Platinum(II) complexes of 8-quinolylmethylphosphonates: Synthesis, characterization and antitumour activity

Polyhedron 29 (2010) 2527–2536

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Platinum(II) complexes of 8-quinolylmethylphosphonates: Synthesis, characterization and antitumour activity Ljerka Tušek-Bozˇic´ a,*, Marina Juribašic´ a, Vito Scarcia b, Ariella Furlani b a b

- Boškovic´ Institute, Bijenicˇka 54, HR-10002 Zagreb, Croatia Division of Physical Chemistry, Ruder Dipartimento di Scienze Biomediche, Universitá degli Studi di Trieste, Via L. Giorgieri 7, 34127 Trieste, Italy

a r t i c l e

i n f o

Article history: Received 1 February 2010 Accepted 27 May 2010 Available online 31 May 2010 Keywords: Platinum(II) complex Quinolylmethylphosphonate complex Spectroscopy Thermal analysis Antitumour activity

a b s t r a c t A series of novel platinum(II) complexes of diethyl (8-dqmp) and monoethyl (8-Hmqmp) ester of 8-quinolylmethylphosphonic acid has been prepared and studied. It was shown that molecular or ionic complexes could be isolated by reaction of these organophosphorus ligands with [PtX4]2 (X = Cl, Br), depending on the acidity of the reaction solution. In the neutral medium diester formed dihalide adducts, trans-[Pt(8-dqmp)2X2] (1 and 2), with N-bonded ligand through the quinoline nitrogen. Under acidic conditions (pH < 3) both ligands gave the quinolinium salt complexes [LH]2[PtX4] (3 and 4, L = 8-dqmp; 7 and 8, L = 8-Hmqmp), with protonated quinoline ligand as cation and tetrahalidoplatinate complex as anion. By heating in methanol complexes 3 and 4 were converted into the corresponding dimeric hexahalidodiplatinum complexes, [8-Hdqmp]2[Pt2X6] (5 and 6). The chelate complex [Pt(8-mqmp)2] (9), with monoester ligand bonded through the quinoline nitrogen and the deprotonated phosphonic acid oxygen and forming two seven-membered {N, O} chelate rings, was obtained in neutral and basic media by reaction of platinum(II) halides either with sodium or hydrochloride salt of this monoester. The complexes were identified and characterized by elemental and thermogravimetric analyses, conductometric measurements, and by spectroscopic studies. In vitro antitumour activity of complexes was evaluated against the human epidermoid KB and murine leukaemia L1210 cell lines. These results were compared with those obtained for the palladium(II) complexes of the same phosphonate ligands and with those of platinum(II) and palladium(II) complexes of diethyl and monoethyl 2-quinolylmethylphosphonate, in order to correlate the structural and biological properties of quinoline-based aminophosphonate compounds. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction There is currently a great interest in the design, synthesis as well as biochemical and biophysical investigations of novel transition metal anticancer coordination compounds in order to improve efficacy and reduce toxic side effects of the cis-diamminedichloridoplatinum(II) (cisplatin) and its analogues, which are still the most widely clinically used anticancer drugs [1,2]. Advances in this research area have been summarized in a number of reviews and books [3–8]. The special attention has been directed to the development of platinum complexes that are structurally different to those agents such as trans-geometry complexes, ionic and polynuclear complexes [9–11], and complexes of the other platinumgroup metals like ruthenium, rhodium, palladium and iridium [12–14]. In addition, few compounds of other transition metals, like titanium, gallium and gold have also been reported to be active and have entered clinical trials [13,15–17]. Over the recent years, alternative approaches were also focused upon metal complexes * Corresponding author. Tel.: +385 1 4571217; fax: +385 1 4680245. E-mail address: [email protected] (L. Tušek-Bozˇic´). 0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2010.05.022

with ligands which are important in medicinal and biological systems. The aim was widening the spectrum of complex activity and the drug targeting based on synthesis of metal complexes linked to bioactive carrier system, affording better modalities of attack to the site of pathology [8]. Very interesting ligand candidates with this regard are derivatives of aminophosphonic acids which are known to possess diverse biological and pharmacological properties as well as high affinity towards bone and other calcified tissues [18,19]. These findings have stimulated our investigations on complexes with alkyl aminophosphonates derived from aniline and quinoline [11,12]. We have designed and structurally characterized a series of molecular and ionic palladium and platinum complexes with this type of organophosphorus ligands and some of them demonstrated notable in vitro cytotoxic activity against various tumour cell lines [20–26]. It is interesting to note that in some cases palladium complexes were more active than their platinum analogues. The complex-forming ability of palladium(II) is very similar to that of platinum(II). The higher liability in ligand exchange at Pd-centre (105-fold with respect Pt) and hence rapid hydrolysis processes that lead to easy dissociation of complex and formation of very reactive species unable to reach their pharmacological

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2528

targets, could be overcome by introduction of the bulky heterocyclic ligands. Thus, a number of palladium complexes with aromatic N-containing ligands, e.g. derivatives of pyridine, quinoline, pyrazole and 1,10-phenanthroline, have shown very promising antitumour characteristics [14,27–30]. Continuing our study on complexes of quinoline-based alkyl phosphonates, in the present work we describe the synthesis and characterization of platinum(II) complexes of diethyl and monoethyl ester of 8-quinolylmethylphosphonic acid shown in Schemes 1 and 2, and a screening of their ability to inhibit the cancer growth in vitro in the epidermoid human KB and murine leukaemia L1210 cell lines. The results obtained were compared with those previously reported for the palladium complexes of the same 8quinolylmethylphosphonates [21,26] as well as with platinum(II) and palladium(II) complexes of diethyl and monoethyl 2-quinolylmethylphosphonates [20,23,25], and were discussed with respect to their metal-binding behaviour and structure–activity relationships. 2. Experimental 2.1. Materials and methods Diethyl 8-quinolylmethylphosphonate (8-dqmp), monoethyl 8-quinolylmethylphosphonate hydrochloride monohydrate (8-Hmqmp
HCl
H2 O) and sodium 8-quinolylmethylphosphonate dihydrate (Na(8-mqmp)
2H2O), prepared as previously described [31], were purified prior to use by recrystallization from absolute ethanol. Platinum halides were either Johnson–Matthey high purity products or were prepared following the literature methods. Solvents were purified and dried according to standard procedures. Melting points were determined on a hot stage microscope and are uncorrected. Infrared spectra were obtained on an ABB Bomem MB102 spectrometer using KBr (4000–250 cm 1) and polyethylene (400–200 cm 1) pellets. The 1H spectra were performed with Varian Gemini 300 Fourier-transform spectrometer in DMF-d7 at 300 K with tetramethylsilane as an internal reference. The twodimensional experiments were performed by standard pulse sequences, using Gemini Data System software Version 6.3 Revision A. Conductance measurements were carried out at room temperature using a CD 7A Tacussel conductance bridge for 10 3 mol 1 solutions in DMF and methanol. The thermogravimetric analyses

