DSpace at VNU: Syntheses, structures and biological evaluation of some transition metal complexes with a tetradentate benzamidine thiosemicarbazone ligand

The potentially tetradentate benzamidine/thiosemicarbazone ligand, Et2N-C=S-NH-CPh=N-o-C4H6-CMe=N-NH-C=S-NH-Me H 2 L readily reacts with NiCH3COO2, [PdCl2CH3CN2], [PtCl2PPh32] and NBu4[

Accepted Manuscript

Syntheses, Structures and Biological Evaluation of some Transition Metal

Com-plexes with a Tetradentate Benzamidine/Thiosemicarbazone Ligand

Thi Bao Yen Nguyen, Chien Thang Pham, Thi Nguyet Trieu, Ulrich Abram,

Hung Huy Nguyen

PII: S0277-5387(15)00223-5

DOI: http://dx.doi.org/10.1016/j.poly.2015.04.026

Reference: POLY 11290

To appear in: Polyhedron

Received Date: 5 March 2015

Accepted Date: 21 April 2015

Please cite this article as: T.B.Y Nguyen, C.T Pham, T.N Trieu, U Abram, H.H Nguyen, Syntheses, Structures and Biological Evaluation of some Transition Metal Complexes with a Tetradentate Benzamidine/

Thiosemicarbazone Ligand, Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.2015.04.026

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Abstract The potentially tetradentate benzamidine/thiosemicarbazone ligand, Et2

N-(C=S)-NH-C(Ph)=N-(o-C4H6)-C(Me)=N-NH-(C=S)-NH-Me (H 2 L) readily reacts with Ni(CH3COO)2, [PdCl2(CH3CN)2], [PtCl2(PPh3)2] and (NBu4)[ReOCl4] under formation of

complexes of the compositions [M(L)] (M= Ni (1), Pd (2), Pt (3)) and [ReO(L)(OMe)] (4)

In all complexes, H2L is doubly deprotonated and bonded to the central meal ion via its

N 2 S 2 donor set Complexes 1, 2 and 3 have distorted square-planar coordination spheres,

while the rhenium compound 4 is an octahedral trans oxido/methoxido complex The H2L proligand shows a medium cytotoxicity with an IC50 value of 21.1 M While the rhenium

complex 4 exhibits a stronger antiproliferative effect (IC50 = 5.52 M), the nickel,

palladium and platinum complexes are almost inactive

Keywords: Transition Metals, Benzamidines, Thiosemicarbazones, X-ray structure, Cytotoxicity

Corresponding Authors: ulrich.abram@fu-berlin.de (Ulrich Abram)

nguyenhunghuy@hus.edu.vn (Hung Huy Nguyen)

Recently we reported the synthesis and structural characterization of a series of tridentate

benzamidine/thiosemicarbazide ligands (H 2L’), which were prepared by reactions of

N-[N’,N’-dialkylamino(thiocarbonyl)]benzimidoyl chlorides (Bzm-Cl) and carbazides [9,10] They form stable complexes with various transition metals [9-12] Additionally, the organic ligands as well as their oxorhenium(V) and gold(III) complexes show a promising cytotoxicity on breast cancer cell lines [10-12] The reaction of 2-

thiosemi-aminoacetophenone-N-(4-methylthiosemicarbazone) with Bzm-Cl results in the formation

of a tetradentate hybrid thiosemicarbazone/benzamidine ligand, H2L [13] This ligand has hitherto only been used for reactions with nitridotechnetium and –rhenium complexes, which resulted in stable, neutral TcVN and ReVN complexes 13

With respect to the stability of these products, H2L should also be suitable for the coordination of other metal ions and the obtained products may possibly show interesting biological properties Here, we report about reactions of H2L with Ni(CH3COO)2,

PdCl2(CH3CN)2, PtCl2(PPh3)2 and (NBu4)ReOCl4, the structures of the obtained products as well as their cytotoxicity against human MCF-7 breast cancer cells

2 Results and Discussion

2.1 Syntheses and structures of the nickel, palladium and platinum complexes

H2L readily reacts with Ni(CH3COO)2, [PdCl2(CH3CN)2] or [PtCl2(PPh3)2] under formation

of complexes of the composition [M(L)] (Scheme 1) Depending on the solubility of the starting materials, the reactions were carried out in MeOH for Ni(CH3COO)2 or in a MeOH/CH2Cl2 mixture (v/v: 1/1) for [PdCl2(CH3CN)2] and [PtCl2(PPh3)2] While the first reaction proceeds very quickly, the two latter reactions require more time The addition of a supporting base such as Et3N accelerates the formation of the complexes and allows the syntheses to be carried out at ambient temperature with good yields The products are only sparingly soluble in alcohols, but well soluble in CH2Cl2 or CHCl3 and can be recrystallized from CH2Cl2/MeOH mixtures

