Thermal and photoreductive elimination from the tellurium center of π-conjugated tellurophenes

Article abstract of DOI:10.1021/ic402485d

This study introduces small molecule tellurophenes that can undergo photoreductive elimination. A tellurophene compound with strong light absorption properties and extended π-conjugation, 2,5-bis[5-(N,N′- dihexylisoindigo)]tellurophene (1), has been synthesized. Halogen oxidative addition to the tellurium center from various halogen sources gives the dibromo- (1Br₂) and dichloro- (1Cl₂) adducts, leading to a red-shift in the optical absorption properties. In the presence of excess opposing halogen, 1Br₂ and 1Cl₂ can interconvert, with equilibrium favoring the dichlorotellurophene adduct. Reductive elimination reactions were studied using optical absorption spectroscopy, NMR spectroscopy, thermogravimetric analysis, and matrix-assisted laser desorption/ionization (MALDI) analysis. Thermal reductive elimination from 1Br₂ and 1Cl₂ occurs in the solid-state to restore 1. Photoreductive elimination occurs under irradiation with green (505 nm) light in solution in the presence of a halogen trap with some decomposition. This is the first example of photoreductive elimination from a mononuclear tellurophene complex.

Full text of DOI:10.1021/ic402485d

Article
pubs.acs.org/IC
Thermal and Photoreductive Elimination from the Tellurium Center
of π‑Conjugated Tellurophenes
Elisa I. Carrera, Theresa M. McCormick, Marius J. Kapp, Alan J. Lough, and Dwight S. Seferos*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, M5S 3H6, Canada
*
S Supporting Information
ABSTRACT: This study introduces small molecule tellur-
ophenes that can undergo photoreductive elimination. A
tellurophene compound with strong light absorption proper-
ties and extended π-conjugation, 2,5-bis[5-(N,N′-
dihexylisoindigo)]tellurophene (1), has been synthesized.
Halogen oxidative addition to the tellurium center from
various halogen sources gives the dibromo- (1Br ) and
2
dichloro- (1Cl ) adducts, leading to a red-shift in the optical
2
absorption properties. In the presence of excess opposing
halogen, 1Br and 1Cl can interconvert, with equilibrium
2
2
favoring the dichlorotellurophene adduct. Reductive elimina-
tion reactions were studied using optical absorption spectros-
copy, NMR spectroscopy, thermogravimetric analysis, and
matrix-assisted laser desorption/ionization (MALDI) analysis. Thermal reductive elimination from 1Br and 1Cl occurs in the
2
2
solid-state to restore 1. Photoreductive elimination occurs under irradiation with green (505 nm) light in solution in the presence
of a halogen trap with some decomposition. This is the first example of photoreductive elimination from a mononuclear
tellurophene complex.
INTRODUCTION
reactions as well as facile reductive elimination to regenerate
■
2
3,25,26
the organotellurium(II) species.
Diorganotellurides such
Photoreductive elimination is of growing interest for reactions
that are important for energy storage such as the generation of
as diphenyl telluride behave as oxidants or reductants
(depending on the oxidation state of Te) for halogenation
1
−7
H from HX or H O.
Transition metal complexes are
2
2
27
and dehalogenation reactions of various organic substrates.
typically used for these types of reactions; however, reactivity is
limited by strong M−X bonds which can slow the reductive
elimination process. Here we show that a novel isoindigo-
substituted tellurophene, 2,5-bis[5-(N,N′-dihexylisoindigo)]-
tellurophene (1), undergoes facile halogen oxidative addition,
thermal reductive elimination (TE), and more importantly
photoreductive elimination (PE) using relatively low energy
Reductive elimination can also be induced thermally as
2
5,26
observed for tellurapyrylium dyes.
The ability to shuttle
between Te(II) to Te(IV) offers potential for catalysis, and
some organotellurium compounds have been used as catalysts
28−34
for organic transformations.
Tellurophene, the tellurium analogue of thiophene, is of
interest as an organic electronic material due to favorable
optoelectronic properties such as a narrow optical gap, large
dielectric constant, high polarizability, and the ability to form
(
505 nm) visible light. This is the first example of photo-
reductive elimination from a mononuclear organotellurium
compound, pointing the way toward using main-group p-block
elements that are in conjugation with strong organic light
absorbers as a means of generating new photoactive and
potentially photocatalytic compounds that may be useful in
energy storage applications, without the use of expensive late
transition metals.
35−45
close intramolecular interactions.
The reactivity of the
tellurium center and its potential utility as a catalyst, however,
have not been extensively studied. Our group has reported the
oxidative addition of bromine to a tellurium-containing
12
polymer and 2,5-substituted tellurophene small molecule
11
with extended π-conjugation. Thermal reductive elimination
was observed for the polymer. On the basis of their strong light-
absorbing properties, we hypothesize that the reactivity of the
tellurium atom as part of 2,5-disubstituted tellurophene small
molecules should be of particular interest for light-driven
transformations.
The incorporation of chalcogen (group 16) atoms into
organic and inorganic compounds and the properties they
8
−22
impart are being increasingly studied.
Incorporation of the
metalloid tellurium, in particular, leads to interesting reactivity
and chemical properties that are not observed in their lighter
chalcogen analogues. Organotellurium(II) compounds are
converted to organotellurium(IV) adducts with oxidants such
2
3−25
as halogens and hydrogen peroxide.
The resultant Te−X
Received: October 2, 2013
Published: November 19, 2013
bond formation is reversible which allows for halogen exchange
©
2013 American Chemical Society
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Figure 1. The main transitions of 1′ and 1Br ′ with oscillator strengths greater than 0.2 predicted by TD-DFT and their associated MOs (isocontour
2
value of 0.02). Excludes transitions at 248 nm (f = 0.42) and 249 nm (f = 0.22) in 1′ and 1Br ′, respectively. The nearly degenerate LUMO and
2
LUMO + 1 MOs have been superimposed for simplification.
Improving the photoreductive elimination of X from late
tional time. Geometry optimizations and time-dependent (TD-
2
53
transition metal complexes has attracted significant recent
DFT) calculations were performed using the Gaussian 09
4
6−51
54
55−57
attention.
containing two precious metals, with quantum yields ranging
from 0.29% to 38%. Gabbaı and co-workers have shown that
The vast majority of these are dinculear systems
software package at the B3LYP level of theory
with a
58
split basis set (LANL2DZ for tellurium, and 6-31G (d) for all
59
̈
other atoms ). Compound 1′ has a calculated −4.92 eV
HOMO energy level (relative to vacuum) and a calculated 2.21
eV HOMO−LUMO energy gap. The molecular orbital (MO)
diagram of the HOMO shows electron density across the
length of the molecule, which is consistent with a delocalized
electronic structure (Figure 1). The LUMO and LUMO + 1
MOs are more localized, with the majority of electron density
residing in the π* orbitals of each isoindigo unit, respectively
one of the transition metals in a bimetallic system can be
replaced by the main group element tellurium to give a main
52
group/late transition metal Te(III)−Pt(I) complex. This
dinuclear complex can oxidatively add and photoreductively
eliminate halogens with a quantum yield of 4.4%. This sole
example of PE from an inorganic tellurium center provides
motivation for studying this process in mononuclear organo-
tellurium compounds, such as tellurophene, removing tran-
sition metals from the system altogether. The ability to design
tellurophenes with extended π-conjugation and strong light-
absorbing properties makes these compounds even more
attractive due to the possibility of undergoing PE using low
energy light. Successful PE from such compounds would
provide the opportunity to explore the utility of tellurophenes
as catalysts for energy storage reactions.
