Cyclometalated Iridium(III) Bipyridine–Phenylboronic Acid Complexes as Bioimaging Reagents and Luminescent Probes for Sialic Acids

Article abstract of DOI:10.1002/asia.201700359

Sialic acids play important roles in mammalian development, cell–cell attachment, and signaling. As cancer cells utilize their overexpressed sialylated antigens to propagate metastases, the development of probes for sialic acids is of high importance. Herein, we report three luminescent cyclometalated iridium(III) bipyridine complexes bearing a phenylboronic acid (PBA) moiety. Spectrophotometric titrations revealed that the PBA complexes displayed higher binding affinity for the most common sialic acid N-acetylneuraminic acid (Neu5Ac) compared with simple sugars that are commonly found on glycoproteins. Notably, cellular imaging and uptake experiments showed that the PBA complexes were able to recognize cellular sialic acid residues, resulting in more efficient uptake than the boronic acid-free analogs. Additionally, one of the PBA complexes was shown to discriminate between cancerous and noncancerous cells.

Full text of DOI:10.1002/asia.201700359

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Cyclometalated Iridium(III) Bipyridine-Phenylboronic Acid
Complexes as Luminescent Probes for Sialic Acids and
Bioimaging Reagents
Authors: Hua-Wei Liu, Wendell Ho-Tin Law, Lawrence Cho-Cheung
Lee, Jonathan Chun-Wai Lau, and Kenneth Kam-Wing Lo
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Chemistry - An Asian Journal
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Cyclometalated Iridium(III) BipyridinePhenylboronic Acid
Complexes as Luminescent Probes for Sialic Acids and
Bioimaging Reagents
Hua-Wei Liu, Wendell Ho-Tin Law, Lawrence Cho-Cheung Lee, Jonathan Chun-Wai Lau, and Kenneth
Kam-Wing Lo*[a]
Abstract: Sialic acids play important roles in mammalian
development, cell-cell attachment, and signaling. Since cancer cells
utilize their overexpressed sialylated antigens to propagate
metastases, the development of probes for sialic acids is of high
importance. Herein, we report three luminescent cyclometalated
iridium(III) bipyridine complexes bearing a phenylboronic acid (PBA)
lectins to in vivo applications due to their immunogenicity and
large molecular size, which refrain them from approaching
vascular compartments. Also, the PAL method necessitates a
non-physiological reaction condition (pH

5).
These
requirements restrict the applicability of these methods in
recognizing and visualizing sialic acid residues on cell surfaces.
Recently, there is a growing interest in the use of phenylboronic
acid (PBA) as a recognition unit for cell-surface sialic acids.[
The PBA unit forms a five- or six-membered cyclic ester with the
exocyclic polyol function of sialic acid. The PBAsialic acid
esters formed are usually more stable than other PBAsugar
moiety.
Spectrophotometric titrations revealed that the PBA
610]
complexes displayed higher binding affinity to the most common
sialic acid N-acetylneuraminic acid (Neu5Ac) compared with simple
sugars that are commonly found on glycoproteins. Notably, cellular
imaging and uptake experiments showed that the PBA complexes
were able to recognize cellular sialic acid residues, resulting in more
efficient uptake than the boronic acid-free analogues. Additionally,
one of the PBA complexes was shown to discriminate between
cancerous and noncancerous cells.
esters at physiological pH (pH 7.4) and even intratumoral pH
(pH 6.5).[
610]
Thus, PBA has been incorporated into
nanoparticles, gold/metal oxide-coated electrodes, organic
compounds, and lanthanide chelates to afford sensors for sialic
acids.[
710]
Notwithstanding these developments, the possibility
of using luminescent transition metal complexes as probes for
sialic acids has not been explored. In view of the interesting
photophysical behavior of cyclometalated iridium(III) polypyridine
complexes,11 we anticipate that functionalization of these
complexes with a PBA moiety will lead to a new class of
luminescent probes and imaging reagents for sialic acids.
Introduction
Sialic acid is a generic term for over 50 structural analogues of
neuraminic acid, with the most common member being N-
acetylneuraminic acid (Neu5Ac, Scheme 1).[1] All sialic acids
share a cyclic nine-carbon structure with a carboxyl group at C1,
providing them a negative charge under physiological pH.
Instead of being an energy source, sialic acids are required for
mammalian development, cell-cell attachment, and signaling.[2]
Cancer cells use their overexpressed sialylated antigens such
as sialyl Lewis X (sLe ) and sialyl Lewis A (sLe ) to facilitate
interactions with the selectins on platelets, innate immune cells,
and endothelium, serving to propagate metastases.[1,3] These
findings have attracted much interest in the development of
probes for sialic acids. However, the lack of distinct reactive
groups in sialic acids is a challenge to the development of
molecular probes. Although sialic acids can be recognized by
antibodies[1,4] and natural lectins,[1,3b,5] and periodate oxidation
O
OH
OH
OH
HO
OH
HO
O
H
N
H3C
O
x
a
Scheme 1. Structure of Neu5Ac.
Herein, we report three luminescent cyclometalated
iridium(III) bipyridylPBA complexes [Ir(N^C) (bpy-TEG-
PBA)](PF )nHCl (bpy-TEG-PBA 4-(N-(13-(3-
boronobenzylamino)-4,7,10-trioxa-tridecanyl)aminocarbonyl-oxy-
methyl)-4’-methyl-2,2’-bipyridine; HN^C 2-phenylpyridine
Hppy), n = 0, (1a), 2-phenylquinoline, n = 0, (Hpq) (2a), 2-
phenyl-4-quinolinecarboxylic acid (Hpqa), n = 1, (3a)) (Scheme
). Their boronic acid-free counterparts [Ir(N^C) (bpy-TEG-
Bn)](PF )nHCl (bpy-TEG-Bn = 4-(N-(13-benzylamino-(4,7,10-
2
6
=
=
[6]
with aniline-catalyzed ligation (PAL), these methods have
limitations; for example, it is difficult to translate antibodies and
(
2
2
6
[
a]
Dr. H.-W. Liu, Dr. W. H.-T. Law, L. C.-C. Lee, J. C.-W. Lau, Prof. K.
K.-W. Lo
trioxa-tridecanyl)aminocarbonyl)-oxy-methyl)-4’-methyl-2,2’-
Department of Biology and Chemistry
City University of Hong Kong
Tat Chee Avenue, Kowloon, Hong Kong, P. R. China
E-mail: bhkenlo@cityu.edu.hk
Prof. K. K.-W. Lo
State Key Laboratory of Millimeter Waves, City University of Hong
Kong, Tat Chee Avenue, Kowloon, Hong Kong, P. R. China
bipyridine; HN^C = Hppy, n = 0, (1b), Hpq, n = 0, (2b), Hpqa, n =
1, (3b)) were also prepared for comparison studies. The
electrochemical and photophysical properties of the complexes
were studied. Also, the sialic acid-binding properties of the PBA
complexes were investigated by spectrophotometric titrations.
Additionally, the lipophilicity, cellular uptake, cytotoxic activity,
and bioimaging capability of the complexes were investigated.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201xxxxxx.
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Results and Discussion
Electrochemical properties
The electrochemical properties of the complexes were
investigated by cyclic voltammetry and the electrochemical data
are summarized in Table 1. The ppy (1a, b) and pq (2a, b)
complexes exhibited a quasi-reversible oxidation couple at 1.23
Synthesis of the ligands and complexes
We selected bpy-TEG-PBA as the diimine ligand (Scheme 2)
since the triethylene glycol (TEG) spacer-arm can increase the
water solubility, polarity, and biocompatibility of the complexes.
Also, the amino group that connects the linker and PBA unit is
positively charged at physiological pH, which can facilitate sialic
acid recognition through electrostatic interaction.[10a,b,e] The
synthesis of bpy-TEG-PBA involved the reaction of carbonate
to 1.26 V versus SCE, which is assigned to an iridium(IV)/(III)
oxidation couple.[
12]
These complexes displayed an additional
quasi-reversible couple or an irreversible wave at 0.95 to 1.00
V, which is associated with the oxidation of the secondary amine
moiety.[12a,b,13] Similar to the ppy and pq analogues, the pqa
complexes 3a, b showed a metal-centered quasi-reversible
couple at 1.35 V (Table 1). The potentials were slightly more
positive than those of the ppy and pq complexes as a result of
the stronger electron-withdrawing effect of the pqa ligand. The
absence of an amine-based quasi-reversible couple/irreversible
wave is due to the protonation of the amine group during
2 2
ester bpy-phen-NO with NH -TEG-NHBoc, followed by removal
of the Boc group to an amine derivative, and its subsequent
reaction with formyl PBA pinacol ester (Scheme S1).
Complexes 1a – 3a were prepared from the reaction of bpy-
2 4 2
TEG-PBA with the precursor complex [Ir (N^C) Cl ] (Scheme 1).
The use of different cyclometalating ligands allows the
complexes to exhibit different emission properties, lipophilicity,
and cellular uptake behavior. Boronic acid-free complexes 1b –
isolation of the complexes.
The first quasi-reversible
couple/irreversible wave of all the complexes at 1.44 to 1.49 V
are assigned to the reduction of diimine ligands.[
12]
The
3b were also prepared for comparison studies. The synthesis of
the pqa complexes 3a, b required an acidification step, resulting
in the secondary amine group of both complexes being isolated
reduction couples or waves that occurred at more negative
potentials should be associated with the reduction of the
cyclometalating ligands.[12,14]
as the protonated form (see below). All the complexes were
1
characterized by ESI-MS,
H NMR spectroscopy, and IR
spectroscopy and gave satisfactory elemental analyses.
Table 1. Electrochemical data of the iridium(III) complexes at 298 K.[a]
Complex
Oxidation E1/2 or E
a
/V
c
Reduction E1/2 or E /V
N
C
C
N
N
0.97,[b] 1.23[b]
1.46,[b] 2.04,[c] 2.21[b]
1.49,[b] 2.06,[c] 2.22[b]
[b]
1a
1b
2a
Ir
0.99,[c] 1.25[b]
N
(PF )•nHCl
6
0.95,[c] 1.26[b]
1.45,
1.76,[b] 2.03,[b]

