Thioamide, urea and thiourea bridged rhenium(I) complexes as luminescent anion receptors

Article abstract of DOI:10.1016/j.ica.2011.02.065

Thioamides, urea 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 the

Full text of DOI:10.1016/j.ica.2011.02.065

Inorganica Chimica Acta 374 (2011) 558–565
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Inorganica Chimica Acta
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Thioamide, urea and thiourea bridged rhenium(I) complexes as luminescent
anion receptors
Maurice O. Odago a, Amanda E. Hoffman a, Russell L. Carpenter a, Douglas Chi Tak Tse a, Shih-Sheng Sun b,
Alistair J. Lees a,
⇑
a Department of Chemistry, State University of New York at Binghamton, Binghamton, New York, NY 13902-6000, USA
b Institute of Chemistry, Academia Sinica, 128 Academia Road, Section 2, Nankang, Taipei 115, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 17 May 2011
Thioamides, urea 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 the anion recognition properties of these complexes have
been studied by luminescence, UV–Vis and 1H NMR spectroscopic methods. The complexes act as recep-
tors for anions via hydrogen bonding and electrostatic interactions. The anion sensing properties of the
complexes are compared to earlier amide-based dinuclear rhenium(I) tricarbonyl complexes.
Ó 2011 Elsevier B.V. All rights reserved.
This article is dedicated to the outstanding
career and research achievements of
Professor Wolfgang Kaim and in
appreciation of our fruitful collaborations.
Keywords:
Thioamide and amidothiourea
Urea and thiourea
Bridged rhenium carbonyl complexes
Luminescence
Anion recognition
Supramolecular chemistry
1. Introduction
advantages over pure organic sensors: (i) transition metal com-
plexes are often positively charged and are therefore electron
The development of anion-sensing systems is of considerable
current interest because of biological, environmental and pharma-
ceutical concerns [1,2]. The design and synthesis of metal–organic
receptors, which are sensitive to interactions between the host
and guest molecules and are able to exhibit either a chromogenic
and/or fluorogenic response upon receptor–anion interaction,
have particularly gained considerable recent attention [2–4]. The
appeal of sensors containing luminescent chromophores, which
are sensitive to interactions between the host and guest mole-
cules, stems from the high sensitivity of luminescence detection
compared to other spectroscopic methods [2,5]. The binding of an-
ionic species to the recognition sites leads to changes in certain
properties of the receptors (such as absorption spectra, lumines-
cence intensity, lifetimes, etc.) and these changes can serve as
indicators of guest host associations. The incorporation of transi-
tion metals into the framework of sensory systems offers several
deficient, this leads to an enhancement of the electrostatic interac-
tions with negatively charged species or the higher probability of
orbital overlap resulting in stronger bonding with anionic species,
(ii) they typically posses precisely defined geometries which can
be exploited in enhancing selectivity of certain shaped anions
and (iii) they possess a variety of unique functionalities such as re-
dox activity, visible spectroscopic features (color), magnetism, cat-
alytic activity and all these functionalities can be used to improve
sensor technology.
While amide groups have been extensively used as a neutral
anion receptor due to their hydrogen bond donor ability [2,5–
7], they can also act as hydrogen bond acceptors and thereby
participate in intramolecular hydrogen bonds. This property of
amide-based receptors can consequently result in a decrease in
receptor affinity toward anions. In this article, we report the syn-
thesis of other promising functional groups, notably thioamide,
Urea, thiourea and amidothiourea based receptors and their rhe-
nium complexes. These receptors have been widely used when
attached to organic chromophores and fluorophores [6]. The thio-
amides were readily synthesized from their respective amides by
thionation, utilizing modified procedures [8]. Thioamides ligands
⇑
Corresponding author.
E-mail address: alees@binghamton.edu (A.J. Lees).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.02.065

