Naphthoxaphospholes as examples of fluorescent phospha-acenes

Article abstract of DOI:10.1039/c2dt30902e

Seven new fluorescent 2-R-naphtho[2,3-d]oxaphospholes (R-NOPs) (4a-g; R = ^tBu (a), Ad (b), C₆H₅ (c), 4-MeC ₆H₄ (d), 4-ClC₆H₄ (e), 4-BrC ₆H₄ (f), 4-MeOC₆H₄ (g)), have been synthesized by cyclocondensation reactions of benzimidoyl chlorides with 3-phosphino-2-naphthol (3). The compounds were characterized by multinuclear NMR, UV-vis, and fluorescence spectroscopy. Compounds 4a-d and 4g were characterized by cyclic voltammetry experiments. The solid state structures of compounds 4b and 4d were also determined by single-crystal X-ray diffraction experiments.

Full text of DOI:10.1039/c2dt30902e

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PAPER
Naphthoxaphospholes as examples of fluorescent phospha-acenes†
Feng Li Laughlin,a Arnold L. Rheingold,b Nihal Deligonul,a Brynna J. Laughlin,c Rhett C. Smith,c
Lee J. Highamd and John D. Protasiewicz*a
Received 25th April 2012, Accepted 6th July 2012
DOI: 10.1039/c2dt30902e
Seven new fluorescent 2-R-naphtho[2,3-d]oxaphospholes (R-NOPs) (4a–g; R = tBu (a), Ad (b), C6H5 (c),
4-MeC6H4 (d), 4-ClC6H4 (e), 4-BrC6H4 (f), 4-MeOC6H4 (g)), have been synthesized by
cyclocondensation reactions of benzimidoyl chlorides with 3-phosphino-2-naphthol (3). The compounds
were characterized by multinuclear NMR, UV-vis, and fluorescence spectroscopy. Compounds 4a–d and
4g were characterized by cyclic voltammetry experiments. The solid state structures of compounds 4b and
4d were also determined by single-crystal X-ray diffraction experiments.
examination of the electrochemistry of these compounds
confirmed that like phosphaalkenes, these materials also feature
Introduction
2-Substituted 1,3-benzoxaphospholes (R-BOPs, Chart 1), first
reported by Heinicke et al.,1–3 are fascinating conjugated hetero-
cyclic molecules featuring phosphorus–carbon double bonds.4 In
particular, they are unusual in that unlike phosphaalkenes
(RPvCR2), they display significant fluorescence and are quite
thermally stable despite lacking sterically demanding groups
that are often required to afford kinetic stability to many
phosphaalkenes.5–9
We have recently expanded on the synthesis of R-BOPs, and
pioneered the synthesis of the difunctional analogues 2,6-
disubstituted-benzo[1,2-d:4,5-d′]bisoxaphospholes (R2-BBOPs,
Chart 1).10 These conjugated materials were determined to have
quantum yields reaching as high as 0.63. In addition, detailed
lower lying LUMO orbitals introduced by the presence of the
PvC functionality. This aspect of phosphaalkene chemistry
makes such compounds more electron deficient compared to
molecules where a CvC bond exists in place of a PvC unit.11
Many multiply bonded organophosphorus compounds having
PvC units behave in a chemical manner similar to CvC con-
taining molecules, hence, such compounds have often been
called “carbon copies”.5 Yet the increased facility that phospha-
alkenes can undergo reduction offers new possibilities for
applications.
Pentacenes (and related acenes) are a well studied class of
molecules having much promise for use as molecular organic
semi-conductors and field effect transistors.12 Pentacene
(Chart 2, left) is a purple material that slowly degrades in air and
light. Much effort has gone into the design and synthesis of new
acenes that can be accessed in high purity and that also possess
increased stability, solubility, and appropriate solid state mor-
phologies. The air stability of such materials could be enhanced
if the overall potentials of the materials are lowered. N-Hetero-
acenes have been investigated for this purpose.13 A lot of recent
work has appeared on the development of conjugated materials
built around phospholes as integrated units in phosphorus-
containing heteroacenes.14,15 In this report we detail the syn-
thesis and full characterization of expanded versions of R-BOPs,
2-R-naphtho[2,3-d]oxaphospholes (R-NOPs, Chart 2, right).
This study represents the first step towards creation of a series of
increasingly extended electron deficient acene-like planar
materials featuring bona-fide PvC bonds.
Chart 1 Structures of previously studied benzoxaphospholes.
aDepartment of Chemistry, Case Western Reserve University, Cleveland,
Ohio, 44106, USA. E-mail: protasiewicz@case.edu
bDepartment of Chemistry and Biochemistry, University of California,
San Diego, La Jolla, California 92093, USA.