2.33[
c]
O
HO
2b
1.00,[c] 1.26[b]
1.49,[b]
1.76,[b]
2.02,[c]

N
C
N
C
N
N
C

2.37
=
C
1.34[b]
1.35[b]
[c]
1.54,[b] 1.81,[b]
1.54,[b] 1.83,[b]
3
a
b
1.44,

2.02[
b]
ppy
pq
pqa
[c]
3
1.48,

2.02[
b]
O
O
N
O
3
N
H
H
[a] In CH CN (0.1 M TBAP), glassy carbon electrode, sweep rate = 100 mV
3
N
N
N

1
s
, all potentials are versus SCE. [b] Quasi-reversible couples. [c]
=
B
Irreversible waves.
N
HO
OH
CH3
Photophysical properties
bpy-TEG-PBA
The electronic absorption spectral data of the iridium(III)
complexes in MeOH and buffer solutions at 298 K are listed in
Table 2. The electronic absorption spectra of complexes 1a 
N^C = ppy, n = 0 (1a)
pq, n = 0 (2a)
pqa, n = 1 (3a)
3
a in MeOH at 298 K are shown in Figure 1. All the complexes
O
showed intense spin-allowed 1IL (  *) (N^C and N^N)
O
N
O
N
H
absorption features in the UV region ( 255  356 nm,  on the
3
H
N
N
N
N
order of 104 dm mol cm ) and weaker spin-allowed MLCT
3
1
1
1
=
1
(d(Ir)

*(N^N and N^C))/ LLCT ((N^C)

*(N^N))
CH3
absorption shoulders or bands in the visible region (> 344
nm).[
12a,c,14c,15]
The weaker absorption tailing beyond  421 nm
bpy-TEG-Bn
3
is assigned to spin-forbidden MLCT (d(Ir)  *(N^N and
N^C = ppy, n = 0 (1b)
pq, n = 0 (2b)
3
[12a,c,14c,15]
N^C))/ LLCT ((N^C)  *(N^N)) transitions.
pqa, n = 1 (3b)
Photoexcitation of the complexes resulted in intense and
long-lived green to orange-red emission in fluid solutions under
Scheme 2. Structures of the iridium(III) complexes.
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ambient conditions and in 77-K alcohol glass. The photophysical
data are listed in Table 3 and the emission spectra of complexes
1
1
2
2
3
a
b
a
b
a
MeOH (298)
O[a] (298)
583
0.32
0.10
0.11
4.74
0.37
0.13
0.13
5.13
2.66
2.04
1.88
7.12
2.78
2.02
1.84
8.22
1.63
0.34
0.36
4.44
1.50
0.36
0.37
4.45
0.048
0.015
0.015
H
2
590
1a  3a in MeOH at 298 K are shown in Figure 2. The ppy (1a,
b) and pqa (3a, b) complexes displayed a broad and featureless
emission band with positive solvatochromism in fluid solutions.
Upon cooling the samples to 77 K, large blue-shifts of emission
Buffer[b] (298)
589
Glass[c] (77)
MeOH (298)
473, 500 (max), 547 sh
3
maxima were observed, suggestive of a MLCT (d(Ir) 
585
0.069
0.021
0.021

*(N^N/N^C)) emissive state.[12a,c,14c,15,16] In contrast, the pq
H
2
O[a] (298)
593
complexes 2a, b exhibited vibronically structured features and
very long emission lifetimes in fluid solutions (Table 3). Also, the
emission properties of these two complexes were not very
Buffer[b] (298)
591
3
Glass[c] (77)
MeOH (298)
sensitive to the polarity of the solvents, indicative of a IL ( 
473, 505 (max), 544 sh
554, 601 sh
557, 602 sh
554, 601 sh
538 (max), 580, 632 sh
555, 601 sh
555, 603 sh
557, 601 sh
541 (max), 581, 630 sh
587

*) (pq) emissive state.[
12a,c,14c,15,16]
0.27
0.25
0.22
H
2
O[a] (298)
Table 2. Electronic absorption spectral data of the iridium(III) complexes at
98 K.
2
Buffer[b] (298)
Glass[c] (77)
MeOH (298)
 )
abs/nm (/dm3 mol–1 cm–1
Complex
Solvent
MeOH
1a
1b
2a
2b
255 (52,160), 275 sh (43,465), 314 sh
18,940), 344 sh (9,445), 385 sh (5,800),
21 sh (3,045)
0.28
0.24
0.21
(
4
H
2
O[a] (298)
Buffer[a]
MeOH
255 (53,610), 269 sh (48,730), 312 sh
20,285), 344 sh (9,465), 381 sh (6,200),
21 sh (3,770)
Buffer[b] (298)
Glass[c] (77)
MeOH (298)
(
4
255 (47,975), 275 sh (40,690), 312 sh
19,465), 344 sh (8,750), 385 sh (5,640),
21 sh (2,950)
0.28
(
4
H
2
O[d] (298)
599
0.012
0.039
Buffer[a]
MeOH
255 (46,195), 269 sh (42,075), 312 sh
17,785), 344 sh (8,155), 381 sh (5,415),
21 sh (3,180)
Buffer[e] (298)
Glass[c] (77)
MeOH (298)
598
(
4
561, 611 sh
586
262 sh (54,205), 284 sh (51,765), 311 sh
(
(
23,910), 336 (25,395), 356 sh (18,705), 438
5,680)
3b
0.23
H
2
O[d] (298)
599
0.010
0.057
Buffer[a]
263 sh (58,340), 284 sh (53,295), 311 sh
(
(
26,535), 336 (26,980), 356 sh (23,120), 438
6,385)
Buffer[e] (298)
Glass[c] (77)
598
563, 613 sh
MeOH
262 sh (56,630), 284 sh (54,220), 311 sh
(
(
25,230), 336 (26,245), 356 sh (19,150), 438
5,885)
[
7
a] H
2
O/MeOH (7:3, v/v). [b] 50 mM potassium phosphate buffer at pH
.4/MeOH (7:3, v/v). [c] EtOH/MeOH (4:1, v/v). [d] H
2
O/MeOH (9:1, v/v).
Buffer[a]
[e] 50 mM potassium phosphate buffer at pH 7.4/MeOH (9:1, v/v).
263 sh (54,130), 284 sh (49,450), 311 sh
(
(
24,545), 336 (24,725), 354 sh (19,745), 435
5,875)
3
a
b
MeOH
Buffer[b]
MeOH
Buffer[b]
264 (48,330), 284 (48,515), 310 sh (22,600),
42 (23,560), 438 (5,280)
6
5
4
3
2
1
3
267 (46,165), 282 (45,215), 310 sh (24,530),
41 (23,205), 444 (5,790)
3
3
263 (54,705), 284 (55,395), 308 sh (26,170),
42 (26,455), 440 (5,870)
3
267 (55,630), 282 (54,570), 310 sh (26,935),
41 (26,585), 444 (5,845)
3

[
a] 50 mM potassium phosphate buffer at pH 7.4/MeOH (7:3, v/v). [b] 50
0
2
50
300
350
400
450
500
550
600
mM potassium phosphate buffer at pH 7.4/MeOH (9:1, v/v).
Wavelength / nm
Table 3. Photophysical data of the iridium(III) complexes.
Figure 1. Electronic absorption spectra of complexes 1a (black), 2a (red), and
a (blue) in MeOH at 298 K.
Complex
Medium (T/K)

em/nm


/s

em
3
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(
Scheme 3).[10a,b,e] Thus, the higher binding affinity of the PBA
complexes to Neu5Ac than other sugars studied (and the
stronger Neu5Ac-binding of the PBA complexes compared with
unmodified PBA) should be due to cooperative two-site binding
through (1) PBA ester formation with the vicinal diol of Neu5Ac
and (2) electrostatic interaction between the ammonium group of
the complexes and carboxylate of Neu5Ac.
5
00
600
700
800
1.00
0.98
0.96
0.94
0.92
Wavelength / nm
Figure 2. Emission spectra of complexes 1a (black), 2a (red), and 3a (blue) in
MeOH at 298 K.
Sugar-binding studies
0.90
.88
The sugar-binding properties of the complexes were studied by
spectrophotometric titrations. Upon addition of Neu5Ac, the
absorbance of the PBA complexes 1a  3a at 309  342 nm was
decreased by  3 to 10% after correction for dilution effect.
Similar changes were not observed for the boronic acid-free
0
0
2
4
6
8
10
[Neu5Ac]/[Ir]
Figure 3. Results of spectrophotometric titrations of complexes 3a (squares)
and 3b (circles) (50 M), respectively, with Neu5Ac in 50 mM potassium
phosphate buffer at pH 7.4/DMSO (9:1, v/v) at 298 K. Absorbance was
monitored at 342 nm and corrected for dilution effect.
complexes 1b  3b.
The absorption titration curves for
complexes 3a, b with Neu5Ac are illustrated in Figure 3. Since
the boronic acid-free analogues did not display a comparable
decrease, we attribute the absorbance changes to the binding of
Neu5Ac to the PBA unit of complexes 1a  3a. The formation of
adducts was supported by ESI-MS (Figure S1). The binding
affinity of the PBA complexes to D-glucose, D-galactose, and D-
mannose, which are commonly found in eukaryotic
glycoproteins,[ was also investigated. Upon addition of D-
glucose, D-galactose, and D-mannose, complexes 1a  3a
required a much higher concentration of sugars to reach the end
point (Figures S2  S4), indicating that the PBA complexes are
more selective to Neu5Ac among the sugars. Treatment of the
titration data of the PBA complexes with the sugars using a 1:1
Table 4. Log K
b
values for the binding of sugars to complexes 1a  3a in 50
mM potassium phosphate buffer at pH 7.4/DMSO (9:1, v/v) at 298 K.
Complex
Neu5Ac
3.61
D-Glucose
2.85
D-Galactose
3.17
D-Mannose
3.21
17]
1a
2a
3a
3.85
2.92
3.05
3.34
3.62
1.68
2.63
2.16
binding model gave satisfactory fits.[18] The plots of A
vs. [Sugar]1 (Sugar = Neu5Ac, D-glucose, D-galactose, and D-
/(A  A)
 
