Synthesis, photochemical decomposition and DFT studies of 2,2,3,3-tetramethyl-1,1-bis(dimethylphenylsilyl)silacyclopropane

Article abstract of DOI:10.1016/j.jorganchem.2015.05.022

The compound, 2,2,3,3-tetramethyl-1,1-bis(dimethylphenylsilyl)silacyclopropane (1), is a potential photochemical source of silylenes either through a 2 + 1 fragmentation of the silacyclopropane ring core or through α-elimination from the aryltrisilane cha

Full text of DOI:10.1016/j.jorganchem.2015.05.022

Journal of Organometallic Chemistry 791 (2015) 163e168
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, photochemical decomposition and DFT studies of
2,2,3,3-tetramethyl-1,1-bis(dimethylphenylsilyl)silacyclopropane
Kothanda Rama Pichaandi, Joel T. Mague, Mark J. Fink*
Department of Chemistry, Tulane University, New Orleans, LA 70118, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The compound, 2,2,3,3-tetramethyl-1,1-bis(dimethylphenylsilyl)silacyclopropane (1), is
a potential
Received 4 March 2015
Received in revised form
5 May 2015
Accepted 7 May 2015
Available online 23 May 2015
photochemical source of silylenes either through a 2 þ 1 fragmentation of the silacyclopropane ring core
or through -elimination from the aryltrisilane chain. Upon 254 nm photolysis, the silacyclopropane (1)
a
exclusively undergoes 2 þ 1 ring fragmentation with the formation of bis(dimethylphenylsilyl)silylene
and tetramethylethylene. This clearly shows the heightened photoreactivity of the silacyclopropane ring
compared with the aryltrisilane chain. The extruded bis(dimethyphenyl)silylene is intercepted with
trapping agents such as triethylsilane, methoxytrimethylsilane and bis(trimethylsilyl)acetylene to form
thermally stable adducts. Time Dependent-Density Functional Theory (TD-DFT) calculations at the
Keywords:
Silacyclopropane
Transient silylene
2 þ 1 fragmentation
Silacyclopropene
TD-DFT
B3LYP/6-31G* level reveal that an electron is excited from a SieC
(HOMO) to empty * orbitals of phenyl group (LUMO). This weakens the SieC bonds of the silacyclo-
propane ring which is believed to result in the observed 2 þ 1 photofragmentation.
© 2015 Elsevier B.V. All rights reserved.
s orbital of the silacyclopropane ring
p
1. Introduction
silacyclopropanes have been used as silylene sources [2,15e20]. In
general, all of these silacyclopropane-based silylene precursors
possess the silacyclopropane ring as their effective chromophore.
However, the study of a single compound possessing two inde-
pendent substructures for silylene generation; a silacyclopropane
ring core and an aryltrisilane chain, remains unnoticed in the
literature.
The photochemical generation of transient silylenes is an
important area of silicon chemistry. There are two common types of
precursors used for this purpose [1]. The first one includes linear,
branched and cyclic polysilanes [2,3]. These precursors can undergo
a
elimination of SieH and SieSi bonds to give transient silylenes.
One example is the aryl substituted oligosilanes, particularly 2-
aryltrisilanes and 2-aryltetrasilanes (Scheme 1). These precursors
are photochemical sources for silylenes, with the chromophore
provided by the aromatic group [4,5].
We report here the synthesis and photochemical decomposition
pathway of 2,2,3,3-tetramethyl-1,1-bis(dimethylphenylsilyl)silacy-
clopropane (1) which possesses both the silacyclopropane ring core
and the aryltrisilane chain. The potential photofragmentation of
this compound may result from either the bis(dimethylphenylsilyl)
silylene (Path A) and tetramethylethylene or the cyclic silylene
2,2,3,3-tetramethylsilacycloprop-1-ylidene (Path B) and tetrame-
thyldiphenyldisilane (Scheme 2). TD-DFT calculations were per-
formed in order to better understand the photochemical fate of (1).