E-mail: arheingold@ucsd.edu
cDepartment of Chemistry and Center for Optical Materials Science and
Engineering Technologies (COMSET), Clemson University, Clemson,
South Carolina 29634, USA
dSchool of Chemistry, Newcastle University, Newcastle upon Tyne,
NE1 7RU, UK. E-mail: lee.higham@ncl.ac.uk
†Electronic supplementary information (ESI) available: Details of syn-
thesis for 1 and 2, crystallographic, electrochemical, UV-vis, fluore-
scence, and computational details. CCDC 878276–878278. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt30902e
Chart 2 Comparison between pentacene and a phospha-acene.
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Compound 2 was thus reduced using LiAlH4 to form com-
pound 3 in 95.6% yield. Interestingly, this white solid displays
much greater air stability than typical primary phosphines. For
example, solutions of 3 in CDCl3 solution at room temperature
persist for a month or more without signs of decomposition
Results and discussion
1. Syntheses and NMR properties
The route to R-NOPs required the synthesis of the previously
unknown 3-phosphino-2-naphthol (3). Following the procedures
developed by Dhawan and Redmore,16 the immediate precursor
to this material, diethyl(3-hydroxy-2-naphthyl) phosphonate (2)
could be accessed by application of an anionic phospho-Fries
rearrangement of readily available diethyl(2-naphthyl)phosphate
(1) (Scheme 1).
1
(as ascertained by H and 31P{1H} NMR spectroscopy). In the
solid state compound 3 is stable for several months or longer.
Compound 3 thus possesses greater air-stability than the
2-phosphino-naphthalene, which decomposes slowly in CDCl3
(72% of 2-phosphino-naphthalene remains after 7 days17). An
interesting predictive model for the enhanced stability of select
primary phosphines has recently been forwarded by Higham and
coworkers.17,18 Two electronic factors were found to be impor-
tant. First, primary phosphines possessing extended conjugation
and/or heteroatoms were found to have HOMOs that were dis-
located from the phosphino functionality and enhanced air-
stability. Second, the products of oxidation of the primary phos-
phines, radical cations, having SOMO energy levels above
−10 eV will have enhanced air-stability. The oxidized phos-
phines ½RPH2ꢀ˙þ , are postulated to be involved in the air oxi-
dation of phosphines. We have thus undertaken analogous
DFT calculations on 3˙þ . DFT studies on the radical cation of
2-phosphino-naphthalene reveals a SOMO energy level of
−10.64 eV, while similar calculations on the 3˙þ show a SOMO
energy level of −10.72 eV (see ESI†). One might initially
predict less air-stability for compound 3 compared to 2-phos-
phino-naphthalene, but as Fig. 2 reveals, the SOMO of 3˙þ has
no significant phosphorus contributions. The special enhanced
stability of 3 might thus be particularly attributed to a shift in the
SOMO away from phosphorus (unlike 2-phosphino-naphtha-
lene). An interesting alternative explanation might lie in the
nature of the hydroxy substituent, for it has been shown that oxi-
dation of secondary phosphines can be inhibited by
hydroquinones.19
Single crystals of 2 suitable for an X-ray diffraction study
were obtained from hexanes at −45 °C. The internal metrics for
2
are unexceptional, but the presence of intermolecular
PvO⋯HO hydrogen bonding leads to chains of molecules
along the c-axis (Fig. 1).
The synthesis of R-NOPs 4a–g was effected by the cyclo-
condensation of benzimidoyl chlorides with 3 in THF under
Scheme 1 Synthesis of R-NOPs.
Fig. 1 Packing diagram of
2 showing intermolecular hydrogen
bonding.
Fig. 2 MO diagram and SOMO for radical cation of 3.
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nitrogen. Yields ranged from modest to good, owing to the diffi-
culty in purifying the products from similarly soluble starting
materials and the anilinium salts. Compounds 4a–g were isolated
as crystalline white (4a,b) or yellow (4c–g) solids that possessed
reasonable air and water stability, especially in the solid state.
The R-NOPs were readily identified by characteristic 31P{1H}
NMR chemical shifts resonating between 72.8–86.6 ppm, con-
sistently ca. 2–3 ppm upfield from similarly substituted
R-BOPs.10 Likewise, 13C{1H} NMR chemical shifts for the
PvC carbons were consistently located 2.8–3.6 ppm upfield
(between 198.3 to 217.9 ppm) for R-NOP versus R-BOP pairs.
