Tunable SiN Hybrid Conjugated Materials

Article abstract of DOI:10.1021/acs.organomet.9b00254

We report the synthesis of a family of N-aryl cyclosilazanes. The reaction of 1,2-bis(trifluoromethanesulfonate)tetramethyldisilane with para-substituted anilines gives six-membered rings with no observed condensation polymerization. X-ray crystallography

Full text of DOI:10.1021/acs.organomet.9b00254

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Tunable SiN Hybrid Conjugated Materials
Carlton P. Folster, Phi N. Nguyen, Maxime A. Siegler, and Rebekka S. Klausen*
Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States
S
* Supporting Information
ABSTRACT: We report the synthesis of a family of N-aryl
cyclosilazanes. The reaction of 1,2-bis(trifluoro-
methanesulfonate)tetramethyldisilane with para-substituted
anilines gives six-membered rings with no observed con-
densation polymerization. X-ray crystallography reveals the
cyclosilazane adopts a twist boat conformation with both
nitrogens in a trigonal planar geometry. Solution phase UV−vis
spectroscopic studies coupled with density functional theory
calculations interrogate the ability of the nitrogen atom to bridge the σ- and π-conjugated moieties. Depending on the aromatic
substituent, the cyclosilazane rings may be either more or less electron rich than an all silicon ring.
because of the analogy to p- and n-type doping of the bulk
semiconductor. Boron-functionalized polymers have been
reported by Matsumi and others,21,22 while we recently
reported the synthesis and optical characterization of SiB
cycles.23 In these cases, density functional theory (DFT)
calculations have supported both the mixing of σ- and p-
orbitals (p,σ-conjugation) and charge transfer.
The relationship between SiN heterocycle structure and
electronic properties is poorly understood, as studies of cyclic
SiN compounds (silazanes)24−26 have focused on their use as
precursors to polysilazanes27−29 by ring-opening polymer-
ization and silicon nitride ceramics by pyrolysis.30,31 In
particular, the influence of nitrogen substituents is underex-
plored.32,33
INTRODUCTION
■
Heteroatom incorporation tunes the properties and function of
a conjugated material.1 A fundamental understanding of the
effect of heteroatom incorporation is essential for rational
design.2−4 Group 15 elements (e.g., nitrogen, phosphorus)
combined with π-conjugated systems can act either in an
electron-withdrawing or in an electron-donating sense depend-
ing on the competing influence of inductive and resonance
effects. For example, while acenes and dihydrophenazines are
donor materials, diazapentacenes are electron-transport
materials due to nitrogen’s greater electronegativity.5,6 In
nitrogen-doped graphene nanoribbons, carbazole and phenan-
thridine subunits act as p- and n-type dopants, respectively,
and the thermal rearrangement of one to another creates an
intrinsic heterojunction.7 Nitrogen functionalization of π-
conjugated systems has enhanced a variety of additional
applications including energy storage, catalysis, and sens-
ing.8−10
Here, we investigate how nitrogen couples σ- and π-
conjugated fragments in N-aryl substituted cyclosilazanes.
Synthetic and X-ray crystallographic studies are reported.
Optical properties are systematically investigated through both
experimental and computational studies.
In oligosilanes and polysilanes, small molecules and
polymers containing Si−Si bonds, the overlap of σ-orbitals
gives rise to σ-conjugation.11,12 Oligosilanes have lower energy
optical transitions than alkanes and show a progressive
narrowing of the HOMO−LUMO gap with increasing
numbers of Si atoms.13 Chemical structures that combine σ-
and π-conjugation, such as phenyl-terminated oligosilanes,
show further bathochromic shifts in absorption.13 The σ,π-
conjugation phenomenon has been explored in the develop-
ment of conductive polymers based on polymethylphenylsi-
lane.14,15 We recently reported a novel class of σ,π-hybrid
compounds that are promising candidates for charge transport
materials and switchable optoelectronic devices.16−18 Ultrafast
spectroscopic characterization demonstrated that these materi-
als undergo intramolecular charge transfer from the σ-
conjugated Si donor chain to the π-conjugated acceptor
group.19,20
RESULTS AND DISCUSSION
■
Synthesis. Cyclosilazane synthesis was achieved through
the reaction of bifunctional silyl triflate 1 with para-substituted
anilines in the presence of triethylamine (NEt3) by adaptation
of a procedure reported by Schmidbaur (Scheme 1).34,35
Scheme 1. Synthesis of Cyclosilazanes
The incorporation of p-orbital-bearing heteroatoms into σ-
conjugated silanes is an intriguing avenue for exploration
Received: April 21, 2019
© XXXX American Chemical Society
A
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Compound 1 is commercially available and can also be readily
prepared by acid-mediated dearylation of known Ph-
(SiMe2)2Ph.36,37 Competitive condensation polymerization of
the primary amine with 1 was not observed. The reaction of
1,2-dichloro-1,1,2,2-tetramethyldisilane under otherwise sim-
ilar reaction conditions gave lower yields and monosilylation of
the aniline. Cyclosilazanes 2b−2f were highly crystalline white
solids, while 2a was a yellow solid.