The second type of silylene precursor includes strained three
membered ring compounds. Silacyclopropanes [6], silacyclopro-
penes [7], disilacyclopropanes [8] and cyclotrisilanes [9] fall into
this category as molecules which undergo 2 þ 1 type photo-
fragmentation to give a silylene and the corresponding multiple
bond compound. These compounds are also known to decompose
thermally to extrude silylenes. Silacyclopropanes as silylene gen-
erators are one of the best studied of these systems originating with
the first reported synthesis by Seyferth of hexamethylsilacyclo-
propane in 1975 [6,7,10e14]. Since this initial synthesis, a variety of
2. Material and methods
2.1. General information
Chlorosilanes, triethylsilane, methoxytrimethylsilane and bis(-
trimethylsilyl)acetylene were purchased from Gelest and used
without further purification. All other reagents were purchased
from Aldrich and were used without purification. Cyclohexane and
* Corresponding author.
E-mail address: fink@tulane.edu (M.J. Fink).
http://dx.doi.org/10.1016/j.jorganchem.2015.05.022
0022-328X/© 2015 Elsevier B.V. All rights reserved.

164
K.R. Pichaandi et al. / Journal of Organometallic Chemistry 791 (2015) 163e168
analysis for C22H36Si3: observed (calculated). C, 68.28 (68.67) H,
9.39 (9.43).
2.3. Synthesis of 2,2-bis(2-bromopropan-2-yl)-1,1,3,3-tetramethyl-
1,3-diphenyltrisilane(2)
Scheme 1. Photolysis of aryltrisilanes to give arylsilylenes
The trisilane (3) (2.8 g 7.4 mmol), N-bromosuccinimide (2.9 g
16 mmol), AIBN (0.040 g, 0.23 mmol) and 45 mL of CCl4 were placed
in a 100 ml Schlenk flask, fitted with a condenser. The mixture was
stirred for 15 min and then heated to reflux for 30 min and the
mixture was then cooled to 0 ꢂC. The precipitated succinimide was
separated from the clear solution by filtration. Rotary evaporation
of the filtrate and subsequent crystallization in toluene yielded (2)
as colorless crystals. M.p. 165e170 ꢂC. Yield 1.7 g (42%) 1H NMR (
C6D12) 0.61e0.75 (s, 12H) 1.74e1.92 (s, 12H) 7.12e7.28 (m, 6H),
7.41e7.56 (m, 4H) 13C{1H} NMR (
, C6D12) 1.3, 35.9, 57.2, 128.1, 129.1,
d,
d
136.1, 138.7 (
d
, C6D12
)
29Si{1H} NMR (
d
, C6D6) ꢁ18.3, ꢁ8.3. MS(EI):
Scheme 2. Potential photochemical decomposition pathways for 1.
m/z (relative abundance) 527 (16, Mþ-Me), 463 (45), 421 (18), 328
(36), 285 (11), 231 (31), 135 (100), 105 (18), 43 (11) HRMS for
21H3179Br2Si3 (Mþ-Me): observed (calculated) 525.00450
C6D12 were dried over sodium, vacuum transferred, and stored
under argon. CDCl3 was distilled from CaH2 and stored under argon.
Hexane and THF were distilled directly from sodium benzophe-
none. Magnesium turnings (~1/8 in, 99.95% on trace metal basis)
were purchased from Aldrich.
C
(525.00401) Elemental analysis for C22H34Br2Si3: observed (calcu-
lated). C, 48.28 (48.70) H, 6.29 (6.32).
2.4. Synthesis of 2,2,3,3-tetramethyl-1,1-bis(dimethylphenylsilyl)
Photolyses were performed using a Rayonet RS photochemical
reactor equipped with 17 low-pressure 254-nm radiation mercury
lamps. All NMR spectra were recorded on a Bruker Avance 300
spectrometer and referenced to TMS as a chemical shift standard.