The 1JPvC coupling constants are ca. 63 Hz for alkyl substituted
R-NOPs, and ca. 55 Hz for aryl substituted R-NOPs.
coplanarity of 7.6°. The angles about the phosphorus atoms in
4b and 4d are very similar (88.0–88.3°). For pentacenes, the
crystal packing is an important consideration for their use. The
crystal packing of 4b is unexceptional, but the packing of
molecules of 4d is more interesting in the crystal lattice as
displayed in Fig. 5. The packing may be described as herring-
bone-like with aromatic CH groups directed towards the aromatic
systems of other molecules (closest CH⋯C distance is 2.78 Å).
3. Absorption and emission spectroscopy of NOPs
The UV-vis absorption spectra of 4a–g are shown in Fig. 6 and
7. The data reveal π–π* transitions for the alkyl-NOP derivatives
4a and 4b both at 333 nm (Fig. 6). Aryl-NOPs (4c–g), by con-
trast, show π–π* transitions varying from 353 to 360 nm
(Fig. 7). Similar bathochromic shifts (∼30 nm) are also observed
between alkyl and aryl substituted BOPs.10 These trends are also
reproduced by DFT calculations (vide infra). Over the concen-
tration range 10−6 to 10−5 M the absorption data showed no evi-
dence for aggregation for 4a–g.
2. Structural studies of NOPs
Single crystals of 4b and 4d were subjected to X-ray diffraction
analyses, and the results are shown in Fig. 3 and 4 in the form of
ORTEP structural diagrams. For compound 4d, the crystal con-
tains two crystallographic independent molecules of 4d.
Within the NOP rings, the atoms are co-planar. The PvC
bond lengths for 4b and 4d are 1.700(3) and 1.716(3)/1.723(2)
Å, respectively, and are consistent with the presence of a phos-
phorus–carbon double bond. Compound 4d has a slightly longer
bond length, possibly attributable to the extended conjugation of
the NOP ring with the attached phenyl group. Notably, this ring
lies in the plane of the NOP system with a maximum twist from
Compounds 4a–g are notably fluorescent under UV light,
appearing violet to blue in color. As for the absorption spectra,
the fluorescence behavior for these compounds break down into
Fig. 3 ORTEP diagram of 4b. Selected bond lengths (Å) and angles
(°): P(1)–C(5), 1.700(3); P(1)–C(6), 1.806(3); O(1)–C(15), 1.373(3);
O(1)–C(5), 1.386(3); C(5)–P(1)–C(6), 88.3(1); C(15)–O(1)–C(5),
110.6(2); O(1)–C(5)–P(1), 116.9(2).
Fig. 5 Packing diagram for compound 4d.
Fig. 4 ORTEP diagram of one of two independent molecules of 4d.
Selected bond lengths (Å) and angles (°): P(1)–C(7), 1.716(3); P(1)–
C(16), 1.789(3); O(6)–C(5), 1.373(3); O(6)–C(7), 1.374(3); C(7)–P(1)–
C(16), 88.0(1); C(5)–O(6)–C(7), 110.9(2); O(6)–C(7)–P(1), 116.6(2).
Fig. 6 UV-vis and fluorescence spectra for 4a and 4b in CH2Cl2.
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Fig. 7 UV-vis and fluorescence spectra for 4c–g in CH2Cl2.
Table 1 UV-vis and fluorescence spectra for 4a–g in CH2Cl2
Fig. 8 Cyclic voltammograms for compounds 4a–d and 4g, in THF
with 0.1 M [nBu4N][BF4].
R
λmax (nm)
λF,max (nm)
ΦF
τ (ns)
4a
4b
4c
4d
4e
4f
tBu
Ad
C6H5
4-MeC6H4
4-ClC6H4
4-BrC6H4
4-MeOC6H4
333
333
353
356
357
357
360
385
381
461
464
465
464
470
0.23
0.26
0.12
0.14
0.13
0.13
0.22
1.24(2)
1.26(1)
0.76(1)
0.75(1)
0.44(1)
0.80(5)
0.84(1)
Table 2 Cyclic voltammograms for compounds 4a–d and 4g, in THF
with 0.1 M [nBu4N][BF4]
Epc
Epa
E(pc + pa)/2
4g
4a
4b
4c
4d
4g
—
—
−1.92
−1.96
−2.02
—
—
−1.61
−1.67
−1.75
—
—
−1.77
−1.82
−1.89
two sets of behavior based on the identity of the R substituents.
The aryl substituted compounds 4c–g displayed λem maxima
from 461–470 nm, while the alkyl substituted derivatives 4a and
4b showed maxima around 383 nm (Table 1). Compared to com-
parably substituted R-BOPs,10 R-NOPs exhibited about a 40 nm
wavelength redshift for λem maxima.
