Synthesis and cytotoxicity of pyridine and quinoline oxorhenium(V) complexes with tridentate (NS2, S3)/monodentate (S) coordination

Article abstract of DOI:10.1007/s10593-012-0989-z

New oxorhenium complexes with tridentate 3-thia- and 3-methylazapentane-1, 5-dithiolate and monodentate pyridine and quinoline derivatives have been synthesized. As a result of investigation of biological activity a high cytotoxicity was found for the synthesized complexes in relation to tumor cells. The specificity of the 2-pyridylthiolato[3-(N-methyl)azapentane-1,5-dithiolato] oxorhenium(V) cytotoxic action towards cells of mouse hepatoma MG-22A on a background of low acute toxicity was established.

Full text of DOI:10.1007/s10593-012-0989-z

Chemistry of Heterocyclic Compounds, Vol. 48, No. 2, May, 2012 (Russian Original Vol. 48, No. 2, February, 2012)
SYNTHESIS AND CYTOTOXICITY OF PYRIDINE AND
QUINOLINE OXORHENIUM(V) COMPLEXES WITH
TRIDENTATE (NS2, S3)/MONODENTATE (S) COORDINATION*
I. Segal1**, A. Zablotskaya1, T. Kniess2, and I. Shestakova1
New oxorhenium complexes with tridentate 3-thia- and 3-methylazapentane-1,5-dithiolate and
monodentate pyridine and quinoline derivatives have been synthesized. As a result of investigation of
biological activity a high cytotoxicity was found for the synthesized complexes in relation to tumor cells.
The specificity of the 2-pyridylthiolato[3-(N-methyl)azapentane-1,5-dithiolato]oxorhenium(V) cytotoxic
action towards cells of mouse hepatoma MG-22A on a background of low acute toxicity was
established.
Keywords: oxorhenium(V) complexes, pyridine, quinoline, rhenium, cytotoxicity.
Derivatives of quinoline and pyridine are widely used for the synthesis of various medicinal agents
including antitumor medications (rogletimide, peldesin, pazelliptine, oxysuran, ledacrine, lurtotecane, emitefur,
brequinar, aconiazide, etc.) [1]. As structural fragments they also form part of the structure of the antitumor
antibiotics nigrin [1], bruneomycin [2], and antitumor alkaloids camptothecin [3] and meridin [4]. On the other
hand, a series of transition metal complexes [5, 6], including quinoline-containing [7-10], and also complexes of
rhenium [11-13], display antitumor activity and are potentially biological transporting agents for medications
[14, 15].
In the present work, with the aim of studying the effect of the ligand nature on antitumor properties,
neutral oxorhenium(V) complexes have been synthesized in which the oxorhenium(V) framework ReO3+ is
coordinated with the tridentate 3-thia- and 3-methylazapentane-1,5-dithiolate, and also with the monodentate
2-mercaptopyridine and 2-mercaptoquinoline. The formation of the complexes was carried out by "3+1"
methodology using two different rhenium precursors 1 and 2 depending on the neutral donor atom of the
tridentate ligand.
Chloro(3-thiapentane-1,5-dithiolato)oxorhenium(V) (4) was obtained from tetra-n-butylammonium
perrhenate as an intermediate for the synthesis of 2-pyridylthiolato(3-thiapentane-1,5-dithiolato)oxo-
rhenium(V) (6). As a result of its reaction with pyridine-2-thiol (5) in refluxing acetonitrile, the chlorine atom in
compound 4 was successfully replaced by a pyridylthiol residue with the formation of the corresponding
rhenium complex 6 in high yield. An alternative approach was used for the synthesis of complexes
_______
*In memory of Professor E. Lukevics.
**To whom correspondence should be addressed, e-mail: seg@osi.lv.
1Latvian Institute of Organic Synthesis, 21 Aizkraukles St., Riga LV-1006, Latvia.
2Forschungszentrum Dresden-Rossendorf, Postfach 51 01 19, Dresden, Germany; e-mail: kniess@fz-rossendorf.de.
_________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 314-319, February, 2012. Original
article submitted March 9, 2011.
296
0009-3122/12/4802-0296©2012 Springer Science+Business Media, Inc.