M.O. Odago et al. / Inorganica Chimica Acta 374 (2011) 558–565
559
are known to be weaker hydrogen-bond acceptors and stronger
2.1. Synthesis of the amide ligands
acids than their amide counterparts [9,10]. Moreover the thioam-
ide-based ligands have shown better solubility in organic sol-
vents such as THF, CH3CN and CH2Cl2 than their amide analogs
[11].
2.1.1. Synthesis ofN,N0-di(pyridin-4-yl)pyridine-2,6-dicarboxamide (1)
To a 250-mL flask containing pyridine-2,6-dicarbonyl dichloride
(1.00 g, 4.90 mmol) and 4-aminopyridine (0.926 g, 9.85 mmol)
were successively added 100 mL of CH2Cl2 and 3 mL of Et3N. The
resulting mixture was stirred at room temperature for 24 h. The
resulting white precipitate was collected on a frit, washed with
CH2Cl2 (3ꢀ 50 mL), and dried in a vacuum oven to afford 880 mg
Also notable is the use of urea and thiourea based receptors. The
works of Wilcox [12] and Hamilton [13] independently observed
that urea is a good receptor for anions. These are potentially useful
because they can establish complementary hydrogen-bonding
interactions with Y-shaped oxoanions, such as carboxylates. Since
then, urea has been widely used in the design of anion receptors.
The thiourea based ligands were also synthesized by reacting de-
sired amines with the corresponding isothiocyanates and then
compared to their urea counterparts prepared similarly from suit-
able amines with isocyanates.
Last but certainly not least, the amidothiourea based receptors
were synthesized by reacting isothiocyanates with suitable hyraz-
ides. The amidothiourea based rhenium complexes that were
prepared, showed preliminary evidence for better solubility in
semi-aqueous solvent media. This has the potential to provide
new sensor systems in aqueous environments and offers the prom-
ise for real-life applications.
One special feature of transition metal atoms is the presence of
partially filled d orbitals in their electronic structure. Molecular
orbitals in metal complexes are built by partially filled d orbitals,
mainly localized in the metal center, and ligand confined orbitals.
Both occupied (bonding orbitals) and unoccupied (antibonding
orbitals) are usually available. In such systems, several transitions
can be observed upon excitation with light. The d–d transitions in-
volve electron transitions between the d orbitals in the metal cen-
ter and ligand to metal charge transfer (LMCT) or metal to ligand
charge transfer (MLCT) transitions are also often observable in
the visible or near visible regions. Coordination of certain anions
to transition metal complex can induce the appearance of new or
modified CT bands. If the new band is located in the visible region,
then a chromogenic sensing of the anion is achieved [14] and it
may also be feasible to detect these CT transitions via their
luminescence.
of white powder. Yield: 56%. 1H NMR (360 MHz, acetone-d6):
3
10.78 (s, 2H, –NH), 8.55 (d, 4H, JH–H = 7.68 Hz, H
a
-py), 8.50 (d,
3
3
2H, JH–H = 9.84 Hz, Hm-py), 8.37 (t, 1H, JH–H = 10.2 Hz, Hp-py),
7.96 (d, 4H, JH–H = 7.68 Hz, H -py). 13C NMR (90 MHz, acetone-
3
a
d6): 163.2, 151.6, 149.8, 145.9, 141.1, 126.9, 115.2.
2.1.2. Synthesis of N,N0-dipyridin-4-yl-isophthalamide (4)
Similarly the isophthalamide derivative compound 4 was pre-
pared by the same procedure as for compound 1 by reacting isoph-
thaloyl chloride with 4-aminopyridine. Yield: 73%. 1H NMR
(360 MHz, DMSO-d6): d 10.36 (s, 2H, –NH), 8.09 (s, 1H, Ph-2), 8.03
(d, 4H, J = 6.72 Hz, H -py), 7.72 (d, 2H, J = 9.36, Ph-4,6) 7.35 (d, 4H,
a
J = 6.2 Hz, Hb-py), 7.28 (t, 1H, J = 9.36 Hz, Ph-5) 13C NMR (90 MHz,
DMSO-d6): d 165.7, 150.2, 145.7, 134.5, 131.3, 128.8, 127.3, 114.0.
2.2. Synthesis of the thioamide derivatives
2.2.1. Synthesis of 2N,6N-di(pyridin-4-yl)pyridine-2,6-
bis(carbothioamide) (2)
Pyridine-2,6-dicarboxylic acid-2-pyridine-3-ylide-6-pyridine-
4-ylamide (0.16 g, 0.50 mmol) and Lawesson’s reagent (0.40 g,
1.0 mmol) were suspended in dry THF (10 mL) and the mixture
was refluxed for 1 day under nitrogen at 80 °C. The solvent was
then evaporated in vacuo and the reaction product was purified
by column chromatography on silica gel (CH2Cl2), and the product
was then recrystallised from C2H4Cl2/pentane. The resulting prod-
uct was 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 amides, due to intermolecular hydrogen bonding, served as
indicators of complete thionation.
2. Experimental
2.2.2. Synthesis of N1,N3-di(pyridin-4-yl)benzene-1,3-
All reagents for the synthesis were obtained commercially from
Sigma–Aldrich and were used without further purification, unless
otherwise noted. Solvents were obtained from Fischer and were
used without further purification, unless otherwise noted. Rhenium
carbonyl was obtained from Strem chemicals and used without
further purifications. The rhenium-complex precursors, (4,40-tBu2b-
py)Re-(CO)3(CH3CN)(PF6), (4,40-tBu2bpy is 4,40-bis-tert-butyl-2,20-
bipyridine), (4,40-Me2bpy)Re-(CO)3(CH3CN)(PF6) (4,40-Me2bpy is
4,40-dimethyl-2,20-bipyridine) and (4,40-Ph2bpy)Re-(CO)3(CH3CN)
(PF6) (4,40-Ph2bpy is 4,40-diphenyl-2,20-bipyridine) were synthe-
sized according to known procedures [15]. 1H NMR spectra were
obtained on a Brucker AM-360 NMR spectrometer using tetrameth-
ylsilane (TMS) as an internal standard.
UV–Vis spectra were recorded on a Hewlett Packard 8453 spec-
trophotometer with a quartz cuvette (1 cm path length) at 298 K.
Infrared spectra were obtained on a Nicolet Magna-IR 760 Spec-
trometer E.S.P. in a KBr pellet while under N2. Emission data were
collected on a FluoratÒ-02-Panorama spectrophotometer from 210
to 840 nm. The instrument utilizes a Xenon flash lamp and a mono-
chromator with 900-l/mm concave diffraction gratings with 100-
mm radius of curvature, irradiating at right angles with light of
the stated wavelengths. Solutions were filtered to eliminate scat-
tering and also deoxygenated by bubbling with nitrogen or argon
for at least 15 min prior to measurements.
bis(carbothioamide) (5)
To a clean dry flask fitted with a magnetic stirrer was charged
with N,N0-di(pyridin-4-yl)isophthalamide (4) (0.30 g, 0.942 mmol)
and Lawesson’s reagent (0.763 g, 1.89 mmol) suspended in 20 mL
of dry THF, and this was refluxed under nitrogen for 1 day at 75 °C.
After removal of solvent, the reaction mixture was dissolved in
dichloromethane and washed twice with water, then dried over
anhydrous sodium sulfate and concentrated to form pale yellow
oil. This was further purified by column chromatography on silica
gel, eluting with 2% methanol in dichloromethane solution, followed
byrecrystallization indichloromethane/pentaneto yield theproduct
(5). Due to the similarity in structure of compounds 4 and 5, NMR
analysis of the product would result in a very closely related spec-
trum and, thus, the compound was characterized by IR spectroscopy.
It is worth noting that the synthesis of compounds 3(a–c) and
6(a–c) was achieved by refluxing two equivalents of the respective
acetonitrile bipyridine rhenium(I) complexes {(bpy)Re-(CO)3
(CH3CN)(PF6)} with the corresponding one equivalent of the appro-
priate bridging ligand for 2 h (see Schemes 1 and 2). 1H NMR spec-
tra were monitored throughout the reaction to determine 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 replacement and coordi-
nation of the bridging ligand to the metal center.

