Alkynyl-functionalised and linked bicyclo[1.1.1]pentanes of group 14

Article abstract of DOI:10.1039/c2cc33045h

We report the synthesis and properties of alkynyl-functionalised and -bridged bicyclo[1.1.1]pentane derivatives consisting of the heavier group 14 elements silicon and tin. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc33045h

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COMMUNICATION
Alkynyl-functionalised and linked bicyclo[1.1.1]pentanes of group 14wz
Timo Augenstein, Pascual Ona-Burgos, Dominik Nied and Frank Breher*
˜
Received 27th April 2012, Accepted 16th May 2012
DOI: 10.1039/c2cc33045h
We report the synthesis and properties of alkynyl-functionalised
and -bridged bicyclo[1.1.1]pentane derivatives consisting of the
heavier group 14 elements silicon and tin.
Rod-like molecules have received considerable attention in
recent years, in particular in view of their relevance in acting as
elementary building blocks in investigations of long-range
interactions such as charge and energy transfer, for instance.1
Most commonly, carbon-rich species composed of polyynes
or poly(aryleneethynylene)s have been investigated.2 More
rigid structures have been obtained by including all-carbon
bicyclo[1.1.1]pentane cages in between the polyynes or by
employing [n]staffane chains. It has been shown that these
scaffolds have the ability to transmit electronic interactions over
long distances, which is astonishing since they are saturated
systems.3 Even larger cages such as the 12-vertex carborane
cluster 1,12-C2B10H10 have been found to permit electronic
communication between two metal centres over large distances.4
Recently, it has been proposed that heavy-core staffanes,
i.e., oligomers of heavy [1.1.1]propellanes of group 14,5 could
have interesting electronic and useful optoelectronic properties,
since the atoms in these chains may couple.6 Although few in
number, some examples of heavy bicyclo[1.1.1]pentanes of
group 14 have been reported in recent years.7 Catenated
versions have, to the best of our knowledge, only been
described once so far. In a recent breakthrough paper, Iwamoto
et al. reported the synthesis and characterisation of persilylated
[n]staffanes (n = 1, 2, 3) and their remarkable conjugation
between bicyclo[1.1.1]pentasilane units.8
Scheme 1 Synthesis of 2.
bicyclo[1.1.1]-pentanes and the first example of an alkynyl-
bridged [1,10]-bis(bicyclo[1.1.1]pentane).
In order to get access to this new family of compounds, we
employed the recently reported trisiladistanna[1.1.1]propellane
(1).7g In a first test reaction, we checked the suitability of 1 to
react with the simple nucleophile MeLi, followed by a treatment
with MeI.10 Indeed, when this simple two-step synthesis was
conducted in THF at low temperature, the dimethyl substituted
cage 2 was obtained after a short workup procedure as yellow
crystals in 82% yield (Scheme 1). Compound 2 is remarkably
stable towards air and moisture and decomposes only when
heated above 200 1C. 2 has been completely characterised,
including single crystal X-ray diffraction. However, several
disordered solvent molecules in the crystal lattice prevented a
detailed discussion of the structural parameters (see ESIz).
Based on this general reactivity of 1, and in order to access
alkynyl-functionalised bicyclo[1.1.1]pentanes, we intended to
employ alkynides of the alkali metals in the first step of the
reaction.11 Initially, we used sodium acetylide as alkynylation
reagent. The reaction of 1 with ca. one equivalent of
NaCRCH was carried out in THF at ꢀ10 1C. The initial
deep purple solution 1 became colourless and an excess of MeI
was added to the reaction mixture. However, instead of clean
formation of the Me/ethynyl terminated bicyclo[1.1.1]pentane
3 we also observed the formation of the propynyl substituted
cage (4; Scheme 2).
As an extension of our studies on heavy [1.1.1]propellanes
of group 147d–g we became interested in utilising these cages
for the well-directed synthesis of catenated bicyclo[1.1.1]pentanes.
As a suitable entry point to this area we have chosen alkynyl
moieties as bridging entities.9 In this paper we report the
synthesis and characterisation of alkynyl-functionalised heavy
Using the sodium acetylide in excess, 4 can be obtained in
up to 69% isolated yield. The formation of 4 can reasonably
be explained by assuming that sodium acetylide is able to
deprotonate the acidic CRCH entity once coordinated to 3;
the resulting carbanion is subsequently converted to CRCMe
Karlsruhe Institute of Technology (KIT), Institute of Inorganic Chemistry,
Engesserstr. 15, D-76131 Karlsruhe, Germany.
