Synthesis and Characterisation of Indium(III) Bis-Thiosemicarbazone Complexes: 18F Incorporation for PET Imaging

Article abstract of DOI:10.1071/CH18559

Several structurally related indium chlorido complexes of bis-thiosemicarbazones were prepared, starting from the appropriately substituted bis-thiosemicarbazones, using sodium methoxide in methanol. Detailed NMR studies were conducted to assign the struc

Full text of DOI:10.1071/CH18559

CSIRO PUBLISHING
Aust. J. Chem. 2019, 72, 383–391
Full Paper
https://doi.org/10.1071/CH18559
Synthesis and Characterisation of Indium(III)
Bis-Thiosemicarbazone Complexes: 18F Incorporation
for PET Imaging
A C
,
B
A
Taracad K. Venkatachalam,
Paul V. Bernhardt, Gregory K. Pierens,
A
A
A
Damion H. R. Stimson, Rajiv Bhalla, and David C. Reutens
A
Centre for Advanced Imaging, University of Queensland, Brisbane, Qld 4072,
Australia.
B
School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane,
Qld 4072, Australia.
C
Corresponding author. Email: t.venkatachalam@uq.edu.au
Several structurally related indium chlorido complexes of bis-thiosemicarbazones were prepared, starting from the
appropriately substituted bis-thiosemicarbazones, using sodium methoxide in methanol. Detailed NMR studies were
conducted to assign the structure including COSY, HSQC, and HMBC techniques. The structures of all indium complexes
were solved using single crystal X-ray diffraction. The chlorido ligand was present at the apex of the square pyramidal
coordination sphere in all indium complexes. In some complexes, an intermolecular hydrogen bond was present between
the chlorine atom and an NH group. Three different indium chlorido complexes were converted into the corresponding
fluorido-derivative by a simple halide exchange method using K18F. These novel complexes, containing the positron
emitting isotope 18F, may have potential applications in positron emission tomography (PET).
Manuscript received: 10 November 2018.
Manuscript accepted: 31 January 2019.
Published online: 13 March 2019.
Introduction
Experimental
Thiosemicarbazones and bis-thiosemicarbazones are used in the
preparation of numerous coordination compounds. The pres-
ence of nitrogen and sulfur donors in their structure contributes
to the coordination versatility of these ligands. Furthermore,
they are easy to synthesise inexpensively and are extremely
stable. Recently, these ligands have been used routinely to
coordinate various metal ions for potential biological applica-
tions.[1–18] For example, some of the complexes have potent
antifungal, antimalarial, antitumour, or antiviral activity. The
metal in the core structure of the complex plays an important
role. For example, copper complexes of thiosemicarbazones
show enhanced cytotoxicity compared with the parent
ligand.[5,19–33] Indium complexes have also been recently pre-
pared and characterised as anticancer agents.[5,34–37] These
complexes may also be useful imaging agents. Pascu et al.[38,39]
have synthesised a series of gallium and indium complexes of
bis-thiosemicarbazones to evaluate their potential in this regard.
Bhalla et al. have also introduced the positron-emitting isotope
18F in the structural core of gallium complexes using an
exchange strategy.[40,41] Furthermore, a synthetic strategy to
obtain indium-polyhedra structural characteristics has been
achieved by Qian et al.[42,43] We recently synthesised the
Ga–18F complex of bis-thiosemicarbazones as potential imag-
ing agents.[44] We have now prepared and characterised new
indium complexes using bis-thiosemicarbazones and introduced
18F using our novel halide exchange method.
Syntheses
All chemicals were purchased from Sigma–Aldrich and used as
received. Anhydrous solvents were obtained in Sure/Seal bottles
and transferred to the reaction vessel via cannula under nitrogen
atmosphere.
