Anion sensing by rhenium(I) carbonyls with polarized N-H recognition motifs

Article abstract of DOI:10.1016/j.ica.2012.02.001

This article describes our recent research results on several rhenium(I) carbonyls complexes with polarized N-H recognition motifs as anion receptors. Structurally simple and easily synthesized luminescent anion receptors containing an amide-type anion binding site and a rhenium(I) tricarbonyl pyridine signaling unit have been developed, and they display outstanding sensitivity and selectivity toward a variety of anionic species. Moreover, thioamides and thiourea derivatives of 2,6-pyridinedicarbonyl dichloride, isophthaloyl dichloride and terephthaloyl dichloride have been synthesized. These ligands have been incorporated in dinuclear rhenium(I) diimine tricarbonyl complexes and provide anion recognition properties in these complexes. Furthermore, the related rhenium(I) complex featuring sulfonamide interacting sites that incorporate the highly chromophoric π-conjugated quinoxaline moiety has been prepared, characterized, and its photophysical properties were also studied.

Full text of DOI:10.1016/j.ica.2012.02.001

Inorganica Chimica Acta 389 (2012) 16–28
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Review
Anion sensing by rhenium(I) carbonyls with polarized N–H recognition motifs
Kai-Chi Chang a, Shih-Sheng Sun a, , Alistair J. Lees b,
⇑
⇑
a Institute of Chemistry, Academia Sinica, 128 Academia Road, Section 2, Taipei 115, Taiwan, ROC
b Department of Chemistry, State University of New York at Binghamton, Binghamton, NY 13902-6016, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 9 February 2012
This article describes our recent research results on several rhenium(I) carbonyls complexes with polar-
ized N–H recognition motifs as anion receptors. Structurally simple and easily synthesized luminescent
anion receptors containing an amide-type anion binding site and a rhenium(I) tricarbonyl pyridine sig-
naling unit have been developed, and they display outstanding sensitivity and selectivity toward a variety
of anionic species. Moreover, thioamides and thiourea derivatives of 2,6-pyridinedicarbonyl dichloride,
isophthaloyl dichloride and terephthaloyl dichloride have been synthesized. These ligands have been
incorporated in dinuclear rhenium(I) diimine tricarbonyl complexes and provide anion recognition prop-
erties in these complexes. Furthermore, the related rhenium(I) complex featuring sulfonamide interact-
Dedicated to Prof. Jon Zubieta
Keywords:
Anion recognition
Rhenium(I) carbonyls
Hydrogen-bonding interaction
Colorimetric sensing
Luminescent sensing
ing sites that incorporate the highly chromophoric
characterized, and its photophysical properties were also studied.
Ó 2012 Elsevier B.V. All rights reserved.
p-conjugated quinoxaline moiety has been prepared,
Kai-Chi Chang received his BS (2002) in chemistry from Tunghai University, Taiwan, a MS (2004) and a PhD (2008) in applied chemistry at National
Chiao Tung University under the supervision of Professor Wen-Sheng Chung. Since then he has been a postdoctoral researcher in the group of Professor
Shih-Sheng Sun at the Institute of Chemistry, Academia Sinica, Taiwan. His research interests include host-guest chemistry and supramolecular self-
assembly. Currently, he is a postdoctoral fellow in Professor Chain-Shu Hsu’s group working on the development of polymer solar cells.
Shih-Sheng Sun studied chemistry at National Cheng Kung University (BS 1989, MS 1991) and completed his PhD (2001) in Supramolecular
Chemistry at State University of New York at Binghamton with Prof. Alistair J. Lees. After two-year postdoctoral studies at Northwestern University
with Prof. Joseph T. Hupp and Prof. SonBin T. Nguyen, he joined the Institute of Chemistry, Academia Sinica as an Assistant Research Fellow in 2003
and was promoted to an Associated Research Fellow in 2009. His research interests focus on supramolecular materials chemistry and their appli-
cations in molecular recognition and sensing and optoelectronic materials.
⇑
Corresponding authors.
E-mail address: alees@binghamton.edu (A.J. Lees).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2012.02.001