5

(TG) were carried out on a Cahn RG electromicrobalance in an air atmosphere at a heating rate of 4 °C min 1 up to 850 °C. Differential thermal analyses (DTA) were performed with a Netzsch 406 differential thermal analyser applying a heating rate of 5 °C min 1 in static air atmosphere. The reference substance was pure alumina. The samples were diluted with the reference substance in the ratio 1:1 by weight. Elemental analyses (C, H and N) were performed on a Perkin–Elmer Analyser PE 2400 Series 2 in the Ruder Boškovic´ Institute. The chlorine and bromine content was determined by the Schöninger method, phosphorus by the vanadomolybdatophosphoric acid spectrophotometric method, and water by the thermogravimetric analysis. 2.2. Synthesis of the complexes 2.2.1. [Pt(8-dqmp)2Cl2] (1) A mixture of 8-dqmp (0.210 g, 0.75 mmol) and Na2[PtCl4] (0.146 g, 0.38 mmol) in dry methanol (5 ml) was stirred under nitrogen at room temperature for 30 h. The reaction was carried out in the absence of light to avoid the presence of some brown to black side products in the final sample. The pale ochre precipitate that gradually formed over this period was filtered off, washed with cold methanol and dried under vacuum over P2O5. Yield 0.232 g, 75%; m.p. 135–137 °C. Anal. Calc. for C28H36Cl2N2O6P2Pt: C, 40.78; H, 4.40; N, 3.40; Cl, 8.60; P, 7.51. Found: C, 40.64; H, 4.20; N, 3.54; Cl, 8.51; P, 7.54%. Molar conductance KM (DMF; MeOH, 22 °C): 2.4; 4.2 S cm2 mol 1. TG data (temperature range, °C): 142–389 (dehalogenation and deesterification, mass loss for 2Cl + 4EtO found 29.1; calc. 30.5%); 287–780 (degradation processes). DTA data (°C): 330 exo, 470 exo. The preparation of this complex obtained by reaction of 8-dqmp with K2[PtCl4] in aqueous-ethanol solution in duration of few days (yield 45%) and its X-ray structure analysis, were previously described [32]. 2.2.2. [Pt(8-dqmp)2Br2] (2) A mixture of 8-dqmp (0.182 g, 0.65 mmol) and Na2[PtBr4] (0.185 g, 0.33 mmol) in dry methanol (5 ml) was stirred for 2 days under the same reaction conditions as for 1. An oily product obtained after evaporation of the solvent was dissolved in CH2Cl2 (5 ml), the solution was extracted with water (5 ml) and dried over anhydrous Na2SO4. Addition of benzene (3 ml) to the concentrated dry solution yielded an ochre precipitate which was separated,

4 3

6 7

N

2

[PtX 4] 2MeOH/N2

CH 2 EtO P O OEt

N CH2 X (EtO) 2OP Pt PO(OEt) 2 X CH 2 N

8-dqmp [PtX 4

] 2- +HX

1: X=Cl; 2: X=Br

MeOH/H 2O

+

+

CH2 EtO P O OEt

X

N H

X

Pt X X

2

3: X=Cl; 4: X=Br

2-

2MeOH Δ / N2

CH2 EtO P O OEt

X

N H

X

X Pt

X Pt

X

2 5: X=Cl; 6: X=Br

Scheme 1. Synthesis of platinum(II) complexes of 8-dqmp (1–6).

X

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5 H2C (EtO)O P

Pt O N

9

O

PO(OEt) CH2

[PtX 4]2pH>6

7 CH2 NaO P O OEt

+

4 3

6

N

2529

N

2 [PtX ]2-+HX 4 pH<3

CH2 HO P O OEt

2-

X

N H

X

Pt

X

X 2

7: X=Cl; 8: X=Br

Na(8-mqmp) Scheme 2. Synthesis of platinum(II) complexes of 8-Hmqmp (7–9).

washed with benzene and diethyl ether, and dried under vacuum over P2O5. Yield 0.208 g, 70%; m.p. 122–124 °C. Anal. Calc. for C28H36Br2N2O6P2Pt: C, 36.82; H, 3.97; N, 3.07; Br, 17.50; P, 6.78. Found: C, 36.62; H, 4.25; N, 3.38; Br, 17.59; P, 7.14%. Molar conductance KM (DMF; MeOH, 22 °C): 2.2; 4.6 S cm2 mol 1. TG data (temperature range, °C): 136–395 (dehalogenation and deesterification, mass loss for 2Br + 4EtO found 37.0; calc. 37.2%); 287–802 (degradation processes). DTA data (°C): 540 exo. 2.2.3. [8-Hdqmp]2[PtCl4]
2H2O (3) To a stirred solution of 8-dqmp (0.175 g, 0.63 mmol) in methanol (5 ml) was added dropwise avoiding any insolubilization a solution of Na2[PtCl4] (0.123 g, 0.32 mmol) in a mixture of methanol (5 ml) and water (1 ml) to which 1:1 HCl (0.5 ml) had been added. After vigorous stirring at room temperature for 4 h, the pale brown precipitate that formed was separated, washed with icecold water and methanol, and dried under vacuum. Yield 0.223 g, 76%; m.p. 173–175 °C. Anal. Calc. for C28H42Cl4N2O8P2Pt: C, 36.02; H, 4.54; N, 3.00; Cl, 15.19; P, 6.64%. Found: C, 36.12; H, 4.34; N, 3.28; Cl, 15.41; P, 6.50. Molar conductance KM (DMF; MeOH, 22 °C): 71.4; 208.2 S cm2 mol 1. TG data (temperature range, °C): 94–121 (dehydration, mass loss for 2H2O found 3.73; calc. 3.86%); 165–250 (dehalogenation and deesterification, mass loss for 2Cl + 4EtO found 25.2; calc. 26.9%); 330–436 (dehalogenation, mass loss for 2Cl found 6.1; calc. 7.6%); 450–816 (degradation processes). DTA data (°C): 120 endo, 185 endo, 430 exo, 455 exo. 2.2.4. [8-Hdqmp]2[PtBr4]
2H2O (4) This complex was prepared as a brown precipitate by almost the same procedure as complex 3 by reaction of 8-dqmp (0.203 g, 0.73 mmol) and a mixture of Na2[PtBr4] (0.207 g, 0.37 mmol) and conc. HBr (0.4 ml). Yield 0.324 g, 80%; m.p. 161–162 °C. Anal. Calc. for C28H42Br4N2O8P2Pt: C, 30.26; H, 3.81; N, 2.52; Br, 28.76; P, 5.57. Found: C, 30.12; H, 4.08; N, 2.28; Br, 28.41; P, 5.50%. Molar conductance KM (DMF, MeOH, 22 °C): 72.1, 207.8 S cm2 mol 1. TG data (temperature range, °C): 87–120 (dehydration, mass loss for 2H2O found 3.31; calc. 3.24%); 170–240 (dehalogenation and deesterification, mass loss for 2Br + 4EtO found 29.3; calc. 30.6%); 340–400 (dehalogenation, mass loss for 2Br found 13.7; calc. 14.4%); 420–816 (degradation processes). DTA data (°C): 100 endo, 155 endo, 480 exo, 520 exo. 2.2.5. [8-Hdqmp]2[Pt2Cl6] (5) This complex was prepared by heating the tetrachloridoplatinum complex 3 in methanol at reflux temperature for 1 h under a stream of nitrogen. The hot solution was filtered to remove some black decomposition products. The ochre precipitate that gradually formed on cooling was isolated, washed with cold methanol and dried under vacuum at room temperature. M.p. 180–183 °C. Anal. Calc. for C28H38Cl6N2O6P2Pt2: C, 28.90; H, 3.29; N, 2.41; Cl, 18.28; P, 5.32. Found: C, 28.62; H, 3.18; N, 2.26; Cl, 18.41; P, 5.51%. Molar