29Si{1H} NMR of all compounds was recorded using inverse gated
coupling technique. Preparative HPLC was performed using a Var-
ian Prostar 210 HPLC with a binary solvent delivery system with a
reverse phase semi-preparative column (column module C18 8U
21.4 ꢀ 250). The eluent had a combined solvent flow rate of 23 mL/
min. GCMS was performed using a Varian Saturn 2100 instrument.
Infrared spectra were recorded as neat samples between NaCl
plates using a Nexus 670 Avatar FTIR Spectrometer. EI-MS and
HRMS were recorded at the Georgia Tech mass spectrometry fa-
cility. Elemental analysis was performed at Schwarzopf Microana-
lytical Laboratory and the Tulane University Coordinated
Instrumentation Facility. UveVis spectra were obtained using a
Cary 50 UveVis spectrophotometer.
silacyclopropane (1)
Magnesium turnings (99%, 0.75 g, 31 mmol) and 20 mL of THF
were placed in a 100 mL Schlenk flask under nitrogen. A small
amount of 1,2-dibromoethane (0.70 mL) was added to the solution
which was then stirred for 1 h in order to activate the magnesium.
Compound (2) (4.0 g, 7.0 mmol), dissolved in 20 mL of THF, was
added to the magnesium slurry over a period of 3 h at 15 ꢂC. The
reaction mixture was stirred for an additional 5 h at room tem-
perature. The solvent was removed from the reaction mixture by
applying vacuum. Chilled hexane (25 mL, 0 ꢂC) was added, stirred
for 10 min, and allowed to settle. The clear solution was decanted
and filtered through a Schlenk fritted funnel. The extraction with
chilled hexane was repeated one more time. The solvent was then
removed from the combined filtrate and dried under vacuum to
give (1) as colorless semi-liquid. Compound (1) was stored
at ꢁ30 ꢂC in a glove box due to its air and thermal instability. Yield
2.2 g (80%) 1H NMR (
7.14e7.23 (m, 6H), 7.43e7.53 (m, 4H). 13C{1H} NMR (
24.2, 28.4, 127.9, 128.7, 134.3, 140.1 29Si{1H} NMR (
d
, C6D6) 0.33e0.40 (s, 12H) 1.33e1.50 (s, 12H)
, C6D6) 0.6,
, C6D6) ꢁ123.9,
2.2. Synthesis of 2,2-diisopropyl-1,1,3,3-tetramethyl-1,3-
diphenyltrisilane (3)
d
d
-13.9. IR (NaCl, cmꢁ1) 3068 (s), 2954 (m), 2854 (s), 1423 (s), 1245 (s),
1111 (s), 1077 (s), 819 (m), 739 (m). MS(EI): m/z (relative abun-
dance) 382 (8, Mþ), 367 (7) 298 (100), 283 (19), 239 (73), 162 (20),
84 (25) 69 (35) HRMS for C22H34Si3 (Mþ): observed (calculated)
382.19412 (382.19683)
Lithium (4.0 g, 0.58 mol) was washed with ether to remove the
oil and was added in pieces to an argon filled 200 mL Schlenk flask
containing 100 mL of dry THF. Phenyldimethylchlorosilane (8.0 g,
0.050 mol) was then added with stirring at room temperature. After
a few minutes of addition, the solution turned dark brown and was
subsequently stirred overnight at room temperature to give the
dimethylphenylsilyllithium. Diisopropyldichlorosilane (6.0 g,
0.030 mol) and 30 mL of THF were placed in a separate 500 mL
Schlenk flask and cooled to ꢁ25 ꢂC. The dimethylphenylsilyllithium
was then added to the flask over a 3 h period and stirred overnight.