K.-C. Chang et al. / Inorganica Chimica Acta 389 (2012) 16–28
17
Alistair J. Lees studied chemistry at the University of Newcastle-upon-Tyne and received his BSc Honors in 1976 and his PhD in 1979. His PhD studies
were in the area of inorganic spectroscopy with Dr. Brian B. P. Straughan. After a two-year postdoctoral fellowship (1979-81) at the University of
Southern California with Prof. Arthur W. Adamson in the area of photophysics and photochemistry of metal complexes, he joined the chemistry faculty
at the State University of New York at Binghamton as an Assistant Professor in 1981. He has been a Full Professor at the University since 1989 and his
research interests are in synthesis, spectroscopy and photochemistry of organometallic complexes, especially studying excited states and luminescence
properties, and the photophysical processes that take place in these materials.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2. Amide bridged rhenium(I) complexes as luminescent anion receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1. Preparation of amide bridged rhenium(I) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2. Photophysical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3. Anion sensing studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3. Thioamide and thiourea bridged rhenium(I) complexes as luminescent anion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.1. Preparation of thioamide and thiourea bridged rhenium(I) complexes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.2. Photophysical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.3. Anion sensing studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4. Quinoxalinebis(sulfonamide) bridged rhenium(I) complex as a novel anion receptor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.1. Preparation of quinoxalinebis(sulfonamide) bridged rhenium(I) complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.2. Photophysical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.3. Anion sensing studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
Incorporating a transition metal into the framework of sensory
systems offers several advantages over pure organic sensors.
Firstly, transition-metal complexes are often positively charged
or electron deficient in nature. This property leads to either the
enhancement of the electrostatic interactions with negatively
charged species or a higher probability of orbital overlap and, thus,
stronger bonding interactions with anionic species. Secondly, tran-
sition-metal complexes typically possess precisely defined geome-
tries. This characteristic is especially useful for enhancing the
selectivity of certain shaped anions or inducing unusual structural
changes upon binding to anions. Thirdly, and perhaps the most
appealing reason, is that transition-metal complexes possess a
variety of unique functionalities such as redox activity, visible
spectroscopic features (color), magnetism, luminescence, and cata-
lytic ability. Incorporation of these functionality properties into
sensory systems provides for not only the improvement of sensor
technology but also some other guest recognition based applica-
tions, such as anion transport, drug delivery, and catalysis.
The development of new chemosensors for anions is an impor-
tant subject in the field of supramolecular chemistry due to their
fundamental role in biological, environmental, and chemical pro-
cesses [1]. Compared to the well-developed field of cation binding
and sensing, anion recognition is a more challenging field because
of the variable size, shape, and strong solvation of the species [2].
As anions are larger than their isoelectronic cations and, thus, have
a smaller charge-to-radius ratio, electrostatic interactions are less
effective for anions than their corresponding isoelectronic cations
[3]. Solvation effects are also more prominent for anions than their
isoelectronic cations, and this is an important factor to consider in
designing artificial anion receptors. Hydroxylic solvents usually
form strong hydrogen bonds with anions, and this will reduce
the affinity of anions toward anion receptors. Hydrophobicity can
also influence the selectivity of an anion receptor as hydrophobic
anions tend to bind more tightly to the hydrophobic sites of the
receptors. Therefore, to achieve the desired sensitive and selectiv-
ity, the combination of electrostatic attraction, hydrogen bonding,
hydrophobic effects, and stacking effects all need to be taken into
consideration when designing artificial anionic hosts [4].
The incorporation of luminescent chromophores into the recep-
tor, which are sensitive to interactions between the host and guest
molecules, has recently gained considerable attention due to their
high sensitivity and low detection limit [5,6]. The appeal of sensors
that contain luminescence chromophores stems from the high sen-
sitivity of luminescence detection compared to other spectroscopic
methods. The binding of anionic species to the recognition sites
leads to changes in certain properties of the receptors (such as
absorption spectra, luminescence intensity, lifetime, etc.) that then
serve as an indicator of guest association.
Herein, we review some of our recent results and focus on dif-
ferent types of rhenium(I) carbonyls complexes with polarized N–
H recognition motifs as anion receptors: (a) new tweezer amide
bridged rhenium(I) complexes [7,8]; (b) two-armed thioamide
and thiourea bridged rhenium(I) complexes [9]; and (c) a novel
quinoxalinebis(sulfonamide) bridged rhenium(I) complex [10].
2. Amide bridged rhenium(I) complexes as luminescent anion
receptors
2.1. Preparation of amide bridged rhenium(I) complexes
The syntheses of amide ligand 1 and complexes 4 and 5 are
shown in Scheme 1. The bridging ligand 1 was prepared in 56%

18
K.-C. Chang et al. / Inorganica Chimica Acta 389 (2012) 16–28
O
CH2Cl2, NEt3
O
2
N
NH2
+
ClOC
COCl
N
NH
N
ClOC
N
O
HN
O
N
CH2Cl2, NEt3
1
HN
2
+
N
NH2
N
NH
3
ClOC
N
(NN)Re(CO)3(CH3CN)(PF6)
(NN)Re(CO)3(CH3CN)(PF6)
THF, reflux
THF, reflux
CO
CO
O
NH
N
N
O
NH
HN
O
N
Re CO
7
N
N
OC Re
N
N
N
N
OC Re
OC
N
O
OC
CO
HN
CO
(PF6)2
=
4
NN
(PF6)2
N
N
N
N
CO
Re CO
5
N
N
N
=
NN
CO
N
N
Scheme 1. Synthesis of the amide ligand 1 and complexes 4 and 5.
Scheme 3. Synthesis of the amide ligand 3 and complex 7.
Similarly, the syntheses of amide ligand 3 and complex 7 are
O
shown in Scheme 3. Ligand 3 was synthesized in 62% yield from
2 equiv. of 4-aminopyridine and terephthaloyl chloride. It has been
found the reaction can be carried out only with a relative small
amount of starting materials and with a large quantity of solvent
to ensure the solubility of the condensation products. Subsequent
reaction of 3 and 2 equiv. of (CH3CN)(4,40-tBu2bpy)Re(CO)3(PF6)
in refluxing THF, followed by recrystallization from CH2Cl2/hexane,
afforded a bright yellow crystalline solid 7 in 78% yield.
N
NH
ClOC
ClOC
O
CH2Cl2, NEt3
2
HN
N
2
N
NH2 +
(NN)Re(CO)3(CH3CN)(PF6)
Complexes 8–10 were synthesized from reaction of
(CH3CN)(4,40-tBu2bpy)Re(CO)3(PF6) and nicotinanilide, 4-NH2-py,
or 0.5 equiv. of BPA in refluxing THF, as shown in Scheme 4, and
purified by recrystallization from CH2Cl2/hexane solution, which
afforded pale yellow microcrystals in 76% yield for 8, 92% yield
for 9, and 86% yield for 10. The identity and purity of all new com-
pounds have been established by NMR, mass spectrometry, and
elemental analysis.
THF, reflux
O
NH
N
N
N
OC Re
OC
O
CO
HN
=
6
NN
(PF6)2
N
N
The N–H protons of complex 4 exhibit a chemical shift of
10.59 ppm in CDCl3. The highly downfield chemical shift in a
non-hydrogen bond donating solvent indicates the presence of
strong intramolecular hydrogen bonding between the N–H protons
and the nitrogen of the central pyridine. This results in ligand 1
having an approximate right angle geometry [11], and the con-
verged structure of complex 4 renders it as an effective anion
receptor through hydrogen bonding. Similarly, the highly down-
field chemical shift (11.06 ppm) of the amide protons is also ob-
served in complex 5. It should be pointed out, however, that the
more downfield shifting of the amide protons in complex 5 com-
pared to those in complex 4 could be simply due to the presence
of trace amounts of water in the deuterated acetone, not really
indicating a stronger hydrogen bonding in complex 5 than in com-
plex 4. In the case of complex 8, the meta connection between the
amide group and nitrogen in the pyridine results in no intramolec-
ular hydrogen bonding, and it is obvious that the chemical shift of
the amide proton appears at the normal position, ca. 8.34 ppm. In
the cases of complexes 6 and 7, there is no chance to form intramo-
lecular hydrogen bonding, but there may be intermolecular
hydrogen bonding between the amide protons and the carbonyl
groups. This idea is tentatively supported by the slightly downfield
shifting of the amide protons at 8.96 and 9.10 ppm for complexes 6
and 7, respectively. Complexes 9 and 10 were also synthesized to
serve as model compounds. Complex 10 is employed to assess
N
CO
N
N
Re CO
CO
Scheme 2. Synthesis of the amide ligand 2 and complex 6.
yield from 2 equiv. of 4-aminopyridine and 2,6-pyridinedicarbonyl
dichloride. Ligand 1 exhibits very poor solubility in most common
organic solvents, presumably because of its potential for intermo-
lecular hydrogen-bonding formation via the amide protons.
Subsequent reaction of 1 and 2 equiv. of (CH3CN)(4,40-tBu2bpy)-
Re(CO)3(PF6) or (CH3CN)(6-Mebpy)Re(CO)3(PF6) in refluxing THF,
followed by recrystallization from CH2Cl2/pentane (4) or CH3CN/
ether (5), afforded bright yellow crystalline solids 4 and 5 in 81%
and 88% yields, respectively.
The syntheses of amide ligand 2 and complex 6 are shown in
Scheme 2. The bridging ligand 2 was prepared in 62% yield from
2 equiv. of 4-aminopyridine and isophthaloyl dichloride. Due to
the extremely low solubility of ligand 2 in THF, the subsequent me-
tal-complexation process needed to reflux for 3 days to ensure the
completion of the reaction. The yellowish powder of complex 6
was isolated after recrystallization from CH2Cl2/hexane in 82%
yield.

