Targeted attachment of functional groups at Ge9 clusters: Via silylation reactions

Article abstract of DOI:10.1039/c7cc06622h

Molecules with low-valent Ge atoms are generally synthesized from organohalogen germanes as precursors. The Zintl phase K₄Ge₉ provides reactive building blocks for a targeted synthesis of germanium-rich molecules. The silylation of Ge9 clusters with chlorosilanes ClSiR₂R′, that carry unsaturated olefin groups R′ leads to the introduction of olefinic side chains of variable lengths allowing for further reactions. The compounds K[Ge₉{Si(SiMe₃)₃}₂(SiPh₂R′)] (R′ = -CHCH₂ (1); -(CH₂)₃CHCH₂ (2)) carry one such functionality, whereas K[Ge₉(SiPh₂R′)₃] (3 and 4) offer the possibility for an interconnection of clusters due to three functional groups on the Ge₉ core. XPS measurements show that the silylated clusters are much more air-stable than the unsubstituted, bare cluster units.

Full text of DOI:10.1039/c7cc06622h

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Targeted Attachment of Functional Groups at Ge9 Clusters via
Silylation Reactions
K. Mayer, a L. J. Schiegerl,a T. Kratky,a S. Günther,a and T. F. Fässlera *
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
9–
Molecules with low-valent Ge atoms are generally synthesized pentameric [Au3Ge45
]
(ref. 21) are formed upon oxidation. Such
from organohalogen germanes as precursors. The Zintl phase interconnected Ge systems are considered as first steps in the
K4Ge9 provides reactive building blocks for a targeted synthesis of formation of possible new Ge allotropes built up by Ge9 clusters.22
germanium-rich molecules. Silylation of chlorosilanes ClSiR2R’, Only recently the link from bare anionic Zintl clusters to
that carry unsaturated olefin groups R’, with Ge9 clusters leads to functionalized germane cage compounds was realized by silylation
the introduction of olefinic side chains of variable lengths allowing with the rather bulky Cl{Si(SiMe3)3}3. The resulting
for further reactions. The compounds K[Ge9{Si(SiMe3)3}2(SiPh2R’)] [Ge9{Si(SiMe3)3}3]− can be obtained from K4Ge9 in good yields,23
(R’ = –CH=CH2 (1); –(CH2)3CH=CH2 (2)) carry one such functionality, although it was for the first time synthesized using Ge(I)Br as
whereas K[Ge9(SiPh2R’)3] (3, 4) offer the possibility for an precursor.24 Since then a fruitful chemistry has been built up around
interconnection of clusters due to three functional groups on the this class of compounds. The reaction with organometallic
Ge9 core. XPS measurements show that the silylated clusters are compounds or salts leads to several complexes in which the
much more air-stable than the unsubstituted, bare cluster units.
silylated cluster acts as a ligand. With group 9 and 10 elements
dimeric compounds [M[Ge9{Si(SiMe3)3}3]2]x− (M = Zn, Cd, Hg, x = 0;
Since the pioneering work of Sekiguchi and Sakurai in the synthesis M = Cu, Ag, Au, x = 1) can be obtained25, 26, but also compounds in
of a Ge6 prismane cage stabilized by bulky bis(trimethylsilyl)methyl which the cluster replaces only one of the ligands are existent, like
groups1 many other germane cage compounds of the general [Ge9{Si(SiMe3)3}3Cr(CO)5]−,27 [Ge9{Si(SiMe3)3}3]ZnCp*,28 or the
formula GenRm (n = 4, 5, 6, 8, 10, 14; n > m) have been isolated.2 neutral Zintl cluster coinage metal NHC compounds
The synthetic protocol generally follows a reductive elimination of [(M−NHC)Ge9{Si(SiMe3)3}3].29 The reaction of K4Ge9 with TMS-C≡C-
halogen atoms from organohalogen germanes. For n = 10 (ref. 3, 4) TMS leads to the mono and bis-vinylated species [Ge9(CH=CH2)]3−
and 14 (ref. 5) four and nine Ge atoms, respectively, that are not and [Ge9(CH=CH2)2]2−, respectively, which can be obtained in good