The IR spectra of all three compounds show a moderate absorption at about 3400 cm-1, which is assigned to the N-H stretch of the MeNH-CS group The absorption band of theC=N vibration is observed as a very intense band around 1550 cm-1 This corresponds to a strong bathochromic shift of the corresponding band in H2L by about 170 cm-1 Such shifts are

commonly explained by a chelate formation with a large degree of π-electron delocalization

within the benzamidine chelate rings [9-12] Expectedly, the 1H NMR spectra of the complexes

no longer show resonances at 12.62 ppm and 8.41 ppm, which are assigned to the NH protons

of the benzamidine and thiocarbazone moieties in the proligand Additionally, a high field shift

of the signal of the NH proton of the CS-NHMe residue from 7.64 ppm in the proligand [13] to

the region around 5.0 ppm in the spectra of the complexes 1, 2 and 3 is observed, which

confirms that the organic ligand is doubly deprotonated and chelated to the central metal ions

A hindered rotation around the C–NEt2 bond is observed and results in two magnetically

non-equivalent ethyl groups Thus, two triplet signals of the methyl groups of the –NEt2 residues are

observed in the 1H NMR spectra of the complexes The signals of the two methylene groups

appear as four well separated multiplet resonances This pattern has previously been

rationalized by the rigid structure of the tertiary amine group, which makes ethylene protons

magnetically nonequivalent with respect to their axial and equatorial positions [9,10]

Single crystals suitable for X-ray studies were obtained by slow evaporation of

CH2Cl2/MeOH mixtures for the nickel and palladium compounds Figure 1 illustrates the

molecular structure of 1 as a representative for this class of compounds The structure of the

analogous palladium compound 2 is virtually identical and, thus, no extra Figure is shown

Selected bond lengths and angles of 1 and 2 are compared in Table 1 The atomic labeling

scheme of the palladium complex has been adopted from that of the nickel complex Both

metal ions are coordinated by the S 2 N 2 donor set of the organic ligand in a distorted

square-planar coordination environment The molecular planes defined by the metal atoms and the

four donor atoms S1, N5, N8 and S11 are slightly distorted with maximum deviations from

the mean least-squares planes of 0.215(1) Å for N8 (in 1) and of 0.098(1) Å for N5 (in 2)

2.1 Synthesis and structures of the oxidorhenium(V) complex

H2L reacts with (NBu4)[ReOCl4] in MeOH in the presence of the supporting base Et3N

under formation of a red crystalline solid of the composition [ReO(OMe)(L)] (4) in high

Figure 1 about here

Table 1 about here

yields (Scheme 2) The reaction can be carried out at room temperature or under reflux

without significant change in the yield

The IR spectrum of 4 is similar to those discussed for the complexes 1 - 3 with a sharp

medium absorption band of the N-H stretch at 3387 cm-1 and a strong bathochromic shift of

the C=N band with respect to the corresponding vibration in the spectrum of H2L The

strong absorption at 941 cm-1 is assigned to the Re=O vibration This wavenumber is

slightly lower than those, which are normally reported for common six-coordinate

oxidorhenium(V) complexes [14], but within the typical range for octahedral trans

oxido/-alkoxido rhenium(V) complexes [15,16] The 1H NMR spectrum of 4 shows the same

features as those of the nickel, palladium and platinum compounds: the absence of two N-H

resonances and a high field shift of the N-H signal of the CS-NHMe moiety to 5.26 ppm

The chelate formation also leads to a strong downfield shift of about 1.1 ppm of the

Me-C=N signal The presence of a methoxy group in 4 is clearly indicated by an additional

singlet at 3.04 ppm The +ESI mass spectrum of 4 show no molecular ion peak, but an

intense peak at m/z = 641.11, which can be assigned to the fragment [M - OMe]+

The X-ray diffraction data of 4 are in a good agreement with the spectroscopic results

Figure 2 shows the molecular structure of the complex The environment of the rhenium

atom is best described as a distorted octahedron with an oxido and a methoxido ligand

arranged in trans positions to each other The L2- ligand is expectedly coordinated to the

metal via its N2 S 2 donor set The Re atom is located only 0.151(2) Å above the mean

least-square plane formed by the four donor atoms of L2- towards the oxido ligand The Re–O20

