A dual-emissive phosphine-borane Lewis pair with a U-shaped linker: Impact of methylation and complexation on fluoride binding affinity

Article abstract of DOI:10.1021/om4011406

To investigate phosphine to borane through-space charge transfer and its utility in anion sensing and the formation of metal complexes, a phosphine-borane Lewis pair (1) connected by a U-shaped linker has been synthesized. 1 could be readily converted to a phosphonium salt (2) and two 2:1 complexes with Au(I) (1-Au) and Pt(II) (1-Pt). The photophysical properties of the new compounds were examined and compared. Compound 1 displayed an intense P → B CT transition and a bright solvent-dependent dual emission that is switchable by fluoride ions. 2 and 1-Pt showed a turn-off and a turn-on fluorescent response, respectively, toward fluoride ions. The binding constant of 2 with F^- was found to be 2 orders of magnitude greater than that of 1. The mechanism that is responsible for the distinct fluorescence response of 1, 2, and 1-Pt toward fluoride ions is proposed.

Full text of DOI:10.1021/om4011406

Article
pubs.acs.org/Organometallics
A Dual-Emissive Phosphine−Borane Lewis Pair with a U‑Shaped
Linker: Impact of Methylation and Complexation on Fluoride Binding
Affinity
Yufei Li,† Youngjin Kang,‡ Jia-Sheng Lu,† Ian Wyman,† Soo-Byung Ko,† and Suning Wang*,†
†Department of Chemistry, Queen’s University, Kingston, Ontario K7M 3N6, Canada
‡Division of Science Education, Kangwon National University, Chuncheon 200-701, Republic of Korea
S
* Supporting Information
ABSTRACT: To investigate phosphine to borane through-
space charge transfer and its utility in anion sensing and the
formation of metal complexes, a phosphine−borane Lewis pair
(1) connected by a U-shaped linker has been synthesized. 1
could be readily converted to a phosphonium salt (2) and two
2:1 complexes with Au(I) (1-Au) and Pt(II) (1-Pt). The
photophysical properties of the new compounds were
examined and compared. Compound 1 displayed an intense
P → B CT transition and a bright solvent-dependent dual emission that is switchable by fluoride ions. 2 and 1-Pt showed a turn-
off and a turn-on fluorescent response, respectively, toward fluoride ions. The binding constant of 2 with F− was found to be 2
orders of magnitude greater than that of 1. The mechanism that is responsible for the distinct fluorescence response of 1, 2, and
1-Pt toward fluoride ions is proposed.
with a distinct P → B CT emission peak and a strong
switchable or “turn-on” response toward fluoride ions.
Compound 1 can be readily converted to a phosphonium salt
and metal complexes that display distinctively different
response toward fluoride ions. The details are presented herein.
INTRODUCTION
■
Donor−acceptor compounds that contain a triarylboryl group
are known to display charge transfer (CT) transitions, which
can lead to bright fluorescence if connected by a conjugated
linker1 or weak fluorescence by a nonconjugated linker.2a−c
Such donor−acceptor compounds have found broad applica-
tions in optoelectronic materials and sensory materials.1,2
Nonconjugated donor−acceptor compounds are particularly
interesting because they can faciliate turn-on fluorescent
sensing for anions2a−c and may possess unusual reactivity as
frustrated Lewis pairs.3 Previously known nonconjugated
donor−acceptor systems that display through-space CT
fluorescence are limited to amino−borane compounds.2a−c
Recently a number of nonconjugated phosphine−borane
compounds incorporating P and B atoms that are directly
attached to a 1,8-naphthyl backbone have been reported.4
However, because the P and B atoms in these systems are
bound to each other or are in very close proximity (P−B
distance 2.08−3.05 Å), they show few interesting photophysical
or chemical properties.4 To force the P and B atoms apart and
achieve through-space CT fluorescence, one effective approach
is to insert a linker between the naphthyl and the P and B
atoms. Such systems would allow us to access not only new
through-space CT systems for sensing applications but also
unusual reactivity introduced by the phosphine group, which is
much more reactive than the conjugated amino group, and the
bulky Lewis acidic boryl group in the vicinity.3 On the basis of
these considerations, we have obtained the first example of a U-
shaped molecule, 1 (Scheme 1), which contains an unbound
phosphine−borane pair and displays an intense dual emission
RESULTS AND DISCUSSION
■
Synthesis and Structures. Compound 1 was synthesized
and isolated in overall 39% yield by the procedure shown in
Scheme 1 via Negishi coupling of P(p-Br-Ph)Ph2 with the
BMes2-phenyl functionalized iodo-naphthyl (BI), using Pd-
(PPh3)4 as the catalyst. Unlike the related N−B molecules with
either an NMe2 or an NAr2 group, which do not bind to a metal
ion and are very difficult to alkylate due to the stabilization of
the lone pair via π conjugation with the aryl ring, the P−B
compound 1 was readily methylated by reacting with CH3I in a
1:1 ratio in refluxing THF, producing the phosphonium
compound 2 in high yield.