Bis-Thiosemicarbazones
Concentrated hydrochloric acid (2 mL) was added to a flask
containing an appropriate mass of diketone (Scheme 1, gener-
ally millimolar amounts) and methanol (50 mL) and stirred at
room temperature to form a homogenous solution. To this
mixture, 4-aminophenyl thiosemicarbazide (dissolved in meth-
anol containing 2 M hydrochloric acid) was added and the
contents stirred at room temperature for 3 days, resulting in
the formation of a colourless precipitate. The bis-thiosemicar-
bazone was filtered off, washed with methanol, and dried under
vacuum.[45]
General Procedure for the Preparation of Chlorido
Complexes of Indium
In a round bottomed flask was placed the appropriate bis-
thiosemicarbazone (1 mmol) and 0.075 g of sodium methoxide
was introduced under dry conditions. Using a dry syringe under
nitrogen atmosphere, 40 mL of anhydrous methanol was added
resulting in a slight yellow coloured solution. To this solution
Journal compilation Ó CSIRO 2019
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384
T. K. Venkatachalam et al.
R1
N
R2
N
Compound 6: Methyl-ethyl amino indium chloride: nmax
/cmꢀ1 3390, 3286, 3163, 1606, 1540, 1477, 1432, 1294, 1210,
1180, 1144, 1055. dH (d6-DMSO) 7.38 (s, 1H), 7.37 (s, 1H),
2.79–2.76 (q, 2H), 2.31 (s, 3H), 1.05–1.03 (t, 3H). dC (d6-
DMSO) 174.2, 173.9, 148.6, 143.8, 21.3, 14.7, 10.6
R1
O
R2
O
H2NNHCSNHR3
EtOH/HCl
NH HN
NHR3
R3HN
Synthesis of Nitrato Precursor of Indium Chloride
Complex
S
S
MeONa
Reflux
MeOH
InCl3
R1
R2
N
In a scintillation vial (20 mL) was placed 28 mg of the
appropriately substituted bis-thiosemicarbazone indium chlor-
ido complex, to which was added 10 mL of methanol. The
contents were stirred for 3 min, then solid silver nitrate (9 mg)
was added and the contents were shaken for 10 min followed by
stirring for an additional 10 min until the silver chloride
precipitate appeared in the vial. The contents of the vial were
transferred into a 20 mL centrifuge tube and centrifuged at 3500
rpm for 5 min. The supernatant liquid was transferred in aliquots
(0.5 mL) into a vial, the solvent was evaporated using nitrogen,
and the vial was sealed under nitrogen atmosphere and stored
before use.
Comp.#
R1 R2 R3
Me Et Ph
Et Et Ph
1
2
3
4
5
6
N
N
N
Ph Ph
Me
In
Me Et Me
Et Et Me
NHR3
R3HN
S
S
Me
Et
H
Cl
Scheme 1. Synthetic steps involved in the preparation of indium bis-
thiosemicarbazone complexes.
was added 0.140 g (0.7 mmol) of indium chloride in 10 mL of
anhydrous methanol, yielding an orange-red coloured solution.
The contents were allowed to reflux for 3 h and the mixture was
cooled to room temperature. The orange-coloured solution was
filtered to remove any unreacted bis-thiosemicarbazone and the
solvent was evaporated, yielding an orange-red coloured solid.
This was further triturated with hexane and dried under vacuum
to yield 0.238 g (60 %) of the desired product. Most of the
indium complexes were freely soluble in common organic
solvents such as chloroform, dichloromethane, tetrahydrofuran,
and dimethyl sulfoxide.
Exchange of Bis-Thiosemicarbazone Indium Nitrato
Complex with KF
An aliquot of the above indium nitrato precursor in a
scintillation vial (containing 1.4 mg of complex) was reacted
with a solution of KF (1 mg) in 2 mL of methanol. The mixture
was allowed to stir for 10 min, filtered, and the solvent
evaporated under nitrogen. The residue was dissolved in d6-
DMSO and 19F NMR performed to confirm the formation of the
indium fluorido complex.
X-Ray Crystallography
Compound 1: Methyl-ethyl diphenylamino bis-thiosemicar-
bazones indium chloride: nmax /cmꢀ1 3402, 3289, 1599, 1541,
1521, 1492, 1453, 1418, 1316, 1237, 1205, 1185, 1097, 1062. dH
(d6-DMSO) 9.66 (s, 1H), 9.64 (s, 1H), 7.77–7.76 (d, J 7.0, 4H),
7.29–7.28 (m, 4H), 6.96–6.95 (t, J 7.0, 2H), 2.89–2.86 (q, 2H),
2.44 (s, 3H), 1.16–1.14 (t, 3H). dC (d6-DMSO) 170.8, 170.7,
152.4, 147.5, 140.6, 140.5, 128.5, 128.4, 122.1, 120.4, 120.2,
22.4, 15.7, 9.9.
