Terpyridine-triarylborane conjugates for the dual complexation of zinc(ii) cation and fluoride anion

Article abstract of DOI:10.1021/om401151p

A series of ditopic terpyridine-triarylborane conjugates (1-3) in which 4′-ethynylterpyridine is linked to the para, meta, and ortho positions of the phenyl ring of dimesitylphenylborane (Mes₂PhB), respectively, were prepared to investigate the dual complexation behavior of the conjugates toward Zn(II) cation and fluoride anion. The crystal structures of the corresponding Zn(II) complexes (L·ZnCl₂, L = 1-3) reveal the formation of a 1:1 adduct between ZnCl₂ and a conjugate, with a five-coordinate Zn(II) center bound to three nitrogen atoms and two chlorine atoms. In particular, the structure of ortho-substituted 3·ZnCl ₂ in comparison with that of 3 indicates the presence of π-π interactions between the mesityl ring and ethynylene-pyridine fragment in 3·ZnCl₂. UV/vis absorption and fluorescence spectra of 1-3 display low-energy bands mainly assignable to a π(Ar) → p _π(B) (Ar = Mes and/or phenylene-ethynylene) charge transfer (CT) transition. The transition in Zn(II) complexes has a π(Mes) → π*(Ar) (Ar = terpyridine-ethynylene) intramolecular CT nature with red shifts of both the absorption and emission bands in comparison to those of free conjugates. These spectroscopic features are further supported by TD-DFT calculations. UV/vis absorption and fluorescence titration experiments of 1-3 toward Zn(II) and fluoride ion, respectively, show that while the absorption and fluorescence bands underwent gradual quenching upon addition of fluoride, the addition of ZnCl₂ gave rise to the red shifts of both bands. Fluoride titration experiments of Zn(II) complexes also resulted in gradual quenching of both the absorption and emission bands accompanied by the disappearance of emission color. Sequential addition of ZnCl₂ and fluoride to the conjugates reproduced the above binding behavior with an emission color change from deep blue to sky blue to dark.

Full text of DOI:10.1021/om401151p

Article
pubs.acs.org/Organometallics
Terpyridine−Triarylborane Conjugates for the Dual Complexation of
Zinc(II) Cation and Fluoride Anion
Young Hoon Lee,† Nguyen Van Nghia,† Min Jeong Go,‡ Junseong Lee,‡ Sang Uck Lee,*,†
and Min Hyung Lee*,†
†Department of Chemistry and EHSRC, University of Ulsan, Ulsan 680-749, Republic of Korea
‡Department of Chemistry, Chonnam National University, Gwangju 500-757, Republic of Korea
S
* Supporting Information
ABSTRACT: A series of ditopic terpyridine−triarylborane
conjugates (1−3) in which 4′-ethynylterpyridine is linked to
the para, meta, and ortho positions of the phenyl ring of
dimesitylphenylborane (Mes2PhB), respectively, were pre-
pared to investigate the dual complexation behavior of the
conjugates toward Zn(II) cation and fluoride anion. The
crystal structures of the corresponding Zn(II) complexes (L·
ZnCl2, L = 1−3) reveal the formation of a 1:1 adduct between
ZnCl2 and a conjugate, with a five-coordinate Zn(II) center bound to three nitrogen atoms and two chlorine atoms. In particular,
the structure of ortho-substituted 3·ZnCl2 in comparison with that of 3 indicates the presence of π−π interactions between the
mesityl ring and ethynylene−pyridine fragment in 3·ZnCl2. UV/vis absorption and fluorescence spectra of 1−3 display low-
energy bands mainly assignable to a π(Ar) → pπ(B) (Ar = Mes and/or phenylene−ethynylene) charge transfer (CT) transition.
The transition in Zn(II) complexes has a π(Mes) → π*(Ar) (Ar = terpyridine−ethynylene) intramolecular CT nature with red
shifts of both the absorption and emission bands in comparison to those of free conjugates. These spectroscopic features are
further supported by TD-DFT calculations. UV/vis absorption and fluorescence titration experiments of 1−3 toward Zn(II) and
fluoride ion, respectively, show that while the absorption and fluorescence bands underwent gradual quenching upon addition of
fluoride, the addition of ZnCl2 gave rise to the red shifts of both bands. Fluoride titration experiments of Zn(II) complexes also
resulted in gradual quenching of both the absorption and emission bands accompanied by the disappearance of emission color.
Sequential addition of ZnCl2 and fluoride to the conjugates reproduced the above binding behavior with an emission color
change from deep blue to sky blue to dark.
binding site for metal ions and shows strong structural stability
with fascinating electrochemical, photophysical, and photo-
chemical properties (I in Chart 1).5 In particular, when a metal
INTRODUCTION
■
Molecular receptors capable of effectively recognizing charged
or neutral species have been actively studied in the field of
supramolecular chemistry, because such receptors can be
utilized in sensing, catalysis, nanoscience, biomimicry, drug
delivery, and separation science.1 In the past several decades, as
artificial cation receptors, a great number of acyclic and
macrocyclic compounds have been reported and evaluated for
their cation recognition ability.2 At the same time, the design
and construction of anion receptors have also attracted
attention due to the increased importance of anions in
physiological and environmental systems.3 Although recog-
nition of individual ionic species was successfully demonstrated
by monotopic receptors, heteroditopic receptors that contain
two disparate recognition sites for both cations and anions may
also be good candidates for sensing purposes because they can
exhibit enhanced affinity for target species in comparison to the
corresponding monotopic receptors probably due to allosteric
effects and favorable electrostatic interactions between the
cobound ions in ion-pair receptors.4
Chart 1
The terpyridine (2,2′:6′,2″-terpyridine) core has been
extensively used as a metal ion receptor in coordination
chemistry. Generally, terpyridine provides a tridentate N∧N∧N
Received: November 26, 2013
© XXXX American Chemical Society
A
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a
Scheme 1.
a
Legend: (i) Pd(PPh3)4, CuI, i-Pr2NH, reflux, 49% (1), 36% (2), 57% (3); (ii) ZnCl2, CH3CN/CH2Cl2 (1/1, v/v), 25 °C, 84% (1·ZnCl2), 95% (2·
ZnCl2), 79% (3·ZnCl2).
