Nickel complexes of some thiosemicarbazones: Synthesis, structure, catalytic properties and cytotoxicity studies

Inorganica Chimica Acta 392 (2012) 118–130

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Nickel complexes of some thiosemicarbazones: Synthesis, structure, catalytic properties and cytotoxicity studies Sayanti Datta a, Dipravath Kumar Seth a, Sudeshna Gangopadhyay b, Parimal Karmakar b, Samaresh Bhattacharya a,⇑ a b

Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India Department of Life Science and Biotechnology, Jadavpur University, Kolkata 700032, India

a r t i c l e

i n f o

Article history: Received 28 December 2011 Received in revised form 12 May 2012 Accepted 31 May 2012 Available online 28 June 2012 Keywords: Thiosemicarbazone Nickel complexes Coordination modes Catalytic activity Cytotoxocity

a b s t r a c t Reaction of salicylaldehyde thiosemicarbazone (H2L1), 2-hydroxyacetophenone thiosemicarbazone (H2L2) and 2-hydroxynaphthaldehyde thiosemicarbazone (H2L3) with Ni(ClO4)26H2O afforded dimeric complexes of type [{Ni(L)}2]. Reaction of these complexes with triphenylphosphine (PPh3), pyridine (py) and 4,40 -bipyridine (bpy) has yielded complexes of type [Ni(L)(PPh3)], [Ni(L)(py)] and [{Ni(L)}2(bpy)], respectively, which have also been obtained from reaction of the thiosemicarbazones with Ni(ClO4)26H2O and PPh3 or pyridine or 4,40 -bipyridine. Structures of the [{Ni(L)}2] complexes have been optimized by DFT calculations. Crystal structures of [Ni(L2)(PPh3)], [Ni(L2)(py)] and [{Ni(L1)}2(bpy)] have been determined. In all these complexes thiosemicarbazone is coordinated to nickel as ONS-donor. All these complexes show characteristic 1H NMR spectra and intense absorptions in the visible and ultraviolet region. Cyclic voltammetry on the complexes shows one irreversible oxidation on the positive side of SCE, and one irreversible reduction on the negative side. The mixed-ligand nickel complexes are found to be efficient catalysts for Heck type C–C coupling reactions. In vitro cytotoxicity screenings of the six mononuclear nickel complexes have been also carried out in a human tumor cell lines, viz. breast carcinoma cell line (MCF-7). [Ni(L3)(py)] shows the lowest LD50 value. An apoptosis study in MCF-7 with all the complexes confirms that at concentrations near LD50 they induce apoptosis. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The chemistry of thiosemicarbazone complexes of the transition metal ions has been receiving significant current attention, primarily because of the bioinorganic relevance of the complexes [1–9]. A large majority of the thiosemicarbazone complexes have found wide medicinal applications owing to their potentially beneficial biological (viz. antibacterial, antimalarial, antiviral and antitumor) activities [10–18]. Systematic studies on the binding of thiosemicarbazones of selected types to different transition metal ions are of considerable importance in this respect. However, we have been exploring the chemistry of transition metal complexes of the thiosemicarbazones [19–31], mainly because of the variable binding mode displayed by these ligands in their complexes, and the present work has emerged out of this exploration. Herein we have chosen three potentially tridentate thiosemicarbazones, viz. thiosemicarbazones of salicylaldehyde, 2-hydroxyacetophenone and 2-hydroxynaphthaldehyde (Chart 1). These ligands are abbreviated in general as H2L, where H2 stands for the two dissociable ⇑ Corresponding author. Tel./fax: +91 33 24146223. E-mail address: [email protected] (S. Bhattacharya). 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2012.05.034

protons, the phenolic proton and the hydrazinic proton. Individual ligand abbreviations are shown in Chart 1. Salicylaldehyde thiosemicarbazone (as well as the two other ligands) is usually expected to bind to a metal center, via dissociation of two acidic protons, as a dianionic tridentate ONS-donor forming stable chelate I, and this mode of binding has been truly observed by us in its complexes of rhodium [27], iridium [25], palladium [23] and platinum [24]. However, upon reaction with [Ru(PPh3)3Cl2] salicylaldehyde thiosemicarbazone has displayed a rather unusual coordination mode (II), where, in spite of having the phenolic oxygen as a potential donor site, it binds to ruthenium as a bidentate NS-donor forming a four-membered chelate ring [30]. The ruthenium-bound thiosemicarbazone in II has been utilized further for the construction of an interesting ruthenium–nickel heterometallic assembly (III), where all the five available donor atoms in the thiosemicarbazone ligand are engaged in coordination along with bridging mode of binding from the sulfur to nickel [28]. This variable mode of binding of salicylaldehyde thiosemicarbazone (and related ligands) has encouraged us to explore its coordination chemistry further, and herein we have chosen nickel as the metal center to interact with the selected thiosemicarbazones. One reason behind the choice of this particular metal center is its ability

