Ring Opening of π-Delocalized 2,5-Diphenyltellurophene by Chemical or Self-Sensitized Aerobic Photooxidation

Article abstract of DOI:10.1021/acs.organomet.7b00240

We report the self-sensitized aerobic photooxidation of 2,5-diphenyltellurophene (PT). Irradiation of PT induces rapid intersystem crossing to the triplet excited state, which undergoes energy transfer to oxygen to form singlet oxygen. Reaction of the heterocycle with singlet oxygen was found to occur at the carbon framework through a 1,4-cycloaddition reaction rather than at tellurium to form the Te(IV) telluroxide, as is commonly observed for other tellurium-containing heterocycles. This leads to oxidative ring-opening of the tellurophene to form (Z)-1,4-diphenylbut-2-ene-1,4-dione ((Z)-ED) with extrusion of TeO₂. In attempts to study the formation of the Te(IV) telluroxide and Te(VI) tellurone by chemical oxidation with m-chloroperoxybenzoic acid (mCPBA), we discovered that PT undergoes an oxidative ring-opening reaction to the same ene-dione product after reaction with 4 equiv of mCPBA. Extrusion of TeO₂ was also observed in this case. This ring-opening reaction was studied in detail by NMR spectroscopy, GC-MS, and density functional theory calculations.

Full text of DOI:10.1021/acs.organomet.7b00240

Article
pubs.acs.org/Organometallics
Ring Opening of π‑Delocalized 2,5-Diphenyltellurophene by
Chemical or Self-Sensitized Aerobic Photooxidation
Elisa I. Carrera and Dwight S. Seferos*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada
S
* Supporting Information
ABSTRACT: We report the self-sensitized aerobic photo-
oxidation of 2,5-diphenyltellurophene (PT). Irradiation of PT
induces rapid intersystem crossing to the triplet excited state,
which undergoes energy transfer to oxygen to form singlet
oxygen. Reaction of the heterocycle with singlet oxygen was
found to occur at the carbon framework through a 1,4-
cycloaddition reaction rather than at tellurium to form the
Te(IV) telluroxide, as is commonly observed for other
tellurium-containing heterocycles. This leads to oxidative
ring-opening of the tellurophene to form (Z)-1,4-diphenyl-
but-2-ene-1,4-dione ((Z)-ED) with extrusion of TeO2. In
attempts to study the formation of the Te(IV) telluroxide and Te(VI) tellurone by chemical oxidation with m-
chloroperoxybenzoic acid (mCPBA), we discovered that PT undergoes an oxidative ring-opening reaction to the same ene-
dione product after reaction with 4 equiv of mCPBA. Extrusion of TeO2 was also observed in this case. This ring-opening
reaction was studied in detail by NMR spectroscopy, GC-MS, and density functional theory calculations.
cells,15−18 organic field effect transistors,19−21 and more
INTRODUCTION
■
recently solid-state phosphors.22−24 Our group has been
The ability of tellurium to adopt various stable oxidation states,
interested in studying the redox chemistry of tellurium as
including +2, +4, and +6, allows for the diverse bonding modes
part of the conjugated tellurophene platform. In particular, we
found in tellurium-containing compounds.1 Organotellurium
have studied the effect of halogen oxidation on the
compounds bearing two bonds to carbon are in the +2
optoelectronic properties of 2,5-disubstituted conjugated
oxidation state, and these compounds easily undergo reaction
tellurophenes in extensive detail.25−30 In contrast to the
with oxidants to form hypervalent, oxidized species. Treatment
tellurapyrylium system, the formation of the Te(IV) tellur-
of Te(II) compounds with oxidants such as Br2 and Cl2 leads to
ophene adducts leads to red-shifted absorption spectra. This is
the rapid formation of Te(IV) hypervalent halogen adducts
attributed to the introduction of charge transfer character in the
(Scheme 1a), with second-order rate constants approaching
excited state, which generally leads to a narrowing of the
diffusion control (≥109 L mol−1 s−1) for some tellurium-
frontier molecular orbital energy gap. In addition, we
containing heterocycles.2−5 Organotellurium compounds can
demonstrated that oxidation is thermally and/or photochemi-
also react with oxygen-based oxidants such as hydrogen
cally reversible for a range of substituted tellurophene
peroxide, m-chloroperoxybenzoic acid (mCPBA), sodium
hypochlorite, and sodium periodate to give the corresponding
compounds. The photolability of the Te−X bond is attributed
to an excited state with significant Te−X σ* antibonding
Te(IV) or Te(VI) telluroxides/dihydroxytelluranes or tellur-
character. The photoredox activity of compounds containing
ones, respectively (Scheme 1a).2,6−13 More recently, Dutton
main-group elements is becoming an emerging field of main
group chemistry.31,32
and co-workers have extended this to iodine-based oxidants
which deliver more exotic ligands upon Te oxidation such as
Organotellurium compounds undergo oxidation to the
triflate and acetate groups.14 In conjugated heterocyclic
telluroxide and/or tellurone upon reaction with singlet oxygen
(1O2).10,33 In the case of simple diaryltellurides, a singlet
systems, oxidation of Te leads to dramatic changes in the
1
optoelectronic properties. Detty and co-workers have studied
oxygen sensitizer is required to produce O2. However, six-
this in detail for the six-membered tellurapyrylium dyes.4,5
membered conjugated tellurium-containing heterocycles are
These compounds, which absorb in the visible to near-IR
regions depending on the extent of conjugation, undergo
dramatic hypsochromic shifts in the absorption spectra upon
oxidation. Thermal reduction restores the Te(II) compound.
Materials containing the five-membered tellurium-containing
heterocycle tellurophene have been receiving increasing
attention for organic electronic applications such as solar
able to undergo efficient intersystem crossing upon excitation,
Special Issue: Tailoring the Optoelectronic Properties of Organo-
metallic Compounds with Main Group Elements
Received: March 31, 2017
© XXXX American Chemical Society
A
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Scheme 1. (a) Oxidation of Organotellurium(II) Compounds by Halogen and Oxygen-Containing Oxidants and (b) Chemical
Structures of PT, PT-O, and PT-O2
Figure 1. (a) TD-DFT calculated excited state transitions of PT, PT-O, and PT-O2 with oscillator strengths greater than 0.1 and their associated
MOs (isocontour value of 0.02; H denotes HOMO, L denotes LUMO). (b) Calculated absorption spectra and vertical transitions for each
compound as indicated.