conductance KM (DMF, MeOH, 22 °C): 80.1, 197.4 S cm2 mol 1. TG data (temperature range, °C): 160–230 (dehalogenation and deesterification, mass loss for 2Cl + 4EtO found 23.5; calc. 21.6%); 260–370 (dehalogenation, mass loss for 4Cl found 7.5; calc. 8.4%); 390–800 (degradation processes). DTA data (°C): 190 endo, 460 exo. 2.2.6. [8-Hdqmp]2[Pt2Br6]
2H2O (6) This complex was prepared as a pale reddish-brown compound by the similar procedure as its chloro analogue 5, by heating the tetrabromidoplatinum complex 4 in methanol at reflux temperature under an inert atmosphere. M.p. 170–174 °C (decomp.). Anal. Calc. for C28H42Br6N2O8P2Pt2: C, 22.94; H, 2.89; N, 1.91; Br, 32.70; P, 4.23. Found: C, 23.22; H, 2.73; N, 2.12; Br, 33.01; P, 4.19%. Molar conductance KM (DMF, MeOH, 22 °C): 79.9, 171.8 S cm2 mol 1. TG data (temperature range, °C): 100–128 (dehydration, mass loss for 2H2O found 3.12; calc. 2.46%); 140–230 (dehalogenation and deesterification, mass loss for 2Br + 4EtO found 26.2; calc. 23.2%); 250–360 (dehalogenation, mass loss for 4Br found 15.7; calc. 15.1%); 370–780 (degradation processes). DTA data (°C): 185 endo, 510 exo. The complexes 7 and 8 were prepared either by reaction of 8Hmqmp
HCl
H2O or Na(8-mqmp)
2H2O with platinum(II) halide compounds. 2.2.7. [8-H2mqmp]2[PtCl4] (7) (a) A concentrated aqueous solution of K2[PtCl4] (0.177 g, 0.43 mmol, 5 ml) was added dropwise to a stirred solution of 8-Hmqmp
HCl
H2O (0.256 g, 0.84 mmol) in water (3 ml). The reaction mixture was continuously stirred for 16 h at room temperature. The pale reddish-brown precipitate that gradually formed was filtered off, washed with ice-cold water and methanol, and dried in vacuum over P2O5. Yield 0.296 g, 84%. (b) PtCl2 (0.112 g, 0.42 mmol) was dissolved in water (3 ml) by addition of 1:1 HCl (0.6 ml) and heating for 30 min at 50 °C. After filtration to remove any impurities, the cooled solution was added dropwise to a concentrated solution of Na(8-mqmp)
2H2O (0.250 g, 0.81 mmol) in water (3 ml). An additional constant stirring for 12 h at 4 °C yielded precipitate, which was separated, washed and dried as described. Yield 0.304 g, 86%; m.p. 178–181 °C. Anal. Calc. for C24H30Cl4-N2O6P2Pt: C, 34.26; H, 3.59; N, 3.33; Cl, 16.86; P, 7.36. Found: C, 34.62; H, 3.84; N, 3.38; Cl, 16.51; P, 7.24%. Molar conductance KM (DMF; MeOH, 22 °C): 74.4; 209.9 S cm2 mol 1. TG data (temperature range, °C): 184–230 (dehalogenation and deesterification, mass loss for 2Cl + 2EtO found 20.9; calc. 19.1%); 240–350 (dehalogenation, mass loss for 2Cl found 13.5; calc. 12.2%); 450–806 (degradation processes). DTA data (°C): 205 endo, 395 exo, 450 exo, 500 exo.

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2.2.8. [8-H2mqmp]2[PtBr4]
2H2O (8) (a) A mixture of K2[PtBr4] (0.255 g, 0.43 mmol) and 1:1 HBr (1.0 ml) in water (4 ml) was slowly added to a stirred solution of 8-Hmqmp
HCl
H2O (0.260 g, 0.85 mmol) in water (5 ml). The reaction mixture was additionally stirred for 24 h at room temperature, the ochre precipitate that formed was filtered off, washed with ice-cold water and dried under vacuum. Yield 0.330 g, 74%; (b) A mixture of PtBr2 (0.149 g, 0.42 mmol) and 1:1 HBr (1.2 ml) in water (4 ml) was stirred for 1 h, filtered and then was added dropwise to a solution of Na(8-mqmp)
2H2O (0.250 g, 0.81 mmol) in water (3 ml). After continuous stirring for 8 h, the formed precipitate was isolated, washed and dried as described. Yield 0.340 g, 80%; m.p. 171–173 °C. Anal. Calc. for C24H34Br4N2O8P2Pt: C, 27.32; H, 3.25; N, 2.66; Br, 30.29; P, 5.87. Found: C, 27.59; H, 3.51; N, 2.79; Br, 30.62; P, 6.06%. Molar conductance KM (DMF, MeOH, 22 °C): 70.1, 206.6 S cm2 mol 1. TG data (temperature range, °C): 80– 114 (dehydration, mass loss for 2H2O found 3.78; calc. 3.41%); 150–230 (dehalogenation and deesterification, mass loss for 2Br + 2EtO found 24.5; calc. 23.7%); 320–400 (dehalogenation, mass loss for 2Br found 23.7; calc. 21.8%); 430–825 (degradation processes). DTA data (°C): 160 endo, 570 exo. 2.2.9. [Pt(8-mqmp)2]
2H2O (9) A concentrated aqueous solution of Na2[PtCl4] (0.146 g, 0.38 mmol, 4 ml) was added dropwise to a stirred solution of Na(8-mqmp)
2H2O (0.228 g, 0.74 mmol) in water (4 ml). The reaction mixture was continuously stirred for two weeks at room temperature and then filtered to remove any impurities. A very hygroscopic residue, obtained after all the solvent was removed, was dried in vacuum and dissolved in DMF (5 ml). A small amount of dark residue was removed by filtration, and addition of CH2Cl2 (3 ml) to the filtrate afforded the dark yellow precipitate that was separated, washed with cold CH2Cl2 and diethyl ether, and dried under vacuum over P2O5. Because of its hygroscopity, the handling of this complex was performed under nitrogen in a drybox. Yield 0.149 g, 55%; m.p. > 200 °C (decomp.). Anal. Calc. for C24H30N2O8P2Pt: C, 39.40; H, 4.13; N, 3.83; P, 8.47. Found: C, 39.72; H, 4.59; N, 4.12; P, 8.39%. Molar conductance KM (DMF, 22 °C): 0.7 S cm2 mol 1. TG data (temperature range, °C): 50–134 (dehydration, mass loss for 2H2O found 5.00; calc. 4.93%); 172– 734 (degradation processes). DTA data (°C): 435 exo, 480 exo. 2.3. In vitro antitumour assays In vitro antitumour activity of the complexes was evaluated against KB cell line derived from an oral epidermoid human carcinoma and murine leukaemia L1210 cell line. As the reference compound was used cispaltin, cis-[Pt(NH3)2Cl2]. The KB or L1210 cells were sown at a density of 3  104 cells ml 1, in 0.2 ml per well in a 96-well plastic plate (Corning Costar, Milano, Italy). After 24 h the platinum compounds were dissolved in sterile acetone and solutions were diluted up to opportune concentrations (2.50, 5.00, 7.50, 10.00, 20.00 lg ml 1) by using a culture medium. Nutritive medium of every well was substituted with 0.2 ml of different solutions. The samples were allowed to incubate for 72 h at 37 °C and the cellular vitality was valued with a colorimetric assay based on the quantification with sulforhodamide B (SRB) of cellular protein component [33]. The optical density was measured at 570 nm with an automated microplate EL311 reader (Bio-Tek Instruments, USA). Each experiment was performed in quintuplicate and repeated twice. Cytotoxic activity was valued as percentage of the cellular growth inhibition in culture treated with complex compounds

with respect to the growth observed in the control culture. Data were analysed using the Student unpaired t-test and significance was accepted at which tumour cell growth showed 50% inhibition (IC50), and was calculated by the PCS programme [34].

3. Results and discussion 3.1. Synthesis and properties The complex-forming ability of 8-quinolylmethylphosphonates towards platinum (II) ion was investigated by reactions of diethyl ester (8-dqmp) and monoethyl ester (8-Hmqmp), either in the form of hydrochloride [8-Hmqmp]
HCl
H2O or sodium salt Na(8mqmp)
2H2O, with platinum(II) halides. Depending on the reaction conditions these organophosphorus ligands form molecular or ionic complexes. Diethyl ester from the methanolic solution gave the square-planar dihalide adducts trans-[Pt(8-dqmp)2X2] (X = Cl, Br) with the ligand bonded through the quinoline nitrogen (1 and 2), which was supported by X-ray structure determination of complex 1 [32]. The ion-pair salt complexes [8-Hdqmp]2[PtX4] (3 and 4) containing the protonated quinoline ligand as cation and tetrahalidoplatinate complex as anion were obtained from the HX acidic methanol-aqueous solution (Scheme 1). By heating in methanol complexes 3 and 4 were converted into the corresponding dimeric hexahalidodiplatinum complexes [8-Hdqmp]2[Pt2X6] (5 and 6). Monoethyl ester could form two types of complexes. The salt complexes [8-H2mqmp]2[PtX4] (7 and 8) were obtained under acidic aqueous solution both, from the sodium and hydrochloride salt of this monoester. The chelate complex [Pt(8mqmp)2] (9), with monoester ligand bonded through the quinoline nitrogen and the deprotonated phosphonic acid oxygen by forming two seven-membered N,O-chelate rings, was obtained from sodium salt in neutral and basic media (Scheme 2). It is worth noting that the corresponding palladium complex was prepared under the same reaction conditions, as recently reported, and its structure was determined by the X-ray diffraction [26]. Comparing the complex-forming behaviour of 8-dqmp and 8-Hmqmp towards platinum that to palladium, there are no differences in types of complexes formed with diester ligand [21], but in the case of monoester ligand, palladium forms four types of complexes with respect to only two types of platinum complexes [26]. Besides the N,O-chelate complex and ionic tetrahalidopalladate complexes, also hexahalidodipalladate complexes [8-H2mqmp]2[Pd2X6] and the quinolinium salt complexes [8-H2mqmp] [Pd(8-Hmqmp)X3] containing the quinoliniummethylphosphonatetrihalidopalladate complex as anion with palladium bonded at the phosphonic acid moiety, were isolated. Duration of most reactions applied for synthesis of platinum complexes is longer than that for synthesis of the corresponding palladium complexes, especially molecular complexes, as palladium is much more reactive than platinum [26]. In addition, it is worth noting that monoethyl ester of 8-quinolylmethylphosphonic acid could not be isolated in the molecular form, but only as hydrochloride salt, and all attempts to prepare platinum and palladium dihalide adducts of this ligand with metal bonded only through the quinoline nitrogen were unsuccessful. When comparing complexation of 8-quinolylmethylphosphonates with respect to the corresponding diethyl and monoethyl 2-quinolylmethylphosphonates, greater differences could be observed. Diester 2-dqmp forms platinum dihalide adducts and tetrahalidoplatinate complexes [25], while palladium forms also dihalide complexes cis-[Pd(2-dqmp)X2] with the ligand bonded through the quinoline nitrogen and the phosphoryl oxygen [20]. Monoester 2-Hmqmp depending on pH exhibits either the molecular, zwitterion or the anionic form, and therefore according to the