ion with a d10 electron configuration (e.g., Zn(II) or Cd(II))
binds to a terpyridine with conjugated aryl substituents,
especially at the 4′-position of the terpyridine core, efficient
intramolecular charge transfer (ICT) from the substituted aryl
donor to the metalated terpyridine acceptor takes place upon
light excitation.6,7 Since metal ion binding enhances the
electron-accepting properties of terpyridine cores from aryl
donors, it leads to a red shift of the ICT emission in
comparison to the metal-unbound state. This effect has been
used to detect and quantify various metal ions, including
Zn(II).6,8,9 On the other hand, triarylboranes have attracted
significant attention as monotopic anion receptors due to their
high Lewis acidity as well as high selectivity endowed by steric
protection of the boron center with ortho substituents on the
aryl group (II in Chart 1).10 Upon anion binding to the boron
center, the extended conjugation is disrupted by the population
of the boron pπ orbital to give rise to changes in the absorption
and/or emission intensity of triarylboranes. Indeed, triarylbor-
anes have been utilized as effective and selective anion sensors
to detect harmful anions such as fluoride and cyanide.11−17
While a number of multitopic receptors have been studied,
ditopic receptors containing terpyridine and triarylborane for
binding of both cationic metal ion and fluoride anion have been
rarely reported. Note that Mes2B-functionalized N∧N chelate
compounds are known for binding Zn(II) or fluoride ions.18,19
Instead, metal chelation effects on the Lewis acidity and optical
properties of triarylboranes have been reported. For instance,
Kitamura and co-workers demonstrated that the triarylborane-
appended terpyridine complex [PtLCl]+ (L = 4′-phenyl-
(dimesitylboryl)-2,2′:6′,2″-terpyridine) shows synergetic effects
of metal-to-ligand charge transfer (MLCT) and π(aryl)−pπ(B)
CT interactions on the enhancement of MLCT absorption and
emission due to effective conjugation between Pt(II)−
terpyridine and arylborane units.20 Wang and co-workers
have reported a series of Mes2B-functionalized
N∧N,14,17,18,21,22 N∧C,12,22−24 and N∧C∧N chelate25 Pt(II)
complexes. They showed that metal chelation not only
enhances the fluoride binding affinity of boron centers but
also alters phosphorescent properties that are useful in anion
sensing16,17,24,26 and OLED applications.25,27
Herein, we report a new series of ditopic terpyridine−
triarylborane conjugates (1−3 in Chart 1) in which 4′-
ethynylterpyridine is linked to the para, meta, and ortho
positions of the phenyl ring of dimesitylphenylborane
(Mes2PhB), respectively, to investigate dual complexation
behavior toward Zn(II) cation and fluoride anion. The changes
in optical properties of the conjugates with Zn(II) and/or
fluoride binding were fully investigated by experimental and
theoretical methods. The synthesis and crystal structures of
corresponding Zn(II) complexes are also provided to under-
stand the dual binding behavior of the conjugates.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Three terpyridine−
triarylborane conjugates (1−3) linked by an ethynylene
group were synthesized by a palladium-catalyzed Sonogashira
cross-coupling reaction between (ethynylphenyl)-
dimesitylborane (1a−3a) and 2,6-bis(pyridin-2-yl)pyridin-4-yl
trifluoromethanesulfonate (tpy-OTf) (Scheme 1). To inves-
tigate the substitution position effect of the dimesitylboryl
(Mes2B) group on the optical properties, as well as the effects
of cation and anion binding, 4′-ethynylterpyridine was
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introduced into the para, meta, and ortho positions of the
phenyl ring of dimesitylphenylborane (Mes2PhB), respectively.
The formation of conjugates 1−3 was characterized by
1
multinuclear NMR spectroscopy and elemental analysis. H
and 13C NMR spectra of 1−3 exhibit the expected resonances
corresponding to mesityl and terpyridine moieties. Two singlets
at δ 2.3 and 2.0 ppm assignable to the p- and o-methyl proton
resonances of mesityl groups, respectively, indicate free rotation
of the Mes2B moiety in solution irrespective of its position on
the phenyl group. The broad 11B NMR signals at δ 74−78 ppm
confirm the presence of a trigonal-planar boron center. The
reaction of each conjugate with zinc(II) chloride in mixed
solvent (CH3CN/CH2Cl2) produced the corresponding Zn(II)
complexes in high yield as white powders. The resulting
complexes were also fully characterized by multinuclear NMR
spectroscopy, elemental analysis, and single-crystal X-ray
diffraction. Although the 11B NMR signals for all Zn(II)
complexes were not observed, the apparent downfield chemical
shifts of the proton resonances for the terpyridine group
indicate binding of the Zn(II) center to terpyridine.28 Finally,
X-ray diffraction studies unequivocally revealed crystal
structures for all Zn(II) complexes (Figure 1 and Table S2
(Supporting Information)). The structures show the formation
of a 1:1 adduct (L·ZnCl2, L = 1−3) between ZnCl2 and
terpyridine−triarylborane conjugate (L).8,29 In all complexes,
the zinc centers adopt a five-coordinate geometry with three
nitrogen atoms and two chlorine atoms. Two chlorine atoms
and one nitrogen atom of the central pyridine ring of the
terpyridine moiety occupy equatorial sites, forming a distorted-
trigonal-bipyramidal geometry around the zinc center. The
equatorial Zn−N bond lengths are shorter than those of the
axial Zn−N bonds in all complexes, as is typical for other
terpyridine Zn(II) complexes (Table S2).8,29,30 While the
central pyridine and phenylene rings linked to the ethylnylene
group are substantially coplanar in 1·ZnCl2 and 2·ZnCl2
(∠pyd−Ph = 15.1(2)° for 1·ZnCl2 and 26.71(6)° for 2·
ZnCl2), the two rings in 3·ZnCl2 are perpendicularly oriented
to each other (∠pyd−Ph = 88.78(8)°). Inspection of the
structure of 3·ZnCl2 further shows that one mesityl ring plane
is positioned right above the ethynylene−pyridine fragment and
the two fragments are in a nearly parallel arrangement (Figure
S1 (Supporting Information)). This feature indicates the
presence of π−π interactions between the two fragments.
