Synthesis, characterisation and electronic properties of naphthalene bridged disilanes

Article abstract of DOI:10.1039/c9dt03058a

Synthesis of naphthalene bridged disilanes 2_R (R = Me, Ph) was performed via catalytic dehydrocoupling. Using RhCl(PPh₃)₃ as a catalyst, an intramolecular Si-Si bond was readily formed from the corresponding disilyl precursors 1_R (R = Me, Ph). For catalytic reactions using (C₆F₅)₃B(OH₂), bridged siloxanes (3_Ph and 3_Me) were observed. Attempts to install the 1,8-naphthalene bridge directly onto a disilane resulted in an unusual product (4), containing two silicon centres bridged through one naphthyl group, and another naphthyl group attached to a single Si centre. In order for this product to form, both a Si to Si hydrogen shift rearrangement as well as Si-Si bond cleavage occurred. The effects of phenyl and methyl substitutions on the structure and electronic properties of the synthesised compounds was investigated by single crystal X-ray diffraction, as well as IR and multinuclear NMR spectroscopic analysis. In addition, theoretical UV-Vis absorption maxima were evaluated using density functional theory (TD-SCF) on a B3LYP/6-31(++)G?? level of theory and compared with experimental UV-Vis spectroscopic data.

Full text of DOI:10.1039/c9dt03058a

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DOI: 10.1039/C9DT03058A
Journal Name
ARTICLE
Synthesis, characterisation and electronic properties of
naphthalene bridged disilanes
Kristel M. Rabanzo-Castillo,a,b Muhammad Hanif,a Tilo Söhnela,b and Erin M. Leitao,*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
Synthesis of naphthalene bridged disilanes 2R (R = Me, Ph) was performed via catalytic dehydrocoupling. Using RhCl(PPh3)3
as a catalyst, an intramolecular Si-Si bond was readily formed from the corresponding disilyl precursors 1R (R = Me, Ph). For
catalytic reactions using (C6F5)3B(OH2), bridged siloxanes (3Ph and 3Me) were observed. Attempts to install the 1,8-
naphthalene bridge directly onto a disilane resulted in an unusual product (4), containing two silicon centres bridged through
one naphthyl group, and another naphthyl group attached to a single Si centre. In order for this product to form, both a Si
to Si hydrogen shift rearrangement as well as Si-Si bond cleavage occurred. The effects of phenyl and methyl substitutions
on the structure and electronic properties of the synthesised compounds was investigated by single crystal X-ray diffraction,
as well as IR and multinuclear NMR spectroscopic analysis. In addition, theoretical UV-Vis absorption maxima were evaluated
using density functional theory (TD-SCF) with standard B3LYP/6-31(++)G** method and basis set combinations and
compared with experimental UV-Vis spectroscopic data.
DOI: 10.1039/x0xx00000x
www.rsc.org/
decreasing HOMO-LUMO gap
Introduction
possibility of forming new Si-Si and Si-E bonds via new catalytic
coupling reactions via Si-H activation would extend the utility
and scope of organosilanes in the preparation of complex
silicon-containing molecules and macromolecules. In particular,
new strategies are necessary to overcome the synthetic
challenges in linking silicon atoms together into a long chain.
For example, the extent and rate of polymerisation decreases
when moving from primary to secondary silanes which can be
rationalised by bond energies and steric hindrance. There is a
difference of ca. 5 kJ mol-1 for the Si-H bond strength between
PhSiH3 (377 kJ mol-1) and PhMeSiH2 (382 kJ mol-1).14 Secondary
silanes are also more sterically demanding, which results in a
higher activation barrier for silicon coupling at the metal centre,
in the case of transition metal-catalysed dehydrocoupling, as
the chain length increases.15 During polymerisation, the
growing polymer chain is subject to Si-Si (327 kJ mol-1) bond
cleavage leading to low molecular weight oligomers and/or
other by-products. Moreover, despite a wide range of well-
established Si–H activation chemistry available, finding the
appropriate experimental conditions to effect the
transformation of Si–H moieties in organosilanes while keeping
the Si–Si bonds intact, still remains a challenge.16
Organosilanes have drawn considerable attention in medicinal1-
5
and materials6 chemistry as useful synthetic building blocks7
and as protecting groups. Organosilicon small molecules are
particularly interesting for applications such as pharmacological
inhibitor design and drug release technology.3-4 Moreover,
oligo- and polysilanes have potential applications as advanced
materials8-12 due to their unique and interesting
physicochemical properties, which arise from sigma bond
electron delocalisation.7
The widespread use of organosilanes can be attributed to the
availability of silane precursors, enabled by the silicon and
silicone industries. To date, many catalysts and synthetic
methods have been used to form Si-Si and Si-element (Si-E)
bonds, however, some of the methods known have drawbacks
including harsh reaction conditions, formation of undesirable
by-products, and poor functional group tolerance.13 The
a. School of Chemical Sciences, University of Auckland, Auckland, New Zealand
b. The MacDiarmid Institute for Advanced Materials and Nanotechnology, New
Zealand
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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One way to potentially support the weak Si-Si bond in these compounds were determined based on HRMS results
View Article Online
DOI: 10.1039/C9DT03058A
polysilanes is to install a bridge between the Si atoms (Figure 1, (Figure S3).
naphthalene bridge). This could allow for more active catalysts
to be employed for dehydrocoupling reactions (using By contrast, a reaction of the 1,8-dilithionaphthalene with
precursors 1R, R = Me, Ph), as the building blocks will be more stoichiometric amounts of chlorodiphenylsilane (ClSiPh2H) or
robust. Moreover, installation of a tether may result in the chlorodimethylsilane (ClSiMe2H) afforded 1Ph and 1Me, in 53%
recombination of any broken Si-Si bonds, due to their close and 48% isolated yield, respectively (Scheme 1; Figures S4-S13).
proximity, particularly in the case of cleavage that results in silyl This reaction is simplified compared to what has been
32
radicals. To this end, research towards making bridged previously reported for the syntheses of 1Ph26 and 1Me
.
disilanes, with the potential to be synthetically manipulated
into polymer building blocks,17 was performed. The polysilane
structure is known to exhibit excellent properties for novel
applications, so increasing the robustness of the polymer will
open up avenues for this.18-24
H
H
Si
H
Si
H
i) Cl3Si-SiCl3
ii)
LiAlH4
In this study, naphthalene bridged disilanes were targeted
(Figure 1, 2R, R = H, Me, Ph). The precursors to these compounds
were either pre-formed disilanes or disilyl naphthalene
compounds, 1R, which can undergo catalytic intramolecular
dehydrocoupling to yield 2R. A comparison of the structural and
electronic properties of the compounds and similar derivatives,
3R, will be discussed.