OH
O
O

B
mannose) for complex 3a and the corresponding theoretical fits
O
H2N
CO2
O
3
N
O
N
HO
H
are shown in Figure S5 as an example. The log K
the binding of sugars to the PBA complexes 1a  3a are listed in
Table 4. The log K values for the binding of Neu5Ac range from
.61 to 3.85 and are larger than those of D-glucose (from 1.68 to
b
values for
N
N
C
C
R
O
Ir
O
H
H3C
N
b
HO
N
O
H3C
3
2
2
.92), D-galactose (from 2.63 to 3.17), and D-mannose (from
.16 to 3.34). These values follow the order: Neu5Ac > D-
Scheme 3. Schematic representation of sialic acid recognition by the
iridium(III) PBA complexes via a cooperative binding mode.
galactose  D-mannose > D-glucose, which is in accordance
with those reported in related studies.[9a,c,10a,c,d] Variation of the
cyclometalating ligands did not substantially change the binding
affinity of the complexes to Neu5Ac and the other three sugars.
Effects of pH values on emission behavior
The possible effects of pH on the emission of the complexes
were studied by emission spectroscopy. When the pH values of
the solutions were tuned from high to low, the emission of the
ppy (1a, b) and pq (2a, b) complexes did not display any pH-
b
Additionally, the log K values for the binding of Neu5Ac to the
PBA complexes are much larger than that of unmodified PBA
19]
(
log K
b
 1.30).[
It is well established that the optimal PBA-
sugar binding requires basic conditions where the concentration
of the boronate anion is maximal.[19,20] Also, recent studies have
shown that the presence of an aminomethyl group at the meta
position of PBA enhances the specificity and stability of the
dependence even at pH values lower than the pK
a
of
benzylamine and that of PBA ( 9.40 and 8.70, respectively)
[21]
(
Figures S6 and S7).
These findings are reasonable in view
of the lack of electronic communication between the
PBA/benzylamine moiety and the iridium(III) polypyridine core.
PBA-Neu5Ac adduct through
a cooperative binding mode
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Figure 4. Emission spectral traces of complexes 3a (top) and 3b (bottom) (50
In contrast, the effects of pH on the emission properties of the
pqa complexes 3a, b were more pronounced. At pH between 12
and 7, the complexes exhibited intense orange-red emission

M) in aerated 100 mM KCl(aq)/MeOH (9:1, v/v) at 298 K upon decreasing the
pH of the solutions.
(

em = 589 nm) with insignificant pH-dependence.
Upon
decreasing the pH to < 7, the emission bands of the complexes
were bathochromically shifted and displayed reduced intensities
25
(
Figure 4). The emission intensities at pH > 7 were  10- to 25-
2
1
1
0
5
0
5
fold higher than those at pH < 3 (Figure 5). Nonlinear least-
square fits of the titration curves of complexes 3a, b in the low-
pH region yielded apparent pK
.02, respectively. The emission quenching is attributed to the
protonation of the carboxylate of pqa ligands of the complexes.
The pK values are slightly smaller than that of
Ir(TPAQCOOH) (pic)] (pK = 5.16  0.05) (HTPAQCOOH = 2-
a
values of 4.57  0.01 and 4.37 
0
a
0
2
[
(
2
a
1
(4-diphenylamino)phenyl)-4-quinolinecaboxylic acid; pic
=
picolinate),[
22]
10
8
which is probably due to the absence of the
electron-donating diphenylamine moiety in complexes 3a, b.
Since it is well known that the MLCT (d(Ir)  *(N^C))
3
6
emission intensity is much lower than that of 3IL emission in
related iridium(III) complexes,[23] the observed emission
quenching upon decreasing the pH should originate from the
increased 3MLCT (d(Ir)  *(pqa)) character in the emissive
states of complexes 3a, b. This is also supported by the fact
that protonation of the carboxylate group will favor charge-
transfer from the iridium(III) center to the cyclometalating ligands.
On the basis of the titration results, the pqa ligands of
4
2
0
2
4
6
8
10
12
pH
Figure 5. Results of the pH titrations of complexes 3a (top) and 3b (bottom)
50 M) in aerated 100 mM KCl(aq)/MeOH (9:1, v/v) at 298 K. The emission
intensity of the complexes at pH = 2 was set as the reference point (I ).