X-ray Crystallography. Given the strong dependence of
oligosilane absorption spectra on conformation,38 we sought
structural insight into the cyclosilazanes via X-ray crystallog-
raphy. Crystals of 2a−2f suitable for X-ray structure
determination were grown from Et2O/MeCN at room
temperature or −30 °C (see the Experimental section).
Crystal structures are shown in Figure 1. Table 1 summarizes
selected structural parameters.
Table 1. Selected Geometrical Parameters for the
Experimental Crystal Structures of 2a−2f
a
∠Si−N−C−C
compound
(deg)
Si−N bond lengths (Å)
1.7405(14), 1.7420(14)
1.7363(12), 1.7412(12), 1.7421(12),
1.7427(12)
2a
2b
79.73(19)
−75.66(16)
2c
2d
2e
84.57(13)
83.00(19)
74.2(4)
1.7384(11), 1.7384(11)
1.7388(14), 1.7434(13)
molecule A: 1.741(3), 1.742(3), 1.746(3),
1.748(3)
molecule B: 1.741(7), 1.744(14), 1.748(7),
1.761(15)
2f
70.3(2)
1.7483(15), 1.7504(16), 1.7521(16),
1.7538(17)
a
Torsion angle; the smallest torsion angle was selected.
Crystal structures of 2a−2f exhibit characteristic features of
a twist-boat conformation.39 Excluding the aryl groups, the
cyclosilazane ring has an approximate D2 symmetry (Figure
S1). All silazane nitrogens on 2a−2f adopt a trigonal planar
geometry with the sum of all three nitrogen bond angles
>359.6°. Looking down the N(1)−N(1)′ axis of 2d (Figure 1)
shows that N(1) and N(1)′ are coplanar with Si(1) and Si(2)
while Si(1)′ and Si(2)′ are displaced away from the plane in
opposite directions. Compounds 2a−2f exhibit typical bond
lengths for both Si−N (1.736−1.752 Å) and Si−Si (2.347−
2.357 Å) single bonds.40 Schmidbauer et al. reported a
cyclosilazane structure where nitrogen is substituted with a tBu
group and silicon is bound to hydrogen rather than methyl
groups.34 Their cyclosilazane also adopts a twist-boat
conformation where the two nitrogen atoms adopt a nearly
planar configuration and the Si−Si bond distances are similar.
Aryl groups on 2a−2f are nearly perpendicular to the Si−
N−Si plane (torsion angles 70.3−84.6°, Table 1). An energy
scan calculation was performed on 2d (Figure S2). An energy
minimum was found at a Si−N−C−C torsion angle of 96.0°,
while an energy maximum was found at a torsion angle of
16.0°. The difference in the energy minimum and maximum
was 10.04 kcal/mol. These results suggest that the energetic
penalty of steric interactions between the Si−Me groups and
ortho C−H bonds on the aryl groups are more significant than
electron delocalization between the N lone pair and π system
of the aryl ring.