Furthermore, despite the highly congested nature of
molecule 1, it could also bind readily to transition-metal ions,
generating the metal complexes in high yields. The Au(I)
complex (1-Au) and Pt(II) complex (1-Pt) were obtained by
the reaction of 1 with Au(SMe2)Cl and Pt(SMe2)2Cl2,
respectively, according to Scheme 2. Both complexes have a
metal to ligand 1 ratio of 1:2. The 1:1 complex for the Au(I)
metal ion was not observed when a 1:1 ratio of 1 and
Au(SMe2)Cl was used in the reaction. The metal complexes
Received: November 25, 2013
© XXXX American Chemical Society
A
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Scheme 1. Synthetic Procedure for 1 and 2
Scheme 2. Synthetic Procedure for 1-Au and 1-Pt
Table 1. Selected Bond Lengths (Å) and Angles (deg)
bond length (Å)
compound
B−CMes
B−CPh
P−C
M−Cl
M−P
1
1.575(4)
1.576(3)
1.561(3)
1.831(2)
1.832(3)
1.833(3)
1-Au
1-Pt
1.59(1)
1.59(1)
1.56(1)
1.58(1)
1.819(7)
2.787(3)
2.301(2)
2.319(2)
2.316(2)
1.822(7)
1.826(7)
1.551(9)
1.57(1)
1.811(6)
1.830(7)
1.851(8)
bond angle (deg)
compound
CMes−B−CMes
CMes−B−CPh
C−P−C
P−M−Cl
P−M−P
Cl−M−Cl
1
118.9(2)
124.0(7)
124.0(6)
121.4(2)
119.7(2)
103.4(1)
100.6(1)
102.9(1)
101.6(3)
104.3(3)
105.2(3)
104.2(3)
106.4(3)
105.7(3)
1-Au
1-Pt
118.0(6)
117.9(7)
100.84(5)
158.31(9)
180.0
117.5(6)
118.5(6)
94.34(7)
85.66(7)
180.0
B
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were fully characterized by NMR and elemental analyses. In the
31P NMR spectra, compound 1 displays a characteristic
phosphine signal at −6.0 ppm, while that of 2 appears at
22.4 ppm, as is typical of phosphonium.5 For the metal
complexes, the 31P chemical shift appears at 33.5 ppm for 1-Au
and 19.5 ppm for 1-Pt, indicating that the phosphine center
donates electrons to the AuCl unit more effectively than the
PtCl2 unit.6 To fully understand the properties of these new
compounds, the crystal structures of 1, 1-Au, and 1-Pt were
determined by X-ray diffraction analysis. Efforts to grow single
crystals of compound 2 were unsuccessful. The crystal
structures of 1, 1-Au, and 1-Pt are shown in Figures 1−3,
respectively. Key bond lengths and angles are given in Table 1.
The P atom in 1 has a typical pyramidal geometry, while the
B atom has a typical trigonal-planar geometry. Because of steric
interactions, the two phenyl linkers are repelled by each other
and the dihedral angles between the peri phenyls and the
naphthyl benzene ring are significantly distorted from 90°
(∼127°). The C−C bond lengths between the phenyl linkers
and the naphthyl carbon atoms are 1.488(3) and 1.489(3) Å,
respectively, which are typical of a C−C single bond. The
separation distance between the B and the P atom is 6.24(1) Å,
a distance that is much longer than those of the closely related
N−B systems (∼5.5 Å)2a−c and can be attributed to the
sterically demanding pyramidal geometry of the P unit. The
naphthyl ring is also considerably distorted from planarity, as
indicated by the 16.2° dihedral angle between the two fused
benzene rings and the side view of the structure in Figure 1.
Figure 2. Diagrams showing the structure of 1-Au with 35% thermal
ellipsoids: (top) side view; (bottom) front view. H atoms are omitted
for clarity.
radii8 of Au(I) and Cl− (2.33 Å). Thus, the Au(I) center in 1-
Au may be best described as a distorted-linear geometry with a
weakly bound Cl−. The significant departure of 1-Au from the
trigonal-planar geometry displayed by Au(PPh3)2Cl may be
attributed to the steric bulkiness of the ligand 1. In the crystal
lattice, there are CHCl3 solvent molecules (4 CHCl3/per 1-
Au), two of which form H bonds with the Cl ion in 1-Au (see
the Supporting Information). Because of the steric congestion,
no short Au···Au interactions are present in the lattice.
The structure of 1-Pt is shown in Figure 3. Like the 1-Au
molecule, the Pt(II) center is also bound by two 1 ligands. The
Figure 1. Diagrams showing the structure of 1 with 35% thermal
ellipsoids: (left) front view; (right) side view. H atoms are omitted for
clarity.
The structure of 1-Au compound is shown in Figure 2. This
molecule has a crystallographically imposed C2 symmetry with a
helical arrangement of the two 1 ligands. The Au(1) atom has a
triangular arrangement with the two B atoms (Au(1)···B(1) =
6.53 Å, B(1)···Au(1)···B(1A) = 122.0°). The Au(I) center is
bound by two P atoms and one Cl anion with an approximate
T shape, as evidenced by the P(1)−Au(1)−P(1A) angle
(158.31(9)°) and the P(1)−Au(1)−Cl(1) angle (100.84(5)°).
Although many examples of three-coordinate Au(I) com-
pounds with a T-shaped geometry have been known
previously,7 the structure of 1-Au is unusual. In comparison
to the closely related compound Au(PPh3)2Cl, which has an
approximate trigonal-planar geometry with a P−Au−P angle of
132.1(1)° and P−Au−Cl angles of 109.2(1) and 112.8(1)°,7b
1-Au is much more distorted from a trigonal-planar geometry.