(
complexes 3a,
b exist in the deprotonated form in the
biologically relevant pH range (pH  7.20  8.10).
Lipophilicity and cellular uptake efficiency
The lipophilicity of the complexes was determined by the shake-
flask method, and the results expressed as log Po/w values are
listed in Table 5. The log Po/w values ranged from 0.71 to 2.18
and were strongly dependent on the cyclometalating ligands and
their substituents. The log Po/w values of the ppy complexes 1a,
b (1.17 and 1.32, respectively) were lower than those of their pq
counterparts 2a, b (1.96 and 2.18, respectively) owing to the
hydrophobic nature of the pq ligands. Also, the pqa complexes
pH = 12
pH = 2
3
a, b (log Po/w = 0.71 and 0.82, respectively) were much less
lipophilic than the pq analogues, reflecting the effect of the polar
carboxyl groups. The polar boronic acid group rendered
450
500
550
600
650
700
750
800
Wavelength / nm
complexes 1a  3a less lipophilic than their boronic acid-free
counterparts complexes 1b  3b, respectively.
Table 5. Lipophilicity and cellular uptake of the iridium(III) complexes.
pH = 12
Complex
Lipophilicity (log Po/w)[a]
Amount of iridium/fmol[b]
pH = 2
1
1
2
a
b
a
1.17
1.32
1.96
2.18
0.71
0.82
2.04  0.11
1.79  0.21
7.17  0.26
6.63  0.31
0.86  0.06
0.58  0.02
450
500
550
600
650
700
750
800
2b
Wavelength / nm
3
a
b
3
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lipophilic complex 2a was internalized into the cells much faster
than complex 1a, as revealed by the time-dependent imaging
studies (Figure S11). Similar to many other cyclometalated
iridium(III) polypyridine complexes, these complexes were not
localized in the nucleus. The ppy complexes were diffusely
distributed in the cytoplasm with punctate staining whereas the
pq complexes fully stained the cytoplasm. Although complexes
[
a] Log Po/w is defined as the logarithmic ratio of the concentration of the
complex in octan-1-ol to that in an aqueous NaCl solution (0.9%, w/v). [b]
Amount of iridium associated with an average HepG2 cell upon incubation
with the complexes (25 M) at 37C for 1 h.
The uptake of all the complexes by HepG2 cells was
determined by ICP-MS measurements. Upon incubation with
the complexes (25 M) at 37C for 1 h, an average HepG2 cell
was found to contain 0.58 to 7.17 fmol of iridium (Table 5), which
is comparable to those reported in the cellular uptake studies of
other inorganic complexes.[24] The uptake efficiency of the
complexes followed the orders: 3a < 1a < 2a and 3b < 1b < 2b,
which is in accordance with their lipophilicity (Table 5).
Importantly, the PBA complexes revealed higher cellular uptake
1a and 2a did not appear to bind to cell-surface sialic residues, it
is noteworthy that co-incubation of the cells with these
complexes and free Neu5Ac resulted in reduced intracellular
emission intensity (Figure 6), whereas similar inhibition was not
observed for their boronic acid-free counterparts complexes 1b
and 2b (Figure S12). These results offer strong evidence that
the uptake of complexes 1a and 2a was facilitated by their
interactions with sialic acid residues on the cell membrane.
compared with the boronic acid-free complexes.
Similar
enhanced uptake has been observed when cells that express N-
azidoacetyl sialic acid are incubated with dibenzocyclooctyne
derivatives.[25] Since hepatic cancer cells such as HepG2 cells
express upregulated levels of sialylated antigens,[9b,e,26] it is likely
that the higher cellular uptake of the PBA complexes is assisted
by their interactions with cellular sialic acid moieties.
Cytotoxic activity
The cytotoxic activity of all the complexes was investigated by
the MTT assay,[27] where the dose dependence of surviving
HepG2 cells after exposure to the complexes for 1 h was
evaluated. The results showed that within a concentration range
of 6.25 – 12.5 M, all the complexes were noncytotoxic
(
percentage of surviving cells  95%) irrespective of their
lipophilicity and cellular uptake efficiencies (Figure S8). Upon
increasing the concentrations to 25 – 50 M, only the cells
treated with the pq complexes 2a, b exhibited a noticeable
decrease in viability, which is in line with their highest cellular
uptake efficiencies among the complexes (Table 5). This effect
was more obvious when the concentration was further increased
to 100 M, where the percentage of surviving cells treated with
the pq complexes reduced to  30%, which is 2- to 5-fold lower
than that of the ppy and pqa complexes, respectively. The
photocytotoxic activity of the complexes was also examined
Figure 6. Laser-scanning confocal images of HepG2 cells upon incubation
with complexes 1a and 2a (25 M, 1 h) in PBS/DMSO (99:1, v/v) containing
1% FBS in the absence (left) and presence (right) of free Neu5Ac (500 M, 1
h) at 37C, respectively.
The cellular uptake of complexes 1a, b and 2a, b was
studied in more detail. In an attempt to alter membrane fluidity
(
and/or decrease cellular energy utilization), HepG2 cells were
incubated at 4C prior to treatment with the complexes.[
28]
The
intracellular emission intensities of the treated cells were
negligible (Figures S13 and S14). Upon preincubation with the
cytoskeletal inhibitor colchicine or the oxidative phosphorylation
inhibitor 3-chlorophenylhydrazone (CCCP), the emission
intensities of the cells loaded with the ppy complexes decreased
substantially (Figure S13) whereas the pq complexes did not
bring a similar change (Figure S14). On the basis of these
results, the cellular uptake of the ppy complexes should occur
through an energy-dependent pathway, which is possibly
endocytosis. Also, the effect of lipophilicity of the pq complexes
appeared to play an important role in their cellular uptake,
suggesting passive diffusion as a major pathway.[28,29]
(
Figure S9). The results showed that the viability of the cells
treated with complexes 1a, b and 3a, b (25 M, 1 h) remained
high (> 70%) under both dark and light conditions (irr = 365 nm,
1
0 min), probably due to the low cellular uptake efficiency of the
complexes (Table 5). In contrast, complex 2a displayed
noticeable photocytotoxic effect (cell viability decreased from
65.6 to 7.3% upon irradiation), while the most lipophilic complex
2b caused extensive cell death with or without irradiation.
Cellular sialic acid-recognition studies
Since the PBA complexes 1a  3a showed interesting Neu5Ac-
binding ability, their capability of recognizing sialic acid residues
expressed on HepG2 cell membrane was evaluated by laser-
scanning confocal microscopy. Treatment of the cells with
On contrary to complexes 1a and 2a, incubation of HepG2
cells with complex 3a under the same conditions led to cell
membrane staining with much weaker cytoplasmic emission
complexes 1a and 2a (25 M) at 37C under a 5% CO
2
(
Figure 7, top left). Importantly, cells that were loaded with the
atmosphere for 1 h led to strong intracellular emission. Similar
results were observed in the cells incubated with the boronic
acid-free complexes 1b and 2b, indicative of very efficient
cellular internalization (Figure S10). Additionally, the more
control complex 3b gave no emission (Figure 7, bottom left).
The absence of extensive cytoplasmic staining should be a
result of the limited uptake of complexes 3a, b due to their much
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Chemistry - An Asian Journal
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lower lipophilicity and electrostatic repulsion between their
carboxylate groups and the phospholipid bilayers. Interestingly,
in the presence of free Neu5Ac, the cells treated with complex
acid residues. To address this point, we selected HEK293T
cells as a normal cell model, which is known to express a lower
level of cancer-associated glycans.[
30]
We found that HEK293T
3
a no longer displayed cell membrane staining (Figure 7, top
cells loaded with complex 3a displayed negligible cellular
emission (Figure S15, left), which is very different to the case of
HepG2 cells (Figure 7, top left) under the same incubation
conditions. Additionally, ICP-MS measurements showed that
the cellular uptake of complex 3a by HEK293T cells was  5.2-
fold lower than that by HepG2 cells (Figure 9, red bars). Again,
treatment of HEK293T cells with complex 3b did not result in
any noticeable cellular emission (Figure S15, right and Figure 9,
blue bars). These interesting findings illustrate that complex 3a
is capable of recognizing cell-surface sialic acid residues and
exhibits cellular discrimination ability.
middle). A similar observation was obtained when the cells were
pretreated with neuraminidase, which specifically catalyzes the
hydrolysis of (2,3) sialic acid linkages, before incubation with
complex 3a (Figure 7, top right). Also, ICP-MS measurements
revealed that the cellular uptake of complex 3a by Neu5Ac- and
neuraminidase-treated cells was decreased by  40 and 30%,
respectively (Figure 8, red bars). In contrast, the cells treated
with complex 3b under the same conditions did not exhibit any
noticeable difference (Figure 7, bottom and Figure 8, blue bars).
On the basis of these findings, we can conclude that the
membrane staining by complex 3a was due to its interaction with
the sialic acid residues on the cell membrane. Additionally,
upon increasing the incubation time, the cells displayed intense
cytoplasmic staining, indicating the internalization of labeled cell-
surface glycans and/or the free complex.[25]
1.2
1.0
0.8
0.6
0.4
0.2
0.0
HepG2
HEK293T
Figure 9. Relative uptake of iridium by HepG2 and HEK293T cells upon
incubation with complexes 3a (red) and 3b (blue) (25 M) at 37C for 1 h. The
uptake of complexes 3a, b by HepG2 cells was set as respective reference
points.
Conclusions
Figure 7. Laser-scanning confocal images of HepG2 cells upon incubation
with complexes 3a, b (25 M, 1 h) at 37C without any treatment (left),
In this work, three luminescent cyclometalated iridium(III)
bipyridine complexes functionalized with a PBA moiety and their
boronic acid-free counterparts were synthesized and
characterized. Their electrochemical, photophysical, lipophilicity,
cellular uptake efficiency, and cytotoxic activity were
investigated. The binding of the PBA complexes to various
sugars was also studied, and the results indicated that the
complexes were selective towards Neu5Ac over other sugars.
The emission of all the complexes was pH-insensitive in the
biologically relevant range (pH  7.20  8.10). Interestingly, ICP-
MS analyses revealed that the ppy (1a, b) and pq (2a, b)
complexes were internalized into live HepG2 cells much more
efficiently than their pqa counterparts (3a, b), which was closely
related to the lipophilicity of the complexes. The cytotoxic
effects of the complexes toward HepG2 cells were negligible at
the concentration used for live-cell imaging (25 M). Also, laser-
scanning confocal images showed that the PBA complexes 1a
and 2a stained the cell cytoplasm readily whereas complex 3a
was located on the surface of plasma membrane. Importantly,
experiments involving free Neu5Ac and neuraminidase
confirmed that the membrane staining was due to the interaction
of complex 3a with cell-surface sialic acids. Furthermore,
complex 3a was found to be capable of distinguishing the
simultaneously treated with free Neu5Ac (500 M,
pretreated with neuraminidase (0.1 U mL , 1 h) (right), respectively.
1
h) (middle), and

1
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Free Neu5Ac
Neuraminidase






Figure 8. Relative uptake of iridium by HepG2 cells upon incubation with
complexes 3a (red) and 3b (blue) (25 M) at 37C for 1 h with or without
treatment with free Neu5Ac (500 M) and neuraminidase (0.1 U mL1) for 1 h.
The uptake of complexes 3a, b in the absence of Neu5Ac and neuraminidase
was set as respective reference points.
Our next question is whether complex 3a can distinguish
cells with different expression levels of membrane-bound sialic
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Chemistry - An Asian Journal
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cancerous HepG2 and noncancerous HEK293T cells. All these
findings demonstrated the innovative applications of the
iridium(III) bipyridinePBA complexes as luminescent probes for
tumor-associated sialic acid residues at physiological pH via the
borate ester chemistry. The cancer cell-targeting properties of
related photofunctional complexes are expected to inspire the
development of novel therapeutics.
4-(N-(13-N’-(tert-Butoxycarbonyl)amino-4,7,10-trioxa-
tridecanyl)aminocarbonyl-oxy-methyl)-4’-methyl-2,2’-bipyridine
bpy-TEG-NHBoc)
(
A mixture of bpy-phen-NO
2
(438 mg, 1.20 mmol), NH
2
-TEG-NHBoc (767
mg, 2.39 mmol), and Et N (1 mL) in CH
3
2
Cl (10 mL) was stirred at room
2
temperature under an inert atmosphere of nitrogen for 24 h. The solvent
was removed by rotary evaporation to give a yellow oil. The crude
product was purified by column chromatography on silica gel using
EtOAc as the eluent. The product bpy-TEG-NHBoc was subsequently
isolated as a colorless oil. Yield: 613 mg (93%). 1H NMR (400 MHz,
Experimental Section
CDCl
3
, 298 K, TMS):  = 8.63 (d, J = 4.8 Hz, 1H, H6 of bpy), 8.51 (d, J =
.2 Hz, 1H, H6’ of bpy), 8.33 (s, 1H, H3 of bpy), 8.21 (s, 1H, H3’ of bpy),
.26 (d, J = 6.0 Hz, 1H, H5 of bpy), 7.14 (d, J = 4.8 Hz, 1H, H5’ of bpy),
.18 (s, 2H, CH on C4 of bpy), 3.64  3.45 (m, 12H, CH O), 3.35  3.31
), 3.18 (d, J = 6.0 Hz, 2H, CH NHBoc), 2.43 (s, 3H,
5
7
5
General considerations
2
2
All solvents were of analytical reagent grade and purified according to
standard procedures.[31] All buffer components were of biological grade
and used as received. 4,4’-Dimethyl-2,2’-bipyridine, selenium oxide,
sodium metabisulfite, 4,7,10-trioxa-1,13-tridecanediamine, di-tert-butyl
(m, 2H, CONHCH
2
2
CH
3
of bpy), 1.81  1.78 (m, 2H, CONHCH
CH NHBoc), 1.41 ppm (s, 9H, CH
2
CH
2
), 1.70 (t, J = 6.0 Hz, 2H,