Indeed, interatomic distances between the mesityl carbon
atoms and ethynylene−pyridine carbon atoms are very short
(3.1−3.4 Å) (Figure S2 (Supporting Information)). The
presence of π−π interactions is also evidenced by the distortion
of terpyridine fragment toward the mesityl ring, resulting in the
deviation of the C25−C26−C27 bond angle from linearity
(170.6(2)°) (Figure S1, left). Note that the angles for 1·ZnCl2
and 2·ZnCl2 are 177.6(6) and 176.26(15)°, respectively.
Figure 1. Crystal structures of 1·ZnCl2 (top), 2·ZnCl2 (middle), and
3·ZnCl2 (bottom) (30% probability ellipsoids). H atoms and one
CH3CN molecule in 1·ZnCl2 are omitted for clarity.
compounds, B(1) boron atoms adopt a trigonal-planar
geometry (∑C−B−C = 360°) in all Zn(II) complexes and in
3.
Absorption and Emission Properties. The optical
properties of conjugates 1−3 and their Zn(II) complexes
were investigated in THF solution (Figure 3 and Table 1). The
absorption spectra of 1−3 exhibit two major bands in the
region of 250−375 nm. A lower energy absorption band
appears in the energy order 2 > 3 > 1. This is in parallel with
the extent of conjugation between terpyridine and triarylborane
groups, although the absorption edges in 1 and 3 are similar in
energy. Zn(II) complexes of 1−3 also have absorption profiles
similar to each other, and the lower energy absorption follows
the same trend as for conjugates 1−3. Interestingly, all
absorption bands are slightly red-shifted in comparison to the
respective absorption bands of conjugates, indicating a decrease
in the band gap energy after Zn(II) complexation.
To elucidate this structural feature in 3·ZnCl2, the crystal
structure of conjugate 3 was determined (Figure 2).
Remarkably, the terpyridine and phenylene rings are nearly
coplanar (∠pyd−Ph = 10.15(14)°) and the ethynylene linker is
linearly connected to both rings in 3 (∠C25−C26−C27 =
177.0(3)°). This finding suggests that the observed π−π
interactions in 3·ZnCl2 originate from an electronic source.
Namely, the decreased electron density of the terpyridine
moiety due to ZnCl2 binding likely facilitates π−π interactions
with the mesityl donor. It is also interesting to note that three
nitrogen atoms of terpyridine in 3 are in a cis,cis conformation
in the solid state. Finally, as observed in other triarylborane
To gain insight into the electronic transitions of 1−3 and
their Zn(II) complexes, TD-DFT calculations on the ground
state (S0) optimized structures of all compounds were
performed with the B3LYP functional and 6-31G(d) basis
sets (Figure 4 and Table 2). Calculation results predict major
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Table 1. Optical Data for Conjugates 1−3 and Their Zn(II)
Complexes
λ /
ex a
λ /
ema
a
ab
,
compound
λabs/nm (ε × 10−3/M−1 cm−1
)
nm
nm
Φem
0.19
1
2
250 (30.4), 285 (38.4), 338 (53.4)
281
294
424
398
253 (33.9), 265 (35.4), 293 (43.4),
310 (35.0)
0.08
3
282 (50.4), 295 (44.2), 325 (25.0)
288 (22.9), 346 (31.7)
284
335
328
332
454
450
408
465
0.03
0.15
0.07
0.03
1·ZnCl2
2·ZnCl2
3·ZnCl2
264 (27.1), 286 (27.3), 328 (31.7)
286 (32.0), 332 (31.2), 344 (28.2)
a
b
Measured in THF (1.0 × 10−5 M). Quinine sulfate as a standard
(0.5 M H2SO4, Φ = 0.55).
conjugation length increases (1 > 3 > 2), contributions from
the π orbital of the phenylene−ethynylene moiety become
significant. It is notable that the HOMO of m-BMes2-
substituted 2 is exclusively contributed by mesityl groups.
Conversely, LUMOs are delocalized over each conjugate with a
substantial contribution from the empty pπ(B) orbital. These
findings suggest that the lower energy absorption is mainly
characterized by a π(Ar) → pπ(B) charge transfer (CT)
transition where Ar is the mesityl and/or phenylene−
ethynylene moiety. Significant contribution from a π →
pπ(B) CT transition in 1−3 may be responsible for (i) the
broad absorption feature (Figure 3a), (ii) the structureless
emission profile (Figure 3b), (iii) the positive solvatochromism
(Figure 5), and (iv) the gradual quenching of the lower energy
absorption band upon fluoride binding to the boron atom
(Figure 7). TD-DFT calculations on the ground state of Zn(II)
complexes 1·ZnCl2−3·ZnCl2 predict major low energy
absorptions at 404, 362, and 395 nm, respectively, similar to
the trend in absorption spectra. The red-shifted absorption in
Zn(II) complexes in comparison to the absorption in free
conjugates is also consistent with experimental results. While
the HOMOs have major contributions from mesityl groups, the
LUMOs have substantial contributions from the central
pyridine ring of terpyridine moieties. Contributions from the
empty pπ(B) orbital to LUMOs decrease in the order 1 > 3 > 2.
Thus, lower energy absorption in the Zn(II) complexes can be
mainly assignable to a π(Mes) → π*(Ar) (Ar = terpyridine−
ethynylene) intramolecular CT (ICT) transition with slight
π(Mes) → pπ(B) CT character. This result reflects the
stabilization of the LUMO of the conjugate upon Zn(II)
complexation. Consequently, the absorption energy of Zn(II)
complexes decreases in comparison to that of free conjugates.
The photoluminescence (PL) spectra of conjugates 1−3 and
their Zn(II) complexes display broad emission bands indicative
of CT transition in nature (Figure 3b and Table 1).6,7 The red-
shifted emission bands for Zn(II) complexes in comparison
with those of 1−3 are consistent with the absorption features.
Among the pairs of conjugates and Zn(II) complexes, a red
shift in the emission wavelength is most apparent for 1 and 1·
ZnCl2 (Δλem = 26 nm). It is notable that the emission energies
of 3 and 3·ZnCl2 are considerably lower than those of 1 and 1·
ZnCl2, respectively. This finding is somewhat different from
that observed in absorption spectra, in which the para-
substituted 1 showed overall lower energy absorption than
the ortho-substituted 3 (see TD-DFT results below).
Figure 2. Side and top views of the crystal structure of 3 (30%
probability ellipsoids). H atoms are omitted for clarity.