Compound 2: Symmetrical diethyl diphenylamino bis-thio-
semicarbazones indium chloride: nmax /cmꢀ1 3352, 3260, 2933,
1508, 1460, 1382, 1257, 1212. dH (d6-DMSO) 9.67 (s, 2H),
7.78–7.77 (d, J 7.0, 4H), 7.29–7.27 (t, 4H), 6.97–6.95 (t, 2H),
2.86–2.83 (q, 4H), 1.19–1.17 (t, 6H). dC (d6-DMSO) 170.9,
151.9, 140.6, 128.4, 122.1, 120.2, 22.2, 10.2.
Compound 3: Symmetrical diphenyl dimethyl amino bis-
thiosemicarbazones indium chloride: nmax /cmꢀ1 3323, 2924,
1476, 1455, 1441, 1398, 1378, 1342, 1254, 1173, 1096, 1073,
1022. dH (d6-DMSO) 9.88 (s, 2H), 7.73–7.72 (d, J 7.0, 4H),
7.43–7.42 (t, 4H), 7.2–7.20 (d, 2H), 3.06–3.05 (d, 6H). dC (d6-
DMSO) 178.2, 140.4, 130.1, 128.9, 127.2, 126.7, 31.4 (contains
some starting material).
Compound 4: Methyl-ethyl aminomethyl bis-thiosemicarba-
zones indium chloride: nmax /cmꢀ1 3352, 3281, 3155, 1631,
1540, 1428, 1210, 1180, 1056. dH (d6-DMSO) 7.38 (s, 2H), 7.37
(s, 2H), 2.78–2.76 (q, 2H), 2.31 (s, 3H), 1.05–1.03 (t, 3H). dC
(d6-DMSO) 174.1, 173.9, 148.6, 143.7, 21.2, 14.6, 10.5.
Compound 5: Symmetrical diethyl dimethylamino bis-thio-
semicarbazones indium chloride: nmax /cmꢀ1 3356, 2929, 1498,
1458, 1380, 1214, 1043. dH (d6-DMSO) 10.33 (s, 2H), 2.88–2.86
(br d, 6H), 2.80–2.77 (q, 4H), 1.12–1.10 (t, 6H). dC (d6-DMSO)
178.5, 151.0, 31.3, 21.3, 10.8 (contains some starting material).
X-Ray quality single crystals were obtained by diffusion of
either THF (compound 1) or dichloromethane (all others) into an
ethanolic solution of each complex. Crystallographic data were
acquired at 190 K on an Oxford Diffraction Gemini CCD
˚
diffractometer employing either Mo Ka (0.71073 A) or Cu Ka
˚
radiation (1.54184 A). Temperature control was achieved with
an Oxford Cryosystems Desktop Cooler. Data reduction and
empirical absorption corrections (multiscan) were performed
with Oxford Diffraction CrysAlisPro software. The structures
were solved by direct methods with SHELXS and refined by
full-matrix least-squares analysis with SHELXL-97[46] within
the WinGX graphical user interface.[47] All non-H atoms were
refined with anisotropic thermal parameters. The molecular
structure diagrams were produced with ORTEP-3.[48] The data
in CIF format have been deposited at the Cambridge Crystallo-
graphic Data Centre with deposition numbers 1545408,
1561253–1561255.
Radiochemistry
No-carrier-added fluorine-18 was manufactured via the 18O
(p, n)18F nuclear reaction using a Cyclone 18 Twin (IBA,
Belgium) dual ion source cyclotron. Approximately 0.7 mL of
pure water enriched to . 98 % [18O] H2O was irradiated with 18
MeV protons at a beam current of 14 mA for up to 15 min
producing up to ,10 GBq of 18F. Aqueous 18Fꢀ was transferred
from the cyclotron target to a 10 mL glass receiving vial located
in a hot cell via 1.6 mm outer diameter polypropylene tubing
,20 m in length under helium pressure. The receiving vial was
measured for radioactive content in a dose calibrator housed
within the hot cell before delivery into a lead pot. The lead pot

Synthesis and Characterisation of Bis-Thiosemicarbazones
385
0.8
0.6
0.4
0.2
0
was then manually transferred to a fume cabinet where aliquots
of the aqueous 18Fꢀ were taken for radiolabelling following the
process described below.