Further, the Au(1)−P(1) bond length (2.319(2) Å) in 1-Au is
similar to those in Au(PPh3)2Cl (2.323(4), 2.339(4) Å), but
the Au(1)−Cl(1) bond (2.787(3) Å) is much longer than that
in Au(PPh3)2Cl (2.500(4) Å) and the sum of the covalent
Figure 3. Diagrams showing the structure of 1-Pt with 35% thermal
ellipsoids. H atoms are omitted for clarity.
molecule of 1-Pt has a crystallographically imposed inversion
center with a trans square-planar geometry. Unlike 1-Au, which
has a very long Au−Cl bond, the Pt−P and Pt−Cl bond lengths
are typical (Table 1). The higher coordination number and the
much shorter Pt−Cl bonds in 1-Pt, in comparison to those of
1-Au, are consistent with the greater donation of the P atoms to
the metal center in 1-Au, as indicated by the 31P NMR data.
The Pt···B distance is very long at 7.31 Å. The chloride ions are
C
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oriented toward the B atom with a separation distance of 6.38
Å. In solution, the trans isomer was observed exclusively, which
is clearly favored by the bulky ligand 1. In the crystal lattice, the
Pt(II) units of 1-Pt are stacked along the a axis with a very long
Pt···Pt separation distance (8.59 Å).
Absorption and Fluorescence Spectra. Compound 1.
Compound 1 has an intense absorption band at λmax 330 nm
that does not change significantly with solvent polarity (see
Table 2). In the fluorescence spectrum, 1 shows dual emission
BMes2 acceptor, which display the N → B CT emission peak
only.2 This difference can be explained by the stabilization of
the amino radical cation through π conjugation with the aryl
group that stabilizes the charge-separated excited state in the
N−B compounds, making the CT emission an efficient and
competitive relaxation pathway in the N−B compounds.
On the basis of the solvent dependence of the dual emission
and the previous study on the related N−B systems, the low-
energy peak of 1 was assigned to a P → B through-space CT
transition while the high-energy peak corresponds to a Mes (π)
→ B-Ph (π*) transition. This assignment was further supported
by the fact that a diboron compound which has a structure
similar to that of 1 except that the phosphine group in 1 is
replaced with a BMes2 displays the high-energy emission peak
only.2d The abnormal behavior of 1 in DMSO, DMF, and
CH3CN was most likely caused by the weak coordination of
these solvent molecules to the B center that blocks the P → B
CT, since a similar phenomenon was observed in the previously
reported U-shaped N−B molecules.2 Thermal quenching of the
CT emission may also be a factor due to the low energy of the
CT state in the high-polarity solvents. The fluorescent quantum
efficiency (ΦFL = 0.07 in acetone, 0.13 in CH2Cl2, and 0.30 in
hexane) of 1 increased with decreasing solvent polarity, as the P
→ B CT peak shifts toward higher energy.
a
Table 2. Photophysical Properties
b
absorption
fluorescence
λem (nm)
compound
solvent
λmax (nm)
log ε
ΦFL
1
hexanes
toluene
CH2Cl2
THF
330
335
330
335
335
335
340
335
330
4.53
4.45
4.50
4.41
4.43
4.40
4.48
4.87
4.60
400 (sh), 460
404 (sh), 493
407, 532
404, 530
415, 554
422
0.30
0.31
0.13
0.04
0.07
0.25
0.55
0.44
acetone
DMSO
CH2Cl2
CH2Cl2
CH2Cl2
2
410
1-Au
1-Pt
402
408
0.02
a
b
All spectra were recorded using a 1.0 × 10−5 M solution. The
fluorescence quantum efficiency was determined using 9,10-
diphenylanthracene as a reference.
The assignment of the dual emission in 1 was also supported
by TD-DFT calculation data on the vertical excitation of the
ground state (Table 3 and Figure 5). The S1 transition of 1 is
mainly the P lone pair (HOMO) → B-Ph (π*, LUMO) CT
transition with significant contributions from the naphthyl to
in most solvents except coordinating solvents such as DMSO,
DMF, and CH3CN, in which the fluorescence spectrum of 1
displays either only one emission peak at ∼410 nm (DMSO,
DMF) or is dominated by the 410 nm emission peak (CH3CN)
(see Figure 4 and the Supporting Information). The high-
Table 3. TD-DFT Calculated Vertical Excitation Energy and
Oscillator Strengths for All Compounds and the Fluoride
Adducts of 1 and 2
energy
(nm)
oscillator strength
a
compound state
transition
(f)
1
S1
S2
S1
S2
H → L (80%)
362
359
369
361
417
369
0.2522
0.0806
0.0113
0.0878
0.0026
0.0490
H-2 → L (80%)
H → L (91%)
b
2
H → L+1 (70%)
H → L (99%)
NMe4[1-F] S1
S3
H-2 → L (77%)
H-3 → L (19%)
H-2 → L (21%)
H-3 → L (76%)
H → L+1 (98%)
H-2 → L (93%)
H-4 → L (22%)
H-3 → L+1 (39%)
H-4 → L+1 (33%)
H-3 → L (25%)
H → L (93%)
S4
358
0.1818
2-F
S2
S4
S4
409
371
369
0.0083
0.0024
0.0159
1-Pt
S5
368
0.1019
Figure 4. Fluorescence spectra and photographs showing the emission
color of 1 in various solvents (λex 330 nm, 1.0 × 10−5 M) with relative
intensities. The fluorescence spectra in DMF and CH3CN can be
found in the Supporting Information.