While there are significant parallels in the absorption and flu-
orescence spectra of R-BOPs and R-NOPs, there are striking
differences in the measured quantum yields (ΦF). In a series of
BOPs, aryl substituted BOPs showed quantum yields (0.56–0.69
for ΦF) that were over ten times greater than alkyl-substituted
analogues.10 By contrast, all of the R-NOPs not only have sig-
nificant fluorescence, but it is the alkyl substituted R-NOPs that
have the greater quantum yields. Specifically, compounds 4a and
4b have quantum yields of 0.23 and 0.26, respectively, while
compounds 4c–f have ΦF in the range of 0.12–0.14, except 4g
which shows ΦF of 0.22. The reasons for these observations are
not clear at this time.
As one might expect, as the electron donating ability of the
p-XC6H4 moves from H to Me and OMe, the reduction becomes
more difficult. Overall, the R-NOPs (R = aryl) reduction occurs
more readily than analogous R-BOPs (by 13–14 mV).10
Many of the above experimental trends observed between sets
of electrochemical data of R-NOPs and R-BOPs can be repro-
duced by DFT calculations. Fig. 9 shows a comparison of the
frontier molecular orbitals of Ph-NOP and Ph-BOP. The nature
of the HOMOs and LUMOs are very similar between the pair of
compounds. The greatest difference lies in the smaller HOMO–
LUMO gap for Ph-NOP (3.39 eV vs. 3.84 eV).
t
A similar comparison can be made for Ph-NOP and Bu-NOP,
in order to illustrate differences between alkyl and aryl substi-
tuted NOP compounds. Fig. 10 shows that the tert-butyl deriva-
tive has HOMO and LUMO orbitals that are localized only on
the NOP ring system, and has a greater HOMO–LUMO gap
(3.97 vs. 3.39 eV). The 0.52 eV higher lying LUMO orbital
correlates with the inability to observe the reduction of the alkyl-
substituted NOP derivatives.
4. Electrochemical and computational studies of NOPs
Electrochemical experiments were performed on several rep-
resentative R-NOPs to probe their facility towards reduction in
THF. Cyclic voltammograms of compounds 4a–d and 4g are
thus shown in Fig. 8.
Two types of electrochemical behavior were displayed for
the NOPs. The alkyl-NOPs 4a and 4b did not show evidence
for reduction within the electrochemical window examined.
By contrast, aryl substituted R-NOPs 4c, 4d and 4g show quasi-
reversible reduction waves (ipa/ipc ∼ 0.6) between −1.8 to
−1.9 V vs. SCE (see Table 2).
Conclusions
The synthesis and detailed characterization of a new class of
fluorescent compounds having low coordinate multiply bonded
phosphorus atoms have been presented. The data support the
conclusion that these materials possess greater conjugation than
previously reported R-BOP species. The synthesis of the
This journal is © The Royal Society of Chemistry 2012
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were made using an EasyLife II lifetime fluorimeter from Photon
Technology International, Inc. The excitation slit width for all
measurements was kept at default settings (5 nm). Melting
points were measured on a Thomas Hoover Capillary Melting
Point Apparatus. Elemental analyses were performed by
Robertson Microlit Laboratories, Ledgewood, New Jersey.
High resolution mass spectrometry was performed by the Uni-
versity of Michigan Mass Spectrometry facility using a VG
(Micromass) 70–250-S magnetic sector spectrometer with EI
technique at 70 eV.
Fig. 9 MO diagrams for Ph-NOP and Ph-BOP.
3-Phosphino-2-naphthol (3)
Diethyl (3-hydroxy-2-naphthyl)phosphonate (2, 4.02 g, 14.3 mmol)
was added to a 500 mL round bottom flask containing a solution
of LiAlH4 (1.09 g, 28.6 mmol) in 100 mL of THF and a stir bar.
The solution was stirred and refluxed under nitrogen for 1 hour.
The flask was opened and an aqueous solution of saturated
ammonium chloride was added drop wise to quench the excess
LiAlH4. The mixture was extracted with chloroform (200 mL)
and the organic layer was separated and then filtered through
celite using a glass fritted filter funnel. The filtrate was evapo-
rated by rotary evaporation to yield white solid 3 (2.41 g,
1
3
95.6%). H NMR (CDCl3): δ 8.04 (d, 1H, JPH = 9.6 Hz), 7.72
(m, 1H), 7.66 (m, 1H), 7.43 (m, 1H), 7.33 (m, 1H), 7.17 (d, 1H,
4JPH = 2.4 Hz), 5.29 (s, 1H, OH), 4.20 (s, 1H, PH), 3.69 (s, 1H,
PH); 31P{1H} NMR (CDCl3): δ −148.3 (s). 13C{1H} NMR
(CDCl3): δ 154.4 (d, JPC = 4.7 Hz), 137.4 (d, JPC = 22.3 Hz),
135.4, 128.9 (d, JPC = 8.0 Hz), 127.3, 127.0, 126.1, 123.9,
117.3 (d, JPC = 12.2 Hz), 109.0 (d, JPC = 1.4 Hz); mp:
134–135 °C; Elemental Analysis: Calc. for C10H9OP (M.W.