CH
2
2
3
of Boc). MS (ESI ): m/z: 569 [M 

Na] .
dicarbonate, NH
Hpqa, LiOHH O, tetra-n-butylammonium hexafluorophosphate (TBAP),
KOH, and MTT were purchased from Aldrich. Sodium carbonate, NaBH
-nitrophenyl chloroformate, pyridine, Et N, trifluoroacetic acid (TFA),
6
NaCl, MgSO 3-formylphenylboronic acid, benzaldehyde, KPF ,
4
OH, pinacol, iridium(III) chloride hydrate, Hppy, Hpq,
2
4-(N-(13-Amino-4,7,10-trioxa-tridecanyl)aminocarbonyl-oxy-methyl)-
4
,
4
’-methyl-2,2’-bipyridine (bpy-TEG-NH
2
)
4
3
4
,
A mixture of bpy-TEG-NHBoc (563 mg, 1.03 mmol) and TFA (2 mL) in
CH Cl (20 mL) was stirred at room temperature under an inert
atmosphere of nitrogen for 2 h. The combined organic layer was
neutralized by addition of NH OH. The mixture was extracted with
CH Cl (100 mL  2). The solution was washed with H O (100 mL  3)
and then saturated NaCl solution (100 mL  2). The combined organic
extract was dried over MgSO , filtered, and the solvent was removed by
rotary evaporation. The product bpy-TEG-NH was subsequently
isolated as a colorless oil. Yield: 401 mg (87%). H NMR (400 MHz,
CDCl , 298 K, TMS):  = 8.56 (d, J = 4.8 Hz, 1H, H6 of bpy), 8.45 (d, J =
.2 Hz, 1H, H6’ of bpy), 8.28 (s, 1H, H3 of bpy), 8.15 (s, 1H, H3’ of bpy),
7.19 (d, J = 3.6 Hz, 1H, H5 of bpy), 7.07 (d, J = 4.8 Hz, 1H, H5’ of bpy),
.10 (s, 1H, CONH), 5.11 (s, 2H, CH on C4 of bpy), 3.57  3.41 (m, 12H,
CH O), 3.28  3.23 (m, 2H, CONHCH ), 2.72  2.69 (m, 2H, CH NH ),
.41 (s, 3H, CH ), 1.62
amberlite IR120 H resin, ferrocene, D-glucose, D-galactose, D-mannose,
KCl, octan-1-ol, colchicine, and CCCP were obtained from Acros.
2
2
Neu5Ac was purchased from Calbiochem.
Neuraminidase from
Clostridium perfringens was obtained from Sigma. All these chemicals
were used without further purification except that TBAP was
recrystallized from hot ethanol and dried in vacuo at 110C before use.
4
2
2
2
The iridium dimers [Ir
2
(N^C)
4
Cl
2
]
(HN^C
=
Hppy, Hpq, 2-
4
phenylcinchoninic acid methyl ester (Hpqe)), 4-formyl-4’-methyl-2,2’-
bipyridine (bpy-CHO), 4-hydroxylmethyl-4’-methyl-2,2’-bipyridine (bpy-
OH), N-(tert-butoxycarbonyl)amino-4,7,10-trioxa-1,13-tridecanediamine
2
1
3
5
(
NH
2
-TEG-NHBoc), and 3-formylphenylboronic acid pinacol ester were
[
12c,15a,16,32]
prepared as previously reported.
Autoclaved Milli-Q water was
6
2
used for preparation of the aqueous solutions. HepG2 and HEK293T
cells were obtained from American Type Culture Collection. Dulbecco’s
modified Eagle’s medium (DMEM), glucose-free DMEM, phosphate
buffered saline (PBS), fetal bovine serum (FBS), trypsin-EDTA, and
penicillin/streptomycin were purchased from Invitrogen. The instruments
for characterization, as well as electrochemical and photophysical
measurements, have been described previously.[12b] The procedures for
emission quantum yield measurements,[33] MTT assays,[27] ICP-MS,[12b]
and live-cell confocal imaging[12b,25c,34] were reported previously.
2
2
2
2
2
3
of bpy), 1.73 (p, J = 6.0 Hz, 2H, CONHCH
2
CH
2


2 2 2
ppm (p, J = 6.4 Hz, 2H, CH CH NH ). MS (ESI ): m/z: 447 [M  H] .
4-(N-(13-(3-boronobenzylamino)-4,7,10-trioxa-
tridecanyl)aminocarbonyl-oxy-methyl)-4’-methyl-2,2’-bipyridine
(bpy-TEG-PBA)
A mixture of bpy-TEG-NH
acid pinacol ester (79 mg, 0.34 mmol), and Et
was heated to reflux under an inert atmosphere of nitrogen for 24 h. The
mixture was cooled to room temperature and NaBH (112 mg, 2.97
2
(132 mg, 0.30 mmol), 3-formylphenylboronic
4
-(4-Nitrophenyloxycarbonyl-oxy-methyl)-4’-methyl-2,2’-bipyridine
3
N (2 mL) in MeOH (15 mL)
(bpy-phen-NO
2
)
4
mmol) was added. The mixture was further stirred for 2 h and then the
solvent was removed by rotary evaporation to give a colorless oil. The
crude product was purified by column chromatography on silica gel using
A mixture of bpy-OH (667 mg, 3.34 mmol), 4-nitrophenyl chloroformate
1.34 g, 6.68 mmol), and pyridine (1.35 mL, 16.7 mmol) in CH Cl (20
mL) was stirred at room temperature under an inert atmosphere of
nitrogen for 2 h. The solution was washed with H O (100 mL  3) and
then saturated NaCl solution (100 mL  2). The combined organic
extract was dried over MgSO , filtered, and the solvent was removed by
(
2
2
CH
TEG-PBA was subsequently isolated as a colorless oil. Yield: 95 mg
55%). 1H NMR (400 MHz, CD
OD, 298 K, TMS):  = 8.61 (d, J = 5.2 Hz,
H, H6 of bpy), 8.50 (d, J = 4.8 Hz, 1H, H6’ of bpy), 8.27 (s, 1H, H3 of
2 2 4
Cl /MeOH/NH OH (1:1:0.1, v/v/v) as the eluent. The product bpy-
2
(
3
4
1
rotary evaporation to give a purple solid. The crude product was purified
by column chromatography on silica gel using EtOAc as the eluent. The
bpy), 8.15 (s, 1H, H3’ of bpy), 7.64  7.53 (m, 2H, H2 and H4 of phenyl
ring of PBA), 7.39 (d, J = 4.8 Hz, 1H, H5 of bpy), 7.33  7.20 (m, 3H, H5’
product bpy-phen-NO
2
was subsequently isolated as a white solid. Yield:
.07 g (88%). 1H NMR (400 MHz, CDCl
, 298 K, TMS):  = 8.72 (d, J =
of bpy, H5 and H6 of phenyl ring of PBA) 5.21 (s, 2H, CH
2
on C4 of bpy),
1
5
3
4
6
.10 (s, 2H, CH
.8 Hz, 2H, CONHCH
of bpy), 1.94 (t, J = 6.0 Hz, 2H, CONHCH
2
on C1 of PBA), 3.60  3.50 (m, 12H, CH
), 3.12 (t, J = 6.8 Hz, 2H, CH NHCH
CH
2
O), 3.24 (t, J =
2
), 2.47 (s, 3H,
), 1.77 ppm (t, J = 6.4
.0 Hz, 1H, H6 of bpy), 8.55 (d, J = 4.8 Hz, 1H, H6’ of bpy), 8.45 (s, 1H,
H3 of bpy), 8.30  8.27 (m, 2H, H3 and H5 of phenyl ring), 8.26 (s, 1H,
H3’ of bpy), 7.42  7.40 (m, 2H, H2 and H6 of phenyl ring), 7.36 (d, J =
2
2
CH
Hz, 2H, CH
3
2
2


2 2 2
CH NHCH ). MS (ESI ): m/z: 581 [M  H] .
4
.8 Hz, 1H, H5 of bpy), 7.17 (d, J = 4.8 Hz, 1H, H5’ of bpy), 5.39 (s, 2H,