1-Au
S1
S3
S7
S8
436
399
379
378
0.0030
0.0030
0.0040
0.0030
H-1 → L+1 (93%)
H → L+3 (95%)
H → L+2 (92%)
H-6 → L+1 (32%)
H-5 → L (33%)
H-6 → L+1 (32%)
energy emission peak of 1 exhibits a small red shift (e.g., λmax
400 nm in hexane and 415 nm in acetone) with increasing
solvent polarity, while the low-energy signal shows a
pronounced red shift (λmax 460 nm in hexane and 554 nm in
acetone). Because of the dual emission, 1 has a white
fluorescence color in THF and CH2Cl2. The dual emissive
behavior of 1 is in sharp contrast to the previously reported U-
shaped amino−BMes2 molecules with a similar 1,8-diphenyl-
naphthyl linker between an NMe2 or an NAr2 donor and a
S9
367
367
0.0112
0.2075
S10 H-6 → L (32%)
H-5 → L+1 (32%)
a
Vertical excitation states with f < 0.001 are not given. For the full
b
listing, please see the Supporting Information. The anion used in the
computation for 2 is PF6 .
−
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Figure 5. Electronic transitions and the associated MO diagrams that
are responsible for the low-energy absorption bands of 1 and NMe4[1-
F]. The MO diagrams are plotted with an isocontour value of 0.03.
Figure 7. Normalized fluorescent emission spectra of 1, 2, 1-Au, and
1-Pt in CH2Cl2.
+
The NMe4 cation was omitted for clarity.
the HOMO and a great oscillator strength (f = 0.2522). The S2
transition (HOMO-2 → LUMO, 80%) is mainly from the Mes
(π) → B-Ph (π*) transition with a smaller oscillator strength
(0.0806). The calculated vertical excitation energies of these
two states are close. In solution, the S1 state could be greatly
stabilized by polar solvent molecules due to the highly polarized
nature of this state, leading to the distinct solvent polarity
dependent CT emission. Thus, the high-energy and low-energy
emission peaks of 1 are mostly likely from the S2 and S1 states,
respectively.
Compounds 2. The absorption spectrum of 2 is red-shifted,
in comparison to that of 1 (Figure 6). The fluorescence
Figure 8. Electronic transitions and the associated MO diagrams that
are likely responsible for the low-energy absorption bands of 2 and 2-
−
F. The MOs are plotted with an isocontour value of 0.03. The PF6
anion in 2 is omitted for clarity.
ring and the phosphonium unit, respectively, while LUMO+1 is
mainly on the phenyl linker between the phosphonium and the
naphthyl with a significant contribution from the empty p
orbital of the boron atom. The fluorescence of 2 may be
therefore attributed to a mixed transition of Mes(π) →
phosphonium(π*) and Mes(π) → Ph(π*).
Metal Complexes. The absorption spectra of the metal
complexes are similar to that of 1 (Figure 6). No
phosphorescence was observed for 1-Au and 1-Pt even at 77
K. The fluorescence spectra of 1-Au and 1-Pt resemble that of
2 with λmax at 402 and 408 nm, respectively (Figure 7),
consistent with the blocking of the P → B CT transition by
metal ion coordination. Surprisingly, the ΦFL value (0.44 in
CH2Cl2) of 1-Au is much higher than that of 1, but that of 1-Pt
is much lower (0.02 in CH2Cl2).
TD-DFT data showed that for 1-Pt the S1−S3 states all have
an oscillator strength of 0.000 involving mostly the transition of
the Cl− lone pair electrons and the Pt(II) d electrons to a Pt−
ligand (Cl and C) σ* orbital (n → σ*); thus, they are unlikely
to be responsible for the observed fluorescence. The S4 and the
S5 states of 1-Pt are degenerate with an appreciable oscillator
strength, involving transitions of HOMO-3/HOMO-4 to
LUMO/LUMO+1 (57−61%) that may be described as Mes(π)
→ B-Ph(π*) transitions, similar to the S2 state of 1 and 2, as
shown in Figure 9. The fluorescence of 1-Pt is therefore most
Figure 6. Absorption spectra of 1, 2, 1-Au, and 1-Pt in CH2Cl2.
spectrum of 2 has only one emission peak at 410 nm in CH2Cl2
that is red-shifted by a few nanometers, in comparison to that
of 1 (Figure 7). The absence of the low-energy emission peak
in the spectrum of 2 further confirmed that the low-energy peak
of 1 is indeed from a P → B CT transition, since such a
transition is not possible in 2. The fluorescent quantum
efficiency (0.55 in CH2Cl2) of 2 is much higher than that of 1
(0.13).
TD-DFT computational results for 2 indicated that the S1
transition is mainly from a HOMO → LUMO transition (91%,
oscillator strength 0.0113) and is 7 nm lower in energy than
that of 1, while the S2 transition that is close in energy is
dominated by a HOMO → LUMO+1 transition (70%,
oscillator strength 0.0878). As shown in Figure 8, the
HOMO and LUMO levels of 2 are localized on the mesityl
E
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and S10 in 1-Au are similar to those of S4 and S5 in 1-Pt. We
therefore suggest that the fluorescence of 1-Au likely has the
same origin as that of 1-Pt: namely, the Mes(π) → B-Ph(π*)
transition. The low fluorescent ΦFL value of 1-Pt is likely
caused by partial quenching of the Mes(π) → B-Ph(π*)
transition by the low-lying n → σ* states. Similar quenching in
1-Au by the low-lying Cl− → π* states is likely to a much lesser
degree due to the weak association of the chloride ion with the
complex, as indicated by the very long Au−Cl bond in the
crystal structure, which is very likely fully dissociated in
solution, giving the propensity of the Au(I) ion to adapt a linear
two-coordinate structure.