The reaction mixture was quenched by the addition of 100 mL of
water and the trisilane (3) was extracted with hexane (2 ꢀ 100 mL)
and dried with anhydrous MgSO4. The solvent was removed by
rotary evaporation. Pure 3 was obtained as a colorless liquid by
vacuum distillation at 125e140 ꢂC (350 mTorr). Yield 5.6 g (62%) 1H
2.5. Thermolysis experiments
In a typical procedure, a mixture of (1) (0.10 g, 0.26 mmol), the
trapping agent (6 times the molar ratio of (1)), and freshly distilled
cyclohexane (4.0 mL) was placed into a pyrex custom-made thick
walled pyrolysis tube (10 ml, 17 mm o.d. ꢀ 180 mm height with a
10 mm o.d. stem). The tube was attached to a teflon stopcock via a
Cajon union. The tube assembly was attached to a high vacuum line
and the solution was degassed in three freeze-pump-thaw cycles.
The tube was then sealed with a torch and heated to 120 ꢂC for 20 h.
The ampule was opened after cooling to room temperature and the
pyrosylate was transferred to a 10 ml Schlenk flask. The solvent as
well as the excess trapping agent were removed by vacuum. Crude
samples (4) and (5) were purified by reverse phase HPLC to give
colorless liquids.
NMR (
d
, C6D6) 0.40e0.47 (s, 12H) 0.99e1.07 (d, 12H, J ¼ 7 Hz) 1.17-
1.3 (m, 2H) 7.12e7.24 (m, 6H), 7.42e7.51 (m, 4H) 13C{1H} NMR (
d,
,
C6D6) 0.5, 13.3, 21.6, 128.4, 129.0, 134.9, 141.3 29Si{1H} NMR (
d
C6D6) ꢁ27.4, ꢁ19.1. GCMS(EI): m/z (relative abundance) 369 (27,
Mþ-Me) 327 (32), 299 (100), 224 (25), 135 (10), 105 (10) Elemental

K.R. Pichaandi et al. / Journal of Organometallic Chemistry 791 (2015) 163e168
165
2.6. Characterization of (4)
solution was degassed in three freeze-pump-thaw cycles. The so-
lution was photolyzed at 254 nm for 16 h at 15 ꢂC. The solvent and
excess triethylsilane were removed by vacuum. Analytically pure
(4) was obtained by HPLC separation using a CH3OH:THF mixture
(93:7). Retention time 8.3e9.6 min.
Analytically pure (4) was obtained by HPLC separation using a
CH3OH:THF mixture (93:7). Retention time 8.3e9.6 min. Yield
0.10 g (80%) 1H NMR (
d
, C6D6) 0.38e0.43 (s, 6H) 0.43e0.5 (s, 6H)
0.52e0.65 (q, 6H, J ¼ 8 Hz) 0.86e0.98 (t, 9H, J ¼ 8 Hz) 2.84e2.94 (s,
1H) 7.08e7.34 (m, 6H) 7.39e7.64 (m, 4H) 13C{1H} NMR (
, C6D6) 0.4,
d
2.11. X-ray structure determinations: general data
6.1, 8.6, 128.2, 129.0, 134.2, 140.8, 29Si{1H} NMR
(d,
C6D6) ꢁ16.8, ꢁ14.0, 3.0. IR (NaCl, cmꢁ1) 3069 (s), 2950 (m), 2069
(SieH, s), 1462 (s), 1428 (s), 1107 (s), 1008 (s), 881 (s), 765 (m).
MS(EI): m/z (relative abundance) 414 (5, Mþ), 399 (14), 299 (70),
278 (100), 239 (37), 197 (23) 135 (99), 59 (18) 43 (5) HRMS for
The solid state structure of (2) was determined by X-ray crys-
tallography. An ORTEP drawing of (2) is given in Fig. 1, while
experimental details and relevant metrical data are presented in
Table 1. General procedures for crystal alignment, unit cell deter-
mination and refinement and collection of intensity data have been
published [21]. All non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were added in idealized positions with fixed
isotropic temperature factors. Each bromoisopropyl group is rota-
tionally disordered over two sites (for Br1 occupancy ratio 91/9, for
Br2, 64/36). These were modeled as rigid groups using metrical
parameters obtained from DFT calculations and with assignment of
approximate isotropic displacement parameters to the methyl
carbon atoms.
