Synthesis of 2,5-substituted siloles and optical study of interactions with mercury(II), copper(II), and nickel(II) cations

Article abstract of DOI:10.1021/om400022f

Several 2,5-diaryl-1,1-dimethyl-3,4-diphenylsiloles (with aryl = p-cyanophenyl (1), m,p-bis(methylthio)phenyl (2), p-N,N-dimethylaminophenyl (3), 2,2′-bipyridin-6-yl (4)) were synthesized and characterized by UV-vis, fluorescence, and NMR spectroscopy, along with crystal structure determinations for 1 and 2. Compounds 1-4 in methylene chloride were combined with 1-10 equiv of M(ClO₄)₂ (M = Ni, Cu, Hg) in methanol, and the resulting solutions were monitored by UV-vis and fluorescence spectroscopy. Compound 1 showed slight effects for each metal, 2 exhibited fluorescence quenching selectively for Hg(II), 3 showed significant changes for all cations, and 4 generally demonstrated fluorescence quenching for all cations. A discussion of the causes behind these changes is also presented.

Full text of DOI:10.1021/om400022f

Article
pubs.acs.org/Organometallics
Synthesis of 2,5-Substituted Siloles and Optical Study of Interactions
with Mercury(II), Copper(II), and Nickel(II) Cations
James B. Carroll and Janet Braddock-Wilking*
Department of Chemistry and Biochemistry and Center for Nanoscience, University of Missouri St Louis, St. Louis, Missouri 63121,
United States
S
* Supporting Information
ABSTRACT: Several 2,5-diaryl-1,1-dimethyl-3,4-diphenylsiloles (with
aryl = p-cyanophenyl (1), m,p-bis(methylthio)phenyl (2), p-N,N-
dimethylaminophenyl (3), 2,2′-bipyridin-6-yl (4)) were synthesized and
characterized by UV−vis, fluorescence, and NMR spectroscopy, along
with crystal structure determinations for 1 and 2. Compounds 1−4 in
methylene chloride were combined with 1−10 equiv of M(ClO4)2 (M =
Ni, Cu, Hg) in methanol, and the resulting solutions were monitored by
UV−vis and fluorescence spectroscopy. Compound 1 showed slight
effects for each metal, 2 exhibited fluorescence quenching selectively for Hg(II), 3 showed significant changes for all cations, and
4 generally demonstrated fluorescence quenching for all cations. A discussion of the causes behind these changes is also
presented.
covalent coordination of metal centers directly to the silole
ring, in both η4 and η5 modes, have been known for many
years,12 coordination of metals at groups bound to the 2,5-
positions has been a relatively recent development. Wang and
co-workers reported the synthesis of two siloles with nitrogen-
containing substituents that could coordinate to zinc,13 and a
short time later Gerbier’s group reported one of the same
siloles along with optical data on coordination with both zinc
and copper;14 the compound exhibited stark differences in
fluorescence depending on the metal center employed.
Recently, Tang and co-workers synthesized a terpyridine-
substituted silole that showed selective fluorescent enhance-
ment for zinc.15 Our group has also investigated some
phosphine-linked siloles and their optical changes upon
coordination to gold and platinum.16 We report herein the
preparation of some known and new siloles chosen for their
potential to bind to metal cations, along with optical spectra
outlining the differences in the changes observed on the basis of
the metal and silole employed. For our initial studies the
perchlorate salts of nickel(II), copper(II), and mercury(II)
were chosen in order to study the effects of paramagnetic or
heavy-atom species. The results of these studies are described
herein.