176.15), C 68.18%, H 5.15%; found: C 67.91%, H 5.10%.
Fig. 10 MO diagrams for Ph-NOP and tBu-NOP.
difunctional analogues of NOPs is now underway to strive
towards new conjugated low band gap analogues of pentacenes
featuring phosphorus atoms participating in conjugation with the
aromatic system.
2-(tert-Butyl)naphtho[2,3-d]oxaphosphole (4a)
Experimental
3-Phosphino-2-naphthol (3, 1.00 g, 5.68 mmol) and N-phenyl-
pivalimidoyl chloride (3.67 g, 18.8 mmol) were added to a
250 mL round bottom flask with a stir bar, and the flask was
outfitted with a reflux condenser and then flushed with nitrogen.
THF (35 mL) was added by cannula to the flask and the solution
was refluxed for 15 hours. The reaction mixture was taken into a
dry box, and filtered using glass fritted filter funnel. The solid
was extracted 2 times with THF (∼5 mL) and the combined
filtrate was evaporated under vacuum yield a brown solid. Purifi-
cation by column chromatography (CH2Cl2–hexanes 1 : 9, Rf =
General considerations
Reactions were conducted under nitrogen using either Schlenk
line techniques or within an MBraun drybox. Tetrahydrofuran,
toluene and diethyl ether were dried by distillation from sodium
and benzophenone ketyl. Hexanes was dried by distillation from
sodium and benzophenone in the presence of tetraethylene
glycol dimethyl ether. Methylene chloride was dried by distilla-
tion from calcium hydride. Methylene chloride (99.8%, for
spectroscopy) and ethanol (200 proof) were degassed prior to
use for UV-vis and fluorescence measurements. Benzimidoyl
chlorides were prepared following literature protocols. NMR
spectra (1H and 31P{1H}) were recorded in CDCl3 on a Varian
INOVA AS-400 spectrometer operating at 399.7 and 161.8 Hz,
respectively, and 31P{1H} NMR were referenced to 85% H3PO4.
13C{1H} NMR spectra were recorded in CDCl3 on a Varian
INOVA AS-600 spectrometer operating at 150.9 Hz. UV-vis and
fluorescence spectra were recorded using a Cary-5G-UV-vis-NIR
spectrophotometer and a Cary Eclipse spectrometer, respectively.
Anthracene in ethanol was used as a standard for quantum yield
measurements (conc. 5.0 × 10−6 M). Lifetime measurements
1
0.8) led to isolation of white solid 4a (0.89 g, 65%). H NMR:
3
δ 8.40 (d, 1H, JPH = 4.0 Hz), 8.08 (s,1H), 7.93 (m, 2H),
4
7.47 (m, 2H), 1.52 (d, 9H, JHH = 1.2 Hz); 31P{1H} NMR:
δ 74.1 (s); 13C{1H} NMR: δ 217.6 (d, JPC = 63.6 Hz), 158.6,
136.2 (d, JPC = 38.1 Hz), 132.4 (d, JPC = 2.3 Hz), 129.9
(d, JPC = 9.2 Hz), 128.4 (d, JPC = 19.2 Hz), 127.7, 127.6, 125.8,
124.4, 108.8, 38.4 (d, JPC = 11.8 Hz), 29.7 (d, JPC = 9.1 Hz);
UV (CH2Cl2): λmax, nm (ε, M−1 cm−1) 333 (12398); Fluore-
scence (CH2Cl2, 5 × 10−6 M): λex, nm (Int.) 385 (504);
Quantum yield (CH2Cl2): ΦF 0.229; mp: 77–80 °C; Elemental
analysis: Calc. for C15H15OP (M. W. 242.25), C 74.37%,
H 6.24%; found: C 73.80%, H 6.20%.
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2-(Adamantyl)naphtho[2,3-d]oxaphosphole (4b)
a 250 mL round bottom flask with a stir bar. The flask
was outfitted with a reflux condenser and flushed with
nitrogen and THF (30 mL) was added by cannula. The solution
was refluxed for 20 hours. The reaction mixture was taken into
a dry box and filtered using glass fritted filter funnel.