C
22H38Si4 (Mþ): observed (calculated) 414.20326 (414.20507).
2.7. Characterization of (5)
Analytically pure (5) was obtained by HPLC separation using a
CH3OH:THF mixture (95:5). Retention time 5.8e7.3 min. Yield
0.080 g (80%) 1H NMR (
d, C6D12) 0.03e0.05 (s, 9H) 0.31e0.45 (d,
12H, J ¼ 6 Hz) 3.33e3.37 (s, 3H) 7.14e7.22 (m, 6H) 7.35e7.42 (m,
4H) 13C{1H} NMR (
139.5 29Si{1H} NMR (
d
, C6D12) ꢁ2.1, -2.1, ꢁ0.3, 54.9, 127.4, 128.1, 133.8,
d
, C6D12) ꢁ19.3, -15.1, 3.0 IR (NaCl, cmꢁ1) 3068
(s), 3048 (s), 2950 (s), 2891 (s), 2817 (s), 1426 (s), 1245 (s), 1106 (s),
1075 (SieOMe, s), 835 (d), 761 (s), 733 (s), 696 (s), 644 (s). MS(EI):
m/z (relative abundance) 402 (11, Mþ), 387 (100), 329 (16), 298
(38), 236 (76), 193 (34), 177 (33), 135 (87) 105 (9), 73 (20) HRMS
2.12. Computational details
All the TD-DFT reported in this paper used the GAUSSIAN03 [22]
program system through WebMo interface [23]. Geometries for
model compound (1) have been optimized by implementing the
density functional calculations using B3LYP method at 6-31G* level.
The GIAO (gauge-independent atomic orbital) nuclear magnetic
shielding tensors were calculated at the B3LYP/6-31G* level using
the optimized structures at the B3LYP/6-31G*. 29Si chemical shift
tensors were converted into the corresponding isotropic chemical
C
20H34OSi4 (Mþ): observed (calculated) 402.16808 (402.16867).
2.8. Characterization of (6)
Compound (6) was isolated after removing the solvent and the
trapping agent from the reaction mixture as an extremely air and
moisture sensitive liquid. It was pure enough to characterize and no
HPLC purification was attempted due to the high reactivity of 6
shifts assuming
s
¼ 386.5 for SiMe4 [24,25].
towards air and moisture. Yield 0.11 g (90%). 1H NMR (
d
, C6D12
)
3. Results and discussion
0.12e0.18 (s, 18H) 0.18e24 (s, 12H) 7.20e7.30 (m, 6H) 7.34e7.42 (m,
4H) 13C{1H} NMR (
164.7 29Si{1H} NMR (
3065 (s), 3046 (s), 2953 (s), 2886 (s),1583 (s),1423 (s),1245 (s),1106
(s), 1066 (m), 836 (m), 739 (m). MS(EI): m/z (relative abundance)
468 (53, Mþ), 454 (34), 391 (19), 303 (23), 236 (62), 155 (22), 135
(100), 43 (6) HRMS for C24H40Si5 (Mþ): observed (calculated)
468.19267 (468.19763).
d
, C6D12) ꢁ0.8, 0.0, 128.6, 129.4, 135.0, 140.5,
3.1. Synthesis of silacyclopropane (1)
d
, C6D12) ꢁ178.9, ꢁ17.0, ꢁ11.1. IR (NaCl, cmꢁ1
)
The synthesis of silacyclopropane (1) is shown in Scheme 3. The
dimethylphenylsilyllithium was prepared by the reaction of dime-
thylphenylchlorosilane with lithium metal in THF by a procedure
reported by Murakami et al. [26]. The silyllithium was then reacted
with dichlorodiisopropylsilane to give 2,2-diisopropyl-1,1,3,3-
tetramethyl-1,3-diphenyltrisilane (2). The trisilane (2) was bromi-
nated using N-bromosuccinimide (NBS) in CCl4 in the presence of
2,20-azobisisobutyronitrile (AIBN) under reflux to give 2,2-bis(2-
bromopropan-2-yl)-1,1,3,3-tetramethyl-1,3-diphenyltrisilane (2).