S. Datta et al. / Inorganica Chimica Acta 392 (2012) 118–130

4

3

5

4

Table 1 Crystallographic data for [Ni(L2)(py)].

3

5

OH

OH

Empirical formula fw Space group a (Å) b (Å) c (Å) a (°) b (°) Y (°) V (Å3) Z k (Å) Crystal size (mm3) T (K) l (mm1) R1a wR2b Goodness-of-fit (GOF)c

6

6 C N R

NH2

7

H

C N

8

N C

H S

NH2 N C

H

1 H2L (R = H) 2 H2L (R = CH3)

119

S

3 H2L

Chart 1.

to take up different coordination environments (such as octahedral, square-planar and tetrahedral), which makes its coordination chemistry very interesting. The other, and more attractive, reason is the demonstrated ability of its complexes to catalyze C–C cross-coupling reactions [32–34]. While various types of palladium complexes are being widely utilized as catalysts for such crosscoupling reactions [34–38], much less attention has been paid to the less expensive nickel complexes. Thus, while the primary objective of this present work has been to prepare nickel complexes of the chosen thiosemicarbazones (Chart 1) and find out binding mode of the thiosemicarbazones in the complexes, the other objective has been to explore catalytic properties of the complexes. Besides, by virtue of being complexes of thiosemicarbazone ligands, the targeted nickel complexes are expected to serve as cytotoxic agents, and hence exploration of their cytotoxicity has also been included as another objective of this study. Reactions of the selected thiosemicarbazones with Ni(ClO4)26H2O, taken as the source of nickel, under different experimental conditions have afforded a family of mononuclear and dinuclear complexes. The chemistry of these complexes is reported in this paper with special reference to their formation, structure and, catalytic and cytotoxic properties.

C14H14N4OSNi 345.05 Monoclinic, P21/c 8.0988(3) 7.1970(2) 25.3321(8) 90 93.636(2) 90 1473.56(8) 4 0.71073 0.19  0.20  0.22 296 1.461 0.0476 0.1995 0.73

a

R1 = R||Fo|  |Fc||/R|Fo|. wR2 ¼ ½RwðF 2o  F 2c Þ2 RwðF 2o Þ1=2 . c GOF ¼ ½RðwðF 2o  F 2c Þ2 Þ=ðM  NÞ1=2 , where M is the number of reflections and N is the number of parameters refined. b

2. Experimental 2.1. Materials Salicylaldehyde, 2-hydroxyacetophenone and 2-hydroxynaphthaldehyde were obtained from S.D. Fine-chem, Mumbai, India. Thiosemicarbazide was procured from Loba Chemie, Mumbai, India. The thiosemicarbazone ligands (H2L1, H2L2 and H2L3) were prepared by condensing equimolar amount of the respective aldehyde or ketone with thiosemicarbazide in hot ethanol. Tetrabutylammonium hexafluorophosphate (TBHP) purchased from Aldrich, and AR grade acetonitrile procured from Merck (India), were used in electrochemical work. All other chemicals and solvents were reagent grade commercial materials and were used as received.