Figure 3. (a) UV/vis absorption and (b) fluorescence spectra of 1−3
and their Zn(II) complexes in THF (1.0 × 10−5 M).
low-energy absorptions at 374, 356, and 363 nm for 1−3,
respectively, which is the same order as for experimentally
observed absorption bands. While HOMOs are mainly located
on both mesityl and phenylene−ethynylene moieties, the
contribution from each fragment is observed differently. As the
Because the CT transition energy is largely affected by
solvent polarity, the effect of solvent on the absorption and
emission spectra of 1 and 3 was further examined. While both
compounds exhibited low-energy absorption at nearly the same
D
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Figure 4. Molecular orbitals for 1−3 and 1·ZnCl2−3·ZnCl2 at their ground state (S0) optimized geometries and lower energy electronic transitions
from TD-DFT calculations (isovalue 0.04). Transition energies (in nm) were calculated using the TD-B3LYP method with 6-31G(d) basis sets.
upon increased solvent polarity is much more apparent for 1
(Δλem = 46 nm for 1 and 10 nm for 3 between toluene and
MeCN solutions), indicating that 1 has a more polar CT
excited state than 3. Thus, 1 is expected to have a transition
dipole moment greater than that of 3, which might be
responsible for the higher molar absorption coefficient (ε) and
quantum efficiency of 1 in comparison to those of 3.20
Comparison of the emission bands of 1 and 3 in the same
solvent shows that the emission energy of 3 is lower than that
of 1 in all solvent systems. This result indicates that the lowest
excited state of 3 should be lower in energy than that of 1.
Since the absorption edge was very similar in energy for both
compounds (Figure 3a), the lowest singlet excited state
structures (S1) of 1 and 3 were optimized, and the transition
energy was compared in detail (Figure 6). For both
compounds, the lowest energy electronic transition is involved
with a HOMO → LUMO transition (98% for 1 and 100% for
3) with substantial π → pπ(B) CT nature. Importantly, the
transition energy of 3 (549 nm, 2.26 eV) is shown to be lower
than that of 1 (492 nm, 2.52 eV), indicating a more stabilized
S1 state for 3. Inspection of the S1 state structures of 1 and 3
reveals that while 1 has essentially identical S1 and S0 structures,
3 undergoes an apparent geometrical distortion in the S1 state.
This is indicated by large distortion between pyridine and
phenylene ring fragments (∠pyd−Ph = 15.4° at S0 and 37.3° at
S1 for 3) (Figure 6a). This result may suggest that the S1 state
of 3 is more stabilized by geometrical changes, leading to lower
emission energy. Furthermore, computation of the transition
dipole moment (μ) and oscillator strength (f) resulted in
higher values for 1 than for 3 (μ = 0.4058 D and f = 0.0250 for
1 and μ = 0.1654 D and f = 0.0092 for 3). This result also
supports that 1 exhibits significant solvatochromism and has
higher quantum efficiency than 3.
Table 2. Lower Energy Electronic Transitions for 1−3 and
a
Their Zn(II) Complexes from TD-DFT Calculations
λcalc
/nm
state
f
assignment
1
2
S2
S1
373.9
356.1
0.6548 HOMO-1 → LUMO (90%)
0.1083 HOMO → LUMO (62%), HOMO
→ LUMO+1 (34%)
3
S2
362.8
0.3054 HOMO-1 → LUMO (80%),
HOMO → LUMO (10%)
1·ZnCl2
2·ZnCl2
S2
S4
404.2
361.7
0.4471 HOMO-1 → LUMO (92%)
0.4140 HOMO-3 → LUMO (56%),
HOMO-4 → LUMO (30%)
3·ZnCl2
S2
394.6
0.2282 HOMO-1 → LUMO (80%),
HOMO → LUMO (12%)
a
Singlet energies for the vertical transition calculated at the optimized
S0 geometries.
wavelength in solvents with different polarities (Figure S3
(Supporting Information)), PL spectra clearly displayed
positive solvatochromism, indicating the polar excited states
of 1 and 3 (Figure 5). This result also indicates that emission is
CT transition in nature. A red shift in the emission wavelength
Cation and Anion Binding Studies. To investigate the
binding properties of 1−3 toward Zn(II) and fluoride ions,
UV/vis absorption and fluorescence titration experiments were
performed in THF. Titration results for 1 are shown in Figure
7, and the results for 2 and 3 are collected in Figures S4 and S5
(Supporting Information). The addition of incremental
amounts of fluoride led to the gradual quenching of the
lower energy absorption band at 338 nm, indicating that
Figure 5. Normalized fluorescence spectra of 1 and 3 in different
solvents.
E
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Figure 6. (a) Overlap of ground state (S0, gray) and lowest excited
state (S1, red) optimized structures of 1 and 3. (b) Molecular orbitals
for 1 and 3 at their S1 optimized geometries with the lowest energy
electronic transition from TD-DFT calculations (isovalue 0.04).
fluoride binding to the boron center of 1 blocks the π → pπ(B)
CT transition (Figure 7a).13,31 Fitting of the absorbance data
resulted in a high binding constant (K) of ca. ∼107 M−1 in
THF, which is comparable to values observed for triarylboranes
with a Mes2PhB group.15,32 Similarly, the broad fluorescence
band at 424 nm decreased in intensity upon fluoride addition
while forming a new weak emission band at the higher energy
region (ca. 360 nm). The structured band shape after fluoride
binding suggests that emission of the fluoride adduct is
attributable to a π−π* transition in the phenylene−
ethynylene−terpyridine fragment. Fluoride titration results for
2 and 3 showed similar quenching of both the lower energy
absorption and fluorescence bands with the appearance of
structured high-energy emission bands (Figure S4). Interest-
ingly, while the K value of 2 is similar to that of 1, the K value
of 3 is one order of magnitude lower than those of 1 and 2 (ca.
1.0 × 106 M−1, Table 3). This result can be ascribed to the
steric hindrance of the ortho-substituted ethynyl group on the
fluoride binding to the boron center in 3.