Compd 1
Compd 2
Compd 3
Compd 4
Compd 5
Compd 6
Preparation of 18Fꢀ/K222/K2CO3
All manual operations were performed behind lead blocks
arranged within protective biological safety cabinets and
employing personal protective equipment (PPE). In the first
process, the drying of 18Fꢀ was achieved by dissolving 1.9 mg of
Kryptofix (222) in 0.5 mL of dry acetonitrile in a 2 mL glass vial.
This was transferred into a 10 mL glass Wheaton vial and 20 mL
of 0.1 M potassium carbonate was added followed by 60 mL of
the 18O water containing cyclotron-produced 18Fꢀ (typically
300–400 MBq). The vial was septum-capped, placed in a dry
block heater maintained at 1008C, and subsequently heated
under nitrogen making sure that the azeotropic evaporation of
the solvent took place smoothly without any adverse bumping or
spurting. The above evaporation process was continued for
,20 min resulting in a dry solid mixture of 18Fꢀ/K222/K2CO3.
400
600
800
1000
Wavelength [nm]
Fig. 1. UV/Vis spectra of all the indium complexes used in the present
study (0.5 mg per 20 mL of MeOH).
dimensions 4.6 ꢁ 150 mm, 5 mm. The temperature of the column
was maintained at 308C during the analysis. The eluent was a
mixture of 0.1 % trifluoracetic acid in water and acetonitrile
under gradient conditions of 10–90 % acetonitrile over 30 min.
The flow rate employed was 1 mL minꢀ1 and the injection
volume was 20 mL. Under these experimental conditions, the
radiolabelled compound eluted between 15–17.5 min depending
on the structure of the indium complex.
Radiolabelling Protocol
The vial containing 18Fꢀ /K222/K2CO3 was removed from the
heater and set aside while the heating block was cooled to 408C
using compressed air. In the meantime, the pre-prepared indium
nitrate bis-thiosemicarbazone (stored under nitrogen in a sealed
vial) was dissolved in anhydrous dimethyl sulfoxide (500 mL)
resulting in a light yellow to orange coloured solution. This
solution was carefully transferred, using a dry syringe (previ-
Results and Discussion
ously flushed with nitrogen), to the vial containing 18Fꢀ/K222
/
Synthesis of indium complexes of bis-thiosemicarbazones was
accomplished following Scheme 1. In brief, appropriately
substituted diketones were condensed with 4-substituted thio-
semicarbazide in ethanol with a catalytic amount of concentrated
hydrochloric acid to furnish the required bis-thiosemicarbazones
as pure compounds. Subsequent treatment of the bis-thiosemi-
carbazones with indium chloride in methanol in the presence of
sodium methoxide yielded the desired final compounds as
orange-red coloured solids. The structures of the indium com-
plexes (compounds 1–6) prepared in the present study are shown
in Scheme 1 illustrating the various combinations of substituents
on the ligand backbone (R1 and R2) and at the N4 positions (R3).
The introduction of aliphatic groups at position N4 was done to
reduce the liphophilicity of the molecule since phenyl-substi-
tuted bis-thiosemicarbazone complexes are highly lipophilic
which potentially limits applications that require compounds that
are to cross the blood–brain barrier. In particular, compound 5
was prepared because of its structural similarity to the well
studied pyruvaldehyde-bis(N4-methylthio semicarbazone)
(PTSM) ligands which, when labelled with the positron emitting
isotope 64Cu, have been used to image hypoxia with PET.
K2CO3 resulting in a red-orange coloured solution. The vial was
then placed in the heater block maintained at 408C and allowed
to react for 2 min. An analytical sample of ,20 mL was drawn
out of the vial and radio-analytical high-pressure liquid chro-
matography (HPLC) performed.
Physical Methods
NMR spectra were recorded on a Bruker 700 MHz instrument
and chemical shifts are reported in ppm relative to the residual
DMSO signal at 2.5 ppm which was used as the internal stan-
dard. 13C NMR spectra were recorded in d6-DMSO using the
proton decoupling technique. Chemical shifts reported for 13C
NMR spectra are referenced to residual DMSO at 39.5 ppm. 19F
NMR spectra were acquired in d6-DMSO solvent at room
temperature. No internal reference was used for this run. Stan-
dard parameters were used to acquire the spectrum. COSY,
HSQC, and HMBC spectra were acquired using a cryoprobe.
