Silicon compounds of 1,1-bis(pyrrol-2-yl)ethenes: Molecular structures and chemical and spectroscopic properties

Article abstract of DOI:10.1021/om400868w

The first examples of silicon compounds of 1,1-bis(pyrrol-2-yl)ethenes have been synthesized via salt metathesis from a 2-fold lithiated dipyrromethene and different diorganodichlorosilanes (i.e., dimethyldichlorosilane, diphenyldichlorosilane, and 1,1-dichlorosilacyclobutane). Herein we report on their molecular structures, their optical properties, and some reactivity patterns.

Full text of DOI:10.1021/om400868w

Article
pubs.acs.org/Organometallics
Silicon Compounds of 1,1-Bis(pyrrol-2-yl)ethenes: Molecular
Structures and Chemical and Spectroscopic Properties
Alexander Kampfe, Edwin Kroke, and Jorg Wagler*
̈
̈
Institut fur Anorganische Chemie, Technische Universitat Bergakademie Freiberg, D-09596 Freiberg, Germany
̈
̈
S
* Supporting Information
ABSTRACT: The first examples of silicon compounds of 1,1-
bis(pyrrol-2-yl)ethenes have been synthesized via salt meta-
thesis from a 2-fold lithiated dipyrromethene and different
diorganodichlorosilanes (i.e., dimethyldichlorosilane, diphenyl-
dichlorosilane, and 1,1-dichlorosilacyclobutane). Herein we
report on their molecular structures, their optical properties,
and some reactivity patterns.
ince the discovery of the excellent photochemical proper-
In 2011 the notable lack of silicon analogues was closed with
Sties of boron-containing dipyrromethene compounds an ONNO-functionalized ligand which had also been used for
the preparation of an Al complex before.15 The extension of the
(Chart 1), such derivatives have been widely explored in the
interim. These so-called BODIPYs1 (boron dipyrromethenes)
classical dipyrrin with adjacent hydroxy anchors promoted
complexation of the Si atom (Chart 1).
have found diverse applications such as protein markers,
selective ion sensors, and solar cell sensitizers.2−4 Moreover, the
BODIPY moiety has also been introduced into a chelating
triphosphane for transition-metal complexation while sustaining
its fluorescence properties.5
Following an analogous reaction protocol with the same
organochlorosilanes and a structurally related oxygen-free
triphenyldipyrromethene, Nabeshima et al.15 only recovered
the unreacted starting material. This hints at certain difficulties
in dipyrromethene silicon complexation. With the confidence
that silicon should nevertheless be capable of accepting
dipyrromethene ligands in its coordination sphere, we
successfully addressed this issue with an alternative synthesis
route (including lithiation of the ligand) and could overcome
preliminary recoils. Unexpectedly, the dipyrromethene ligand
used for our investigations gave rise to a 2-fold deprotonated
1,1-bis(pyrrol-2-yl)ethene, the motif of which is retained in the
resulting silicon compounds. Therefore, we now report on the
first examples of silicon 1,1-bis(pyrrol-2-yl)ethene compounds
in detail.
Chart 1
RESULTS AND DISCUSSION
■
As starting materials we have chosen the easily accessible
diethylpentamethyldipyrromethene hydrochloride derivative 1
(Scheme 1),16 keeping in mind the difficulties reported earlier
with meso-phenyl-substituted compounds.15 Despite their wide
exploration as BF2 complexes, compounds such as 1 and the
analogous perchlorate salt 1′ still lack crystallographic
characterization. We obtained 1 as a bright red powder and
were able to convert it into the hydroperchlorate 1′, which
formed large crystals suitable for X-ray diffraction analysis
(Figure 1). The crystal structure of 1, which comprises the
same cation, is given in the Supporting Information.
In addition to boron as the most frequently explored central
element of dipyrromethene chelates there have also been a
considerable number of compounds reported in literature
which involve transition metals (e.g., Co,6 Ni,7 and Cu8) and
Zn9,10 coordinated by this bidentate ligand system.
In terms of group 13 elements beyond boron the central
elements aluminum,11 gallium,12 and indium12a,b have been
successfully introduced into this class of compounds, and
dipyrromethene compounds with group 14 elements (germa-
nium13 and tin13,14) have been synthesized as well (Chart 1).
The free base, the 1,1-bis(pyrrol-2-yl)ethene compound 2,
was obtained by treatment of 1 with triethylamine (Et3N), as
Received: August 29, 2013
Published: December 23, 2013
© 2013 American Chemical Society
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Scheme 1
Table 1. Selected Bond Lengths (Å) and Torsion Angles
a
(deg) of 2′ and Related Parameters from 217
2
2′
C2−C3
C1−C2
C2−C4
N1−C1−C2−C3
N2−C4−C2−C3
1.512(5)
1.415(4)
1.415(4)
179.1(3)
179.1(3)
1.338(3)
1.471(3)
1.477(3)
32.8(3)
1.346(3)
1.458(3)
1.473(3)
32.6(3)
133.6(2)
144.4(2)
a
2 contains two independent molecules in the crystallographic
asymmetric unit.
the two tautomers. Apparently, in solution they coexist in
equilibrium, as in both cases they were obtained from similar
solvents but at different temperatures. However, variable-
temperature NMR studies of 2 in deuterated [D14]n-hexane
provided no hints of the presence of any equilibrium.