connected to a ligand group, are present. 6 Bare anionic clusters Gen yields, and derivatives of this compound with different side chains
with n = 9 are easily accessible by reacting the soluble binary as e.g. [Ge9(CH=CH-CH(CH2)2]− are also known.30-35 The linkage of
compounds A4Ge9 and A12Ge17 (A = heavier alkali metal), which then two Ge9 clusters through a conjugated π system was realized in the
can act as building blocks in further reactions.2, 7-10 The treatment of Zintl triade [RGe9−CH=CH−CH=CH−Ge9R]4− (R=(2Z,4E)-7-amino-5-
these units with organometallic compounds revealed a multifaceted aza-hepta-2,4-dien-2-yl),36, 37 and also the bridging of two silylated
chemistry including a large number of transition metal complexes cluster units via a bis-silyl group was successful, leading to the
such as [Ge9-Ni(CO)]3−.11 Examples in which low oxidation states are recently reported compound K2[{Si(SiMe3)3}3Ge9-SiMe2-(C6H4)-
stabilized, and which even contain an unsupported Zn−Zn bond, like SiMe2-Ge9{Si(SiMe3)3}3].38 The hypersilyl group [Si(SiMe3)3]+ provides
in [Ge9−Zn−Zn−Ge9]6−,12 have been realized as well as endohedrally a high stability for tris-silylated compounds. With the chloro-silane
filled clusters with deltahedral shape, [Ni@Ge9]3−,11 or the non- derivatives ClSiR3 (R = Ph, iBu, iPr, Et, ClSiHtBu2) also sterically less
3−
39, 40
deltahedral [Co@Ge10
]
(ref. 13). Larger clusters such as demanding groups can be introduced,28,
resulting in the bis-
(K-18c6)2[Ge9{Si(iBu)3}2]39
and
[Pd−Pd@Ge18]4−,14 oligomeric [Ge9
]
(m
=
2, 3, 4)15-20 or silylated41
[(CuPiPr3)4{Ge9(SiPh3)2}2].28
compounds
2−
m
a Department Chemie, Technische Universität München
Lichtenbergstraße 4, 85747 Garching, Germany
E-mail: thomas.faessler@lrz.tum.de
The silylated clusters being saturated and the vinyl derivatives being
still highly charged did not allow besides forming metal
-
Electronic supplementary information (ESI) available: Experimental details,
crystallographic details, XPS study details, ESI-MS spectra, NMR spectra, IR- and Raman
spectra. CCDC 1569447 (1) 1569446 (2). See DOI: 10.1039/x0xx00000x
complexes - for a systematic development of synthetic protocols for
subsequent reactions. Taking advantage of the stabilization of
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clusters with silyl substituents we intended now to introduce side Furthermore, the structure shows that the vinyl group at the silyl
chains with reactive double bonds via silylation (Scheme 1). substituent should be accessible for further reactions despite the
Here we describe the reaction of [Ge9]4− and [Ge9{Si(SiMe3)3}2]2− presence of the bulky hypersilyl groups (Figure 1a). However, to
(ref. 41) with the chlorosilanes ClSiPh2CH=CH2 and improve the accessibility of the double bond, a longer alkenyl group
ClSiPh2(CH2)3CH=CH2 to afford the target molecules with functions should be advantageous. Therefore, a pentenyl rest was attached
either for cluster interconnection or for immobilization of cluster on the silyl group. The reaction of the chloro-silane
units on surfaces.
ClSiPh2(CH2)3CH=CH2 with the bis-silylated cluster in thf or acn leads
The reaction of the bis-silylated cluster [Ge9{Si(SiMe3)3}2]2− with to the formation of K[Ge9{Si(SiMe3)3}2(SiPh2(CH2)3CH=CH2)] (2). The
ClSiPh2CH=CH2 in tetrahydrofuran (thf) or acetonitrile (acn) leads to product was characterized by ESI-MS, NMR, Raman and IR
the
formation
of
a
mixed
silylated
cluster41 spectroscopy (Supporting Information). Red plate-shaped crystals of
K[Ge9{Si(SiMe3)3}2(SiPh2CH=CH2)] (1) with two hypersilyl groups and the composition 2ꢀ(thf) were obtained from a toluene solution
one vinylated silyl group. NMR investigations showed that the (Figure 1b). The symmetry of the Ge9 cluster core is again Cs, but
reaction is stoichiometric and proceeds quantitatively.