UV−vis Spectroscopy. Cyclosilazane optical properties
were investigated to elucidate structure−property relation-
ships. UV−vis absorption spectra of 2a−2f were recorded in n-
pentane at room temperature (Figure 2). The absorption
spectrum of 2d was blue-shifted by 43 nm relative to aniline
(Figure S3). The twisting of the nitrogen atom out of planarity
upon silylation likely accounts for the blue shift. In acidic
media, aniline’s absorption spectrum undergoes an ∼30 nm
hypsochromic shift due to nitrogen protonation and loss of
conjugation.41
All six cyclosilazanes exhibited three absorption bands (λ,
Table 2). Analysis of structure−property relationships
suggested that the highest energy feature involves the sigma
framework, while lower energy transitions include the aromatic
moiety. The position of the highest energy transition (194−
201 nm) was largely insensitive to the aryl substituent. The
position of the second transition for compounds bearing
electron-donating substituents 2a,b (232−261 nm) was red-
shifted relative to 2c−2f (218−226 nm). The lowest energy
Figure 1. Crystal structures of compounds showing a molecule of
compounds 2a−2f. Hydrogens and disorder (for compounds 2e and
2f only) are omitted for clarity. View through the N(1)−N(1)′ axis
included for 2d. Yellow = silicon, blue = nitrogen, gray = carbon, red
= oxygen, lime green = chlorine, yellow-green = fluorine. The
asymmetric unit for 2e has two crystallographically independent
molecules; the more disordered molecule was omitted for clarity (see
the Supporting Information for details).
B
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Table 3. Electrochemical Data for 2a−2f
a
b
c
compound
Eox
(V)
HOMO (eV)
LUMO (eV)
onset
d
e
2a
2b
2c
2d
2e
2f
0.093
0.431
0.562
0.567
0.659
0.822
−4.879
−1.155
−1.157
−0.602
−0.666
−0.843
−1.248
−5.304
−5.427
−5.435
−5.522
−5.692
a
b
Onset of oxidation (vs Fc/Fc+). HOMO energy calculated from eq
c
1. LUMO energy calculated from the HOMO energy and optical
d
band gap. Eoxonset relative to Ag/Ag+. Eoxonset relative to Fc/Fc+ could
not be determined due to overlap (see Figures S34 and S35).
e
Calculated from eq 1 with Eoxonset of 2a from Figure S34 and Eox
onset
of Fc from Figure S35.
vacuum, and X is the onset of oxidation of ferrocene under the
CV conditions (see the Experimental Section).42
HOMO = [−Eox
− (4.8 − X)] eV
(1)
onset
A linear relationship (R2 = 0.95, Figure 3a, solid line) was
observed between the HOMO energy and the Hammett
Figure 2. UV−visible absorbance spectra of 2a−2f. Spectra recorded
in n-pentane at room temperature ([analyte] = 1.0 × 10−5 M).
Table 2. Onset of Absorbance and Optical Band Gap for
2a−2f
a
b
compound
λ (nm)
λonset (nm)
Eg (eV)
2a
2b
2c
2d
2e
2f
203, 261, 305
197, 232, 289
196, 222, 239
194, 218, 242
196, 226, 247
196, 221, 258
333
299
257
260
265
279
3.72
4.14
4.82
4.76
4.67
4.44
a
λonset = wavelength of light corresponding to the onset of absorbance.
Eg = optical band gap.
b
feature is also influenced by the aromatic substituent.
Compounds with NMe2, OMe, and CF3 substituents (2a,
2b, and 2f) are bathochromically shifted (258−305 nm)
relative to compounds with weak donors and acceptors (2c−
2e, 239−247 nm).
Figure 3. Experimental (solid) and theoretical (dashed) energy levels
(eV) for the HOMO (a) and LUMO (b) with respect to the
Hammett σp value for cyclosilazanes 2a−2f. Theoretical energy levels
were calculated at the B3LYP/6-311G(d) level.
More polar solvents (THF, CH2Cl2, MeCN) were also
investigated (Figures S4−S6). Modest solvatochromism was
apparent in the lowest energy transition, with a ca. 5 nm
bathochromic shift from the least to the most polar solvent.
Cyclic Voltammetry and Density Functional Theory
(DFT). Cyclic voltammetry (CV), coupled to computational
studies, was pursued to further probe cyclosilazane substituent
effects. Energies were referenced to ferrocene (Fc). Irreversible
oxidation was observed for all compounds. The onset of
oxidation increased as the substituent became more electron
withdrawing (Table 3). The HOMO energy was calculated
parameter σp.43−45 While the voltammograms of 2a−2f exhibit
oxidation peaks, no reduction peaks were observed for 2b−2f,
complicating direct electrochemical determination of the
LUMO energy (see the Supporting Information). The
LUMO energy was estimated from the optical band gap and
the electrochemical HOMO energy (Table 3). In this case, no
linear relationship (R2 = 0.003, Figure 3b, solid line) is
observed between the LUMO energy and σp. The LUMO
energy is lowered when the R group is either a strong electron
donor (2a,b) or a strong electron acceptor (2f).