Unexpectedly, only a set of signals characteristic of 2 was
detected in solution in a temperature range from −55 to +18
°C.
At this point we applied computational methods18 to address
this issue. The molecular structures of 2 and 2′ were optimized
as gas-phase structures of isolated molecules at the DFT
(MPW1PW91 6-311G(d,p)) level, thus revealing an energy
difference between both tautomers of only 0.96 kcal/mol.
Single-point energy calculations of the optimized structures
with alternative methods and basis sets are in support of the
small energetic differences between both tautomers (Table 2).
Although 2′ is predicted to represent the more stable tautomer
in each case, these energy differences point at the coexistence of
both tautomeric forms in solution.
Figure 1. Molecular structure of 1′ in the crystal state. The ellipsoid
probability level is set at 50%, and carbon-bound hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and torsion angles
(deg): C2−C3 1.507(2); N1−C1−C2−C3 24.7(2), N2−C4−C2−C3
21.0(2). Note that simple atomic labels have been chosen for clarity;
they do not correspond to the labels in the CIF file in the Supporting
Information.
Table 2. Calculated Energy Differences between 2 and 2′
a
method
ΔE (kcal/mol)
MPW1PW91
0.96
2.09
3.53
1.23
MP2 6-31G(d)
confirmed by NMR spectroscopy (in accord with data reported
earlier by Thompson et al.,17 who also reported the crystal
structure which corresponds to 2 as drawn in Scheme 1).
Thompson et al.17 had obtained crystals of 2 by slow
evaporation of a pentane solution. Upon cooling a saturated
solution of 2 in hexane, we also obtained crystals which, to our
surprise, were revealed to consist of the alternative tautomer,
the dipyrromethene 2′ (Figure 2).
B3LYP 6-31G(d,p)
B3LYP 6-311G+(d,p)
a
ΔE = E(2) − E(2′).
Comparison of the calculated absorption spectra of 2 and 2′
(DFT TD-SCF, B3LYP 6-311G*(d,p)) with the experimental
spectrum reveals that both 2 and 2′ should exhibit two
absorption bands in the range 250−500 nm, but the
experimental spectrum shows three bands, which can be
interpreted as the superposition of the spectra of 2 and 2′
(Figure 3). Therefore, we assume that in solution tautomers 2
and 2′ both coexist in a rapid exchange equilibrium, thus giving
rise to the recorded absorption spectrum as a sum of the
spectra of both forms, even though we could not detect 2′ by
means of NMR spectroscopy.
For comparison Table 1 gives selected structural features of 2
and 2′, which clearly illustrate structural differences between
In a first attempt at synthesizing a silicon complex out of the
dipyrromethene ligand 2′, we confirmed the earlier finding of
Nabeshima et al.:15 i.e., upon mixing of 2′ with SiCl4 in the
presence of Et3N we found the starting materials unreacted.
Therefore, to enhance reactivity, we used nBuLi to deprotonate
the ligand. Even though we were aiming at the monolithiated
dipyrromethene-like ligand (examples had been reported in the
literature by Thompson19), only the dilithiated 1,1-bis(pyrrol-2-
yl)ethene 3 was isolated despite the 1:1 stoichiometry of 2′ and
nBuLi used: i.e., compound 3 formed while 50% of the starting
Figure 2. Molecular structure of 2′ in the crystal state. The ellipsoid
probability is set at 50%. The C2−C3 bond is situated on a
crystallographically imposed 2-fold axis. Therefore, the N and NH sites
are disordered in a 50:50 ratio by symmetry. Only one N···NH
combination is depicted. Note that simple atomic labels have been
chosen for clarity; they do not correspond to the labels in the CIF file
in the Supporting Information.
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significantly longer than in the related structures of N-
indolyllithium THF solvate (2.57 Å) and N-carbazolyllithium
THF solvate (2.66 Å).20 The Li−N bond lengths vary in a wide
range (1.98−2.30 Å). In contrast to the structure of 2, both
pyrrole nitrogen atoms point to the same side of the molecule,
thus resembling a preorganization of the ligand arrangement
suitable for formation of silicon complexes.
In further pursuits, compound 3 reacted with a variety of
diorganodichlorosilanes with LiCl elimination to furnish the
organosilicon compounds 4a−c (Scheme 3). These were
Scheme 3
Figure 3. Comparison of absorption spectra recorded for the reaction
product 2′ and calculated for the different tautomers 2 and 2′ (using
DFT TD-SCF B3LYP 6-311G*(d,p)).
Scheme 2
obtained as deep yellow oils and could be recrystallized from
hexane, thus yielding crystals suitable for single-crystal X-ray
diffraction analysis (Figure 5 and Table 3).