L. Tušek-Bozˇic´ et al. / Polyhedron 29 (2010) 2527–2536

complex. The structure of the complexes, as well as their stability and mode of decomposition, were deduced mainly from their spectroscopic and thermal studies.

acidity of the reaction solution forms different types of platinum(II) complexes; dihalide adducts, ion-pair tetrahalidoplatinate and hexahalidodiplatinate salt complexes as well as N,O-chelate complex [25]. On the other side, palladium forms dihalide adducts, ionic trihalidopalladates and hexahalidodipalladates, and the chelate complex [35]. Differences in the complex-forming behaviour between diester and monoester ligands mainly arise from the presence of the phosphonic acid group in monoester derivatives, which enables these ligands to coordinate to the metal ion in either their molecular or anionic form. On the other side, differences between complexation of 8-Hmqmp and 2-Hmqmp could be ascribed partly to different basicity of the quinoline nitrogen in these ligands and hence their different structural properties. Thus, under the same experimental conditions 8-Hmqmp was isolated as the hydrochloride salt, while 2-Hmqmp has inner-salt character with the quinoline group being protonated and the phosphonic group being ionized [35]. The prepared complexes are yellow to brown microcrystalline or powder like diamagnetic compounds, slightly soluble or soluble in DMF and DMSO and slightly soluble or insoluble in other common solvents. The molar conductance data in methanol and DMF are consistent with the proposed structure of complexes. The values of KM < 5 S cm2 mol 1 for complexes 1 and 2 in both solvents correspond to those of non-electrolyte compounds, all salt complexes (3–8) exhibit conductivities comparable to those of 2:1 electrolytes (KM = 172–210 in methanol and 70–80 in DMF), while the very low conductance of complex 9 (KM < 1 in DMF) confirms the presence of deprotonated phosphonic acid group in this chelate

3.2. IR spectra The IR frequencies associated with the main functional groups of the complexes along with those of the free 8-dqmp and hydrochloride and sodium salt of 8-Hmqmp are given in Table 1. Generally, upon complexation marked changes in the ligand bands may be noticed in the region of 1600–1500 cm 1 where occur bands arising from the stretching modes of the quinoline C@C and C@N bonds [21,26]. The variation in position and intensity of these absorptions could be ascribed to p electronic redistribution in the heterocyclic ligand caused either by metallation (complexes 1, 2 and 9) or protonation (complexes 3–8) of the quinoline nitrogen. Thus, while free 8-dqmp has three rather weak bands around 1600 cm 1 and a more intense one at 1500 cm 1 [21], its dihalidoplatinum complexes 1 and 2 contain only one band at 1600 cm 1 and that around 1515 cm 1. There are no great changes in the position of absorption bands of the phosphonic ester group as it is not involved in the metal coordination. Tetrahalidoplatinum and hexahalidodiplatinum complexes with the protonated quinoline nitrogen, show a sharp medium band at about 1635 cm 1 and a very broad complex progression of absorptions covering the whole 2900–2400 cm 1 region due to the m(NH+) vibration. Differences in position of the phosphoryl absorption in these complexes may arise from the weak hydrogen bonding between the NH+ and

Table 1 Selected infrared spectroscopic data for quinolylmethylphosphonates and their platinum complexes (cm Compound

m(OH)b (H2O)

m(P@O) mas(PO2 )

m(PO–C) msym(PO2 ) m(PO–H)

w w w m-s w m-s w m-s m-s m-s m-s m-s m-s m-s

1265 sh 1245 s

1050 vs 1022 vs

1248 s

2900–2400 m-s, br 1634 m 2900–2400 m-s, br 1638 m 2900–2400 m-s, br 1636 m

1616 1598 1578 1500 1600 1516 1601 1514 1596 1559 1603 1564 1603 1570

1060 1028 1055 1020 1054 1018 1058 1028 1056 1028

1602 1567 1598 1560

m m s s

2

4

m-s m-s m-s m-s

5

6 (8-Hmqmp)
HCl
H2O

3480 m-s 3340 m-s 3250 w m, br

Na(8-mqmp)
2H2O

3400 w m, br

7

a b c

8

3358 m 3280 m

9

3385 m-s, br

).

m(C@C) m(C@N)

1

3490 3320 3508 3320

1 a

m(NH+) m[P(O)OH]

8-dqmp

3

2531

1240 s

2900–2400 m, br 2360 w, br 1635 s

1598 s 1568 s

1240 1212 1240 1218 1264 1233 1222 1266 1222 1234 1220 1210 1250 1225 1200 1235 1228

2900–2400 m, br 2350 w, br 1630 s

1596 s 1567 s

1220 m 1192 vs

1598 w 1513 m-s

1225 m-s 1217 vs 1196 s

2900–2400 m-s, br 1636 m 2900–2400 m, br 2050–1750 w, br 1630 m-s

1598 w 1575 m

sh vs, br sh vs, br s m m-s s s vs s sh sh s s m-s, br sh

s vs s s s vs s vs vs vs

m(Pt–X)b

m(Pt–N)

344 m

252 w

250 w, brc

250 w, brc

319 m-s 230 m 322 w298 w

1052 vs 1028 vs 1040 sh 1035 s, br 994 vs, br 1055 vs, br

230 m

1039 m-s 1020 s 1002 vs 989 vs 1032 vs 1021 vs 987 s 1044 vs 1032 vs 1015 s-vs 991 m-s

329 m-s

As KBr pellets. Abbreviations: vs, very strong; s, strong; m, medium; w, weak; br, broad; vbr, very broad; sh, shoulder. X = Cl, Br. Overlapped absorptions.

243 m

261 w

L. Tušek-Bozˇic´ et al. / Polyhedron 29 (2010) 2527–2536

2532

[Pt2X6]2 (5 and 6) anions [38]. Thus, one m(Pt–X) band is visible for the complexes 1–4, 7 and 8, two bands there are for the chloride complex 5, while for the corresponding bromide complex 6 only one band is found in the examined range, as the other band is expected to be bellow 200 cm 1. One m(Pt–N) band within 250–258 cm 1 in complexes 1, 2 and 9 confirms platinum coordination through the quinoline nitrogen in a trans-orientation. It is worth noting that the presented spectroscopic data of complexes are in good agreement with those obtained for the corresponding types of the molecular and ionic palladium complexes of the same 8-quinolylmethylphosphonate ligands [26], as well as with those of the palladium and platinum complexes of 2-quinolylmethylphosphonates [20,23,25].

