Synthetic tuning of electronic and photophysical properties of 2-aryl-1,3-benzothiaphospholes

Article abstract of DOI:10.1021/jo400947u

A series of 2-aryl-1,3-benzothiaphospholes have been synthesized from 1-mercapto-2-phosphinobenzene and a variety of acid chlorides. The structure of 2-phenyl-1,3-benzothiaphosphole was established using X-ray diffraction. The electrochemical and photophysical properties of each benzothiaphosphole are reported and some of these molecules exhibit reversible 1-electron reductions.

Full text of DOI:10.1021/jo400947u

Article
pubs.acs.org/joc
Synthetic Tuning of Electronic and Photophysical Properties of
2‑Aryl-1,3-Benzothiaphospholes
Joshua C. Worch, Danielle N. Chirdon, Andrew B. Maurer, Yunyan Qiu, Steven J. Geib,‡ Stefan Bernhard,
and Kevin J. T. Noonan*
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania, 15213, United States
‡Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania, 15260, United States
S
* Supporting Information
ABSTRACT: A series of 2-aryl-1,3-benzothiaphospholes have been synthesized from 1-mercapto-2-phosphinobenzene and a
variety of acid chlorides. The structure of 2-phenyl-1,3-benzothiaphosphole was established using X-ray diffraction. The
electrochemical and photophysical properties of each benzothiaphosphole are reported and some of these molecules exhibit
reversible 1-electron reductions.
of the structures shown in Figure 1 bear a 3-coordinate
phosphorus atom and facile modulation of the solid-state
packing and band gap can be achieved by oxidation,
quaternization, or complexation of the phosphorus. Recent
device fabrication with phosphole-based dopants has yielded
light-emitting diodes capable of emitting white light.14
INTRODUCTION
■
Organic π-conjugated materials have garnered enormous
attention over the past 40 years as solution processable
components for photovoltaics, light-emitting diodes, and field-
effect transistors.1 The incorporation of main group elements
including boron,2,3 silicon,4 selenium,5 tellurium,6 and
phosphorus7 has evolved as a versatile strategy for tuning the
electronic properties of π-conjugated architectures.
The use of phosphorus for this purpose is particularly
intriguing due to its variable oxidation states and coordination
modes. Recent reports of conjugated materials bearing
phosphorus heterocycles include: dithienophospholes,8 benzo-
furan-fused phospholes,9 dibenzophosphapentaphenes,10 bi-
phospholes,11 dithienodiketophosphepins,12 and diazadibenzo-
phosphole oxides13 (several examples depicted in Figure 1). All
The ability of phosphorus to form π-bonds15 has led to the
investigation of phosphaalkenes (PC bonds) in extended
conjugated structures. Gates16 and Protasiewicz17 both
prepared poly(p-phenylene vinylene) (PPV) analogs where
the vinyl unit between the phenyl groups in the polymer main
chain was replaced by a PC bond (Figure 2). Protasiewicz
has also described a PPV analog with a diphosphene unit (P
P) between the aryl rings,17a and Ott has investigated PC
bonds in conjugation with acetylenic moieties (Figure 2).18
However, to our knowledge, very few reports of aromatic
systems containing PC bonds have been investigated as π-
conjugated materials, though a great deal of research on
aromatic heterocycles bearing phosphorus atoms has been
conducted.15,19 Protasiewicz and co-workers recently prepared
a series of photoluminescent benzoxaphospholes and benzobi-
soxaphospholes with the PC bond participating in the
conjugated aromatic architecture (Figure 2).20 This work
provided inspiration to investigate the related benzothiaphosp-
holes since polythiophenes are an important class of π-
conjugated material. The benzothiaphosphole resembles
benzothiophene except a CH moiety is replaced by a P atom.
Herein, we report a new synthetic procedure to prepare 2-aryl-
Figure 1. Several examples of π-conjugated building blocks that
incorporate a 3-coordinate phosphorus atom.
Received: May 1, 2013
Published: June 19, 2013
© 2013 American Chemical Society
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Scheme 2. Synthesis of 1,3-Benzothiaphospholes from 1-
Mercapto-2-phosphinobenzene (2) and Aryl Acid Chlorides
phenyl-1,3-benzothiaphosphole 3a) which exhibits a 31P{1H}
signal at 192 ppm.