In general, the coordination spheres around the silicon atoms
are close to tetrahedral; only the C5−Si1−C6 angle in the
silacyclobutane ring of 4c is noticeably smaller due to steric
constraints associated with the four-membered ring. The sum
of angles about C2 is close to or equal to 360° (>359° in all
cases), and the six-membered rings Si1N1C1C2C4N2 exhibit
boat conformations. For example, in compound 4a the
deviations from the least-squares plane of this six-membered
ring are 0.208(1) and 0.231(1) Å in one direction for Si1 and
C2, respectively, whereas the other four atoms are displaced
into the opposite direction to similar extents (0.112(1),
0.108(1), 0.075(1), and 0.144(1) Å for N1, C1, C4, and N2,
respectively). As a result of this boat conformation in
compounds 4a−c, the angles between the least-squares planes
of the pyrrole rings (α) are similar. Nonetheless, one can still
find differences between the angles α and one might expect
closer analogy between 4a and 4b, since both silicon centers
carry acyclic substituents. Thus, differences in their molecular
structures hint at some degree of flexibility of this kind of ligand
system, which might be of interest in terms of optical
properties, especially fluorescence, which is most important
for applications of BODIPY derivatives. A planar conjugated
system is regarded to enhance the efficiency of fluorescence
processes.15
Figure 4. Molecular structure of 3·4THF in the crystal state. The
ellipsoid probability level is set at 50%. Most hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and torsion angles
(deg): Li1−N1 1.976(2), Li1−N2 2.068(2), Li2−N1 2.296(2), Li2−
N2 2.035(2); N1−C1−C2−C3 140.7(1), N2−C4−C2−C3 173.7(1).
Note that simple atomic labels have been chosen for clarity; they do
not correspond to the labels in the CIF file in the Supporting
Information.
material 2′ remained unreacted (Scheme 2). Compound 3
precipitates almost quantitatively from hexane solution as a
bright red and highly air and moisture sensitive powder and can
be recrystallized from THF, yielding the corresponding THF
solvate 3·4THF. The molecular structure of the latter was
confirmed by single-crystal X-ray diffraction analysis (Figure 4).
The lithium atoms in the solvate 3·4THF, which each exhibit
a tetrahedral coordination sphere, bridge the two pyrrolide
nitrogen atoms and both contain two molecules of THF in the
coordination sphere. The Li1···Li2 separation (2.803(3) Å) is
Optical Characterization. Figure 6 shows the absorption
spectra of 2 and 4a−c (see also Table 4). All of these
compounds exhibit significant absorbances between 400 and
500 nm, thus appearing yellow to the eye. In the UV region the
absorbance of 4b distinctly exceeds the others at around 220
nm due to π → π* transitions of its phenyl substituents.
As Nabeshima et al.15 observed fluorescence of their
SIDIPYs, which comprise the “regular” dipyrromethene
backbone (analogously to BODIPYs which are best known
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Figure 6. Absorption spectra of 2 and 4a−c (0.05 mM solutions in n-
hexane, recorded at room temperature, d = 10 mm, quartz cuvette).
Table 4. Absorption Wavelengths λmax (nm) and Extinction
Coefficients ε (L mol−1 cm−1) of Compounds 2 and 4a−c
(Figure 6)
λmax
455
ε
2/2′
4a
5133
297
466
14069
819
302
8546
4b
503/410
302
1275/1286
10649
224
4c
502
Figure 5. Molecular structures of (from top) compounds 4a−c in the
crystal state. 4a crystallized as the hexane solvate 4a·0.5(n-hexane); the
solvent molecule is omitted for clarity. For 4c only one of the two
crystallographically independent molecules from the modification of
4c in space group P21 is shown. The ellipsoid probability level is set at
50%. Note that simple atomic labels have been chosen for clarity; they
do not correspond to the labels in the CIF files in the Supporting
Information.
304
11753
4b the fluorescence quantum yield Φ4b = 0.011 (λex 350 nm)
was determined by applying a comparative method22 using
anthracene as reference (Φanthracene = 0.27).23 Hence, the
fluorescence of 4b is rather poor with respect to what is known
for BODIPYs where Φ might reach high values such as 0.95 for
1,3,5,7,8-pentamethyldipyrromethene boron difluoride.24
a
Table 3. Selected Angles (deg) of Compounds 4a−c and 3
In order to elucidate the essential differences between the
electronic transitions in compounds 4a,b, we have optimized
their gas-phase molecular structures and performed TD-DFT
calculations (see the Supporting Information). For the SiMe2
compound 4a we found the UV/vis absorptions around 300
nm (Figure 6) to originate from HOMO → LUMO and
HOMO-1 → LUMO transitions. Whereas HOMO and
HOMO-1 are predominantly composed of contributions of
the pyrrole π systems, LUMO represents the π* orbital of the
ethene CC bond; thus, the UV/vis absorption of 4a at ca.