The proton and carbon sweep width used in all experiments were
14 and 180 ppm respectively. The 2D NMR experiments were
acquired with 256 increments in the T1 direction. The HSQC
spectra were acquired with a 1J coupling constant of 140 Hz and
the HMBC had a long-range coupling constant of 6 Hz.
UV-Vis Spectroscopy
The UV/vis spectra of the indium complexes in MeOH are
shown in Fig. 1. All spectra show a pair of absorption maxima
around 300 and 450 nm which were sensitive to the substituents
(R1, R2, and R3, Scheme 1). For example, compounds 1 and 2
showed a much longer wavelength (480 nm) of absorption
compared with the other derivatives. These two compounds
possess phenyl rings which are attached to the N4 atom. In
contrast, compound 3 bears phenyl substituents on the ligand
backbone and the wavelength maxima shift to shorter wave-
length. The red shift occurring in the complexes containing
multiple phenyl rings are rationalised due to further
UV/Visible spectra were obtained using a Thermo Scientific
Nanodrop model 2000C spectrophotometer using methanol as
solvent. The IR spectra were acquired using a Nicolet Diamond
model 5700 Fourier Transform Infrared spectrometer. The
samples were used as powders and introduced in the spectrome-
ter. HPLC analysis was performed using a Shimadzu model-LC
20AD instrument with a diode array, a quaternary pump,
thermostatic column compartment, and a radio detector (Bio-
scan Flow Count with a pin-diode detector) in conjunction with
LCsolutions software computer operated system. The analytical
column used was a Zorbax Eclipse Plus C18 (Agilent) of

386
T. K. Venkatachalam et al.
delocalisation of electrons compared with alkyl substituted
indium complexes
similar to those reported for analogous In complexes of thiose-
micarbazone ligands.[49] A particular feature are the acute
coordinate angles (,808) enforced by the three consecutive
five-membered chelate rings and exacerbated by the relatively
long coordinate bonds.
NMR Analysis
A detailed NMR investigation of all compounds was conducted
in d6-dimethyl sulfoxide at room temperature. As an example,
the asymmetrical compound 1 (Supplementary Material, Fig.
S1) showed that the NH protons attached to the phenyl ring are
slightly inequivalent (1H NMR singlets at 9.66 and 9.64 ppm).
These were followed by a doublet at 7.77 ppm, a multiplet at
7.29 ppm, and a triplet at 6.95 ppm corresponding to the aro-
matic ring protons, which gave rise to a single set of peaks from
each ring. The ethyl group yields a quartet at 2.87 ppm and a
triplet at 1.15 ppm, while the isolated methyl group gave a
singlet at 2.44 ppm.
The crystal structure of the symmetrical compound 2 was
also determined (Fig. 3). The compound crystallised as its hemi-
ethanol solvate, which is disordered about a 2-fold axis. The
structure has few H-bonding interactions; the only one of note is
donated by N6 to the EtOH hydroxy group establishing 2-fold
symmetric H-bonded dimers (Supplementary Material, Fig. S2).
The bond lengths and angles (Table 1) are again consistent with
InIII thiosemicarbazone complexes. The coordinate bonds in
compound 2 are also significantly longer than those observed for
the corresponding gallium(III) complex[44] as expected.
1H NMR of the symmetrical compound 2 showed that the NH
protons adjacent to the phenyl ring gave a singlet at 9.67 ppm.
The aromatic protons were between 7.7 and 6.5 ppm. The ortho
and para protons from both rings appeared as a doublet and
triplet respectively. The meta protons showed there was a slight
difference in chemical shift between the two aromatic rings. The
ethyl protons were at 2.9 and 1.2 ppm. The structure was
confirmed by 2D NMR experiments (COSY, HSQC, and
HMBC). The NMR spectra of all remaining compounds (1H,
13C, COSY, HSQC, and HMBC) are shown in the Supplemen-
tary Material.
N5
N4
N3
Cl1
N6
N2
In1
N1
S2
S1
Fig. 2. ORTEP view of compound 1 (30 % probability ellipsoids). The
Mass Spectra
THF solvent molecule is omitted.