The P1−C1 and P1−C2 bond lengths are 1.677(9) Å and
1.789(9) Å, respectively (Figure 3). One of these bonds is
slightly longer than a typical PC bond (1.60−1.70 Å)15a and
the observed lengths are comparable to a previously reported
1,3-thiaphosphole which exhibited bond lengths of 1.691(5) Å
and 1.719(5) Å.21e The bond lengths for 3a are in accord with
the benzobisoxaphospholes reported recently that have
phosphorus−carbon bond lengths of 1.694(1) Å and
1.782(1) Å.20a One of the most interesting features of the
structure is that very little twisting is observed between the aryl
rings. The angle between the planes of the benzothiaphosphole
and phenyl group is only 3.75°. This structural feature suggests
that crystalline packing in extended conjugated structures may
be possible and extended delocalization could result. Following
the synthesis of 3a, a series of 1,3-benzothiaphospholes were
prepared (3b−3f, Scheme 2) to investigate how electron
donating and electron withdrawing groups modify the
electronic and photophysical properties of the ring.
Figure 2. Extended π-conjugated materials that incorporate
phosphaalkene units.
1,3-benzothiaphospholes and describe their electrochemical
and photophysical behavior.
RESULTS AND DISCUSSION
■
Several synthetic strategies to prepare 1,3-thiaphospholes have
been reported;21 however, 1,3-benzothiaphospholes have been
described only once.22 To synthesize the desired heterocycle,
we prepared diisopropyl (2-mercaptophenyl)phosphonate (1)
according to a literature procedure.23 Reduction of compound
1 using lithium aluminum hydride afforded the desired 1-
mercapto-2-phosphinobenzene 2 (Scheme 1).
Scheme 1. Reduction of Diisopropyl (2-
Mercaptophenyl)phosphonate to Prepare 1-Mercapto-2-
phosphinobenzene
Benzannulated variants of 1,3-heterophospholes can be
prepared from a phosphine precursor and a variety of carboxylic
acid derivatives such as imidoyl chlorides, iminoester hydro-
chlorides, or amide acetals.15b,24 Issleib and co-workers
reported the synthesis of 2-phenyl-1,3-benzothiaphosphole
from compound 2 and benzaldehyde; however, efforts to
reproduce this reaction proved challenging.22 The wide
commercial availability of acid chlorides and a previous report
describing 1,3-benzoxaphospholes from acid chlorides led us to
investigate the reaction of 2 with benzoyl chloride.25 The
combination of 2 (31P δ = −127) with benzoyl chloride
proceeded smoothly in toluene at 85 °C, with nearly
quantitative conversion to a single product as evidenced by
31P{1H} NMR spectroscopy (δ = 192). Upon cooling of the
reaction mixture, yellow crystals formed and were collected by
filtration to yield the desired 2-phenyl-1,3-benzothiaphosphole
3a (Scheme 2).
Figure 3. Solid-state molecular structure of 3a. Thermal ellipsoids at
50%. Selected bond lengths (Å) and bond angles (deg): S1−C1
1.757(10); S1−C3 1.800(9); P1−C1 1.677(9); P1−C2 1.789(9);
C1−C8 1.473(11); C1−S1−C3 92.8(5); C1−P1−C2 95.3(5); C8−
C1−P1 122.8(7); C8−C1−S1 119.7(7); P1−C1−S1 117.3(6).
For the 2-aryl-1,3-benzothiaphospholes, redox properties and
frontier orbitals were probed via cyclic voltammetry (Table 1).
Each compound in the series exhibited an irreversible oxidation
indicative of phosphaalkenes (Figure 4). For the parent species
(3a), the irreversibility of the process persists even at elevated
scan rates (5−10 V/s) signaling that the oxidized radical cation
quickly undergoes chemical reaction. High scan rates also reveal
a new reduction process near 0.17 V which is too shifted to be a
reversal of the oxidation and instead is consistent with redox
activity of a product formed from the oxidized species.
Throughout the 2-aryl-1,3-benzothiaphosphole series, rela-
tively high oxidation potentials (Table 1) highlight the electron-
poor nature of these materials as compared to their
oxaphosphole analogs20b and C,C-diacetylenic phosphaalke-
nes.18b DFT calculations for all variants of compound 3 indicate
that the HOMO is largely concentrated on the phosphaalkene
The 31P NMR signal of compound 3a does not match the
one previously reported (31P δ = 55.3).22 However, that report
only includes 31P NMR data without supporting evidence to
prove the formation of the 2-phenyl-1,3-benzothiaphosphole.
