersion of chemical shifts of 3 (Dd = 448.8 ppm) is
even larger than in the case of the recently reported charge-
separated tetrasilacyclobutadiene.[5a] The highest-field reso-
nance at d = À274.2 ppm was assigned to the unsubstituted
bridgehead silicon atoms on the basis of a 2D-29Si/1H
correlation spectrum. Further corroboration for this assign-
ment is provided by comparison with Breherꢂs Si5Mes6, which
shows an almost identical 29Si NMR shift for the bridgehead
atoms at d = À273.2 ppm.[18] At first glance the low-field
resonance of 3 at d = 174.6 ppm seemed to be at odds with the
solid-state structure, suggesting a formally sp2-hybridized
silicon atom such as in tetrasilyl disilenes[2] or silylium cations
(e.g. Mes3Si+B(C6F5)4À in [D6]benzene: d = 225.5 ppm).[22] On
the basis of the 2D-29Si/1H correlation of 3, however, this
signal is assigned to one of the SiTip2 units and hence to a
tetracoordinate silicon atom. The 29Si NMR shifts computed
for 3Dip (d = 207.0, 32.1, 20.4, and À267.0 ppm)[21] are very
similar to the experimental NMR shifts of 3 and confirm that
the most deshielded silicon atom is that of the untethered
“propeller blade” (Si6). To the best of our knowledge, this
represents the by far most deshielded 29Si NMR signal of a
tetracoordinate silicon atom in a molecular environment void
of transitions metals.[23] For comparison, even the silicon
atoms of donor-stabilized silylium cations show resonances at
far higher field (e.g. [Et3Si(C6H6)]+B(C6F5)4À in [D6]benzene:
d = 92.3 ppm).[24]
et al.[18] All other Si Si bond lengths are within the typical
À
range for single bonds. The presence of the additional
bridging silicon unit (Si3) between Si2 and Si2’ results in a
smaller angle between these two “propeller blades” and a
widened angle to the untethered “blade” (Si1-Si1’-Si2/Si1-
Si1’-Si2’: 96.78; Si1-Si1’-Si2/Si1-Si1’-Si6: 131.648). The angles
at the bridging silicon atoms are, as to be expected,
approximately the same (Si1-Si2-Si1’: 69.74(2)8; Si1-Si6-Si1’:
69.32(2)8).
In light of the recent attention devoted to diradical(oid)
main group compounds in general[19] and heavier Group 14
propellanes in particular,[20] the UV/Vis spectrum of 3
deserved special consideration as an approximate measure
of the HOMO–LUMO gap. For their unbridged persilapro-
pellane Breher et al. observed a very weak band at lmax
=
546 nm that they assigned to the lowest energy singlet–triplet
transition (S–T).[18] Conversely, the longest wavelength
absorption of 3 at lmax = 473 nm is relatively intense (e =
700mÀ1 cmÀ1) and therefore unlikely to be associated with a
formally forbidden transition. Indeed, TD-DFT calculations
on the slightly simplified model compound 3Dip (R = Dip =
2,6-iPr2C6H3) show that this absorption band is due to the
HOMO–LUMO vertical singlet excitation (lmax,calc
=
474.6).[21] The corresponding band for Si5Mes6 was found at
lmax = 396 nm by Breher and co-workers.[18] As in the case of
Si5Mes6, the HOMO of 3Dip does not correspond to the
predominantly nonbonding orbitals on the bridgehead atoms,
but rather to cluster bonding electrons (Figure 2). The large
redshift of this transition in 3 compared to that of Si5Mes6 is
due to the lifted degeneracy of the HOMO on grounds of a
The topology of the magnetically induced currents readily
explained the 29Si NMR chemical shifts of the dismutational
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between Si1 and Si1’) in 3H amounts to a purely diamagnetic
contribution of 10.0 nATÀ1 (0.0 nATÀ1 paramagnetic).