300 nm reflects electron transfer from the conjugated 1,1-
bis(pyrrol-2-yl)ethene system into the ethene π* level. In sharp
contrast, the UV/vis absorptions observed for compound 4b in
the range 300−370 nm (see Figure 6), which give rise to the
luminescence of this compound (see Figure 7), originate from
transitions from HOMO and HOMO-1 into the orbitals
LUMO+1/+2/+3/+4. Whereas HOMO and HOMO-1 of
compound 4b resemble the same orbitals as in 4a, the orbitals
LUMO+1/+2/+3/+4 of 4b are predominantly composed of π*
interactions within the SiPh2 phenyl groups; thus, the UV/vis
absorptions of 4b at ca. 300−370 nm reflect electron transfer
from the conjugated 1,1-bis(pyrrol-2-yl)ethene system into the
phenyl π* level. Therefore, our calculations confirm that the
4a
4b
4c
3
N1−Si1−N2
C5−Si1−C6
α
100.6(1)
111.4(1)
27.1
101.2(1)
113.8(1)
19.5
101.1(1)−102.2(1)
80.7(1)−82.0(1)
18.7−29.1 (av 22.6)
46.5
a
α is the angle between the least-squares planes of the pyrrole rings
relative to one another.
for this property), we expected our compounds to exhibit
similar optical features. Surprisingly, for compounds 4a,c as well
as for 2 we could not observe any significant fluorescence, as
shown by their 3D fluorescence spectra (Figure 7). This might
originate from the cross-conjugated 1,1-bis(pyrrol-2-yl)ethene
π system. In this context we were then surprised to observe
fluorescence for 4b with absorption and emission maxima at
350 and 534 nm, respectively. We thus conclude that this
fluorescence is predominantly due to the phenyl substituents at
the silicon center, which seems reasonable since other “simple”
diphenylsilicon compounds can also exhibit fluorescence (for
example, the emission spectrum of 1,1-dimethyl-3,3,5,5-
tetraphenylcyclotrisiloxane shows absorption and emission
maxima at 388 and 459 nm, respectively).21 For compound
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Figure 7. Fluorescence 3D scan spectra of (a) 2, (b) 4a, (c) 4b, and (d) 4c (0.05 mM solutions in hexane, recorded at room temperature, 10 × 10
mm quartz cuvette). Please note the different scaling for 4b. A linear scale was applied in each case.
SiPh2 moiety alters the nature of electronic transitions in
compounds 4.
Scheme 4
For a comparison of electronic transitions in dipyrromethene
vs 1,1-bis(pyrrol-2-yl)ethene systems, the model compound
LHSiMe2 (a 4a analogue which lacks the alkyl substituents at
the pyrrole rings) and the related dipyrromethene HLHSiMe2F
were analyzed by TD-DFT. In general, LHSiMe2 exhibits the
same electronic transitions as 4a (slightly blue shifted), whereas
HLHSiMe2F exhibits transitions from HOMO/-1/-2 to LUMO,
all orbitals of which are composed of π contributions of the
dipyrromethene backbone. Whereas the dipyrromethene back-
bone is rather rigid (as a prerequisite for efficient fluorescence),
relaxation of the excited 1,1-bis(pyrrol-2-yl)ethene system
(calculated by optimization of the triplet state molecule)
involves 90° torsion of the CCH2 group with formation of a
SOMO which is no longer perpendicular to the idealized plane
of the ligand backbone’s π-system. Thus, we attribute the
nonfluorescence of compound 4a to this kind of mobility of the
ligand backbone upon electronic excitation.
dimethylbis(p-nitrophenoxy)silane (5) (Scheme 5), which was
obtained as an almost colorless crystalline material suitable for
Scheme 5
Reactivity. It has been demonstrated for silicon enamine
complexes that addition of Brønsted acids can convert formally
covalent Si−N bonds into dative Si←N bonds and enhance the
Si coordination number by Si−X bond formation25 (Scheme
425a). Since the molecular features of 1,1-bis(pyrrol-2-yl)ethene
compounds 4 offer prerequisites for an analogous approach, we
tried to convert compounds 4a−c in the manner shown in
Scheme 4: i.e., reacting them with a HX equivalent to achieve
addition of X− to the silicon center and H+ to the CH2
moiety, a sequence which should result in the formation of
silicon complexes with a “regular” dipyrromethene backbone
(as in BODIPYs).
For convenience, as an easy to dispense form of an HX
derivative we have chosen p-nitrophenol, which was intended
to add to compound 4a with formation of a pentacoordinate
silicon complex. However, despite conduction of the reaction in
a 1:1 stoichiometric ratio, this reaction led to the formation of
X-ray diffraction (its structure is given in the Supporting
Information). Therefore, to rule out the acidity of p-
nitrophenol as the source of failure (due to complete
displacement of the dipyrromethene ligand), we have chosen
the less acidic p-tert-butylphenol for further reactions (which
were carried out as NMR scale experiments). Despite these
considerations of the phenol acidity, we not only observed the
formation of multiple compounds but also 29Si NMR spectra
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were lacking signals in a typical range for pentacoordinate
silicon compounds. Hence, the reaction did not yield products
as expected from Scheme 4 (bottom). Furthermore, in each
case also the starting material (4a−c) could be detected in the
resulting mixture.
Scheme 7
Obviously, as a result of the deficit of Lewis acidity induced
by the electron-rich dipyrromethene ligand, the silicon center
refused to accept a fifth donor atom, thus sustaining
tetracoordination. Therefore, only the most Lewis acidic Si
center among compounds 4 (in 4c, due to the ring strain and
lower steric demand) was subjected to another reaction which
could enhance the coordination number of its Si atom. (The
spatial accessibility of the Si atoms of compounds 4 for
nucleophiles is illustrated by the space-filling models in Figure
8.)