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M.O. Odago et al. / Inorganica Chimica Acta 374 (2011) 558–565
N
O
N
H
ClOC
N
CH2Cl2, NEt3
+
N
2
N
NH2
1
HN
O
ClOC
N
NN = 4,4'- Bu bpy 3a
t
2
Lawesson's
reagent
THF
4,4'-Me2bpy 3b
4,4'Ph2bpy 3c
N
N
Re N
OC
OC
S
N
S
CO
N
H
N
N
H
N
(NN)Re(CO)3(CH3CN)(PF6)
THF,reflux
2
(PF6)2
HN
S
HN
S
N
CO
N
N
N
Re CO
CO
3
Scheme 1. Synthesis of the thioamide derivatives (3a–c).
N
O
N
H
ClOC
4
CH2Cl2, NEt3
+
2
N
NH2
HN
N
O
ClOC
NN = 4,4'- Bu bpy 6a
t
2
THF
Lawesson's
reagent
4,4'-Me2bpy 6b
4,4'-Ph2bpy 6c
N
N
Re N
OC
S
OC
N
S
CO
N
H
N
H
(NN)Re(CO)3(CH3CN)(PF6)
THF,reflux
(PF6)2
HN
N
S
5
HN
S
CO
N
N
N
Re CO
CO
6
Scheme 2. Synthesis of the isophthaloyl derivatives (6a–c).
2.3. Synthesis of simple urea and thiourea ligands
pyridineisothiocyanate (0.250 g, 2 equiv.) over 1 h, then the reac-
tion was refluxed for 2 h. The resulting solution was filtered and
10 mL of hexanes was added to precipitate the dithiourea ligand.
This was collected and dried in vacuum to yield product (0.138 g,
40% yield). 1H NMR (360 MHz, DMSO-d6): d 13.81 (s, 2H, –NH)
10.60 (s, 2H, –NH), 8.76 (s, 2H, –NCH), 8.39 (dd, JH–H = 2.88 Hz,
2H, –NCH), 8.20 (d, JH–H = 4.32 Hz, 2H, –NCH), 7.41 (m, 2H,
2.3.1. Synthesis of 1,10-(pyridine-2,6-diyl)bis(3-(pyridin-3-
yl)thiourea) (7)
To a clean dry 25 mL flask was added 2,6-diaminopyridine
(0.100 g, 0.916 mmol) in 15 mL of dry THF until the reaction
started to reflux. This was followed by dropwise addition of 3-