P@O oxygen or that with the lattice water present in the hydrated complexes. In the ion-pair salt complexes 7 and 8 there is extensive vibrational coupling and overlapping of absorptions arising from the presence of the P(O)OH group. In the m(NH+) regions occur also absorptions characteristic for the intermolecular hydrogen bonded phosphonic acidic group [36,37], giving a complex band within 3360–3280 cm 1, less intense broad absorption at about 2350 cm 1, and a strong sharp band at 1635 cm 1 to which also contribute deformation modes of the lattice water. Furthermore, the spectra show a great complexity between 1250 and 950 cm 1. Displayed bands in this region are attributed to stretching modes of the P@O, P–O–C and PO–H vibrations, along with those of the PO2 group. As expected, the spectrum of complex 9 with the N,O-bidentate bonded ligand in the ionic form shows changes in the quinoline ring stretching vibrations and is characterized by the strong m(PO2 ) absorptions within 1235–1190 cm 1 for the antisymmetric and within 1040–990 cm 1 for the symmetric mode of this vibration. In the latter frequency range the m(P–O–C) absorption is superimposed upon that of the symmetric PO2 vibration. The similar spectral pattern was observed for the corresponding chelate palladium(II) complex [26]. The hydrated ionic complexes 3, 4 and 6–8 in the m(OH) region show two medium to strong bands in the 3500–3280 cm 1 region, while in the complex 9, similarly as in the hydrochloride and sodium salt of 8Hmqmp, only one broad band between 3400 and 3250 cm 1 is present, indicating strong hydrogen bonding in these compounds. The far-IR spectra of the complexes are consistent with the spectral data obtained for the trans-square-planar dihalidoplatinum complexes (1 and 2) as well as with those containing the squareplanar monomeric [PtX4]2 (3, 4, 7 and 8) or dimeric halidobridged

3.3. NMR spectra The 1H NMR spectral data for platinum complexes obtained in DMF-d7 solutions are summarized in Table 2 according to the numbering scheme shown in Schemes 1 and 2. Spectral data of the free 8-dqmp and sodium salt Na(8-mqmp)
2H2O provided a basis for interpreting spectra of the complexes [26,39]. In the spectra of complexes the resonances of the most of protons show a certain high-frequency shift with respect to the uncomplexed quinolylmethylphosphonates. These changes could be ascribed to the influence of the electronic and steric effects and are related to the decrease in the electron density in the heterocyclic aromatic ring caused by the metal coordination of the quinoline nitrogen in the neutral complexes (1, 2 and 9) or by its protonation in the ionic complexes (3–8), as well as by the proximity of the platinum atom which is known to be magnetically anisotropic bringing

Table 2 1 H NMR spectral data [d (ppm), J (Hz)].a Proton

8-dqmp

1b

2c

3

4

5

6

Na(8-mqmp)
2H2O

7

8

9b

H-2

9.15 dd J = 4.0, 1.6

10.31 d JHH = 4.6 (9.85–10.35) 7.79d (7.93–8.21) 9.12 d 3 JHH = 8.3 (9.02–9.28) 8.13 d 3 JHH = 8.2 (8.10–8.35) 7.81d (7.70–8.02)

10.16 d 3 JHH = 4.8

9.65 d 3 JHH = 4.6

9.46 d 3 JHH = 4.5

3

9.21 d JHH = 4.6

3

9.41 d JHH = 4.6

9.10 dd J = 4.6, 1.6

9.51 d 3 JHH = 4.8

9.64 d 3 JHH = 4.2

7.81 dd J = 8.1, 4.6 9.14 d 3 JHH = 8.4

8.21 dd J = 8.2, 4.9 9.18 d 3 JHH = 8.5

8.11 dd J = 8.2, 4.7 9.03 d 3 JHH = 8.3

7.92 dd J = 8.2, 4.6 8.84 d 3 JHH = 8.4

7.94 dd J = 8.2, 4.7 8.82 d 3 JHH = 8.3

7.59 dd J = 7.5, 3.6 8.45 dd J = 8.4, 1.6

8.22e 9.14 d 3 JHH = 8.3

8.38 dd J = 8.4, 5.1 9.12 d 3 JHH = 8.1

10.18 d JHH = 4.6 (9.88–10.31) 8.14f

8.11 d 3 JHH = 8.1

8.23 d 3 JHH = 8.0

8.19 df

8.15 d 3 JHH = 8.3

3

8.14 d JHH = 8.3

7.88 d 3 JHH = 8.1

8.38 d 3 JHH = 7.8

8.49 d 3 JHH = 8.3

7.87 t 3 JHH = 8.0

8.00 t 3 JHH = 7.7

7.95 t 3 JHH = 7.7

3

7.88 t JHH = 7.8

3

7.90 t JHH = 7.7

7.60 t 3 JHH = 7.5

8.00 t 3 JHH = 7.5

8.05 t 3 JHH = 7.8

8.88 d JHH = 7.2 (8.65–9.02) 6.66 d 2 JPH = 21.2 (6.32–6.70) 4.40 m (3.60–4.51) 1.30 t 3 JHH = 7.1 (0.80–1.35)

8.77 d 3 JHH = 7.1

8.39 d 3 JHH = 8.0

8.32 d 3 JHH = 7.7

8.28 d JHH = 7.2

8.36 d JHH = 7.3

3

8.22 d 3 JHH = 7.5

8.23e

3

8.31 dd J = 6.6, 4.1

5.82 d 2 JPH = 20.2

4.25 dg 2 JPH = 24.0

4.24 dg JPH = 21.6

4.26 dg 2 JPH = 23.8

4.25 dg 2 JPH = 24.0

3.94 dg JPH = 20.1

4.17 dg 2 JPH = 21.6

4.21 dg 2 JPH = 21.3

4.52 m

4.27 mg

4.21 mg

4.29 mg

4.19 mg

H-3 H-4

H-5

H-6

7.75 ddd J = 8.4, 4.9 8.58 dd J = 8.1, 1.6 8.09 dd J = 8.1, 1.6 7.77 td JHH = 7.8

3

H-7

PCH2

8.04 dd J = 6.7, 3.7 4.20 dg JPH = 22.0

2

OCH2

4.17 mg,h JHH  3JPH  7 1.28 t 3 JHH = 7.1

3

CH3

NH+, H2O a b c d e f g h

3

3

2

2

3.93 mg

1.30 t 3 JHH = 7.1

1.30 t 3 JHH = 7.1

3

1.29 t JHH = 7.1

3

1.29 t JHH = 7.1

0.98 t 3 JHH = 6.9

4.18 mg JHH  3JPH  7 1.32 t 3 JHH = 7.1

4.32 s br

6.71 s br

7.4 s br

5.68 s br

3.60 s br

7.81 s br

3

1.22 t 3 JHH = 7.1

4.23 mg JHH  3JPH  7 1.31 t 3 JHH = 7.0

3

9.09 d JHH = 8.1 (8.82–9.11) 8.52 d 3 JHH = 8.0 (8.41–8.60) 7.71 t 3 JHH = 7.6 (7.67–7.82) 8.17f 3

6.90 d 2 JPH = 23.2 (6.58–7.02) 3.62–4.10 m

3

8.92 s br

Spectra recorded in DMF-d7 at room temperature: s, singlet; d, doublet; t, triplet; dd, doublet of doublets; m, multiplet; br, broad signal. Data for the most abundant isomer. Presence of more isomeric species brings about multiple signals in the region given in parentheses. One isomer is predominant. Partly overlapped H-3 and H-6. Tentative value obtained from a COSY experiment. Partly overlapped H-3 and H-7. Tentative value obtained from a COSY experiment. Partly overlapped DMF-d7 absorption. Tentative value obtained from a COSY experiment. Overlapped OCH2 and PCH2. Visible as quintet.