K.-C. Chang et al. / Inorganica Chimica Acta 389 (2012) 16–28
19
N
N
L
THF, reflux
O
(NN)Re(CO)3(CH3CN)(PF6)
Re
L
OC
OC
PF6
N
HN
8
9
CO
L =
1/2 N
CH2CH2
N
N
NH2
N
N
OC Re
CO
CO
N
CH2CH2
N
Re CO
OC
N
CO
N
=
(PF6)2
NN
N
N
10
Scheme 4. Synthesis of the complexes 8–10.
Table 1
Absorption and emission spectral data and calculated photophysical parameters of complexes 4–10 in CH2Cl2 at 293 K.
Complexes
Absorption
Emission
kmax, nm (10ꢁ3
e
, Mꢁ1 cmꢁ1
)
kem, nm
Uem
s,
ls
10ꢁ5kr, sꢁ1
10ꢁ6knr, sꢁ1
4
5
6
7
8
9
10
254 (63.6), 281(61.5), 380 (sh, 7.70)
536
544
538
538
524
552
532
0.20
0.083
0.16
0.22
0.39
0.042
0.27
0.48
0.38
0.62
0.54
1.10
0.21
0.59
4.2
2.2
2.6
4.1
3.5
2.0
4.6
1.7
2.4
1.3
1.4
0.56
4.6
1.2
252 (45.6), 300 (47.6), 324 (sh, 35.9), 368 (6.02)
255 (50.1), 285 (53.0), 315 (sh, 33.6), 362 (7.84)
255 (52.7), 288 (56.4), 303 (sh 54.5), 376 (sh, 8.03)
250 (35.2), 274 (34.7), 300 (sh, 25.6), 315 (22.1), 373 (sh, 5.11)
270 (30.6), 316 (sh, 9.82), 368 (4.09)
254 (40.9), 271 (41.6), 303 (sh, 28.1), 315 (24.8), 348 (sh, 9.42)
the position of MLCT band, while complex 9 is expected to form the
weakest hydrogen bonding, if there is any, among all six complexes
(4–9) studied in this paper.
2.2. Photophysical properties
The absorption and emission spectral data, including lifetimes
and emission quantum yields, in deoxygenated CH2Cl2 at 293 K
are summarized in Table 1. In general, the absorption spectra of
all the complexes exhibit a series of strong absorption bands below
335 nm. These bands are assigned to ligand-based
p–
p⁄ transi-
tions. The absorption spectra of complexes 4 and 10, as well as
their difference spectrum, are displayed in Fig. 1. The low-energy
shoulder around 380 nm in complex 4 disappears in the difference
spectrum, and this confirms that the lowest excited state is a Re
Fig. 1. Absorption spectra of complexes 4 (curve a), 10 (curve b), and the difference
spectrum between complexes 4 and 10 (curve c) in CH2Cl2 at 293 K.
(dp
) to 4,40-tBu2bpy (p⁄) charge-transfer band. The low-energy fea-
tures around 370 nm in complexes 6–8 are also assigned to Re (dp)
to 4,40-tBu2bpy (p⁄) charge-transfer bands. This assignment is
2.3. Anion sensing studies
supported by the appearance of an absorption band at 368 nm in
complex 9, where the only low-energy transition is the Re (d
p
)
The anionic sensory system studied here comprises an amide–
pyridine moiety, which provides the anion binding site through
amide-anion hydrogen bonding and a signaling unit which is
(4,40-tBu2bpy)Re(CO)3. The positive-charged character of the
(4,40-tBu2bpy)Re(CO)3 moiety is also expected to provide an addi-
tional electron-withdrawing effect on the amide binding site
[13]. Addition of different halides or inorganic polyatomic anions
(as tetrabutylammonium salts) into 2 ꢀ 10ꢁ6 M solutions of com-
plexes 4–9 was observed to cause different degrees of quenching
of the luminescence intensities. Except for complex 9, the N–H pro-
tons in 1H NMR spectra all showed significant downfield shifts
(>1.5 ppm), indicating the strong hydrogen-bonding formation be-
tween the amide protons of the complexes and the anions. In the
case of complex 9, there is no significant decrease in the emission
to 4,40-tBu2bpy (p⁄) charge-transfer band. Similarly, the shoulder
appearing at 368 nm in complex 5 is assigned to a Re (d
6-Mebpy (p⁄) charge-transfer band.
p) to
All six complexes exhibit fairly strong luminescence in CH2Cl2
solution at room temperature. The structureless spectral profiles
and submicrosecond lifetimes indicate that the emissions of all
six complexes originate from the 3MLCT excited states. The calcu-
lated kr values are also very typical for triplet MLCT emissions, ca.
1 ꢀ 105 sꢁ1 [12]. The photophysical parameters of complexes 4, 6,
and 7 are virtually the same. This is not unexpected given that
the lowest excited states in both these complexes are Re (dp) to
4,40-tBu2bpy (p⁄) CT states and the bridging ligands are structurally
similar amide–pyridines.