2H
not formed
Br
Br
Li
Li
2 eq n-BuLi
Et2O, -78 o
C
25oC, 10 min
H
H
R
R
Si
Si
R
R
2 eq ClSiR2H
22oC, 22 h
R = Ph, Me
1R
H
H
R
R
R
R
R
R
O
Si
Si
Si
Si
Scheme 1. Synthesis of 1,8-disilylnaphthalene derivatives 1R, R = Ph,
Si
Si
R
R
R
R
R
R
Me
Catalytic dehydrocoupling of 1R was then performed to link the
two Si centres together in order to form bridged
disilylnaphthalenes (Scheme 2, 2R, Table 1 and Figures S14-S22)
containing a 5-membered ring with an internal Si-Si bond. Four
different dehydrocoupling catalysts were considered,
B(C6F5)3,33 Wilkinson’s catalyst (RhCl(PPh3)3), 2nBuLi/Cp2TiCl2,
and 2nBuLi/Cp2ZrCl2.34-36 All reactions were conducted using
dilute concentrations of the substrate to favour an
1R
2R
R = H, Me, Ph
3R
Figure 1. Naphthalene bridged target compounds 2R and 3R with
precursors 1R
Results and Discussion
Synthesis, spectroscopic and crystallographic characterisation
of bridged 1,8-disilylnaphthalene derivatives
intramolecular
dehydrocoupling
reaction
over
an
intermolecular one, which would yield the undesirable disilane
oligomers. The reaction progress could easily be tracked by
1
monitoring the disappearance of the Si-H signal by H NMR
A naphthalene bridge was chosen for two reasons i) to increase
overall conjugation (, ) in the molecule and ii) because the 5-
membered ring in 2R is unstrained as the distance between the
silicon atoms matches nicely with a standard Si-Si bond length.1
The first reported preparation of 1H wasby hydride-reduction of
a minor component of the product mixture generated in the
copyrolysis of trichlorosilylnaphthalene and silicochloroform in
a hot tube at 650oC.25 More practical synthetic routes to 1,8-
disilylnaphthalene compounds (1R) are now readily available via
literature methods.1, 25-30
Several attempts were made to synthesise the 1,8-
disilylnaphthalene derivatives. One of which, unsuccessfully
involved the reaction of 1,8-dilithionaphthalene with
stoichiometric amounts of hexachlorodisilane (Cl3Si-SiCl3), a
disilane which decomposes easily upon exposure to air and is
relatively unstable in ethereal solutions.31 The reaction resulted
in a mixture of products as shown by NMR spectroscopy,
wherein the major products are disilanes with two naphthyl
units and silanol functionality (Figures S1-S2). The structures of
spectroscopy, δ 6.13 ppm (Table 1, s, Si-H, 2H, 1JSiH = 102. 55 Hz).
Use of Wilkinson’s catalyst, RhCl(PPh3)3, afforded the
naphthalene bridged disilane (Scheme 2, 2R) as a major product
along with a minor amount of the naphthalene bridged siloxane
(Scheme 2, 3R). The two products could be separated from each
other by extracting 3R using dichloromethane (Table 1 and
Figures S23-S35). The successful synthesis of 2R suggests that,
unlike the sluggish dehydrocoupling typically observed with
secondary and tertiary silanes, crowding in the peri- or 1,8-
positions in the 1,8-disilylnaphthalene precursor, 1R, did not
limit the formation of a naphthalene bridged disilane.27
Reacting 5 mol% of the water adduct of B(C6F5)3 with 1R,
exclusively afforded the naphthalene bridged siloxane (Scheme
2, 3R), irrespective of the substrate concentration. While the
reaction was performed under a constant flow of nitrogen, the
water B(C6F5)3 adduct has caused partial hydrolysis of 1R prior
to an intramolecular dehydrocoupling reaction from
a
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naphthalene bridged silanol (R2SiOH)/silane (R2SiH) Table 1 1H and 29Si {1H} NMR chemical shifts (in CDCl3),
View Article Online
intermediate (Scheme 3).
characteristic infrared absorption freDqOuI:e1n0.c1i0e3s9/Ca9nDdT0305m8aAx
determined by UV-Vis spectroscopy of 1R, 2R, and 3R (R = Ph, Me)
R
R
δH (ppm)
IR Absorption Frequency
(cm-1)
max
Si
Si
δSi
R
R
5 mol% RhCl(PPh3)3
Silane
(nm)
(ppm)
toluene, 90oC, 70 h
Aromatic
Si-H
Si-H
Si-R2
Si-O-Si-R2
3R
1Ph
1Me
2Ph
2Me
3Ph
7.94-7.27 6.13
7.86-7.45 5.04
-20.2
-22.0
-24.9
-21.9
-16.5
2154 1103
2167 1257
-
295
292
296
290
295
2R
H
H
-
-
R
R
R
Si
Si
2Ph R = Ph; 38%
2Me R = Me; 26%
8.01-7.17
7.83-7.48
7.97-7.28
-
-
-
-
-
-
1154
1249
-
R
-
1110-
1030
984
R
R
1R
O
3Me
7.89-7.47
-
3.14
-
1251
289
Si
Si
R
R
1Ph R = Ph; 53%
1Me R = Me; 48%
5 mol% (C6F5)3B(OH2)
toluene, 90oC, 70 h
Single crystals were grown for each compound in the series 1Ph,
2Ph, 3Ph by slow evaporation of solvent from dichloromethane;
the crystal structures were solved by single crystal X-ray
diffraction. The molecular structures are shown in Figures 2a,
b, d, respectively, and crystallographic data are given in Table
S1. In compound 1Ph, there is a slight twisting of the naphthyl
ring to accommodate the bulky Ph substituents as
demonstrated by the torsion angles of 12.7° and 17.0° for Si1-
C13-C18-C22 and Si2-C22-C18-C13, respectively (see Table 2 for
selected distances and angles). In compounds 2Ph (with a Si-Si
bond contained in a 5-membered ring) and 3Ph (with a Si-O-Si
bond contained in a 6-membered ring), the planarity of the
naphthyl ring is increased relative to 1Ph with torsion angles of
less than 10°. The Si centres in all three molecules are nearly
tetrahedral, with the greatest amount of deviation observed in
2Ph. The angles from Si1-C13-C18 and Si2-C22-C18 are
123.36(12)° and 129.35(9)° (1Ph), 124.57(10)° and 125.49(10)°
for 3Ph (C1 symmetry molecule), which are much wider than the
angles in 2Ph with 117.30(10)° and 11.50(10)°, respectively.