INTRODUCTION
■
Siloles, or 1-silacyclopentadienes, and systems containing the
silole motif continue to receive appreciable interest. In
comparison to analogous cyclopentadienes, siloles exhibit red-
shifted absorption and emission spectra and greatly increased
electron mobility1 and affinity;2 these differences are attributed
to a lowered LUMO energy due to overlap of the σ* and π*
orbitals from the silylene and butadiene fragments, respec-
tively.3 Further, the observed physical and electronic effects can
be greatly influenced by the substituents chosen. The degree of
radiative relaxation in the solution and solid states can be
controlled through judicious selection of moderately and
significantly bulky substituents,4 which can greatly affect the
observed quantum yield.5 Though physical effects can be
manipulated at all positions along the silole ring, the most
significant contributions to electronic properties (e.g., absorp-
tion and emission maxima) come from the substituents at the
2,5-positions.6 Both strong π-donors and acceptors on aryl
substituents at these sites can induce substantial bathochromic
shifts, and asymmetric siloles have been synthesized to take
advantage of these donor and acceptor substituents in concert.7
Because of these varied avenues for manipulation of the desired
optoelectronic properties, siloles and related molecules have
been studied as components in diverse applications such as
organic field-effect transistors (OFETs),8 organic light-emitting
diodes,9 photovoltaics,10 and sensors.11
RESULTS AND DISCUSSION
■
Synthesis. Siloles 1−4 were prepared using the standard
Tamao one-pot reductive cyclization procedure,17 outlined in
Scheme 1. The new siloles 1 and 2, along with known siloles 3
and 4, were isolated as yellow powders in moderate yields (39−
53%, based on starting silane). The reagent to synthesize 2, 4-
Because of the rich electronic variations that siloles present,
we desired to investigate several changes in optoelectronic
properties upon interaction with metal ions. A significant
amount of substituents that affect the properties of the silole
can also be envisioned to have some coordinating ability to
metal ions. Appreciable changes in properties can then be
explored for potential chemosensor applications. Though
Received: January 11, 2013
Published: March 1, 2013
© 2013 American Chemical Society
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Scheme 1
Figure 1. Molecular structure of 1. Hydrogen atoms have been
removed for clarity. Thermal ellipsoids are drawn at the 50%
probability level. Selected bond distances (Å), bond angles (deg),
and torsion angles (deg) for 1: Si1−C1 = 1.873(1), Si1−C2 =
1.859(1), Si−C3 = 1.883(1), Si−C6 = 1.880(1), C3−C4 = 1.366(1),
C5−C6 = 1.361(1), C31−N2 = 1.146(2), C32−N1 = 1.150(2); C3−
Si1−C6 = 91.54(4), C2−Si1−C1 = 110.09(5), C10−C32−N1 =
179.4(1), C28−C31−N2 = 177.3(2); C12−C7−C3−C4 = −36.5(2),
C3−C4−C13−C14 = −58.2(1), C6−C5−C19−C24 = −56.5(1),
C5−C6−C25−C26 = −53.7(1).
bromo-1,2-bis(methylthio)benzene, was not available commer-
cially and was prepared according to the literature procedure.18
1
The new siloles 1 and 2 were characterized by H, 13C{1H},
and 29Si{1H} NMR, UV−vis, and fluorescence spectroscopy,
elemental analysis, and X-ray crystallography. The known
siloles 317b and 419 were also characterized by 1H and 13C{1H}
NMR; the spectra showed good agreement with the literature
values. All of the siloles exhibited characteristic NMR shifts,
including the 13C{1H} shifts at ca. 152 and −3.0 ppm for the
3,4-silole carbons and methyl carbons attached at the silicon
center, respectively. The two new siloles also exhibited 29Si
shifts at 9.3 and 7.9 ppm for 1 and 2, respectively; these values
are similar to other reported silole 29Si shifts.16
On occasion, after addition of the ZnCl2(TMEDA) to make
the intermediate dizinc silole (see Scheme 1), the reaction
solution would become viscous and gel-like, at times to the
point of preventing stirring. The cause of this gelation could
not be fully identified. Subjecting the reaction contents to
sonication with a sonicating bath for 10−15 s dispatched the
viscous aspect, and the mixture was then able to be stirred to
complete the reaction. Curiously, whether this gelation
occurred or not did not have a significant impact on the
overall yields observed. Rigorous purification of starting
materials did not reduce the chances of this phenomenon
occurring; however, increasing the time stirred after the
addition of the silane (vide infra) from the standard time of
10 min17b to at least 30 min prior to addition of
ZnCl2(TMEDA) also reduced the chances for gelation
considerably. Notably, reactions using lithium without higher
sodium content (0.5−1.0%) cause the reductive cyclization to
fail, as noted by Morra and Pagenkopf;20 lithium with added
copper stabilizer also seems to give much lower yields.
X-ray Crystal Structures. Crystals of sufficient quality for
the new siloles 1 and 2 were grown by slow evaporation and
slow diffusion, respectively (see the Experimental Section), as
thin needles. Compound 2 refined with three molecules of
methylene chloride per molecule of 2. The structures were
solved and refined to reasonable geometries and residual
electron densities. Table S-1 in the Supporting Information
gives the crystallographic data for compounds 1 and 2.