2.9. NMR tube photolysis experiments
In typical procedure, a solution of (1) (0.030 mM) and trapping
reagent (6 times the molar ratio of (1)) in cyclohexane-d12 (0.70 mL)
was placed in a quartz NMR tube fitted with a J-Young valve. The
tube was connected to a Teflon high vacuum stopcock. The stopcock
tube was then attached to a high vacuum line and the solution was
degassed by three freeze-pump-thaw cycles. The solution was then
photolyzed in a Rayonet photochemical reactor at 254 nm. The
progress of the photolysis was monitored by NMR spectroscopy in
2 h time intervals throughout the experiment. Quantitative for-
mation of tetramethylethylene with respect to decomposition of (1)
was calculated by comparing 1H NMR ratio of trapping agent to (1)
(decreasing) and trapping agent to tetramethylethylene
(increasing) before and after photolysis.
2.10. Photolysis of (1) in the presence of triethylsilane
Fig. 1. Molecular structure of (2) with atomic numbering (ORTEP, 50% probability el-
lipsoids; H atoms are omitted for clarity). Selected bond distances (Å) and bond angles
(deg) for 2: Si1eSi2 ¼ 2.381(4), Si2eSi3 ¼ 2.389(4), Si2eC17 ¼ 1.93 (1), Si2eC20 ¼ 1.93
(1) Si1eSi2eSi3 ¼ 106.1 (1), Si1eSi2eC17 ¼ 112.2 (3), Si1eSi2eC20 ¼ 109.1 (3),
C17eSi2eC20 ¼ 109.7 (4).
Silacyclopropane (1) (0.10 g, 0.26 mmol), triethylsilane (0.20 g,
3.2 mmol), and 8.0 mL of hexane were placed in a quartz tube. The
quartz tube was then attached to a high vacuum line and the

166
K.R. Pichaandi et al. / Journal of Organometallic Chemistry 791 (2015) 163e168
Table 1
structure revealed a mild disorder arising from the positional
scrambling of the methyl and bromine atoms within the bromoi-
sopropyl groups. Despite this disorder, the refinement was well
behaved and proceeded to an R value of 0.0537. The Si1eSi2
(2.381 (4) Å) and Si2eSi3 bonds (2.389 (4) Å) are slightly longer
than the normal SieSi bond length of 2.33 Å [27]. The silicon to
bromoisopropyl carbon bond lengths (Si1eC17 and Si3eC20) of
1.93(1) Å are also longer than a typical SieC single bond length of
1.87 Å [28]. The longer SieSi and SieC bonds reflect the sterically
congested nature of this species.
Experimental crystallographic data for compound (2).
Compound (2)
Formula
C22 H34 Br2 Si3
542.58
Molecular weight
Crystal dimensions [mm]
Crystal system
Space group
Cell dimensions
a (Å)
0.18 ꢀ 0.29 ꢀ 0.34
Monoclinic
P21/c (No. 14)
9.0647(5)
12.2012(7)
22.930(1)
90
94.880(1)
90
2526.9(2)
4
1.426
b (Å)
c (Å)
a
b
g
(ꢂ)
(ꢂ)
(ꢂ)
3.3. Silylene trapping reactions
V (Å3)
Z
3.3.1. Thermolysis of (1) and the characterization of trapping
products
The thermolysis of (1) was carried out in cyclohexane at 120 ꢂC
for 20 h in the presence of trapping agents. The silylene was trap-
r
(calc) (g/cm3)
Radiation
(
l
¼ 0.71073)
Mo K
q
a
(mm)
3.357
ped by
a 6-fold excess of either triethylsilane, methoxy-
range (ꢂ)
2.3, 28.8
trimethylsilane or bis(trimethylsilylacetylene) to give the expected
adducts (4), (5) and (6), respectively, from the reaction with the
transient bis(silyl)silylene (Scheme 4).