M.O. Odago et al. / Inorganica Chimica Acta 374 (2011) 558–565
561
CH-py), 6.23 (s, 1H, CHp-py), 6.33 (d, JH–H = 7.56 Hz, 1H, CH), 6.12
N
(d, JH–H = 7.72 Hz, 2H, CH).
O
N
C S
NH2
H
N
H
N
2.3.2. Synthesis of [(4,40-Ph2bpy)Re(CO)3]2-(
l-L7) (PF6)2 (8)
N
H
EtOH
+
N
H
O
Ligand 7 (0.009 g, 2.36 ꢀ 10ꢁ2 mmol) was dissolved in dry THF
5 mL together with two equivalents of (4,40-Ph2bpy)Re(CO)3
NCCH3(PF6), (0.035 g, 0.046 mmol) and this was purged with nitro-
gen and refluxed for 2 h. This resulted to a bright yellow solution
which was then cooled to room temperature and then filtered,
and dried over anhydrous Na2SO4 before concentrating to yield
yellowish orange crystals, which was then recrystallised in
CH2Cl2/n-pentane (see Scheme 3).
N
S
9
-
[4,4'-t Bu2bpy)Re(CO)3NCMe] PF6
THF, Heat
2.4. Synthesis of N-amidothiourea based ligands
CO
PF6
OC
N
2.4.1. 2-Isonicotinoyl-N-(naphthalen-1-yl)hydrazinecarbothioamide-
based anion receptors
Re
OC
N
An amidothiourea-based receptor, which has the amidothiourea
site closer to the rhenium linkage, was prepared. The ligand (2-
isonicotinoyl-N-(naphthalen-1-yl)hydrazine carbothioamide) (9)
was prepared by the reaction of isoniazid and 1-isothiocyanato-
naphthalene in ethanol, according to the literature procedure with
modifications [16]. This was then treated with [(4,40-tBub-
py)Re(CO)3NCMe]PF6 in refluxing THF (see Scheme 4).
N
H
N
H
N
N
H
O
S
10
Scheme 4. Synthesis of the amidothiourea derivative (10).
2.4.2. Synthesis of 2-isonicotinoyl-N-(naphthalen-2-
yl)hydrazinecarbothioamide (9)
Isoniazid (0.137 g, 1.00 mmol) and 1-isothiocyanatonaphtha-
lene (0.185 g, 1.00 mmol, 1 equiv.) were dissolved in ethanol and
the mixture allowed to stir at room temperature for 3 h. A white
precipitate was observed to form with continued stirring and this
was collected by filtration followed by drying the compound under
vacuum to yield product (0.312 g, 97% yield). 1H NMR (360 MHz,
DMSO-d6) spectrum was consistent with the known values.
1 equiv.). The reaction was allowed to reflux for 6 h. The solvent
was then removed by evaporation to give compound 10.
3. Results and discussion
3.1. FTIR studies of thioamide ligands
2.4.3. Synthesis of amidothiourea-based rhenium complex (10)
In dry THF solvent, was added [(4,40-tBu2bpy)Re(CO)3CNCH3]PF6
(0.193 g, 0.266 mmol) and 2-isonicotinoyl-N-(naphtalen-2-
yl)hydrazinecarbothioamide (9) (8.58 ꢀ 10ꢁ2 g, 0.266 mmol,
Infrared studies were carried out to determine the formation
of the thioamide ligands from their amide counterparts by look-
ing for the presence of the C@S stretching vibration, and indeed
N
S
Dry THF
2
+
S
NH
N
C
N
H2N
N
NH2
Reflux 2 h
NH
N
H
N
NH
7
N
S
Dry THF
(NN)Re(CO)3(CH3CN)(PF6)
4;4'-tBu
8a
2bpy
4,4'-Me2bpy 8b
8c
=
reflux, 2 h
NN
4,4-ph2bpy
CO
OC
N
N
Re
N
OC
NH
S
CO
N
N
(PF6)2
Re
S
N
N
N
N
N
OC
H
H
H
CO
8
Scheme 3. Synthesis of the thiourea derivative (8).