Herein, we report a new, strong light-absorbing isoindigo-
containing tellurophene. The extended π-conjugation of this
tellurophene and resultant low-energy absorption introduces
the possibility for PE using relatively low-energy light. The
photophysics and calculated electronic structure of this
compound are determined, the reactivity with various halogen
sources is evaluated, and the TE and PE reactions are studied.
(
the MO diagrams have been superimposed in Figure 1 for
simplification). These MOs are essentially degenerate (−2.71
and −2.70 eV, respectively). Upon bromination, the HOMO
energy level is stabilized significantly (by 0.50 eV; to −5.41 eV),
while the LUMO energy level is stabilized only slightly (by 0.18
eV; to −2.52 eV), leading to an overall increase in the
HOMO−LUMO energy gap upon bromination (2.89 eV).
Similar to compound 1′, the HOMO of 1Br ′ is a delocalized
2
π-state, while electron density is localized on the π* orbitals of
each isoindigo unit for the nearly degenerate LUMO and
LUMO + 1 MOs (−2.89 and −2.88 eV, respectively).
TD-DFT calculations were performed on 1′ and 1Br ′ to
2
determine the excited state transitions (Figure 1; see Figure S1
for the calculated absorption spectra). The HOMO → LUMO
transition is not expected to contribute significantly to the
absorption spectrum of 1′ due to an extremely low oscillator
strength (f < 0.01, 703 nm). Instead, the main transition (the
transition with highest oscillator strength) is the 13th excited
state which occurs at 380 nm (f = 0.63), which is HOMO →
LUMO + 2 in nature. The MO diagrams indicate that this is a
delocalized π → π* transition. The HOMO → LUMO
RESULTS AND DISCUSSION
■
Computational Modeling. Density functional theory
DFT) was used to calculate the model compound 2,5-bis[5-
(
(
N,N′-dimethylisoindigo)]tellurophene (1′) and its halogen-
ated adducts 1Br ′ and 1Cl ′ prior to synthesis. The hexyl
2
2
chains were replaced with methyl groups to reduce computa-
transition in 1Br ′ occurs at a higher energy than 1′ (597 nm)
2
1
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Scheme 1. Synthetic Route to 2,5-Bis[5-(N,N′-dihexylisoindigo)]tellurophene (1)
Scheme 2. Oxidative Addition of Br , PhICl , or ICl To Afford 1Br and 1Cl and Reductive Elimination by Heat or Light
2
2
2
2
which is consistent with an increase in the HOMO−LUMO
energy gap. However, the oscillator strength of this transition is
too low to contribute significantly to the absorption spectrum
dihexyl-5-iodoisoindigo in 57% yield. Hexyl chains are required
for solubility due to the strong pi-stacking of isoindigo-type
63
conjugated molecules. The target compound 2,5-bis[5-(N,N′-
dihexylisoindigo)]tellurophene (1) was prepared through a
Stille-type coupling between N,N′-dihexyl-5-iodoisoindigo and
2,5-bis(trimethylstannyl)tellurophene (prepared from literature
(
→
f = 0.03). The main transition (3rd excited state) is a HOMO
LUMO + 2 transition at 535 nm (f = 0.37), which is red-
shifted compared to 1′ due to a large stabilization of the
LUMO + 2 energy (by 1.33 eV). The MO diagrams indicate
that this transition has charge transfer character, from a
delocalized π-state in the HOMO to localized electron density
on the dibromotellurophene in the LUMO + 2, consistent with
6
5,66
methods
) to afford the desired product in 41% yield.
Oxidative Addition and Competition Experiments.
The absorption spectrum of compound 1 in chloroform shows
a dual-band absorption with peaks occurring at 274 and 390
nm, respectively. The charge-transfer nature of the lower
energy transition (as calculated by DFT) was probed by
obtaining the absorption spectrum of 1 in solvents of increasing
2
0,60,61
a donor−acceptor type structure.
Higher energy
transitions with significant oscillator strength also contribute
to the calculated electronic spectra of 1′ and 1Br ′ (see Figure
2
61,67
1
). Similar results are obtained for 1Cl ′ (Figure S2, Supporting
polarity;
however, solvatochromism was not observed in
2
Information).
the absorption spectra (Figure S3, Supporting Information).
Since solvatochromic effects are a result of the excited state
nature of the compound, solvatochromism is more commonly
observed in fluorescence spectroscopy; however, 1 does not
exhibit appreciable emission to study this phenomenon due to
Interestingly, some electron-density in the LUMO + 2 state
for 1Br ′ (the strongest predicted transition) resides within the
2
Te−Br antibonding orbitals. This suggests that populating this
state (i.e., through photoexcitation) may induce reductive
elimination, which has previously been reported for Te−Pt
6
8,69
the heavy atom effect imparted by tellurium.
Halogen addition to 1 was attempted using various halogen
5
2
complexes but never for a tellurophene. Because of the strong
light-absorbing isoindigo units and extended π-conjugation, the
use of visible light to induce reductive elimination from the
tellurophene can be studied.
70
sources: bromine (Br ), iodobenzene dichloride (PhICl2),
2
iodine (I ), and iodine monochloride (ICl) (Scheme 2).
2
Dramatic color changes are observed upon Br and PhICl
2
2
Synthesis. The synthetic route utilizes N,N′-dihexyl-5-
iodoisoindigo for coupling with a 2,5-difunctionalized tellur-
ophene (Scheme 1). Accordingly, 5-iodoisatin was prepared by
addition, from yellow (for a dilute solution of 1) to red and
reddish-orange for the bromine and chlorine adducts,
respectively. Stoichiometric optical absorption titration experi-
treating isatin with I and periodic acid under acidic
ments indicate clean conversion from 1 (λ = 389 nm, ε = 3.8
× 10 L cm mol ) to the halogenated adducts by the
presence of clear isosbestic points (340 and 431 nm for Br2
2
max
6
2
63
4
−1
−1
conditions (54% yield). Condensation between 5-iodoisatin
and oxindole in acetic and hydrochloric acid afforded 5-
iodoisoindigo in 96% yield. Finally, 5-iodoisoindigo was
hexylated at the N positions by treatment with sodium hydride
titration, 323 and 430 nm for PhICl titration; Figure 2a,b).
2
Complete conversion to 1Br and 1Cl is achieved after the
2
2
64
followed by the addition of iodohexane to afford N,N′-
addition of 1.25 equiv of Br and PhICl , respectively. 1Br and
2 2 2
1
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copy, where the addition of 1 and 2 equiv of ICl give 50% and
1
00% conversion, respectively, to a product that is
spectroscopically identical to 1Cl (Figure S4b, Supporting
2
Information). No reaction was observed for I2.