In this work, mass spectrometry data and X-ray analysis (Figure
3) are used to confirm the identity of the heterocycle (2-
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Table 1. Photophysical and Electrochemical Properties of 2-Aryl-1,3-Benzothiaphospholes
a
−1
−1
Entry λmax1 abs (nm) ε1 (cm−1
M
)
λmax2 abs (nm) ε2 (cm−1
M
)
λmax emission (nm)
Φ
(%) Eox (V vs SCE) Ered (V vs SCE) ΔEred (mV)
b
3a
3b
3c
3d
3e
3f
271
279
273
271
272
270
34300
330
347
337
331
340
324
36800
60500
443
445
439
430
433
4.4
1.44
1.24
1.49
1.52
1.50
1.56
−1.76
−1.85
−1.62
−1.54
−1.41, −1.91
−1.55
81
b
b
b
b
b
26800
38900
41200
31800
47200
5.4
3.2
1.5
1.7
1.5
82
b
c
49900
39400
32000
43800
-
85
61, 94
c
432
b
77
Quantum yields were measured according to the procedure of Abergel et al.26 Process is completely irreversible. Process is quasi-reversible.
a
c
Figure 5. Frontier orbitals of compound 3b (left) and 3e (right)
generated via DFT calculations performed with a B3LYP/6-31G (d,p)
basis set.
complete reversibility is observed for the one electron
reductions of compounds 3a, 3b, and 3e (Figure 4). The
reversibility in these compounds indicates that the anionic
radicals are stable in solution on the time scale of the anodic
and cathodic sweeps. Such stability suggests potential for these
conjugated materials to conduct electrons and this behavior is
not observed in many types of phosphaalkenes.18b,28
In comparison to the oxidation peaks, the reductions are
roughly half as intense. Thus, while several of the reduction
processes are clearly reversible, one electron processes, the
Figure 4. (Top) Cyclic voltammograms of 3a (red) and 3f (black).
(Bottom) Cyclic voltammogram of 3e showing the most positive first
reduction of the benzothiaphosphole series and a second reduction
unique to 3e. All voltammograms were collected in 0.10 M N(n-
Bu)4PF6 (MeCN) solution.
oxidation may represent a two electron process facilitated by
the favorability of the P(III)/P(V) redox couple. Reduction
also differs from oxidation in that its potentials show a more
defined pattern: they become incrementally more positive as
the electron deficiency of the 2-aryl substituent increases and
enhances LUMO stabilization. DFT calculations explain the
more pronounced effect of electron deficient groups on
reduction by showing significantly more participation of those
substituents in the LUMO than in the HOMO (Figure 5). The
greatest LUMO stabilization is afforded by the C6H4-p-CN
group which shifts 3e’s reduction potential by +0.35 V
compared to the parent 3a and causes observation of a second
reduction within the range of the solvent window (Figure 4-
bottom). In contrast to 3e, compound 3f does not show two
reversible reductions despite its similar C6H4-m-CN group.
This observed difference in reduction occurs because
conjugation allows for stabilization of radicals at the PC
bond by the cyano functionality at the para position but not at
the meta position. This is also reflected in the DFT calculations
which illustrate that the substituent at the para position of the
2-aryl group of the benzothiaphospholes exhibits orbital density
in the LUMO along with the phosphorus−carbon double bond
while the meta position does not. Additionally, DFT
calculations explain the facile reduction of 3e by showing that
its LUMO is stabilized by distribution across its entire C6H4-p-
double bond (Supporting Information). Similar HOMO
assignments have been made previously with other phosphaal-
kene systems.20b,27 The lowest oxidation potential is observed
for compound 3b (1.24 V) due to destabilization of its HOMO
via the C6H4-p-OMe group (Figure 5). Compounds 3c−f have
higher oxidation potentials (1.49 V-1.56 V) as the HOMOs are
stabilized by the electron withdrawing substituent (C6H4-p-Br,
C6H4-p-CF3, C6H4-p-CN, C6H4-m-CN). However, the oxida-
tion potentials do not scale consistently with the electron
withdrawing power of the substituent group suggesting that
they exhibit limited participation in the HOMO. DFT
calculations support this claim as only minor contributions to
the HOMO are observed if the 2-aryl group has an electron
withdrawing substituent (Figure 5 Compound 3e).
In addition to oxidation, the benzothiaphospholes undergo
reduction with varying extents of reversibility. Reduction of
compound 3c is irreversible resulting most likely from
elimination of its halide after radical anion formation.20b Single
quasi-reversible reductions are observed for 3d and 3f while
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CN group (Figure 5). Across the benzothiaphosphole series,
each material is slightly more easily reduced than its
oxaphosphole analogue likely due to facilitation of PC
reduction by the more polarizable sulfur.20b
The reduction potentials of the benzothiaphospholes
correlate with moderate accuracy to the Hammett parameters
of the employed substituents (Supporting Information).