20
K.-C. Chang et al. / Inorganica Chimica Acta 389 (2012) 16–28
436 nm upon addition of anion into the receptor solution (see
Fig. 3).
However, in the case of Iꢁ, there were no apparent changes in
the absorption spectrum. The red shift of the absorption and
emission positions, as well as the quenching of the emission
intensities upon addition of anions, are not apparent, but this
is likely due to a switching of the lowest excited state from
a 4,40-tBu2bpy-based 3MLCT state to a amide–pyridine-based
3MLCT state through hydrogen-bonding formation, thereby
enhancing nonradiative decay. This conclusion is partially sup-
ported by low-level extended Hückel calculations, where the
LUMO changes from p⁄ orbital localized on 4,40-tBu2bpy to the
p⁄ orbital localized on the amide–pyridine ligand upon formation
of hydrogen bonding between the anion and amide protons.
Scheme 5 depicts possible qualitative excited-state diagrams rep-
resenting the respective charge-transfer energy levels before and
after anion binding. Similar red shifts on the emission spectra
and quenching of the emission intensities upon anion binding
were also observed in Ru(II)-polypyridine-based sensing systems
[6b,15]. It appears that a dynamic uenching mechanism does
not occur in this system, given the fact that there was no appar-
ent quenching between complex 10 and the various anions stud-
ied. Another possible explanation for the observed emission
quenching behavior is anion-enhanced reductive quenching of
the MLCT excited state; this would also be expected to lower
the energy of the Re(I) to pyridine–amide ligand charge-transfer
excited state.
Fig. 2. Titration curve of the addition of Fꢁ anion (as tetrabutylammonium salt) to
complex 4 in CH2Cl2 solution as monitored by luminescence spectroscopy. The inset
showed the changes to the emission spectrum of complex 4 in CH2Cl2 solution upon
addition of Fꢁ anion. Emission spectra of the two highest anion concentrations were
not included for clarity. The excitation wavelength is 360 nm.
Table 2 summarizes the binding constants measured for com-
plexes 4–8 toward different anions. Clearly, complexes 4 and 5
show strong binding affinity toward halides, cyanide, or acetate
anions, only moderate binding affinity toward dihydrogen phos-
phate, and very weak binding affinity to nitrate or perchlorate
anions. The overall order determined for binding affinity is the
CNꢁ > Fꢁ > Iꢁ > Clꢁ ꢂ Brꢁ ꢂ OAcꢁ ꢃ H2PO4ꢁ > NO3
>
ꢁ
following:
ClO4ꢁ. This finding is significant as it is unusual for charged recep-
tors to exhibit such outstanding selectivity for anion species
Fig. 3. Changes in the absorption spectra of complex 4 (6.7 ꢀ 10ꢁ6 M) in CH2Cl2
solution upon addition of Fꢁ anion (as tetrabutylammonium salt). The concentra-
Table 2
tions of Fꢁ anion are 0, 4.0 ꢀ 10ꢁ5, 5.9 ꢀ 10ꢁ5, 9.8 ꢀ 10ꢁ5, 1.9 ꢀ 10ꢁ4, 3.7 ꢀ 10ꢁ4
6.9 ꢀ 10ꢁ4, 1.3 ꢀ 10ꢁ3, and 2.2 ꢀ 10ꢁ3 M, respectively.
,
Association constants Ka of complexes 4–8 toward different anions determined by
luminescence titration in CH2Cl2 at 298 K.
Anions
10ꢁ3Ka, Mꢁ1
4
5
6
7
8
intensity when the complex is exposed to anions. The titration
curves were observed to fit well to a 1:1 binding isotherm, and
the binding stoichiometry was further confirmed by Job plots [14].
Fig. 2 shows a typical 1:1 titration curve for the luminescence
intensity upon addition of Fꢁ to a CH2Cl2 solution of complex 4.
Concomitant with the quenching, the luminescence maximum
red shifts approximately 6–10 nm in each case. The lowest-energy
band (1MLCT) in the absorption spectrum also red shifts 10–30 nm
CNꢁ
Fꢁ
88
38
4.0
14
1.5
2.5
2.3
6.0
1.5
1.1
0.33
0.22
1.6a
2.8
1.6
2.4
2.7
1.4
Clꢁ
Brꢁ
Iꢁ
0.41
0.38
5.7a
0.71
18
0.75
0.44
1.1a
0.73
5.5
3.9
15a
a
OAcꢁ
3.4
1.6
1.6
ꢁ
H2PO4
0.015
6.3 ꢀ 10ꢁ3
8.4 ꢀ 10ꢁ4
0.010
ꢁ
NO3 ꢁ
5.9 ꢀ 10ꢁ3
0.47
0.52
0.34
ClO4
7.4 ꢀ 10ꢁ4
4.5 ꢀ 10ꢁ3
2.0 ꢀ 10ꢁ3
3.1 ꢀ 10ꢁ3
in each case along with the decrease of the
and a new band developed around 347 nm with a shoulder around
p–
p⁄ band at 281 nm,
a
Possibly involves an excited-state association process. See text for details.
Scheme 5. Qualitative excited-state diagrams for charge-transfer energy levels before and after anion binding.