Next, the binding properties of 1−3 toward ZnCl2 were
examined. Upon gradual addition of ZnCl2 to a solution of 1
(1.0 × 10−5 M in THF), the absorption band at 338 nm
increased in intensity with a red shift (Figure 7b). The
absorbance data, which were best fitted with a 1:1 binding
isotherm, resulted in a K value of 1.5 × 106 M−1. The distinct
isosbestic points at 286 and 326 nm also support 1:1 binding
between 1 and ZnCl2, in parallel with the crystal structure of 1·
ZnCl2. The final absorption curve after ZnCl2 titrations was
identical with that of 1·ZnCl2, as shown in Figure 3a. PL
titrations exhibited similar quenching of the emission band with
an apparent red shift upon ZnCl2 addition. The emission band
shift accompanied an emission color change of the solution
from deep blue to sky blue. Unlike the case with fluoride
binding, the emission intensity after ZnCl2 binding remained
Figure 7. Spectral changes in UV/vis absorption and fluorescence of 1
(1.0 × 10−5 M in THF) upon addition of (a) TBAF ((0−1.5) × 10−5
M in THF) and (b) ZnCl2 ((0−2.4) × 10−5 M in CH3CN) and of 1·
ZnCl2 (1.0 × 10−5 M in THF) upon addition of (c) TBAF ((0−1.4) ×
10−5 M in THF).
Table 3. Binding Constants (K) of Conjugates 1−3 and
Their Zn(II) Complexes in THF
compound
Zn(II) (M−1
)
F− (M−1
)
1
1.5 × 106
5.0 × 106
5.0 × 106
1.0 × 107
3.0 × 107
1.0 × 106
5.0 × 107
1.0 × 107
4.0 × 106
2
3
1·ZnCl2
2·ZnCl2
3·ZnCl2
relatively strong. This is consistent with the slightly lower
quantum efficiency of 1·ZnCl2 in comparison to that of 1
(Table 1). Similar absorption quenching with the appearance of
a new lower energy band, as well as emission quenching with a
red shift, was observed for 2 and 3 upon ZnCl2 addition (Figure
S5 (Supporting Information)). This result indicates that both
compounds have binding properties similar to those of 1
toward ZnCl2. Compounds 2 and 3 also exhibited a level of
high binding constants (K = 5.0 × 106 M−1) comparable with
that of 1. This is a consequence of Zn(II) binding to the
terpyridine moieties with a similar steric environment in all
conjugates.
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To examine the dual binding abilities of 1−3 toward Zn(II)
and fluoride ions, fluoride titration experiments were further
carried out with Zn(II) complexes 1·ZnCl2−3·ZnCl2. As shown
in Figure 7c, the addition of fluoride to the solution of 1·ZnCl2
led to gradual quenching of the low-energy absorption band at
346 nm along with formation of a new lower energy band at
390 nm with a distinct isosbestic point at 365 nm. This result
indicates fluoride binding to the boron center of 1·ZnCl2. A
high fluoride binding constant of ca. 5.0 × 107 M−1 was also
estimated. Complexes 2·ZnCl2 and 3·ZnCl2 showed similar
absorption changes upon fluoride binding (Figure S6
(Supporting Information)). Although the estimated fluoride
binding constant of 3·ZnCl2 (K = 4.0 × 106 M−1) was still
lower than those of 1·ZnCl2 and 2·ZnCl2 (K = ca. >107 M−1)
due to steric effects, it was higher than that of free conjugate 3
(K = 1.0 × 106 M−1). The increased K value for 3·ZnCl2
strongly indicates that the Lewis acidity of a boron center was
enhanced by Zn(II) complexation.17 It is worth mentioning
that the high binding constants of >107 M−1 obtained by fitting
are difficult to compare accurately. Furthermore, it is known
that Zn(II) centers in bipyridine complexes are susceptible to
fluoride ions.18 However, as shown in Figure 7 and Figure S6
(Supporting Information), the present terpyridine−Zn(II)
complexes did not show abnormal spectral changes up to 2
equiv of fluoride addition, indicating relatively high stability. 1H
NMR titration results of 1·ZnCl2 also support the preferential
binding of fluoride to the boron center (Figure S7 (Supporting
Information)). After full binding of fluoride to the boron
center, however, excess fluoride addition caused dissociation of
Zn(II) centers. While PL titrations of 1·ZnCl2 exhibited
quenching of the emission band at 450 nm, the band was
gradually blue-shifted upon fluoride binding. It finally became a
weak structured emission centered at 370 nm with a broad tail
to 600 nm (Figure 7c). The structured emission may indicate
that the π−π* transition in the phenylene−ethynylene−
terpyridine fragment is a dominant emission process in the
fluoride adduct, although the broad emission might be due to
the newly formed lower energy CT state caused by elevation of
π(Mes) level upon fluoride binding, as observed in the
absorption titrations above. As a result, the sky blue emission
color of the solution of 1·ZnCl2 almost disappeared at the end
of titrations. Similar emission changes were also observed for 2·
ZnCl2 and 3·ZnCl2 with an apparent increase in the emission
intensity of high-energy bands (Figure S6 (Supporting
Information)).
Figure 8. Spectral changes in UV/vis absorption (left) and
fluorescence (right) of 1 (1.0 × 10−5 M in THF) upon sequential
addition of ZnCl2 (red curves, 0−2.0 equiv) and TBAF (blue curves,
0−1.5 equiv). The pictures show the emission color change of a
solution of 1 under UV illumination upon sequential addition of ZnCl2
and TBAF.
be potentially useful for sensing both Zn(II) cation and fluoride
anion, as well as each ionic species.
CONCLUSION
■
Ditopic terpyridine−triarylborane conjugates (1−3) and their
Zn(II) complexes were prepared and characterized to
investigate the dual complexation behavior of conjugates
toward Zn(II) cation and fluoride anion. Experimental and
theoretical studies showed that the π(Ar) → pπ(B) (Ar = Mes
and/or phenylene−ethynylene) charge transfer (CT) transition
in 1−3 switched to π(Mes) → π*(Ar) (Ar = terpyridine−
ethynylene) intramolecular CT in the Zn(II) complexes,
resulting in red shifts of both the absorption and emission
bands in comparison to those of the free conjugates. UV/vis
absorption and fluorescence titration experiments of 1−3
exhibited gradual quenching of the absorption and fluorescence
bands upon addition of fluoride, while the addition of ZnCl2
gave rise to red shifts of both bands. The fluoride titrations of
Zn(II) complexes also led to quenching of both the absorption
and emission bands. Finally, sequential addition of ZnCl2 and
fluoride to the conjugates caused an emission color change
from deep blue to sky blue to dark. This suggests that the
conjugates are potentially useful as dual receptors for Zn(II)
cation and fluoride anion.