Moreover, redox properties are in agreement with the orbital
energies obtained for 3a−3f through DFT calculations. The
LUMO energies of compounds 3a-3f form a moderately linear
relationship with their related reduction potentials (Figure 6).
The investigated compounds exhibit strong light absorption
in the UV which tails into the visible region of the spectrum
explaining the yellow color of these materials in solution and in
solid-state. Two absorption signals are observed at 271 and 330
nm for the parent compound (Table 1). The λmax of the lower
energy signal is influenced by substituent effects and ranges
from 324 nm (3f) to 347 nm (3b). TD-DFT calculations
confirm the π−π* character of this transition showing that it
involves the HOMO and LUMO orbitals exclusively, with
strong involvement of the PC bond. All of the derivatives
luminesce near 440 nm upon excitation at 330 nm, and the
fluorescence spectrum of 3b along with its absorption are
depicted in Figure 7. The emission of 3b is the most red-shifted
Figure 7. Absorption spectrum (blue) and luminescence spectrum
(red) of 3b collected for a 10 μM MeCN solution. Luminescence was
measured following excitation at 330 nm.
Figure 6. Linear correlation of experimental reduction potentials for
▲
3a−3f with DFT calculations. Blue triangles ( ): LUMO energies
◆
correlated to reduction potentials. Black diamonds ( ): SOMO
in the series because of destabilization of the HOMO by the
methoxy group. However, as with oxidation potentials, the
emission maxima for more electron deficient derivatives do not
follow a clear trend due to minimal participation of their
substituents in the HOMO. Emission quantum yields vary
greatly across the series (Table 1). Electron withdrawing groups
diminish quantum yield while the methoxy functionality
markedly increases it.
energies of the radical anions correlated to reduction potentials. Red
■
rectangles ( ): The calculated energy difference between the radical
anion and neutral benzothiaphosphole species correlated to reduction
potentials.
Strong correlation is also seen between the reductions and the
radical anion SOMO energies calculated to better model
reduction products (Figure 6). Surprisingly, the calculated
radical anions of the 2-aryl-1,3-benzothiaphospholes show
lower absolute energies than the corresponding uncharged
systems. This observation is likely explained by the electro-
negative nature of the heterocyclic phosphorus ring.
Furthermore, all of the radical anions optimize to a flat
geometry, which is in contrast to the 30° twisting observed in
the calculated structure of the neutral species. The resulting
increase in aromaticity could partially explain the observed
stabilization effect for the radical anions. However, perhaps the
best indicator of reduction potentials is the SCF energy
difference of the radical anion and the neutral molecule. The
SCF energy difference between radical anions and neutral
molecules is plotted versus reduction potentials in Figure 6.
The quality of the observed linearity is extraordinary, so
reduction potentials of similar compounds can be predicted
from DFT with great confidence. Interestingly, neither the
HOMO energy nor the energy difference between the radical
cation and the uncharged structure show any relationship with
the measured oxidation potentials for the 2-aryl-1,3-benzothia-
phospholes. The irreversible nature of the oxidation hints at the
involvement of more complex processes, which are under-
standably problematic to evaluate by simple DFT calculations.
CONCLUSION
■
The preparation of a series of 2-aryl-1,3-benzothiaphospholes
has been reported and their redox and photophysical behavior
has been evaluated. Electronic modulation of the benzothia-
phosphole materials was employed as a strategy to alter the
reduction and oxidation potentials of the ring system. Some of
the benzothiaphosphole derivatives yielded accessible, rever-
sible reductions highlighting the potential of these compounds
as n-type charge transport systems.
EXPERIMENTAL SECTION
■
Materials and Methods. All reactions and manipulations of air or
water sensitive compounds were carried out under dry nitrogen using a
glovebox or standard Schlenk techniques unless otherwise specified.
Diisopropyl (2-mercaptophenyl)phosphonate (1) was prepared
according to a published procedure from thiophenol.23 In the
synthesis of 1, diisopropyl bromophosphate (prepared from
triisopropyl phosphite and bromine [(31P{1H} δ = −11]), was used
as opposed to diisopropyl chlorophosphate. All solvents (toluene,
diethyl ether, tetrahydrofuran, hexanes) were degassed with argon and
dried prior to use. Acetonitrile, used for photophysical characterization
and electrochemistry measurements, was degassed prior to use. CDCl3
was dried using P2O5 and distilled prior to use. C6D6 was dried over 4
Å sieves prior to use. Melting points are uncorrected.