K.-C. Chang et al. / Inorganica Chimica Acta 389 (2012) 16–28
21
O
S
N
NH
N
NH
Lawesson's reagent
O
S
HN
N
HN
N
2
12
(NN)Re(CO)3(CH3CN)(PF6)
S
Scheme 6. An excited-state association mechanism for iodide-induced emission
quenching.
THF, reflux
N
N
OC Re
OC
N
NH
O
S
S
CO
HN
N
NH
N
N
NH
N
NN = 4,4'-tBu2bpy
4,4'-Me2bpy
Lawesson's reagent
14a
14b
14c
(PF6)2
O
S
4,4'-Ph2bpy
HN
HN
N
CO
Re CO
N
N
N
N
CO
1
11
14
Scheme 8. Synthesis of the thioamide ligand 12 and complexes 14a–14c.
(NN)Re(CO)3(CH3CN)(PF6)
S
effectively quenched by as much as 10%, even in the presence of
only 10ꢁ8 M cyanide or fluoride anions.
THF, reflux
It is surprising that Iꢁ has higher association constant than both
the Clꢁ and Brꢁ anions. Given the fact that there was no apparent
change in the absorption spectrum upon addition of Iꢁ to the
receptor solution (vide infra), the efficient emission quenching is
likely due to the involvement of an excited-state association pro-
cess (see Scheme 6).
N
N
N
OC Re
OC
NH
N
S
CO
HN
NN
4,4'-tBu2bpy
=
13a
13b
13c
(PF6)2
4,4'-Me2bpy
4,4'-Ph2bpy
N
CO
To account for the unusual selectivity observed in complexes 4
and 5, several structurally similar complexes were also synthesized
and their anion binding studies were conducted under the same
experimental conditions. Although the general trend of sensitivity
still holds in complexes 6–8, the selectivity toward different anions
is greatly reduced. Except for ClO4ꢁ, which is expected to form the
weakest hydrogen bond, all other anions studied in this research
exhibit similar binding affinities within two orders of magnitude.
The similar association constants of complexes 6 and 7 toward an-
ions indicate that the two amide groups in complex 6 are arranged
in an anti-fashion. The intramolecular hydrogen-bonding capabil-
ity of ligand 1, which forces a cleft conformation, seems to be the
N
Re CO
N
CO
13
Scheme 7. Synthesis of the thioamide ligand 11 and complexes 13a–13c.
[11,16]. In fact, the combination of interactions involving electro-
static forces, hydrogen-bonding strength, hydration energy, and
steric effects all apparently influence the binding affinities toward
anions in complexes 4 and 5. Importantly, the sensitivities of
complexes 4 and 5 are so high that the emission intensity can be
N
Dry THF
reflux
NH
S
2
S
+
N
C
N
H2N
N
NH2
N
S
N
N
N
H
N
H
H
15
(NN)Re(CO)3(CH3CN)(PF6)
THF, reflux
CO
Re
N
N
N
OC
OC
NN = 4,4'-tBu2bpy
4,4'-Me2bpy
16a
16b
16c
NH
S
CO
4,4'-Ph2bpy
N
N
N
(PF6)2
S
N
N
N
H
N
H
Re
H
OC
CO
16
Scheme 9. Synthesis of the thiourea ligand 15 and complexes 16a–16c.

22
K.-C. Chang et al. / Inorganica Chimica Acta 389 (2012) 16–28
N
O
N
C S
H
N
NH2
EtOH
H
N
N
H
+
N
H
O
N
S
17
(NN)Re(CO)3(CH3CN)(PF6)
CO
THF, reflux
N
OC
OC
Re
N
N
PF6
H
N
H
N
N
O
H
NN
=
S
N
N
18
Scheme 10. Synthesis of the amidothiourea ligand 17 and complex 18.
key to its metal complexes being efficient as an anionic sensory
system.
3. Thioamide and thiourea bridged rhenium(I) complexes as
luminescent anion
3.1. Preparation of thioamide and thiourea bridged rhenium(I)
complexes
The syntheses of thioamide ligands 11–12 and complexes 13(a–
c) and 14(a–c) are shown in Schemes 7 and 8. The thioamide li-
gands 11 and 12 were prepared from 2 equiv. of Lawesson’s re-
agent and amide ligands 1 and 2, respectively. The resulting
products were characterized by IR spectroscopy to compare with
the amide and thioamide ligands. The disappearance of the C@O
stretching bands at about 1600 cmꢁ1 and the two bands associated
with amide, due to intermolecular hydrogen bonding, served as
indicators of complete thionation. It is worth noting that the syn-
thesis of complexes 13(a–c) and 14(a–c) was achieved by refluxing
2 equiv. of the respective acetonitrile bipyridine rhenium(I) com-
plexes {(bpy)Re(CO)3(CH3CN)(PF6)} with the corresponding one
equivalent of the appropriate bridging ligand for 2 h. 1H NMR
spectra were monitored throughout the reaction to determine
Fig. 5. Emission titration spectra of complex 16c (1.6 ꢀ 10ꢁ5 M) in CH2Cl2 at 298 K
with increasing concentrations of TBA-Cl: (a) the initial spectrum without added
anion, (b) 1, (c) 2, (d) 3, (e) 4, (f) 5, (g) 6, and (h) 7 equiv. Excitation wavelength is
330 nm.
the extent of product formation. The disappearance of the peak
at 2.3 ppm, which is attributed to the bound acetonitrile ligands,
served as an indicator of successful ligand and coordination of
the bridging ligand to the metal center.
Fig. 6. Emission titration spectra of complex 16c (1.0 ꢀ 10ꢁ5 M) in CH2Cl2 at 298 K
with increasing concentrations of TBA-I: (a) the initial spectrum without added
anion, (b) 0.5, (c) 1.0, (d) 2.0, (e) 2.5, (f) 3.0 (g) 3.5, (h) 4.0, (i) 4.5, (j) 5.0, and (k)
5.5 equiv. Excitation wavelength is 385 nm.
Fig. 4. Absorption spectrum (blue) and emission spectrum (pink) of 5.0 ꢀ 10ꢁ6 M of
complex 13a in CH2Cl2 at 298 K. Excitation wavelength is 360 nm.