To confirm the addition of two chlorine atoms to Te from
ICl, a crystal structure of the phenyl-substituted analogue, 2,5-
bis(phenyl)dichlorotellurophene (2Cl ) was determined by X-
2
ray crystallography (attempts to obtain crystal structures of
1
2
Br and 1Cl were unsuccessful). Orange block crystals of
Cl were obtained by allowing a solution of 2 in chloroform to
2
2 2
slowly diffuse into a solution of 2 equiv of ICl in carbon
tetrachloride (Figure 3). The structure confirms the addition of
Figure 3. ORTEP drawing of compound 2Cl A shown at 50%
2
probability.
two chlorine atoms to the tellurium center. Two molecules of
2
Cl are in the asymmetric unit (2Cl A and 2Cl B; Figure S5,
2
2
2
Supporting Information), with the major difference being the
dihedral angles between the phenyl rings and the tellurophene
ring.
Competition experiments were carried out where Br was
2
added to solutions of 1Cl and ICl was added to solutions of
2
1
Br . The progress of the reaction was monitored through
2
optical absorption spectroscopy (Figure 4). In both cases,
addition of excess opposing halogen readily yields the
corresponding opposing halogen adduct, demonstrating that
halogen addition is reversible, and an equilibrium exists
between the two halogen adducts. Clear isosbestic points are
observed in the competition experiments, indicating direct
conversion from one halogenated adduct to the other.
Approximately 10 equiv of Br is required to convert 1Cl to
2
2
1
Br , while 2.5 equiv of ICl is required to convert 1Br to 1Cl .
2
2
2
Figure 2. Optical absorption spectra of 1 treated with (a) Br in
carbon tetrachloride, (b) PhICl2 in chloroform, and (c) ICl in
chloroform in 0.25 equiv increments.
2
Given that 2.25 equiv of ICl was required to initially convert 1
to 1Cl , 2.5 equiv is only a slight excess to carry out the
2
exchange reaction. On the other hand, exchange for Br2
requires a much larger excess. Stronger bonds for Te−Cl as
compared to Te−Br have been experimentally observed for
1
Cl both exhibit a red-shift in absorption compared to 1, with
2
4
−1
23
the lowest energy maxima at 486 nm (ε = 1.7 × 10 L cm
other organotellurium compounds and explains the trends in
−1
4
−1
−1
mol ) and 471 nm (ε = 2.0 × 10 L cm mol ), respectively.
Several higher energy absorption maxima are also observed,
which is consistent with the TD-DFT calculations. The slight
excess of halogen required for complete conversion is likely due
to halogen loss from either evaporation or reaction with
the exchange reactivity. Elemental analyses of 1, 1Br , and 1Cl
2
2
(prepared with ICl) confirm the identity of each compound but
also indicate the presence of water which is likely the result of
hydrogen bonding between water and the two isoindigo units.
Electrochemical Behavior. The positions of the frontier
solvent. Immediate addition of 1 equiv of Br from a more
orbital energies of 1 and 1Br were determined by cyclic
voltammetry (Figure S6, Table S3, Supporting Information).
2
2
concentrated stock solution to 1 shows complete conversion at
1
1
equiv by H NMR spectroscopy (Figure S4a, Supporting
The HOMO and LUMO energy levels of 1 occur at −5.37 eV
Information).
and −3.60 eV, and the HOMO and LUMO energies of 1Br
2
Upon addition of the mixed halogen, ICl, an immediate color
change to reddish-orange is observed. A titration experiment
shows that complete conversion is achieved at 2.25 equiv, and
occur at −5.76 eV and −3.59 eV versus vacuum, respectively.
When 1 is converted to 1Br , the oxidation potential shifts to a
2
higher potential and is irreversible, while the reduction
potential is roughly equal to 1 and is quasi-reversible. As a
result, the HOMO−LUMO gap increases from 1.77 to 2.17 eV
the absorption spectrum of the product is identical to 1Cl
2
1
(Figure 2c). This is further confirmed by H NMR spectros-
1
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spectroscopy; however, no reductive elimination was observed
for temperatures just below the boiling point of the solvent (70
°
C). Solid-state thermolysis experiments were carried out
instead on 1Br , 1Cl , and 1 (used as a control). Thin films of
2
2
the compounds were heated on a hot plate set to 140 °C in air,
and the solid state optical absorption spectra were recorded at
several time intervals (Figure S7, Supporting Information).
Within 10 min, a significant change is observed in the optical
absorption spectra of 1Br and 1Cl . The intensity of the low
2
2
energy peak in the halogenated adduct decreases, while a peak
at 389 nm, corresponding to the λmax of 1, increases. The solid
state optical absorption spectra of 1Br and 1Cl are identical
2
2
to 1 at the end of the experiment (Figure 5). In control
experiments, only a slight decrease in intensity of the λmax of 1
is observed during heating (Figure S8a, Supporting Informa-
tion).
1H NMR spectroscopy experiments were carried out to
further confirm the chemical transformations observed when
1
Br and 1Cl are heated in the solid state. Solids of 1Br were
2
2
2
heated to 140 °C in air as well as under a nitrogen atmosphere.
After 1 h of heating in air, the sample contained a mixture of 1
and the brominated compound (∼60:40 1/1Br by integration
2
of tellurophene protons; Figure S9, Supporting Information),
along with new aromatic resonances that we assign as minor
decomposition products. Further heating led to nearly
complete disappearance of 1Br and restoration of 1 with
2
some decomposition (Figure 5b). After 1 h of heating under
nitrogen, more complete conversion to 1 (∼80:20 1/1Br ) was
2
Figure 4. Optical absorption spectra of competition experiments in
chloroform (a) 1Cl treated with Br , and (b) 1Br treated with ICl.
observed, with a few new aromatic signals corresponding to
decomposition products (Figure S9, Supporting Information).
It should be noted, however, that some of the decomposition
products from the sample heated under nitrogen were not
2
2
2
upon bromination which is consistent with the trends in the
DFT-calculated MO energy levels.
soluble in CDCl and are therefore not observed in the NMR
3
Thermal Reductive Elimination. Initial thermolysis
spectrum. Insoluble material was not observed for the sample
heated in air. The integration of some of the isoindigo protons
relative to the tellurophene protons are too high in the restored
reactions were attempted for solutions of 1Br in CCl with
2
4
1
-hexene as a halogen trap and monitored by optical absorption
Figure 5. Solid-state optical absorption spectra of 1Br (a) and 1Cl (c) before and after heating at 140 °C. Compound 1 is included for reference.
2
2
1
H NMR spectra (aromatic region) of 1Br (b) and 1Cl (d) before and after heating at 140 °C under air and nitrogen. Compound 1 is included for
2
2
reference. The CDCl residual peak is marked by asterisks.