1
followed by triplet energy transfer to oxygen to generate O2,
which can then effect oxidation of Te.34−37 Te oxidation by this
self-sensitized method is interesting, as it does not require the
addition of sensitizers or chemical oxidants. In addition, it
introduces potential applications in photodynamic therapy, as
demonstrated by Detty and co-workers,38,39 and in catalytic
aerobic oxidation of organic substrates, as demonstrated by
Ando and co-workers for simple diaryltelluroxide/tellurone
compounds.8−10,40−42 In these systems, sensitizers were
required since the diaryltellurides were unable to sensitize
1O2 formation. Replacement of the diaryltelluride with a
conjugated tellurium-containing heterocycle that is able to
generate 1O2 would eliminate the need for the sensitizer,
leading to self-sustained aerobic oxidation photocatalysis.
Our group demonstrated that 2,5-diphenyltellurophene (PT;
Scheme 1b) derivatives are able to sensitize singlet oxygen
generation under aerobic flash photolysis conditions.29 We have
also previously reported a water-soluble tellurophene system,
which showed evidence for reactivity between the tellurophene
and O2 generated under irradiative conditions. Thus, we
were interested in investigating this further for the PT systems
to fully characterize the effect of oxidation to the telluroxide
(PT-O) and tellurone (PT-O2; Scheme 1b) on optoelectronic
properties. We were also interested in investigating the
11
1
B
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photoreduction reaction for this series and exploring the
possibility of using PT as a photocatalyst for aerobic oxidation
reactions. However, the discovery of alternative reactivity of the
tellurophene prevented the latter investigations.
DPA undergoes 1,4-addition of 1O2 to form the 9,10-
endoperoxide with a rate constant of 3 × 106 M−1 s−1.47 On
observation by optical absorption spectroscopy, formation of
the endoperoxide is accompanied by a decrease in absorbance
intensity around 355 nm, and this can therefore be used to
In this work, we show that 2,5-diphenyltellurophene
undergoes self-sensitized photooxidation by singlet oxygen at
the carbon framework of the tellurophene rather than at the
tellurium atom, leading to a ring-opening reaction. This results
in a widening of the frontier molecular orbital (FMO) energy
gap and a resultant blue shift in the absorption spectrum. A
similar ring-opening reaction occurs by reaction with excess
chemical oxidant, mCPBA. The products of the ring-opening
reaction were characterized and the mechanism was inves-
tigated through experiments and through density functional
theory calculations. We also report that in the absence of
oxidant (i.e. 1O2), tellurophene displays some photolytic
sensitivity when irradiated at 365 nm under an inert
atmosphere.
1
confirm the presence of O2.
A solution of 9,10-DPA and a known singlet oxygen
sensitizer, Eosin Y, was irradiated with a 505 nm LED. A
decrease in absorbance at 355 nm was observed as the
endoperoxide was formed (Figure S1a in the Supporting
Information). Next, a solution of PT in CHCl3 was prepared.
Since PT and 9,10-DPA have overlapping absorption spectra, a
baseline scan was obtained for the PT solution for
simplification. After addition of 9,10-DPA and irradiation
with white light from a fiber optic illuminator, a decrease in
absorbance at 355 nm was observed over 65 min, confirming
1O2 generation from PT (Figure S1b). However, the
absorbance in this region fell below 0, suggesting a loss of
absorbance from PT. Additionally, an increase in absorbance
intensity was observed between 288 and 316 nm, which was
not observed in the control experiment. These results suggest
the depletion of PT during irradiation, which may occur by
reaction with singlet oxygen to form an oxidized species. To
further confirm this, a solution of PT in the presence of Eosin Y
was irradiated at 505 nm. This also resulted in a loss of
absorbance intensity associated with PT (Figures S1c). Long
irradiation times (65 min) were required when using white light
from the fiber optic illuminator due to relatively low light
intensity. Thus, a 365 nm LED was used for subsequent
experiments in order to reduce irradiation time.
Photolysis experiments were carried out at 365 nm on
aerated chloroform solutions of PT. Over a 150 s irradiation
period, a blue shift in the absorption spectrum was observed,
along with a decrease in absorbance intensity above 300 nm
and an increase in intensity below 300 nm (Figure 2a). Visually,
the solution remained clear and colorless. On the basis of the
TD-DFT calculations, formation of either PT-O or PT-O2
would be expected to give a red-shifted dual-band absorption
spectrum. Thus, it seemed unlikely that the observed
photoproduct could be assigned to either the telluroxide or
the tellurone. To probe this, a control experiment was repeated
on an argon-purged solution, which was expected to result in a
lack of spectral change. In contrast, the control experiment
showed similar changes in the absorption spectrum as the
sample was irradiated under aerobic conditions (Figure 2b).
However, the final solution appeared to be slightly gray after
photolysis, which is indicative of the presence of elemental
tellurium and is responsible for the significant amount of
scattering observed in the spectrum. Additionally, a spectral
change was observed over a longer irradiation period of 480 s.
Similar results were obtained when these experiments were
repeated on chloroform solutions of 2,5-bis(4-(tert-butoxy)-
phenyl)tellurophene (PT-OtBu) and 2,5-bis(4-
(trifluoromethyl)phenyl)tellurophene (PT-CF3; Figure S2 in
the Supporting Information). On the basis of the experiments
carried out under argon, it was clear that photochemical
decomposition of the diphenyltellurophene compounds was
occurring. The similarity of the spectra obtained under argon
versus air suggests that decomposition (rather than formation
of the telluroxide or tellurone) also likely occurred in the
aerated solutions. However, the lack of elemental tellurium
formation in the aerated solutions suggests that different
mechanisms are at play under the different conditions. At this
RESULTS AND DISCUSSION
■
Computational Modeling of Excited State Properties.
Geometry optimizations and time-dependent density functional
theory (TD-DFT) calculations were performed on PT, PT-O,
and PT-O2 in order to gain insight into the effect of tellurium
oxidation on the optoelectronic properties. Calculations were
carried out using the B3LYP functional43,44 (LANL2DZ45
effective core potential for Te, 6-311+g(d)46 basis set for all
other atoms) with a chloroform solvent model. Similar to
previously reported oxidation of tellurophenes by halogens,
formation of the Te(IV) oxide PT-O leads to a significant
decrease in the energy gap between the highest occupied
molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) from 3.96 to 3.43 eV (Figure 1a).
This is due to a stabilization of the HOMO energy and a larger
stabilization of the LUMO energy by 0.32 and 0.85 eV,
respectively. A second oxidation to the Te(VI) dioxide PT-O2
further stabilizes the HOMO and LUMO energies by roughly
equal magnitudes (0.40 and 0.44 eV, respectively). Thus, the
HOMO−LUMO gap of PT-O2 (3.38 eV) is quite similar to
that of PT-O.