Fluoride Sensing. Fluoride titration experiments for 1, 2,
and 1-Pt were performed. 1-Au was not investigated for
fluoride sensing, because the loosely bound chloride ion on the
Au center can be readily replaced by a fluoride ion and
complicates the matter. The response of 1, 2, and 1-Pt toward
fluoride ions was examined in both absorption and fluorescence
modes.
Figure 9. Electronic transitions and the associated MO diagrams that
are likely responsible for the low-energy absorption bands of 1-Pt. The
MOs are plotted with an isocontour value of 0.03.
likely from these transitions. For the 1-Au compound, TD-DFT
data indicated that the S1−S8 states all have either a 0 or a very
small oscillator strength (<0.004) involving transitions mostly
from the lone pairs of the chloride ion to the B-Ph π* orbitals
(Cl− → π*) (see the Supporting Information). They are
therefore not likely to be responsible for the bright fluorescence
of 1-Au. Similar to those observed in 1-Pt, the S9 and S10
transitions in 1-Au are degenerate, involving HOMO-5/
HOMO-6 to LUMO/LUMO+1 (64−65%) transitions, and
may be ascribed as Mes(π) → B-Ph(π*) transitions, as shown
in Figure 10. Furthermore, the vertical excitation energies of S9
In the absorption spectra, the addition of fluoride ions caused
a substantial intensity increase of the 330 nm peak for 1 (Figure
11). In the fluorescence spectrum, the addition of fluoride ions
led to quenching of the low-energy CT peak and enhancement
of the high-energy emission peak of 1, with ΦFL increasing from
0.13 to 0.29 and an emission color change from whitish yellow
to deep blue, as shown in Figure 11. This response could be
ascribed as a “turn-on” or “switchable” fluorescence sensing, a
phenomenon similar to that observed in the related N−B
compounds, although the high-energy emission peak only
appears after the addition of fluoride ions in the N−B
compounds.2b,c The more than doubled fluorescent quantum
efficiency of 1 after the addition of fluoride ions is in sharp
contrast to the case for the related N−B compounds,2b,c which
have either a small increase or no increase at all in fluorescent
quantum efficiency with fluoride addition, making 1 more
sensitive as a ratiometric probe for fluoride ions than the related
N−B compounds. Indeed, ∼2 equiv of F− was needed to reach
the saturation point for 1, while more than 20 equiv of F− was
needed to achieve the same effect for the related N−B
compounds.2b,c The large emission energy difference between
the P → B CT peak in 1 and the fluorescent peak in [1-F]− in
comparison to that for the related N−B compounds may be
partially responsible for the great quantum efficiency increase.
The increase of the absorbance of the excitation band at 330
nm with fluoride addition, as shown in Figure 11, was clearly a
Figure 10. Electronic transitions and the associated MO diagrams that
are likely responsible for the low-energy absorption bands of 1-Au.
The MOs are plotted with an isocontour value of 0.03.
Figure 11. Absorption (left) and fluorescence (right) spectral changes of 1 in CH2Cl2 upon the addition of NBu4F. The fluorescence spectrum was
recorded using 330 nm excitation.
F
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Figure 12. Absorption (left) and fluorescence (right) spectral changes of 2 in CH2Cl2 upon the addition of NBu4F. The fluorescence spectrum was
recorded using 340 nm excitation.
Figure 13. Absorption (left) and fluorescence (right) spectral changes of 1-Pt in CH2Cl2 upon the addition of NBu4F. The fluorescence spectrum
was recorded using 330 nm excitation.
key factor for the enhancement of the high-energy emission
peak in 1. The quenching of the CT band was clearly caused by
the blocking of the empty p orbital of the B center by the
fluoride ion. The binding of fluoride to the B center in 1 was
confirmed by the characteristic four-coordinate 11B chemical
shift (3.8 ppm) observed for [1-F]− and the 19F chemical shift
(−175.7 ppm; see the Supporting Information), typical for
fluoride ions bound to a BMes2Ar unit.2
As shown by the TD-DFT data in Table 3 and Figure 5, the
S1 state of the fluoride adduct [1-F]− is lower in energy than
that of 1 and involves the transition from the BFMes2 unit
(HOMO) to the naphthyl π* (LUMO) with a very weak
oscillator strength (0.0026). This is in agreement with the
appearance of a very weak absorption band at about 430 nm in
the absorption spectrum of NBu4[1-F] (Figure 11). The fact
that [1-F]− is nonemissive on excitation at 430 nm supports
that the fluorescence of [1-F]− is not from the S1 state.
Therefore, we suggest that the fluorescence of [1-F]− is likely
from the S3 or the S4 state that involves a transition from either
the BFMes2-Ph (π, HOMO-2) or the PPh2-Ph (π, HOMO-3)
to the naphthyl (π*, LUMO), which happens to have a energy
similar to that of the S2 state of 1.
and the emission quantum efficiency. The binding of fluoride to
the B center in 2 was also confirmed by the 11B chemical shift
(3.5 ppm) observed for [2-F] and the 19F chemical shift
(−167.7 ppm; see the Supporting Information) that are similar
to those observed for [1-F]−.
TD-DFT data showed that the S1−S15 states in 2-F all have
either 0.000 or very weak oscillator strengths (Table 3), which
is in agreement with the observed quenching in absorption and
fluorescence spectra of 2 upon the addition of fluoride ions.
The first two excited states S2 and S4 in 2-F that have
appreciable oscillator strengths are shown in Figure 8 and Table
3.