Linking the Si atoms together contained in a 5-membered ring,
as in 2Ph, resulted in bond angles to naphthalene of 92.15(5)°
(C13-Si1-Si2) and 92.44(5)° (C22-Si2-Si1), while the bond angles
in 3Ph are 107.82(5)° (C13-Si1-O1) and 107.38(5)° (C22-Si2-O1).
The Si1-Si2 bond distance of 2.3512(13) Å is in fact similar to the
distance from C13 to C22 (2.506(3) Å), confirming that a 1,8-
naphthalene bridge is the ideal bridge distance for a disilane.
However, the Si1-Si2 bond length in 2Ph (2.3512(13) Å) is longer
3R
3Ph R = Ph; 43%
3Me R = Me; 28%
Scheme 2. Catalytic routes to naphthalene bridged disilane, 2R, and
siloxane, 3R
To test the potential formation of a silane/silanol intermediate,
a control reaction was performed. This included the intentional
addition of a ½ equivalent of H2O to 1Ph in the presence, and
absence, of (C6F5)3B(OH2) (Scheme 3). There was no observable
change in the NMR spectra when 1Ph reacted with H2O in the
absence of the catalyst. However, in the presence of
(C6F5)3B(OH2) with the addition ½ equivalent of H2O/Si-H bond,
3Ph was produced rapidly, even under much milder conditions
(22°C) than in the original catalytic synthesis, providing
evidence for the formation of a silanol intermediate.37-39
H
H
OH
Si
H
Ph
Ph
Ph
Ph
Ph
O
Ph
Ph
Si
Si
Si
Si
Si
residual H2O
from the catalyst
(C6F5)3B(OH2)
toluene, 90oC, 70 h
Ph
Ph
Ph
Ph
Ph
1Ph
silanol
intermediate
3Ph
1
/
2 H2O, (C6F5)3B(OH2)
toluene, 22oC
Scheme 3. (C6F5)3B(OH2) catalysed synthesis of 3Ph in the presence
25
than in the reported structures for 2H (2.309(1) Å) and 2Me
of H2O
(2.338(2) Å40, 41 or 2.329(2) Å in this work, Figure 2c). This can
be explained by the increased steric bulk due to the phenyl
substituents and the effect was observed in a similar
anthracene bridged diphenyldisilane.42 Additional electron
density was observed above the Si-Si bond during refinement
for both compounds 2Ph and 2Me and this was determined to be
due to co-crystallisation of minor amounts of 3Ph (10.3(2)%) and
3Me (9.4(4)%), respectively (Figures S42-S43), and was
confirmed with analytical methods (see Experimental). Single
crystals of 3Me were grown from slow diffusion of hexanes into
dichloromethane. The molecular structure of 3Me (Figure 2e)
shows similar bond angles to 3Ph. Both of the six membered
rings containing Si-O-Si connectivity are similar, exhibiting a
Using RhCl(PPh3)3, 1R is converted to 2R, in parallel with
hydrolysis (competitive reaction) which undergoes
dehydrocoupling to yield 3R. By contrast, (C6F5)3B(OH2) catalyst
readily facilitates the production of silanol, favouring a synthetic
route which involves hydrolysis prior to dehydrocoupling.
Notably, intermolecular dehydrocoupling was not observed
even when the reactions were performed under higher
concentrations of 1R and catalyst relative to solvent. This
suggests that the steric bulk in 1R precludes oligomerisation or
that the oligomers which form are susceptible to Si-Si bond
cleavage.
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slight twist away from planarity; with one Si above and one Si Attempted synthesis of bridged building blocks that are
View Article Online
DOI: 10.1039/C9DT03058A
below a plane created with the napthyl ring (Figure 2d, e).
polymerisable
A polymerisable bridged disilane should have at least two
hydride
substituents
to
undergo
intermolecular
a)
dehydrocoupling. In an attempt to remove one phenyl
substituent from each Si centre in 1Ph via the step-wise reaction
with 2 equivalents TfOH28 and LiAlH4, the reaction resulted to
the cleavage of the naphthalene Si-C bond rather than the
phenyl Si-C bond. Diphenylsilane (Ph2SiH2) and naphthalene
were formed as the major products. Based on this observation,
we expected a similar reaction if 2Ph was used, instead. The
naphthyl substituent would be preferentially cleaved over the
phenyl groups. The step wherein the phenyl substituents are
being replaced by hydride substituents is crucial to obtain a
bridged-disilane monomer suitable for dehydrocoupling
reactions. Thus, an alternative route to prepare the desired
bridged disilane monomer was employed (Scheme 4).
b)
H
H
H
H
Ph Si Si Ph
Li
Li
Si
Si
Ph
Ph
OTf OTf
X
22oC, 24 h
c)
desired product
2Ph'
Ph
Si
H
H
Ph
Si
actual product
d)
4
Scheme 4. Attempted synthesis of polymerisable building block 2Ph’
which resulted in the redistribution product 4
A
reaction was performed where 1,2-bis(triflate)-1,2-
diphenylsilane and 1,8-dilithionaphthalene were each
separately synthesised in situ and then combined to form a
43-46
naphthalene bridged disilane, 2Ph’
.