from eq 1, where Eox
is the onset of oxidation of the
DFT calculations provided insight into the origin of
substituent effects in cyclosilazanes. Geometry optimization
onset
analyte, 4.8 eV is the HOMO energy of ferrocene below
C
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Figure 4. Calculated HOMO/LUMO orbital densities of 2a−2f and Si6Ph2. Initial geometries were obtained from the corresponding crystal
structure for 2a−2f. Hydrogens omitted for clarity. Gray = carbon, teal = silicon, blue = nitrogen, red = oxygen, green = chlorine, light blue =
fluorine. B3LYP/6-311G(d).
was performed using the crystal structures as a starting point
(B3LYP/6-311G(d)). B3LYP/6-311G(d) was selected to
allow for comparison to the literature, as Michl et al. reported
analysis of linear alkylsilanes with this basis set and
functional.46 Significant changes were not observed at higher
levels of theory.
Good agreement was observed between calculated and
experimental HOMO energies (Figure 3a, compare the solid
and dashed lines). Both showed a linear relationship between
the Hammett parameter σp and the HOMO energy. The
agreement between theory and experiment was less strong for
the LUMO energies (Figure 3b), but the overall good
agreement between calculated and experimental HOMO−
LUMO gap and λonset (see Figure S7) suggested that the
computational results were of sufficient accuracy.
TD-DFT calculations of 2d with the CAM-B3LYP func-
tional support the involvement of the HOMO−LUMO
transition in the low energy features (see Figure S8 and
Table S1). We found that the CAM-B3LYP functional
outperformed PBE0 and B3LYP at predicting the overall
absorption spectrum of 2d, although the predicted spectrum
was hypsochromically shifted by ca. 15 nm relative to
experiment.
D
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Organics. Chloro(dimethyl)phenylsilane was purchased from Gelest.
p-Toluidine, p-chloroaniline, and p-anisidine were sublimed under a
vacuum prior to use. NEt3 was freshly distilled from KOH under Ar
prior to use. TBABF4 was recrystallized from an 80/20 (v/v) water/
ethanol mixture. Ferrocene was sublimed at 100 °C. Compound 1 was
synthesized from known Ph(SiMe2)2Ph which was prepared by
adaptation of a literature procedure.47 All other chemicals were
obtained from commercial suppliers and used without further
purification. 1H NMR, 13C {1H} NMR, 29Si {1H} NMR, and 19F
{1H} NMR spectra were recorded on a Bruker Avance 300 or 400
MHz Spectrometer, and chemical shifts are reported in parts per
million (ppm). NMR spectra were recorded in CDCl3 with
tetramethylsilane (TMS) or the residual solvent as the internal
standard (1H NMR: CHCl3 δ = 7.26). Multiplicities are as indicated:
s (singlet), q (quartet), p (pentet), and m (multiplet). Coupling
constants, J, are reported in Hertz, and integration is provided. High
resolution mass spectrometry (HRMS) was performed at the
Columbia University mass spectrometry facility using a Waters
XEVO 62XS QToF mass spectrometer equipped with a UPC2 SFC
inlet, electrospray ionization (ESI) probe, atmospheric pressure
chemical ionization (APCI) probe, and atmospheric solids analysis
probe (ASAP). All column chromatography was performed on a
Teledyne ISCO Combiflash Rf+ using Redisep Rf silica columns.
Elemental analysis was performed by Robertson Microlit Laborato-
ries. For 2b and 2d, carbon content was lower than expected by more
than 0.4%. Although these results are outside the range viewed as
establishing analytical purity, they are provided to illustrate the best
values obtained to date. Additionally, low carbon content in
organosilane elemental analysis has previously been attributed to
incomplete combustion related to SiC formation.48,49 UV−vis
spectroscopy was performed on a Shimadzu UV-1800 UV−vis
spectrophotometer. The UV−vis spectra were measured at room
temperature in pentane in a quartz cuvette (10 mm). Cyclic
voltammetry (CV) measurements were performed inside a glovebox
under a dry nitrogen atmosphere. CV measurements were obtained
with a CH Instruments, Inc., 600 E Electrochemical Analyzer with a 2
mm diameter platinum working electrode, a platinum wire counter
electrode, and a quasi-internal silver wire reference electrode
submersed in 0.01 M AgNO3/0.1 M TBABF4 in anhydrous
acetonitrile. Voltammograms were obtained with an analyte
concentration of 3.5 mM and 0.1 M TBABF4 in anhydrous
acetonitrile with a scan rate of 100 mV s−1 using ferrocene as an
internal reference.