For 1-Pt, the addition of fluoride ions enhanced the
absorption band at 330 nm and the fluorescence peak at 408
nm with a ΦFL increase from 0.02 to 0.10, in a manner similar
to that observed for 1 (Figure 13). The increase of the
fluorescence intensity in the fluoride adduct of 1-Pt was
attributed to the increase of absorbance of the excitation band.
A low-energy absorption band at ∼425 nm was observed for
the fluoride adduct of 1-Pt, which likely shares the same origin
as that of [1-F]− (BFMes2 → naphthyl). Although TD-DFT
calculations were not performed for the fluoride adduct of 1-Pt
due to the large size of the molecule, on the basis of the
similarity of the absorption and fluoreescence profiles and
energies, we suggest that the fluorescence of the fluoride adduct
of 1-Pt is likely from the state of BFMes2-Ph → naphthyl,
similar to that observed in [1-F]−.
In contrast to 1, the addition of fluoride ions to 2 caused
partial quenching of the 340 nm peak in the absorption
spectrum and the emission peak at 410 nm in the fluorescence
of 2 with a ΦFL change from 0.55 to 0.06 and the disappearance
of the deep blue emission color (Figure 12). Clearly the
decrease of absorbance at 340 nmthe excitation energy
usedwas responsible for the decrease of the emission peak
G
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Figure 14. Proposed fluorescence change mechanism of 1, 2, and 1-Pt upon the addition of fluoride ions. The arrows indicate the charge transfer
states that are likely responsible for the fluorescence of these molecules.
The impact of fluoride on fluorescence and the possible
origin of fluorescence for 1, 2, and 1-Pt and their fluoride
adducts are illustrated in Figure 14.
CONCLUSIONS
■
In summary, an unbound phosphine−borane compound that
displays a distinct through-space CT transition and intense dual
emission has been obtained. The use of the new P−B
compound in turn-on/switchable fluorescent sensing of fluoride
has been demonstrated. The P−B compound was found to
have a ratiometric response toward fluoride ions greater than
that of the related N−B compounds. Converting the P−B
compound to its phosphonium salt was found to greatly
enhance the binding affinity of the borane center toward
fluoride ions by 2 orders of magnitude. Despite the great steric
congestion, the new P−B molecule was found to be an effective
ligand for metal ions such as Au(I) and Pt(II). The utility of the
new P−B molecule as a bulky and bifunctional ligand in
enhancing the chemical reactivity of transition-metal complexes
is currently being examined, and the results will be reported in
due course.
To examine the effect of the Coulombic force on the fluoride
binding strength of the borane center in 2, the binding
constants of 1 and 2 with F− were determined and compared.
The binding constant of 1 ((4.3 0.5) × 104 M−1) was found
to be greater than that of a closely related N−B compound
((1.6 0.5) × 104 M−1),2c while the binding constant of 2 with
F− ((3.2 1.0) × 106 M−1) was found to be about 100 times
greater than that of 1 (see the Supporting Information). The
large binding constant increase from 1 to 2 can be attributed to
the Coulombic attraction exerted by the phosphonium ion,
despite its long distance from the borane group. Combining
Coulombic interactions with a borane center by attaching a
peripheral cationic group such as a phosphonium has been
demonstrated previously to be highly effective in enhancing the
5,9
EXPERIMENTAL SECTION
−
̈
■
binding strengh of F to the sensor by Gabbaı and others.
The K1 and K2 binding constants of 1-Pt with F− were found to
be (5.2 1.0) × 104 and (7.9 2.0) × 103 M−1, respectively
(see the Supporting Information), indicating that the attach-
ment of a neutral metal unit to the phosphine center in the U-
shaped molecule 1 has little influence on the sensitivity of the
borane center toward fluoride ions, perhaps due to the greatly
increased steric congestion and the long separation distance
between the metal and the boron atom.
General Procedures. All reactions were carried out under a
nitrogen atmosphere. Reagents were purchased from Aldrich Chemical
Co. and used without further purification. Thin-layer and flash
chromatography were performed on silica gel. H, 13C and 31P NMR
1
spectra were recorded on Bruker Avance 400 and 500 MHz
spectrometers. Deuterated solvents were purchased from Cambridge
Isotopes. UV−vis spectra were recorded on a Varian Cary 50 UV−
visible spectrophotometer. Emission spectra were recorded using a
Photon Technologies International QuantaMaster Model 2 spectrom-
eter. Fluorescent quantum yields were determined using optically
H
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dilute solutions (A ≈ 0.1) relative to 9,10-diphenylanthracene (Φ =
0.90). Fluorescence titrations were carried out by adding stock
solutions of TBAF in CH2Cl2 to the solutions of 1, 2, and 1-Pt in
CH2Cl2. The starting material BI was prepared using a modified
literature procedure.2
observed, and the full assignment of the 13C NMR spectrum could not
be completed because of the low intensity and poor resolution of the
13C chemical shifts caused by the poor solubility of this molecule. The
11B NMR chemical shift for 1-Au was not observed, perhaps because
of the congested nature and the low symmetry of the molecule. HRMS
(EI): calcd for C104H92B2P2Cl [M − Cl]+ 1622.6577, found 1621.6555.
Synthesis of 1-Pt. 1 (50 mg, 0.07 mmol) and Pt(SMe2)2Cl2 (14
mg, 0.035 mmol) were dissolved in 15 mL of dry and degassed THF.