The mass spectrometric analysis of a few of the indium com-
plexes was done to confirm certain structural features of the
dithiosemicarbazone complexes. The mass spectrometric data
of a few of the selected chloride indium thiosemicarbazone
complexes are attached in the Supplementary Material. The
mass spectrum of compound 2 showed a peak at m/z 525 cor-
responding to the molecular ion devoid of chlorine. Compound 3
similarly showed a mass peak at m/z 497 showing the absence of
two CH2 groups (a decrease in the mass value by 28 units) in
comparison with the structure of compound 2 which is expected
based on its structure. A similar trend was observed for com-
pound 5 which showed a peak at m/z 401. Based on the above we
confirm that the compounds contained indium along with other
substituents in the structure of the dithiosemicarbazone com-
plexes. This will be elaborated in the following discussion where
the X-ray crystal structure of the complexes provided concrete
evidence for the structure of the compounds.
˚
Table 1. Coordinate bond lengths (A) and angles (8) for compounds 1,
2, 3, and 5
˚
Bond length [A]
1ꢂTHF
2ꢂ1/2EtOH 3ꢂ1/2CH2Cl2
5
In–N3
2.247(7)
2.234(6)
2.454(2)
2.469(2)
2.383(2)
2.234(2)
2.225(2)
2.4489(7)
2.4572(7)
2.4039(7)
2.243(4)
2.261(4)
2.458(1)
2.433(1)
2.405(1)
2.219(2)
2.229(2)
2.4753(7)
2.4453(7)
2.4426(7)
In–N4
In–S1
In–S2
In–Cl1
Bond angle [deg.]
N4–In–N3
N4–In–Cl1
N3–In–Cl1
N4–In–S1
N3–In–S1
Cl1–In–S1
N4–In–S2
N3–In–S2
Cl1–In–S2
S1–In–S2
70.4(2)
102.1(2)
98.6(2)
70.89(7)
101.91(5)
96.03(5)
141.15(5)
78.60(5)
104.73(3)
78.84 (5)
144.06(5)
109.04(3)
129.4(7)
70.8(1)
104.3(1)
96.2(1)
71.61(8)
101.25(6)
96.20(6)
136.7(2)
77.1(2)
138.2(1)
77.1(1)
144.13(6)
77.92(6)
110.60(8)
77.7(2)
105.25(4)
78.8(1)
100.42(2)
79.66(6)
X-Ray Crystallography
141.2(2)
109.40(8)
115.21(7)
144.8(1)
108.30(5)
118.21(5)
149.03(6)
100.59(3)
123.56(3)
Compound 1ꢂTHF was characterised by single crystal X-ray
diffraction. As seen in Fig. 2, the chloride is positioned at the
apex of a square pyramidal coordination sphere and the phenyl
rings are coplanar with the tetradentate bis-thiosemicarbazone
ligand. The structure also includes a molecule of THF (not shown
in Fig. 2). There is also disorder in the positions of the methyl and
ethyl groups andoneofthephenylrings is rotationally disordered
about its C1–C4 axis. This disorder is shown in the Supple-
mentary Material (Fig. S1). The THF molecule accepts a H-bond
from N1 while the other donor N6 forms an intermolecular
H-bond to S2 to define H-bonded dimers (Fig. S1).
Cl1
N5
N6
N4
N2
N1
N3
In1
S2
Table 1 includes selected bond lengths and angles for this
structure and all others. The In–S and In–Cl bond lengths are
similar, while the In–N bonds are somewhat shorter due to the
smaller covalent radius of the N atom. The bond lengths are
S1
Fig. 3. ORTEP view of Compound 2 (30 % probability ellipsoids). The
EtOH solvent molecule is omitted.

Synthesis and Characterisation of Bis-Thiosemicarbazones
387
The crystal structure of compound 3 as its hemi-dichloro-
methane solvate was also determined (Fig. 4). The solvent is
disordered about a 2-fold axis. The phenyl rings necessarily
rotate out of the plane of the N2S2 ligand but their presence does
not have a marked influence on the bond lengths or angles
relative to compounds 1 and 2. H-bonding in this structure
comprises a polymeric chain linked by N–H?Cl interactions
originating from N1 (Supplementary Material, Fig. S3).
Finally, the crystal structure of compound 5 was determined
(Fig. 5). This structure was devoid of solvent. Unlike the
aromatic substituted compounds 1–3 discussed earlier, this
compound only bears aliphatic substituents. Interestingly
the In–Cl bond length is significantly longer than in compounds
1–3. The origin of this can be seen in the packing diagram
(Supplementary Material, Fig. S4) where the chloride ligand
accepts two H-bonds from the thiosemicarbazone NH groups
originating at N1 and N6. This evidently weakens the In–Cl
bond.