Figure 2. Molecular structure of 2. Hydrogen atoms and solvent
molecules have been removed for clarity. Thermal ellipsoids are drawn
at the 50% probability level. Selected bond distances (Å), bond angles
(deg), and torsion angles (deg) for 2: Si1−C1 = 1.873(4), Si1−C2 =
1.887(3), C2−C3 = 1.362(4), C3−C3 = 1.500(5), C8−S1 = 1.768(3),
S1−C16 = 1.798(4); C1−Si1−C1 = 108.7(2), C2−Si1−C2 = 92.1(1),
Si1−C2−C3 = 107.4(2), C8−S1−C16 = 103.1(2), C7−S2−C17 =
102.4(2); Si1−C2−C4−C5 = −28.0(4), C2−C3−C10−C11 =
−64.0(4), C9−C8−S2−C17 = −11.5(3).
tetraphenylsiloles and their analogues.21 Given that the cyano
substituent on the 2,5-phenyl groups is para to the silole ring,
the steric influence on the geometry is minimal. The
magnitudes of the phenyl torsion angles for 1 at the 2,5-
positions are 36.5 and 53.7°, while for TPS they are 34.8 and
48.8°.21 The torsion angles at the 2,5-positions of 2, however,
are slightly smaller, with a magnitude of 28°. Counter to an
assumption that 2 would pack to minimize steric strain, the
methylthio substituent meta to the silole ring turns toward the
3,4-phenyl groups. On the basis of steric assumptions alone,
this observation is unexpected. The difference Fourier map
suggests that the hydrogen atoms on the thiomethyl groups are
accurately placed; one of these atoms is oriented toward the
centroid of the 3,4-phenyl rings in a manner that suggests some
The molecular structures of 1 and 2 are shown in Figures 1
and 2, respectively. Compound 1 exhibits a structure that is
very similar to that of the unsubsituted phenyl compound 1,1-
dimethyl-2,3,4,5-tetraphenylsilole (TPS).21 Both compounds
exhibit the propeller-like arrangement characteristic of 2,3,4,5-
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possible C−H···π interactions.22 Compound 2 also has a larger
torsional magnitude of the 3,4-phenyl substituents (64.0°) in
comparison to 1 (56.5°), perhaps to accommodate for the
thiomethyl substituent. These observations, viz., the smaller
2,5-torsional angle and weak intersubstituent interactions, may
help explain some of the photometric data recorded for 2 (vide
infra).
Electronic Transitions. The UV−visible and fluorescence
spectra and quantum yields for the new siloles 1 and 2 were
recorded and are given in Table 1. Also given are the optical
spectral data for known siloles 3 and 4 along with the literature
values.
for 4). Compound 3 exhibits significant bathochromic shifts
from TPS due to the strong π-symmetric donating ability of the
dimethylamino substituent.6 Compound 4 exhibits a red shift in
the absorbance maximum in comparison to TPS, but a
fluorescence blue shift. In the known crystal structure,23 the
torsional angles between the silole core and the adjoining
pyridine moieties in 4 are between 26 and 36°, in the
approximate range of the silole structures presented herein.
Therefore, the extended conjugation brought on by the
bipyridine moiety does not significantly affect the electronics
of the silole, probably due to its own strong tendencies as a π-
accepting unit. The siloles containing 2,5-bis(pyridin-3-yl) and
2,5-bis(pyridin-2-yl) groups17b also had a slight bathochromic
shift in the absorbance spectra and had little effect on the
fluorescence spectrum in comparison to TPS.
Table 1. UV−Visible and Fluorescence Spectral Data for the
a
Reported Compounds
Studies with Metal Salts. All of the reported compounds
were tested with nickel(II), copper(II), and mercury(II)
perchlorate salts. Copper(II) was chosen because of its
paramagnetic nature; typically paramagnetic ions cause
significant quenching, and we desired to determine to what
end this occurred with the siloles reported herein. Nickel(II)
also has some known quenching ability,25 given its ability to
form diamagnetic or paramagnetic complexes. Mercury(II) was
chosen for several reasons. Being an environmentally harmful
ion, the potential for selective and sensitive detection has been
a steady area of research.26 Also, it too is generally seen as an
efficient quencher due to the phenomenon known colloquially
as the “heavy-atom effect”.27 As shown below, these metal ions
have differing abilities to quench (or enhance) fluorescence on
the basis of the 2,5-substituent chosen.
Figure 3 shows the results of each silole upon interaction
with 10 equiv of the metal ion. For clarity, the absorbance and
emission spectra of 3 with the metals were separated. Also
tested were 1, 2, and 5 equiv (see Experimental Section), and
these spectra are shown in the Supporting Information. During
these experiments, the total concentration of silole in the
system was kept constant at 5 × 10−5 M, and the volume of
silole solution (in methylene chloride) to metal solution (in
methanol) was also 2 mL to 2 mL in order to directly compare
the changes on using different metals and different
equivalencies. Equivalencies of the metal solution were adjusted
by changing the concentration of the metal perchlorate in
methanol. In order to get an accurate representation of the
silole in the absence of metal ion, the spectrum of each silole
was measured with 1/1 methylene chloride/methanol to keep
the concentration and overall solvent polarity constant.