Compound 2Ph’ contains
a HSi-SiH unit, which is required for the intermolecular
dehydrocoupling reaction aimed at producing a polysilane.
However, the reaction did not yield the desired compound 2Ph’
and formed 4 instead (crystals were grown from slow
evaporation of hexanes). The structure of 4 is confirmed by
analytical and spectroscopic data (see Figures S36-S41).
Compound 4 results from Si-Si bond cleavage showing an
unusual structure as it contains a highly strained 4-membered
ring from a naphthyl substituent bonding Si2 (Figure 3). The
bond angles to the naphthalene are 87.38(9)° (C28-C32-Si2) and
88.21(9)° (C28-C23-Si2). A second naphthalene is bridged to
both Si atoms and each Si atom contains a phenyl substituent.
This creates a highly congested environment for Si2, Si1 is
connected to two hydrides, one phenyl and one naphthyl
substituent.
e)
Figure 2. Molecular structures of a) 1Ph, b) 2Ph, c) 2Me, d) 3Ph, and e)
3Me (50% probability ellipsoids). For 3Ph and 3Me only one of the two
crystallographically independent molecules found in the crystal
structures is shown (3Ph: molecule 1 has C1 symmetry, molecule 2
has C2 symmetry; 3Me both molecules have C1 symmetry). Selected
distances and angles are given in Table S2.
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reactions. The possibility of having insufficiently VliietwhiAarttiecledO1n,li8ne-
DOI: 10.1039/C9DT03058A
naphthalene (e.g. 1-bromo-8-lithionaphthalene) in the reaction
mixture as well as the tendency for silicon compounds to
undergo redistribution reactions complicates the reaction
chemistry, but this is hard to pin down due to the number of
variables and sensitive nature of the chemistry.
ROUTE I
Li
Li
O
F3C
O
H
H
S
H
H
O
Si Si Ph
Ph
Li
TfO Si Si Ph
Ph OTf
- LiOTf
Figure 3. Molecular structure of 4 with thermal ellipsoids drawn at
the 50% probability level. Selected distances and angles are given in
Table 2.
Ph
H
H
Si
Ph
Si
- LiOTf
When considering a potential mechanism for the formation of
4, keeping in mind that both starting materials (e.g. 1,2-
bis(triflate)-1,2-diphenylsilane and 1,8-dilithionaphthalene)
were formed in situ when they were combined together, there
are three postulated routes.
Br
Li
Ph
Si
H
H
Ph
Si
The first route involves the installation of one side of the 1,8-
dilithionaphthalene bridge along with concomitant loss of LiOTf
(Route I, Scheme 5). Carbanion attack at the Si centre then
forms the silicycle which induces a 1,2-hydride shift, moving the
hydride to the other Si centre, and results in the loss of the
second triflate group (as LiOTf). There is literature precedence
for this part of the mechanism as it has been proposed by
Söldner and co-workers to explain a cyclisation reaction which
involves a Si to Si hydride shift.28,47 The resulting disilane
intermediate is then susceptible to the installation of the
bridging naphthalene from 1-bromo-8-lithionaphthalene
simultaneously with the cleavage of the Si-Si bond.48 Further
support for this mechanism is from the literature reports on
nucleophilic aromatic substitution reactions to form new aryl
silanes.49,50 In addition, catalytic routes which involve Si-Si bond
cleavage during a reaction with aromatic substrates containing
both OTf and SiMe3 substituents demonstrate the potential for
the latter part of the mechanism.49-54
- LiBr
4
ROUTE II
H
H
H
H
Ph Si Si Ph
OTf OTf
Li
Li
Si
Si
Ph
Ph
- 2 LiOTf
2Ph'
- LiBr
Br
Li
Ph
Si
H
H
Ph
Si
4
ROUTE III
A second mechanistic option is where the desired compound is
formed (2Ph’, Route II, Scheme 5), and assuming that a mono-
lithiated species is also present, this will be followed by a
nucleophilic attack on one of the Si centres to install the other
naphthalene while simultaneously cleaving the Si-Si bond and
undergoing a 1,2-hydride shift to move a hydride to a
neighbouring Si centre as observed in 4.
X
Si
H
X
H
Ph
Li
Li
Si
Ph
X
Si
Ph
2
+
X
H
- 2 LiX
X = Cl, OTf
Li
Li
A final route is based on the independent formation of
compound 4 in a reaction of 1,8-dilithionaphthalene with two
equivalents of Cl2SiPhH in the presence of LiAlH4. In this case, it
was clear that cleavage of a Si-Si bond was not necessary in
order to yield 4. In this reaction, the first step would most likely
be formation of 1XPh (e.g. 1,8-dichlorophenylsilyl naphthalene)
followed by a 1,2-hydride shift which is induced during the
silacycle formation to produce 4 (Route III, Scheme 5).
Li
X
H
H
Ph
Si
Si
Ph
Si
Ph
H
H
Ph
- LiX
Si
- LiX
4
Despite the stoichiometry, there seems to be a preference for
the addition of two equivalents of naphthalene in these
Scheme 5- Postulated mechanisms to form 4
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Si-Si σ-conjugation (Table 2; 2H-series; Figures S48, S53-S56;
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2Ph’-series Figures S47 and S57).57, 58 The optimised structures
DOI: 10.1039/C9DT03058A
DFT Calculations
containing the bridged disilanes also show that the -
The calculated band gap for the precursor molecule 1Me (4.46 conjugated bridge makes the oligomer conformationally
eV), was found to be 0.02 eV smaller than 1Ph (4.48 eV) which is constrained resulting in an anti-turn which extends the σ-
substantially lower than free naphthalene (4.73 eV; Figure S44). conjugation.