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NMR Analysis. NMR spectra were recorded on either a 300 MHz
or a 500 MHz Spectrometer. The 31P{1H} NMR spectra were
referenced to an external standard (85% H3PO4). The 1H NMR
spectra are referenced to residual protio solvents (7.24 for CHCl3 and
charged with 1-mercapto-2-phosphinobenzene (0.375 g, 2.64 mmol),
5 g of dry toluene, and an aryl acid chloride (1 equiv, 2.64 mmol). The
Schlenk flask was removed from the glovebox and immersed in an oil
bath at 85 °C. The solution was stirred overnight and the colorless
solution turned bright yellow. An aliquot of the reaction mixture was
removed using a syringe and analyzed using 31P NMR spectroscopy.
Complete consumption of the starting material was confirmed by
disappearance of the 31P{1H} NMR signal at −127 ppm. Formation of
the desired benzothiaphosphole was confirmed by the appearance of a
31P{1H} NMR signal in the range of 185−203 ppm. The reaction
vessel was removed form the oil bath, and cooled to room
temperature. The flask was placed in a refrigerator at 4 °C for several
hours and, upon removal from the fridge, yellow crystals were
observed in the reaction flask. The yellow crystals were collected via
filtration and washed with cold ether (3 × 10 mL).
General Procedure B: Synthesis of 2-Aryl-1,3-benzothiaphosp-
holes 3c and 3d. In a N2 filled glovebox, a 50 mL Schlenk flask was
charged with 1-mercapto-2-phosphinobenzene (0.375 g, 2.64 mmol),
5 g of dry toluene, and an aryl acid chloride (1 equiv, 2.64 mmol). The
Schlenk flask was removed from the glovebox and immersed in an oil
bath at 85 °C. The solution was stirred overnight and the colorless
solution turned bright yellow. An aliquot of the reaction mixture was
removed using a syringe and analyzed using 31P NMR spectroscopy.
Complete consumption of the starting material was confirmed by
disappearance of the 31P{1H} NMR signal at −127 ppm. Formation of
the desired benzothiaphosphole was confirmed by the appearance of a
31P{1H} NMR signal in the range of 185−203 ppm. The volatiles of
the reaction mixture were removed in vacuo and the crude mixture was
brought into the glovebox. The residue was dissolved in a minimal
amount of solvent, and placed in a 20 mL scintillation vial. The vial
was stored in the freezer at −35 °C overnight. Yellow crystals were
observed in the scintillation vial and they were collected by filtration.
The filtrate was placed back in the freezer and a second batch of
crystals was obtained. These were also collected by filtration.
General Procedure C: Synthesis of 2-Aryl-1,3-benzothiaphosp-
holes 3e and 3f. In a N2 filled glovebox, a 50 mL Schlenk flask was
charged with 1-mercapto-2-phosphinobenzene (0.375 g, 2.64 mmol),
5 g of dry toluene, and an aryl acid chloride (1 equiv, 2.64 mmol). The
Schlenk flask was removed from the glovebox and immersed in an oil
bath at 85 °C. After stirring overnight, the colorless solution had
turned bright yellow and some precipitate was observed. The hot
reaction mixture was separated from the precipitate by a hot gravity
filtration. The yellow filtrate was collected and immediately transferred
to a 20 mL scintillation vial. The vial was allowed to cool to room
temperature and placed in a refrigerator at 4 °C for several hours and,
upon removal from the fridge, yellow crystals were observed in the
reaction flask. The yellow crystals were collected via filtration and
washed with cold ether (3 × 10 mL).
2-Phenyl-1,3-benzothiaphosphole (3a). was prepared following
general procedure A. The combination of compound 2 (0.375 g, 2.64
mmol) and benzoyl chloride (0.370 g, 2.64 mmol) afforded 3a as
yellow crystals (0.234 g, 39%). Mp 142−145 °C dec 31P{1H} NMR
(202 MHz, C6D6) δ 191.9. 1H NMR (500 MHz, C6D6): δ 8.06 − 8.00
(m, 1H), 7.89 − 7.84 (m, 2H), 7.63 (br d, J = 7.8 Hz, 1H), 7.11 − 6.98
(m, 5H). 13C NMR (126 MHz, CDCl3) δ 181.1 (d, JPC = 52.8 Hz),
153.8 (d, JPC = 39.8 Hz), 148.4 (d, JPC = 10.2 Hz), 137.0 (d, JPC = 16.1
Hz), 130.9 (d, JPC = 27.5 Hz), 129.4 (d, JPC = 3.5 Hz), 129.3, 126.9 (d,
JPC = 14.6 Hz), 126.7 (d, JPC = 3.5 Hz), 124.1 (d, JPC = 13.4 Hz),
123.4. HR-EIMS (m/z): [M]+ calculated for C13H9PS: 228.0163;
found 228.0153.