The TD-DFT calculations indicate that the first transition
with significant oscillator strength for all three compounds is
HOMO → LUMO in nature, and therefore the decrease in
HOMO−LUMO energy gap results in a red shift of the lowest
energy absorption band in the calculated spectrum of PT-O
(λmax 436 nm; Figure 1b) in comparison to that of PT (λmax 360
nm; Figure 1b). Upon further oxidation to PT-O2, the
calculated spectrum is quite similar to that of PT-O, with a
small shift in the low-energy absorption band (λmax 434 nm;
Figure 1b). The molecular orbital (MO) diagrams indicate a
delocalized π−π* transition for PT and charge-transfer
behavior upon oxidation from a delocalized HOMO to a
localized LUMO, with electron density residing on the oxidized
tellurophene for both PT-O and PT-O2 (Figure 1a). For PT-O,
this includes the Te−O σ* orbital, similar to the halogenated
PT derivatives described previously. On the other hand, the
Te−O σ* orbitals are excluded in the LUMO of PT-O2, and a
contribution from the oxygen p orbitals is observed.
Preliminary Experiments for PT Oxidation with Self-
Sensitized Singlet Oxygen. The formation of singlet oxygen
under white light irradiation of PT was first studied through a
control experiment involving a known singlet oxygen trap, 9,10-
1
diphenylanthracene (9,10-DPA). In the presence of O2, 9,10-
C
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In order to gain more insight into the reactivity, the mCPBA
oxidation reaction was studied by NMR spectroscopy. The
telluroxide was prepared on a preparative scale by adding 1
equiv of mCPBA to a concentrated CHCl3 solution of PT. In
order to remove the mCPBA byproduct, m-chlorobenzoic acid
(CBA), the chloroform solution was added to cold hexanes
followed by centrifugation and washing with fresh hexanes. The
1H NMR spectrum of the yellow solid obtained from this
process had broad peaks and evidence of a small amount of PT
starting material. In addition, the 125Te NMR spectrum showed
a downfield shift from 747.5 ppm for PT to 1123.3 ppm upon
addition of mCPBA. Thus, conversion from PT to a new
oxidized product was clear. The optical absorption spectrum
obtained from a solution of the yellow compound was
consistent with the spectrum previously obtained from the
addition of mCPBA to PT in dilute solution and was thus
attributed to the telluroxide.
Next, a large excess of mCPBA (8 equiv) was added to PT.
Immediately after addition, the solution became bright yellow,
indicating formation of the telluroxide. However, a slow loss of
color was observed over time. After it was stirred overnight, the
reaction mixture was colorless and contained a white
precipitate, which was insoluble in organic solvents. Purification
of the soluble components of the reaction mixture by column
chromatography yielded a colorless solid with an optical
1
absorption λmax at 261 nm in CHCl3. The H NMR spectrum
contained aromatic peaks corresponding to the phenyl protons
(10 H total integration) along with a singlet (2 H), which was
shifted significantly upfield in comparison to the tellurophene
proton signal (7.16 ppm versus 7.83 ppm for the tellurophene
proton). Analysis of the product by mass spectrometry yielded
a relatively clean spectrum with a peak at m/z 237.1. The
absence of the diagnostic isotope pattern for tellurium in the
mass spectrum confirmed that the purified product did not
contain tellurium. The mass spectrum of the crude reaction
mixture before chromatography also did not show any evidence
for tellurium-containing products. This was supported by a lack
of signals in the 125Te NMR spectrum. This further confirms
that the tellurone is not formed upon addition of excess
mCPBA, and tellurium is not present in any soluble organic
compounds obtained in the reaction mixture.
Figure 2. Photolysis of PT with 365 nm light in CHCl3 under (a)
aerobic conditions and (b) argon. (c) Titration of PT with mCPBA in
CHCl3 (0.5 equiv increments). Spectra for 0.5−1.0 equiv of mCPBA
were collected immediately after addition. Spectra for 1.5−4.0 equiv of
mCPBA were collected 30 min after addition.
point, we decided to investigate this unexpected chemistry in
further detail.
Chemical Oxidation of PT by mCPBA. Prior to
determining the identity of the photolysis product, the
synthesis of the telluroxide and/or tellurone was attempted
by chemical oxidation for spectral comparison and for further
confirmation that the tellurone had not been formed
photochemically. Addition of 1 equiv of mCPBA to a solution
of PT in CHCl3 led to an immediate color change from
colorless to yellow and a resultant red shift in the absorption
spectrum (λmax 388 nm, 1.42 × 104 M−1 cm−1; Figure 2c) as
expected for the telluroxide (PT-O). Increasing titration with
mCPBA (up to 4 equiv) led to a decrease in absorbance at 388
nm and an increase in absorbance below 300 nm. However, in
contrast to the rapid reaction with the first equivalent of
mCPBA, the subsequent additions required up to 30 min to
reach completion. This result suggests that continued reaction
with excess mCPBA occurs at a much slower rate than the
initial telluroxide formation. No further changes in the
spectrum were observed after the addition of 4 equiv of
mCPBA. On the basis of stoichiometry, reaction beyond 2
equiv of mCPBA would not be expected if the tellurone (PT-
O2) were being formed as the final product. Similar to the
photolysis experiment, the loss of color and blue-shifted final
spectrum obtained from chemical oxidation strongly suggests
that the tellurone is not being formed experimentally and
instead PT is prone to alternative reactivity under both
oxidation conditions.
The product was confirmed as (Z)-1,4-diphenylbut-2-ene-
1,4-dione, (Z)-ED, by comparison of the molar mass, optical
absorption, and NMR data to literature values.48 Evidence of
the E isomer, (E)-ED, was also observed in the crude NMR
spectrum and coeluted with some of the fractions containing
the Z isomer. This oxidative ring-opening reaction by mCPBA
has in fact been previously reported for other five-membered
heterocycles, including furans and selenophenes,49,50 leading to
the ene-diones as the major product. This has also been shown
for aryl-substituted benzo[c]thiophenes, yielding diaroylben-
zenes as the ring-opened products.51 The lack of soluble
products or byproducts containing Te suggests that Te is
removed from solution as the observed white precipitate. It can
be concluded that the tellurium was not extruded as Te(0),
which is gray. Instead, it is likely that Te was extruded as TeO2,
and indeed, a previous report on the oxidative ring opening of
tetrasubstituted selenophenes by mCPBA confirmed formation
of SeO2 as the byproduct. Similar ring-opening reactions of five-
membered heterocycles have also been reported using other
52
1
oxidants, including O2.