with a stronger relation to C2v than in compound 1, since the two
ESI-MS, Raman- and IR spectroscopy (Supporting Information) regarding prism heights are more similar (h1 = 3.146(1) Å, h2
=
confirmed the composition of the product (1), and yellow needle- 3.249(1) Å, h3 = 3.733(1) Å); the “square base” with the lowest α1
shaped crystals of 1ꢀ(Tol)1.23 have been obtained from toluene. The angle (17.0°) is again situated between one hypersilyl and the new
crystals contain two crystallographically independent cluster units, silyl group. The Ge−Si bond of the pentenyl silyl group is slightly
with only slightly different bond lengths and angles. Both Ge9 shorter than that of the hypersilyl groups: 2.367(2) Å vs. 2.372(2) Å
cluster cores possess slightly distorted Cs symmetry,42 as shown by and 2.373(2) Å, respectively. The double bond of the pentenyl
three very different prism heights (average values for both clusters: substituent is with 1.206(17) Å quite short, probably as a result of
h1 = 3.07 Å, h2 = 3.30 Å, h3 = 3.79 Å). However, a relation to C2v the relatively large displacement ellipsoid of the “dangling”, end-
symmetry is still existing, since there is a “square base” with a standing C atom.
smaller α1 angle (average value: 16.2°). Interestingly, this side is not In the crystal structures of 1 and 2 (for details see Supporting
located between the two hypersilyl groups like in the bis-silylated Information) the functionalized clusters are arranged in one-
precursor,41 but lies between one hypersilyl group and the vinylated dimensional strands: The clusters are connected via the counter ion
silyl substituent. The Ge−Si bond lengths are unaffected by the K, which also coordinates to one phenyl ring of the functionalized
substituents and range from 2.372(2) to 2.395(2) Å. The C-C double silyl group and to a solvent molecule.
bonds with 1.289(12) Å resp. 1.307(13) Å are in the normal range.
For instance, the vinyl groups of the bis-vinylated clusters range
between 1.21–1.34 Å.30
b)
a)
2-
4-
Hyp
Hyp
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
2 HypCl
- 2 Cl-
Ge
Ge
+ ClSiPh2(CH3)nCH=CH2
+ 3 ClSiPh2(CH3)nCH=CH2
- 3 Cl-
- Cl-
-
-
n
n
SiPh2
Ge
Hyp
Hyp
Ph2Si
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
Ge
SiPh2
n
SiPh2
n
Scheme 1: Reaction pathways for the attachment of olefin functionalities at
Ge9 clusters by reaction with with ClSiR2R’ [R’ = –CH=CH2 ; –(CH2)3CH=CH2]
4
−
chloro-silanes a) Silylation of [Ge9] with three ClSiR2R’ groups under
formation of
a
tris-functionalized species. b) Silylation step on
Figure 1: Molecular structures of the cluster anions of compound 1 (a) and 2
(b). Ge atoms are in blue, Si atoms in orange and C atoms in grey,
displacement ellipsoids are shown at a probability level of 50%. [Ge9] is
shown as polyhedron. For reasons of clarity H atoms and methyl groups at
the hypersilyl units are omitted.
[Ge9{Si(SiMe3)3}2]2− resulting in a mixed tris-silylated Ge9 species with the
olefin functionality on one silyl group group (Hyp = [Si(SiMe3)3]).
.
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(3) and K[Ge9(SiPh2(CH2)3CH=CH2)3] (4) could be identified via ESI-
MS (Figure 3).