Almost identical values were found for 2H.[15]
First investigations with regards to further functionaliza-
tion confirmed the high reactivity of the bridgehead silicon
atoms of 3. Treatment of 3 with elemental bromine or iodine
affords the 1,5-dihalogenated derivatives 4a,b in moderate to
good isolated yields as orange crystals (Scheme 2).[17] It is
noteworthy that the color of the halogenated 4a,b is not as
strongly affected by halogenation as commonly observed
upon saturation of free valences in silicon compounds. The
blue shift of longest wavelength absorptions in the UV/Vis
compared to that of 3 (3: lmax = 473; 4a: lmax = 436; 4b: lmax
=
463 nm) is small to negligible. Conversely, the 29Si NMR shifts
of both compounds are observed between d = 21.2 and
À77.4 ppm and hence within the common range for small
silicon rings, which provided further corroboration that the
unusual shifts of 3 are indeed due to its nonbonding electron
density being engaged in the cluster current.
The solid-state structures of 4a,b were determined by X-
ray crystallography on single crystals (Figure S8 and S13 in
the Supporting Information).[17] Remarkably, the molecular
structures of 4a,b reveal a small but significant shortening of
the distance between the bridgehead silicon atoms compared
to the unsubstituted propellane-type structure 3 (3·thf3:
2.7076(8); 4a: 2.6547(7); 4b: 2.6810(17) ꢁ). This is in sharp
contrast to the substantial elongation of the interbridgehead
distance in the case of carbon-based propellanes upon
substitution ([1.1.1]propellanes: 1.577 to 1.605 ꢁ; bicyclo-
[1.1.1]pentanes: 1.80 to 1.891 ꢁ),[26] but qualitatively similar
to previous observations for the addition of more electro-
positive reagents to heavier propellanes whereupon only
slight elongations were found.[27] It is conceivable that the
decrease in the distance between the bridgehead silicon atoms
is associated with an increased s character of the cluster
bonds—and thus an overall contraction of the silicon frame-
work. Such an increased s character should be expected for
electronegative substituents on the basis of Bentꢂs rule.[28] We
recently observed a related substituent effect in cyclotrisi-
lanes.[29]
Figure 3. Magnetically induced current density field vectors in 3H,
calculated with the GIMIC[25] (gauge-including magnetically induced
À
currents) method. The magnetic field (B vector) is parallel to Si2 Si2’,
hence diatropic vortices are clockwise, paratropic vortices are counter-
clockwise. Very small currents and very intense currents are omitted.
The largest arrows displayed arise from the diatropic atomic Larmor
currents of closed atomic subshells: a) Vectors in the Si6-Si1-Si1’-Si3
plane. b) Vectors in the Si1-Si1’-Si2’ plane. c) Dominating (non-
Larmor) current vortices; experimental 29Si NMR chemical shifts of 3
in [D6]benzene [ppm].
In summary, with the bridged propellane 3 we have
reported a stable derivative of the assumed global minimum
of the Si6H6 potential energy surface and thus the thermody-
namic counterpart of benzene in the case of silicon. The
magnetically induced cluster current in 3 is responsible for an
unprecedented dispersion of 29Si NMR shifts and the most
deshielded tetracoordinate silicon atom to date. The remark-
able thermal stability of 3 allows its ready transfer into the gas
phase suggesting its suitability for gas phase deposition
techniques. Conversely, the halogenation of the bridgehead
positions confirms the high reactivity, offering the possibility
of further functionalization of the tricyclic silicon scaffold.
hexasilabenzene isomer 2.[16] Similarly, the cluster current
present in the bridged propellane derivative 3H (R = H,
Figure 3a, see the Supporting Information) includes Si1 and
Si1’ into a diatropic current loop, which thus exerts a magnetic
shielding effect by back-induction. This strong current vortex
excludes the close-by lying Si6 atom, which is in turn
surrounded by a distinct paratropic current vortex, leading
to the observed unusual 29Si NMR low-field resonance of d =
174.6 ppm for 3. The main current vortex, interestingly,
branches around Si2 and Si2’ (Figure 3b). As a consequence
shielding effects are approximately canceled out, yielding the
comparably small chemical shifts of d = 7.5 ppm. Like in the
case of 2,[16] the innermost current vortex in 3H is also
diatropic, a typical feature of cluster currents as opposed to
ring currents in 2D aromatic systems. The total induced
molecular current (integrated through a half-plane parallel to
the B-field and passing through the central vortex located
Received: April 15, 2011
Published online: July 4, 2011
Keywords: cluster compounds · density functional calculations ·
.
inorganic synthesis · isomerization · silicon
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