Detailed Investigation of PT Oxidation with Self-
Sensitized Singlet Oxygen. To confirm whether a similar
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oxidative ring-opening reaction was occurring under aerobic
photolysis, an air-purged 4.8 mM solution of PT in CDCl3 was
irradiated with 365 nm light and the reaction was followed by
1H NMR spectroscopy and mass spectrometry (Figure 3). After
no evidence of the ene-dione as would be expected due to a
lack of oxygen in the system. Some evidence for the ene-yne
(Z)-1,4-diphenylbut-1-en-3-yne53 was observed in the 1H NMR
spectrum and the mass spectrum (m/z 205.1) of the photolysis
products obtained under argon. The formation of a similar ene-
yne was reported by Barton and co-workers for 2-phenyl-
tellurophene.54 They showed that irradiation of 2-phenyl-
tellurophene in argon-purged ether solutions led to formation
of the corresponding ene-yne, phenylvinylacetylene (PVA), and
deposition of a tellurium mirror on the walls of the reaction
vessel. Through deuterium-labeling studies, they deduced a
mechanism for the formation of PVA involving homolytic
cleavage of the Te−C bond followed by intermolecular
hydrogen abstractions and extrusion of Te(0) (Scheme S2 in
the Supporting Information).
A similar mechanism could be at play for 2,5-diphenyltellur-
ophene; however, the severe decomposition and formation of
multiple products suggests that other decomposition pathways
are also at play. Unlike aerobic photolysis, which led to
complete formation of the ene-dione followed by decom-
position, the experiments carried out under argon suggest that
other decomposition products are formed simultaneously with
the ene-yne. This can be deduced by the presence of signals in
the 1H NMR spectrum at low conversion that are not
attributed to either the E or Z isomers of the ene-yne (e.g.,
at 7.59 and 7.54 ppm). Barton and co-workers did not report
the formation of any other decomposition products, which is in
contrast to the results for PT.
In light of the rapid photooxidative ring-opening reaction, we
did not pursue the original goal of self-sensitized singlet oxygen
generation and aerobic oxidation catalysis using the PT
compounds. However, we were interested in further investigat-
ing the mechanism of the oxidative ring-opening reaction by
mCPBA.
Possible Mechanisms for Oxidative Ring Opening of
Tellurophene with mCPBA. Experimental Study. As
mentioned above, a study by Nakayama and co-workers
reported mCPBA oxidation of tetraarylselenophenes to give
the ene-dione compounds cis-1,2-diaroyl-1,2-diarylethylenes as
the major products along with formation of SeO2.50 Similar to
the case for PT, this reaction required 4 equiv of mCPBA for
complete conversion. The authors proposed a mechanism in
which the first equivalent of mCPBA reacts to form the
selenoxide. This was observed for the tellurophene by the
abrupt color change to bright yellow immediately after addition
of mCPBA. The additional 3 equiv was proposed to react with
selenophene at each double bond along with a second oxidation
of Se to form a selenone diepoxide intermediate (Scheme S3 in
the Supporting Information). Rearrangement and expulsion of
selenium dioxide led to the formation of the corresponding
ene-dione.
Figure 3. 1H NMR spectra obtained during photolysis of an air-purged
solution of PT with a 365 nm LED. Peaks corresponding to PT, (Z)-
ED, and other decomposition products are presented in black, blue,
and red, respectively. The asterisk denotes the CDCl3 residual peak.
1
30 min of irradiation, the H NMR spectrum showed peaks
corresponding to PT and (Z)-ED in a 3:1 ratio. The same
species were observed after 60 min, in a ratio of approximately
1:4. These results were further supported by the mass spectra.
Up to this point, conversion from PT to (Z)-ED was fairly
clean; however, complete decomposition was observed with
continued irradiation up to 90 min, as indicated by a loss of
signals corresponding to PT and (Z)-ED. Once again,
formation of a white precipitate was observed during
photolysis, which is attributed to the formation of TeO2.
Reactions of singlet oxygen with other five-membered
heterocycles, such as substituted furans, occur by 1,4-addition
1
of O2 across the two double bonds, forming an unstable
endoperoxide species. From here, the endoperoxide can
undergo various rearrangements to different products (Scheme
S1 in the Supporting Information).52 Most of these products
retain the original heteroatom in the final structure. On the
other hand, the heteroatom is eliminated upon rearrangement
1
to the ene-dione product. In the reaction of PT with O2, the
absence of organotellurium-containing products in the mass
spectra (in combination with the lack of other product signals
in the NMR spectra) suggests that rearrangement of the
endoperoxide to other products does not occur. Instead, the
ene-dione is formed relatively cleanly until near-full conversion
is reached. The elimination of tellurium as TeO2 and
rearrangement to the ene-dione requires reaction of the
endoperoxide with a second equivalent of oxygen.
The requirement of 4 equiv of mCPBA in the PT titration
experiment and the observation of the ene-dione by NMR for
the reaction of PT with excess mCPBA are consistent with the
proposed selenophene mechanism. To further confirm this, the
PT oxidation reactions were repeated using 2.1 and 4.2 equiv of
mCPBA and studied by NMR spectroscopy and GC-MS (the
expected products and their relative ratios are shown in Scheme
S4 in the Supporting Information). On the basis of the titration
experiment, the first oxidation is much faster than subsequent
oxidations. Therefore, complete (or near complete) conversion
to PT-O would be expected before subsequent oxidations
occur. In the case of the experiment involving 2.1 equiv, the
Investigation of Air-Free Photolysis of PT. Photolysis of
a concentrated argon-purged solution of PT showed very slow
decomposition. After irradiation overnight, the 1H NMR
spectrum still contained a significant amount of PT; however,
the deposition of a tellurium mirror occurred on the walls of
the reaction vessel, which prevented light from entering the
sample. Dilution and continued photolysis for 6 h still showed
1
the presence of some starting material by H NMR; however,
the spectrum was quite messy and contained a series of new
signals in the aromatic region, as well as in the allylic and alkyl
regions, indicating a significant amount of other decomposition
products (Figure S3 in the Supporting Information). There was
E
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remaining mCPBA equivalent would be expected to react in a
3:1 ratio with PT-O, leaving two-thirds unreacted. The
expected ratio of the mCPBA byproduct, CBA, to the ene-
dione and PT-O would be 6:1 and 3:1, respectively. On the
other hand, reaction with 4 equiv of mCPBA would first lead to
complete conversion to PT-O, followed by complete oxidation
to the ene-dione, resulting in a 4:1 ratio of CBA to ene-dione.