On the basis of the titration results of conjugates 1−3 and
their Zn(II) complexes toward ZnCl2 and/or fluoride, we
finally tested the conjugates as dual receptors for Zn(II) cation
and fluoride anion. The absorption and fluorescence changes of
1−3 were examined upon sequential addition of ZnCl2 and
fluoride (Figure 8 and Figure S8 (Supporting Information)). As
shown in Figure 8, the absorption and emission bands of 1
underwent red shifts upon addition of ZnCl2, with an emission
color change from deep blue to sky blue. The subsequent
addition of fluoride into the solution led to gradual quenching
of both the absorption and emission bands with the growth of a
new lower energy absorption band accompanied by disappear-
ance of the emission color. In fact, this experiment reproduced
the results of previous binding studies of 1 with ZnCl2 and of 1·
ZnCl2 with fluoride. Conjugates 2 and 3 also showed similar in
situ binding behavior (Figure S8 (Supporting Information)).
Therefore, the terpyridine−triarylborane conjugates 1−3 could
EXPERIMENTAL SECTION
■
General Considerations. All operations were performed under an
inert nitrogen atmosphere using standard Schlenk and glovebox
techniques. Anhydrous grade solvents (Aldrich) were dried by passing
them through an activated alumina column and stored over activated
molecular sieves (5 Å). Spectrophotometric grade THF and CH3CN
(Merck) were used for absorption and emission measurements.
Commercial reagents were used as obtained without any further
purification from Aldrich (n-BuLi (2.5 M solution in n-hexanes), tetra-
n-butylammonium fluoride (TBAF), diisopropylamine (i-Pr2NH),
Pd(PPh3)4, CuI, dimesitylboron fluoride (Mes2BF), sodium hydroxide,
sodium carbonate, ZnCl2), TCI (1-bromo-4-iodobenzene, 1-bromo-3-
iodobenzene, 1-bromo-2-iodobenzene), and Alfa Aesar
((trimethylsilyl)acetylene). (4-Ethynylphenyl)dimesitylborane (1a),33
(2-(3-bromophenyl)ethynyl)trimethylsilane,34 (2-ethynylphenyl)-
G
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Organometallics
Article
dimesitylborane (3a),35 and 2,6-bis(pyridin-2-yl)pyridin-4-yl trifluor-
omethanesulfonate (tpy-OTf)36 were synthesized according to the
reported procedures. Deuterated solvents from Cambridge Isotope
Laboratories were used. NMR spectra were recorded on a Bruker 300
that for 1 was employed with tpy-OTf (0.33 g, 0.86 mmol) and 2a
(0.33 g, 0.87 mmol) to afford a white powder of 2 (0.18 g, 36%). H
1
NMR (CDCl3): δ 8.76 (d, J = 4.2 Hz, 2H, Py-H), 8.67 (d, J = 8.0 Hz,
2H, Py-H), 8.57 (s, 2H, Py-H), 7.94 (td, J = 7.7, 1.6 Hz, 2H, Py-H),
7.74 (s, 1H, Ph-H), 7.71 (dt, J = 7.6, 1.5 Hz, 1H, Ph-H), 7.56 (dt, J =
7.5, 1.2 Hz, 1H, Ph-H), 7.43 (m, 3H, Py-H and Ph-H), 6.87 (s, 4H,
Me-H), 2.36 (s, 6H, Mes-CH3), 2.05 (s, 12H, Mes-CH3). 13C NMR
(CDCl3): δ 156.54, 156.37, 150.09, 141.76, 140.36, 139.94, 137.93,
137.60, 135.99, 134.38, 129.35, 129.18, 124.98, 123.78, 123.19, 122.20
(Ar-C), 95.01, 88.48 (ethynylene-C), 24.44, 22.25 (Mes-CH3). 11B
NMR (CDCl3): δ 74 (br). Mp: 240 °C. Anal. Calcd for C41H36BN3: C,
84.68; H, 6.24; N, 7.23. Found: C, 84.16; H, 6.28; N, 6.99.
Synthesis of 2-(4-(2-(2-(Dimesitylboryl)phenyl)ethynyl)-6-
(pyridin-2-yl)pyridin-2-yl)pyridine (3). A procedure analogous to
that for 1 was employed with tpy-OTf (0.40 g, 1.05 mmol) and 3a
(0.45 g, 1.28 mmol) to afford a white powder of 3 (0.35 g, 57%).
Single crystals suitable for an X-ray diffraction study were obtained by
slow evaporation of a CH2Cl2 solution of 3, giving colorless blocklike
crystals. 1H NMR (CDCl3): δ 8.80 (d, J = 4.5 Hz, 2H, Py-H), 8.64 (d,
J = 7.9 Hz, 2H, Py-H), 8.18 (s, 2H, Py-H), 7.94 (td, J = 8.0, 1.3 Hz,
2H, Py-H), 7.63 (dt, J = 7.6, 0.8 Hz, 2H, Ph-H), 7.48 (m, 1H, Ph-H),
7.42 (m, 2H, Py-H), 7.36 (d, J = 0.8 Hz, 1H, Ph-H), 7.34 (m, 1H, Ph-
H), 6.82 (s, 4H, Mes-H), 2.21 (s, 6H, Mes-CH3), 2.07 (s, 12H, Mes-
CH3). 13C NMR (CDCl3): δ 156.81, 156.08, 150.07, 141.72, 140.20,
137.77, 135.28, 134.46, 134.17, 131.24, 129.42, 127.25, 124.81, 123.95,
122.09 (Ar-C), 95.18, 91.32 (ethynylene-C), 24.19, 22.16 (Mes-CH3).
11B NMR (CDCl3): δ 76 (br). Mp: 208 °C. Anal. Calcd for
1
AM spectrometer (300.13 MHz for H, 75.48 MHz for 13C, 96.29
MHz for 11B) at ambient temperature. Chemical shifts are given in
ppm and are referenced against external Me4Si (1H, 13C) and BF3·
OEt2 (11B). Elemental analyses were performed on an EA1110
(FISONS Instruments) by the Environmental Analysis Laboratory at
KAIST. Melting (mp) or decomposition points (dec pt) of solid
compounds were measured using a MEL-TEMP II instrument
(Laboratory Devices, Inc.). UV/vis absorption and PL spectra were
recorded on a Varian Cary 100 and a HORIBA FluoroMax-4P
spectrophotometers, respectively.