The solid on the frit was extracted 2 times with THF (∼5 mL)
and the combined filtrate was evaporated under vacuum yield a
brown solid. The solid was dissolved in CH2Cl2 (∼75 mL),
washed successively with degassed solutions of aqueous HCl
(1.2 M, 100 mL), aqueous NaOH (1 M, 100 mL) and distilled
H2O (100 mL). The solvent was removed under vacuum to yield
a yellow solid which was washed with degassed ethanol
(∼150 mL) and recrystallized from toluene at −45 °C to yield
yellow solid 4d (0.910 g, 58.0%). 1H NMR: δ 8.47 (d, 1H,
3JPH = 2.8 Hz), 8.13 (s, 1H), 7.98 (m, 2H), 7.93 (m, 2H), 7.47
3-Phosphino-2-naphthol (3, 0.600 g, 3.41 mmol) and N-adaman-
tyl-phenylbenzimidoyl chloride (1.73 g, 6.32 mmol) were added
in a 250 mL round bottom flask with a stir bar, and the flask was
outfitted with a reflux condenser and then flushed with nitrogen.
THF (25 mL) was added to the flask by cannula under nitrogen.
The solution was refluxed for 15 hours. The reaction mixture
was taken into a dry box, and filtered using glass fritted filter
funnel. The solid was extracted 2 times with THF (∼5 mL) and
the combined filtrate was evaporated under vacuum yield a
brown solid. Purification by column chromatography (CH2Cl2–
hexanes 1 : 9, Rf = 0.8) led to white solid 4b (0.72 g, 66%).
3
1H NMR: δ 8.44 (d, 1H, JPH = 4.0 Hz), 8.07 (s, 1H), 7.93
(m, 2H), 7.46 (m, 2H), 2.13 (s, 9H), 1.83 (s, 6H); 31P
{1H} NMR: δ 72.8 (s); 13C{1H} NMR: δ 217.9 (d, JPC
62.8 Hz), 158.4, 136.1 (d, JPC = 38.1 Hz), 132.3 (d, JPC
=
=
3
(m, 2H), 7.25 (d, 2H, JHH = 5.2 Hz), 2.41 (s, 3H); 31P{1H}
NMR: δ 80.1 (s); 13C{1H} NMR: δ 200.3 (d, JPC = 55.6 Hz),
2.1 Hz), 129.8 (d, JPC = 9.4 Hz), 128.4 (d, JPC = 19.2 Hz),
127.7, 127.6, 125.7, 124.3, 108.7, 41.8 (d, JPC = 9.4 Hz), 40.3
(d, JPC = 10.9 Hz), 36.7, 28.3 (d, JPC = 1.4 Hz); UV (CH2Cl2):
λmax, nm (ε, M−1 cm−1) 333 (10972); Fluorescence (CH2Cl2, 5 ×
10−6 M): λex, nm (Int.) 381 (755); Quantum yield (CH2Cl2):
ΦF 0.256; mp: 145–147 °C; Elemental analysis: Calc. for
C21H21OP (M. W. 320.36), C 78.73%, H 6.61%; found: C
78.52%, H 6.82%.
158.0 (d, JPC = 3.5 Hz), 140.7 (d, JPC = 5.0 Hz), 136.8 (d, JPC
=
37.1 Hz), 132.8 (d, JPC = 2.6 Hz), 132.0 (d, JPC = 13.0 Hz),
130.1 (d, JPC = 10.3 Hz), 129.6, 128.6 (d, JPC = 19.2 Hz),
127.7, 127.7, 126.0, 125.1 (d, JPC = 14.6 Hz), 124.6, 109.0,
21.6; UV (CH2Cl2): λmax, nm (ε, M−1 cm−1) 356 (28654);
Fluorescence (CH2Cl2, 5 × 10−6 M): λex, nm (Int.) 464 (689);
Quantum yield (CH2Cl2): ΦF 0.140; mp 170–174 °C; Elemental
analyses: Calc. for C18H13OP (M. W. 276.27), C 78.25%,
H 4.74%; found: C 78.36%, H 4.96%.