MS to determine the retention times and corresponding mass
spectra. PT had the longest retention time (14.3 min), followed
by (E)-ED (12.9 min), (Z)-ED (12.6 min), and CBA (8.2
min). The chromatogram and corresponding mass spectra for
the reaction with 2.1 equiv showed peaks corresponding to
CBA, (E)-ED, and (Z)-ED, as well as a peak at 14.06 min. This
retention time is similar to that of PT but is still different
enough to suggest that it is a different species (i.e., PT-O). The
corresponding mass spectrum matches that of PT, which is
commonly observed for oxidized tellurophene adducts, as they
usually fragment during ionization.
1
Analysis of the crude reaction mixtures by H NMR showed
1
the expected products (Figure 4). The H NMR spectrum of
A rough estimate of the relative ratio of the CBA to the ring-
opened products and PT-O can be obtained by integrating the
chromatogram. For the reaction with 2.1 equiv, the relative
integration of CBA to ring-opened products (both Z and E
isomers) was 5.3:1, which is close to the expected 6:1 ratio.
However, the ratio of CBA to PT-O was roughly 1:1, which is
lower than the expected 3:1 ratio. This discrepancy may be due
to the difference in response factors for the different
compounds. For the reaction with 4.2 equiv of mCPBA, the
ratio of CBA to ring-opened products was 4.4:1 by GC
integration, which is in good agreement with the expected 4:1
ratio. For the 2.1 and 4.2 equiv experiments, the ratios of (Z)-
ED to (E)-ED were 1.6:1 and 1.1:1, respectively. Overall, the
NMR and GC-MS experiments generally support a mechanism
analogous to the proposed tetraphenylselenophene mechanism.
Aside from the initial oxidation to the chalcogen(IV) oxide, the
order of the oxidation reactions (e.g., chalcogen oxidation vs
epoxidation) was not determined in the selenophene study.
Thus, we employed density functional theory calculations to
explore the potential oxidative pathways for the PT system.
Computational Study. As described above, experimental
evidence supports the formation of PT-O as the first oxidation
step. From there, three possible pathways exist for the
remaining oxidation steps (Scheme 2a). PT-O may undergo
oxidation at Te to form PT-O2, followed by oxidation at each
CC bond to form the PT-O2-epoxide (IntC) and PT-O2-
diepoxide (IntD; path C). Alternatively, PT-O can undergo
Figure 4. 1H NMR spectra from reaction or PT with 2.1 and 4.2 equiv
of mCPBA.
the 2.1 equiv reaction showed evidence of PT-O, CBA, (Z)-ED,
and (E)-ED. There was no evidence of unreacted PT. For the
reaction with 4.2 equiv, the 1H NMR spectrum contained peaks
corresponding to the ring-opened products and CBA, with no
evidence of PT or PT-O, as expected. Due to overlapping
signals, it was difficult to accurately verify the relative yields of
each by integration.
The presence of each of these compounds was further
supported by GC-MS analysis (Figures S4−S10 in the
Supporting Information). As a reference, PT, (Z)-ED, (E)-
ED, and CBA were prepared separately and subjected to GC-
Scheme 2. (a) Initial Proposed Pathways for the Formation of (Z)-ED from PT and (b) Modified Proposed Pathways for the
Formation of (Z)-ED from PT Involving a Diphenylfuran Intermediate
F
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Figure 5. Resulting structures from forward (IRCfor) and reverse (IRCrev) IRC calculations for indicated transition states, and minimum energy
structures obtained from optimization of the forward direction IRC structures.
Figure 6. Relative energy diagram for paths B′ (blue) and C′ (red) calculated by DFT.
oxidation at the CC bond to form the PT-O-epoxide (IntA).
From here, oxidation may occur at the second CC bond to
form the PT-O-diepoxide (IntB; path A) or at Te to form the
PT-O2-epoxide (IntC; path B) followed by a final oxidation to
form IntD. Finally, ring opening and extrusion of TeO2 would
afford the final product (Z)-ED.
In order to explore each of these reaction pathways,
geometry optimizations were carried out to locate each of the
possible minimum energy structures in Scheme 2a as well as the
transition state (TS) structures connecting each intermediate
(transition states are indicated above the reaction arrows in
Scheme 2). For clarity, the transition state structures are named
according to the general nomenclature TSx‑y, where x and y
indicate the two intermediates that the TS structure connects.
All structures and transition states were successfully located
(Figure S11 and Cartesian coordinates in the Supporting
Information) with the exception of TSD‑(Z)‑ED, the transition
state connecting the PT-O2-diepoxide to the final product.
Frequency calculations carried out on each TS structure
contained one negative frequency, which is consistent with a
saddle point structure.
Intrinsic reaction coordinate (IRC) calculations are required
in order to verify that a saddle point structure is the true
transition state connecting two minimum energy structures.55
This type of calculation follows the reaction pathway from the
TS to the minima on either side. Thus, the final structures
obtained from the IRC calculation should reflect the minimum
energy structures in the case of a true transition state. In most
G
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cases, IRC calculations carried out on each of the transition
state structures led to the expected geometries in the forward
and reverse directions (Figures S12−S19 in the Supporting
Information). Interestingly, however, IRC calculations for
TSA‑C and TSPTO2‑C led to a ring-opened geometry in the
product direction (Figure 5) rather than the PT-O2-epoxide
depicted in Scheme 2a. Geometry optimizations from these
ring-opened structures led to 2,5-diphenylfuran with extrusion
of TeO2 (IntC′; Figure 5). These results suggest an alternative
pathway for paths B and C that requires consideration (paths
B′ and C′, Scheme 2b).
oxidation of the tellurophene ring leads to ring opening and
extrusion of TeO2 to form the ene-dione. This mechanism has
been studied through a combination of experimental and
computational experiments. The ring opening occurs through
oxidation to the telluroxide, followed by oxidation of both C
C bonds and formation of the tellurone, which is then expelled
as TeO2. The DFT calculations suggest that, after formation of
the telluroxide, the activation energies for subsequent
oxidations at the various reactive sites are quite similar. This
suggests that the different potential pathways may be in
competition with one another. The calculations also suggest
that a diphenylfuran intermediate is involved in the reaction
pathway. Photolysis under aerobic conditions also leads to
oxidative ring-opening of tellurophene, likely through the
formation of the endoperoxide and subsequent ring-opening to
the ene-dione with precipitation of TeO2. In the absence of
oxygen, the Te−C bond is susceptible to homolytic cleavage
under photolytic conditions, resulting in complete decom-
position of the heterocycle. At this point, the photooxidative
ring-opening reaction has prevented further study of the
optoelectronic properties of the telluroxide/tellurone species,
and the potential to use these compounds as photocatalysts for
aerobic oxidation chemistry. Thus, a challenge for the research
community remains, which is to design robust tellurophene
systems that are capable of undergoing self-sensitized aerobic
oxidation at Te and subsequently oxidizing a substrate.