Figure 8. Space-filling plots of 4a−c. Color code: silicon, pink;
nitrogen, blue; carbon, gray; hydrogen, white. Due to the different
steric demands of the substituents, the silicon center of 4c exhibits the
best accessibility for nucleophiles.
Earlier, it was demonstrated that silacyclobutane derivatives
support penta- and hexacoordination of the silicon atom.26,27 In
general, the acceptance of one or two additional donors D
could release the ring strain of silacyclobutanes, as the newly
formed geometries bear 90° angles (Scheme 6).
Scheme 6
Figure 9. Molecular structure of 6c′ in the crystal state (one of the
three independent molecules of the asymmetric unit). The ellipsoid
probability level is set at 30%. Hydrogen atoms are omitted for clarity.
Note that simple atomic labels have been chosen for clarity; they do
not correspond to the labels in the CIF file in the Supporting
Information.
To attain such a geometry, we applied the acac analogue 2-
hydroxy-4-methoxybenzophenone in a reaction with 4c to
obtain 6c including either a pentacoordinate (6c5) or a
hexacoordinate (6c6) silicon center (Scheme 7). Once again,
this reaction gave rise to a product other than that initially
expected, as revealed by a single-crystal X-ray diffraction study
(Figure 9). The isolated compound 6c′ resulted from a ring-
opening reaction of the silacyclobutane and its attack of the
dipyrromethene moiety to furnish a new six-membered ring.
The 2-oxy-4-methoxybenzophenone ligand only acts as a
monodentate donor, thus leaving the silicon center tetracoordi-
nated. Nevertheless, the intended proton shift to the CH2
moiety of 4c has taken place.
Scheme 8
Ring-opening reactions of silacycloalkanes bearing a higher
coordinate silicon atom have already been reported (Scheme
8).26 Corresponding examples also comprise the reclosure of
the opened ring in an extended manner together with lowering
of the silicon coordination number. The relevant cases
illustrated by some examples in Scheme 8 also have the
(pseudo)imine carbon atom with its partially positive character
as the atom in common, where ring reclosure takes place.
Again, in compound 6c′ the former pseudo imine carbon atom
of the dipyrromethene system (which formally appears in
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The sample 2 derived from 1 (0.10 g, 0.32 mmol) and triethylamine
(0.06 g, 0.6 mmol) was recrystallized from the minimum amount of
hexane (0.15 mL) and brought to crystallization upon storage at 5 °C
overnight.
compounds 6c) has been the target of the alkyl shift. Even
though we were not able to isolate or detect an intermediate
with a higher coordinate silicon atom, the reaction might
proceed via 6c6 or 6c5 as a transient stage. Inspired by this
partial success in the reaction of compound 4c with 2-hydroxy-
4-methoxybenzophenone, we monitored the reaction of a 1:1
stoichiometric solution of dimethylsilicon compound 4a with 2-
Synthesis of 3. Excess triethylamine (0.40 g, 3.9 mmol) was added
to a stirred suspension of 1 (0.61 g, 2.0 mmol) in hexane (20 mL), and
the mixture was stirred at 45 °C for 1 h to afford a yellow solution of 2
and a pale precipitate of Et3NHCl. After it was cooled to room
temperature, the mixture was filtered and the residue was washed with
hexane (10 mL). From the combined filtrate and washings the volatiles
were evaporated under reduced pressure (condensation into a cold
trap) and the resulting brownish yellow oil was dissolved in hexane (20
mL). To this solution was added dropwise a solution of nBuLi in
hexanes (1.65 mL of a 2.5 M nBuLi solution, 4.1 mmol) at room
temperature, whereupon a bright red precipitate was formed which,
after stirring for an additional 6 h, was collected on a Schlenk sinter
tube and washed with pentane (5 mL). The deep red solid was dried
in vacuo and stored under dry argon, as it appeared to be extremely
sensitive to air and moisture. Yield: 0.56 g (2.0 mmol, quantitative).
1H and 13C NMR spectra indicate the absence of solvent (hexane).
Crystals suitable for X-ray diffraction analysis were obtained upon
reflux extraction with THF.
1
hydroxy-4-methoxybenzophenone by H, 13C, and 29Si NMR
spectroscopy, because a related methyl shift (from Si to NC)
is less likely. Unfortunately, this attempt also failed to produce a
silicon dipyrromethene compound. Instead, we observed a
reaction related to Scheme 5 (i.e., formation of Me2Si(2-oxy-4-
methoxybenzophenone)2).
CONCLUSION
■
In hexane solution 1,1-bis(pyrrol-2-yl)ethene 2 was found to
coexist with its dipyrromethene tautomer 2′. Reaction with
nBuLi, even in a 1:1 stoichiometric ratio, caused 2-fold
lithiation and thus transformation into the more reactive
lithiated 1,1-bis(pyrrol-2-yl)ethene derivative 3, which was
suitable for synthesizing diorganosilicon compounds 4a−c.