Calculated HOMO and LUMO isodensity plots are shown
in Figure 4. In all cases, HOMO orbital density is seen on both
the σ and π fragments of the cyclosilazane. This suggests that
nitrogen can effectively couple these fragments in the HOMO.
The degree of σ−π coupling is substituent dependent. As the
aromatic substituent becomes less electron-donating, the
HOMO orbital density on the anilinic component lessens,
and in 2f (R = CF3), the HOMO density is mostly
concentrated on the silazane ring. At the other extreme, the
compound with the most electron-donating substituent (2a, R
= NMe2) has a different HOMO isodensity plot from the other
compounds in the series. For compounds 2b−2f, a nitrogen p-
orbital is apparent and bridges the conjugation associated with
the Si−Si σ-conjugation and the aromatic π-system, while the
2a HOMO does not include a nitrogen p-orbital and exhibits
σ−π conjugated structure similar to an all Si analogue (Figure
S9).
Only in some cases is LUMO orbital density seen on both
the SiN and aromatic ring: all compounds have LUMO density
on the aromatic ring, while only comparatively electron-poor
substituents 2d−2f show orbital density on the SiN system.
This likely accounts for the weaker linear free energy
relationship between σp and experimental LUMO energies.
Finally, we sought to understand how SiN rings 2a−2f
compare to all silicon rings, in analogy to the n-type doping of
bulk silicon by elements from group V. Quantum chemical
calculations shed light on how nitrogen incorporation perturbs
HOMO and LUMO energies in cyclosilanes. trans-1,4-
Diphenyldecamethylcyclohexasilane (Si6Ph2) was selected as
a control compound for these computational studies. Si6Ph2
and its cyclosilazane analogue 2d have HOMOs of similar
energy, although the LUMO of 2d is ca. 0.3 eV higher in
energy. Electron-withdrawing substituents shift both HOMO
and LUMO energies down, while electron-donating sub-
stituents shift HOMO and LUMO energies up relative to
Si6Ph2 (Figure 4). Thus, depending on the aromatic
substituent, the effect of nitrogen incorporation can be n- or
p-type.
Synthesis of 1,2-Diphenyl-1,1,2,2-tetramethyldisilane (Ph-
(SiMe2)2Ph). Chloro(dimethyl)phenylsilane (4.0 mL, 23.7 mmol)
was added dropwise to a Schlenk flask containing lithium granules
(0.66 g, 95.1 mmol) suspended in THF (24 mL) at 0 °C. After
complete addition, the reaction was warmed to room temperature and
stirred overnight. The next day, the solution was filtered under Ar to
another Schlenk flask. iPrMgCl (2 M in THF, 11.9 mL, 23.7 mmol)
was added to the filtrate at 0 °C dropwise and stirred for 20 min.
Chloro(dimethyl)phenyl-silane (4.0 mL, 23.7 mmol) was added and
stirred at room temperature for 3 h. The reaction mixture was
quenched at 0 °C by dropwise addition of water followed by saturated
ammonium chloride solution. The aqueous and organic layers were
separated, and the aqueous layer was extracted three times with Et2O.
The combined Et2O layers were washed with brine solution followed
by Na2SO4 before being concentrated by rotary evaporation to yield a
yellow liquid. Purification was achieved by column chromatography
on silica gel with hexanes to yield a white solid (4.61 g, 72%). NMR
spectra agree with a previous report.50
CONCLUSION
■
In conclusion, we have demonstrated a high yielding synthesis
of N-aryl cyclosilazanes. We varied the para-substituent of the
anilinic subunit in the cyclosilazanes and spectroscopically and
electronically investigated substituent effects. Our experimental
and computational results showed that the aniline substituent
systematically perturbs HOMO energies. A Hammett analysis
of the silazane energies revealed a correlation of σp with the
HOMO energy. Calculations of HOMO/LUMO densities
revealed the nitrogen nonbonding p-orbital can bridge σ- and
π-conjugated systems and that SiN rings can be tuned to either
more or less electron rich than all silicon rings.