3
1
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Figure 6. Optical absorption spectra of 1Br (a) and 1Cl (c) before and after irradiation with 505 nm light under nitrogen atmosphere. Compound
2
2
1
1
is included for reference (normalized to spectrum of PE product at 390 nm). H NMR spectra (aromatic region) of 1Br (b) and 1Cl (d) before
2 2
and after after irradiation with 505 nm light under nitrogen atmosphere. Compound 1 is included for reference. The CDCl residual peak is marked
3
by asterisks.
compound. For example, integrating the tellurophene protons
to 1.0, isoindigo protons that should integrate for 1.0 instead
integrate for 1.4 or 1.7 (see Figure S11a, Supporting
Information for proton integrations). This suggests that some
of the decomposition products may have common resonances
that cannot be distinguished from 1. 1 is stable at 140 °C in air
further suggests that the liberated halogen plays a role in the
decomposition pathway.
To further confirm TE, thermogravimetric analysis (TGA)
was performed on 1 (as a control), 1Br , and 1Cl prepared
2 2
from ICl. The samples were heated to 200 °C (5 °C/min) and
held at this temperature for 2 h (Figure S12, Supporting
Information). The mass of 1 remained stable over the entire
(Figure S8b, Supporting Information), which suggests that the
experiment. For 1Br and 1Cl , the weight percent values at the
liberated halogens play a role in the decomposition pathway.
These experiments were repeated at lower temperature (hot
plate set to 120 °C) to determine whether TE could occur with
less decomposition (Figure S10, Supporting Information). The
reaction proceeds notably more slowly (over ∼5 h). Once
again, more complete conversion occurs under nitrogen for a
2
2
end of the experiment were 85.1% and 95.4%, respectively,
which are in good agreement with the expected mass loss for
halogen elimination to restore 1 (expected values are 86.6%
and 93.6%, respectively). In addition, TGA further confirms the
formation of 1Cl from ICl as opposed to a mixed halogen
2
adduct since the corresponding mass loss would vary
dramatically for a I−Te−Cl adduct.
given amount of time (∼90:10 1/1Br compared to ∼55:45 in
2
air); however relative integrations of isoindigo protons to
tellurophene protons are similar to the experiment carried out
at 140 °C. Even at lower temperature, it appears that some
decomposition and insoluble material occur.
Overall, TE occurs slowly at 120 °C and more rapidly at 140
°
C, with some decomposition occurring at both temperatures.
1
On the basis of the differences in the H NMR spectra, these
decomposition products are the same regardless of the
atmosphere of the experiment but are different for the bromine
and chlorine adducts. Surprisingly, a greater amount of
decomposition occurs when the samples are heated under
nitrogen than in air, leading to insoluble material (in the case of
Similar results were obtained with 1Cl , except that neither
2
experiment (air vs nitrogen atmosphere) led to insoluble
products. As a result, all of the decomposition products are
1
observed by H NMR spectroscopy, which shows a significantly
greater amount of decomposition in the sample heated under
nitrogen compared to the sample heated in air. Less complete
conversion was again observed after heating for 1 h in air
compared to nitrogen (Figure S9b, Supporting Information).
Further heating in air (2 h total) led to nearly complete
conversion to 1 (Figure 5d) with some decomposition as
indicated by the high isoindigo proton integration compared to
the tellurophene protons. Interestingly, the minor decom-
position products appear to be different for the 1Br and 1Cl
thermolysis of 1Br ) or the observation of significant
decomposition products in the H NMR spectrum (in the
2
1
case of thermolysis of 1Cl ). This suggests that oxygen may be
2
involved in a process that is in competition with the
decomposition pathway. Oxygen can react with halogen species
to form unstable halogen oxides of various X O composi-
n
7
1,72
tions.
In this way, oxygen may act as a trap for liberated
halogens during the TE process, preventing reaction with 1 to
form decomposition products. This competing pathway is
removed when the experiment is carried out under nitrogen,
2
2
experiments (Figure S11a, Supporting Information), which
1
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and a greater amount of decomposition is observed. Further
discussion of the decomposition products are provided below.
Photoreductive Elimination. Photolysis experiments were
carried out using a 505 nm light-emitting diode (LED) light
source. This wavelength coincides with the long wavelength
absorption maximum for the halogenated adducts and was used
to selectively excite that species. Solutions of 1Br and 1Cl
were prepared in chloroform (1 × 10 M) with 1-hexene as a
halogen trap (0.1 M). The solutions were irradiated and
monitored periodically by optical absorption spectroscopy both
under nitrogen and ambient conditions (Figures S13 and S14,
Supporting Information). The optical absorption spectra of the
present; however, the sample contains decomposition products
rather than restored 1 (Figure S16, Supporting Information).
The disappearance of 1Br shows that reductive elimination of
2
the halogens from Te is a light-driven rather than a thermal
pathway. The reaction between free halogen and 1-hexene is
slower at cooler temperature, leading to decomposition
products rather than restored 1.
2
2
−5
The use of a more reactive halogen trap, 2,3-dimethyl-1,3-
butadiene (DMBD), during photolysis of 1Br2 leads to
conversion at much shorter irradiation times and with less
1
decomposition. The H NMR spectrum after 5 h of irradiation
under nitrogen atmosphere shows some new aromatic proton
resonances, similar to those observed in the 1-hexene
photolysis experiment. The relative integrations of the isoindigo
protons to the tellurophene protons are still higher than in 1
but much less so than in experiments with 1-hexene as the trap,
indicating that decomposition products are present but in a
much lesser amount (Figure S17a, Supporting Information).
The greater reactivity of DMBD with halogens compared to 1-
hexene makes it a more effective halogen trap, limiting
unwanted reactions between liberated halogens and the
tellurophene compounds. The optical absorption spectrum of
photolysis products of 1Br and 1Cl are similar. As was
2
2
observed in the thermolysis experiments, the intensity of the
low energy absorption corresponding to the halogenated
adduct decreases, while the absorption near 390 nm intensifies
as the sample is irradiated. By the end of the experiment,
however, a low intensity absorption near 520 nm remains, and a
shifted λmax (relative to 1) occurs at 365 nm (Figure 6). The
high energy absorption around 275 nm intensifies with
photolysis, which indicates a possible contribution from
decomposition products; however, the shoulder at 390 nm
suggests that restored 1 is also present. A slight decrease in the
intensity of the absorption at 365 nm (from decomposition
products) is observed for the sample irradiated under nitrogen
compared to air for both 1Br and 1Cl ; however the spectra
the photolysis of a dilute solution of 1Br with 0.1 M DMBD
2
under nitrogen is similar to that of the 1-hexene experiment,
indicating both the presence of restored 1 and some
decomposition products (Figure S17b, Supporting Informa-
tion).
2
2
are very similar (Figure S14c,d, Supporting Information).
Overall, the optical absorption spectra suggest that PE occurs
but also leads to the formation of decomposition products both
in air and under nitrogen. The optical absorption spectrum of 1
after irradiation shows a slight decrease in intensity compared
to before irradiation but is otherwise identical (Figure S15a,
Supporting Information).
Mechanistic studies on TE of halogens from organotellurium
compounds suggest a unimolecular mechanism is more
26
probable than a bimolecular mechanism. In this case,
steady-state intermediates were not observed spectroscopically
upon PE from Te(IV) to Te(II), which would suggest a
unimolecular PE pathway, although a bimolecular radical
mechanism cannot be discounted. If such a mechanism occurs,
it would involve the formation of the unstable Te(III)
intermediate, which would likely be too short-lived to observe
using standard NMR and optical absorption spectroscopy.