2-(4-Methoxyphenyl)-1,3-benzothiaphosphole (3b). was prepared
following general procedure A. The combination of compound 2
(0.375 g, 2.64 mmol) and 4-methoxybenzoyl chloride (0.450 g, 2.64
mmol) afforded 3b as yellow crystals (0.170, 25%). Mp 164−165 °C
dec 31P{1H} NMR (202 MHz, CDCl3): δ 184.6. 1H NMR (500 MHz,
CDCl3) δ 8.23 (br t, J = 7.0 Hz, 1H), 8.02 (br d, J = 8.1 Hz, 1H), 7.83
− 7.78 (AA′BB′ system, A centered on 7.82, A′ centered on 7.80, 2H),
7.42 (br t, J = 7.6 Hz, 1H), 7.36 (br t, J = 7.5 Hz, 1H), 6.94 − 6.90
(AA′BB′ system, B centered on 6.94, B′ centered on 6.92, 2H), 3.84
(s, 3H). 13C NMR (126 MHz, CDCl3) δ 181.2 (d, JPC = 53.1 Hz),
1
7.16 for C6D5H). In the H NMR spectra of 3a−3f, significant fine
splittings (∼1−2 Hz) were observed from long-range coupling
1
(1H−1H and H−31P) on the fused aromatic rings. Signals that are
listed as broad typically exhibited fine splitting that was left unassigned.
1
The H NMR spectra of all compounds, except for 3a and 3f, have
shown the typical ABCD spin systems pattern for ring A and an
AA′BB′ spin system characteristic of p-disubstituted aromatic rings.
13C NMR spectra are referenced to the solvent signal (δ 77.23 CDCl3)
and JPC coupling is observed for most signals.
Photophysical Characterization. Photoluminescence measure-
ments were performed at room temperature using 10 μM solutions in
acetonitrile using a capped quartz cuvette (1.0 cm). UV−vis
absorption spectra were measured using a dual-beam scanning
spectrophotometer. Photoluminescence emission spectra were meas-
ured on a fluorimeter equipped with dual monochromators and a
photomultiplier tube (PMT) at right angle geometry. The emission
spectra were recorded at an excitation wavelength of 330 nm.
Photoluminescent quantum yields (Φ) were measured against 100 μM
quinine sulfate in 0.5 M H2SO4,Φ = (Is/Iref)(Aref/As)(ηs/ηref),26 where
Φ is the quantum yield for the sample, Is and Iref represent the points
of maximum intensity in the emission spectra of the sample and
reference, As and Aref are the absorbance of the sample and the
reference at the excitation wavelength, and ηref and ηs are the refractive
indices of the solvents of the reference and of the sample used for
UV−vis absorption measurements.
Electrochemical Analyses. Electrochemical potentials were
determined using an electrochemical analyzer at a potential sweep
rate of 100 mV/s. A 1 mm2 platinum working electrode, a platinum
coil counter electrode, and a silver wire pseudoreference electrode
were employed for the measurements. Ferrocene was used as an
internal standard referenced against SCE at 0.4 V (EFc/Fc+).29 Solutions
of the benzothiaphosphole were prepared at ∼1−2 mM and tetra-n-
butylammonium hexafluorophosphate (electrochemical grade) served
as the supporting electrolyte at a concentration of 0.1 M. The MeCN
solutions with the supporting electrolyte were degassed for 5 min with
bubbling Ar prior to adding the benzothiaphosphole.
Computational Studies. Hybrid density functional theory (DFT)
calculations were performed for compounds 3a−3f using a B3LYP/6-
31G (d,p) basis set in the Gaussian 03 suite.30
Mass Spectrometry. High-resolution mass spectrometry data was
obtained using a double focusing sector instrument.