EXPERIMENTAL SECTION
■
General Procedures. All experiments were performed using
either standard Schlenk line techniques under a dry argon atmosphere
or a UNIlab Plus Glove Box by MBRAUN under a dry nitrogen
atmosphere. All glassware was oven-dried overnight in a 175 °C oven.
All reaction solvents were dried and degassed on a J. C. Meyer Solvent
Dispensing System. Granular lithium, triethylamine, isopropylmagne-
sium chloride (2 M in THF), dichlorodimethylsilane, aniline, p-
toluidine, 4-chloroaniline, p-anisidine, 4-(trifluoromethyl)aniline,
tetrabutylammonium tetrafluoroborate (TBABF4), ferrocene, and
anhydrous acetonitrile were purchased from Sigma-Aldrich. Trifluor-
omethanesulfonic acid (triflic acid, TfOH) was purchased from Acros
Synthesis of 1,2-Bis(trifluoromethanesulfonate)-
tetramethyl-disilane (1). Triflic acid (8.03 mL, 90.7 mmol) was
added dropwise via addition funnel to 1,2-diphenyl-1,1,2,2-tetrame-
thyldisilane (12.26 g, 45.4 mmol) dissolved in dichlormethane (75
mL) at 0 °C. The reaction was stirred at 0 °C for 30 min followed by
room temperature for 1 h. Volatile materials were removed under a
vacuum to yield a yellow liquid. The resulting compound was distilled
from the crude liquid under a vacuum (45 °C, 0.7 Torr) to yield a
1
colorless liquid (18.72 g, 99%). H NMR (400 MHz, CDCl3) δ 0.73
E
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(s, 12H). 13C {1H} (101 MHz, CDCl3) δ 118.3 (q, J = 317.7 Hz),
−0.6 (s). 29Si {1H} NMR (79 MHz, CDCl3) δ 33.6. 19F {1H} (282
MHz, CDCl3) δ −75.90.
Computational Details. All calculations were carried out using
the Gaussian 09 package.51 Geometries were optimized from the
crystal structure using the B3LYP functional with the 6-311G(d) basis
set with forced nonsymmetry. All optimized structures possess zero
imaginary frequencies. The TD-DFT calculation of 2d was performed
using the CAM-B3LYP functional with the 6-311G(d) basis set using
the geometry optimized coordinates of 2d from the B3LYP/6-
311G(d) functional. The 20 lowest excitations were calculated.
General Procedure for the Synthesis of 2a−2f. Disilane 1
(1.00 g, 2.41 mmol) was added dropwise via syringe to the
corresponding substituted aniline (2.41 mmol) and NEt3 (0.68 mL,
4.82 mmol) dissolved in Et2O (24 mL) and stirred at room
temperature for 2 h. A biphasic mixture formed after stirring for 30
min. After 2 h, the bottom ionic liquid layer was frozen at −78 °C.
The top Et2O layer was cannulated to an oven-dried Schlenk flask,
and volatile materials were removed under a vacuum to yield the
crude cyclosilazane.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00254.
Synthesis of 2a. Synthesized according to the above general
procedure (0.330 g of N,N-dimethyl-p-phenylenediamine). Pure 2a
was obtained by slow evaporation recrystallization in Et2O/MeCN at
1
room temperature (0.512g, 85%). H NMR (400 MHz, CDCl3) δ
Computed Cartesian coordinates of all of the molecules
reported in this study (XYZ)
6.78 (d, J = 8.9 Hz, 4H), 6.65 (d, J = 8.9 Hz, 4H), 2.91 (s, 12H), 0.09
(s, 24H). 13C {1H} (101 MHz, CDCl3) δ 147.4, 135.4, 130.5, 113.3,
41.3, 0.7. 29Si {1H} NMR (79 MHz, CDCl3) δ −6.8. HRMS (ASAP+)
Calcd [M + H] for C24H44N4Si4 501.2721, found 501.2727. Anal.
Calcd for C24H44N4Si4: C, 57.54; H, 8.85; N, 11.18. Found: C, 57.33;
H, 8.99; N, 11.06.