K.-C. Chang et al. / Inorganica Chimica Acta 389 (2012) 16–28
23
Fig. 10. Emission titration spectra of 13b (5.0 ꢀ 10ꢁ6 M) in CH2Cl2 at 298 K with
TBA-H2PO4, in increments of 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 equiv. The
initial spectrum has no added anion. Excitation wavelength is 360 nm.
Fig. 7. Emission titration spectra of complex 13a (2.0 ꢀ 10ꢁ5 M) in CH2Cl2 at 298 K
adding Clꢁ anion up to 6 equiv. Excitation wavelength is 360 nm.
The syntheses of thiourea ligand 15 and complexes 16(a–c) are
shown in Scheme 9. The bridging ligand 15 was synthesized in 40%
yield from 2,6-diamionpyridine and 2 equiv. of 3-pyridineisothio-
cyanate. Subsequent reaction of 15 and 2 equiv. of the respective
acetonitrile bipyridine rhenium(I) complex, {(bpy)Re(CO)3 (CH3CN)
Fig. 11. Emission titration of compound 18 (1.6 ꢀ 10ꢁ5 M) in 95% H2O/DMSO (v/v)
at 298 K with TBA-CN (0.2, 0.4, 0.6, and 0.8 equiv.). The initial spectrum has no
added anion. Excitation wavelength is 360 nm.
Fig. 8. Emission titration spectra of complex 13a (2.0 ꢀ 10ꢁ5 M) in CH2Cl2 at 298 K
adding Brꢁ anion in increments of 1 up to 9 equiv., with the initial spectrum having
no added anion. Excitation wavelength is 360 nm.
(PF6)}, in refluxing THF, followed by recrystallization from CH2Cl2/
pentane, afforded yellowish orange crystals 16(a–c).
The syntheses of the amidothiourea ligand 17 and the corre-
sponding complex 18 are shown in Scheme 10. Ligand 17 was
synthesized in 97% yield from isoniazid and 1-isothiocyanatonaph-
thalene in ethanol. Subsequent reaction of 17 and (CH3CN)
(4,40-tBu2bpy)Re(CO)3(PF6) in refluxing THF afforded the amid-
othiourea-based rhenium complex 18.
3.2. Photophysical properties
It was observed that these dinuclear rhenium complexes of the
type 13a–c, 14a–c and 16a–c, are yellow compounds with consid-
erable absorption in the range 300–400 nm and only slight varia-
tions depending on the pyridyl ligand attached to the rhenium
metal. Also noted was their strong emission centered at about
600 nm. Fig. 4 shows typical absorption and emission spectra of
a representative complex 13a. Both the absorption and emission
spectra are dominated by higher lying intraligand (IL) and lowest
energy lying metal-to-ligand charge transfer (MLCT) transitions
[7].
Fig. 9. Emission titration spectra of complex 13b (5.0 ꢀ 10ꢁ6 M) in CH2Cl2 at 298 K
with TBA-Br: adding in increments of 0.2, 0.4, 0.6, 0.8, 1.0, and 2.0 equiv. The initial
spectrum has no added anion. Excitation wavelength is 360 nm.

24
K.-C. Chang et al. / Inorganica Chimica Acta 389 (2012) 16–28
O
S
N
H2N
NH2
O
O
N
N
HN
HN
HOAc
reflux
O
+
N
N
O
TsHN
NHTs
S
O
N
19
BrRe(CO)5
THF, reflux
O
S
Re(CO)3Br
N
N
HN
HN
O
O
N
S
O
N
Re(CO)3Br
20
Scheme 11. Synthesis of the sulfonamide ligand 19 and complex 20.
CH2Cl2 at 298 K, while exciting at 385 nm. The intensities of the
two emission peaks changed with increasing addition of the iodide
anion, added in the form of TBA-I. The relatively strong emission
peak at 575 nm showed quenching, while the initially less emissive
band at 465 nm exhibited significant enhancement resulting in the
formation of an isoemissive point at 530 nm (see Fig. 6). The asso-
ciation constant for 16c with TBA-I in CH2Cl2 was calculated to be
2.96 ꢀ 104 Mꢁ1 by a Stern–Volmer plot.
Table 3
Photophysical properties of probes 19 and 20 in CH3CN at 293 K.
Probe Absorption
kmax, nm (10ꢁ3
261 (37.5), 355 (13.2), 415 (0.6)
324 (19.3), 439 (11.4), 550 (sh, 5.7) 604
Emission
e
, Mꢁ1 cmꢁ1
)
kem, nm Uem
s
, ns
19
20
530
0.05
1.2
2.6 ꢀ 10ꢁ5 <0.1a
a
The excited-state lifetime is too short to be precisely measured in our
instrument.
Emission titration studies of the complex 13a were carried out
in CH2Cl2, where the addition of tetrabutylammonium chloride re-
sulted in the emission quenching of the band at about 560 nm fol-
lowing excitation at 352 nm (see Fig. 7). A similar quenching
pattern was observed for the other anions such as fluoride and bro-
mide, with fluoride showing the most significant response in this
category.
In order to study the interactions of anions with this receptor,
UV–Vis spectrophotometric titrations in dichloromethane were
carried out. It should be noted that dichloromethane was chosen
because it is relatively nonpolar, aprotic and does not interfere
with the receptor–anion hydrogen bonding. These results did not
differ much from the amide counterparts reported earlier [7]. How-
ever it should be pointed out that the solubility of these new thio-
amide, urea and thiourea ligands in organic solvents was relatively
better than their amide counterparts.
3.3. Anion sensing studies
The thiourea-bridged dinuclear complex 16c, exhibited a maxi-
mum absorption at 330 nm. Upon irradiation of this compound
with 330 nm light, two emission bands were observed, centered
at 400 and 575 nm, respectively. Titration of complex 16c with tet-
rabutylammonium chloride (see Fig. 5) resulted in emission
enhancement of the higher energy band (at 400 nm) with a corre-
sponding quenching of the lower energy band at 575 nm, and an
isoemissive point at about 495–500 nm. Similar experiments were
also carried out with TBA-I (tetrabutylammonium iodide) and
again an enhancement of the upper energy band and a concomi-
tant quenching of the lower energy band was observed (see Fig. 6).
The thiourea-based rhenium(I) complex 16c was found to emit
strongly at 575 nm and weakly at 465 nm upon excitation at
385 nm. Emission titration studies were carried out with TBA-I in
The emission titration shown in Fig. 8 indicates how the
addition of bromide to complex 13a with the triflate (OTfꢁ) counter
Fig. 12. Absorption (a) and emission (b) spectra of ligand 19 (4.6 ꢀ 10ꢁ5 M) in acetonitrile solutions upon the addition of n-Bu4NF [(0–4.6) ꢀ 10ꢁ5 M].