2 3
CH ), 2.45 ppm (s, 3H, CH ). MS (ESI ): m/z: 366 [M  H] .
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Chemistry - An Asian Journal
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4
-(N-(13-benzylamino-(4,7,10-trioxa-tridecanyl)aminocarbonyl)-oxy-
bpy), 8.14 (d, J = 8.4 Hz, 2H, H3 of pyridyl ring of ppy), 7.98 (d, J = 5.6
Hz, 1H, H6 of bpy), 7.89  7.83 (m, 5H, H6’ of bpy, H4 and H6 of pyridyl
ring of ppy), 7.65 (d, J = 5.6 Hz, 2H, H3 of phenyl ring of ppy), 7.48 
methyl)-4’-methyl-2,2’-bipyridine (bpy-TEG-Bn)
7
.41 (m, 7H, H5 and H5’ bpy, H2, H3, H4, H5 and H6 of phenyl ring of
Bn), 7.09  7.03 (m, 4H, H5 of pyridyl ring and H4 of phenyl ring of ppy),
.91 (t, J = 6.8 Hz, 2H, H5 of phenyl ring of ppy), 6.31 (t, J = 7.6 Hz, 2H,
H6 of phenyl ring of ppy), 5.30 (s, 2H, CH on C4 of bpy), 4.23 (s, 2H,
CH on C1 of Bn), 3.67  3.49 (m, 12H, CH O), 3.24  3.20 (m, 4H,
of bpy), 2.02  1.96 (m, 2H,
A mixture of bpy-TEG-NH
.19 mmol), and Et N (2 mL) in MeOH (15 mL) was refluxed under an
inert atmosphere of nitrogen for 24 h. The mixture was cooled to room
temperature and NaBH (73 mg, 1.93 mmol) was added. The mixture
was further stirred for 2 h and then the solvent was removed by rotary
evaporation to give a colorless oil. The crude product was purified by
2
(86 mg, 0.19 mmol), benzaldehyde (19.7 L,
0
3
6
2
4
2
2
CONHCH
2
and CH
CH ), 1.80  1.73 ppm (m, 2H, CH
OD, 298 K, TMS):  = 168.0, 162.0, 161.7, 161.5, 156.2,
155.3, 152.2, 150.9, 150.3, 150.1, 149.6, 148.5, 143.9, 143.8, 138.2,
2 2 3
NHCH ), 2.61 (s, 3H, CH
13
CONHCH
2
2
2 2 2
CH NHCH ).
C NMR
2 2 4
column chromatography on silica gel using CH Cl /MeOH/NH OH
(
150 MHz, CD
3
(
10:1:0.1, v/v/v) as the eluent.
The product bpy-TEG-Bn was
1
subsequently isolated as a colorless oil. Yield: 87 mg (84%). H NMR
1
1
5
8
38.1, 131.5, 131.4, 130.1, 130.1, 129.4, 129.3, 129.0, 128.9, 125.3,
24.6, 124.5, 123.0, 122.2, 119.5, 70.0, 69.9, 69.8, 69.7, 68.7, 68.3, 63.6,
1.0, 46.3, 37.9, 29.5, 19.9 ppm. IR (KBr): ṽ = 3428 (NH), 1719 (C=O),
(
8
400 MHz, CD
3
OD, 298 K, TMS):  = 8.58 (d, J = 5.2 Hz, 1H, H6 of bpy),
.47 (d, J = 4.8 Hz, 1H, H6’ of bpy), 8.26 (s, 1H, H3 of bpy), 8.12 (s, 1H,
H3’ of bpy), 7.35 (d, J = 4.4 Hz, 1H, H5 of bpy), 7.31  7.25 (m, 4H, H2,
H3, H5 and H6 of phenyl ring), 7.24 (d, J = 5.2 Hz, 2H, H5’ of bpy and H4

1

 
6
] . Elemental analysis
43 (PF
6
) cm . MS (ESI ): m/z: 1038 [M  PF
PF 4H O1.5CH Cl : C 46.51, H 4.89, N 6.08;
found: C 46.12, H 5.18, N 6.47.
calcd (%) for IrC52
H N O
57 6 5
6
2
2
2
of phenyl ring of Bn), 5.19 (s, 2H, CH
C1 of Bn), 3.56  3.47 (m, 12H, CH
CONHCH ), 2.69  2.66 (m, 2H, CH NHCH
.80  1.73 ppm (m, 4H, CONHCH CH
2
on C4 of bpy), 3.73 (s, 2H, CH
O), 3.23 (t, J = 6.8 Hz, 2H,
), 2.42 (s, 3H, CH of bpy),
and CH CH NHCH ). MS
2
on
2
2
2
2
3
1
(
2
2
2
2
2
2 6
[Ir(pq) (bpy-TEG-PBA)](PF ) (2a)


ESI ): m/z: 537 [M  H] .
The synthetic procedure was similar to that of complex 1a except that
[Ir (pq) Cl ] (81.6 mg, 64.1 mol) was used instead of [Ir (ppy) Cl ]. The
complex was isolated as orange crystals. Yield: 130 mg (76%). H NMR
400 MHz, CD OD, 298 K, TMS):  = 8.43  8.38 (m, 4H, H3 of quinoline
of pq, H3 and H6 of bpy), 8.24  8.17 (m, 5H, H3’ of bpy, H3 of phenyl
ring and H4 of quinoline of pq), 8.10 (d, J = 5.6 Hz, 1H, H6’ of bpy), 7.84
[
Ir(ppy) (bpy-TEG-PBA)](PF
2
6
) (1a)
2
4
2
2
4
2
1
(
3
A mixture of [Ir
mg, 87.7 mol) in CH
temperature under an inert atmosphere of nitrogen in the dark for 24 h.
The reaction mixture was cooled to room temperature and stirred for 30
2 4
(ppy) Cl
2
] (47 mg, 43.8 mol) and bpy-TEG-PBA (50.9
/MeOH (25 mL, 1:4, v/v) was stirred at room
2
Cl
2
(d, J = 7.6 Hz, 2H, H8 of quinoline of pq), 7.59  7.22 (m, 10H, H5 and
H5’ of bpy, H5 and H7 of quinoline of pq, H2, H4, H5 and H6 of phenyl
ring of PBA), 7.18 (t, J = 7.2 Hz, 2H, H4 of phenyl ring of pq), 7.08  7.02
min after addition of KPF
removed by rotary evaporation to give a yellow solid. Subsequent
recrystallization of the solid from CH Cl /diethyl ether afforded the
complex as yellow crystals. Yield: 40 mg (50%). 1H NMR (400 MHz,
CD OD, 298 K, TMS):  = 8.71 (s, 1H, H3 of bpy), 8.66 (s, 1H, H3’ of
bpy), 8.14 (d, J = 8.4 Hz, 2H, H3 of pyridyl ring of ppy), 7.97 (d, J = 5.6
Hz, 1H, H6 of bpy), 7.93  7.83 (m, 6H, H6’ of bpy, H4 and H6 of pyridyl
ring of ppy, and H2 of phenyl ring of PBA), 7.65 (d, J = 5.6 Hz, 2H, H3 of
phenyl ring of ppy), 7.52  7.48 (m, 2H, H5 of bpy and H4 of phenyl ring
of PBA), 7.41 (d, J = 5.2 Hz, 1H, H5’ of bpy), 7.21  7.02 (m, 6H, H5 of
pyridyl ring and H4 of phenyl ring of ppy and H5 and H6 of phenyl ring of
PBA), 6.90 (t, J = 7.2 Hz, 2H, H5 of phenyl ring of ppy), 6.31 (t, J = 6.8
6
(8.88 mg, 48.2 mol). The solvent was
(
m, 2H, H6 of quinoline of pq), 6.81 (t, J = 7.2 Hz, 2H, H5 of phenyl ring
of pq), 6.51 (t, J = 8.8 Hz, 2H, H6 of phenyl ring of pq), 5.14 (s, 2H, CH
on C4 of bpy), 4.12 (s, 2H, CH on C1 of PBA), 3.61  3.48 (m, 12H,
O), 3.21  3.12 (m, 4H, CONHCH and CH NHCH ), 2.46 (s, 3H,
of bpy), 1.97  1.91 (m, 2H, CONHCH CH ), 1.77  1.71 ppm (m, 2H,
OD, 298 K, TMS):  = 170.3,
2
2
2
2
3
CH
CH
CH
2
3
2
2
2
2
2
2
13
2 2 3
CH NHCH ). C NMR (150 MHz, CD
1
1
1
1
1
61.9, 156.4, 156.0, 155.2, 152.1, 150.9, 150.8, 147.6, 147.4, 147.3,
47.1, 145.8, 139.9, 139.8, 134.2, 130.5, 130.3, 128.9, 128.5, 127.8,
27.1, 127.0, 126.4, 126.3, 125.0, 124.7, 124.6, 124.5, 122.6, 121.2,
17.6, 117.5, 70.1, 69.9, 69.8, 69.7, 65.4, 63.4, 52.1, 45.9, 38.0, 29.4,
9.7 ppm. IR (KBr): ṽ = 3428 (NH), 1718, (C=O), 845 (PF
6
) cm1. MS
] . Elemental analysis calcd (%) for
3
OH: C 51.89, H 5.32, N 5.76; found: C
Hz, 2H, H6 of phenyl ring of ppy), 5.30 (s, 2H, CH
2
on C4 of bpy), 3.97 (s,

 
(
ESI ): m/z: 1181 [M  PF
BPF 2H O3CH
2.17, H 5.10, N 5.99.
6
2
2

H, CH
H, CONHCH
1.86 (m, 2H, CONHCH
2
on C1 of PBA), 3.61  3.51 (m, 12H, CH
), 2.98 (s, 2H, CH NHCH
CH ), 1.79  1.76 ppm (m, 2H, CH
C NMR (150 MHz, CD OD, 298 K, TMS):  = 168.0, 162.0, 161.7,
2
O), 3.24 (t, J = 6.8 Hz,
IrC60
H N O
61 6 7
6
2
2
2
2
), 2.61 (s, 3H, CH
3
of bpy), 1.94
CH NHCH ).
5
2
2
2
2
2
1
3
3
1
1
1
4
61.5, 156.2, 155.4, 152.2, 151.0, 150.2, 152.1, 150.0, 149.6, 148.5,
42.9, 143.8, 138.2, 138.1, 131.4, 131.3, 130.1, 130.0, 128.9, 125.4,
24.6, 124.5, 123.0, 122.2, 120.0, 70.1, 70.0, 69.8, 69.7, 68.4, 63.6, 48.5,
6.2, 38.0, 29.4, 20.0 ppm. IR (KBr): ṽ = 3423 (NH), 1719 (C=O), 843
[Ir(pq)
2 6
(bpy-TEG-Bn)](PF ) (2b)
The synthetic procedure was similar to that of complex 1b except that
[
Ir
2
(pq)
4
Cl
2
] (50.8 mg, 39.9 mol) was used instead of [Ir
2
(ppy)
4
Cl
2
]. The