Scan type
u/2q
44078
6530
5659
1112
100
0.0537
0.1460
Total data collected
Unique data
Observed data [I > 2.0
F(000)
s
(I)]
Compounds (4), (5) and (6) were fully characterized by 1H NMR,
13C{1H} NMR including DEPT135, 29Si{1H} NMR spectroscopy,
infrared spectroscopy (IR) and mass spectroscopy. The 29Si{1H}
NMR spectrum of the silacyclopropene (6), showed a resonance
at ꢁ178.9 ppm which is very typical for a silacyclopropene ring
[2,7,29] and is also consistent with the chemical shift
of ꢁ168.5 ppm calculated from DFT. The other two silicon reso-
nances are observed at ꢁ17.0 and ꢁ11.1 ppm.
Temperature (K)
R
a
Rw
n
ꢀ
ꢁ
h
i
P
P
a
Rw
¼
w
F20 ꢁ F2c
=
wF20Þg1=2; w ¼ 1= s2ðF20þðxPÞ2
;
ꢀ
ꢁ
where P ¼ F20 þ Fc2 =3.
Silacyclopropane (1) was obtained in 80% yield from the reac-
tion of (2) with activated magnesium in dry THF following the
procedure pioneered by Seyferth [10]. Compound (1) is an air and
moisture sensitive colorless semi-liquid and is thermally unstable
3.3.2. Photolysis of silacyclopropane (1) in the presence of trapping
reagents
Compound (1) was photolyzed at 254 nm in cyclohexane-d12 at
15 ꢂC in the presence of the trapping agents triethylsilane,
methoxytrimethylsilane and bis(trimethylsilyl)acetylene. The
photochemical reaction was monitored by 1H, 13C{1H} and 29Si{1H}
NMR spectra. Loss of tetramethylethylene and the formation of (4),
(5) and (6) were observed from their respective trapping agents
(Scheme 4). In the case of triethylsilane, the adduct (4) was
photochemically stable and was isolated. Adducts (5) and (6),
however, underwent decomposition upon further photolysis to
give uncharacterized products. However the formation of (5) and
(6) was confirmed by comparing the 1H, 13C{1H} and 29Si{1H} NMR
spectra at an early photolysis stage (5.5 h), with authentic samples
of (5) and (6) obtained from their respective thermolysis reactions
(Section 3.3.2). Quantitative formation of tetramethylethylene with
respect to the decomposition of (1) was observed in the 1H NMR by
comparing the integrated ratio of trapping agent to (1) and trap-
ping agent to tetramethylethylene before and after photolysis.
Neither the adduct of the cyclic silylene nor (Me2PhSi)2, products
at room temperature. The structure of (1) is fully supported by 1
H
NMR, 13C{1H} NMR and 29Si{1H} NMR and EI-MS spectra. The 29Si
{1H} NMR chemical shift of ꢁ123.9 ppm is very typical for the ring
silicon atom of silacyclopropanes [2,10,15,16,18] and is also consis-
tent with the chemical shift of ꢁ110.4 calculated from Density
Functional Theory (DFT). The EI-MS of (1) shows the parent ion
(Mþ) at 382 amu and the base peak, corresponding to bis(dime-
thylphenylsilyl)silylene, at 298 amu.
3.2. X-ray crystal structure of 2
A single crystal X-ray structure diffraction study was carried out
on the dibromide (2) (Fig. 1). Compound (2) crystallized in a
monoclinic unit cell with P21/C space group symmetry. The crystal
from a SieSi a
-elimination, were observed in the 1H NMR spectrum.
