Atomically Precise Expansion of Unsaturated Silicon Clusters

Article abstract of DOI:10.1002/anie.201811331

Small- to medium-sized clusters occur in various areas of chemistry, for example, as active species of heterogeneous catalysis or as transient intermediates during chemical vapor deposition. The manipulation of stable representatives is mostly limited to

Full text of DOI:10.1002/anie.201811331

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Accepted Article
Title: Atomically precise expansion of unsaturated silicon clusters
Authors: Kinga I. Leszczyńska, Volker Huch, Carsten Präsang,
Jan Schwabedissen, Raphael J. F. Berger, and David
Scheschkewitz
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201811331
Angew. Chem. 10.1002/ange.201811331
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10.1002/anie.201811331
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Atomically precise expansion of unsaturated silicon clusters
Kinga I. Leszczyńska,*[a] Volker Huch,[a] Carsten Präsang,[a] Jan Schwabedissen,[b] Raphael J. F.
Berger,*[b] David Scheschkewitz*[a]
Dedicated to Prof. Robert West on occasion of his 90th birthday
Abstract: Small- to medium-sized clusters occur in various areas of
chemistry, e.g. as active species of heterogeneous catalysis or
transient intermediates during chemical vapor deposition. The
manipulation of stable representatives is mostly limited to the
stabilizing ligand periphery, virtually excluding the systematic
variation of the property-determining cluster scaffold. We now report
the deliberate expansion of a stable unsaturated silicon cluster from
six to seven and finally eight vertices. The consecutive application of
lithium/naphthalene as reducing agent and decamethylsilicocene as
electrophilic source of silicon results in the expansion of the core by
precisely one atom with the potential of infinite repetition.
far the lowest energy structure on the C6H6 potential energy
surface.[11]
Small to medium-sized clusters assume key roles in various
technologically important areas. In heterogeneous reactions,
metal and metal compound clusters are powerful catalysts for
chemical transformations on an industrial scale.[1] Metal and
semi-metal clusters constitute key intermediates during the
chemical vapor deposition of thin films and bulk[2] as well as the
bottom-up synthesis of nanoparticles.[3] Although a remarkable
number of stable molecular clusters has been reported, e.g.
– [4]
2– [6]
[7]
Si32R45
,
(CdSe)34,[5] R20Al77
,
R44Au102 (R = ligand or
functional group), they are typically obtained in a non-systematic
or even serendipitous manner, often entailing difficulties with
yield and reproducibility.
Chart 1. Selected saturated and unsaturated silicon clusters. 1, 3, 4, 5a: R =
Tip = 2,4,6-iPr3C6H2, 2: R = 2,4,6-Me3C6H2, 5b: R = 2,6-iPr2C6H3; 6a, 7a, 8: R
Unsaturated silicon clusters with partial substitution
(siliconoids) have attracted attention for their role as proposed
intermediates in gas phase deposition processes. With the
unsubstituted vertices, they share important surface features of
both, nanoparticles and the elemental bulk.[8] The preference of
silicon for single bonds as opposed to the competitive strength
of multiple bonds in the corresponding carbon systems leads to
a growing dominance of cluster motifs when the number of
=
tBu3Si; 6b, 7b:
R
=
((Me3Si)2CH)2(Me)Si; 9, 11: (R)2
=
C(SiMe3)2CH2CH2C(SiMe3)2, R´ = tBu; 10: (R)2 = C(SiMe3)2C(Me)2C(SiMe3)2;
11: D = p-(dimethylamino)pyridine; a: KC8, b: ICl.
While solvated Zintl ions – charged, polyhedral clusters – of
germanium and tin can be used as well-defined precursors for
the synthesis of partially substituted molecular clusters,[12] the
strongly reducing silicon congeners have only been employed as
ligands towards a few redox-stable transition metal fragments.[13]
and, very recently, protonated to the partially hydrogen-
substituted clusters [HSi9]3− and [H2Si9]2−.[14] Therefore,
siliconoids are typically accessed by reductive cleavage of
competent leaving groups from classical saturated precursors.
This approach had initially been demonstrated by the
preparation of saturated polyhedral silicon clusters such as
hexasilaprismane R6Si6 5a,b[15] and tetrasilatetrahedrane R4Si4
6a,b[16] by reductive dimerization of the corresponding linear
R2Si2X4 or cyclic R3SiX3 building blocks. Functionalization of the
saturated Si4-core was achieved with KC8 to yield isolable
silicon core atoms is increased.[9] For example, the first stable
[10a]
siliconoid, Si5R6
(1, Chart 1), adopts a tricyclic framework
with one unsubstituted silicon vertex and the isomeric
pentasilapropellane
2 even features two "naked" cluster
atoms.[10b] The low-energy Si6R6 isomers 3[10c] and 4[10d] are both
characterized by a cluster framework with two unsaturated
vertices, in marked contrast to the iconic benzene, which is by
[a]
Dr. K. Leszczyńska, Dr. V. Huch, Dr. C. Präsang, Prof. Dr. D.