This can be attributed to the slightly twisted geometry of
naphthalene in 1Ph (Figure S45) to accommodate the bulky Table 2 B3LYP/6-31(++)G**-calculated HOMO-LUMO gaps (eV).
phenyl substituents, which results in a reduction of -
HOMO-LUMO
Gap (eV)
4.73
conjugation in the naphthyl ring, compared to the more planar
geometry found in 1Me (Figure S46). Once the Si-Si bond has
been formed, the calculations indicate that the HOMO-LUMO
energy band gap in the naphthalene bridged
bis(disubstituted)disilanes decreases slightly with an increasing
number of phenyl groups and a decreasing number of hydride
or methyl substituents: 2Me = 2H > 2Ph’ > 2Ph (where 2H is
naphthalene bridged tetrahydrodisilane, Table 3; Figures S47-
S50).55, 56 This trend is explained by the increased -conjugation
with increasing numbers of phenyl substituents, however, is a
minor effect compared to strong contribution provided by
naphthalene (Figure 4). Unlike in 2Me, the HOMO of 2Ph includes
contributions from the phenyl rings. By contrast, the LUMO in
both 2Ph and 2Me is located over the naphthalene ring. The
calculations also reveal the formation of 3Ph to have a more
positive Gibbs free energy than 2Ph. This supports the argument
that 1Ph has to undergo a two-step (partial hydrolysis and
intramolecular dehydrocoupling) reaction to form 3Ph.
Compound
naphthalene
1Ph
1Me
2Ph’
2H
2Ph
2Me
3Ph
4.48
4.46
4.54
4.56
4.44
4.56
4.51
4.54
4.41
4.32
4.31
4.23
3Me
2H-dimer
2H-trimer
2H-tetramer
2H-pentamer
2Ph’-dimer
4.26
In summary, for the 2H-series, the decreasing trend in energy
gap as the chain increases can be explained by the resulting
anti-turn, however, this effect is diminished by the cisoid
configuration (e.g. when going from trimer to tetramer there is
only 0.01 eV difference in band gap).58
HOMO-2Ph
LUMO-2Ph
2H-dimer
2Ph'-dimer
H
H
Si
H
Ph
Si
H
Ph
Si
Si
Si
Si
H
Ph
Si
H
Si
Ph
H
H
HOMO-2Me
LUMO-2Me
4.41 eV
4.26 eV
2H-pentamer
H
H
Si
2H-tetramer
H
H
Si
H
2H-trimer
Si
Si
H
H
H
Si
H
Si
H
Si
Si
H
Si
H
H
H
Si
Si
H
Si
Si
H
Si
H
Si
H
H
H
Si
Si
H
Si
H
Si
Si
H
Si
Figure 4. HOMOs and LUMOs calculated from the optimised
structures of 2Ph (top) and 2Me (bottom)
H
H
Si
Si
H
H
H
Si
H
H
As expected, for the corresponding oligomers of 2H and 2Ph’ the
energy gap decreases as the oligomer length increases (Figure
5; monomer = 4.56 eV; dimer = 4.41 eV; trimer = 4.32 eV,
tetramer = 4.31 eV, pentamer = 4.23 eV) due to the increased
4.32 eV
4.31 eV
4.23 eV
Figure 5. Chemical structures and HOMO-LUMO gaps, eV, of 2H- and
2Ph’-oligomers
6 | J. Name., 2012, 00, 1-3
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ARTICLE
The experimental UV-vis spectrum of 2Ph in THF was compared with
those of naphthalene, 1Ph and 3Ph (Table 3 and Figure S34; Figure S35
for Me-series). These results, correlate nicely with the
aforementioned calculations which demonstrate that the main
contributor to the calculated band gap is naphthalene. The addition
of silyl groups to the naphthalene results in a red shift of the -*
transition (e.g. 277 nm to 296 nm for 2Ph), observed both
experimentally and in the calculated TD-DFT spectra (Figures S58-
S61). Maeda and coworkers demonstrated a similar trend in their
studies of naphthelene substituted molecules.59 The contribution
from σ–σ* transition to the band gap is not prevalent until the
disilanes are linked into dimers (see Figure S60-S61) due to addition
of -conjugation.
Conclusions
View Article Online
DOI: 10.1039/C9DT03058A
Compounds 1R, 2R, 3R, (R = Me, Ph), 4 containing naphthalene
bridged disilanes were synthesised and characterised.
Wilkinson’s catalyst, RhCl(PPh3)3, generally promotes an
intramolecular dehydrocoupling reaction to form a Si-Si bond.
On the other hand, (C6F5)3B(OH2) catalyses siloxane formation.
Structural analysis and multinuclear NMR spectroscopic data
indicate the effects of phenyl and methyl substituents on the
bond lengths and bond angles of 1R, 2R and 3R. DFT calculations
revealed that tethered disilanes with aromatic functionalities
have a relatively wide band gap of about 3.9-4.5 eV. However,
increasing the Si-Si chain length would result in a lower energy
band gap. Knowledge on the synthesis of organosilanes, and
structural and electronic properties of bridged disilanes is useful
for the design of silanes, in addition to polysilanes with
extended σ-conjugation, and as semi-conducting inorganic
polymers.
There is very little influence from solvent as the calculated TD-
DFT spectra (Figure S62-S65) in THF do not deviate much from
the gas phase (data in Table 3). In summary, the experimental
and theoretical UV-Vis spectra are in good agreement and
corroborate the previous findings that the presence of the
naphthalene substituent has a greater impact on lowering the
HOMO-LUMO gap than the subtle changes observed when
Experimental
changing another substituent (e.g. Ph vs. Me vs. H). General Procedure. All reactions and manipulations were done
Futhermore, increasing the number of Si atoms in the chain under a nitrogen atmosphere in an MBraun Unilab 1200/780
lowers the band-gap.
glovebox or using conventional Schlenk techniques. Dry
solvents were obtained using a solvent purification system.