K.-C. Chang et al. / Inorganica Chimica Acta 389 (2012) 16–28
25
Table 4
to 50% of the initial value, exhibiting a comparable level of sensitiv-
ity to anion as observed in the corresponding amide system [7].
In the absorbance spectrum of complex 13a, the intensity of a
band at ca. 230 nm was noted to increase when CNꢁ was added
to the solution. Also observed was a slight bathochromic shift of
approximately 10 nm, when up to 0.6 equiv. of TBA-CN was added.
However, a lower energy band at 360 nm remained relatively un-
changed both in position and intensity.
Equilibrium constants for probes 19 and 20 in CH3CN at 293 K as determined by
absorption spectroscopic titrations.
Anion
Equilibrium constant Ka
Probe 19
Probe 20
Fꢁ
Ka = 2.5 ꢀ 104
Ka = 3.1 ꢀ 106
Kd = 1.1 ꢀ 104
Kd1 = 6.4 ꢀ 106
Kd2 = 1.2 ꢀ 106
Kd1 = 1.4 ꢀ 106
Kd2 = 1.8 ꢀ 103
Ka = 2.8 ꢀ 104
Ka = 6.5 ꢀ 103
Kd1 = 5.5 ꢀ 106
Kd2 = 2.6 ꢀ 106
CNꢁ
OAcꢁ
Ka = 2.3 ꢀ 105
Ka = 1.3 ꢀ 105
1H NMR titrations studies of compounds 13a and 16a with fluo-
ride anion, introduced as tetrabutylammonium fluoride (TBA-F) in
CDCl3, showed significant chemical shift changes of the thioamide
and the thiourea (N–H) protons, respectively, upon addition of Fꢁ.
This is indicative of strong hydrogen bonding interactions between
the N–H groups and the anions.
Both anions showed significant quenching to the emission at
low concentrations of 10ꢁ5–10ꢁ6 M (see Figs. 9 and 10). Addition-
ally, 13b-OTf was synthesized by known procedures and yielded
the desired product which showed enhanced absorbance upon
addition of TBA-H2PO4 in CH2Cl2 at 354 nm. The emission titration
results were similar; therefore, the hexfluorophosphate counter
ion could be interchangeably used with triflate.
Ka = 2.3 ꢀ 103
Ka = 1.2 ꢀ 102
ꢁ
H2PO4
Clꢁ
OHꢁ
a
The anions were added as tetrabutylammonium salts. Ka and Kd represent the
association constants of anion–probe hydrogen-bonding complexes and proton
dissociation constants of probe molecules, respectively.
ion resulted in significant luminescent quenching, with a quench-
ing constant of Ksv = 2.38 ꢀ 104 Mꢁ1, obtained from the Stern–Vol-
mer plot. It required about 2 equiv. of Brꢁ to quench the emission
Fig. 13. Electronic absorption spectra of complex 20 (2.5 ꢀ 10ꢁ5 M) in CH3CN upon the addition of n-Bu4NF: (a) [Fꢁ] = (0–2.8) ꢀ 10ꢁ5 M; (b) [Fꢁ] = 2.8 ꢀ 10ꢁ5–1.1 ꢀ 10ꢁ4 M.
Fig. 14. 1H NMR spectra of 20 in CD3CN with the addition of n-Bu4NF.

26
K.-C. Chang et al. / Inorganica Chimica Acta 389 (2012) 16–28
quent reaction of 19 and BrRe(CO)5 in refluxing THF, followed by
washing with THF and diethyl ether, afforded an orange-red pow-
der of Re(I) complex 20.
4.2. Photophysical properties
The absorption and emission spectral data along with lifetimes
and emission quantum yields of ligand 19 and complex 20 are
summarized in Table 3. Ligand 19 displays a series of absorption
bands in the UV–Vis region, where the absorptions can be assigned
to p–
p⁄ transitions. In contrast to the white color of ligand 19, the
Re(I) complex 20 is orange-red in the solid state. For complex 20 in
CH3CN, the absorption bands at 324 and 439 nm are assigned to li-
gand-centered
der appearing at ca. 550 nm is assigned to a spin-allowed metal
(Red
)-to-ligand (p⁄) charge-transfer (1MLCT) transition [17]. We
p–
p⁄ transitions. The less intense absorption shoul-
p
envision that the incorporation of MLCT character in the probe
molecules which is particularly sensitive to the surrounding micro-
environment [18], a property that is essential for molecular recog-
nition, would effectively promote the optical detecting limit and
provide a more sensitive way to distinguish the incoming anionic
substrates.
Fig. 15. Absorption (left, y axis) and 1H NMR (right, y axis) spectral profiles of
complex 20 with the addition of CNꢁ (green triangle curve), OAcꢁ (red diamond
curve), and OHꢁ (blue square curve).
Ligand 19 shows a weak emission at 530 nm and a relatively
short lifetime at 1.2 ns. The short lifetime of 19 suggests that the
emission arises from a singlet p–
p⁄ excited state. Upon coordina-
A modified complex based on the amidothiourea ligand 17, was
synthesized. This complex has the amidothiourea binding site lo-
cated closer to the rhenium complex and the metal is bound para
to the binding group. This is attached to a naphthyl group. Further
observations indicate that this complex does have potential in cya-
nide anion recognition in an aqueous environment. Fig. 11 shows
the results of emission quenching of compound 18 upon titration
with CNꢁ in water. Other anions such as AcOꢁ, BzOꢁ, Brꢁ, Clꢁ,
tion to Re(I), the luminescence for complex 20 is only detectable
in a deoxygenated solution with extremely low quantum yields,
while the emission is completely quenched in an air-equilibrated
solution. The luminescence of the rhenium(I) polypyridyl complex
typically originates from the lowest 3MLCT excited state because of
the large spin orbital coupling exerted by Re(I), and the deactiva-
tion follows the energy gap law, where the nonradiative decay pro-
cess becomes more efficient when the energy gap between the
ground state and the emissive excited state is smaller because of
greater vibrational overlap between the two electronic states
[17]. Therefore, the luminescence decay for complex 20 is assigned
to the 3MLCT excited state and is anticipated to be dominated by a
H2PO4ꢁ, Fꢁ, Iꢁ and ClO4 did not show any significant response.
ꢁ
4. Quinoxalinebis(sulfonamide) bridged rhenium(I) complex as
a novel anion receptor
nonradiative process, with the fully p-conjugated structural frame-
4.1. Preparation of quinoxalinebis(sulfonamide) bridged rhenium(I)
complex
work of the ligand 19 effectively lowering the ligand p⁄ level,
resulting in a smaller energy gap. The consequence here is the
observation of the low luminescence quantum yield and short life-
time for complexes 20, even in a deoxygenated solution.
The syntheses of the sulfonamide ligand 19 and complex 20 are
shown in Scheme 11. The bridging ligand 19 was synthesized from
reaction of sulfonamide-functionalized diaminobenzene and the
corresponding diketone in refluxing acetic acid in 95% yield. Subse-
4.3. Anion sensing studies
Fig. 12 illustrates absorption and emission titrations of ligand
19 upon the addition of Fꢁ in a CH3CN solution. In the electronic
absorption spectrum, the tail at ꢂ415 nm starts to grow accompa-
nied with a decrease of the band at 355 nm and, concomitantly, the
emission intensity at 530 nm shows an enhancement upon the
addition of Fꢁ ions. The appearance of a set of distinct isosbestic
points indicates that the binding of Fꢁ to ligand 19 is a clean equi-
librium process. With reference to the spectroscopic studies of re-
lated sulfonamide compounds [19], it is likely that the observed
changes in the absorption and emission spectra are a consequence
of the hydrogen bonding of Fꢁ to the sulfonamide N–H protons.
Similar trends of spectral changes have been observed in absorp-
tion and emissꢁion spectra for ligand 19 upon the addition of OAcꢁ,
CNꢁ, or H2PO4 in a CH3CN solution.
The UV–Vis absorption traces were further analyzed over the
entire wavelength range using a nonlinear least-squares fit algo-
rithm as implemented in the SPECFIT software package to extract
the corresponding equilibrium constants. The obtained equilib-
rium constants are compiled in Table 4. In general, ligand 19 exhib-
its high sensitivity to CNꢁ, OAcꢁ, and Fꢁ but, nevertheless, lacks
good selectivity to explicitly differentiate these three anions.
Fig. 16. Changes of
n-Bu4NF in a CH3CN solution.
m(CO) of complex 20 in the IR spectra upon the addition of