To quantify the efficiency of the PE reactions, the
Photolysis experiments were also carried out on more
concentrated solutions (∼2 mg/mL) under dry, oxygen-free
1
conditions for analysis by H NMR spectroscopy. In order to
reduce the reaction time, the concentration of 1-hexene was
increased to 2 M, and the solution was irradiated overnight,
after which the solvent and excess trap were removed by
photochemical quantum yields were determined using
evaporation, and the products were redissolved in CDCl . A
3
73−75
1
potassium ferrioxalate standard actinometry.
Using
comparison of the H NMR spectra of 1 and the halogenated
DMBD as the halogen trap at a concentration of 2 M, the
adducts before and after photolysis clearly shows the
disappearance of the halogenated compound and the
restoration of signals from 1 (Figure 6). For the photolysis of
both 1Br and 1Cl , new aromatic signals appear. Some of
quantum yields for PE of 1Br and 1Cl are 0.18% (std = 0.01)
2
2
and 0.19% (std = 0.04), respectively. To put these numbers
into context, they are lower than those obtained with recent
2
2
4
9−51
bimetallic gold and platinum complexes
but similar to
these resonances overlap with the isoindigo proton signals from
restored 1, which causes the isoindigo proton integrations to be
significantly higher than the tellurophene proton integrations.
47
those obtained with recent bimetallic rhodium complexes.
The long irradiation times and low quantum yields, even with
the more reactive DMBD trap, indicate that the reaction is not
very efficient. As previously discussed, the main transitions in
the parent and halogenated compounds are HOMO → LUMO
1
The H NMR spectra once again show that the decomposition
products are not the same for 1Br and 1Cl (see Figure S11b,
2
2
Supporting Information for a comparison of spectra including
relative integrations). In a control experiment, irradiation of 1
for 18 h led to a small amount of decomposition (Figure S15b,
Supporting Information) as indicated by isoindigo proton
integrations that are slightly high relative to the tellurophene
protons.
+
2 transitions. The existence of lower energy excited states that
do not contain Te−X antibonding character may limit PE.
Discussion of Decomposition Products. In attempts to
determine the decomposition products, several observations
were made: (1) Decomposition products are not the same for
In order to further confirm that reductive elimination is light-
1Br
2
and 1Cl
2
in the TE experiments; (2) decomposition
and 1Cl in the PE
driven in these experiments, a solution of 1Br and 1-hexene
products are not the same for 1Br
2
2
2
1
was kept in the dark for an equal amount of time as the
experiments; (3) there are some common H NMR signals
between the TE and PE decomposition products for a given
adduct; (4) more decomposition (and more decomposition
species) occurs for PE compared to TE; (5) the isoindigo
protons integrate too high compared to the tellurophene
1
photolysis experiment. The H NMR spectrum does not show
any evidence for the restoration of 1 in the absence of light.
Next, a solution of 1Br and 1-hexene was irradiated at 0 °C.
2
1
The H NMR spectrum indicates that 1Br is no longer
2
1
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Inorganic Chemistry
Article
1
protons following reductive elimination, and this discrepancy is
more pronounced in the PE experiments.
indicated by the tellurophene proton signal. The H NMR
spectrum shows that the major decomposition product for
photolysis carried out with and without a halogen trap contain
common resonances (Figure S19, Supporting Information).
Additional minor aromatic signals are also observed including a
singlet at 7.92 ppm which may be due to a tellurophene
decomposition product. Peaks corresponding to the mass of a
ring-opened dibrominated compound (m/z = 1069.4) and a
monohalogenated tellurophene product (m/z = 1117.3) are
once again observed by mass spectrometry.
Several types of decomposition products are possible from
these reactions. The high temperature or radiative environment
may provide favorable conditions in which free halogens can
react with 1 at sites other than the tellurium center, such as the
aromatic carbons of the isoindigo units. In this case,
halogenation of the isoindigo unit(s) would likely result in
dramatic changes in the resonances of the remaining isoindigo
1
protons. The low intensity decomposition peaks in the H
NMR spectra (6.8−6.9, 7.5−8.0 ppm) are likely a result of
these types of decomposition products. The resonances of
these protons may also be different depending on the type of
halogen that is appended to the isoindigo unit, which is
consistent with the observed difference in decomposition
products for 1Br and 1Cl in the isoindigo region of the
CONCLUSION
■
We have synthesized a new tellurophene compound, 2,5-bis[5-
N,N′-dihexylisoindigo)]tellurophene, that has strong light-
(
absorbing properties and extended π-conjugation. The
oxidative addition of halogens to the tellurium center and
both thermal and photoreductive elimination were studied. The
dibromo- and dichloro-Te(IV) compounds are rapidly formed
and can be readily isolated. These compounds undergo
halogen-exchange reactions in solutions containing excess
opposing halogen, demonstrating that in the case of
2
2
spectrum.
The appearance of overlapping resonances with the isoindigo
proton resonances indicates that some of decomposition
products have similar isoindigo environments to 1. This, in
conjunction with the low tellurophene proton integration
relative to isoindigo, suggests two other possible types of
decomposition products: tellurophene halogenated at the C3 or
C4 position(s) or a ring-opened product. In both of these cases,
the positions of the isoindigo proton resonances would likely
not change dramatically, and the integration of the tellurophene
protons would be lower than expected for 1. Halogenation at
both C3 and C4 positions of the tellurophene would lead to a
loss of the tellurophene proton signal, while halogenation at
only one of these positions would shift the tellurophene proton
signal and reduce the integration by one proton. The
observation of a new singlet near the tellurophene signal in
tellurophene, the Te−X bond is labile. Compounds 1Br and
2
1
Cl undergo TE and PE to restore compound 1; however,
2
both reductive elimination pathways also lead to the formation
of decomposition products. The reactivity of the halogen trap
influences the reaction rate as well as the extent of
decomposition, with shorter times and less decomposition
observed for the more reactive DMBD trap. Several types of
1
decomposition products have been observed by H NMR
spectroscopy and mass spectrometry of the TE and PE
products. This is the first detailed study of TE from a
tellurophene center and the first example of PE from a
mononuclear organotellurium compound. Despite the short-
comings of this particular system, we achieve PE efficiencies
that are close to certain binuclear late transition metal
complexes. Changing the flanking substituents so that the
main transition is HOMO → LUMO should improve the PE
efficiency by removing efficiency losses from relaxations that do
not promote Te−X dissociation. Preliminary work from our
laboratory is yielding promising results along these lines, which
will be the subject of a future publication. The realization of
more efficient PE of halogens from organotellurium com-
pounds could lead to transition metal-free catalysts for energy
storage reactions.
1
the H NMR spectra (more pronounced for some experiments
than others) may be due to a C3 or C4 mono-halogenated
tellurophene. A ring opened product would lead to a loss of the
tellurophene proton signals. A combination of decomposition
products is also possible (e.g., ring-opened and halogenated).