Optical characterization of 4a−c showed intense absorbance as
for the related BODIPY dyes but significantly different
fluorescence properties. From reactions of 4a−c with selected
phenolates, thus aiming at the formation of silicon compounds
comprising a higher coordinate silicon atom and a dipyrrome-
thene ligand backbone, the expected products could not be
isolated. Reaction of 4c with an acac-like ligand led to ring
opening of the silacyclobutane ring and formation of the
polycyclic product 6c′.
1H NMR ([D8]THF, 500 MHz): δ 0.98 (t, 6 H, 7.4 Hz, CH2CH3),
2.16 (s, 6 H, pyrrole CH3), 2.17 (s, 6 H, pyrrole CH3), 2.39 (q, 4 H,
7.4 Hz, CH2CH3), 4.64 (s, 2H CCH2). 13C{1H} NMR ([D8]THF,
125 MHz): δ 14.1 (pyrrole 4 CH3), 15.5 (pyrrole 2 CH3), 17.2
(CH2CH3), 19.6 (CH2CH3), 100.2 (CCH2), 115.4, 121.5, 130.9,
136.3, 142.3, (pyrrole ring atoms, methene atom). 7Li NMR
([D8]THF, 194.4 MHz): δ 0.0.
Synthesis of 4a·0.5(hexane). To a solution of 3 (0.56 g, 2.0
mmol) in THF (10 mL) was added dropwise dichlorodimethylsilane
(0.26 g, 2.0 mmol) at room temperature to yield a yellow solution
which was stirred for 3 h at room temperature. Thereafter, the volatiles
were removed under reduced pressure (cold trap condensation) and
the residue was dissolved in hexane (10 mL) followed by filtration
through a plug of diatomaceous earth. After washing with hexane (2 ×
1 mL) the combined filtrate and washings were evaporated to dryness
under reduced pressure. The resulting brownish yellow oil was
dissolved in hexane (0.5 mL) and stored at −24 °C to afford crystals of
4a·0.5(hexane), which were separated from the supernatant by
decantation and briefly dried under vacuum. Yield: 0.25 g (0.68
mmol, 34%). Elemental analysis indicates loss of solvent upon drying.
Anal. Found: C, 73.76; H, 9.62; N, 8.14. Calcd for C20H30N2Si·
0.1C6H14 (Mr = 335.169): C, 73.82; H, 9.44; N, 8.36. 1H NMR (C6D6,
500 MHz): δ 0.27 (s, 6 H, SiCH3), 1.15 (t, 6 H, 7.6 Hz, CH2CH3),
2.06 (s, 6 H, pyrrole 4 CH3), 2.33 (s, 6 H, pyrrole 2 CH3), 2.45 (q, 4
H, 7.6 Hz, CH2CH3), 5.52 (s, 2H CCH2). 13C{1H} NMR (C6D6,
125 MHz): δ 2.25 (SiCH3), 12.7 (pyrrole 4 CH3), 12.8 (pyrrole 2
CH3), 15.9 (CH2CH3), 18.3 (CH2CH3), 109.9 (CCH2), 118.1,
125.8, 127.4, 131.3, 132.3, (pyrrole ring atoms, methene CCH2
atom). 29Si{1H} NMR (C6D6, 99.4 MHz): δ −1.0.
EXPERIMENTAL SECTION
■
General Considerations. Chemicals commercially available were
used as received without further purification. Dimethylethylpyrrole,28
1,16 and 217 were prepared according to literature procedures. THF,
hexane, and toluene were distilled from sodium benzophenone and
were stored over sodium wire (hexane, toluene) or activated molecular
sieves 3 Å under an argon atmosphere (THF). All reactions involving
organochlorosilanes and/or free dipyrromethene base as well as those
interconverting dimethylethylpyrrole were carried out under an
atmosphere of dry argon utilizing standard Schlenk techniques.
NMR spectra were recorded on a Bruker Avance III 500 MHz
spectrometer (Me4Si as internal standard, 1 M LiCl in D2O as external
reference).
UV/vis spectra were recorded on a Jasco V-650 spectrophotometer
in 10 mm quartz cuvettes, and fluorescence spectra were recorded on a
Jasco FP 6500 spectrofluorometer in 10 × 10 mm quartz cuvettes.
Elemental analyses were performed using an Elementar vario
MICRO cube instrument. Single-crystal X-ray diffraction data were
collected on a Bruker X8 APEX2 CCD (1′, 2′, 3) or a STOE IPDS 2/
2T single-crystal diffractometer (1, 4a−c, 5, 6c′) using Mo Kα
radiation. The structures were solved by direct methods using
SHELXS-97 and refined with full-matrix least-squares methods of F2
against all reflections with SHELXL-97.29 All non-hydrogen atoms
were anisotropically refined. C-bound hydrogen atoms were refined in
idealized positions (riding model), and N-bound H atoms were
located as residual electron density peaks and were refined
isotropically. Ellipsoid plots of the molecular structures were generated
with ORTEP30 and POV-Ray.31 Space-filling plots were generated
with MERCURY.32
4b,c were synthesized analogously to 4a.