Table 3 Experimental and theoretical comparison of HOMO- Reagents were purchased from Sigma-Aldrich, AK Scientific,
LUMO gaps from UV-Vis and TD-SCF(DFT) spectra
Arcos and TCI and used as received. 1H, 13C, and 29Si NMR
spectra were recorded on a Bruker DPX-400 (400MHz)
spectrometer. Chemical shifts for protons are reported in parts
per million (ppm) downfield from tetramethylsilane and are
referenced to residual protium in the NMR solvent (e.g. CHCl3 =
7.26 ppm). Chemical shifts for carbon are reported in ppm
downfield from CDCl3 (77.3 ppm). Chemical shifts for silicon are
reported in ppm downfield to the silicon resonance of
tetramethylsilane (TMS δ 0.0). The silicon NMR resonances
were determined with a DEPT pulse sequence. Data are
represented as follows: chemical shift, multiplicity (app =
apparent, br = broad, s = singlet, d = doublet, t = triplet, q =
quartet, m = multiplet), coupling constants in Hertz (Hz), and
integration. High resolution mass spectrometry measurements
were made on a Bruker microTOF-QII mass spectrometer,
equipped with a KD Scientific syringe pump, in positive ion ESI
mode. Infrared spectral data were obtained using Bruker Vertex
70 FTIR spectrometer. Elemental analysis measurements were
conducted on a Vario EL cube (Elementar Analysensysteme
GmbH, Hanau, Germany). X-ray diffraction measurements of
single crystals were performed on a Rigaku Oxford Diffraction
XtaLAB-Synergy-S single-crystal diffractometer with a PILATUS
200K hybrid pixel array detector using Cu Kα radiation (λ =
1.54184 Å; Table S1). The data were processed with the
SHELX2016 and Olex2 software packages.60, 61 All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were
inserted at calculated positions or if possible based on
difference Fourier analysis and refined with a riding model or
without restrictions. Mercury 4.1.0 was used to visualize the
molecular structure.62 UV-vis spectra were recorded on a
SHIMADZU UV-3600 Plus spectrophotometer. All DFT
HOMO-
LUMO
Gap
HOMO-
LUMO
Gap
(eV)
Theoreticalb
HOMO-
LUMO
Gap
max
(nm)
max
(nm)
max
(nm)
(eV)
(eV)
Experimentala
Gas Phase Solvent = THF
Naphtha-
lene
277
4.48
285
4.36
287
4.33
1Ph
295
296
295
292
290
289
-
4.20
4.19
4.20
4.25
4.28
4.29
-
304
316
303
307
297
298
299
328
4.07
3.93
4.10
4.04
4.17
4.16
4.15
3.78
306
316
304
310
299
300
301
327
4.05
3.92
4.07
4.01
4.14
4.13
4.12
3.79
2Ph
3Ph
1Me
2Me
3Me
2Ph’
2Ph’
-
-
-
dimer
2H
-
-
-
-
297
312
4.17
3.98
299
313
4.14
3.96
2H
-
dimer
2H
-
-
-
-
-
-
-
319
321
324
3.89
3.86
3.83
320
322
325
3.87
3.84
3.81
trimer
2H
-
tetramer
2H-
pentamer
a solution in THF (Ph-series, 0.33 mM; Me-series, 0.50 mM)
b TD-SCF(DFT), B3LYP/6-31(++)G**
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Page 8 of 11
ARTICLE
Journal Name
calculations were done using Gaussian 09.64 The geometries of [C34H26NaSi2]+ cal’d 513.1471 m/z; observed 513.1477 m/z. IR:
View Article Online
each of the subject molecules were optimised using the B3LYP ⱱ, cm-1 3044 (arene), 1426 (Si-Ph), 1154 (Si-Ph2), 760-690 (Si-
DOI: 10.1039/C9DT03058A
functional and the 6-31(++)G** basis set for all elements. At the Ph). CHN Anal. Calc. C34H26Si2.20%C34H26Si2O: C: 82.67, H: 5.31.
63
optimised geometries, specific properties such as Gibbs free Found: C: 82.32, H: 5.46. 2Me
:
1H NMR (400 MHz, CDCl3): δ
energies and frontier orbital energies using 6-31(++)G** were 7.83-7.82 (m, H4/H5, 2H), δ 7.76-7.74 (m, H2/H7, 2H), δ 7.52-
calculated. In addition, the information about the UV-vis 7.48 (m, H3/H6, 2H), δ 0.42 (s, CH3, 12H). 13C{1H} NMR (100.6
spectra of the molecules were obtained using TD-SCF(DFT) MHz, CDCl3): δ 131.5 (s, C2/C7/C9), δ 128.8 (s, C1/C8/C10), δ
method, B3LYP functional and 6-31(++)G** basis set.
125.7 (s, C3/C6/C4/C5), δ -3.4 (s, CH3). 29Si{1H} NMR (79.5 MHz,
CDCl3):  -21.9 ppm (s). IR: ⱱ, cm-1 3044 (arene), 2949 (alkane),
Synthesis of 1,8-bis(disubstitutedsilyl)naphthalenes, 1Ph and 1249 (Si-CH3), 781 (Si-CH3). Note: use of other catalysts, namely
1Me. To a cooled (-78oC) solution of 1,8-dibromonaphthalene 2nBuLi/Cp2TiCl2
and 2nBuLi/Cp2ZrCl2 showed no
(4.2 mmol, 1.2 g) in diethyl ether (20 mL), 2.0 M nBuLi (in dehydrogenative coupling (only starting materials remained).