8 and 9, which were obtained as a result of the interaction of the previously synthesized phosphine-containing
oxorhenium(V) precursor 2 with the tridentate ligand 3 and monodentate thiols 5 or 7 in a ratio 1:1:1.1 in
refluxing methanol. The tridentate 3-methylazapentane-1,5-dithiol (3) was synthesized from N-methyl-
diethanolamine as a result of a series of sequential conversions. The corresponding dichloro derivative was
obtained by the reaction of N-methyldiethanolamine with thionyl chloride in chloroform, the interaction of
which with thiourea in refluxing ethanol and subsequent alkaline hydrolysis of the resulting isothiouronium salt
was completed by the formation of the desired dithiol 3.
SH
S
O
S
N
SH
+
Cl
S
O
S
N
H
S
5
SH
Re
n-Bu4N+ ReOCl4–
Re
MeCN, 80°C
CHCl3–MeOH,
Ar
S
Cl–
S
1
S
4
1) n-Bu4N+ Br–
2) г. HCl, EtOH
6
(NH4)ReO4
O
PPh3, conc. HCl,
EtOH, 78°C
S
N
S
5
Re
S
N
O
Me
Ph3P
Cl
PPh3
Cl
3, MeOH, 65°C
8
Re
Cl
O
2
N
7
SH
S
N
S
Re
S
N
1) SOCl2, CHCl3, 60°C
2) (H2N)2CS, EtOH, 78°C
Me
OH
OH
SH
9
3) NaOH, 
Me
N
Me
N
SH
3
The vibrational frequency (Re=O) in the IR spectra of the synthesized complexes 6, 8, and 9 depends
more on the central donor atom of the tridentate ligand (complexes 6 and 8) than on the donor properties of the
monodentate ligand (complexes 8 and 9). The signals varied from 960 (6) to 956 (8) and 953 cm-1 (9), which is
typical for oxorhenium(V) complexes with mixed ligands [16].
The biological properties of the synthesized compounds were studied in four tumor cell lines: HT-1080
(human fibrosarcoma), MG-22A (mouse hepatoma), B16 (mouse melanoma), and SH-SY-5Y (human
neuroblastoma), and also in relation to normal fibroblasts NIH-3T3, which also served for assessment of the
compounds' toxicity (alternative method of determining LD50 [17]) (Table 1).
The investigations results showed that the cytotoxic activity of 2-pyridyl- and 2-quinolylthiolate
complexes of oxorhenium(V) depends on the nature of both the monodentate and tridentate ligands, and in
several cases selectivity is displayed in relation to certain cell line. Differences were also revealed in the type of
action: the CV test reveals effect on cell membranes, and the MTT test reveals effect on the activity of
mitochondrial enzymes in the cell.
297

TABLE 1. Cytotoxicity (LC50) and Ability to Generate NO within Cells
by Oxorhenium(V) Complexes 6, 8, and 9*
HT-1080
LC50, g/ml
CV MTT
MG-22A
B16
SH-SY-5Y
NIH-3T3
Com-
pound
LC50, g/ml
LC50, g/ml LC50, g/ml LC50 LD50,
CV MTT CV MTT
NO, %
NO, %
NR mg/kg
CV MTT
4
12
3
3
23
2
100
86
2
2
3
3
3
3
133
250
250
3
91
3
3
63
3
3
15
2
2
10
2
52
445
4
801
1877
256
6
8
9
167
_______
*CV – crystal violet; MTT – 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-
2H-tetrazolium bromide; NR – neutral red; NO – degree of NO generation,
determined and calculated by the procedure of [22].
Rhenium(V) complexes 6 and 9 possess high cytotoxicity (2-3 μg/ml) in relation to each tumor cell line
studied in each test. In turn, complex 8 displayed high tissue specificity. Cells of hepatoma MG-22A proved to
be the most sensitive towards compound 8. Its inhibiting action was moderate in relation to HT-1080 and
SH-SY-5Y, low relative to B-16, and very low towards normal NIH-3T3 fibroblasts (Table 1).
All of the studied complexes were nontoxic compounds. The least toxic proved to be 2-pyridylthiolato-
[3-(N-methyl)azapentane-1,5-dithiolato]oxorhenium(V) (8). Replacement of the monodentate ligand in complex
8 by quinolylthiolate (complex 9) led to a more than sevenfold increase in toxicity. In turn, replacement of the
tridentate ligand, namely 3-(N-methyl)azapentane-1,5-dithiolate by 3-thio-1,5-dithiolate (complex 6), increased
the toxicity of the compound more than twice (Table 1).
It has therefore been established that the studied compounds possess a selective cytotoxic action in
relation to the studied tumor cells. A specificity of the cytotoxic effect towards line MG-22A was displayed by
compound 8.
EXPERIMENTAL
The IR spectra were recorded on a Perkin Elmer FTIR Specord 2000 spectrometer using KBr pellets.
The 1H and 13C NMR spectra were recorded on a Varian Inova 400 (at 400 and 100 MHz, respectively), internal
standard was the signal of the residual solvent protons (CDCl3 δH 7.26 ppm, δC 77.0 ppm, DMSO-d6 δH 2.5 ppm,
δC 39.5 ppm). Elemental analyses were carried out on a LECO CHNS 932 analyzer. Reactions progress and the
purity of the obtained compounds were monitored by TLC on Machery-Nagel plates in the system CHCl3–MeOH.
Merck type silica gel (0.040-0.063 mm) was used for column chromatography.
Tetra-n-butylammonium tetrachlorooxorhenate(V) (1) was obtained by the procedure of [18] with
minor amendments. n-Bu4N·ReOCl4 (4.51 g, 9.17 mmol), obtained from an aqueous NH4ReO4 solution by
precipitation with n-Bu4N+Br-, was dissolved in abs. EtOH (70 ml). The obtained solution was saturated with
gaseous HCl. The reaction mixture was then stirred for 3 h, the solvent was partially evaporated, and the
reaction mixture was cooled to -18oC. The precipitated solid was filtered off and washed with ether, then dried
in vacuum for 4 h. Yield 5.05 g (94%); mp 143-145oC (decomp.). The IR spectral data agreed with those
described in the literature [18].
trans-Monooxotrichlorobis(triphenylphosphine)rhenium(V) (2) was obtained by the procedure of
[19]. NH4ReO4 (1.61 g, 6 mmol) and PPh3 (9.00 g, 34 mmol) were dissolved in abs. EtOH (100 ml) and conc.
HCl (10 ml) was added. The obtained reaction mixture was refluxed for 1 h, then cooled, and filtered. The solid
was washed with hot EtOH. The yield of crude product was 4.92 g (98%). Product 2 was used for the synthesis
of compounds 8 and 9 without further purification.
298