2.2. Preparation of the complexes O M C N H

S N C NH2 I

OH C N H

NH2 N C S Ru II

O Ni C N H

NH N C S Ru III

The [{Ni(L)}2] complexes (L = L1, L2 and L3) were prepared by following a general procedure. Specific details are given below for a particular complex. [{Ni(L1)}2]. To a solution of H2L1 (53 mg, 0.27 mmol) in hot ethanol (30 mL) was added triethylamine (55 mg, 0.54 mmol) followed by Ni(ClO4)26H2O (100 mg, 0.27 mmol). The mixture was heated at reflux for 3 h. [{Ni(L1)}2] precipitated as a brown solid, which was collected by filtration, washed thoroughly with ethanol and then dried in air. Yield: 87%. Anal. Calc. for C16H14O2N6S2Ni2: C, 38.14; H, 2.78; N 16.69. Found C, 38.24; H, 2.89; N, 16.52%. Mass: 505, [M+H]+. 1H NMR (d6-DMSO, 300 MHz)1: d = 5.73 (2NH2), 6.35 (2 C5–H), 7.07 (2 C6–H), 7.27 (2 C4–H), 7.52 (2 C3–H), 7.91 (2 azomethine-H). [{Ni(L2)}2]. Yield: 91%. Anal. Calc. for C18H18O2N6S2Ni2: C, 41.11; H, 2.28; N, 15.99. Found C, 41.24; H, 2.16; N, 16.09%. Mass: 533, [M+H]+. 1 H NMR (d6-DMSO, 300 MHz) 1 d = 2.28 (2CH3), 5.66 (2NH2), 6.18 (2 C5–H)}, 6.85 (2 C6–H)}, 7.15 (2 C4–H)}, 7.50 (2 C3–H)}. [{Ni(L3)}2]. Yield: 83%. Anal. Calc. for C24H18O2N6S2Ni2: C, 47.73; H, 2.98; N, 13.92. Found C, 47.51; H, 3.15; N, 14.04%. Mass: 605, [M+H]+. 1H NMR (d6-DMSO, 300 MHz)1 d = 5.79 (2NH2), 6.23 (2 C5–H)}, 7.00 (2 C6–H)}, 7.19 (2 C7–H)}, 7.30 (2 C8–H)}, 7.71 (2 C4–H)}, 7.89 (2 C3–H)}, 8.09 (2 azomethine-H). 1 Chemical shifts are given in ppm. Multiplicity of the signals did not resolve in all the spectra. Multiplicity of the signals (wherever resolved) along with the associated coupling constants (J) are given in parentheses. Overlapping signals are marked with an asterisk. The numbering in the coordinated thiosemicarbazone ligands is given according to, Chart 1.