Matrix-assisted laser desorption/ionization (MALDI) anal-
ysis was performed on the TE and PE products of 1Br and
2
1
Cl . In all cases, a high intensity m/z peak corresponding to
2
the parent compound 1 is observed (m/z = 1038), further
confirming that both TE and RE occur. Several other signals are
also observed (Table S4, Supporting Information), including
those corresponding to (1) a ring-opened, mono-halogenated
product (- Te + 1X); (2) a ring-opened, di-halogenated
product (- Te + 2X); (3) a mono-halogenated tellurophene
product (1 + X); (4) a di-halogenated tellurophene product (1
EXPERIMENTAL SECTION
■
+
2 X, for 1Cl photolysis only). For 1Br , the intensity of the
General Considerations. All reagents were used as
received. Isatin, oxindole, periodic acid, sodium hydride,
iodohexane, palladium tetrakis(triphenylphosphine) (Pd-
(PPh ) ), iodine monochloride (ICl), 1-hexene, 2,3-dimethyl-
2
2
ring-opened dihalogenated signals are comparable for both the
TE and PE experiments; however, a greater amount of mono-
halogenated tellurophene is observed in the PE experiment. For
3
4
1
Cl , the intensity of the ring-opened product signals are
1,3-butadiene, bromine, chloroform, and carbon tetrachloride
were purchased from Sigma-Aldrich. Sulfuric acid, acetic acid,
DMF, MgSO , and NaHCO were purchased from Fisher
2
significantly higher in the PE experiment compared to TE and
are also significantly higher for the photolysis of 1Cl compared
2
4
3
to 1Br . On the basis of these observations, several possible
decomposition products are proposed (Figure S18, Supporting
Information). We have also performed mass analysis on the
Scientific. Iodine and hexanes were purchased from Caledon.
CuI and ethyl acetate were purchased from Alfa Aesar and
EMD, respectively. Silica gel was purchased from Silicycle.
2
65
66
insoluble material obtained from thermolysis of 1Br under
Tellurophene, 2,5-bis(trimethylstannyl)tellurophene, iodo-
70 11
2
nitrogen. The absence of signals at double or triple the
molecular weight of 1 indicates that oligomers are not formed
as a decomposition product.
benzene dichloride, and 2,5-bis(phenyl)tellurophene were
synthesized according to literature procedures.
Optical absorption spectroscopy was carried out using a
Varian Cary 5000 UV−vis-NIR spectrophotometer. NMR
spectra were recorded on a Varian Mercury 300 spectrometer
Irradiation of 1Br in the absence of halogen trap leads to
2
complete disappearance of 1Br but very little restored 1 as
2
1
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Inorganic Chemistry
300 MHz), Varian Mercury 400 spectrometer (400 MHz) or
Article
(
brownish-yellow. 1-Iodohexane (5.02 mL, 34.0 mmol) was
added, and the flask was sealed and stirred for 18 h. Brine was
added and the mixture was extracted three times with ethyl
acetate. The combined organic layers were washed three times
with brine and dried over MgSO . The crude product was
purified by column chromatography (silica gel, 1:25 ethyl
Agilent DD2 500 MHz spectrometer as noted. Mass
spectrometry was performed using a Waters GCT Premier
TOF mass spectrometer. MALDI-TOF was carried out on a
Waters MALDI micro MX using a 10000:1 dithranol matrix/
analyte ratio. Electrochemistry was carried out using a BASi
Epsilon potentiostat. Thermogravimetric analyses were per-
formed on a TA Instruments SDT Q600 Simultaneous TGA/
DSC.
4
acetate/hexanes) to yield 5.37 g (9.65 mmol, 56.8%) of N,N′-
dihexyl-5-iodoisoindigo as a red solid. H NMR (400 MHz,
1
CDCl ) δ 0.86−0.91 (m, 6H), 1.26−1.40 (m, 12H), 1.63−1.74
3
Computational Methods. DFT calculations were per-
(m, 4H), 3.73−3.79 (m, 4H), 6.58 (d, J = 8.4 Hz, 1H), 6.79
1
54
formed using the Gaussian 09 Software package, at the
(d, J = 8.0 Hz, 1H), 7.04 (dd, J = 8.0 Hz, J = 8.0 Hz, 1H),
1
1
1
5
6,57
B3LYP level of theory
with a split basis set (LANL2DZ for
7.36 (dd, J = 7.6 Hz, J = 7.6 Hz, 1H), 7.66 (d, J = 8.0 Hz,
1H), 9.16 (d, J = 8.4 Hz, 1H), 9.57 (s, 1H). C NMR (100
1 1 1
5
8
59
13
Te and 6-31G (d) for all other atoms ). Structures were
1
created using Gaussview 5.0. Geometries were optimized to a
minimum, and time-dependent DFT calculations were
MHz, CDCl ) δ 26.65, 13.99, 22.50, 27.33, 31.47, 40.06, 84.63,
108.00, 109.73, 121.43, 122.18, 123.62, 130.17, 131.71, 132.87,
3
5
3
performed on the optimized geometries to determine the first
134.87, 137.99, 140.41, 143.99, 145.09, 167.16, 167.60. TOF-
MS (DART +) m/z 557.2 [M + H] .
+
8
0 singlet transitions for 1′ and the first 100 singlet transitions
for 1Br ′ and 1Cl ′.
2,5-Bis[5-(N,N′-dihexylisoindigo)]tellurophene (1). A 25
mL flame-dried Schlenk flask was charged with N,N′-dihexyl-
5-iodoisoindigo (200 mg, 0.359 mmol), Pd(PPh ) (19.3 mg,
2
2
Synthesis. 5-Iodoisatin. A 250 mL three-neck flask was
charged with isatin (5.0 g, 34.0 mmol) and periodic acid (1.94
g, 8.50 mmol) and equipped with a condenser and addition
funnel. A solution of water (14 mL), concentrated sulfuric acid
3
4
0.0167 mmol), and copper iodide (6.4 mg, 0.0334 mmol), and
placed under vacuum. Dry DMF (8 mL) was degassed by
bubbling nitrogen through for 45 min. A nitrogen atmosphere
was introduced to the Schlenk flask, and the DMF was added.
The mixture was stirred for several minutes. 2,5-Bis-
(trimethylstannyl)tellurophene (84.4 mg, 0.167 mmol) was
then added, and the deep-red reaction mixture was heated to 60
°C for 16 h until the color changed to dark brown. The flask
was removed from the heat and allowed to cool to room
temperature. Methanol was added, and the brown precipitate
was collected by vacuum filtration. The crude product was
redissolved in a minimum amount of dichloromethane,
recrystallized by addition to cold methanol, and centrifuged
to remove any unreacted starting material, yielding 71 mg of
(
8 mL), and glacial acetic acid (150 mL) was prepared. 75 mL
of this solution was added to reaction flask. Iodine (4.40 g, 17.3
mmol) was dissolved in the remaining solvent, forming a dark-
brown solution, with some solid iodine which remained
undissolved. The solution was transferred to the addition
funnel, and the remaining solid iodine was added directly to the
reaction flask. The dark-red reaction mixture was heated to 65
°C with vigorous stirring, and the iodine solution was added
dropwise. After stirring of the solution overnight (∼22 h), a red
precipitate was observed. The reaction mixture was cooled to
room temperature, filtered, and washed with 0.5 M NaHCO3
and water. The crude product was purified by column
chromatography (silica gel, 1:2 ethyl acetate/hexanes) to
yield 5.05 g of 5-iodoisatin (18.5 mmol, 54%) as a red solid.