3.3.3. UVeVis spectrum and TD-DFT studies of silacyclopropane (1)
In order to better understand the dominance of silacyclopro-
pane ring cleavage in the photochemical silylene extrusion of (1),
TD-DFT calculations were performed on (1) and compared with its
UVeVis spectrum. Figs. 2 and 3 represent the experimental UVeVis
spectrum and the calculated MO diagram of the frontier orbitals of
(1) based on TD-DFT calculations, respectively. Table 2 is the com-
parison of lmax obtained from experiment and the MOs responsible
for the transition as calculated by TD-DFT. The longest wavelength
absorption (270 nm) in silacyclopropane (1) can be assigned to the
transitions of the HOMO to LUMO (dark transition), LUMO þ 1 and
LUMO þ 2. Careful analysis revealed that the HOMO has a pre-
dominant contribution from the p orbitals of the SieC bonds of the
Scheme 3. Synthetic route to the silacyclopropane (1).

K.R. Pichaandi et al. / Journal of Organometallic Chemistry 791 (2015) 163e168
167
Scheme 4. Trapping reactions for the bis(phenyldimethylsilyl)silylene generated from the decomposition of (1).
significant weakening of the silacyclopropane SieC bonds. This
results in the breaking of a SieC bond leading to an intermediate
singlet diradical (D) (Scheme 5) on the way to the extrusion of the
silylene. This mechanism is analogous to the one proposed by Kira
et al. for the microscopic reverse reaction: formation of the sila-
cyclopropane from the silylene and the alkene [30].
4. Conclusion
Silacyclopropane (1) upon photolysis, undergoes exclusively a
2 þ 1 fragmentation to extrude bis(dimethylphenylsilyl)silylene
(A), thereby demonstrating that the silacyclopropane ring is a
highly effective photochemical silylene generator. TD-DFT calcula-
tions show that the low energy excitations results in the promotion
of an electron from the SieC bonds of the silacyclopropane ring
Fig. 2. UVeVis spectrum of silacyclopropane (1) in CH2Cl2.
silacyclopropane ring, whereas the LUMO is predominantly carbon
2p in character derived from the
p* orbitals of phenyl ring. Since
photoreactivity stems from the lowest singlet state LUMO accord-
ing to Kasha's rule, we propose that the excitation of electron from
(HOMO) to a set of delocalized
p* orbitals on the phenyl rings
(LUMO). The subsequent weakening of the silacyclopropane SieC
bonds thereby favors the 2 þ 1 fragmentation. The extruded sily-
lene can be conveniently trapped with triethylsilane,
the silacyclopropane ring to the
p* orbitals of phenyl group creates
a net partial positive charge on the silacyclopropane ring (C) and a
Fig. 3. Partial MO diagram of silacyclopropane (1) based TD-DFT calculations.

168
K.R. Pichaandi et al. / Journal of Organometallic Chemistry 791 (2015) 163e168
Table 2
Comparison of lmax and excitation energy from experimental and TD-DFT calculation for silacyclopropane (1).
Expt
DFT
lmax
Exitation energy(eV)
4.6
lmax (dft)
Exitation energy(eV)
Orbital transitions
Oscillator strength
% Contribution
270
281
264
253
4.4
4.7
4.9
HOMO e LUMO
0.004
0.035
0.059
98
91
86
HOMO e LUMO þ 1
HOMO e LUMO þ 2
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Scheme 5. Proposed stepwise mechanism for the photochemical generation of silylene
(A) from (1).
[19] P. Boudjouk, U. Samaraweera, R. Sooriyakumaran, J. Chrusciel, K.R. Anderson,
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Acknowledgments
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7
The financial support of NSF (CHE- 0445637) is greatly
appreciated.
[22] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb,
J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson,
H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino,
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J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J.J.A. Montgomery, J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers,
K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi,
N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo,
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C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski,
G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas,
J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian, Inc, Wallingford CT,
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2015.05.022.
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