1

 
(
(
PF
%) for IrC52
6
) cm . MS (ESI ): m/z: 1082 [M  PF
6
] . Elemental analysis calcd
1
complex was isolated as orange crystals. Yield: 72 mg (70%). H NMR
H
57
N
6
O
7
BPF H O2CH OH: C 49.58, H 5.16, N 6.42; found:
6
2
3
3
(400 MHz, CD OD, 298 K, TMS):  = 8.43  8.38 (m, 4H, H3 of quinoline
C 49.65, H 5.40, N 6.75.
of pq, H3 and H6 of bpy), 8.23  8.16 (m, 5H, H3’ of bpy, H3 of phenyl
ring and H4 of quinoline of pq), 8.10 (d, J = 5.6 Hz, 1H, H6’ of bpy), 7.84
(d, J =7.6 Hz, 2H, H8 of quinoline of pq), 7.50 (d, J = 5.6 Hz, 1H, H5 of
bpy), 7.44  7.28 (m, 10H, H5’ of bpy, H5 and H7 of quinoline of pq, H2,
H3, H4, H5 and H6 of phenyl ring of Bn), 7.18 (t, J = 7.6 Hz, 2H, H4 of
phenyl ring of pq), 7.08  7.02 (m, 2H, H6 of quinoline of pq), 6.81 (t, J =
[Ir(ppy) (bpy-TEG-Bn)](PF ) (1b)
2
6
A mixture of [Ir
mg, 80.5 mol) in CH
temperature under an inert atmosphere of nitrogen in the dark for 24 h.
The reaction mixture was cooled to room temperature and stirred for 30
min after addition of KPF
removed by rotary evaporation to give a yellow solid. Subsequent
recrystallization of the solid from CH Cl /diethyl ether afforded the
complex as yellow crystals. Yield: 55 mg (58%). 1H NMR (400 MHz,
CD OD, 298 K, TMS):  = 8.64 (s, 1H, H3 of bpy), 8.57 (s, 1H, H3’ of
2
(ppy)
4
Cl
2
] (43.1 mg, 40.2 mol) and bpy-TEG-Bn (43.1
/MeOH (25 mL, 1:4, v/v) was stirred at room
2
Cl
2
6
.8 Hz, 2H, H5 of phenyl ring of pq), 6.51 (t, J = 8.4 Hz, 2H, H6 of phenyl
ring of pq), 5.15 (s, 2H, CH on C4 of bpy), 3.87 (s, 2H, CH on C1 of
phenyl ring of Bn), 3.58  3.48 (m, 12H, CH O), 3.19 (t, J = 6.4 Hz, 2H,
CONHCH ), 2.81 (s, 2H, CH NHCH ), 2.46 (s, 3H, CH of bpy), 1.85 
.82 (m, 2H, CONHCH CH ), 1.75  1.72 ppm (m, 2H, CH CH NHCH ).
C NMR (150 MHz, CD OD, 298 K, TMS):  = 170.4, 161.9, 161.7,
61.4, 156.0, 155.2, 152.1, 150.9, 150.8, 147.6, 147.4, 147.3, 147.1,
2
2
2
6
(8.14 mg, 44.2 mol). The solvent was
2
2
2
3
1
2
2
2
2
2
2
2
13
3
1
3
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Chemistry - An Asian Journal
10.1002/asia.201700359
FULL PAPER
1
1
1
2
45.8, 139.9, 139.8, 134.2, 134.1, 130.5, 130.3, 128.9, 128.5, 128.4,
28.3, 127.8, 127.0, 126.9, 126.4, 126.3, 125.1, 124.7, 124.6, 124.5,
22.6, 121.2, 117.5, 70.1, 70.0, 69.9, 69.8, 69.3, 63.4, 52.6, 46.2, 38.0,
(84%). 1H NMR (400 MHz, CD
3
OD, 298 K, TMS):  = 8.68 (d, J = 7.2 Hz,
2H, H3 of quinoline of pqa), 8.51 (t, J = 8.0 Hz, 2H, H5 of quinoline of
pqa), 8.23  8.21 (m, 4H, H3 of phenyl ring of pqa and H6 and H3 of bpy),
8.15 (s, 1H, H3’ of bpy), 8.10 (d, J = 5.6 Hz, 1H, H6’ of bpy), 7.76  7.41
(m, 10H, H5 and H5’ of bpy, H6 and H8 of quinoline of pqa, H2, H4, H5
and H6 of phenyl ring of PBA), 7.21 (t, J = 7.2 Hz, 2H, H4 of phenyl ring
of pqa), 7.09  7.02 (m, 2H, H7 of quinoline of pqa), 6.85 (t, J = 7.2 Hz,
9.4, 19.7 ppm. IR (KBr): ṽ = 3416 (NH), 1718, (C=O), 845 (PF
6
) cm1
] . Elemental analysis calcd (%) for
3
OH: C 54.99, H 4.99, N 6.31; found: C 54.73,
.

 
MS (ESI ): m/z: 1137 [M  PF
PF H OCH
H 5.35, N 6.73.
6
IrC60
H N O
60 6 5
6
2
2
H, H5 of phenyl ring of pqa), 6.56 (t, J = 8.4 Hz, 2H, H6 of phenyl ring of
pqa), 5.20  5.11 (m, 2H, CH on C4 of bpy), 4.23 (s, 2H, CH on C1 of of
PBA), 3.64  3.46 (m, 12H, CH O), 3.22  3.16 (m, 4H, CONHCH and
CH NHCH ), 2.46 (s, 3H, CH of bpy), 2.00 1.97 (m, 2H,
CONHCH CH ), 1.74  1.71 (m, 2H, CH CH NHCH
). 13C NMR (150
MHz, CD OD, 298 K, TMS):  = 170.1, 167.8, 161.9, 16.6, 161.4, 156.4,
55.9, 150.0, 152.4, 150.8, 150.7, 148.0, 147.9, 147.5, 147.0, 145.6,
45.5, 134.4, 134.3, 130.7, 130.4, 129.4, 128.8, 128.2, 127.2, 127.1,
2
[Ir(pqe) (bpy-TEG-PBA)](Cl) (4a)
2
2
2
2
2
2
3

2 4 2
A mixture of [Ir (pqe) Cl ] (54 mg, 35.9 mol) and bpy-TEG-PBA (42.0
mg, 72.4 mol) in MeOH (15 mL) was stirred at room temperature under
an inert atmosphere of nitrogen in the dark for 24 h. The solvent was
2
2
2
2
2
3
1
1
removed by rotary evaporation to give
Subsequent recrystallization of the solid from CH
afforded the complex as reddish brown crystals. Yield: 81 mg (77%). 1H
NMR (400 MHz, CD OD, 298 K, TMS):  = 8.78 (d, J = 6.4 Hz, 2H, H3 of
quinoline of pqe), 8.55  8.52 (m, 2H, H5 of quinoline of pqe), 8.26  8.23
m, 3H, H3 of phenyl ring of pqe and H3 of bpy), 8.18 (d, J = 6.4 Hz, 1H,
H6 of bpy), 8.15 (s, 1H, H3’ of bpy), 8.07 (d, J = 6.0 Hz, 1H, H6’ of bpy),
a
reddish brown solid.
2
Cl /diethyl ether
2
127.0, 126.9, 125.3, 125.1, 125.0, 124.5, 124.4, 124.4, 122.9, 121.3,
117.4, 117.2, 70.0, 69.8, 69.8, 68.7, 68.7, 68.3, 63.3, 51.0, 46.2, 37.9,
3
2
9.5, 19.8 ppm. IR (KBr): ṽ = 3416 (NH), 3046 (OH), 1723 (C=O), 847

1



(
PF
analysis calcd (%) for IrC62
.90, N 5.37; found: C 50.11, H 5.26, N 5.55.
6
) cm . MS (ESI ): m/z: 1267 [M  2  H  PF
6
 HCl] . Elemental
(
H
62
N
6
O
11BClPF H O3MeOH: C 49.89, H
6
2
4
7.59  7.56 (m, 3H, H8 of quinoline of pqe and H5 of bpy), 7.51  7.43 (m,
5H, H6 of quinoline of pqe, H5’ of bpy, H2 and H4 of phenyl ring of PBA),
7.36  7.32 (m, 2H, H5 and H6 of phenyl ring of PBA), 7.25  7.21 (m, 2H,
2 6
[Ir(pqa) (bpy-TEG-Bn)](PF )HCl (3b)
H4 of phenyl ring of pqe), 7.11  7.07 (m, 2H, H7 of quinoline of pqe),
.88  6.85 (m, 2H, H5 of phenyl ring of pqe), 6.58  6.53 (m, 2H, H6 of
phenyl ring of pqe), 5.13 (s, 2H, CH on C4 of bpy), 4.61 (s, 2H, CH on
C1 of PBA), 4.13 (s, 6H, COOCH ), 3.62  3.46 (m, 12H, CH O), 3.19 
NHCH ), 2.46 (s, 3H, CH of bpy), 1.96
), 1.74  1.71 ppm (m, 2H, CH CH NHCH ).
6
The synthetic procedure was similar to that of complex 3a except that
complex 4b was used instead of complex 4a. The complex was isolated
2
2
as red crystals. Yield: 64 mg (77%). 1H NMR (400 MHz, CD
3
2
3
OD, 298 K,
3.12 (m, 4H, CONHCH
2
and CH
2
2
3
TMS):  = 8.68 (d, J = 5.2 Hz, 2H, H3 of quinoline of pqa), 8.51 (t, J = 8.0
Hz, 2H, H5 of quinoline of pqa), 8.23  8.21 (m, 4H, H3 of phenyl ring of
pqa and H6 and H3 of bpy), 8.15 (s, 1H, H3’ of bpy), 8.10 (d, J = 6.0 Hz,