Figure 2 about here

Table 2 about here

Scheme 2

N NH N S N HN HN S (NBu4)[ReOCl4] +

N N

N S N N HN S Re O

OMe MeOH, Et3N

- NBu4Cl, HNEt3Cl

absorption in the IR spectrum of 4

2.3 In vitro cell tests

We investigated the antiproliferative effects of the ligand H2L and its complexes on human MCF-7 breast cancer cells in a concentration response assay This allows the determination

of their IC50 values The uncoordinated H2L shows an IC50 value of 21.1(9) M reflecting a medium antiproliferative effect The complexation of H2L with Ni2+, Pd2+ and Pt2+ ions

dramatically decreases the cytotoxicity of the compound Thus, complex 1 only causes a

very weak reduction of the growth of human MCF-7 breast cancer cells (IC50 = 258(10)M), while complexes 2 and 3 show almost no antiproliferative effect The low

cytotoxicity of the square-planar complexes reflects their limited dissociation inside the cell and the stable metal chelates themselves exhibit no antiproliferative effects This can be

understood by the rigid tetradentate N2 S 2 coordination of the ligand, which is expected to

form stable complexes with Ni2+, Pd2+ and Pt2+ Additionally, no labile coordination site is available, as they are present in all metal complexes of the previously studied tridentate thiosemicarbazone/benzamidine hybrid ligands, for which promising antiproliferative effects have been found [10-12] Such a potentially labile ligand is present in the rhenium

complex 4, where the methoxo ligand can readily be hydrolyzed, which enables the

resulting complex fragment for interactions with biological targets In fact,

compound 4 exhibits an IC50 value of 5.52(14) mM, which is markedly lower than that of

H2L The cytotoxicity of 4 is in the magnitude of those reported for oxidorhenium(V) and

gold(III) complexes with tridentate H2L’ ligands [10,12], which also possess a labile (chlorido) ligand in their coordination spheres, and it is almost equal to that of cisplatin (IC50 = 7.10) determined under the same experimental conditions [17]

3 Conclusions

In the present communication, we could show that the benzamidine/thiosemicarbazone hybrid ligand H2L forms stable complexes with Ni2+, Pd2+ and Pt2+ and ReO3+ metal centers The proligand H2L and its oxidorhenium(V) complex show some antiproliferative effects on human MCF-7 breast cancer cells Since H2L is the hitherto only representative

of this new class of potentially bioactive hybrid ligands, studies with other derivatives of this class are recommended and are in preparation in our laboratories

4 Experimental

4.1 Materials

All reagents used in this study were reagent grade and used without further purification

H2L was prepared as reported previously 13

4.3 Syntheses

Ni(CH3COO)2 4 H2O (25 mg, 0.1 mmol) was added to a solution of H2L (44 mg, 0.1 mmol)

in 5 mL of MeOH A deep red solution was obtained after the addition of three drops of Et3N, and a dark red solid deposited within a few minutes The mixture was stirred for additional 2 h

at room temperature and then the precipitate was filtered off, washed with MeOH and dried in

vacuum Single crystals of 1 were obtained by slow evaporation of a CH2Cl2/MeOH (v/v: 1/1) solution Yield 84% (42 mg) Elemental analysis: Calcd for C22H26N6S2Ni: C, 53.13; H, 5.27;

N, 16.90; S, 12.90% Found: C, 53.01; H, 5.14; N, 16.95; S, 12.63% IR (KBr, cm-1): 3407 (m),

3086 (w), 2923 (m), 1547 (vs), 1502 (vs), 1486 (s), 1424 (s), 1361 (s), 1230 (m), 1022 (m), 810 (w), 702 (m), 630 (m) 1H NMR (500 MHz, CDCl3, ppm): 1.30 (t, J = 7.0 Hz, 3H, CH3), 1.31

(t, J = 7.0 Hz, 3H, CH3), 2.70 (s, 3H, N=C-CH3), 2.97 (d, J = 5.0, 3H, NCH3), 3.69 (m, 1H, NCH2), 3.74 (m, 1H, NCH2), 3.86 (m, 1H, NCH2), 4.09 (m, 1H, NCH2), 4.84 (s, br, NH), 6.41

(d, J = 8.0 Hz, 1H, C6H4), 6.78 (t, J = 7.7 Hz, 1H, C6H4), 6.86 (t, J = 7.7 Hz, 1H, C6H4), 7.12 (t,

J = 7.5 Hz, 2H, Ph), 7.20 (t, J = 7.3 Hz, 1H, Ph), 7.32 (d, J = 7.0 Hz, 2H, Ph), 7.52 (d, J = 8.0

Hz, 1H, C6H4) +ESI MS (m/z): 497.09, 100%, [M + H]+

4.3.2 Synthesis of Pd(L), 2

[PdCl2(CH3CN)2] (26 mg, 0.1 mmol) was added to a solution of H2L (44 mg, 0.1 mmol) in