Scheschkewitz
Krupp-Chair of Inorganic and General Chemistry
Saarland University
tetrasilatetrahedranides K+[R3Si4 ] 7a,b.[16b,c] Oxidation of 7a with
-
Campus Saarbrücken C41, 66123 Saarbrücken (Germany)
E-mail: scheschkewitz@mx.uni-saarland.de
Dr. R. J. F. Berger, Dr. J. Schwabedissen
Division of Materials Chemistry
iodine monochloride gave siliconoid R6Si8 8,[17] which had
previously been synthesized from a different R3Si4 precursor by
reduction.[18] Further examples of isolable siliconoids were
reported by Iwamoto[19] (9, 11) and Kyushin[20] (10). Despite the
isolated reports of core expansion of siliconoids,[15b,19] the
number of atoms in the scaffold, once assembled, cannot be
increased in a systematic way.
[b]
University of Salzburg
5020 Salzburg (Austria)
Supporting information for this article is given via a link at the end of
the document.
1
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Based on our report on the first anionic siliconoid with partial
substitution Si6R5− (12, R = Tip = 2,4,6-triisopropylphenyl),[21] we
sought to develop an atomically precise cluster expansion
protocol. As the leaving group properties of the
pentamethylcyclopentadienyl ligand (Cp*) are well-established in
main group chemistry,[22] we speculated that Jutzi's
decamethylsilicocene (SiCp*2)[23] (despite the decacoordinate
silicon atom) might react with 12 as an electrophilic source of
satisfactory agreement was achieved with the B3LYP functional;
see Table S9, Supporting Information).
Table 1. Experimental (13, 14·Li+(thf)3, 15) and calculated (13Tip
14Tip·Li+(thf)3, 15Tip 29Si NMR chemical shifts [ppm] at the OLYP/def2-TZVP
level of theory. Atoms of 15 and 15Tip are numbered as in 16.
,
)
13
13Tip
14
14Tip
15
15Tip
precisely one silicon atom. Previously, only the corresponding
Si1
Si2
Si3
Si4
Si5
Si6
Si7
Si8
−229.6 −237.6 −191.9 −197.0 −138.9 −151.9
−241.9 −255.5 −195.7 −204.8 −165.1 −160.5
−
cation Cp*Si+BAr4 (Ar
=
pentafluorophenyl)[24] had been
employed as electrophile.[25] As proof-of-principle, we carried out
the reaction of R2Si=Si(R)Li with SiCp*2. Indeed, the literature-
known Cp*-substituted cyclotrisilene, cyclo-Si3Tip3Cp*, is
obtained in 90% spectroscopic yield.[26]
181.9
2.6
180.3
10.6
284.3
10.8
292.1
18.1
18.7
33.5
5.9
41.0
−138.4 −139.4
−66.1
60.1
−73.1
78.4
−81.7
111.8
60.3
−82.2
115.5
80.2
15.0
20.2
156.0
154.6
148.0
150.7
206.9
201.0
The structure of siliconoid Si7R5Cp* in the solid state was finally
confirmed as the first stable example for a neutral siliconoid with
three adjacent unsubstituted vertices 13 by X-ray diffraction on
orange-red crystals obtained in 53% yield (Figure 1).[29] The
cluster scaffold is almost indistinguishable from that predicted for
[27]
13H and 13Tip
.
The three unsubstituted vertices form an
isosceles triangle with its base and apex bridged on both sides
by electron-precise disilanyl units.
Scheme 1. Cluster expansion from anionic Si6 siliconoid 12 to Si7 siliconoids
13, 14 and finally Si8 siliconoids 15 and 16. R = Tip = 2,4,6-triisopropylphenyl,
Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl.
More importantly, treatment of the anionic siliconoid 12 with one
equivalent of SiCp*2 leads to a uniform product 13 accompanied
by precipitation of Cp*Li (Scheme 1). 1H NMR of 13 in C6D6
confirms the presence of five nonequivalent Tip groups. The
additional singlet resonance at 1.69 ppm corresponds to 15
hydrogen atoms and is assigned to the remaining Cp*
substituent. The characteristic wide distribution of 29Si NMR
chemical shifts of siliconoid 4 and its derivatives (typically from
about −270 ppm for the "naked" vertices to +175 ppm for the
adjacent R2Si bridge)[8] is retained in the new product. A third
strongly shielded resonance at −138.4 ppm without a cross-peak
in the 29Si-1H correlation NMR spectrum suggests the presence
of an additional silicon vertex without substituent. As the
Figure 1. Molecular structure of siliconoid 13 in the solid state. Hydrogen
atoms are omitted for clarity. Thermal ellipsoids represent 50% probability.