cyclohexane, 8.4 mmol, 4.2 mL) was added dropwise and the
mixture was stirred for 10 min at 25°C. The mixture was cooled Synthesis of tethered siloxanes, 3Ph and 3Me. A mixture of 1 (1.0
again to -78°C and the monochlorosilane (8.4 mmol, 1.6 mL mmol, 0.49 g 1Ph; 1.0 mmol, 0.24 g 1Me) and catalyst (4.9 mol%;
ClSiHPh2 or 0.93 mL ClSiHMe2) was added with stirring. The dry 0.026 g (C6F5)3B(OH2)) was dissolved in toluene (30 mL) and
ice-acetone bath was removed and the mixture was then heated with stirring for 70 h. All volatiles were removed under
allowed to stir for 24 h at room temperature. The diethyl ether vacuum to give an off-white solid. The product was purified by
layer was removed and dried under vacuum to form a mixture Florisil filtration using hexane as the eluent and was crystallised
of white and yellow-green solids. The yellow green impurities by slow evaporation from DCM and hexanes (3Ph, 41-46% yield;
were removed by dissolution in hexane (2x10mL) leaving a 3Me, 28% yield). 3Ph: 1H NMR (400 MHz, CDCl3): δ 7.97-7.94 (m,
white solid product (1Ph, 53% yield; 1Me, 48% yield). For 1Ph, H4/H5, 2H), δ 7.77-7.75 (m, H2/H7, 2H), δ 7.61-7.59 (m, Ph-H[o],
crystals suitable for diffraction were grown from 8H), δ 7.51-7.48 (m, H3/H6, 2H), δ 7.42-7.28 (m, Ph-H[m,p],
dichloromethane. 1Ph:26 1H NMR (400 MHz CDCl3): δ 7.94-7.92 12H). 13C{1H} NMR (100.6 MHz, CDCl3): δ 139.79 (s, C9), δ 135.94
(m, H4/H5, 2H), δ 7.78-7.77 (m, H2/H7, 2H), δ 7.40-7.27 (m, (s, Ph(C)-Si), δ 135.21 (s, Ph-C[o]), δ 132.52 (s, C1/C8), δ 132.12
H3/H6, 2H; Ph-H[o,m,p], 20H), δ 6.13 (s, Si-H, 2H, JSi-H = 102. 6 (s, C2/C7), δ 131.23 (s, C10), δ 130.02 (s, Ph-C[p]) , δ 129.28 (s,
Hz). 13C{1H} NMR (100.6 MHz, CDCl3): δ 139.8 (s, C9), δ 135.9 (s, C4/C5) , δ 127.72 (s, Ph-C[m]), δ 124.86 (s, C3/C6). 29Si{1H} NMR
C2/C7, Ph-C[o]), δ 135.5 (s, C10), δ 132.1 (s, Ph(C)-Si), δ 129.3 (s, (79.5 MHz, CDCl3):  -16.5 ppm (s). HRMS-ESI: [C34H26NaOSi2]+:
C1/C8), δ 127.8 (s, Ph-C[m,p], C4/C5), δ 124.51 (s, C3/C6). cal’d: 529.1420 m/z; observed 529.1420 m/z. IR: ⱱ, cm-1 3024
29Si{1H} NMR (79.5 MHz, CDCl3):  -20.2 ppm (s). HRMS-ESI: (arene), 1427 (Si-Ph), 1110-1030 (Si-O-Si-Ph2), 760-690 (Si-Ph).
[C34H28NaSi2]+ cal’d: 515.1627 m/z; observed 515.1618 m/z. IR: CHN Anal. Calc. C34H26OSi2.H2O: C: 77.82, H: 5.38. Found: C:
ⱱ, cm-1 3050 (arene), 2154 (Si-H), 1426 (Si-Ph), 1103 (Si-Ph2), 77.96, H: 5.25. 3Me: H NMR (400 MHz, CDCl3): δ 7.89-7.87 (m,
1
760-690 (Si-Ph). CHN Anal. Calc: C: 82.87, H: 5.73. Found: C: H4/H5, 2H), δ 7.61-7.59 (m, H2/H7, 2H), δ 7.51-7.47 (m, H3/H6,
82.91, H: 5.96. 1Me:
32 1H NMR (400 MHz, CDCl3): δ 7.86-7.81 (m, 2H), δ 0.43 (s, CH3, 12H). 13C{1H} NMR (100.6 MHz, CDCl3): δ
H2/H4/H5/H7, 4H), δ 7.48-7.45 (m, H3/H6, 2H), δ 5.04 (s, Si-H, 130.3 (s, C2/C7/C9), δ 128.4 (s, C1/C8/C10), δ 122.9 (s,
2H, JSi-H = 96.6 Hz), δ 0.43-0.42 (d, CH3, 12H). 13C{1H} NMR (100.6 C3/C6/C4/C5), δ -0.0036 (s, CH3). 29Si{1H} NMR (79.5 MHz,
MHz, CDCl3): δ 141.3 (s, C9), δ 138.2 (s, C2/C7), δ 135.6 (s, CDCl3):  3.14 ppm (s). IR: ⱱ, cm-1 3043 (arene), 2957 (alkane),
C1/C8), δ 133.6 (s, C10), δ 131.0 (s, C4/C5), δ 124.3 (s, C3/C6), δ 1251 (Si-CH3), 984 (Si-O-Si), 786 (Si-CH3). CHN Anal. Calc: C:
-0.8 (s, CH3). 29Si{1H} NMR (79.5 MHz, CDCl3):  -22.0 ppm (s). IR: 65.06, H: 7.02. Found: C: 65.03, H: 7.34.
ⱱ, cm-1 3066 (arene), 2961 (alkane), 2167 (Si-H), 1257 (Si-CH3),
781 (Si-CH3).