Oxidation of the diphenylfuran IntC′ may lead to formation
of the ene-dione through a furan-epoxide intermediate. Thus,
geometry optimizations were carried out to locate the
diphenylfuran-epoxide (IntE; not shown) and a transition
state connecting IntC′ to IntE, both of which were successful.
However, similar to the case for TSA‑C and TSPTO2‑C, IRC
calculations from this transition state led directly to (Z)-ED
rather than IntE (TSC′‑(Z)‑ED; Figure 5). Thus, it appears that
oxidation of IntC′ at the CC bond does not go through an
epoxide intermediate and instead leads directly to the ring-
opened product.
Single-point energy calculations were performed using higher
accuracy basis sets and effective core potentials in order to
compare relative energies for each structure involved in paths
B′ and C′ (Figure 6). Since a transition state structure
connecting IntD to the product could not be located, path A
has been excluded (see Figure S20 in the Supporting
Information for an energy diagram including path A). The
overall conversion of PT to (Z)-ED is energetically favorable
(Gibbs free energy of reaction, ΔGrxn = −865 kJ/mol). In
addition, the formation of each intermediate is downhill in
energy with respect to the previous intermediate for both
reaction pathways, and the activation energies connecting each
intermediate are relatively low (Tables S1 and S2 in the
Supporting Information). After the first oxidation to form PT-
O, oxidation at Te versus CC is almost equally energetically
favorable (ΔG⧧ = 41.9 vs. 49.6 kJ/mol, respectively). The
activation energy required for conversion of PT-O2 to IntC′ is
higher than that required for conversion of IntA to IntC′ (70.7
vs. 41.8 kJ/mol); however, both of these barriers are relatively
low. The final calculated energetic barrier for conversion of
IntC′ to (Z)-ED is 71.3 kJ/mol.
EXPERIMENTAL SECTION
■
General Considerations. Chloroform, dichloromethane, m-
chloroperoxybenzoic acid, Eosin Y, and 9,10-diphenylanthracene
were purchased from Sigma-Aldrich. Magnesium sulfate and methanol
were purchased from Fisher Scientific. m-Chloroperoxybenzoic acid
was concentrated slightly under vacuum, and the weight percent of
active oxidant was determined by iodometric titration prior to use. All
other reagents were used as received. 2,5-Diphenyltellurophene (PT),
2,5-bis(4-(tert-butoxy)phenyl)tellurophene (PT-OtBu), and 2,5-bis(4-
(trifluoromethyl)phenyl)tellurophene (PT-CF3) were previously
synthesized.29
NMR spectra were recorded on a Varian Mercury 400 MHz
spectrometer or on Agilent 500 and 600 MHz spectrometers, as
indicated. 125Te NMR signals were referenced to diphenyl ditelluride
(422 ppm). Compound purity was established by NMR spectroscopy.
Optical absorption spectroscopy experiments were performed on a
Varian Cary 5000 UV−vis−NIR spectrophotometer, and mass
spectrometry was performed using a Waters GCT Premier TOF
mass spectrometer. GC-MS experiments were performed on a
PerkinElmer Clarus 600C GC-MS instrument. Photochemistry
experiments were carried out using an Oriel Instruments 66088
Fiber Optic Illuminator equipped with a 150 W QTH broad-band
lamp, a Luxeon Star SR-03-E0070 505 nm tri-LED assembly (366 lm
at 700 mA), or a LED Engin LZ1-10UV00-0000 365 nm LED (800
mW luminous flux).
Because the activation energies involved in each reaction
pathway are relatively close in energy, it would be inaccurate to
conclude that one pathway is more likely than the other. In
addition, although the final transition state structure in path A
was not successfully located, the reactions leading to IntD are
energetically competitive with those involved in paths B′ and
C′, and given the challenges associated with saddle point
optimizations, path A should not be discounted as a viable
mechanism. While the calculations should be regarded as
suggestive rather than conclusive, they have indeed been useful
for providing insight into an additional pathway that was
previously unconsidered, which involves the formation of a
furan intermediate.
Photochemistry Experiments. Singlet Oxygen Trapping with
9,10-Diphenylanthracene. Control: a solution of Eosin Y (2 × 10−5
M) and 9,10-DPA (2 × 10−5 M) in 3 mL of CHCl3 was prepared and
sparged with air for 5 min. The solution was irradiated with a 505 nm
LED, and the absorption spectrum was collected at various irradiation
time intervals.
PT solution: a solution of PT (2 × 10−5 M) in CHCl3 was prepared
and collected as a baseline scan. 9,10-DPA was then added (2 × 10−5
M) and the solution was irradiated with white light from a fiber optic
illuminator (Oriel Instruments Model 6608, 150 W QTH, Newport).
The absorption spectrum was collected at various irradiation time
intervals.
CONCLUSIONS
■
Overall, this study demonstrates that the tellurophene ring
system is susceptible to oxidative ring opening by both
chemical and photochemical means. The decomposition
pathway varies depending on whether oxygen (or an oxygen
source) is present. In the presence of 4 equiv of mCPBA,
Irradiation of PT Derivatives under Air and Argon. Optical
Absorption Experiments. Solutions of PT derivatives (2 × 10−5 M) in
CHCl3 were prepared and sparged with air or argon for 15 min. The
H
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solutions were irradiated at 365 nm and monitored by optical
absorption spectroscopy.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00240.
■
S
NMR Experiments. For irradiation under air, a solution of PT in
CDCl3 (1.6 mg/mL) was prepared, sparged with air, and irradiated at
1
365 nm. Aliquots were taken at 30 min intervals and analyzed by H
NMR spectroscopy and mass spectrometry. For irradiation under
argon, the solution was sparged with Ar for 20 min. After the first
aliquot was obtained, no change was observed in the 1H NMR
spectrum, and thus irradiation was continued overnight. Due to
Additional optical absorption and NMR spectra,
schemes, GC-MS data, intrinsic reaction coordinate
(IRC) plots, additional DFT data, and additional tables
(PDF)
1
deposition of a tellurium mirror and little observed change in the H
NMR spectrum at this time, the sample was diluted (∼0.25 mg/mL)
and irradiation was continued for 6 h followed by analysis by 1H NMR
and mass spectrometry.
(DFT optimized geometries XYZ)
Oxidation of PT with mCPBA. Optical Absorption Titration.