Synthesis of 4b. A 2.0 mmol portion of 3 and 2.0 mmol of
dichlorodiphenylsilane were used as starting materials. Yield: 0.35 g
(0.78 mmol, 39%).
Anal. Found: C, 80.22; H, 7.64; N, 5.90. Calcd for C30H34N2Si (Mr
= 450.690): C, 79.95; H, 7.60; N, 6.22. 1H NMR (C6D6, 500 MHz): δ
1.07 (t, 6 H, 7.6 Hz, CH2CH3), 1.85 (s, 6 H, pyrrole 4 CH3), 2.37 (s, 6
H, pyrrole 2 CH3), 2.38 (q, 4 H, 7.6 Hz, CH2CH3), 5.53 (s, 2H C
CH2), 7.0 (m, 4 H, oPh) 7.08 (m, 2 H, pPh),7.60 (m, 4 H, mPh).
13C{1H} NMR (C6D6, 125 MHz): δ 12.6 (pyrrole 4 CH3), 13.2
(pyrrole 2 CH3), 15.8 (CH2CH3), 18.2 (CH2CH3), 110.5 (CCH2),
118.6, 127.2, 128.7, 131.2, 132.0, 132.2, 132.4, 135.8 (pyrrole ring
atoms, methene CCH2 atom, phenyl ring atoms). 29Si{1H} NMR
(C6D6, 99.4 MHz): δ −25.1.
In order to obtain X-ray-quality crystals of compound 1′, excess
70% aqueous HClO4 was added to a sample of 1 in ethanol. The
solution thus obtained was then subjected to slow evaporation of the
solvent at room temperature. 1′ was obtained as large orange crystals
with a purplish metallic luster.
Synthesis of 4c. A 2.0 mmol portion of 3 and 2.0 mmol of 1,1-
dichlorosilacyclobutane were used as starting materials. Yield: 0.37 g
(1.1 mmol, 55%).
Anal. Found: C, 74,62; H, 9.08; N, 8.08. Calcd for C21H30N2Si (Mr
= 338.562): C, 74.50; H, 8.93; N, 8.27. 1H NMR (C6D6, 500 MHz): δ
118
dx.doi.org/10.1021/om400868w | Organometallics 2014, 33, 112−120

Organometallics
Article
1.14 (t, 6 H, 7.6 Hz, CH2CH3), 1.64 (m, 4H, SiCH2−), 1.78 (m, 2 H,
SiCH2CH2), 2.16 (s, 6 H, pyrrole 4 CH3), 2.31 (s, 6 H, pyrrole 2
CH3), 2.45 (q, 4 H, 7.6 Hz, CH2CH3), 5.51 (s, 2H CCH2). 13C{1H}
NMR (C6D6, 125 MHz): δ 11.6 (SiCH2), 12.1 (pyrrole 4 CH3), 12.6
(pyrrole 2 CH3), 15.8 (CH2CH3), 18.2 (CH2CH3), 23.3 (SiCH2CH2),
110.2 (CCH2), 118.5, 127.0, 127.7, 131.1, 131.9 (pyrrole ring
atoms, methene CCH2 atom). 29Si{1H} NMR (C6D6, 99.4 MHz): δ
−4.1.
Reactions of 4a−c with Phenols. To a precooled (0 °C)
solution of 4a prepared from 560 mg (2 mmol) of 3 and 260 mg (2
mmol) of dichlorodimethylsilane in toluene (15 mL) was added a
solution of 280 mg (2 mmol) of p-nitrophenol in toluene (5 mL).
After the mixture was warmed to room temperature (within 30 min),
the solvent was removed under reduced pressure. The resulting dark
residue was recrystallized from a mixture of toluene (0.2 mL) and
hexane (0.5 mL). Crystals appeared after 1 day of standing at room
temperature and appeared to be dimethylbis(p-nitrophenoxy)silane
(5). This compound has been reported in the literature before
(synthesized by reaction of p-nitrophenol with dialkoxydimethylsi-
lanes).33 For an efficient synthetic procedure toward dimethylbis(p-
nitrophenoxy)silane (5) as well as NMR data see the Supporting
Information.
CH2CH3), 1.12 (s, 3 H CH3), 1.46 (m, 2 H, CH2 ring), 1.66 (m, 2 H,
CH2 ring), 1.84 (m, 2 H, CH2 ring) 1.86 (s, 3H, CH3), 2.15 (q (low
resolution), 2 H, CH2CH3), 2.16 (s, 3 H, CH3), 2.31 (s, 3 H, CH3),
2.37 (q, 2 H, 7.6 Hz CH2CH3), 2.42 (s, 3 H, CH3), 3.28 (s, 3 H,
OCH3), 5.66 (s, 1H), 6.42 (d, 1 H, 8.6 Hz), 7.05 (m, 1H), 7.14 (m,
2H), 7.47 (d, 1 H, 8.3 Hz), 7.90 (d, 2H, 7.2 Hz) (aromatic H atoms).