Selected bond distances [pm] and angles [°]: Si1-Si2 264.8(1), Si1-Si5
234.41(9), Si1-Si3 231.63(9), Si2-Si5 234.19(9) Si3-Si4 241.81(9), Si4-Si5
238.86(9), Si5-Si6 238.42(9), Si6-Si7 241.55(9), Si1-Si5-Si2 68.82(3), Si1-Si3-
Si2 69.45(3), Si4-Si5-Si6 173.77(4).
predicted global minimum isomer of the Si7H6 potential energy
[27]
The Si1-Si2 distance of 264.8(1) pm is relatively long, while all
Si-Si bonds of the cluster core (231.8 to 241.8 pm) show typical
Si-Si single bond distances. Alternatively, the Si7 scaffold of
siliconoid 13 can be described as persilapropellane with twofold
R2Si bridging of the propeller blades and an unprecedented
seesaw-type tetracoordination of Si5 as shown by the close-to-
linear bond angle Si4-Si5-Si6 of 173.88(4)°.
surface 13H
has precisely three "naked" vertices, we
calculated the 29Si NMR spectrum of a B3LYP/def2-TZVP-
optimized model system for 13 corresponding to the
experimental case (13Tip) at the OLYP/def2-TZVP level of
theory.[28] The excellent agreement of the calculated chemical
shifts of 13Tip with those experimentally observed for 13
suggests very similar constitutions (Table 1; much less
2
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The residual Cp* group in 13 should allow for the
regeneration of the anionic functionality after the first core
expansion step. The Si7 siliconoid 13 was therefore treated with
two equivalents of lithium/ naphthalene in tetrahydrofuran (thf) at
−100 °C in order to obtain the anionic 14. 1H NMR spectroscopy
of the uniform reaction mixture confirmed the presence of five
Tip substituents, while the resonance of the Cp* group had
indeed completely vanished. Instead, characteristic signals for
three equivalents of coordinated thf suggested the incorporation
of a lithium cation into the molecule. The 29Si NMR spectrum
exhibits a similar distribution of the seven signals as the starting
material 13. The most remarkable difference is the downfield
shift of the formerly Cp*-bonded silicon vertex by about  = 100
ppm accompanied by substantial line broadening due to
Si3 due to the absence of direct interaction with the quadrupolar
7Li nucleus, all other resonances remain essentially unchanged.
Similarly, the solid state structure of 14[Li+(dme)3] is hardly
influenced by the spatially distant Li+ counter cation (Figure 2).
With anionic siliconoid 14 in hand, the obvious next step was
the incorporation of an eighth silicon vertex. Similar to the
synthesis of neutral Si7 cluster 13 from anionic 12, the reaction
of 14 with SiCp*2 yields
a uniform product 15 (90%
spectroscopic purity). In this case, the observation of only three
of the eight resonances at high field ( = −165.1, −138.9, −81.7
ppm) suggested the migration of an aryl group from
a
disubstituted SiR2 unit to a formerly "naked" vertex. Indeed, a
1H-29Si correlation spectrum reveals only one SiR2 unit ( = 33.5
ppm), while four resonances ( = 111.8, 60.3, 35.5, 18.7 ppm)
are due to SiR vertices bonded to just one substituent.
7
coupling to the quadrupolar Li nucleus (7Li = −0.15 ppm). In
contrast, the unsubstituted vertices are only shifted by  = 40 to
70 ppm. A similar relative deshielding had been observed
between the neutral Si6 siliconoid 4 and the corresponding
lithiated species 12.[21] Applying the OLYP functional, the
calculated 29Si chemical shifts of the B3LYP optimized model
Tip5Si7(Li)thf3 (14Tip·Li+(thf)3) nicely reproduce the experimental
data of 14·Li+(thf)3 (Table 1).
In one crystallization attempt, a few orange single crystals
were collected and investigated by x-ray diffraction (Figure 3).
Instead of the Si8R5Cp* siliconoid 15, however, its water adduct
16 with a central unit of three unsubstituted silicon atoms was
obtained. The formerly almost linear bond angle of the Si7
siliconoids 13 and 14 at Si5 is with 149.53(5)° much more acute
in 16. Apparently, the release of strain results in an
approximation of regular tetrahedral coordination at Si5.