3-(N-Methyl)azapentane-1,5-dithiol (3) was obtained by the procedure of [20] with modifications. A
solution of SOCl2 (26.7 ml, 369.2 mmol) in CHCl3 (10 ml) was added dropwise with stirring to a solution of
N-methyldiethanolamine (20.0 g, 167.8 mmol) in CHCl3 (100 ml) cooled to 0oC. The reaction mixture was
refluxed for 24 h. The lighter fractions were then removed by evaporation of the mixture on a rotary evaporator.
Ethanol (50 ml) and thiourea (28.3 g, 369.2 mmol) were then added to the residue. The obtained mixture was
refluxed for 6 h, cooled, and the alcohol was removed by evaporation on a rotary evaporator. A 3 N NaOH
solution (280 ml) was added to the residue, and the mixture was refluxed with stirring in an atmosphere of argon
for 2 h. After cooling to room temperature, the reaction mixture was neutralized with 2 N HCl solution, the
product was extracted with ether (3×200 ml), the ether extract was dried over MgSO4, then evaporated on the
rotary evaporator, and the residue was distilled in vacuum. Yield 10.64 g (48%); bp 65-67oC (4.1·10-1 mm Hg),
which corresponded to the literature data of [20].
Chloro(3-thiapentane-1,5-dithiolato)oxorhenium(V) (4) was obtained by the procedure of [21] with
minor amendments. n-Bu4N·ReOCl4 (1) (1.17 g, 2 mmol) was dissolved in a mixture of CHCl3–MeOH, 10:1
(100 ml), and cooled to 0oC. A solution of bis(2-mercaptoethyl) sulfide (0.31 g, 2 mmol) in CHCl3 (50 ml) was
added dropwise with stirring to the obtained solution in an atmosphere of argon. Afterwards the reaction
mixture was brought to room temperature; the precipitated solid was filtered off, washed with CHCl3 and with
ether, and dried. The yield of product was 0.61 g (78%). The physicochemical properties corresponded to
literature values [21].
2-Pyridylthiolato(3-thiapentane-1,5-dithiolato)oxorhenium(V) Hydrochloride (6). Compound 4
(85.6 mg, 0.22 mmol) was dissolved with stirring in hot acetonitrile (5 ml), and a solution of pyridyl-2-thiol (5)
(48.9 mg, 0.22 mmol) in acetonitrile (5 ml) was carefully added dropwise to the refluxing solution. The reaction
mixture was refluxed for an additional 30 min, cooled, and filtered. The solid was washed with ether (520 ml)
1
and dried. Yield 105.6 mg (96%); mp 190-192oC. IR spectrum, , cm-1: 960 (Re=O). H NMR spectrum
(DMSO-d6), δ, ppm (J, Hz): 2.42 (2H, m), 3.12 (2H, td, J = 14.0, J = 4.3), 4.17 (2H, dd, J = 10.8, J = 3.0), and
4.27 (2H, dd, J = 13.0, J = 4.5, 2SCH2CH2S); 7.21 (1H, m, H-5); 7.65 (1H, d, J = 7.6, H-3); 7.74 (1H, td, J = 7.6,
J = 1.8, H-4); 8.50 (1H, dd, J = 4.7, J = 0.9, H-6); 13.46 (1H, br. s, NH). 13C NMR spectrum (DMSO-d6), δ, ppm:
43.2 (SCH2); 46.1 (SCH2); 121.3; 129.7; 135.9; 149.2 (C Ar); 167.6 (C-2). Found, %: C 21.56; H 2.58; N 2.75;
Cl 7.02; S 25.36. C9H13ClNOReS4. Calculated, %: C 21.57; H 2.61; N 2.80; Cl 7.07; S 25.59.