E-mail: breher@kit.edu; Fax: +49 721 608 47021;
Tel: +49 721 608 44855
w This article is part of the ChemComm ‘Frontiers in Molecular Main
Group Chemistry’ web themed issue.
z Electronic supplementary information (ESI) available: Experimental
details and characterisation data, including crystallographic data for 3,
4, and 5 and details of the DFT studies. CCDC 879397–879399. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2cc33045h
Scheme 2 Synthesis of 3 and 4 using NaCRCH.
Chem. Commun., 2012, 48, 6803–6805 6803
c
This journal is The Royal Society of Chemistry 2012

Fig. 1 Displacement ellipsoid plots (30% probability) of 3 (a) and 4
(b); Mes ligands without hydrogen atoms and with carbon atoms drawn
with arbitrary radii for clarity (for bond lengths and angles, see ESIz).
Scheme 3 Synthesis of 3, 4, and 5 using LiCRCHꢁen.
were consistent with the structure shown in Scheme 3 (for a
further discussion, see below). Single crystals suitable for
X-ray diffraction were obtained from a concentrated toluene
solution, which was layered with acetonitrile. The molecular
structure of 5 is shown in Fig. 2. Each individual molecule is
located on a special crystallographic position. By inspecting a
space-filling model of the molecular structure of 5 it becomes
evident that the alkynyl bridge is comfortably lodged in
by the treatment with MeI. When changing from sodium
acetylide to the less basic and more soluble lithium acetylide
ethylenediamine complex (LiCRCHꢁen), we were able to
isolate 3 in similar yields (73%). In order to suppress the
formation of 4, the reaction was performed at ambient
temperature with slow addition of a dilute THF solution of
LiCRCHꢁen. The addition was stopped precisely at the
point when the deep purple solution of 1 became colourless;
afterwards, an excess of methyl iodide was added.
between both Sn2Si3 moieties [d(CRC)
= 116.1 pm;
d(Sn–C) = 213.8(4) pm]. The tin–silicon distances fall within
the expected ranges and are only slightly different as compared
to 1. The Sn–Si–Sn angles of 74.9–75.01 and the interbridgehead
distance of 318.7 pm are similar to that determined for 3 or 4.
119Sn NMR investigations of 2–5 in C6D6 and d8-thf (5)
revealed that the unsymmetrically 1,3-disubstituted bicyclo-
[1.1.1]pentanes 3 and 4 consist of two signals (d = ꢀ245 and
ꢀ282 ppm for 3; ꢀ241 and ꢀ281 ppm for 4) and the dimethyl
derivative 2, as expected, consists of only one with a chemical
shift of d = ꢀ222 ppm. The symmetrical vs. unsymmetrical
Compounds 3 and 4 are colourless and pale yellow solids,
respectively. The 1H, 13C, 29Si, and 119Sn NMR data are
consistent with unsymmetrically 1,3-disubstituted bicyclo[1.1.1]-
pentane structures (see ESIz and below for more details). As
already observed for 2, both cages are stable to air and
moisture as well as thermally very stable. The molecular
structures of 3 and 4 are shown in Fig. 1.
The distances between the bridgehead tin atoms were detected
as follows: 315.7 pm for 3 and 315.8 pm for 4. Compared to the
starting material 1 [d(Snꢁ ꢁ ꢁSn) = 309.7 pm],7g these values are
increased by ca. 6 pm. The tin silicon distances of 260.5–262.5 pm,
however, are similar to 1 (260.3–265.3 pm). As expected, the
Sn–Si–Sn angles of 74.1–74.41 are slightly enlarged as compared
to 1 (av. 72.11). Overall, only small structural changes are
observed confirming the structural integrity upon addition of
organic substituents to the bridgehead tin atoms in 1.
1
substitution pattern is also reflected in the H and 13C NMR
data. Interestingly, the alkynyl-bridged compound 5 gave three
119Sn NMR signals at d = ꢀ237, ꢀ295, and ꢀ296 ppm in d8-thf
at room temperature. The latter two resonances (integration
B1 : 1) differ only slightly. Our current interpretation is that
different stereoisomers of 5 are present in solution, which differ
in the relative orientation of the mesityl substituents in both
bicyclopentane subunits. Note that in Mes-substituted cages of
this type, the aryl rings adopt a propeller-type arrangement
(D3 symmetry; Scheme 4 (left) and Fig. 2b). If two of these
cages are connected by a bridge (CRC in the case of 5),
different relative orientations of the Mes substituents of the
two Sn2Si3 subunits are possible. The resulting pairs of
In order to employ the ethynyl-substituted cage 3 as starting
material for the construction of catenated structures, we
checked the suitability of the –CRCH moiety in 3 to be
employed for further manipulation. To this end, we treated 3
with 1 equiv. of a dilute THF solution of LiN(SiMe3)2 and, in a
second step, with MeI as alkylating reagent (Scheme 3). As
expected, the propynyl substituted cage 4 was obtained in almost
quantitative yield. Based on these results we performed another
reaction, but this time we added the [1.1.1]propellane 1 to the
deprotonated cage before treating the reaction mixture with
MeI. This short synthetic sequence furnished the first alkynyl-
bridged [1,10]-bis(bicyclo[1.1.1]pentane) (5; Scheme 3). The use
of the sterically demanding base LiN(SiMe3)2 is crucial for the
success of this reaction; no side reactions with 1 can occur.