1.28 t JHH = 7.0 (0.84–1.32) 4.35 s br 3

L. Tušek-Bozˇic´ et al. / Polyhedron 29 (2010) 2527–2536

2533

Fig. 1. The H-2 proton region in the 1H NMR spectra of 8-dqmp and its platinum(II) bromide complexes: (a) 8 dqmp; (b) 2; (c) 4 and (d) 6.

about deshielding effects [40]. In general, the magnitude of the shift changes decreases with the distance from the coordination or protonation site and is smaller in the ionic complexes due to the lack of platinum bonding to the quinoline ligand. Therefore, the most pronounced shift was found for the aromatic protons H-2, H-4 and H-7 and PCH2 protons, which varies in the molecular complexes within 0.78–1.16, 0.56–0.74, 0.68–0.84 and 1.62– 3.06 ppm, respectively. In the ionic complexes these protons exhibit a smaller frequency shift from 0.04 to 0.69 ppm. As an example, the H-2 proton region of 8-dqmp and its bromide complexes is presented in Fig. 1. The spectra of the molecular complexes show a greater complexity than those of the ionic complexes, as in these compounds the restricted rotation around the Pt–N coordination bond may give rise to the presence of more isomeric forms with different magnetic environment in dynamic equilibrium. The similar spectroscopic feature was found for a lot of platinum(II) and palladium(II) complexes of various quinoline and aniline derivatives, which was confirmed by variable temperature measurements [41–43]. The distribution of rotamers depends on the halogen present in the complex and on the solvent used. In dibromide adduct 2 one isomer predominates while in the spectrum of dichloride adduct 1 more isomers are visible. The abundance of the main isomeric form is ca. 80% in DMF and could be ascribed to the less sterically hindered form. The connectivity from 2D NMR experiments corroborate the assignments of all protons in the major isomer which are given in Table 2 along with the tentative spectral range covering absorption of all isomers of each proton. The solvent redistribution of rotamers for H-4 proton in the spectra of complex 1 in DMF and chloroform solutions is shown in Fig. 2. It is worth noting that orientation of the phosphonate groups in the chloride and bromide complexes could be markedly different, as it was found in the case of the palladium dichloride and dibromide adducts of 2-quinolylmethylphosphonate (2-dqmp) by their crystal structure analysis [20]. The similar situation may be proposed for complexes 1 and 2, in spite of fact that only for complex 1 the molecular structure could be determined by crystallographic study [32]. A very short methylene proton–platinum distance was found in this complex, which is in accordance with a great downfield shift of 2.42 ppm observed for PCH2 protons with respect to

the free 8-dqmp ligand. In the dibromide complex this shift is only 1.62 ppm. Concerning the hindered metal–ligand rotation, the situation is even more complicated in complex 9 due to the proximity of the bulky phosphonate ligands groups as a result of the N,O-ligand bonding to platinum. As a consequence either the broadening or the presence of few sets of signals could be observed, and population of the major isomeric species is about 70%. The greatest downfield shift was observed for the aromatic H-2 (1.08 ppm) and PCH2 (2.96 ppm) protons. The signal of NH and OH protons in all complexes gives a broad singlet. It should be noted that in the molecular complexes owing to the presence of more isomers the long-range couplings with platinum could not be resolved. An evidence for the 4J(Pt,H) coupling was indicated only in complex 2 for the H-2 resonance by small broad shoulders at the base of the central peak (Fig. 1b).

Fig. 2. The H-4 proton region in the 1H NMR spectra of 1 in (a) DMF-d7 and (b) CDCl3.

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3.4. Thermal study Decomposition behaviour of the complexes was investigated by the thermal measurements (TG, DTA) accompanied by the infrared spectroscopy study. The spectra were recorded every 50 °C and are compared with the corresponding spectra obtained at room temperature. In the case of metal halide complexes information about dehalogenation of the complex is interesting in view of the importance of the Pt/Pd–halogen bond strength for the binding of the complexes to DNA strands, which are supposed to be the prime target for the Pt drugs. A smaller effect might be expected when the leaving ability of the labile ligands is lower. In the biological milieu the drug undergoes hydrolyses whereby the halide ligands are displaced by water molecules, thus yielding charged monoand diaqua complexes, which in turn promptly react with the cellular microphyles [3]. Thermal decomposition of the complexes takes place through a multistep process and the results obtained are summarized in Experimental section. The hydrated complexes 3, 4, 6 and 8 exhibit a single dehydration step between 80 and 130 °C with the mass loss close to the evolution of two water molecules. In the IR spectra this is indicated by a disappearance of rather strong m(OH) absorptions between 3500 and 3280 cm 1. Degradation of the halide complexes begins by a dehalogenation process overlapped with the ligand deesterification, and is influenced by the type of complex. In the DTA curves these processes are visible as endothermic peaks between 160 °C and 205 °C. In dihalide adducts of 8-dqmp this step covers approximately the range between 140 and 400 °C. The weight loss corresponds well with evolution of two halide ions and four ethyl ester groups. Dehalogenation in tetrahalidoplatinum and hexahalidodiplatinum complexes occurs in two steps, as revealed by the very distinct inflections in the TG curves. In tetrahalidoplatinum complexes of 8-dqmp, the first step ranged up to 230–250 °C corresponds to the loss of two halogens and four ethyl ester groups, while in those of 8-Hmqmp this step displayed up to ca. 200–230 °C corresponds to the loss of two halogens and two ethyl ester groups. In hexahalidodiplatinum complexes 5 and 6 the first step ranged up to 210–230 °C corresponds to evolution of two halogens and four ethyl ester groups. The mass loss in the second step in the ionic complexes, which ranged up to ca. 350–400 °C, corresponds approximately to the release of two and four, respectively, remaining halogens. The complete dehalogenation and deesterification of the complexes is confirmed by the absence of the m(Pt–Cl/Br), P–O–Et and C–C ethyl absorptions in the IR spectra. The pyrolytic decomposition of the complexes is followed by a continuous mass loss with exothermic or unclear peaks in the DTA curves. In the chelate complex 9 a broad dehydration step between 55 and 125 °C, which indicates that two water molecules are lattice-held [26,44], is followed by decomposition of the complex including deesterification and other ligand degradation processes in the 200–500 °C region visible in the DTA curve as a broad exothermic effect around 460 °C. The pyrolytic residue in all the complexes is a mixture of Pt and P2O5. The platinum metal was identified as the pyrolytic residue in a number of platinum complexes [45,46]. Difference obtained between the calculated and found residue value in complexes could be ascribed to partial sublimation of P2O5, which takes place at higher temperatures [47]. 3.5. Antitumour activity The antitumour activity of the 8-dqmp and 8-Hmqmp complexes was assayed in vitro against the human epidermoid KB and murine leukaemia L1210 cell lines. The results of the platinum(II) complexes 1–9, expressed as IC50 values, are listed in Table 3 together with those of the palladium(II) complexes of the same

8-quinolylmethylphosphonate ligands [21,26] as well as with the corresponding data of the platinum(II) and palladium(II) complexes of diethyl (2-dqmp) and monoethyl (2-Hmqmp) 2-quinolylmethylphosphonates [20,23,25]. These previously reported data are given in order to correlate the structural factors that influence on the complex activity of this class of organophosphorus compounds. As the cytotoxicity of free 8-dqmp and Na(8mqmp)
2H2O tested against the L1210 line was found to be very low, with IC50 value of >50 and >100 lM, respectively, it could be presumed that the free ligands do not participate a lot in activity of the complexes. It was shown that in the case of platinum(II) complexes of diester 8-dqmp there are no significant differences in antiproliferative activity between the molecular complexes (1 and 2) and the two types of the ionic complexes (3–6), neither between the chloride and bromide analogues. In general, most of the complexes are slightly less cytotoxic to the KB cell line (IC50 9.7–15.4 lM) than to the L1210 cell line (IC50 7.0–11.4 lM). The similar findings were observed in the platinum(II) complexes of diester 2-dqmp with the IC50 concentrations within 13.3–14.8 in KB and 8.3–9.8 lM in L1210 cells [25]. The only exception is tetrabromidoplatinum complex with IC50 values higher than 100 lM. All platinum complexes of monoester 8-Hmqmp (7–9) as well as all types of platinum complexes of 2-Hmqmp have also shown marginal effectiveness with concentration values of >100 lM [25]. The situation is somewhat different in the case of palladium complexes of the same phosphonate ligands. While all types of palladium complexes of 8-dqmp were found to exhibit cytotoxicity in a similar concentration range as their platinum analogues [21], greater differences could be observed in palladium complexes of 2-dqmp [20]. There is much greater difference in response between the chloride and bromide complexes as well as between the KB and L1210 cell lines. Thus, dibromido and tetrabromidopalladium complexes of 2-dqmp are ca. 3–24 times more effective than their chloride analogues, and are the most effective investigated palladium complexes with IC50 values in the lower micromolar range in the L1210 cell (3 lM). Only dibromidopalladium complex displays a strong cytotoxicity in both test systems (12.7 and 3.3 lM, respectively). With respect to the platinum complexes of 8-Hmqmp and 2-Hmqmp, which activity is very weak with IC50 >100 lM, most of palladium complexes of these monoesters showed a medium antiproliferative activity against the examined cell lines (15.3–63.4 lM in KB and 8.1–30.1 lM L1210 cells) [23,26]. Only exceptions are tetrachloridopalladium and hexachloridodipalladium complexes of 8-Hmqmp and dibromidopalladium complex of 2-Hmqmp with IC5 values >100 lM in KB cell line. Some structure–activity correlations could be derived when comparing results of the antitumour assays related to quinolylmethylphosphonate complexes. The nature of the phosphonate ligand appears to have important influence on overall activity of the complexes, since complexes of diethyl phosphonates in general appear to be more active than those of monoester derivatives, which may be partly explained by the greater solubility and lipophilicity of the former complexes. These differences are more pronounced in the platinum than in the palladium complexes as well as in the KB than in the L1210 cell line. The lipophilicity of the complexes increases with increasing bulkiness from monoester to diester derivatives and may facilitate transport through the cellular membranes [48,49], while the presence of the acidic phosphonate group in monoester complexes may cause the opposite effect. Similar findings were observed for the antitumour activity of palladium complexes with dialkyl and monoalkyl esters of some aniline-based phosphonates [22,24]. The differences between activity of chloride and bromide complexes, observed in some cases, may be ascribed to their different solubility as well to differences in lability between the chloride and bromide groups, as the