Figure 2. Molecular structure of the solvent-separated anionic siliconoid 14 in
the solid state. Hydrogen atoms and [Li+(dme)3] counter-cation are omitted for
clarity. Thermal ellipsoids represent 50% probability. Selected bond distances
[pm] and angles [°]: Si1-Si2 255.96(8), Si1-Si5 233.77(8), Si1-Si3 239.37(8),
Si2-Si5 231.96(8), Si3-Si4 244.82(8), Si4-Si5 235.68(8), Si5-Si6 237.59(8),
Si6-Si7 241.12(7), Si1-Si5-Si2 66.68(2), Si1-Si3-Si2 64.16(2), Si4-Si5-Si6
174.00(3).
Figure 3. Molecular structure of siliconoid 16 in the solid state. Hydrogen
atoms (except OH) and co-crystalized solvent molecules are omitted for clarity.
Thermal ellipsoids represent 50% probability. Selected bond distances [pm]
and angles [°]: Si1-Si2 263.8(1), Si1-Si5 234.2(1), Si1-Si8 235.70(13), Si2-Si5
233.97(13), Si2-Si8 232.7(1), Si7-Si8 236.9(1), Si6-Si7 237.6(1), Si5-Si6
232.3(1), Si6-O 169.0(3), Si1-Si5-Si2 68.60(4) Si4-Si5-Si6 149.53(5).
A crystalline sample of 14 was obtained from pentane in 42%
yield. As shown by single crystal X-ray diffraction, the lithiated
siliconoid 14·Li+(thf)3 crystallizes as contact ion pair from
pentane solution (see Supporting information). The structural
parameters of the cluster scaffold of 14·Li+(thf)3 are similar to
those of the neutral precursor 13 except for the noticeably
shortened base of the central triangle (Si1-Si2 = 254.48(5) pm; 
= 10.3 pm). In order to assess the influence of the close contact
of the lithium cation to the anionic Si7 siliconoid, the solvent
separated ion pair 14[Li+(dme)3] was prepared by crystallization
from a mixture of hexane and 1,2-dimethoxyethane in 73% yield.
Except for significant sharpening of the 29Si NMR resonance of
Figure 4. Calculated HOMO (left hand side) and LUMO (right hand side) of Si8
siliconoid 15Tip
.
3
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The deliberate addition of one equivalent of H2O to 15 leads to
the near quantitative formation of 16 (85% spectroscopic yield).
Using the structure of 16 (minus H2O) as a reasonable starting
point, we optimized the geometry of 15Tip at the B3LYP/def2-
TZVP level of theory. The resulting R6Si8 cluster is related to the
Si7 species 13 inasmuch as one of the SiR2 moieties is formally
replaced by an exohedral unsaturated Si=Si bridge (Si6 and Si7).
The calculated HOMO and LUMO of 15 are predominantly
constituted by the  and * components at this Si=Si moiety,
which readily explains the selective H2O addition across this
bond (Figure 4). The experimental 29Si NMR signals of 15 at  =
111.8 (Si6) and 60.3 ppm (Si7) can be assigned to the
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experimental data. Those of the exohedral Si=Si moiety at  =
115.5 (Si6) and 80.2 ppm (Si7) are (as usually for atoms with
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bonding including
a
pronounced -component) slightly
overestimated, but otherwise nicely reproduce this trend (Table
1). Indeed, the calculated structure of 15Tip confirms appreciable
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even eight vertices 13, 14 and 15, we provide proof-of-concept
for the stepwise and repeated expansion of clusters with atomic
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electrophilic
source
of
divalent
silicon
with
pentamethylcyclopentadienide ligands (Cp*) as anionic leaving
groups. As our protocol allows for the regeneration of the anionic
functionality after the expansion step, a rapid increase of
available siliconoids (including heteroatom-doped variations) can
be expected. The ubiquitous use of Cp* throughout the Periodic
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This work was supported by Deutsche Forschungsgemeinschaft
(DFG SCHE906/4-1 and 4-2) and COST (Action CM1302). We
thank Susanne Harling for elemental analysis and the reviewers
whose comments contributed significantly to the improvement of
the computational results. J.S. acknowledges the generous
provision of computational resources by the Paderborn Center
for Parallel Computing (PC2)
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Keywords: anions • cluster compounds • main group elements •
silicon • subvalent compounds
4
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5
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10.1002/anie.201811331
Angewandte Chemie International Edition
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COMMUNICATION
A stable unsaturated Si6R6 cluster is
consecutively expanded to stable
neutral and anionic clusters Si7R6,
Si7R5− and Si8Rg. Key to this protocol
is the use of decamethylsilicocene
(Cp*2Si) as electrophile with Cp*- as
leaving group. s
Kinga I. Leszczyńska,* Volker Huch,
Carsten Präsang, Jan Schwabedissen,
Raphael J. F. Berger,* David
Scheschkewitz*
Page No. – Page No.
Atomically precise expansion of
unsaturated silicon clusters
6
This article is protected by copyright. All rights reserved.