5 is stable against air and moisture, has a high decomposi-
tion temperature of >270 1C, and is accessible in 65% yield in
analytically pure form. The symmetrical n(CRCstretch) vibration
was observed in the Raman spectrum (powder) at 2043 cmꢀ1
.
This frequency is in good accordance with the values obtained
Fig. 2 (a) Displacement ellipsoid plot (50% probability) of 5; Mes
ligands without hydrogen atoms and with carbon atoms drawn with
arbitrary radii for clarity (see also ESIz); (b) view along the C3 axis.
for Me3Sn–CRC–SnMe3 (cf. 2065 cmꢀ1).12 The NMR data
c
6804 Chem. Commun., 2012, 48, 6803–6805
This journal is The Royal Society of Chemistry 2012

gap decreases by 0.23 eV when going from q2 to q5.16 In order
to confirm this calculated decrease, we tried to extract the
relevant experimental data from the UV/Vis spectra of 2 and 5.
However, since these p–s* transitions are too high in energy,
they are covered by other transitions of the Mes rings in the
UV region.
Scheme 4 Relative orientations of the aryl rings of the D3-symmetric
Sn2Si3 subunits in 5; formation of pairs of enantiomers and diastereomers.
To conclude, we report the first alkynyl-functionalised
bicyclo[1.1.1]pentanes of heavy group 14 elements, including
a linked member of this new family. NMR investigations
revealed that the title compounds show significant ‘‘through
cage’’ communication effects, which can most likely be attributed
to ‘‘back-lobe-to-back-lobe’’1,14 interactions. For the bridged
[1,10]-bis(bicyclo[1.1.1]pentane) 5, quantum chemical calculations
predicted some conjugation along the rod-like scaffold. Studies in
our lab continue to further explore the catenations of bicyclo-
pentanes of this type.
enantiomers and diastereomers may be visualised as shown in
Scheme 4. It is reasonable to assume that the diastereomeric
forms give slightly different 119Sn NMR resonances. The Sn
atoms bonded to the alkynyl bridge are obviously more
affected (signals at d = ꢀ295 and ꢀ296 ppm), whereas the
resonances of the Sn–Me moieties overlap. Note that we also
observed different 1H NMR signals (likewise in other solvents
like C6D6), albeit not clearly resolved (see ESIz). Currently we
are not able to comment on whether these isomers are in
equilibrium or not. The single crystal used for the X-ray
structure analysis of 5, however, only contained the forms
III and IV (Scheme 4 and Fig. 2b).
We thank Joachim Reimer for preparative assistance. P.O.B.
and D.N. thank the AvH foundation for Postdoctoral Fellowships.
Notes and references
The 29Si NMR chemical shifts of the bridging silicon groups
of 2–5 are almost equal (d = ꢀ56 to ꢀ59 ppm), but clearly
different from 1 (d = 98 ppm).7g Interestingly, we found an
(almost) linear correlation when the 117Sn,119Sn coupling
constants between the bridgehead atoms are correlated with
the interbridgehead distance. As can be seen from Table 1, a
larger distance is associated with a smaller 117Sn,119Sn coupling
constant.13 Without going into too much details, this correlation
nicely shows the significance of the interbridgehead distance for
the strength of the interaction between the bridgehead atoms –
even in these cases, where both are formally not bonded and
‘‘simply’’ arranged in the same region of space. For all-carbon
bicyclo[1.1.1]pentanes, the term ‘‘back-lobe-to-back-lobe’’ inter-
action has been coined for this phenomenon.1,14 In this line of
thought, similar effects are observable for 3 and 4: the alkyne
proton in 3 consists of two pairs of 117/119Sn satellites, the
smaller one of which (5 Hz) belongs to a coupling to the tin
bridgehead on the opposite side of the cage (most likely
‘‘through cage’’). For 4, the corresponding coupling is only
evident as shoulders flanking the central signal.
1 P. F. H. Schwab, M. D. Levin and J. Michl, Chem. Rev., 1999, 99, 1863.
2 Selected reviews: (a) S. Szafert and J. A. Gladysz, Chem. Rev.,
2003, 103, 4175; (b) M. I. Bruce and P. J. Low, Adv. Organomet.
Chem., 2004, 50, 179; (c) U. H. F. Bunz, Chem. Rev., 2000,
100, 1605; (d) M. Akita and T. Koike, Dalton Trans., 2008, 3523.