The mixture was stirred and refluxed for 5 h. After the removal of the
solvent in vacuo the residue was purified on a silica gel column
(CH2Cl2 as the eluent), producing 49 mg of 1-Pt as a pale yellow solid
Synthesis of 1. 1-(Diphenylphosphino)-4-bromobenzene (316
mg, 0.926 mmol) was dissolved in 20 mL of dry degassed THF in a 50
mL Schlenk flask with a stir bar. After the solution was cooled to −78
°C for 15 min, n-BuLi (0.64 mL, 1.6 M in hexanes, 1.020 mmol) was
added dropwise. The mixture was stirred 1 h at −78 °C.
ZnCl2(TMEDA) (292 mg, 1.158 mmol) was added. The mixture
was warmed to room temperature slowly and stirred for 1 h. BI (359
mg, 0.620 mmol) and Pd(PPh3)4 (80 mg, 0.069 mmol) were added,
and the reaction mixture was stirred and refluxed for 40 h. After the
removal of the solvent in vacuo, the residue was partitioned between
CH2Cl2 and water and the aqueous layer was extracted twice with
CH2Cl2. The combined organic layers were dried over MgSO4 and
filtered. Purification on a silica gel column (5/1 hexanes/CH2Cl2 as
1
3
(83% yield). H NMR (400 MHz, CDCl3, δ): 7.81 (dd, JH−H = 8.08
4
Hz, JH−H = 3.28 Hz, naph, 4H), 7.41 (m, naph and Ph, 16H), 7.24
3
(m, naph and Ph, 12H), 7.12 (t, JH−H = 7.58 Hz, Ph, 8H), 7.02 (d,
3JH−H = 6.31 Hz, Ph, 4H), 7.93 (d, 3JH−H = 6.06 Hz, Ph, 4H), 6.63 (s,
Mes, 8H), 2.18 (s, Me, 12H), 1.76 (s, Me, 24H) ppm. 31P{1H} NMR
(162 MHz, CDCl3, δ): 19.5 ppm (JP−Pt = 1323 Hz). 13C NMR (100
MHz, CDCl3, δ): 147.9, 145.8, 141.4, 141.0, 140.6, 139.9, 138.2, 137.2,
135.5, 134.9, 131.0, 130.6, 130.3, 129.7, 129.2, 128.1, 127.7, 125.5,
125.3, 26.7 (Me), 21.2 (Me) ppm. Some of the carbon chemical shifts
were not observed, and the full assignment of the 13C NMR spectrum
could not be completed because of the low intensity and poor
resolution of the 13C chemical shifts caused by the poor solubility of
this molecule. The 11B NMR chemical shift for 1-Pt was not observed,
perhaps because of the congested nature and the low symmetry of the
molecule. Anal. Calcd for C104H92B2P2PtCl2·0.5CH2Cl2: C, 72.39; H,
5.41. Found: C, 71.97; H, 5.66.
1
eluent) afforded 257 mg of 1 as a white solid (39% yield). H NMR
(500 MHz, CD2Cl2, δ): 7.98 (d, 3JH−H = 7.88 Hz, naph, 2H), 7.61 (m,
naph, 2H), 7.43 (t, 3JH−H = 5.6 Hz, naph, 2H), 7.26 (m, Ph, 12H), 7.10
(t, 3JH−H = 7.0 Hz, Ph, 4H), 7.01 (t, 3JH−H = 6.94 Hz, Ph, 2H), 6.88 (s,
Mes, 4H), 2.36 (s, Me, 6H), 2.03 (s, Me, 12H) ppm. 31P{1H} NMR
(202 MHz, CD2Cl2, δ): −6.0 ppm. 11B{1H} NMR (160 MHz, C6D6,
70 °C, δ): 74.3 ppm. 13C NMR (126 MHz, CD2Cl2, δ): 148.4 (Ph),
144.6 (naph), 142.1 (Mes), 141.4 (Mes), 140.6 (d, Ph, JC−P = 29.3
Hz), 138.9 (Mes), 138.2 (d, Ph, JC−P = 12 Hz), 137.3 (ph), 136.2
(Ph), 134.0 (d, Ph, JC−P = 19.5 Hz), 133.5 (d, Ph, JC−P = 19.4 Hz),
131.9 (Ph), 131.7 (naph), 130.1 (Ph), 129.7 (Ph), 129.0 (naph), 128.9
(naph), 128.7 (Mes), 125.8 (d, Ph, JC−P = 11 Hz), 24.4 (Me), 21.5
(Me) ppm. Anal. Calcd for C52H46BP: C, 87.60; H, 6.40. Found: C,
87.72; H, 6.56.
Computational Study. The DFT calculations were performed
using the Gaussian 09, revision B.01,10 software package and the High
Performance Computing Virtual Laboratory (HPCVL) at Queen’s
University. The ground-state geometries were fully optimized at the
B3LYP11 level using the LANL2DZ basis set for platinum and gold
metal atoms and the 6-31G(d) basis set for all other atoms.12 The
initial geometric parameters in the calculations were employed from
crystal structure data for geometry optimization except for compound
2 and the fluoride adducts of 1 and 2, for which the initial geometry
parameters were established by Gauss View (version 3.08). TD-DFT
calculations were performed to obtain the vertical singlet and triplet
excitation energies.
X-ray Crystallographic Analysis. Single crystals of 1, 1-Au, and
1-Pt were mounted on glass fibers and were collected on a Bruker
Apex II single-crystal X-ray diffractometer with graphite-monochro-
mated Mo Kα radiation, operating at 50 kV and 30 mA and at 180 K.
Data were processed on a PC with the aid of the Bruker SHELXTL
software package (version 6.10)13 and corrected for absorption effects.