NMR spectra of compounds 1 and 2a−2f, cyclic
voltammograms, crystal measurement and refinement
data for compounds studied by XRD, and TD-DFT
results (PDF)
Synthesis of 2b. Synthesized according to the above general
Accession Codes
procedure (0.308 g of p-anisidine). Pure 2b was obtained by
1
CCDC 1892007−1892012 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
recrystallization in MeCN at −30 °C (0.361 g, 63%). H NMR (400
MHz, CDCl3) δ 6.85−6.76 (m, 8H), 3.79 (s, 6H), 0.10 (s, 24H). 13
C
{1H} (101 MHz, CDCl3) δ 156.2, 138.6, 130.7, 114.0, 55.5, 0.6. 29Si
{1H} NMR (79 MHz, CDCl3) δ −6.5. HRMS (ASAP+) Calcd [M +
H] for C22H38O2N2Si4 475.2089, found 475.2095. Anal. Calcd for
C22H38O2N2Si4: C, 55.64; H, 8.07; N, 5.90. Found: C, 54.22; H, 7.79;
N, 5.69.
Synthesis of 2c. Synthesized according to the above general
procedure (0.259 g of p-toluidine). Pure 2c was obtained by slow
evaporation recrystallization in Et2O/MeCN at room temperature
AUTHOR INFORMATION
Corresponding Author
*E-mail: klausen@jhu.edu.
■
1
(0.405, 76%). H NMR (400 MHz, CDCl3) δ 7.07−7.01 (m, 4H),
6.82−6.77 (m, 4H), 2.31 (s, 6H), 0.11 (s, 24H). 13C {1H} (101 MHz,
CDCl3) δ 143.1, 133.0, 129.7, 129.4, 21.0, 0.7. 29Si {1H} NMR (79
MHz, CDCl3) δ −6.6. HRMS (ASAP+) Calcd [M + H] for
C22H38N2Si4 443.2190, found 443.2190. Anal. Calcd for C22H38N2Si4:
C, 59.66; H, 8.65; N, 6.33. Found: C, 59.39; H, 8.80; N, 6.19.
Synthesis of 2d. Synthesized according to the above general
procedure (0.22 mL of aniline). Pure 2d was obtained by
recrystallization in MeCN at −30 °C (0.455 g, 91%). 1H NMR
(400 MHz, CDCl3) δ 7.27−7.22 (m, 4H), 7.10−7.05 (m, 2H), 6.94−
6.89 (m, 4H), 0.13 (s, 24H). 13C {1H} (101 MHz, CDCl3) δ 146.1,
129.8, 128.8, 123.7, 0.7. 29Si {1H} NMR (79 MHz, CDCl3) δ −6.2.
HRMS (ASAP+) Calcd [M + H] for C20H34N2Si4 415.1877, found
415.1873. Anal. Calcd for C20H34N2Si4: C, 57.91; H, 8.26; N, 6.75.
Found: C, 57.13; H, 8.02; N, 6.54.
ORCID
Maxime A. Siegler: 0000-0003-4165-7810
Rebekka S. Klausen: 0000-0003-4724-4195
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S Department of Energy
(DOE), Office of Science, Basic Energy Sciences under Award
No. DE-SC0013906 (molecular synthesis). DFT calculations
were conducted using scientific computing services at the
Maryland Research Computing Center (MARCC). We thank
Prof. Lan Cheng (Johns Hopkins University) for helpful
discussions.
Synthesis of 2e. Synthesized according to the above general
procedure (0.308 g of p-chloroaniline). Pure 2e was obtained by slow
evaporation recrystallization in Et2O/MeCN at room temperature
1
(0.379 g, 65%). H NMR (400 MHz, CDCl3) δ 7.24−7.18 (m, 4H),
6.87−6.80 (m, 4H), 0.12 (s, 24H). 13C {1H} (101 MHz, CDCl3) δ
144.7, 130.9, 129.3, 129.0, 0.6. 29Si {1H} NMR (79 MHz, CDCl3) δ
−5.7. HRMS (ASAP+) Calcd [M + H] for C20H32Cl2N2Si4 483.1098,
found 483.1106. Anal. Calcd for C20H32Cl2N2Si4: C, 49.66; H, 6.67;
N, 5.79. Found: C, 49.87; H, 6.57; N, 5.79.
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