Compound 1 showed slight changes overall in the optical
spectra (Figure 3a). The absorbance λmax upon addition of the
metals is essentially unchanged. This suggests that, through the
combination of the slight communication of the cyano
substituent and the general lability of the group with metal
ions, the metal ions studied here do not have significant
contributions to the ground state of the silole. The fluorescence
spectrum with the metal ions showed variable differences, from
none with copper(II) to the most with mercury(II).
Interestingly, the fluorescence features are preserved between
nickel(II) and mercury(II), and there is no quenching with 10
equiv of copper(II). The slight hypsochromic shift in the
fluorescence data along with no discernible changes in the
absorbance spectrum might indicate that some excited-state
decoordination is occurring,28 as the changes in the overall
electronics of the system for this phenomenon generally reflect
the observed data for 1.
λmax, absorbance
λmax, fluorescence
compd
(nm)
log ε
4.12
(nm)
Φf, %
b
1
2
365
482
0.25
c
390
4.29
511
1.58
d
3
415 (423)
4.32 (4.33)
549 (529)
(0.251)
e
e
f
4
382 (386)
3.99
472 (489)
(<0.1)
a
Spectral data from siloles in a 1/1 CH2Cl2/MeOH mixture, 5 × 10−5
b
c
M. With reference to 9,10-diphenylanthracene. With reference to
d
e
fluorescein. Literature values are given in parentheses.17b Literature
values are given in parentheses.19 Literature values are given in
f
parentheses.23
The optical data of 1 indicate that the cyano substituent has a
relatively modest effect on the electronics of the silole; in
comparison to TPS,21 there is a 7 and 12 nm bathochromic
shift in the absorbance and fluorescence λmax values,
respectively. This corroborates with the very similar structural
features of 1 and TPS (vide supra). It appears that the cyano
substituent is not suitably positioned to be an effective π-
withdrawing group (relative to the 2,5-phenyl moieties); the
ability to either be a π donor or acceptor has the most effect on
the electronics of siloles.6 Tang’s group used dicyanovinyl
substitution, allowing the withdrawing cyano groups to interact
more effectively with the π system of the silole; this group
affected significant bathochromic shifts relative to the parent
silole TPS.24 The moderate communication of the cyano group
in 1 will also be addressed in the metal interaction section (vide
infra).
The optical spectra of 2 show further shifts to lower energy
in comparison to 1. In solution, the largest contribution to this
observation is likely the modest donating ability of the lone
pairs of the sulfurs on the methylthio substituents. The related
methoxy substituent, situated para to the silole ring on the 2,5-
phenyl groups, induces a bathochromic shift of 12 nm in the
absorption λmax in comparison to the methyl analogue for this
reason.17b Compound 2 also exhibits a significantly higher
solution quantum yield. While higher steric hindrance from a
methylthio group meta to the silole could help explain this
observation,5 the solid-state arrangement of this substituent,
preferring an orientation that suggests some interaction with
the 3,4-phenyl groups (Figure 1), could also help explain this
increase.
Siloles 3 and 4 both were characterized by UV−vis and
fluorescence spectroscopy and agreed reasonably well with the
literature values. The slight differences in λmax of the absorbance
and fluorescence are most likely from differences in solvent
polarity (literature sources used chloroform17b for 3 and THF19
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Figure 3. Optical spectra of each of the reported siloles and the mixtures with 10 equiv of the metal perchlorate: (a) UV−vis (dashed) and
fluorescence (solid) spectra of 1 with metal solutions; (b) UV−vis (dashed) and fluorescence (solid) spectra of 2 with metal solutions; (c) UV−vis
spectra of 3 with metal solutions; (d) fluorescence spectra of 3 with metal solutions; (e) UV−vis (dashed) and fluorescence (solid) spectra of 4 with
metal solutions. Legend: (red) mercury(II); (blue) copper(II); (green) nickel(II); (black) pure silole. The wavelengths for fluorescence excitation
are 365 nm (1), 390 nm (2), 415 nm (3), and 383 nm (4).
Silole 2 was synthesized to affect some selectivity among the
metals studied. The absorbance spectrum of 2 also exhibits little
change in the presence of the metal ions (Figure 3b), indicating
that the metals involved do not change the electronics of the
silole core significantly. The fluorescence is unaffected by both
nickel(II) and copper(II), but there is significant quenching of
the emission from mercury(II). It should be noted that
quenching was not appreciable on 2 until 10 equiv of
mercury(II) was added (see the Supporting Information).
Complexes containing the bis(methylthio)phenyl moiety with
mercury ions and compounds are known elsewhere,29 but the
reported spectra here suggest that even binding to mercury on
2 is rather labile. Nevertheless, the selectivity of the methylthio
substituents for mercury(II), as predicted by the well-known
Pearson acid−base theory (HSAB theory),30 could make
analogues based on 2 a useful template for selective detection
of this ion.