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M.O. Odago et al. / Inorganica Chimica Acta 374 (2011) 558–565
the disappearance of the carbonyl C@O stretch at ca. 1710 cmꢁ1
of the amide was attributed to the complete thionation of the
amide C@O bond. As expected, there was complete disappearance
of the amide carbonyl stretch with the corresponding appearance
of the thioamide peak. The infra-red absorption band observed at
ca. 3400 cmꢁ1 is due to the N–H stretching vibration of the thio-
amide and is consistent with the literature value [17]. It can also
be noticed that the amide counterpart shows broad N–H stretch-
ing vibrations at the relatively lower wavenumber of ca.
3150 cmꢁ1 in the solid state. These observations were consistent
with the literature values, thus confirming the formation of the
thioamide ligands, and similar results were observed for the other
thioamide ligands based on isophthaloyl and terephthaloyl moie-
ties (see Fig. 1).
Fig. 1. IR spectra of ligands (amide 1) (blue) and (thioamide 2) (pink). (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this paper.)
3.2. Absorption and luminescence studies of the ligand-bridged
dinuclear complexes
It was observed that these dinuclear rhenium complexes of the
type 3(a–c), 6(a–c) and 8(a–c), are yellow compounds with absorp-
tion in the range 300–400 nm with only slight variations depend-
ing on the pyridyl ligand attached to the rhenium metal. Also
noted was their strong emission centered at about 600 nm. Fig. 2
shows typical absorption and emission spectra of a representative
complex (3a). Both the absorption and emission spectra are domi-
nated by higher lying intraligand (IL) and lowest energy lying
metal-to-ligand charge transfer (MLCT) transitions [2].
The thiourea-bridged dinuclear complex [(Ph2bpy)Re(CO)3]2-
(l-(L7)](PF6)2 (8c), exhibited a maximum absorption at 330 nm.
Upon irradiation of this compound with 330 nm light, two emis-
sion bands were observed, centered at 400 nm and 575 nm, respec-
tively. Titration of complex 8c with tetrabutylammonium chloride
(see Fig. 3) resulted in emission enhancement of the higher energy
band (at 400 nm) with a corresponding 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 (tet-
rabutylammonium iodide) and again an enhancement of the upper
energy band and a concomitant quenching of the lower energy
band was observed (see Fig. 4).
Fig. 2. UV–Vis absorption spectrum (blue) and emission spectrum (pink) of
5.0 ꢀ 10ꢁ6 M of complex 3a in CH2Cl2 at 298 K. Excitation wavelength is 360 nm.
(For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this paper.)
The thiourea-based rhenium(I) complex 8c 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
Emission studies with {(Ph 2 bpy)Re(CO)3 ]2 L2 (PF)6 }
with TBA-Cl
0.30
0.25
0.20
0.15
0.10
0.05
0.00
a
b
c
d
e
f
g
h
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 3. Emission titration spectra of complex 8c (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.