Solutions of PT (2 × 10−5 M) and mCPBA (3 mM) in CHCl3 were
prepared. mCPBA was added in 0.5 equiv increments and monitored
by optical absorption spectroscopy. For the addition of 0.5 and 1.0
equiv, the spectrum was obtained immediately after mixing. For all
subsequent additions, the solution was allowed to equilibrate for 30
min before obtaining the absorption spectrum to ensure complete
reaction.
AUTHOR INFORMATION
Corresponding Author
*E-mail for D.S.S: dseferos@chem.utoronto.ca.
ORCID
Elisa I. Carrera: 0000-0002-9801-1685
Dwight S. Seferos: 0000-0001-8742-8058
Notes
■
Stoichiometric Reactions. A reaction with 8 equiv of mCPBA was
first carried out by adding 250 mg of mCPBA to a solution of PT (50
mg) in a minimum amount of DCM. The reaction mixture
immediately changed from colorless to bright yellow. The reaction
mixture was stirred overnight, over which time the reaction gradually
returned to colorless with formation of a white precipitate. After
centrifugation, the supernatant was separated and the solvent removed
by rotary evaporation. After purification by column chromatography
(5% MeOH in CHCl3), (Z)-1,4-diphenylbut-2-ene-1,4-dione was
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
D.S.S. is grateful to the NSERC, DuPont for a Young Professor
Grant, and the A. P. Sloan Foundation for a research fellowship
in chemistry. E.I.C. is grateful to the NSERC for a CGS D and
the Walter Sumner Foundation. 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 thank Mark Taylor for the use of his GC-MS
instrument and Ross Mancini for instrument training.
1
3
4
isolated. H NMR (400 MHz, CDCl3): δ 7.94 (dd, J = 7.2 Hz, J =
1.2 Hz, 4H), 7.57, (t, J = 7.2 Hz, 2H), 7.45 (t, J = 7.6 Hz, 4H), 7.16 (s,
2H). 13C NMR (100 MHz, CDCl3): δ 192.5, 136.2, 135.8, 133.7,
128.9, 128.8. DART-TOF HRMS calcd for C16H13O2 (M + H)+:
expected, 237.09155; found, 237.09096. Δ = −2.51 ppm.
For more detailed NMR and GC-MS analysis, the reaction was
repeated on 10 mg scales. Two solutions of PT in CDCl3 (2 mg/mL)
were prepared, and to each was added 2.1 or 4.2 equiv of mCPBA. The
reaction mixture was wrapped in foil and stirred at room temperature
REFERENCES
■
(1) Chivers, T.; Laitinen, R. S. Chem. Soc. Rev. 2015, 44, 1725−1739.
(2) Detty, M. R.; Friedman, A. E. Organometallics 1994, 13, 533−540.
(3) Detty, M. R.; Friedman, A. E.; McMillan, M. Organometallics
1994, 13, 3338−3345.
1
overnight. The crude reaction mixtures were analyzed by H NMR
spectroscopy and GC-MS. For reference, solutions of PT, (Z)-1,4-
diphenylbut-2-ene-1,4-dione, (E)-1,4-diphenylbut-2-ene-1,4-dione
(prepared according to a literature procedure),48 and m-chlorobenzoic
acid were also analyzed by GC-MS.
(4) Detty, M. R.; Luss, H. R. Organometallics 1986, 5, 2250−2256.
(5) Detty, M. R.; McKelvey, J. M.; Luss, H. R. Organometallics 1988,
7, 1131−1147.
Computational Methods. All calculations were performed using
the Gaussian 09 software suite.56 Structures were created and
visualized using GaussView 5.0. Geometry optimizations and time-
dependent calculations were performed with a chloroform polarizable
continuum model for all of the tellurophene derivatives at the
B3LYP43,44 level of theory using the LANL2DZ45 effective core
potential (ECP) for tellurium and the 6-311+g(d)46 basis set for all
other atoms. The TD-DFT calculations on PT, PT-O, and PT-O2
were performed on the optimized geometries to determine the first 20
singlet transitions. All structures involved in the mechanistic study
were subject to frequency calculations to obtain thermochemical data,
to which a scaling factor of 0.9887 was applied.57 Transition state
optimizations were carried out by using an initial guess for the starting
geometry and the TS keyword or by using appropriate initial final
geometries as input with the QST2 keyword. Transition state
structures were verified by performing intrinsic reaction coordinate
calculations,55 and final structures obtained from IRC were optimized
to confirm that these are the expected structures. Single-point energy
calculations were performed on each structure at the B3LYP level of
theory using the SDD ECP for Te and 6-311+g(2df,2p) for all other
atoms. The energy of each structure was determined by adding the
zero-point energy correction from the frequency calculation to the
SCF energy from the SPE calculation. These energies were then
converted to kJ/mol and plotted relative to the total energy of the
reactants (PT + 4 equiv of mCPBA).
(6) Detty, M. R.; Zhou, F.; Friedman, A. E. J. Am. Chem. Soc. 1996,
118, 313−318.
(7) Khurana, J. M.; Kandpal, B. M.; Chauhan, Y. K. Phosphorus, Sulfur
Silicon Relat. Elem. 2003, 178, 1369−1375.
(8) Oba, M.; Endo, M.; Nishiyama, K.; Ouchi, A.; Ando, W. Chem.
Commun. 2004, 1672−1673.
(9) Oba, M.; Okada, Y.; Nishiyama, K.; Shimada, S.; Ando, W. Chem.
Commun. 2008, 5378−5380.
(10) Oba, M.; Okada, Y.; Endo, M.; Tanaka, K.; Nishiyama, K.;
Shimada, S.; Ando, W. Inorg. Chem. 2010, 49, 10680−10686.
(11) McCormick, T. M.; Carrera, E. I.; Schon, T. B.; Seferos, D. S.
Chem. Commun. 2013, 49, 11182−11184.
(12) Alberto, E. E.; Muller, L. M.; Detty, M. R. Organometallics 2014,
33, 5571−5581.
(13) Annaka, T.; Nakata, N.; Ishii, A. Organometallics 2015, 34,
1272−1278.
(14) Aprile, A.; Iversen, K. J.; Wilson, D. J. D.; Dutton, J. L. Inorg.
Chem. 2015, 54, 4934−4939.
(15) Carrera, E. I.; Seferos, D. S. Macromolecules 2015, 48, 297−308.
(16) Lee, W.-H.; Kyu Lee, S.; Suk Shin, W.; Moon, S.-J.; Kang, I.-N. J.
Polym. Sci., Part A: Polym. Chem. 2013, 51, 2753−2758.
(17) Park, Y. S.; Kale, T. S.; Nam, C. Y.; Choi, D.; Grubbs, R. B.