We performed X-ray photoelectron spectroscopy (XPS) studies on 1
and 2, i.a. in order to get an idea about the air sensitivity of these
compounds (for data of 2 see Supporting Information). As a
reference, we used the rather sensitive K4Ge9. Both species, the
silylated cluster and the bare cluster compound, were sputtered
with Ar+ before the first XPS measurement. K4Ge9 is characterized
,
as
required
for
by
a Ge 2p3/2 core level peak that still shows conspicuous
components related to oxidized Ge (GeII, GeIV), while the main peak
component of the silylated compounds is related to non-oxidized
Ge. We conclude that the compounds mainly consist of Ge0 with
only small amounts of germanium oxides (for the ratios of all
species see Supporting Information). After an exposition of the
cluster compounds to air for 30 min the Ge signals of K4Ge9 had
disappeared, accompanied by an increase of the K and the O
signals; this hints for a complete oxidation and decomposition of
the compound and a dense potassium oxide layer covering the
surface. The silylated compounds, however, are significantly more
air-stable. The oxide fractions increased slightly, but the main
species is still Ge0 (Figure 2). This points out that the silylated
compounds have an enhanced stability on air than the bare Ge9
clusters, which raises their potential as candidates for further
applications even in oxygen- and moisture-containing
environments.
Figure 2: XP spectra in the Ge 3d region of compound 1 after sputtering
(left) and after exposure to air for 30 min (right).
The longer pentenyl group of compound 2 promises a better
accessibility of the double bond at the cluster, but the introduction
of smaller substituents than phenyl at the silyl groups would be a
step in the same direction. Therefore, [Ge9{Si(SiMe3)3}2]2– was
treated
with
ClSiMe2CH=CH2,
and
we
obtained
Figure 3: ESI-MS spectra of solutions of
3 (left, m/z = 1282
K[Ge9{Si(SiMe3)3}2(SiMe2CH=CH2)] with sterically less demanding
methyl groups on the vinylated silyl substituent. (see Supporting
Information) The formation of the product proceeds analogously to
1 and 2, as shown by ESI-MS and NMR experiments, whereas the
crystallization of the product has not been successful so far.
Further, less bulky substituents than hypersilyl at the cluster could
also enhance the accessibility of the double bond. The reaction of a
bis-silylated cluster equipped with tri-phenyl-silyl groups,
[Ge9{SiPh3}2]2−,28 with ClSiPh2CH=CH2 leads to the formation of the
mixed cluster K[Ge9(SiPh3)2(SiPh2CH=CH2)], indicating that this type
of reaction is not limited to the sterically demanding hypersilyl
groups. The anion was characterized by ESI-MS, and as a side
[Ge9(SiPh2CH=CH2)3]−) and 4 (right, m/z = 1408 [Ge9{SiPh2(CH3)3CH=CH2}3]−).
NMR spectroscopy showed that the reactions proceeded
quantitatively, no side products were detected. The obtained
products are stable in solution, however, the crystallization of the
compounds has not been successful as yet. Since there is only one
set of signals for the silyl groups in the NMR spectra, indicative for
three magnetically identical groups, a similar structure as found for
other tris-silylated clusters with the three groups attached at the
caps of the trigonal prism can be assumed.
The presented compounds fulfill many demands for various
applications. They allow access to clusters with a localized double
bond (compound 1 and 2) by adding functionalized silyl groups with
side chains of different lengths on a bis-silylated cluster. Those
cluster species are ideal for controlled reactions with one double
bond or for an immobilization of the clusters on surfaces. Especially
the enhanced air-stability qualifies them to test them for further
product
the
tris-silylated
clusters
[Ge9{SiPh3}3]−
and
[Ge9(SiPh3)(SiPh2CH=CH2)2]− were observed.
The formation of compounds 1 and 2 show that a targeted
introduction of one or two functional groups on the Ge9 clusters is
possible which opens the possibility for immobilization of such units
on surfaces or to interconnect such clusters. To build up three-
dimensional Ge9 networks, however, more functional groups would
be useful. For this reason, we synthesized clusters with a double
bond at each of the three silyl groups of the tris-silylated cluster.
The reaction of three equivalents of ClSiPh2CH=CH2 or
ClSiPh2(CH2)3CH=CH2 with K4Ge9 in acn or thf led to the formation of
red solutions, and the tris-silylated clusters in K[Ge9(SiPh2CH=CH2)3]
applications. Moreover, the compounds
3 and 4 with three
functional groups on one cluster can be obtained. They are perfect
for the potentially formation of Ge9 cluster networks. In order to
further improve the accessibility of the olefin chains, first strategies
to decrease the steric demand of the olefin chains are presented.
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