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Silole 3 has the most significant spectral changes of any in the
sample of siloles studied herein. With all metals studied, there is
a relatively intense absorption peak ca. 85 nm lower in energy
than that for the parent silole (Figure 3c). Notably, a smaller
peak ca. 65 nm higher in energy also exists in all cases. The
local maxima for the silole with each metal do not vary greatly;
this suggests that the new transitions are mainly due to
electronic changes in the silole. Figure 4 shows an example of 3
depends on the solvent’s coordinating ability. From these data,
both bands can be attributed to interaction of 3 with the metal
ion. The reduced intensity, higher energy band (compared to
3) is characteristic of reducing the donating abilities of amino
substituents (a photoinduced charge transfer, or PCT, effect);31
the absorbing peak energy is now much closer to that of TPS,
suggesting deactivation of the amino group due to coordina-
tion. The increased intensity of the lower energy band in
noncoordinating solvents also indicates some manipulation of
the PCT effect, but through interaction with the acceptor (i.e.,
the silole ring) by making the PCT from the amino group to
the silole more efficient, thus increasing the absorption
coefficient.31 Given that the silole ring and accompanying
aromatics of the “acceptor” region will likely coordinate with
ions through arene interactions, the decrease in intensity in
THF can be justified, as this coordination mode can be
considered more labile than amine coordination and will be
much more affected by the competing coordinating ability of
the solvent (for a model of the coordination modes, see Figure
S-35 in the Supporting Information). Studies on the specific
nature of this bonding are ongoing.
The fluorescence spectrum of 3 with the metals is notable in
the differences upon changing the ion used. Every metal ion
studied induces a similar hypsochromic shift in the fluorescence
maximum, suggesting that this change is again due to
coordination to, and deactivation of, the amino donating
group;31 this shift to higher energy corresponds to the initial
higher energy shift of 3 in THF after addition of metal ion
solution (Figure 5). However, the “acceptor-bound” bath-
ochromic shift in the UV−visible spectrum of 3 (Figure 3c)
does not seem to have a correlated shift in the fluorescence
spectrum; this is possibly due to the nature of the intraligand
charge transfer on excitation. While nickel(II) and copper(II)
exhibit a lower intensity than 3, mercury(II) is exceptional in its
significant increase in the fluorescence intensity. Part of this
difference might be due to self-absorption of the fluorescence
by the absorbance band near 500 nm, shown in the UV−visible
spectrum (Figure 3c), which is more intense for copper(II) and
nickel(II). The increase in intensity on addition of mercury(II)
to 3 could be due to oligomeric species, as 3 has two possible
coordinating amino subsitutients. An increase in the quantum
yield on addition of zinc(II) to a terpyridine-substituted silole,
reported by Yin et al., was attributed to oligomer formation that
would reduce intramolecular rotations and thus decrease the
amount of nonradiative decay.15
Figure 4. Compound 3 (5 × 10−5 M in methylene chloride) with 1−
10 equiv of copper(II) perchlorate solution (in methanol).
with various amounts of copper. From this plot the two new
absorption bands seem to appear at the same time. In a
preliminary titration experiment (Figure 5), THF was used for
The UV−visible spectrum of 4 (Figure 3e) follows a trend
similar to that for the other siloles, in that differences among
the metal ions are slight with regard to λmax values. In all cases, a
new band appears at ca. 315 nm. Though this band is higher in
energy than the silole absorbance at 383 nm, it has a significant
increase in intensity, which more closely matches the pure silole
peak at 275 nm. Pyridine-based groups are considered stronger
π-acceptors than siloles,15 and therefore this metal-induced shift
is likely from the higher energy pure silole band. These factors
combined suggest that the metal−silole band is, again, a
consequence of the metal binding to the bipyridine moieties
(for a model of the coordination see Figure S-35 in the
Supporting Information) and making intramolecular PCT more
efficient by increasing the accepting ability of those moieties.31
The fluorescence spectra with each metal show that there is
very significant quenching. The UV−visible spectra indicate
that the silole band completely disappears, and therefore
excitement at the silole peak probably causes a charge transfer
Figure 5. Selected titration data of a solution of 3 in THF (6 × 10−5
M), titrated with 1.3 × 10−2 M mercury(II) perchlorate solution in
THF. The values in parentheses in the legend represent the
equivalents of mercury(II) in solution.
both the silole and metal solutions in order to determine the
effects on coordination with a more coordinating solvent. The
blue-shifted peak in this experiment appears first, with a relative
intensity similar to that before, but the lower energy band
appears after much more metal ion solution has been added, at
a much lower intensity.