2-(Phenyl)naphtho[2,3-d]oxaphosphole (4c)
3-Phosphino-2-naphthol (3, 0.350 g, 1.99 mmol) and
4-methoxy-N-phenylbenzimidoyl chloride (1.28 g, 5.93 mmol)
were added in a 250 mL round bottom flask with a stir bar, and
the flask was outfitted with a reflux condenser and then flushed
with nitrogen. THF (25 mL) was added to the flask by cannula
under nitrogen. The solution was refluxed for 20 hours. The
reaction mixture was taken into a dry box and filtered using glass
fritted filter funnel. The solid on the frit was extracted 2 times
with THF (∼5 mL) and the combined filtrate was evaporated
under vacuum yield a brown solid. This solid was dissolved in
CH2Cl2 (∼75 mL), and the solution was washed successively
with degassed solutions of aqueous HCl (1.2 M, 100 mL),
aqueous NaOH (1 M, 100 mL) and distilled H2O (100 mL). The
solvent was removed under vacuum to yield a yellow solid,
which was washed with degassed ethanol (∼150 mL) and then
recrystallized from toluene at −45 °C to yield yellow solid 4c
2-(4-Chlorophenyl)naphtho[2,3-d]oxaphosphole (4e)
3-Phosphino-2-naphthol (3, 0.480 g, 2.72 mmol) and 4-chloro-
N-phenylbenzimidoyl chloride (2.22 g, 8.87 mmol) were added
in a 250 mL round bottom flask with a stir bar and the flask was
outfitted with a reflux condenser and flushed with nitrogen. THF
(25 mL) was added by cannula and the solution was refluxed for
20 hours. The reaction mixture was taken into a dry box and
filtered using glass fritted filter funnel. The solid was extracted 2
times with THF (∼5 mL) and the combined filtrate was evapor-
ated under vacuum yield a brown solid. The solid was dissolved
in CH2Cl2 (∼75 mL), washed successively with degassed
solutions of aqueous HCl (1.2 M, 100 mL), aqueous NaOH
(1 M, 100 mL) and distilled H2O (100 mL). The solvent was
removed under vacuum to yield a yellow solid which was
washed with degassed ethanol (∼150 mL) and then recrystallized
from toluene at −45 °C to yield yellow solid 4e (0.350 g,
1
3
(0.150 g, 28.8%). H NMR: δ 8.51 (d, 1H, JPH = 3.2 Hz), 8.17
(s, 1H), 8.11 (m, 2H), 7.96 (m, 2H,), 7.47 (m, 5H); 31P{1H}
NMR: δ 83.7 (s); 13C{1H} NMR: δ 199.9 (d, JPC = 55.3 Hz),
158.0, 136.6 (d, JPC = 37.3 Hz), 134.6 (d, JPC = 12.8 Hz), 132.9
1
3
43.2%). H NMR: δ 8.51 (d, 1H, JPH = 4.8 Hz), 8.15 (s, 1H),
8.03 (m, 2H), 7.95 (m, 2H), 7.50 (m, 2H), 7.43 (m, 2H);
(d, JPC = 2.9 Hz), 130.3 (d, JPC = 5.0 Hz), 130.1 (d, JPC
10.3 Hz), 128.9, 128.7, 127.8, 127.7, 126.1, 125.1 (d, JPC
=
=
)
31P{1H} NMR: δ 85.8(s); 13C{1H} NMR: δ 198.3 (d, JPC
=
14.8 Hz), 124.7, 109.1; UV (CH2Cl2): λmax, nm (ε, M−1 cm−1
54.7 Hz), 158.0 (d, JPC = 3.8 Hz), 136.4 (d, JPC = 37.0 Hz),
136.0 (d, JPC = 6.0 Hz), 133.1 (d, JPC = 13.1 Hz), 133.0
(d, JPC = 2.6 Hz), 130.1 (d, JPC = 10.6 Hz), 129.1, 129.0 (d,
JPC = 19.0 Hz), 127.8, 127.8, 126.3, 126.3 (d, JPC = 14.9 Hz),
353 (24246); Fluorescence (CH2Cl2, 5 × 10−6 M): λex, nm (Int.)
461 (516); Quantum yield (CH2Cl2): ΦF 0.121; mp:
173–176 °C; Elemental analysis: Calc. for C17H11OP (262.24),
C 77.86%, H 4.23%; found: C 77.63%, H 4.16%.
124.8, 109.1; UV (CH2Cl2): λmax
, )
nm (ε, M−1 cm−1
357 (28 710); Fluorescence (CH2Cl2, 5 × 10−6 M): λex, nm (Int.)
465 (661); Quantum Yyield (CH2Cl2): ΦF 0.128; mp
190–194 °C; Elemental analysis: Calc. for C17H10ClOPC
(M. W. 296.69), C 68.82%, H 3.40%; found: C 69.01%,
H 3.32%.
2-(4-Methylphenyl)naphtho[2,3-d]oxaphosphole (4d)
3-Phosphino-2-naphthol (3, 1.00 g, 5.68 mmol) and 4-methyl-N-
phenylbenzimidoyl chloride (1.95 g, 8.52 mmol) were added in
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 12016–12022 | 12021

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2-(4-Bromophenyl)naphtho[2,3-d]oxaphosphole (4f)
Acknowledgements
3-Phosphino-2-naphthol (3, 0.600 g, 3.41 mmol) and 4-bromo-
N-phenylbenzimidoyl chloride (3, 1.86 g, 6.31 mmol) were
added in a 250 mL round bottom flask with a stir bar. The flask
was outfitted with a reflux condenser and flushed with nitrogen.