M.O. Odago et al. / Inorganica Chimica Acta 374 (2011) 558–565
563
Emission enhancing/quenching with TBA-I
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
a
b
c
d
e
f
g
h
i
j
k
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Fig. 4. Emission titration spectra of complex 8c (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.
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. 4). The Stern–
Volmer plot is shown in Fig. 5, and illustrates linearity, with the
determined quenching constant Ksv = 2.96 ꢀ 104 Mꢁ1
.
Emission titration studies of the complex (4,40-tBu2bpy)Re(-
CO)3(l-L2)(OTf)2 (3a) were carried out in CH2Cl2, where the addi-
tion of tetrabutylammonium chloride resulted in the emission
quenching of the band at about 560 nm following excitation at
352 nm (see Fig. 6). A similar quenching pattern was observed
for the other anions such as fluoride and bromide, 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 [2]. How-
ever it is worth noting that the solubility of these new thioamide,
urea and thiourea ligands in organic solvents was relatively better
than their amide counterparts.
Fig. 6. Emission titration spectra of complex 3a (2.0 ꢀ 10ꢁ5 M) in CH2Cl2 at 298 K
adding Clꢁ anion up to 6 equiv. Excitation wavelength is 360 nm.
The emission titration shown in Fig. 7 indicates how the addi-
tion of bromide to complex 3a with the triflate (OTfꢁ) counter
ion resulted in significant luminescent quenching, with a quench-
ing constant of Ksv = 2.38 ꢀ 104 Mꢁ1, obtained from the Stern–
Volmer plot in Fig. 8. It required about 2 equiv. of Brꢁ to quench
the emission to 50% of the initial value, exhibiting a comparable
level of sensitivity to anion as observed in the corresponding amide
system [2].
In the absorbance spectrum of complex 3a, 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.
Stern-Volmer plot
7
6
5
4
y = 29579x + 0.8713
R2 = 0.9988
3
2
1
0
1H NMR titrations studies of compounds 3a and 8a with fluoride
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.
0
0.00005
0.0001
0.00015
0.0002
0.00025
[Q]
Fig. 5. Stern–Volmer plot of the titration data in Fig. 4.

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M.O. Odago et al. / Inorganica Chimica Acta 374 (2011) 558–565
Fig. 7. Emission titration spectra of complex 3a (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.
Fig. 10. Emission titration spectra of 3b (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. 8. Stern–Volmer plot of the titration data in Fig. 7.
Both anions showed significant quenching to the emission at
low concentrations of 10ꢁ5 to 10ꢁ6 M (see Figs. 9 and 10). Addition-
ally, {(Me2bpy)Re(CO)3}2L (CF3SO3)2 3b-OTf was synthesized by
known procedures and yielded the desired product which showed
enhanced absorbance upon addition of TBA-H2PO4 in CH2Cl2 at
Fig. 11. Emission titration of compound 10 (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.
354 nm. The emission titration results were similar; therefore,
the hexfluorophosphate counter ion could be interchangeably used
with triflate.
A modified complex based on the 2-isonicotinoyl-N-(naphtha-
len-1-yl)hydrazine carbothioamide ligand 9, was synthesized. This
complex had the amidothiourea binding site located closer to the
rhenium complex and the metal was bound para to the binding
group. This was attached to a naphthyl group. Further observations
indicate that this complex has potential in cyanide anion recogni-
tion in an aqueous environment. Fig. 11 shows the results of emis-
sion quenching of compound 10 upon titration withꢁCNꢁ in water.
Other anions such as AcOꢁ, BzOꢁ, Brꢁ, Clꢁ, H2PO4ꢁ, F , Iꢁ and ClO4
ꢁ
did not show any significant response.
4. Conclusion
Thioamide, urea and thiourea ligands were synthesized, and
their bridged rhenium(I) tricarbonyl diimine complexes were sub-
sequently prepared and characterized. The absorption and emis-
sion studies of the thioamide complexes showed comparable
anion sensitivity results to those of their amide counterparts [2].
Fig. 9. Emission titration spectra of complex 3b (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.

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565
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This work was made possible by the financial support of the
Department of Chemistry at the State University of New York at
Binghamton and the Binghamton University Foundation for Clif-
ford Myers and Keith Innes summer research fellowships.
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