1
the desired product (0.068 mmol, 41%). H NMR (500 MHz,
CDCl ) δ 0.84−0.94 (m, 12H), 1.25−1.47 (m, 24H), 1.68−
3
1
H NMR (300 MHz, DMSO-d ) δ 6.75 (d, J = 8.1 Hz, 1H),
1.75, (m, 8H), 3.80 (q, J = 7.5 Hz, 8H), 6.76 (d, 8.0 Hz, 2H),
6
1
1
7
.76 (s 1H), 7.88 (d, J = 8.1 Hz, 1H), 11.10 (s, 1H, N−H).
6.80 (d, J = 8.0 Hz, 2H), 7.05 (dd, J = 8.0 Hz, J = 8.0 Hz,
1
1
1
1
1
3
C NMR (100 MHz, DMSO-d ) δ 85.36, 114.57, 119.93,
2H), 7.36 (dd, J = 7.5 Hz, J = 7.5 Hz, 2H), 7.47 (d, J = 8.5
6
1
1
1
1
32.37, 145.74, 149.93, 158.66, 183.04.
Hz, 2H), 7.86 (s, JTe = 20.0 Hz, 2H), 9.19 (d, J
9.53 (s, 2H). 13C NMR (125 MHz, CDCl
) δ 14.19, 14.24,
3
1
= 8.0 Hz, 2H),
5-Iodoisoindigo. Glacial acetic acid (68 mL) and concen-
trated hydrochloric acid (0.34 mL) were added to a 250 mL
round-bottom flask containing 5-iodoisatin (5.05 g, 18.5 mmol)
and oxindole (2.55 g, 19.2 mmol). The mixture was heated to
reflux and stirred for 24 h. The resultant black mixture was
cooled to 0 °C and filtered. The solids were washed with water,
ethanol, and ethyl acetate, and dried under vacuum to yield
22.70, 22.75, 26.85, 26.88, 27.59, 27.68, 31.69, 31.71, 40.19,
40.42, 108.07, 108.19, 121.81, 122.24, 122.36, 128.07, 130.19,
130.73, 132.69, 133.19, 133.53, 134.51, 134.81, 144.03, 145.15,
+
147.36, 167.87, 168.01. MALDI-TOF MS (LD ) m/z 1038.6.
CHN: C60H N O Te·H O Expected: C, 68.32; H, 6.69; N,
68 4 4 2
5.31. Found: C, 68.06; H, 6.56; N, 5.21.
6
.90 g (17.8 mmol, 96%) of 5-iodoisoindigo as a dark-purple
Oxidative Addition. 2,5-Bis[5-(N,N′-dihexylisoindigo)]-
1
solid which was used without further purification. H NMR
dibromotellurophene (1Br ). Compound 1 (23 mg, 0.022
2
(
400 MHz, DMSO-d ) δ 6.70 (d, J = 8.4 Hz, 1H), 6.86 (d, J =
mmol) was dissolved in a minimum amount of CHCl . A
6
1
1
3
8
=
=
.0 Hz, 1H), 6.97 (dd, J = 8.0 Hz, J = 8.0 Hz, 1H), 7.36 (dd, J
solution of bromine (10 μL in 1 mL CCl , 0.194 M) was
1
1
1
4
7.6 Hz, J = 7.6 Hz, 1H), 7.67 (d, J = 8.0 Hz, 1H), 9.05 (d, J
prepared, and 1 equiv (0.022 mmol) was added to the solution
of 1. After the solution was stirred for 5 min, the product was
recrystallized by addition to cold hexanes, centrifuged, and
1
1
1
8.4 Hz, 1H), 9.46 (s, 1H), 10.92 (s, 1H, N−H), 11.01 (s, 1H,
13
N−H). C NMR (100 MHz, DMSO-d ) δ 84.9, 110.6, 112.8,
6
1
1
1
22.2, 122.4, 124.8, 130.5, 132.7, 134.1, 135.5, 138.0, 141.2,
44.4, 145.4, 169.3, 169.9.
collected as solid 1Br . H NMR (500 MHz, CDCl ) δ 0.84−
2
3
0.94 (m, 12H), 1.25−1.47 (m, 24H), 1.69−1.76, (m, 8H), 3.80
(q, J = 7.5 Hz, 8H), 6.81 (d, 8.0 Hz, 2H), 6.89 (d, J = 8.5 Hz,
N,N′-Dihexyl-5-iodoisoindigo. 5-Iodoisoindigo (6.60 g, 17.0
1
1
mmol) and 95% sodium hydride (897 mg, 37.4 mmol) were
added to a flame-dried 1 L Schlenk flask under nitrogen. Dry
DMF (645 mL) was added, and the solution was stirred at
room temperature under an open nitrogen atmosphere (an
open needle was placed in the septum and a nitrogen stream
was maintained). After 1 h the solution had turned dark
2H), 7.06 (dd, J = 7.5 Hz, J = 7.5 Hz, 2H), 7.39 (dd, J = 8.0
1 1 1
Hz, J = 8.0 Hz, 2H), 7.53 (s, J = 5.5 Hz, 2H), 7.59 (d, J =
1
Te
1
1
3
8.5 Hz, 2H), 9.21 (d, J = 8.0 Hz, 2H), 9.78 (s, 2H). C NMR
1
(125 MHz, CDCl ) δ 14.20, 14.26, 22.70, 22.76, 26.86, 26.88,
3
27.58, 27.69, 31.66, 31.71, 40.29, 40.57, 108.19, 108.80, 121.64,
122.40, 123.04, 127.31, 128.63, 130.50, 131.59, 131.79, 133.23,
1
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Inorganic Chemistry
35.76, 137.28, 145.43, 146.03, 162.02, 167.82, 167.93. CHN:
Article
1
set to 140 °C, and the solid-state optical absorption spectrum
was recorded at several time intervals. For NMR spectroscopy
experiments, 5 mL round-bottom flasks containing ∼2.5 mg of
1Br or 1Cl were heated in a 140 °C hot oil bath. Experiments
C H N O TeBr ·H O Expected: C, 59.33; H, 5.81; N, 4.61.
60
68
4
4
2
2
Found: C, 59.17; H, 5.76; N, 4.68.
,5-Bis[5-(N,N′-dihexylisoindigo)]dichlorotellurophene,
1Cl ). Compound 1 (25 mg, 0.024 mmol) was dissolved in a
2
2
2
(
were carried out in air as well as under nitrogen flow with an
2
minimum amount of CHCl . One equivalent of iodobenzene
dichloride (6.63 mg, 0.024 mmol) was added to the solution of
open needle. After cooling, the samples were dissolved in
3
1
CDCl and a H NMR spectrum was obtained. TE experiments
3
1
and allowed to stir for 1 h. The product was recrystallized by
monitored by TGA were performed on 1 (control), 1Br , and
2
addition to cold hexanes, centrifuged, and collected as solid
1Cl . The samples were heated under nitrogen to 200 °C at a
2
1
1
1
8
Cl . H NMR (500 MHz, CDCl ) δ 0.84−0.94 (m, 12H),
rate of 5 °C/min and held at 200 °C for 2 h.