Synthesis of 1-Mercapto-2-phosphinobenzene (2). A 1000
mL Schlenk flask was charged with lithium aluminum hydride (14.25
g, 375 mmol) and dried in vacuo for 15 min. To this was added 400
mL THF and the mixture was cooled to 0 °C using an ice bath. Slow
addition of 1 (33.40 g, 122 mmol) over a period of 8 min produced a
vigorous reaction with significant gas evolution (the addition of 1 was
conducted slowly so as to control the rate of gas evolution). Upon
completion of the addition, the ice bath was removed and the reaction
mixture was stirred overnight at room temperature. The mixture was
quenched with 6 M HCl (100 mL) and the layers were allowed to
separate. The organic layer was cannula transferred through a filter
funnel containing Celite into a 1000 mL Schlenk flask containing
magnesium sulfate. This extraction process was repeated using dry,
degassed hexanes (4 × 75 mL). Then the dried organic layer was
transferred to another 1000 mL Schlenk flask via a filter cannula. After
removing volatiles in vacuo, the crude product was distilled using a
short path column (64 °C, 0.1 Torr) to afford 2 as colorless oil. The
31
1
31P{1H} and H NMR spectra were compared to a previous report.
1
Often, the H spectrum of the distilled product contained impurities
that are suspected to be aluminum isopropoxide salts. Loading the 1-
mercapto-2-phosphinobenzene onto silica gel under an atmosphere of
N2 and eluting with degassed hexanes helped remove the impurity
(7.51 g, 43%).
General Procedure A: Synthesis of 2-Aryl-1,3-benzothiaphosp-
holes 3a and 3b. In a N2 filled glovebox, a 50 mL Schlenk flask was
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160.9 (d, JPC = 3.7 Hz), 153.8 (d, JPC = 39.7 Hz), 148.0 (d, JPC = 10.1
ASSOCIATED CONTENT
* Supporting Information
NMR spectra, cyclic voltammograms, absorption and emission
spectra, crystallographic data, calculated energies, coordinates,
and orbitals images. This material is available free of charge via
the Internet http://pubs.acs.org.
■
Hz), 130.7 (d, JPC = 27.6 Hz), 130.0 (d, JPC = 16.2 Hz), 128.1 (d, JPC
=
S
14.7 Hz), 126.3 (d, JPC = 3.4 Hz), 124.1 (d, JPC = 13.2 Hz), 123.2,
114.6, 55.6. HR-EIMS (m/z): [M]+ calculated for C14H11OPS:
258.0268; found 258.0260. Anal. Calcd for C14H11OPS: C, 65.10; H,
4.29. Found: C, 64.84; H, 4.15.
2-(4-Bromophenyl)-1,3-benzothiaphosphole (3c). was prepared
following general procedure B using compound 2 (0.375 g, 2.64
mmol) and 4-bromobenzoyl chloride (0.579 g, 2.64 mmol). 3c was
obtained as a yellow powder (0.246 g, 30%) after recrystallization from
Toluene/THF (10:1). Mp 176−177 °C dec 31P{1H} NMR (202
AUTHOR INFORMATION
Corresponding Author
*E-mail: noonan@andrew.cmu.edu
Notes
■
1
MHz, C6D6): δ 194.0. H NMR (500 MHz, CDCl3) δ 8.26 (br t, J =
7.1 Hz, 1H), 8.04 (br d, J = 8.1 Hz, 1H), 7.73 − 7.69 (AA′BB′ system,
A centered on 7.72, A′ centered on 7.70, 2H), 7.53 − 7.49 (AA′BB′
system, B centered on 7.52, B′ centered on 7.51, 2H), 7.46 (br t, J =
7.6 Hz, 1H), 7.38 (br t, J = 7.5 Hz, 1H). 13C NMR (126 MHz, CDCl3)
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
δ 179.1 (d, JPC = 52.6 Hz), 153.7 (d, JPC = 39.7 Hz), 148.4 (d, JPC
=
The authors are grateful to Carnegie Mellon for financial
support of this work and to Prof. Roberto Gil for help with
NMR assignments. NMR Instrumentation at Carnegie Mellon
was partially supported by the National Science Foundation
(CHE-0130903 and CHE-1039870). The authors also
acknowledge support from the National Science Foundation
through CHE-1055547 and the Princeton MRSEC grant DMR-
0819860. D.N.C. gratefully acknowledges the support of the
U.S. Department of Energy Office of Science Graduate
Research Fellowship. The mass spectrometer was purchased
in part with a grant from the Division of Research Resources,
National Institutes of Health (RR 04648).