2-Pyridylthiolato[3-(N-methyl)azapentane-1,5-dithiolato]oxorhenium(V) (8). Compound 2 (1.25 g,
1.50 mmol) was dissolved in MeOH (100 ml), the solution was stirred at room temperature in an argon
atmosphere, and solutions of pyridyl-2-thiol (5) (0.18 g, 1.65 mmol) in MeOH (5 ml) and 3-(N-methyl)-
azapentane-1,5-dithiol (3) in MeOH (2 ml) were added dropwise. The reaction mixture was refluxed with
stirring for 2 h, and after cooling, the solvent was evaporated. The product was isolated by column
chromatography, eluent was CHCl3–MeOH, 10:1. Yield 0.39 g (56%); mp 268-269oC. IR spectrum, , cm-1:
956 (Re=O). 1H NMR spectrum (DMSO-d6), δ, ppm (J, Hz): 2.74 (2H, m), 2.93 (2H, m), 3.19 (2H, m), and 3.57
(2H, m, 2SCH2CH2N); 3.36 (3H, s, NCH3); 7.15 (1H, m, H-5); 7.53 (1H, d, J = 7.7, H-3); 7.73 (1H, td, J = 6.3,
J = 1.1, H-4); 8.44 (1H, dd, J = 4.9, J = 0.9, H-6). 13C NMR (DMSO-d6), δ, ppm: 40.5 (SCH2); 53.9 (NCH3);
67.1 (NCH2); 120.7; 128.3; 135.8; 149.0 (C Ar); 175.0 (C-2). Found, %: C 26.00; H 3.26; N 5.99; S 20.87.
C10H15N2OReS3. Calculated, %: C 26.02; H 3.28; N 6.07; S 20.84.
2-Quinolylthiolato[3-(N-methyl)azapentane-1,5-dithiolato]oxorhenium(V) (9). Compound
2
(1.13 g, 1.4 mmol) was dissolved in MeOH (100 ml), and the solution was stirred at room temperature in an
atmosphere of argon. Solutions of quinolyl-2-thiol (7) (0.24 g, 1.5 mmol) in MeOH (5 ml) and 3-(N-methyl)-
azapentane-1,5-dithiol (3) in MeOH (5 ml) were added dropwise. The reaction mixture was refluxed with
stirring for 2 h, cooled, and then the solvent was evaporated. The product was isolated by column
chromatography, eluent was CHCl3–MeOH, 5:1. Yield 0.33 g (48%); mp 220-221oC. IR spectrum, , cm-1: 953
(Re=O). 1H NMR spectrum (DMSO-d6), δ, ppm (J, Hz): 2.75 (2H, m), 2.94 (2H, m), 3.22 (2H, m), and 3.57 (2H,
m, 2SCH2CH2N); 3.35 (3H, s, NCH3); 7.54 (1H, m, H-6); 7.65 (1H, d, J = 8.5, H-3); 7.74 (1H, m, H-5); 7.91
(2H, m, H-4,7); 8.26 (1H, d, J = 8.7, H-8). 13C NMR spectrum (CDCl3), δ, ppm: 41.6 (SCH2); 54.2 (NCH3);
299

68.2 (NCH2); 126.0; 126.9; 127.5; 129.1; 129.5; 135.1; 148.1 (C Ar). Found, %: C 32.84; H 3.36; N 5.43;
S 18.74. C14H17N2OReS3. Calculated, %: C 32.86; H 3.35; N 5.47; S 18.80.
Cytotoxicity of compounds 6, 8, and 9 (Table 1) in vitro in relation to monolayers of tumor cells
HT-1080, MG-22A, B16, SH-SY-5Y, and normal NIH-3T cells was determined in 96-hole plastic plates using
dyestuffs CV, MTT, and NR according to the procedures of [22, 23]. The concentration of compounds causing
death of 50% cells (LC50, μg/ml) is given in Table 1. The expected acute toxicity (LD50, mg/kg) was calculated
by the procedure of [17].
The work was carried out with the financial support of the Latvian Council of Science (projects 09.1321
and 10.0030).
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