Synthesis of 3Ph using H2O.38 A mixture of 1Ph (1 mmol, 0.49 g)
and catalyst (4.9 mol %; 0.026 g (C6F5)3B(OH2)) was dissolved in
Synthesis of tethered disilanes, 2Ph and 2Me. A mixture of 1 (1.0 toluene (2 mL) and 0.5 eq H2O/Si-H (1 mmol, 0.018 mL) was
mmol, 0.49 g 1Ph; 0.24 g 1Me) and catalyst (5.0 mol%; 0.046 g added with stirring at 22°C. The mixture was allowed to react
RhCl(PPh3)3) was dissolved in toluene (30 mL) and heated with for 70 h to form 3Ph. A second trial was conducted to convert
stirring for 70 h. All volatiles were removed under vacuum to 1Ph to 3Ph using the same conditions (0.5 eq H2O/Si-H and 5.0
give an off-white solid. The product was purified by Florisil mol% (C6F5)3B(OH2) at 22oC) but this time monitoring the
filtration using hexane as the eluent (2Ph, 38% yield; 2Me, 26% reaction once the H2O has been added into the reaction
yield). 2Ph was separated from 3Ph by washing with cold (0oC) mixture. The reaction was observed to be done soon after the
dichloromethane (3x5mL) and slow evaporation allowed addition of water. The product from both trials were purified by
1
crystallisation. 2Ph: H NMR (400 MHz, CDCl3): δ 8.01-7.98 (m, Florisil flitration and gave a much higher isolated yield of 93%
H4/H5, 2H), δ 7.91-7.89 (m, H2/H7, 2H), δ 7.62-7.59 (m, H3/H6, and 89%, respectively. Formation of 3Ph was confirmed by NMR
2H), δ 7.40-7.17 (m, Ar-H[o,m,p], 20H). 13C{1H} NMR (100.6 spectroscopy.‡
MHz, CDCl3): δ 138.9 (s, C9), δ 136.2 (s, Ph-C[o]), δ 135.3 (s, C10),
δ 134.6 (s, C2/C7), δ 133.6 (s, C1/C8), δ 129.9 (s, C4/C5), δ 129.2 Synthesis of compound 4. The 1,2-bis(triflate)-1,2-
(s, Ph-C[p]), δ 127.8 (s, Ph-C[m]), δ 126.3 (s, C3/C6). 29Si{1H} diphenyldisilane was prepared by dissolving 1,1,2,2-
NMR (79.5 MHz, CDCl3):  -24.9 ppm (s). HRMS-ESI: tetraphenyldisilane (4.0 mmol, 1.5 g) in toluene and cooled to
8 | J. Name., 2012, 00, 1-3
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ARTICLE
3. A. K. Franz and S. O. Wilson, J. Med. Chem., 2013, 56, 388-405.
0°C. Triflic acid (8.0 mmol, 0.71 mL) was added and the reaction
mixture was stirred for 1 h at 0°C.
4. S. M. Sieburth, T. Nittoli, A. M. Mutahi and L. Guo, VAienwgeArwtic.leCOhnelmine.
DOI: 10.1039/C9DT03058A
Int. Ed., 1998, 37, 812-814.
In a separate Schlenk flask, a 2.0 M solution of nBuLi (in
cyclohexane, 8.0 mmol, 4.0 mL) was added dropwise to a cooled
solution of 1,8-dibromonaphtalene (4.0 mmol, 1.1 g) in diethyl
ether, and the mixture was stirred for 10 min at -78°C and
another 10 min at 0oC. The 1,8-dilithionaphthalene solution was
added dropwise to 1,2-bis(triflate)-1,2-diphenyldisilane
solution at -78°C and stirred for 24 h at room temperature. The
resulting mixture was filtered through Florisil using diethyl
ether as an eluent. The filtrate was collected and the solvents
were removed under high vacuum (41% yield, crude). The
residue was dissolved and recrystallized using hexanes to obtain
a white crystalline product (36% yield). 1H NMR (400 MHz,
CDCl3): δ 8.69-8.67 (m, Ha4, 1H), δ 8.02-7.96 (m, Hb4/Hb5, 2H),
δ 7.85-7.83 (m, Hb2/Hb7, 2H), δ 7.80-7.77 (m, Ha5, Ha2/Ha7, 3H),
δ 7.61-7.55 (m, Ha3, Hb3/Hb6, 3H), δ 7.47-7.43 (m, Ha6, 1H), δ
7.36-7.11 (m, Ph-H[o,m,p], 20H), δ 5.59 (s, Si-H, 2H, JSi-H = 105.1
Hz). 13C{1H} NMR (100.6 MHz, CDCl3): δ 139.9 (s, Ca9), δ 138.8 (s,
Cb9), δ 135.7 (s, Ph[o(SiH2)]), δ 134.84 (s, Ca2/ Ca7/ Cb2/ Cb7), δ
133.1 (s, Ph[o(Si(ring)]), δ 132.4 (s, Ph(C)-SiH2), δ 130.0 (s, Ca10), δ
129.6 (s, Cb10), δ 128.6 (s, Ca1/ Ca8/ Cb1/ Cb8), δ 127.9 (s, Ph-
C[p]), δ 127.9 (s, Ph-C[m]), δ 126.7 (s, Ca4/ Ca5/ Cb4/ Cb5), δ
125.4 (s, Ca3/ Ca6/ Cb3/ Cb6), δ 124.9 (s, Ph(C)-Si(ring)).
29Si{1H}NMR (79.5 MHz, CDCl3):  5.1 ppm (s), -36.4 ppm (s). IR:
ⱱ, cm-1 3041 (arene), 2212, 2143 (Si-H), 1428 (Si-Ph), 1106 (Si-
Ph2), 760-690 (Si-Ph). CHN Anal. Calc. C32H24Si2.1/2H2O: C: 81.13,
H: 5.32. Found: C: 81.61, H: 5.20.
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There are no conflicts to declare
Acknowledgements
The work was supported by the School of Chemical Sciences,
University of Auckland, New Zealand. KRC and EML thank
L’Oréal Women in Science, Royal Society of New Zealand
Marsden Fast-Start grant and the MacDiarmid Institute for
financial support. This research was enabled in part by
calculation services provided by New Zealand e-Science
Infrastructure (NeSI). We thank Tatiana Groutso for collecting
the single crystal X-ray diffraction data. We would also like to
thank Paul Hume (Victoria University of Wellington) and Martijn
Wildervanck (University of Auckland) for help with the DFT
calculations.
Notes and references
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DOI: 10.1039/C9DT03058A
R
R
Si
Si
5 mol%
R
R
RhCl(PPh3)3
3R
toluene, 90oC, 70 h
H
H
R
Si
R
R
Si
2R
R
5 mol%
(C6F5)3B(OH2)
R
R
O
Si
Si
R
R
1R
= Ph, Me
toluene, 90oC, 70 h
3R