Cl1
Exchange Reaction of Indium Chlorido Complexes with KF
N2
N3
We then introduced fluoride into the coordination sphere using a
halogen exchange strategy. This involved first replacement of
chloride by nitrate (using AgNO3) then treating the nitrato
complex with potassium fluoride in methanol and isolation of
the resultant fluoride complex. The 19F NMR spectrum in
dimethyl sulfoxide showed a peak at ꢀ73.4 (Fig. 6) demon-
strating the formation of the fluoride complex. This chemical
shift was similar to that obtained for a gallium fluoride complex
where in CF3COOH (0.1 %) was used as a capillary insert.[44]
We then introduced 18Fꢀ using a similar protocol (Scheme 2).
Efforts are continuing to crystallise this compound to obtain X-
ray quality crystals. However, based on our earlier work[44,50] on
gallium compound radiolabelling using a similar strategy along
with isolation and X-ray crystal structure determination of the
fluoride derivative, we are confident that the obtained radio peak
in this study corresponds to the fluorinated indium species.
Using 18F the radio peak of the fluorido derivative of
compound 2 emerged at 17.5 min retention time (Fig. 7). The
corresponding UV trace for this compound is shown in Fig. 8
and shows a peak at the same retention time as the starting
material.
N1
N4
S2
N5
In1
N6
S1
Fig. 4. ORTEP view of compound 3 (30 % probability ellipsoids). Disor-
dered dichloromethane omitted.
Cl1
N2
N3
N4
N1
N5
N6
In1
S1
S2
Fig. 5. ORTEP view of compound 5 (30 % probability ellipsoids).
19F NMR
Ϫ67
Ϫ68
Ϫ69
Ϫ70
Ϫ71
Ϫ72
Ϫ73
Ϫ74
Ϫ75
Ϫ76
Ϫ77
Ϫ78
Ϫ79
Ϫ80
Ϫ81
Ϫ82
f1 [ppm]
Fig. 6. 19F NMR of the exchange reaction between KF and the nitrato complex of compound 3 in d6-DMSO. (Note: the small peak at
72.9 ppm is due to the free KF dissolved in the water present in the NMR solvent, since KF is insoluble in dimethyl sulfoxide. We have also
eliminated this peak using Al2O3 cartridges earlier, see refs [44] and [50] for details.)

388
T. K. Venkatachalam et al.
R1
N
R2
N
R1
N
R2
N
AgNO3
N
N
N
N
MeOH
RT
In
In
NHR3
NHR3
R3HN
R3HN
S
S
S
ONO2
S
Cl
DMSO KF
R1
R2
N
N
N
N
In
F
NHR3
R3HN
S
S
Scheme 2. Introduction of Fꢀ into the core structure of bis-thiosemicarbazones indium chloride complexes. The
same procedure is used for both 19Fꢀ and 18Fꢀ.
Et
N
Et
Et
N
Et
N
Et
N
Et
N
18
AgNO
[
F][K(Kryptofix)]F
DMSO
3
N
N
N
N
N
N
N
290
240
190
140
90
MeOH
In
In
In
NHPh
NHPh
NHPh
PhHN
PhHN
PhHN
18
S
S
S
S
S
S
Cl
ONO2
F
Radiotrace
40
Ϫ10
0
2
4
6
8
10
12
14
16
18
20
22
24
Retention time [min]
Fig. 7. Radiochromatogram of the 18F derivative of compound 2 after halide exchange.
To assess the general applicability of this halide exchange
protocol, we examined two structurally different indium com-
plexes; the results are shown in Figs 9 and 10. Exchange took
place for each complex, albeit with differing yields. The yields
are estimated based on the area under the curve obtained for
individual compounds. We attribute the more facile exchange to
the apical position of the chlorine atom in the structure of these
compounds which avoids steric hindrance for the approach of
the 18F. This is the first report of the introduction of 18F directly
covalently bound to an indium complex. This simple halogen
exchange process may be of value for the preparation of PET
imaging agents. The radiochemical yields are summarised in
Table 2.
analysis. The variation in radiochemical yield for each com-
pound is most likely due to the nitro-derivative being generated
in situ before reaction with 18Fꢀ. We state here that there was no
attempt made to separate the starting materials, the chloro and
nitro complexes, from the fluorinated derivative because of the
need of semi-preparative HPLC columns.