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2535

Table 3 In vitro antitumour activity of Pt(II) and Pd(II) complexes with quinolylmethylphosphonates. Complex

IC50 (lg/mL)a KB cellsb

L1210 cellsc

d

e

8Q [Pt(dqmp)2Cl2] [Pd(dqmp)2Cl2] [Pt(dqmp)2Br2] [Pd(dqmp)2Br2] [Pd(dqmp)Cl2] [Pd(dqmp)Br2] [Hdqmp]2[PtCl4]
2H2O [Hdqmp]2[PdCl4]
2H2O [Hdqmp]2[PtBr4]
2H2O [Hdqmp]2[PdBr4]
2H2O [Hdqmp]2[Pt2Cl6] [Hdqmp]2[Pd2Cl6] [Hdqmp]2[Pt2Br6] [Hdqmp]2[Pd2Br6] [Pt(mqmp)2Cl2] [Pd(mqmp)2Cl2] [Pd(mqmp)2Br2] [H2mqmp]2[PtCl4]2H2O [H2mqmp]2[PdCl4] [H2mqmp]2[PtBr4]
2H2O [H2mqmp]2[PdBr4]
2H2O [H2mqmp]2[Pt2Cl6] [H2mqmp]2[Pd2Cl6] [H2mqmp]2[Pt2Br6] [H2mqmp]2[Pd2Br6] [H2mqmp][Pd(Hmqmp)Cl3] [H2mqmp][Pd(Hmqmp)Br3] [Pt(mqmp)2]
2H2O [Pd(mqmp)2]
2H2O

8Qd

2Q f

12.68 6.65 13.40 10.03

(15.38) (9.03)h (14.67)f (12.12)h

12.98 16.00 12.12 9.89 13.14 18.72 13.90 17.61

(13.90)f (18.98)h (10.90)f (10.04)h (11.29)f (18.98)h (9.72)f (14.04)h

>100 >100 >100 41.01

>100f >100k >100f (42.42)k

>100

>100k

31.40 45.34 32.00 >100 18.40

(25.30)k (63.39)k (37.66)k >100f (28.62)k

g

12.20 47.74 13.08 10.51 25.23 23.07 13.13 >100 >100 35.15

(14.79) (64.91)i (13.31)g (12.72)i (51.22)i (42.22)i (14.29)g >100i >100g (35.69)i

>100 18.10 >100 >100

>100g (26.72)j >100j >100g

>100

>100g

>100

>100g

>100 18.26 43.60 27.20 >100 25.43

>100g (15.31)j (61.11)j (32.02)j >100g (42.01)j

2Qe f

9.42 7.92 9.81 12.72

(11.42) (10.76)h (10.74)f (15.42)h

9.51 11.51 9.36 9.83 9.70 16.12 10.01 14.79

(10.19)f (13.65)h (8.42)f (9.98)h (8.34)f (16.33)h (7.00)f (11.80)h

>100 15.32 >100 12.31

>100f (20.35)k >100f (12.74)k

18.25

(19.62)k

10.03 17.23 11.80 >100 17.30

(8.14)k (24.05)k (13.88)k >100f (26.91)k

7.51 57.20 7.57 2.76 5.68 7.08 8.99 44.29 >100 3.00

(9.11)g 77.71i (8.29)g (3.30)i (11.52)i (13.01)i (9.82)g (52.47)i >100g (3.05)i

>100 15.81 11.10 >100

>100g (25.89)j (14.53)j >100g

>100

>100g

>100

>100g

>100 12.60 22.11 15.90 >100 15.75

>100g (10.51)j (30.91)j (18.73)j >100g (25.91)j

a Concentration to inhibit the growth of cells by 50%. Micromolar concentration of the complexes are given in the parentheses (lM). The IC50 values obtained for 8-dqmp and Na(8-mqmp)
2H2O against L1210 line are >50 and >100 lM, respectively. b KB – epidermoid human carcinoma cell line. c L1210 – murine leukaemia cell line. d Complexes of diethyl (8-dqmp) and monoethyl (8-Hmqmp) quinolylmethylphosphonates. e Complexes of diethyl (2-dqmp) and monoethyl (2-Hmqmp) quinolylmethylphosphonates. f Data from this work. g Data taken from Ref. [25]. h Data taken from Ref. [21]. i Data taken from Ref. [20]. j Data taken from Ref. [23]. k Data taken from Ref. [26].

breaking ability of the Pt/Pd–halogen bond is presumed to be the crucial step in complex reaction with DNA strands. Our results of greater halide lability in the molecular diester complexes of 2dqmp with respect to the corresponding complexes of monoesters 2-Hmqmp, obtained by the thermal study, are in agreement with this assumption [46]. It may be presumed that the complex stability decreases with the number of the ethyl ester groups, most probably due to the increase of the steric hindrance effects. On the other side, extensive hydrogen bonding in monoester derivatives may contribute to the increased stability of the monoester complexes. In the case of the ion-pair salt complexes, it is difficult to investigate the stability and the possible structure/activity relationship with respect to the leaving ability of the halogen ligands. Besides the fact that complexes of diesters and monoesters form ion-pair complexes with different structural properties, most probably these complexes possess also different properties in hydrolysis, transport mechanism inside the cells and different mode of DNA binding and nicking ability to that proposed for the halide adducts [50]. In addition, it is worth pointing out that palladium complexes of both monoesters are much more effective than their platinum complexes. Platinum and palladium complexes of diester 8-dqmp show more or less comparable growth-inhibitory effects, while this difference is a bit greater between complexes of diester

2-dqmp. It may be assumed that the very low solubility of some particular complexes could be critical for their very low activity.

4. Conclusion On the basis of the present publication, as well as on our previous works, it was summarized the complex-forming properties of diethyl and monoethyl esters of quinolylmethylphosphonic acids towards platinum and palladium ions. By forming various types of molecular and ionic complexes, this class of organophosphorus compounds cause attention from the viewpoint of their physicochemical, structural and biological properties. Although almost all of the complexes were found to exhibit antitumour activity above therapeutically meaningful concentrations (IC50 > 10 lM) and hence has not proven sufficiently active to warrant clinical trials, the knowledge obtained about structural and biological properties of these compounds could be considered in the future design of new potent inhibitors. The results obtained by alternation of the metal ion (platinum/ palladium), the kinetically inert quinolylmethylphosphonate ligand (diester/monoester), the kinetically labile ligand (chloride/ bromide) as well as tumour type (KB/L1210) provide valuable