Hence, the blue-shifted absorption is considerably less
dependent on the solvent, but the low-energy band greatly
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over 15 min via an addition funnel. This mixture was stirred for 20 min
and cooled to 0 °C. Then, ZnCl2(TMEDA) (2.1 g, 8.3 mmol) was
added as a solid at once, followed by additional dry THF (15 mL). If
gelation occurred (vide supra), the contents were sonicated for several
seconds in a sonicating bath. After the mixture was stirred for 1.5 h, 4-
bromobenzonitrile (0.73 g, 4.0 mmol) and Pd(PPh3)2Cl2 (60 mg,
0.085 mmol) were successively added, and the mixture was refluxed
overnight. The solution was quenched with 1 M HCl. The aqueous
layers were combined and extracted with Et2O (2 × 20 mL), and the
organics were combined, washed with water, sodium bicarbonate
solution, and brine, and dried with MgSO4. The solvents were
removed by rotary evaporation, and the crude mixture was subjected
to silica gel column chromatography (hexanes then hexanes/Et2O, 10/
1) to obtain compound 1 as a yellow powder (0.35 g, 40% yield). Mp:
229−230 °C. X-ray-quality crystals were obtained by slow evaporation
of an Et2O solution of 1 at room temperature. 1H NMR (500 MHz): δ
7.43−7.39 (m, 4H, ArH), 7.09−6.98 (m, 10H, ArH), 6.75−6.73 (m,
4H, ArH), 0.48 (s, 6H, SiMe). 13C{1H} NMR (125 MHz): δ 156.2,
144.9, 141.7, 137.4, 132.1, 129.8, 129.3, 127.9, 127.2, 119.3, 109.3,
−4.0. 29Si{1H} NMR (99 MHz): δ 9.3. Anal. Calcd for C32H24N2Si: C,
82.72; H, 5.21. Found: C, 82.51; H, 5.14.
Preparation of 2,5-Bis[3,4-bis(methylthio)phenyl]-1,1-di-
methyl-3,4-diphenylsilole (2). The compound was prepared by a
procedure similar to that for compound 1 above except for using 1-
bromo-3,4-bis(methylthio)benzene (0.99 g, 4.0 mmol) as the cross-
coupling reagent. The material was subjected to silica gel column
chromatography (hexanes then hexanes/ethyl acetate, 1/2) to obtain
compound 2 as a yellow powder (0.58 g, 51% yield). Mp: 187−189
°C. X-ray-quality crystals were grown from a slow diffusion of
methanol into a CH2Cl2 solution of 2. 1H NMR (500 MHz): δ 7.05−
7.03 (m, 8H, ArH), 6.86 (dd, J = 6.86, 1.85 Hz, 2H, ArH), 6.84−6.81
(m, 4H, ArH), 6.64 (d, J = 1.85 Hz, 2H, ArH), 2.44 (s, 6H, SMe), 1.97
(s, 6H, SMe), 0.51 (s, 6H, SiMe). 13C{1H} NMR (125 MHz): δ 154.4,
140.6, 139.2, 137.4, 137.2, 134.0, 130.1, 128.1, 126.67, 126.64, 126.60,
126.58, 16.5, 15.7, −3.1. 29Si{1H} NMR (99 MHz): δ 7.9. Anal. Calcd
for C34H34S4Si: C, 68.18; H, 5.72. Found: C, 68.10; H, 5.67.
to the bipyridine moiety such that fluorescence is attenuated.
Bipyridine is a chelating ligand, and so coordination to each ion
with 4 is very efficient; the fluorescence quenchings for
copper(II) and mercury(II) are very similar even at 2 equiv
(see the Supporting Information), with nickel(II) exhibiting
less quenching. The exception for nickel(II) is most likely due
to its relatively slow ligand exchange kinetics, as ligand
exchange of the hexakis(methanol) complex of nickel(II) is
approximately 4 orders of magnitude slower than for the
copper(II) complex.32
In conclusion, two new siloles along with two known siloles
were prepared according to the standard Tamao reductive
cyclization procedure. The substituents at the 2,5-positions
were chosen for the potential to coordinate with metal ions,
and the siloles were initially studied for interactions using
nickel(II), copper(II), and mercury(II) perchlorate salts. While
siloles were affected to various degrees depending on the metal,
most changes in the spectra were consistent between metals,
but their intensity varied, sometimes significantly. Compound 1
showed slight changes to its fluorescence on addition of the
metal perchlorate solutions. Compound 2 differed in that
mercury(II) selectively began to quench fluorescence. Com-
pound 3 exhibited significant changes in both the absorption
and emission spectra, and intensity of fluorescence differed
significantly between the various metal ions. Compound 4
displayed significant quenching with all metal ions studied, and
the absorbance suggested that contributions from the silole ring
are essentially removed. Studies on the nature of changes
observed for these siloles are in progress, along with expanding
the metal centers to observe whether the changes observed
continue to occur.