3 See e.g. (a) R. Gleiter, K.-H. Pfeifer, G. Szeimies and U. Bunz,
Angew. Chem., Int. Ed. Engl., 1990, 29, 413; (b) A. J. McKinley,
P. N. Ibrahim, V. Balaji and J. Michl, J. Am. Chem. Soc., 1992,
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2000, 38, 115; (d) Y. A. Berlin, G. R. Hutchison, P. Rempala,
M. A. Ratner and J. Michl, J. Phys. Chem. A, 2003, 107, 3970;
(e) E. W. Della, I. J. Lochert, N. M. Peruchena, G. A. Aucar and
R. H. Contreras, J. Phys. Org. Chem., 1996, 9, 168.
4 M. A. Fox, R. L. Roberts, T. E. Baines, B. Le Guennic, J.-F.
Halet, F. Hartl, D. S. Yufit, D. Albesa-Jove, J. A. K. Howard and
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P. J. Low, J. Am. Chem. Soc., 2008, 130, 3566.
5 D. Nied and F. Breher, Chem. Soc. Rev., 2011, 40, 3455.
6 N. Sandstrom and H. Ottosson, Chem.–Eur. J., 2005, 11, 5067.
¨
7 (a) Y. Kabe, T. Kawase, J. Okada, O. Yamashita, M. Goto and
S. Masamune, Angew. Chem., Int. Ed. Engl., 1990, 29, 794;
(b) L. R. Sita and R. D. Bickerstaff, J. Am. Chem. Soc., 1989,
111, 6454; (c) I. Kinoshita and L. R. Sita, J. Am. Chem. Soc., 1990,
112, 8839; (d) D. Nied, W. Klopper and F. Breher, Angew. Chem.,
Int. Ed., 2009, 48, 1411; (e) D. Nied, R. Koppe, W. Klopper,
¨
H. Schnockel and F. Breher, J. Am. Chem. Soc., 2010, 132, 10264;
¨
(f) D. Nied, E. Matern, H. Berberich, M. Neumaier and F. Breher,
Organometallics, 2010, 29, 6028; (g) D. Nied, P. Ona-Burgos,
W. Klopper and F. Breher, Organometallics, 2011, 30, 1419;
(h) K. Abersfelder, A. J. P. White, R. J. F. Berger, H. S. Rzepa
and D. Scheschkewitz, Angew. Chem., Int. Ed., 2011, 50, 7936.
8 T. Iwamoto, D. Tsushima, E. Kwon, S. Ishida and H. Isobe,
Angew. Chem., Int. Ed., 2012, 51, 2340.
9 Alkynyl-bridged all-carbon bicyclo[1.1.1]pentanes have, to the best of
our knowledge, not been described so far. For related structures see for
instance (a) O. Schafer, M. Allan, G. Szeimies and M. Sanktjohanser,
Chem. Phys. Lett., 1992, 195, 293; (b) M. N. Paddon-Row and
K. D. Jordan, J. Am. Chem. Soc., 1993, 115, 2952.
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V. S. Nikitin, Russ. Chem. Bull., 2000, 49, 620.
13 This particularly holds true if other (published or unpublished)
1,3-disubstituted trisiladistannabicyclo[1.1.1]pentane derivatives
known from our group are included in this correlation.
14 M. D. Levin, P. Kaszynski and J. Michl, Chem. Rev., 2000, 100, 169.
15 C. Cauletti, C. Furlani, G. Granozzi, A. Sebald and B. Wrackmeyer,
Organometallics, 1985, 4, 290.
In order to shed some light on conceivable conjugation
effects in 5,15 a density functional theory (DFT) calculation
was conducted on q5 (a Ph-substituted model for 5) at the
(RI)-DFT/BP86/def2-TZVP level (see ESIz). The highest
occupied molecular orbitals are of p-type and located on both
the CRC bridge and the Sn2Si3 entities (Fig. S26, ESIz). The
lowest unoccupied orbital was found to be of s*-type. Most
importantly, the calculations revealed that the HOMO–LUMO
Table 1 Comparison of the interbridgehead distances detected by
X-ray diffraction and the 117Sn,119Sn coupling constants of 2–5a
Compound
d(Snꢁ ꢁ ꢁSn) [pm]
J(117Sn,119Sn) [Hz]
2
3
4
5
319.4(1)b
315.71(9)
315.75(9)
318.7(2)
4877
7127
6942
6770
a
Note that the 1J(29Si,119Sn) coupling constants do not show a linear
b
correlation with the interbridgehead distance. Some uncertainties
due to the quality of the refinement, see ESI.
16 Note that the HOMO–LUMO gap decreases by only 0.02 eV when
going from q2 to q4.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6803–6805 6805