Compounds 1 and 1-Au belong to the monoclinic crystal space groups
P21/c and C2/c, respectively. The crystals of 1-Pt belong to the
Synthesis of 2. Compound 1 (50 mg, 0.07 mmol) was dissolved in
10 mL of dry and degassed THF. Iodomethane (4.4 μL, 0.07 mmol)
was added to the solution dropwise. The mixture was stirred and
refluxed overnight. After the solvent was removed in vacuo and 5 mL
of hexanes was added, a precipitate was obtained and washed with
diethyl ether to afford 53 mg of 2 as a colorless solid (89% yield).
1
Compound 2 was recrystallized from CH2Cl2/hexane. H NMR (400
MHz, CD2Cl2, δ): 8.06 (d, 3JH−H = 8.06 Hz, naph, 1H), 8.03 (d, 3JH−H
3
= 8.31 Hz, naph, 1H), 7.84 (t, JH−H = 7.55 Hz, naph, 2H), 7.63 (m,
naph and Ph, 6H), 7.51 (m, Ph, 6H), 7.39 (m, Ph, 6H), 7.14 (d, Ph,
2
3JH−H = 7.81 Hz, 2H), 6.85 (s, Mes, 4H), 2.81 (d, Me, JH−P = 13.09
Hz, 3H), 2.35 (s, Me, 6H), 1.85 (s, Me, 12H) ppm. 31P{1H} NMR
(162 MHz, CD2Cl2, δ): 22.4 ppm. 13C NMR (100 MHz, CD2Cl2, δ):
150.9 (Ph), 148.0 (naph), 147.0 (Ph), 141.5 (Mes), 141.3 (Ph), 140.0
(Mes), 139.6 (naph), 138.9 (Mes), 137.6 (naph), 137.0 (Ph), 134.9
(d, Ph, JC−P = 10.6 Hz), 132.9 (d, Ph, JC−P = 10.9 Hz), 132.4 (d, Ph,
JC−P = 14.6 Hz), 131.1 (naph), 130.9 (d, Ph, JC−P = 12.8 Hz), 130.1
(Ph), 129.0 (Ph), 128.3 (Mes), 128.2 (naph), 125.4 (naph), 125.2
(naph), 118.0 (d, Ph, JC−P = 88.9 Hz), 114.0 (d, Ph, JC−P = 90.3 Hz),
24.3 (Me), 21.5 (Me), 11.7 (d, J = 57.9 Hz, P-CH3) ppm. The 11B
NMR chemical shift for 2 was not observed, perhaps because of the
congested nature and the low symmetry of the molecule. Elemental
analysis, calcd for C53H49BPI·1H2O: C 72.95, H 5.89; found: C 73.19,
H 6.04.
triclinic crystal space group P1. All non-hydrogen atoms were refined
̅
anisotropically. Complete crystal structure data can be found in the
Supporting Information. In the crystal lattice of 1-Au, CHCl3 solvent
molecules (4 CHCl3/per 1-Au) were located and refined successfully.
Two of the CHCl3 solvent molecules form H bonds with the Cl ligand
bound to the Au(I) atom. The crystal structure data of 1, 1-Au, and 1-
Pt have been deposited at the Cambridge Crystallographic Data
Center (CCDC Nos. 967364−967366).
Synthesis of 1-Au. 1 (50 mg, 0.07 mmol) and Au(SMe2)Cl (10
mg, 0.035 mmol) were dissolved in 15 mL of dry and degassed THF.
The mixture was stirred overnight at ambient temperature. After the
removal of the solvent in vacuo the residue was recrystallized from
CHCl3 and hexane, affording a colorless crystalline solid of 1-Au (49
mg, 84%). 1H NMR (400 MHz, CDCl3, δ): 7.96 (d, 3JH−H = 7.05 Hz,
naph, 4H), 7.59 (td, 3JH−H = 7.3 Hz, 4JH−H = 3.27 Hz, naph, 4H), 7.46
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, tables, and CIF files giving NMR spectra for all
compounds, crystal structure data for 1, 1-Au, and 1-Pt,
fluoride titration data, binding constant calculations, and TD-
DFT computational data. This material is available free of
charge via the Internet at http://pubs.acs.org.
(m, naph and Ph, 4H), 7.35 (m, naph and Ph, 28H), 7.25 (d, 3JH−H
=
3
6.29 Hz, Ph, 4H), 7.08 (d, JH−H = 7.81 Hz, Ph, 4H), 6.84 (s, Mes,
8H), 2.34 (s, Me, 12H), 1.91 (s, Me, 24H) ppm. 31P{1H} NMR (162
MHz, CDCl3, δ): 33.5 ppm. 13C NMR (100 MHz, CDCl3, δ): 147.9,
147.5, 140.8, 140.2, 138.9, 138.6, 137.0, 135.5, 133.8, 133.6, 131.7,
131.5, 131.2, 129.2, 129.1, 129.0, 128.9, 128.5, 128.3, 125.6, 125.4, 24.0
(Me), 21.2 (Me) ppm. Some of the carbon chemical shifts were not
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for S.W.: wangs@chem.queensu.ca.
I
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(10) 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.; Keith, T.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.;
Cossi, M.; N. 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.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian09,
revision B.01; Gaussian, Inc., Wallingford, CT, 2010.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Natural Sciences and Engineering Research
Council for financial support and the High Performance
Computing Virtual Laboratory for computing facilities. S.W.
thanks the Canada Council for the Arts for the Killam Research
Fellowship.
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dx.doi.org/10.1021/om4011406 | Organometallics XXXX, XXX, XXX−XXX