5 mL of a CH2Cl2/MeOH (v/v 1/1) mixture, and then three drops of Et3N were added The mixture was stirred for 3 h at room temperature to obtain a clear yellow solution Slow

evaporation of the solvents gave light yellow crystals of 2 The product was filtered off, washed

with MeOH and dried in vacuum Yield 65% ( 35 mg) Elemental analysis: Calcd for

C22H26N6S2Pd: C, 48.48; H, 4.81; N, 15.42; S, 11.77% Found: C, 48.21; H, 4.85; N, 15.35; S, 11.60% IR (KBr, cm-1): 3434 (m), 3052 (w), 2967 (m), 1547 (vs), 1503 (vs), 1467 (s), 1420 (s),

1379 (s), 1223 (m), 1089 (m), 749 (m), 618 (m) 1H NMR (500 MHz, CDCl3, ppm): 1.33 (t, J

= 7.0 Hz, 3H, CH3), 1.36 (t, J = 7.0 Hz, 3H, CH3), 2.78 (s, 3H, N=C-CH3), 3.03 (d, J = 5.0, 3H,

4.3.2 Synthesis of Pt(L), 3

The compound was prepared as described for complex 2, but starting from [PtCl2(PPh3)2] (79 mg, 0.1 mmol) to obtain yellow crystals Yield 62% ( 39 mg) Elemental analysis: Calcd for C22H26N6S2Pt: C, 41.70; H, 4.14; N, 13.26; S, 10.12% Found: C, 41.63; H, 4.05; N, 13.32;

S, 10.10% IR (KBr, cm-1): 3415 (m), 3058 (w), 2966 (m), 1548 (vs), 1502 (vs), 1467 (s), 1421 (s), 1375 (s), 1230 (m), 1081 (m), 747 (m), 623 (m) 1H NMR (500 MHz, CDCl3, ppm): 1.32

(t, J = 7.0 Hz, 3H, CH3), 1.36 (t, J = 7.0 Hz, 3H, CH3), 2.80 (s, 3H, N=C-CH3), 3.01 (d, J = 5.0,

3H, NCH3), 3.63 (m, 1H, NCH2), 3.72 (m, 1H, NCH2), 4.03 (m, 1H, NCH2), 4.25 (m, 1H, NCH2), 4.96 (s, br, NH), 6.50 (d, J = 8.0 Hz, 1H, C6H4), 6.81 (t, J = 7.7 Hz, 1H, C6H4), 6.86 (t,

solution was slowly evaporated to give red crystals of 4 Yield 60% (40 mg) Elemental

analysis: Calcd for C23H29N6O2S2Re: C, 41.12; H, 4.35; N, 12.51; S, 9.55% Found: C, 40.71; H, 4.09; N, 12.56; S, 9.21% IR (KBr, cm-1): 3387 (m), 3070 (w), 2970 (m), 2924 (m), 2800 (w), 1558 (s), 1512 (s), 1425 (m), 1359 (m), 1288 (m), 1250 (m), 1227 (m), 1172 (w),

1110 (m), 942 (s), 810 (w), 771 (w) 1H NMR (400 MHz, CDCl3, ppm): 1.36 (t, J = 7.1 Hz,

3H, CH3), 1.41 (t, J = 7.2 Hz, 3H, CH3), 2.92 (s, 3H, N=C−CH3), 3.04 (s, 3H, OCH3), 3.14

The intensities for the X-ray determinations were collected on a Bruker D8-QUEST (1 and

2) and a STOE IPDS 2T instrument (4) with Mo K radiation ( = 0.71073 Å) Standard

procedures were applied for data reduction and absorption correction Structure solution

and refinement were performed with SHELXS and SHELXL 17] Hydrogen atom

positions were calculated for idealized positions and treated with the ‘riding model’ option

of SHELXL More details on data collections and structure calculations are contained in

Table 3

4.5 In vitro cell tests

The cytotoxic activity of the compounds was determined using a MTT assay Human

cancer cells of the cell line MCF-7 were obtained from the American Type Culture

Collection (Manassas, VA) ATCC Cells were cultured in medium RPMI 1640

supplemented with 10% FBS (Fetal bovine serum) under a humidified atmosphere of 5%

CO2 at 37 °C The testing substances were initially dissolved in DMSO, then diluted to the

desired concentration by adding cell culture medium The samples (100 L) of the

complexes with different concentrations were added to the wells on 96-well plates Cells

were detached with trypsin and EDTA and seeded in each well with 3  104 cells per well

After incubation for 48 h, a MTT solution (20 L, 4 mg mL-1) of phosphate buffer saline

(8 g NaCl, 0.2 g KCl, 1.44 g Na2HPO4 and 0.24 g KH2PO4/L) was added into each well

Table 3 about here