EXPERIMENTAL SECTION
Preparation of 1,1-Dimethyl-2,5-bis[p-(N,N-dimethylamino)-
phenyl]-3,4-diphenylsilole (3).17b The compound was prepared by
a procedure similar to that for compound 1 above except for using 4-
bromo-N,N-dimethylaniline (0.80 g, 4.0 mmol) as the cross-coupling
reagent. The reaction mixture was quenched with 0.1 M NaOH and
extracted with CH2Cl2 (2 × 30 mL), and the organics were combined
and washed with water and brine and finally dried over magnesium
sulfate. After filtration of the solution through a silica plug, the solvents
were removed and the crude mixture was triturated with Et2O to yield
■
General Procedures. Reactions were carried out under an argon
atmosphere using standard Schlenk techniques with solvents dried and
purified by standard methods, unless otherwise notified. The
compounds 4-bromo-N,N-dimethylaniline, 4-bromobenzonitrile, n-
butyllithium (2.5 M in hexanes), 6-bromo-2,2′-bipyridine, nickel(II)
perchlorate hexahydrate, copper(II) perchlorate hexahydrate, and
mercury(II) perchlorate hexahydrate were purchased from Aldrich
Chemical Co. and used as received. Dichlorodimethylsilane was
purchased from Aldrich Chemical Co. and purified by distillation over
K2CO3 under an inert atmosphere prior to use. Phenylacetylene was
purchased from GFS Chemicals, Inc., and used as received.
Chloroform-d was purchased from Cambridge Isotopes, Inc., and
dried over activated sieves prior to use. The following compounds
were prepared according to the literature: dimethylbis(phenylethynyl)-
silane,2 0 1-bromo-3,4-bis(methylthio)benzene,1 8 bis-
(triphenylphosphine)palladium(II) chloride,33 and zinc(II) chloride−
1
compound 3 as a bright yellow powder (0.50 g, 53% yield). H NMR
(500 MHz): δ 7.06−7.01 (m, 6H, ArH), 6.89−6.81 (m, 8H, ArH),
6.52−6.48 (m, 4H, ArH), 2.89 (s, 12 H, NMe), 0.51 (s, 6H, SiMe).
13C{1H} NMR (125 MHz): δ 151.8, 148.4, 140.5, 139.2, 130.3, 130.2,
128.0, 127.7, 125.9, 112.1, 40.5, −2.6.
Preparation of 1,1-Dimethyl-2,5-bis(2,2′-bipyridin-6-yl)-3,4-
diphenylsilole (4).19 The compound was prepared by a procedure
similar to that for compound 1 above except for using 6-bromo-2,2′-
bipyridine (0.47 g, 2.0 mmol) as the cross-coupling reagent and a
smaller scale (1.0 mmol in dimethylbis(phenylethynyl)silane). The
solvents of the reaction mixture were evaporated, and the crude
material was subjected to silica gel column chromatography (hexanes/
ethyl acetate 9/1). The eluted product was crystallized by slow
evaporation from the eluting solvents to isolate compound 4 as a
yellow crystalline material (0.22 g, 39%). 1H NMR (500 MHz): δ 8.67
(ddd, J = 4.65, 1.83, 0.91 Hz, 2H, pyH), 8.42 (dt, J = 7.85, 1.36 Hz,
2H, pyH), 8.10 (dd, J = 7.85, 0.91 Hz, 2H, pyH), 7.85 (td, J = 7.85,
1.83 Hz, 2H, pyH), 7.39 (t, J = 7.85 Hz, 2H, pyH), 7.30 (ddd, J = 7.45,
4.67, 0.91 Hz, 2H, pyH), 7.17−7.14 (m, 6H, ArH), 7.00−6.97 (m, 4H,
ArH), 6.58 (dd, J = 8.11, 0.86, 2H, ArH), 0.80 (s, 6H, SiMe). 13C{1H}
NMR (125 MHz): δ 157.7, 156.9, 156.1, 155.4, 149.2, 144.3, 139.8,
137.0, 136.5, 129.4, 128.4, 126.9, 123.6, 123.2, 121.2, 118.1, −2.2.
Spectroscopy Studies with Metal Ions. Solutions of com-
pounds 1−4 were prepared at a concentration of 1.0 × 10−4 M in
spectrophotometric-grade methylene chloride. The metal perchlorate
TMEDA.34 The H, 13C{1H}, and 29Si{1H} NMR (DEPT) spectra
1
were recorded on a Bruker ARX-500 MHz spectrometer in CDCl3.