Synthesis of 2-(4-(2-(4-(Dimesitylboryl)phenyl)ethynyl)-6-
(pyridin-2-yl)pyridin-2-yl)pyridine (1). In a Schlenk flask charged
with tpy-OTf (0.50 g, 1.31 mmol), (4-ethynylphenyl)-dimesitylborane
(1a; 0.50 g, 1.44 mmol), Pd(PPh3)4 (30 mg, 0.026 mmol), and CuI (5
mg, 0.026 mmol) was added 30 mL of anhydrous diisopropylamine
under a nitrogen atmosphere, and the reaction mixture was heated at
reflux for 24 h. After it was cooled to room temperature, the resulting
dark brown solution was concentrated and purified by silica gel
column chromatography (eluent: ethyl acetate/n-hexane 1/4) to give a
1
white powder of 1 (0.37 g, 49%). H NMR (CDCl3): δ 8.77 (dq, J =
4.8, 0.8 Hz, 2H, Py-H), 8.67 (dt, J = 8.0, 0.9 Hz, 2H, Py-H), 8.62 (s,
2H, Py-H), 7.93 (td, J = 7.7, 1.8 Hz, 2H, Py-H), 7.57 (d, J = 1.5 Hz,
4H, Ph-H), 7.41 (m, 2H, Py-H), 6.87 (s, 4H, Mes-H), 2.30 (s, 6H,
Mes-CH3), 2.05 (s, 12H, Mes-CH3). 13C NMR (CDCl3): δ 155.51,
155.46, 149.12, 140.86, 138.96, 137.10, 136.09, 133.27, 131.45, 128.28,
125.60, 124.11, 122.97, 121.33 (Ar-C), 94.05, 89.29 (ethynylene-C),
23.48, 21.27 (Mes-CH3). 11B NMR (CDCl3): δ 78 (br). Mp: 226 °C.
Anal. Calcd for C41H36BN3: C, 84.68; H, 6.24; N, 7.23. Found: C,
84.80; H, 6.21; N, 7.26.
C41H36BN3: C, 84.68; H, 6.24; N, 7.23. Found: C, 84.32; H, 6.26; N,
7.25.
Synthesis of 1·ZnCl2. To a solution of 1 (50 mg, 0.086 mmol) in
CH2Cl2/CH3CN (1/1, v/v, 10 mL) was added ZnCl2 (15 mg, 0.11
mmol) in CH3CN (3 mL). After the mixture was stirred for 2 h at
room temperature, the solvent was evaporated off under reduced
pressure and the resulting white powder was washed with CH3CN and
dried under vacuum at room temperature (52 mg, 84%). Single
crystals suitable for an X-ray diffraction study were obtained by slow
evaporation of a DMF/CH3CN solution of 1·ZnCl2, giving colorless
needlelike crystals. 1H NMR (CDCl3): δ 9.13 (dq, J = 4.9, 0.7 Hz, 2H,
Py-H), 8.27 (s, 2H, Py-H), 8.17 (d, J = 8.0 Hz, 2H, Py-H), 7.94 (td, J =
7.7, 1.7 Hz, 2H, Py-H), 7.59 (m, 6H, Py-H and Ph-H), 6.89 (s, 4H,
Mes-H), 2.36 (s, 6H, Mes-CH3), 2.05 (s, 12H, Mes-CH3). 13C NMR
(CDCl3): δ 151.08, 150.37, 147.41, 142.26, 141.74, 140.54, 140.29,
138.88, 136.89, 132.78, 129.36, 128.20, 124.47, 124.15, 122.02 (Ar-C),
100.50, 87.91 (ethynylene-C), 24.44, 22.22 (Mes-CH3). 11B signal was
not observed. Dec pt: 246 °C. Anal. Calcd for C41H36BCl2N3Zn: C,
68.60; H, 5.05; N, 5.85. Found: C, 68.00; H, 4.98; N, 5.98.
Synthesis of (3-Ethynylphenyl)dimesitylborane (2a). To a
solution of (2-(3-bromophenyl)ethynyl)trimethylsilane (0.50 g, 2.0
mmol) in dry THF (20 mL) was added n-BuLi (0.8 mL, 2.0 mmol)
dropwise at −78 °C. After it was stirred for 1 h at this temperature, a
THF solution (5 mL) of Mes2BF (0.54 g, 2.0 mmol) was slowly added
to the mixture. The reaction mixture was stirred for 1 h at −78 °C and
then gradually warmed to room temperature. After it was stirred
overnight, the mixture was quenched by the addition of saturated
aqueous NH4Cl (30 mL). The organic layer was separated, and the
aqueous layer was extracted with Et2O (3 × 30 mL). The combined
organic layers were dried over MgSO4, filtered, and concentrated
under reduced pressure. The crude product was purified by silica gel
column chromatography (eluent: n-hexane) to yield dimesityl(3-(2-
(trimethylsilyl)ethynyl)phenyl)borane (2b) as a white powder (0.55 g,
Synthesis of 2·ZnCl2. A procedure analogous to that for 1·ZnCl2
was employed with 2 (40 mg, 0.069 mmol) and ZnCl2 (10 mg, 0.073
mmol) to afford a white powder (47 mg, 95%) of 2·ZnCl2. Single
crystals suitable for an X-ray diffraction study were obtained by slow
evaporation of a MeOH/CH2Cl2 solution of 2·ZnCl2, giving colorless
1
65%). H NMR (CDCl3): δ 7.62 (s, 1H, Ph-H), 7.58 (dt, J = 7.6, 1.5
Hz, 1H, Ph-H), 7.43 (dt, J = 7.5, 1.3 Hz, 1H, Ph-H), 7.29 (t, J = 7.6
Hz, 1H, Ph-H), 6.80 (s, 4H, Mes-H), 2.30 (s, 6H, Mes-CH3), 1.96 (s,
12H, Mes-CH3), 0.21 (s, 9H, TMS-CH3).