1.93 (m, 2H, CONHCH
2
CH
2
2
2
2

 
MS (ESI ): m/z: 1297 [M  Cl ] .
1H, H6’ of bpy), 7.56  7.41 (m, 11H, H6 and H8 of quinoline of pqa, H5
and H5’ of bpy, and H2, H3, H4, H5 and H6 of phenyl ring of Bn), 7.22 (t,
J = 7.2 Hz, 2H, H4 of phenyl ring of pqa), 7.09  7.02 (m, 2H, H7 of
quinoline of pqa), 6.85 (t, J = 7.2 Hz, 2H, H5 of phenyl ring of pqa), 6.56
[
2
Ir(pqe) (bpy-TEG-Bn)](Cl) (4b)
The synthetic procedure was similar to that of complex 4a except that
bpy-TEG-Bn (40.0 mg, 74.7 mol) was used instead of bpy-TEG-PBA.
The complex was isolated as reddish brown crystals. Yield: 90 mg (87%).
(
t, J = 8.0 Hz, 2H, H6 of phenyl ring of pqa), 5.20  5.11 (m, 2H, CH
2
on
C4 of bpy), 4.23 (s, 2H, CH on C1 of Bn), 3.64  3.47 (m, 12H, CH
2
2
O),
3
.22  3.17 (m, 4H, CONHCH
.00  1.97 (m, 2H, CONHCH
2
and CH
2
NHCH
2
), 2.47 (s, 3H, CH
3
of bpy),
1H NMR (400 MHz, CD
of quinoline of pqe), 8.53 (d, J = 8.0 Hz, 2H, H5 of quinoline of pqe), 8.26
8.15 (m, 5H, H3 of phenyl ring of pqe, H6, H3 and H3’ of bpy), 8.07 (d,
J = 5.6 Hz, 1H, H6’ of bpy), 7.58  7.51 (m, 2H, H8 of quinoline of pqe),
.48  7.41 (m, 8H, H6 of quinoline of pqe, H5 and H5’ of bpy, H2, H3,
3
OD, 298 K, TMS):  = 8.78 (d, J = 5.6 Hz, 2H, H3
2
2
CH ), 1.76  1.71 (m, 2H, CH
2
2
CH
2
NHCH ).
2
13
C NMR (150 MHz, CD OD, 298 K, TMS):  = 170.1, 167.8, 161.9,
3

1
1
1
1
7
61.7, 161.4, 156.4, 155.9, 155.0, 152.4, 151.1, 150.8, 150.7, 148.1,
47.9, 147.6, 147.0, 145.6, 145.5, 142.3, 134.4, 134.3, 131.2, 130.7,
30.4, 129.4, 129.3, 129.0, 128.8, 127.2, 127.1, 127.0, 126.9, 126.9,
25.3, 125.1, 125.0, 124.5, 124.4, 124.4, 122.9, 121.3, 117.4, 117.2,
0.1, 69.9, 69.8, 69.7, 68.6, 68.3, 63.4, 50.9, 46.1, 37.9, 29.4, 25.5, 19.8
7
H5 and H6 of phenyl ring of Bn), 7.25  7.17 (m, 3H, H4 of phenyl ring of
pqe and H4 of phenyl ring of Bn), 7.11  7.05 (m, 2H, H7 of quinoline of
pqe), 6.89  6.85 (m, 2H, H5 of phenyl ring of pqe), 6.58  6.53 (m, 2H,
ppm. IR (KBr): ṽ = 3422 (NH), 3049 (OH), 1719 (C=O), 844 (PF
cm1. MS (ESI ): m/z: 1223 [M  2  H  PF
analysis calcd (%) for IrC62 ClPF H
6
)
 HCl] . Elemental
O3MeOH: C 51.33, H 4.97,
H6 of phenyl ring of pqe), 5.15 (s, 2H, CH
CH on C1 of Bn), 4.13 (s, 6H, COOCH ), 3.63  3.43 (m, 12H, CH
.19  3.15 (m, 4H, CONHCH and CH NHCH ), 2.46 (s, 3H, CH of bpy),
.97 1.94 (m, 2H, CONHCH CH ), 1.74 1.71 ppm (m, 2H,
CH ). MS (ESI ): m/z: 1253 [M  Cl ] .
2
on C4 of bpy), 4.63 (s, 2H,



6
2
3
2
O),
H N O
61 6 9
6
2
3
1
CH
2
2
2
3
N 5.53; found: C 50.99, H 5.29, N 5.80.

2
2


 +
2
2 2
NHCH
Sugar-binding studies
2 6
[Ir(pqa) (bpy-TEG-PBA)](PF )HCl (3a)
In the spectrophotometric titrations, the iridium(III) complex (50 M) in 50
mM potassium phosphate buffer at pH 7.4/DMSO (9:1, v/v) was titrated
with sugar of interest, which was dissolved in the same solvent mixture.
The electronic absorption spectrum of the solution was measured after
successive additions (2-L aliquots) of the sugar solution at 1-min
A mixture of complex 4a (80.0 mg, 60.0 mol) and LiOHH
2
O (5.0 mg,
1
12 mol) in THF/H O (12 mL) (4:1, v/v) was stirred at room temperature
2
under an inert atmosphere of nitrogen in the dark for 2 h. The pH of the
mixture was adjusted to 3 using amberlite IR120 H resin. The mixture
was then filtered and the filtrate was rotary evaporated to dryness,
affording a reddish brown solid. The solid was dissolved in MeOH (15
b
intervals. The binding constants, K , of the sugar to the iridium(III)
complex as described in the following equilibrium assuming 1:1
complexation:
mL) containing KPF
room temperature under an inert atmosphere of nitrogen in the dark for
0 min. The solvent was then removed by rotary evaporation to give a
6
(12.1 mg, 66.0 mol) and the solution was stirred at
3
reddish brown solid. Subsequent recrystallization of the solid from
MeOH/diethyl ether afforded the complex as red crystals. Yield: 67 mg
Ir  Sugar ⇌ Ir −Sugar
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Chemistry - An Asian Journal
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was determined by fitting the titration data to the following equation:[18]
We thank the Hong Kong Research Grants Council (Project Nos.
T42-103/16-N and CityU 11302116) and the Hong Kong
Research Grants Council, National Natural Science Foundation
of China (Project No. N_CityU113/15) for financial support.
W.H.-T.L and L.C.-C.L. both acknowledge the receipt of a
Postgraduate Studentship, a Research Tuition Scholarship, and
an Outstanding Academic Performance Award administrated by
City University of Hong Kong. K. K.-W. Lo is grateful to the
Croucher Foundation for the award of a Croucher Senior
Research Fellowship.
퐴
퐴  퐴ꢀ


1
=
ꢁ
ꢂꢃꢁ
ꢂ  1ꢆ



퐾bꢄSugarꢅ
where A
absence and presence of sugar at a concentration [Sugar], and 
are the absorption coefficients for the free and sugar-bound iridium(III)
complex, respectively. The K was determined from the ratio of the y-

and A are the dilution effect-corrected absorbance of Ir in the

and 
b
 
intercept to the slope of the linear fit plot of A /(A
 A) vs. [Sugar]1.
pH-dependent emission studies
A 20 mL stock solution of the iridium(III) complex (50 M) in a mixture of
0 mM KOH and 100 mM KCl(aq)/MeOH (7:3 or 9:1, v/v) was prepared.
The pH of the stock solution was adjusted to a desired value by addition
of appropriate amounts of 10, 5, 2.5, 1.25, 0.5, 0.25, 0.13, 0.07, 0.04 or
Keywords: imaging agents • iridium • luminescent probes •
phenylboronic acid • sialic acids
1
0
.02 N HCl(aq). After that, 2 mL of the stock solution was transferred to
[1]
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a quartz cuvette and the emission spectrum was measured. The solution
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apparent pK
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a
values of the complexes were determined from the
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equal volume of aqueous NaCl (0.9%, w/v) (saturated with octan-1-ol).
The mixture was swirled at 60 rpm for 30 min, and subsequently allowed
to partitioning at 298 K. The solution was then centrifuged, and the
[
4]
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5
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Chemistry - An Asian Journal
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Entry for the Table of Contents
FULL PAPER
Cancer Cells
Normal Cells
We report here three luminescent
cyclometalated iridium(III)
bipyridinePBA complexes that are
capable of binding sialic acid
residues, which are overexpressed
on tumors. With suitable
Hua-Wei Liu, Wendell Ho-Tin Law,
Lawrence Cho-Cheung Lee, Jonathan
Chun-Wai Lau, and Kenneth Kam-Wing
Lo*
HO
O
-
B
H2N-
+
O
HO
CO
2
O
O
O
HO
-
OHO-
B
AcNH
B
HO
O
H2N-
+
H2N+
CO2
-
HO
CO
HO
O
2
O
O
O
O
AcNH
AcNH
HO
HO
Page No. – Page No.
modification of their molecular
structures, these complexes have a
high potential to be developed as
novel theranostic reagents.
Cyclometalated Iridium(III)
BipyridinePhenylboronic Acid
Complexes as Luminescent Probes
for Sialic Acids and Bioimaging
Reagents
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