2
3
.25−1.47 (m, 24H), 1.68−1.76, (m, 8H), 3.80 (q, J = 7.5 Hz,
Photoreductive Elimination. Photolysis experiments were
carried out using 505 nm Rebel LEDs mounted on a Tri-Star
CoolBase with 7.2 W output from Luxeon Star LEDs. The LED
assembly was mounted onto a high efficiency heatsink and
operated at constant current using a 700 mA driver.
Experiments were carried out with a cooling fan. Solutions of
1
H), 6.80 (d, 8.0 Hz, 2H), 6.88 (d, J = 8.0 Hz, 2H), 7.05 (dd,
1
J = 7.5 Hz, J = 7.5 Hz, 2H), 7.38 (dd, J = 7.5 Hz, J = 7.5 Hz,
1
1
1
1
2
2
2
1
1
1
6
H), 7.58−7.63 (m, 4H), 9.21 (d, J = 8.0 Hz, 2H), 9.78 (s,
1
H). 13C NMR (125 MHz, CDCl ) δ 14.18, 14.23, 22.67,
3
2.73, 26.83, 26.85, 27.54, 27.65, 31.64, 31.69, 40.26, 40.54,
08.18, 108.79, 121.59, 122.37, 123.03, 127.15, 128.72, 130.48,
31.53, 131.74, 133.21, 135.70, 136.93, 145.40, 145.97, 163.87,
67.79, 167.89. CHN: C H N O TeCl ·4H O Expected: C,
−5
1Br
and 1Cl
(∼1 × 10 M) in dry, degassed CHCl with 0.1
2
2
3
M halogen trap (1-hexene or DMBD) were irradiated and
monitored periodically using optical absorption spectroscopy.
Similar experiments were also carried out under ambient
conditions without drying the solvent. For NMR spectroscopy
6
0
68
4
4
2
2
1.08; H, 6.49; N, 4.75. Found: C, 61.32; H, 6.03; N, 4.78.
,5-Bis(phenyl)dichlorotellurophene (2Cl ). 2,5-Bis-
2
2
(
phenyl)tellurophene, 2, (50 mg, 0.151 mmol) was dissolved
experiments, ∼2 mg/mL solutions in dry, degassed CHCl
3
in a minimum amount of CHCl . Two equivalents of iodine
were prepared with 2 M halogen trap in a dry 10 mL Schlenk
flask under nitrogen. Samples were irradiated (15 h for 1-
hexene, 5 h for DMBD), after which the solvent and excess trap
3
monochloride (15.8 μL, 0.302 mmol) was dissolved in a
minimum amount of CCl . The solution containing 2 was
4
layered over the ICl solution and allowed to slowly diffuse over
were removed by evaporation. The solid products were
1
several days. Orange crystals were obtained and examined by
dissolved in CDCl
3
and analyzed by H NMR spectroscopy.
1
single crystal X-ray crystallography. H NMR (500 MHz,
Photolysis at 0 °C was carried out in an NMR tube submerged
in a beaker containing isopropanol and irradiated through the
beaker wall. The bath was maintained at 0 °C using a Thermo
Scientific EK45 immersion cooler.
1
3
CDCl ) δ 7.46−7.52 (m, 8H), 7.61−7.64 (m, 4H). C NMR
3
(
125 MHz, CDCl ) δ 126.90, 127.61, 129.11, 129.85, 130.79,
3
1
32.81, 134.07, 138.35.
Titration Experiments. A 1 × 10 M solution of 1 was
prepared (in CCl for bromine titration, CHCl for PhICl and
−5
Quantum Yield Determination. Quantum yield experi-
ments were carried out using potassium ferrioxalate standard
4
3
2
7
3−75
−5
actinometry.
Standard and unknown samples were
ICl titrations). 7.75 × 10 M halogen stock solutions were
prepared (Br and ICl in CCl , PhICl in CHCl ) and added in
irradiated using the same 505 nm LED used for the photolysis
reactions. Full experimental details for quantum yield measure-
ments can be found in the Supporting Information.
2
4
2
3
10 μL aliquots (0.25 equiv) with stirring. The absorption
spectrum was recorded after each aliquot.
−5
Competition Experiments. 1 × 10 M solutions of 1Br2
and 1Cl were prepared in CHCl . Opposing halogens (ICl or
ASSOCIATED CONTENT
2
3
■
−5
Br , respectively) were added in increments from 7.75 × 10
2
*
S
Supporting Information
M stock solutions in CCl4 and monitored using optical
absorption spectroscopy.
Electrochemistry. Cyclic voltammetry experiments were
Additional experimental, computational, and crystallographic
data are available free of charge via the Internet at http://pubs.
acs.org.
carried out on dilute solutions of 1 and 1Br in DCM in the
2
glovebox, using 0.1 M tetrabutylammonium hexafluorophos-
phate as the electrolyte. The setup involved a platinum wire
counter electrode, platinum gauze working electrode, and silver
wire pseudoreference electrode. Scans were carried out from
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dseferos@chem.utoronto.ca.
−
1.25 to 1.25 V at a scan rate of 100 mV/s. Cyclic
Notes
voltammograms were obtained for 1 and 1Br both with and
2
The authors declare no competing financial interest.
without ferrocene added as an internal reference. The oxidation
and reduction potentials were obtained by taking the E1/2 of
oxidation and reduction peaks, and subtracting E1/2 of the
ferrocene peaks to give a potential versus the ferrocene/
ferrocenium redox couple. To convert to potentials referenced
to the normal hydrogen electrode (NHE), 0.4 V was added to
ACKNOWLEDGMENTS
■
This work was supported by the University of Toronto,
NSERC, the CFI, and the Ontario Research Fund. D.S.S. is
grateful to the Ontario Research Fund (for an Early Researcher
Award) and DuPont Central Research (for a Young Professor
Grant). We wish to acknowledge the Canadian Foundation for
Innovation, Project Number 19119, and the Ontario Research
Fund for funding of the Centre for Spectroscopic Investigation
of Complex Organic Molecules and Polymers. We would also
like to thank Tyler B. Schon, Lisa M. Kozycz, and Jon Hollinger
for their help with CV, TGA, and MALDI experiments.
76
the potential versus ferrocene. To convert to potentials
referenced to vacuum, 4.5 V was added to the potential versus
NHE (see Table S3, Supporting Information for conver-
7
7
sions).
Thermal Reductive Elimination. Thin films of 1Br and
Cl were spin-cast at 1000 rpm onto glass slides from solution
2
2
1
(
∼2 mg/mL in CHCl ). The films were heated on a hot plate
3
1
3788
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Inorganic Chemistry
Article
(
37) Patra, A.; Wijsboom, Y. H.; Leitus, G.; Bendikov, M. Org. Lett.
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