THF (30 mL) was added by cannula and the solution was
refluxed for 20 hours. The reaction mixture was taken into a dry
box and filtered using glass fritted filter funnel. The solid was
extracted 2 times with THF (∼5 mL), the combined filtration
was evaporated under vacuum yield a brown solid. The solid
was dissolved in CH2Cl2 (∼75 mL), washed successively with
degassed solutions of aqueous HCl (1.2 M, 100 mL), aqueous
NaOH (1 M, 100 mL) and distilled H2O (100 mL). The solvent
was removed under vacuum to a yield yellow solid, which was
washed with degassed ethanol (∼150 mL) and recrystallized
from toluene at −45 °C to yield yellow solid 4f (0.380 g,
We thank the National Science Foundation for support
(CHE-0748982 and CHE-1150721).
Notes and references
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3
7.97 (m, 4H), 7.59 (d, 2H, JHH = 8.4 Hz), 7.50 (m, 2H);
31P{1H} NMR: δ 86.7 (s, 1P); 13C{1H} NMR: δ 198.3 (d, JPC
=
54.8 Hz), 158.0 (d, JPC = 3.6 Hz), 136.4 (d, JPC = 37.0 Hz),
133.5 (d, JPC = 13.1 Hz), 133.0 (d, JPC = 2.7 Hz), 132.1, 130.2
(d, JPC = 10.3 Hz), 129.0 (d, JPC = 19.2 Hz), 127.8, 127.8,
126.5 (d, JPC = 14.9 Hz), 126.3, 124.9, 124.3 (d, JPC = 6.0 Hz),
109.2; UV (CH2Cl2): λmax, nm (ε, M−1 cm−1) 357 (28086);
Fluorescence (CH2Cl2, 5 × 10−6 M): λex, nm (Int.) 464 (633);
Quantum yield (CH2Cl2): ΦF 0.125; mp 202–205 °C; HRMS
m/z: 339.9655 (calc. 339.9653).
2-(4-Methoxyphenyl)naphtho[2,3-d]oxaphosphole (4g)
3-Phosphino-2-naphthol (3, 0.500 g, 2.84 mmol) and 4-
methoxy-N-phenylbenzimidoyl chloride (1.68 g, 6.84 mmol)
were added in a 250 mL round bottom flask with a stir bar. The
flask was outfitted with a reflux condenser and flushed with
nitrogen. THF (25 mL) was added by cannula and the solution
was refluxed for 20 hours. The reaction mixture was taken into a
dry box and filtered using glass fritted filter funnel. The solid
was extracted 2 times with THF (∼5 mL) and the combined
filtrate was evaporated under vacuum yield a brown solid.
The solid was dissolved in CH2Cl2 (∼75 mL), washed succes-
sively with degassed solutions of aqueous HCl (1.2 M, 100 mL),
aqueous NaOH (1 M, 100 mL) and distilled H2O (100 mL).
The solvent was removed under vacuum to yield a yellow solid
which was washed with degassed ethanol (∼150 mL) and recrys-
tallized from toluene at −45 °C to yield yellow solid 4g
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3
(0.600 g, 72.3%). 1H NMR: δ 8.47 (d, 1H, JPH = 4.8 Hz),
8.13 (s, 1H), 8.05 (m, 2H), 7.94 (m, 2H), 7.48 (m, 2H), 6.98
(m, 2H); 31P{1H} NMR: δ 74.9 (s, 1P); 13C{1H} NMR: δ 200.2
(d, JPC = 56.2 Hz), 161.4 (d, JPC = 4.8 Hz), 157.9 (d, JPC
3.2 Hz), 142.0(d, JPC = 20.1 Hz), 136.9 (d, JPC = 37.3 Hz),
132.7 (d, JPC = 2.4 Hz), 130.1 (d, JPC = 9.8 Hz), 128.4 (d, JPC
19.3 Hz), 127.7, 127.7, 126.9 (d, JPC = 14.5 Hz), 125.9, 124.6,
114.3, 108.8, 55.5; UV (CH2Cl2): λmax, nm (ε, M−1 cm−1
=
=
)
360 (23566); Fluorescence (CH2Cl2, 5 × 10−6 M): λex, nm
(Int.) 470 (876); Quantum yield (CH2Cl2): ΦF 0.219; mp
195–198 °C; Elemental analysis: Calc. for C18H13O2P
(M. W. 292.27), C 73.91%, H 4.48%; found: C 73.40%, H
4.16%.
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12022 | Dalton Trans., 2012, 41, 12016–12022
This journal is © The Royal Society of Chemistry 2012