Proton and carbon NMR shifts were referenced internally to residual
peaks of the solvent used. Silicon NMR shifts were referenced
externally to SiMe4. UV−visible spectra were recorded on a Cary 50
Bio UV−visible spectrometer, and fluorescence spectra were recorded
on a Cary Eclipse fluorescence spectrometer. X-ray structural
determinations were performed using a Bruker Apex II diffractometer
equipped with a CCD area detector. Elemental analyses of the
compounds were obtained from Atlantic Microlabs, Inc., Norcross,
GA.
Preparation of 2,5-Bis(p-cyanophenyl)-1,1-dimethyl-3,4-di-
phenylphenylsilole (1). Lithium (55 mg, 7.9 mmol), naphthalene
(1.1 g, 8.6 mmol), and dry THF (12 mL) were combined and
sonicated20 for 2 h to form a deep green solution of lithium
naphthalenide. A solution of dimethylbis(phenylethynyl)silane (0.50 g,
1.9 mmol) in dry THF (8 mL) was added dropwise to the solution
1910
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Article
solutions were prepared in the following concentrations (equivalents)
in spectrophotometric-grade methanol: 1.0 × 10−4 M (1 equiv), 2.0 ×
10−4 M (2 equiv), 5.0 × 10−4 M (5 equiv), 1.0 × 10−3 M (10 equiv).
The solutions were not degassed. To 2 mL of the silole solution in a
quartz cuvette was added 2 mL of the above metal perchlorate
solutions, and the solution was then analyzed by UV−visible and
fluorescence spectroscopy.
ACKNOWLEDGMENTS
■
We thank Dr. Nigam P. Rath for his generous assistance in
analyzing the crystallographic data and models. Funding from
the National Science Foundation (MRI, CHE-0420497) for the
Apex-II diffractometer is acknowledged and appreciated.
X-ray Structure Determination of 1 and 2. Crystals of X-ray
diffraction quality were grown by slow evaporation of an ether solution
of 1 or by slow diffusion of methanol into a methylene chloride
solution of 2. Crystals with appropriate dimensions were mounted on
a glass capillary in a random orientation. Preliminary examination and
data collection were performed using a Bruker Kappa Apex II charge
coupled device (CCD) detector system single-crystal X-ray diffrac-
tometer using an Oxford Cryostream LT device. Data were collected
using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å)
from a fine focus sealed X-ray tube source. Preliminary unit cell
constants were determined using a set of 36 narrow frame scans. Data
sets consist of combinations of φ scan frames with a typical scan width
of 0.5° and an exposure time of 15 s/frame at a crystal to detector
distance of 4.0 cm. The collected frames were integrated using an
orientation matrix determined from the narrow frame scans. Apex II
and SAINT software packages were used for data collection and
integration. Final cell constants were determined through global
refinement of reflections from the complete data set. Systematic errors
in the data were corrected using SADABS, on the basis of the Laue
symmetry using equivalent reflections. Crystal data and intensity
parameters are given in Table S-1 in the Supporting Information.
Structure solutions for 1 and 2 were carried out using SHELXS, and
refinement was performed against the reduced data using SHELXL,35
using the OLEX2 graphical interface software.36 The structures were
solved as orthorhombic unit cells by direct methods (1, Pbca; 2, Pbcn)
and refined with full matrix least-squares refinement by minimizing
∑w(Fo2 − Fc2)2. All non-hydrogen atoms were refined to convergence.
Compound 2 crystallized with three methylene chloride solvent
molecules per molecule. One chlorine atom on one solvent molecule
was disordered; this was addressed by refinement of the chlorine
molecules over two positions at a ratio of 93/7 of their partial
occupancies. Another methylene chloride molecule has a chlorine
atom that was symmetrically generated, and the special position
constraints were suppressed; the molecule also has a partial occupancy
of 0.5. EADP constraints were employed on the chlorine atoms. The
silicon atom in 2 is on a special position. All hydrogen atoms were
added in the calculated positions and were refined using appropriate
riding models. The models were refined to convergence to the final
residual values of R1 = 4.5% and wR2 = 12.1% for 1 and R1 = 6.1% and
wR2 = 14.3% for 2. Complete listings of geometrical parameters,
positional and isotropic displacement coefficients for hydrogen atoms,
and anisotropic displacement coefficients for the non-hydrogen atoms
are submitted as Supporting Information within the associated CIF
files.
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ASSOCIATED CONTENT
* Supporting Information
■
S
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AUTHOR INFORMATION
Corresponding Author
*J.B.-W.: e-mail, wilkingj@umsl.edu; tel, 1-314-516-6436; fax,
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