This research builds on recent advances of the use of
complexes of group 13 metals to form high affinity complexes
with 18Fꢀ.[51] Furthermore, Bhalla et al. highlighted they had
observed a high stability of group 13 metal fluoride complexes
with Al3þ, Ga3þ, and In3þ. While researchers have reported the
synthesis of 18Fꢀ complexes of Al3þ and Ga3þ [52]
surprisingly
,
there are currently no examples of 18Fꢀ complexes of indium in
the literature. This current paper presents the first examples
of the formation of a fluorine-18 complex of indium.
The radiochemical yield is based on the percentage area of
the peaks obtained from the radio-HPLC chromatogram of each

Synthesis and Characterisation of Bis-Thiosemicarbazones
389
Et
N
Et
Et
N
Et
N
Et
Et
N
18
AgNO
[
F][K(Kryptofix)]F
3
N
N
N
N
N
N
N
N
MeOH
In
In
In
DMSO
4450
3950
3450
2950
2450
1950
1450
950
NHPh
NHPh
NHPh
PhHN
PhHN
PhHN
18
S
S
S
S
S
S
Cl
ONO2
F
UV 254 nm
450
Ϫ50
0
2
4
6
8
10
12
14
16
18
20
22
24
Retention time [min]
Fig. 8. UV trace of 18F derivative of compound 2. (Note: the UV peak of the 19F derivative is buried under the large starting material peak.)
Ph
N
Ph
N
Ph
N
Ph
N
Ph
N
Ph
N
AgNO
18
3
[
F][K(Kryptofix)]F
DMSO
N
N
N
N
N
N
MeOH
In
In
In
145
NHMe
NHMe
NHMe
MeHN
MeHN
MeHN
18
S
S
S
S
S
S
ONO
Cl
F
2
Radiotrace
95
45
Ϫ5
0
2
4
6
8
10
12
14
16
18
20
22
24
Retention time [min]
Fig. 9. Radiochromatogram of 18F derivative of compound 3.
While currently radiochemical yields for are still low, this work
highlights the scope of further exploring In–18F complexes of
the thiosemicarbazone family of ligands and should inspire
other researchers to make In–18F complexes of other ligands.
is positioned at the apex of the molecule. In some, the chlorine
atom is hydrogen bonded to the NH group of the bis-thiosemi-
carbazones moiety. We introduced 18F into the coordination
sphere of the indium bis-thiosemicarbazone complexes using a
novel and facile halogen exchange method. The technique may
enable the development of kit-based PET imaging agents.
Conclusions
Several structurally related indium chloride complexes of
substituted bis-thiosemicarbazones were prepared and char-
acterised using high resolution NMR spectroscopy. Single crystal
X-ray analysis of the complexes indicates that the chlorido ligand
Supplementary Material
Supplementary information for the indium complex is available
on the Journal’s website.

390
T. K. Venkatachalam et al.
Me
N
Et
N
Me
N
Et
Me
N
Et
N
18
AgNO
[
F][K(Kryptofix)]F
N
3
N
N
95
N
N
N
N
MeOH
In
In
In
DMSO
NHPh
NHPh
NHPh
PhHN
PhHN
PhHN
18
S
S
S
S
S
S
Cl
F
ONO2
Radiotrace
45
Ϫ5
0
2
4
6
8
10
12
14
16
18
20
22
24
Retention time [min]
Fig. 10. Radiochromatogram of 18F derivative of compound 1.
Table 2. Summary of radiolabelling of indium complexes with 18F2
Compound No.
18F radiolabelling study
Radio-HPLC retention time [min]
Radiochemical yield [%]
1
1
2
3
1
2
3
1
2
16.1
16.1
16.1
17.3
17.3
17.3
15.1
15.1
11
8
4
2
3
2
12
11
14
1
Conflicts of Interest
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The authors declare no conflicts of interest.
Acknowledgements
This work was supported by an ARC Linkage Project Grant LP 130100703
(DR, RB). The authors thank their colleagues at the Centre for Advanced
Imaging for help during this investigation.
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