10.4 Hz), 136.0 (d, JPC = 16.6 Hz), 132.3, 131.0 (d, JPC = 27.6 Hz),
128.2 (d, JPC = 14.8 Hz), 126.9 (d, JPC = 3.5 Hz), 124.3 (d, JPC = 13.5
Hz), 123.5, 123.4. HR-EIMS (m/z): [M]+ calculated for C13H8BrPS:
305.9268; found 305.9267. Anal. Calcd for C13H8BrPS: C, 50.84; H,
2.63. Found: C, 50.58; H, 2.38.
2-(4-(Trifluoromethyl)phenyl)-1,3-benzothiaphosphole (3d). Fol-
lowing the general procedure B using compound 2 (0.375 g, 2.64
mmol) and 4-(trifluoromethyl)benzoyl chloride (0.550 g, 2.64 mmol)
afforded 3d as yellow crystals (0.256 g, 33%) after recrystallization
from THF. Mp 215−217 °C dec 31P{1H} NMR (202 MHz, C6D6): δ
199.5. 1H NMR (500 MHz, C6D6) δ 8.02 (br t, J = 6.8 Hz, 1H), 7.63
(br d, J = 7.9 Hz, 1H), 7.60 (br d, J = 7.3 Hz, 2H), 7.24 (br d, J = 8.2
Hz, 2H), 7.08 − 6.99 (m, 2H). 13C NMR (126 MHz, CDCl3) δ 178.4
(d, JPC = 52.7 Hz), 153.9 (d, JPC = 40.1 Hz), 148.8 (d, JPC = 10.5 Hz),
140.5 (d, JPC = 16.8 Hz), 131.2 (d, JPC = 27.7 Hz), 127.3 (d, JPC = 3.6
Hz), 127.1 (d, JPC = 14.7 Hz), 126.3 and 126.2 (overlapping doublets),
124.5 (d, JPC = 13.6 Hz), 124.3, (q, JFC = 271.5 Hz), 123.5. HR-EIMS
(m/z): [M]+ calculated for C14H8F3PS: 296.0036; found 296.0031.
Anal. Calcd for C14H8F3PS: C, 56.76; H, 2.72. Found: C, 56.48; H,
2.49.
2-(4-Cyanophenyl)-1,3-benzothiaphosphole (3e). was prepared
following general procedure C. The combination of 2 (0.375 g, 2.64
mmol) and 4-cyanobenzoyl chloride (0.437 g, 2.64 mmol) afforded a
crude yield of 0.254 g of 3e as yellow crystals. Recrystallization from
toluene yielded analytically pure 3e (0.139 g, 21%). Mp 178 °C dec
31P{1H} NMR (202 MHz, C6D6): δ 202.7. 1H NMR (500 MHz,
C6D6) δ 7.98 (br t, J = 6.9 Hz, 1H), 7.60 (br d, J = 7.8 Hz, 1H), 7.44 −
7.39 (AA′BB′ system, A centered on 7.43, A′ centered on 7.41, 2H),
7.07 − 6.97 (m, 2H), 6.94 − 6.89 (AA′BB′ system, B centered on 6.92,
B′ centered on 6.91, 2H). 13C NMR (126 MHz, CDCl3) δ 177.2 (d,
JPC = 52.7 Hz), 153.7 (d, JPC = 39.7 Hz), 148.7 (d, JPC = 10.5 Hz),
141.3 (d, JPC = 16.8 Hz), 133.0, 131.3 (d, JPC = 27.6 Hz), 127.5 (d, JPC
= 3.7 Hz), 127.1 (d, JPC = 15.4 Hz), 124.5 (d, JPC = 13.8 Hz), 123.6,
118.8, 112.4 (d, JPC = 4.2 Hz). HR-EIMS (m/z): [M]+ calculated for
C14H8NPS: 253.0115; found 253.0111. Anal. Calcd for C14H8NPS: C,
66.39; H, 3.18; N, 5.53. Found: C, 66.09; H, 2.91; N, 5.29.
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following general procedure C. The combination of 2 (0.375 g, 2.64
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1
MHz, C6D6) δ 198.5. H NMR (300 MHz, C6D6) δ 7.98 (br td, J =
6.7, 1.9 Hz, 1H), 7.80 (br d, J = 1.8 Hz, 1H), 7.65 − 7.57 (m, 2H),
7.09 − 6.97 (m, 2H), 6.87 (dd, J = 7.8, 1.1 Hz, 1H), 6.60 (br t, J = 7.8
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=
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