13C{1H} NMR (C6D6, 125 MHz): δ 11.5, 11.8, 13.0, 14.3, 14.9, 16.0,
16.4, 18.1, 18.3, 19.8, 23.7, 37.7 (12 aliphatic C atoms), 54.9 (OCH3),
69.7 (quat C atom), 95.6 (methene C atom), 102.9, 109.8, 114.8,
115.5, 123.1, 123.7, 128.1, 129.8, 131.8, 132.3, 134.1, 134.8, 140.7,
141.7, 151.2, 154.2, 163.8 (17 aromatic C atom), 195.1 (CO).
29Si{1H} NMR (C6D6, 99.4 MHz): δ −39.1.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, and CIF files giving details of the synthesis and
characterization of compound 5 (1H, 13C, and 29Si NMR data),
NMR spectra (1H, 13C, 7Li) of compound 3, 1H NMR
spectrum of the reaction mixture obtained from 4a and 2-
hydroxy-4-methoxybenzophenone, atomic coordinates of the
optimized molecular structures of compounds 2, 2′, and 4a,b
Reactions of 4a−c with p-tert-butylphenol were carried out on a
NMR scale: to the dissolved samples of 4a−c (0.1226 mmol) in
benzene-d6 (0.5 mL) was added the corresponding 1:1 stoichiometric
amount of 4-tert-butylphenol as a 10% solution in benzene-d6 (180 mg,
0.1226 mmol). Immediately, mixtures were mixed vigorously and
subsequently subjected to NMR analysis.
and model compounds LHSiMe2singlet, LHSiMe2
,
triplet
HLHSiMe2F, electronic transitions calculated for compounds
2, 2′, 4a,b, LHSiMe2
and HLHSiMe2F and color
singlet
representations of the relevant MOs involved in these
transitions, parameters of data collection and structure
refinement for the crystal structure studies and crystallographic
data of compounds 1·0.2H2O, 1′, 2′, 3, 4a·0.5(hexane), 4b, 4c
(two modifications), 5, and 6c′. This material is available free of
charge via the Internet at http://pubs.acs.org. The CCDC
entries 957875 (1·0.2H2O), 957874 (1′), 957876 (2′), 957877
(3), 957880 (4a·0.5(hexane)), 957882 (4b), 957873 (4c,
modification in P21), 957878 (4c, modification in P21/n),
957881 (5), and 957879 (6c′) contain supplementary
crystallographic data for this paper. These data can be obtained
free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
4a: 29Si{1H} NMR (C6D6, 99.4 MHz) δ 0.8, −1.0 (starting
material), −5.6 (Me2Si(O-ptertBuC6H4)2).
4b: 29Si{1H} NMR (C6D6, 99.4 MHz) δ −8.0, −11.0, −25.1
(starting material), −28.2.
4c: 29Si{1H} NMR (C6D6, 99.4 MHz) δ −4.1 (starting material),
−10.5, −21.1, −28.9, −38.4 (presumably an analogue of 6c′).
Reaction of 4a with 2-Hydroxy-4-methoxybenzophenone.
To a solution of 4a·0.5(hexane) (132 mg, 0.32 mmol) in C6D6 (0.4
mL) was added a solution of 2-hydroxy-4-methoxybenzophenone (72
mg, 0.32 mmol) in C6D6 (0.4 mL). The resulting mixture was stirred
for 10 min at room temperature and subsequently subjected to NMR
analysis. The signal in the 29Si{1H} NMR (C6D6, 99.4 MHz) spectrum
1
at δ −1.0 confirmed the presence of 4a, and the H NMR spectrum
(C6D6, 500 MHz) revealed the signals of 4a and 2-hydroxy-4-
methoxybenzone. On reinvestigation after 1 day, in addition to the
29Si{1H} NMR signal at δ −1.0 (4a), a new peak at δ −3.9 was
detected and assigned to Me2Si(2-oxy-4-methoxybenzophenone)2. In
the 1H NMR spectrum, which still showed signals of 4a and 2-
hydroxy-4-methoxybenzophenone, signals of the free ligand 2, a new
signal of SiMe groups, and a new set of 2-oxy-4-methoxybenzophe-
none signals appeared. The identity of the reaction product as
Me2Si(2-oxy-4-methoxybenzophenone)2 was finally confirmed by its
deliberate synthesis from Me2SiCl2 (0.29 g, 2.25 mmol), 2-hydroxy-4-
methoxybenzophenone (oxybenzone; 0.92 g, 4.0 mmol), and
triethylamine (0.45 g, 4.5 mmol) in THF (25 mL), at room
temperature; removal of Et3NHCl precipitate by filtration and washing
with THF (15 mL) afforded 0.55 g (4.0 mmol) of Et3NHCl. From the
filtrate the solvent was removed under reduced pressure (cold trap
AUTHOR INFORMATION
Corresponding Author
*E-mail for J.W.: joerg.wagler@chemie.tu-freiberg.de.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was performed within the Cluster of Excellence
“Structure Design of Novel High-Performance Materials via
Atomic Design and Defect Engineering (ADDE)” that is
financially supported by the European Union (European
regional development fund) and by the Ministry of Science
and Art of Saxony (SMWK).
1
condensation) and 29Si and H NMR spectroscopy was performed
with a C6D6 solution of the oily residue. The signals observed in the
1
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