Chem. Commun. 2014, 50, 7964−7967.
(18) Park, Y. S.; Wu, Q.; Nam, C.-Y.; Grubbs, R. B. Angew. Chem., Int.
Ed. 2014, 53, 10691−10695.
I
DOI: 10.1021/acs.organomet.7b00240
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(19) Takimiya, K.; Kunugi, Y.; Konda, Y.; Niihara, N.; Otsubo, T. J.
Am. Chem. Soc. 2004, 126, 5084−5085.
(51) Nandakumar, M.; Sivasakthikumaran, R.; Mohanakrishnan, A. K.
Eur. J. Org. Chem. 2012, 2012, 3647−3657.
(52) Iesce, M.; Cermola, F.; Temussi, F. Curr. Org. Chem. 2005, 9,
109−139.
(20) Jung, E. H.; Bae, S.; Yoo, T. W.; Jo, W. H. Polym. Chem. 2014, 5,
6545−6550.
(53) Zhu, Y.; Li, T.; Qu, X.; Sun, P.; Yang, H.; Mao, J. Org. Biomol.
Chem. 2011, 9, 7309.
(54) Barton, T. J.; Tully, C. R.; Roth, R. W. J. Organomet. Chem.
1976, 108, 183−195.
(55) Fukui, K. Acc. Chem. Res. 1981, 14, 363−368.
(21) Kaur, M.; Lee, D. H.; Yang, D. S.; Um, H. A.; Cho, M. J.; Kang,
J. S.; Choi, D. H. Chem. Commun. 2014, 50, 14394−14396.
(22) He, G.; Torres Delgado, W.; Schatz, D. J.; Merten, C.;
Mohammadpour, A.; Mayr, L.; Ferguson, M. J.; McDonald, R.; Brown,
A.; Shankar, K.; Rivard, E. Angew. Chem., Int. Ed. 2014, 53, 4587−
4591.
(23) He, G.; Wiltshire, B. D.; Choi, P.; Savin, A.; Sun, S.;
Mohammadpour, A.; Ferguson, M. J.; McDonald, R.; Farsinezhad,
S.; Brown, A.; Shankar, K.; Rivard, E. Chem. Commun. 2015, 51,
5444−5447.
(24) Braun, C. A.; Zomerman, D.; de Aguiar, I.; Qi, Y.; Delgado, W.
T.; Ferguson, M. J.; McDonald, R.; de Souza, G. L. C.; He, G.; Brown,
A.; Rivard, E. Faraday Discuss. 2017, 196, 255−268.
(25) Jahnke, A. A.; Howe, G. W.; Seferos, D. S. Angew. Chem., Int. Ed.
2010, 49, 10140−10144.
(26) McCormick, T. M.; Jahnke, A. A.; Lough, A. J.; Seferos, D. S. J.
Am. Chem. Soc. 2012, 134, 3542−3548.
(56) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
(27) Carrera, E. I.; McCormick, T. M.; Kapp, M. J.; Lough, A. J.;
Seferos, D. S. Inorg. Chem. 2013, 52, 13779−13790.
(28) Carrera, E. I.; Seferos, D. S. Dalton Trans 2015, 44, 2092−2096.
(29) Carrera, E. I.; Lanterna, A. E.; Lough, A. J.; Scaiano, J. C.;
Seferos, D. S. J. Am. Chem. Soc. 2016, 138, 2678−2689.
(30) Li, P.-F.; Carrera, E. I.; Seferos, D. S. ChemPlusChem 2016, 81,
917−921.
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision A.1; Gaussian, Inc., Wallingford, CT, 2009.
(57) Merrick, J. P.; Moran, D.; Radom, L. J. Phys. Chem. A 2007, 111,
11683−11700.
(31) Lin, T.-P.; Gabbaï, F. P. J. Am. Chem. Soc. 2012, 134, 12230−
12238.
(32) Yang, H.; Gabbaï, F. P. J. Am. Chem. Soc. 2014, 136, 10866−
10869.
(33) Serguievski, P.; Detty, M. R. Organometallics 1997, 16, 4386−
4391.
(34) Detty, M. R.; Merkel, P. B. J. Am. Chem. Soc. 1990, 112, 3845−
3855.
(35) Detty, M. R.; Gibson, S. L. Organometallics 1992, 11, 2147−
2156.
(36) Kryman, M. W.; Schamerhorn, G. A.; Yung, K.; Sathyamoorthy,
B.; Sukumaran, D. K.; Ohulchanskyy, T. Y.; Benedict, J. B.; Detty, M.
R. Organometallics 2013, 32, 4321−4333.
(37) Kryman, M. W.; Schamerhorn, G. A.; Hill, J. E.; Calitree, B. D.;
Davies, K. S.; Linder, M. K.; Ohulchanskyy, T. Y.; Detty, M. R.
Organometallics 2014, 33, 2628−2640.
(38) Powers, S. K.; Walstad, D. L.; Brown, J. T.; Detty, M.; Watkins,
P. J. J. Neuro-Oncol. 1989, 7, 179−188.
(39) Detty, M. R.; Merkel, P. B.; Hilf, R.; Gibson, S. L.; Powers, S. K.
J. Med. Chem. 1990, 33, 1108−1116.
(40) Okada, Y.; Oba, M.; Arai, A.; Tanaka, K.; Nishiyama, K.; Ando,
W. Inorg. Chem. 2010, 49, 383−385.
(41) Oba, M.; Tanaka, K.; Nishiyama, K.; Ando, W. J. Org. Chem.
2011, 76, 4173−4177.
(42) Oba, M.; Okada, Y.; Nishiyama, K.; Ando, W. Org. Lett. 2009,
11, 1879−1881.
(43) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.
(44) Becke, A. D. J. Chem. Phys. 1996, 104, 1040−1046.
(45) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283.
(46) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639−
5648.
(47) Monroe, B. M. J. Phys. Chem. 1978, 82, 15−18.
(48) Jadhav, A. M.; Gawade, S. A.; Vasu, D.; Dateer, R. B.; Liu, R.-S.
Chem. - Eur. J. 2014, 20, 1813−1817.
(49) Merino, P.; Franco, S.; Merchan, F. L.; Tejero, T. In Recent
Research Development in Synthetic Organic Chemistry; Pandalai, S. G.,
Ed.; Transworld Research Network: Trivandum, India, 2000; Vol. 3,
pp 65−74.
(50) Nakayama, J.; Matsui, T.; Sato, N. Chem. Lett. 1995, 24, 485−
486.
J
DOI: 10.1021/acs.organomet.7b00240
Organometallics XXXX, XXX, XXX−XXX