K.-C. Chang et al. / Inorganica Chimica Acta 389 (2012) 16–28
27
The pKa value of the sulfonamide N–H protons in ligand 19 is
expected to become smaller upon coordination of Lewis acidic me-
tal centers on the pyridylquinoxaline moiety of 19. The overall ef-
fect is to effectively increase the sensitivity of the metal complex
20 to anions. In addition, the increasing acidity of the sulfonamide
N–H protons also facilitates the tendency of complete proton
transfer from probe molecule to basic anions.
bands of complex 20 shift to lower frequencies upon anion
addition. Fig. 16 illustrates the shift of the IR
m(CO) bands of
complex 20 upon the addition of Fꢁ in a CH3CN solution. The shift
of the carbonyl group frequency is in the order of CNꢁ > Fꢁ ꢄ
OAcꢁ > H2PO4ꢁ > Clꢁ, which is consistent with the results obtained
from 1H NMR and UV–Vis spectrophotometric titrations.
Indeed, the addition of Fꢁ to an acetonitrile solution of complex
20 resulted in an increase in the 1MLCT absorption band with sharp
isosbestic points at 305 and 354 nm (see Fig. 13a), indicating a
clean well-defined equilibrium in solution. After more than
1 equiv. of Fꢁ was added, however, the 1MLCT absorption band
was further intensified and red-shifted, with a new set of isosbestic
points appearing at 287, 330, 396, 452, and 492 nm (see Fig. 13b).
Similar two-step equilibrium phenomena were also observed in
the titration of CNꢁ or OAcꢁ to the solution of complex 20. Job plots
confirm the 1:2 stoichiometry between complex 20 and these an-
ions. However, only a single equilibrium was observed in the UV–
Vis absorption response similar to the profile shown in Fig. 13a
5. Concluding remarks
We have demonstrated in this review that different types of
rhenium(I) carbonyls complexes with polarized N–H recognition
motifs can be designed and synthesized as anion receptors. In par-
ticular, the selectivity for different anions is greatly enhanced upon
coordination of organic probe to transition metals. The incorpora-
tion of potential luminescent chromophores into the structure pro-
vides a very sensitive way to detect the presence of the anions. The
degrees of probe–anion interactions can be easily visualized via
naked eye colorimetric or luminescent responses.
upon the addition of H2PO4 or Clꢁ ions, and the Job plots show
ꢁ
Acknowledgments
a 1:1 stoichiometry for both anions.
The stepwise process observed in the spectrophotometric titra-
We are grateful to the Division of Chemical Sciences, Office of
Basic Energy Sciences, Office of Energy Research, US Department
of Energy for support of the research carried out at State University
of New York at Binghamton and the National Science Council of
Taiwan for support of the research carried out at Academia Sinica.
Dr. K.-C. Chang thanks the postdoctorate fellowship sponsored by
Academia Sinica.
tion with Fꢁ can be described by two stepwise equilibria: (1) for-
mation of
a hydrogen-bound complex via sulfonamide N–H
protons with an incoming anion followed by (2) deprotonation of
the acidic sulfonamide N–H proton to form mono-deprotonated
complex 20 and HF2ꢁ. Fig. 14 displays the 1H NMR titration spectra
of complex 20 with n-Bu4NF, and a hydrogen-bond-induced up-
field shift was observed in the singlet signal of quinoxaline upon
the addition of a first 1 equiv. of Fꢁ in a CD3CN solution of complex
20. The proton signals hardly move between additions of 1 and
2 equiv. of Fꢁ. Subsequently, further upfield shifts of the proton
signals are observed again with the amount of added Fꢁ over
2 equiv.
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In the cases of CNꢁ and OAcꢁ, the spectrophotometric responses
are attributed to a stepwise double deprotonation. Complex 20 was
further titrated by n-Bu4NOH to confirm that the double deproto-
nation indeed occurred during the course of CNꢁ and OAcꢁ titra-
tions. The very similar titration isotherms of the absorption band
at 570 nm as well as the upfield shift of the quinoxaline proton sig-
nals upon the addition of OHꢁ, CNꢁ, and OAcꢁ are shown in Fig. 15.
These results suggest that complex 20 undergoes a stepwise dou-
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H2PO4 and Clꢁ ions show no ability to deprotonate the sulfon-
ꢁ
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ꢁ
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Fig. 16) and, thus, form only simple 1:1 hydrogen bonding com-
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The stretching frequencies of the metal carbonyl in complex 20
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p
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p back-bonding to the Re(I)–C bond and, thus, reduce
the IR stretching frequencies of the metal carbonyl.
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m(CO)

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