1
Next, 2b (0.30 g, 0.71 mmol) was dissolved in THF (10 mL) and
treated with TBAF (ca. 1.0 M in THF, 5 mL). After the mixture was
stirred for 1 h at room temperature, aqueous HCl (ca. 3.0 M, 10 mL)
was added and the mixture stirred for 0.5 h. The resulting solution was
extracted with diethyl ether (10 mL), and the organic portions were
washed with water (3 × 10 mL). The combined organic layers were
dried over MgSO4, filtered, and dried under reduced pressure. The
solid mixture obtained was purified by silica gel column chromatog-
raphy (eluent: n-hexane) to give a white powder of 2a (0.23 g, 92%).
1H NMR (CDCl3): δ 7.58 (s, 1H, Ph-H), 7.54 (dt, J = 7.7, 1.4 Hz, 1H,
Ph-H), 7.42 (dt, J = 7.5, 1.3 Hz, 1H, Ph-H), 7.27 (td, J = 7.6, 0.6 Hz,
1H, Ph-H), 6.75 (s, 4H, Mes-H), 2.96 (s, 1H, ethynyl-H), 2.24 (s, 6H,
Mes-CH3), 1.91 (s, 12H, Mes-CH3). 13C NMR (CDCl3): δ 141.80,
140.23, 139.95, 137.39, 136.27, 129.29, 129.04, 122.85 (Ar-C), 84.86,
78.22 (ethynyl-C), 24.43, 22.24 (Mes-CH3). 11B NMR (CDCl3): δ 74
(br). Anal. Calcd for C26H27B: C, 89.15; H, 7.77. Found: C, 88.70; H,
7.86.
rodlike crystals. H NMR (CDCl3): δ 9.18 (dq, J = 5.0, 0.8 Hz, 2H,
Py-H), 8.30 (s, 2H, Py-H), 8.20 (dt, J = 8.0, 0.8 Hz, 2H, Py-H), 8.06
(td, J = 7.7, 1.7 Hz, 2H, Py-H), 7.83 (s, 1H, Ph-H), 7.76 (dt, 7.7, 1.4
Hz, 1H, Ph-H), 7.70 (dd, J = 5.0, 2.8 Hz, 1H, Py-H), 7.68 (dd, J = 5.0,
1.0 Hz, 1H, Py-H), 7.65 (dt, J = 7.4, 1.4 Hz, 1H, Ph-H), 7.49 (t, J = 7.5
Hz, 1H, Py-H), 6.89 (s, 4H, Mes-H), 2.37 (s, 6H, Mes-CH3), 2.05 (s,
12H, Mes-CH3). 13C NMR (CDCl3): δ 151.13, 150.36, 147.53,
141.87, 140.58, 140.26, 140.19, 139.13, 138.85, 136.58, 129.55, 129.37,
128.21, 124.09, 121.94, 121.58 (Ar-C), 100.67, 86.69 (ethynylene-C),
24.46, 22.24 (Mes-CH3). 11B signal was not observed. Dec pt: 232 °C.
Anal. Calcd for C41H36BCl2N3Zn: C, 68.60; H, 5.05; N, 5.85. Found:
C, 68.53; H, 5.26; N, 6.17.
Synthesis of 3·ZnCl2. A procedure analogous to that for 1·ZnCl2
was employed with 3 (38 mg, 0.065 mmol) and ZnCl2 (10 mg, 0.073
mmol) to afford a greenish yellow powder (37 mg, 79%) of 3·ZnCl2.
Single crystals suitable for an X-ray diffraction study were obtained by
slow evaporation of a CH3CN/CH2Cl2 solution of 3·ZnCl2, giving
1
colorless rodlike crystals. H NMR (CDCl3): δ 9.18 (dt, J = 4.8, 1.2
Synthesis of 2-(4-(2-(3-(Dimesitylboryl)phenyl)ethynyl)-6-
(pyridin-2-yl)pyridin-2-yl)pyridine (2). A procedure analogous to
Hz, 2H, Py-H), 8.14 (m, 4H, Py-H), 7.77 (s, 2H, Py-H), 7.74 (m, 3H,
H
dx.doi.org/10.1021/om401151p | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Py-H and Ph-H), 7.57 (td, J = 7.1, 1.8 Hz, 1H, Ph-H), 7.49 (td, J = 7.7,
1.4 Hz, 1H, Ph-H), 7.43 (m, 1H, Ph-H), 6.85 (s, 4H, Mes-H), 2.28 (s,
6H, Mes-CH3), 2.05 (s, 12H, Mes-CH3). 13C NMR (CDCl3): δ
151.19, 150.04, 147.67, 142.04, 140.55, 140.39, 139.33, 135.85, 134.64,
131.74, 131.01, 129.37, 128.26, 125.41, 123.97, 121.71 (Ar-C), 100.96,
86.11 (ethynylene-C), 24.27, 22.38 (Mes-CH3). 11B signal was not
observed. Dec pt: >300 °C. Anal. Calcd for C41H36BCl2N3Zn: C,
68.60; H, 5.05; N, 5.85. Found: C, 68.54; H, 5.02; N, 5.95.
program (No. 2009-0093818) through the National Research
Foundation of Korea (NRF) funded by the Ministry of
Education. S.U.L. expresses sincere thanks to the crews of the
Center for Computational Materials Science of the Institute for
Materials Research, Tohoku University, for their continuous
support of the SR11000 supercomputing facilities.
UV/Vis Absorption and PL Measurements. UV/vis and PL
measurements were performed with a 1 cm quartz cuvette at ambient
temperature. A solution of conjugate or Zn(II) complex (3.0 mL, 1 ×
10−5 M) was titrated with incremental amounts of ZnCl2 in CH3CN or
TBAF in THF (∼10−3 M). The absorbance data obtained were fitted
to a 1:1 binding isotherm to estimate a binding constant (K). Detailed
conditions are given in the figure captions. Quantum efficiencies of
compounds were measured with reference to that of quinine sulfate
(0.5 M H2SO4, Φ = 0.55).37
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ASSOCIATED CONTENT
* Supporting Information
■
S
Tables, figures, and CIF files giving UV/vis absorption and
fluorescence titration results of 1−3 and their Zn(II) complexes
and crystallographic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: suleechem@ulsan.ac.kr (S.U.L.); lmh74@ulsan.ac.kr
(M.H.L.).
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the Basic Science Research
Program (NRF-2012R1A1A2039773 for M.H.L. and NRF-
2013R1A1A2058722 for Y.H.L.) and Priority Research Centers
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