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information for the correlation between the structural and biological properties of the complexes studied, and support the general assumption that there are many factors that influence activity of the compound. The structure–activity relationship is extremely complex and for a better understanding how structural and physicochemical properties of complexes impact their biological activity, further studies on the potential mechanism of antitumour action of complexes as well as more detailed pharmacological evaluation are needed. Acknowledgements The financial support of the Croatian Ministry of Science, Education and Sports (Grant No. 098-0982915-2950) is gratefully acknowledged. The authors would like to thank Mr. Zˇ. Marinic´ for recording the NMR spectra and Dr. R. Trojko for TG/DTA measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2010.05.022. References [1] B. Rosenberg, Platinum Metals Rev. 15 (1971) 42. [2] P.J. O’Dwyer, P. Stevenson, S.W. Johnson (Clinical status of cisplatin, carboplatin, and other platinum-based antitumor drugs), in: B. Lippert (Ed.), Cisplatin: Chemistry and Biochemistry of a Leading Anticancer Drug, Verlag Helvetica Chimica Acta, Zurich, 1999, pp. 31–72. [3] B. Lippert (Ed.), Cisplatin: Chemistry and Biochemistry of a Leading Anticancer Drug, Wiley–VCH, Weinheim, 1999. [4] S. van Zutphen, J. Reedijk, Coord. Chem. Rev. 249 (2005) 2845. [5] Y. Jung, S.J. Lippard, Chem. Rev. 107 (2007) 1387. [6] B. Desoize, C. Madoulet, Crit. Rev. Oncol. Hematol. 42 (2002) 317. [7] L. Kelland, Nat. Rev. Cancer 13 (2007) 573. [8] J. Reedijk, Eur. J. Inorg. Chem. (2009) 1301, and references cited therein. [9] G. Natile, M. Coluccia, Coord. Chem. Rev. 216–217 (2001) 383. [10] G. Momekov, A. Bakalova, M. Karaivanova, Curr. Med. Chem. 12 (2005) 2177. [11] S.M. Aris, N.P. Farrell, Eur. J. Inorg. Chem. (2009) 1293. [12] G. Sava, A. Bergamo, S. Zorzet, B. Gava, C. Casara, M. Cocchieto, A. Furlani, V. Scarcia, B. Serli, E. Iengo, E. Alessio, G. Menstroni, Eur. J. Cancer 38 (2002) 427. [13] M.A. Jakupec, M. Galanski, V.B. Arion, C.G. Hartinger, B.K. Kepler, Dalton Trans. (2008) 183. [14] G. Zhao, H. Lin, Curr. Med. Chem. – Anticancer Agents 5 (2005) 137. [15] A. Alama, B. Tasso, F. Novelli, F. Sparatore, Drug Discov. Today 14 (2009) 500. [16] A. Casini, C. Hartinger, C. Gabbiani, E. Mini, P.J. Dyson, B.K. Keppler, L. Messori, J. Inorg. Biochem. 102 (2008) 564. [17] B. Cebrian-Losantos, E. Reisner, C.R. Kowol, A. Roller, S. Shova, V.B. Arion, B.K. Keppler, Inorg. Chem. 47 (2008) 6513.

[18] P. Kafarski, B. Lejczak, in: V.P. Kukhar, H.R. Hudson (Eds.), Aminophosphonic and Aminophosphinic Acids. Chemistry and Biological Activity, John Wiley & Sons, Chichester, 2000, pp. 407–442. [19] T. Klenner, P. Valenzuela-Paz, F. Amelung, H. Münch, H. Zahn, B.K. Keppler, H. Blum (Platinum phosphonato complexes with particular activity against bone malignancies. An evaluation of an experimental model highly predictive for the clinical situation), in: B.K. Keppler (Ed.), Metal Complexes in Cancer Chemotherapy, VCH, Weinheim, 1993, pp. 85–127. [20] Lj. Tušek–Bozˇic´, I. Matijašic´, G. Bocelli, G. Calestani, A. Furlani, V. Scarcia, A. Papaioannou, J. Chem. Soc., Dalton. Trans. (1991) 195. [21] Lj. Tušek-Bozˇic´, I. Matijašic´, G. Bocelli, G. Sgarabotto, A. Furlani, V. Scarcia, A. Papaioannou, Inorg. Chim. Acta 185 (1991) 229. [22] M. C´uric´, Lj. Tušek–Bozˇic´, D. Vikic´–Topic´, V. Scarcia, A. Furlani, J. Balzarini, E. De Clercq, J. Inorg. Biochem. 63 (1996) 125. [23] Lj. Tušek-Bozˇic´, A. Furlani, V. Scarcia, E. De Clercq, J. Balzarini, J. Inorg. Biochem. 72 (1998) 201. [24] Lj. Tušek-Bozˇic´, M. Komac, M. C´uric´, A. Lycˇka, M. D’Alpaos, V. Scarcia, A. Furlani, Polyhedron 19 (2000) 937. [25] Lj. Tušek-Bozˇic´, F. Frausin, V. Scarcia, A. Furlani, J. Inorg. Biochem. 95 (2003) 259. [26] Lj. Tušek–Bozˇic´, M. Juribašic´, P. Traldi, V. Scarcia, A. Furlani, Polyhedron 27 (2008) 1317. [27] D. Kovala-Demertzi, M.A. Demertzis, E. Filiou, A.A. Pantazaki, P.N. Yadav, J.R. Miller, Y. Zheng, D.A. Kyriakidis, Biometals 16 (2003) 411. [28] E. Budzisz, U. Krajewska, M. Rozalski, A. Szulawska, M. Czyz, B. Nawrot, Eur. J. Pharmacol. 502 (2004) 59. [29] J. Kuduk-Jaworska, A. Puszko, M. Kibiak, M. Pelczynska, J. Inorg. Biochem. 98 (2004) 1447. [30] D. Kovala-Demertzi, A. Bocarelli, M.A. Demertzis, M. Coluccia, Chemotherapy 53 (2007) 148. [31] V. Jagodic´, B. Bozˇic´, J. Heterocycl. Chem. 17 (1980) 685. [32] I. Matijašic´, Lj. Tušek–Bozˇic´, Croat. Chem. Acta 72 (1999) 531. [33] P. Skehan, R. Storeng, D. Scudiero, A. Monks, J. McMahon, D. Vistica, J.T. Warren, H. Bokesch, S. Kenney, M.R. Boyd, J. Natl. Cancer Inst. 82 (1990) 1107. [34] R.J. Tallarida, R.B. Murray, Manual of Pharmacological Calculations with Computer Programs, Springer, New York, 1987. [35] Lj. Tušek–Bozˇic´, M. D’Alpaos, Polyhedron 17 (1998) 1481. [36] L.C. Thomas, Interpretation of Infrared Spectra of Organophosphorus Compounds, Plenum Press, New York, 1974. pp. 39–45. [37] Lj. Tušek–Bozˇic´, Vibr. Spectrosc. 28 (2002) 235. [38] K. Nakamoto (Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part B: Applications in Coordination Organometallic and Bioinorganic Chemistry), 5th ed., Wiley–Interscience, 1997. [39] Zˇ. Marinic´, M. C´uric´, D. Vikic´–Topic´, Lj. Tušek–Bozˇic´, Magn. Reson. Chem. 41 (2003) 969. [40] A. Albinati, P.S. Pregosin, F. Wombacher, Inorg. Chem. 29 (1990) 1812. [41] U. Bierbach, N. Farrell, Inorg. Chem. 36 (1997) 3657. [42] R.M. Scolaro, A. Mazzaglia, A. Romeo, M.R. Plutino, M. Castriciano, R. Romeo, Inorg. Chim. Acta 330 (2002) 189. [43] Lj. Tušek-Bozˇic´, A. Lycˇka, Magn. Reson. Chem. 40 (2002) 175, and references cited therein. [44] S.P. Perleps, P. Jacops, H.O. Desseyn, J.M. Tsangaris, Spectrochim. Acta, Part A 43 (1987) 771. [45] G. Matuschek, A.A. Kettrup, A. Prior, J. Therm. Anal. Cal. 56 (1999) 471. [46] Lj. Tušek-Bozˇic´, R. Trojko, J. Therm. Anal. Cal. 81 (2005) 153. [47] D.R. Lide (Ed.), Handbook of Chemistry and Physics, The Chemical Rubber Co., Boca Raton, FL****, 1998–1999, p. 76. [48] J.P. Souchard, T.T.B. Ha, S. Cros, N.P. Johnson, J. Med. Chem. 34 (1991) 863. [49] J. Reedijk, Chem. Commun. (1996) 801. [50] Y.A. Lee, Y.K. Chung, Y.S. Sohn, J. Inorg. Biochem. 68 (1997) 289.