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The [Ni(L)(PPh3)] complexes (L = L1, L2 and L3) were prepared by following a general procedure. Specific details are given below for a particular complex. [Ni(L1)(PPh3)]. Method A. The dimeric [{Ni(L1)}2] species (100 mg, 0.20 mmol) and triphenylphosphine (52 mg, 0.20 mmol) were taken together in ethanol (30 mL) and the solution was refluxed for 3 h to yield a reddish-brown solution. Partial evaporation of solvent from this solution gave [Ni(L1)(PPh3)] as a crystalline reddish-brown solid. It was collected by filtration, washed with ethanol and dried in air. Yield: 74%. Method B. To a solution of H2L1 (53 mg, 0.27 mmol) in hot ethanol (30 mL) was added triethylamine (55 mg, 0.54 mmol) followed by triphenylphosphine (72 mg, 0.27 mmol) and finally Ni(ClO4)26H2O (100 mg, 0.27 mmol). The mixture was heated at reflux for 3 h to obtain a reddish-brown solution. Partial evaporation of solvent from this solution gave [Ni(L1)(PPh3)] as a crystalline solid. It was collected by filtration, washed with ethanol and dried in air. Yield: 70%. Anal. Calc. for C26H22N3OPSNi: C, 60.74; H, 4.28; N, 8.18. Found C, 60.82; H, 4.15; N 8.33%. 1H NMR (CDCl3, 300 MHz)1 d = 4.69 (s, NH2), 6.43 (d, C5–H, J = 9.0), 6.60 (t, C6–H, J = 7.5), 7.07 (t, C4–H, J = 7.5), 7.25 (d, C3–H, J = 9.0), 7.33–7.82⁄ (PPh3), 8.24 (azomethine H). [Ni(L2)(PPh3)]. Yield: 75%. C27H24N3OPSNi: C, 61.40; H, 4.55; N, 7.96. Found C, 61.56; H, 4.68; N, 7.83%. 1H NMR (CDCl3, 300 MHz)1 d = 2.77 (s, CH3), 4.60 (s, NH2), 6.32 (d, C5–H, J = 9.0), 6.61 (t, C6–H, J = 7.5), 6.99 (t, C4–H, J = 7.5), 7.62 (d, C3–H, J = 9.0), 7.38–7.83⁄ (PPh3). [Ni(L3)(PPh3)]. Yield: 73%. Anal. Calc. for C30H24N3OPSNi: C, 63.86; H, 4.26; N, 7.45. Found C, 63.74; H, 4.37; N, 7.59%. 1H NMR (CDCl3, 300 MHz)1 d = 4.42 (s, NH2), 6.29 (d, C5–H, J = 8.4), 6.50 (d, C6–H, J = 9.0), 7.36–7.82⁄ (PPh3 + C7–H + C8–H + C4–H), 8.05 (d, C3–H, J = 8.7), 9.31 (azomethine-H). The [Ni(L)(py)] complexes (L = L1, L2 and L3) were prepared by following a general procedure. Specific details are given below for a particular complex. [Ni(L1)(py)]. Method A. The dimeric [{Ni(L1)}2] species (100 mg, 0.20 mmol) and pyridine (16 mg, 0.20 mmol) were taken together in ethanol (30 mL) and the solution was refluxed for 3 h to yield a reddish-brown solution. Partial evaporation of solvent from this solution gave [Ni(L1)(py)] as a crystalline solid. It was collected by filtration, washed with ethanol and dried in air. Yield: 73%. Method B. To a solution of H2L1 (53 mg, 0.27 mmol) in hot ethanol (30 mL) was added triethylamine (55 mg, 0.54 mmol) followed by pyridine (22 mg, 0.27 mmol) and finally Ni(ClO4)26H2O (100 mg, 0.27 mmol). The mixture was heated at reflux for 3 h to obtain a reddish-brown solution. Partial evaporation of solvent from this solution gave [Ni(L1)(py)] as a crystalline solid. It was collected by filtration, washed with ethanol and dried in air. Yield: 71%. Anal. Calc. for C13H12N4OSNi: C, 47.17; H, 3.63; N, 16.93. Found C, 47.22; H, 3.71; N, 16.84%. 1H NMR (CDCl3, 300 MHz)1 d = 4.57 (s, NH2), 6.56 (t, C5–H, J = 6.3), 6.72 (C6–H), 7.12 (t, C4– H, J = 7.5), 7.36 (2 py-H), 7.46 (azomethine-H), 7.59 (d, C3–H, J = 6.6), 7.68 (1 py-H), 8.81 (2 py-H). [Ni(L2)(py)]. Yield: 74%. C14H14N4OSNi: C, 48.74; H, 4.06; N, 16.25. Found C, 48.61; H, 4.13; N, 16.17%. 1H NMR (CDCl3, 300 MHz)1 d = 2.64 (s, CH3), 4.68 (s, NH2), 6.65 (t, C5–H, J = 7.5), 6.85 (d, C6–H, J = 6.6), 7.13 (t, C4–H, J = 7.5), 7.31 (2 py-H), 7.58 (d, C3–H, J = 9.0), 7.74 (t, 1 py-H, J = 7.5), 8.86 (2 py-H). [Ni(L3)(py)]. Yield: 72%. Anal. Calc. for C17H14N4OSNi: C, 53.59; H, 3.68; N, 14.71. Found C, 53.48; H, 3.73; N, 14.76%. 1H NMR (CDCl3, 300 MHz)1 d = 4.64 (s, NH2), 6.44 (C5–H), 6.53 (t, C6–H, J = 7.5), 6.66 (C7–H), 7.12–7.52⁄ (2 py-H + C8–H + C4–H + C3– H + azomethine-H), 7.94 (t, 1 py-H, J = 7.5), 8.86 (2 py-H). The [{Ni(L)}2(bpy)] complexes (L = L1, L2 and L3) were prepared by following a general procedure. Specific details are given below for a particular complex.

[{Ni(L1)}2(bpy)]. Method A. The dimeric [{Ni(L1)}2] species (100 mg, 0.20 mmol) and 4,40 -bipyridine (16 mg, 0.10 mmol) were taken together in ethanol (30 mL) and the solution was refluxed for 3 h to yield a reddish-brown precipitate, which was collected by filtration, washed thoroughly with ethanol and then dried in air. Yield: 92%. Method B. To a solution of H2L1 (53 mg, 0.27 mmol) in hot ethanol (30 mL) was added triethylamine (55 mg, 0.54 mmol) followed by 4,40 -bipyridine (21 mg, 0.13 mmol) and finally Ni(ClO4)26H2O (100 mg, 0.27 mmol). The mixture was heated at reflux for 3 h. [{Ni(L1)}2(bpy)] precipitated as a reddish-brown solid, which was collected by filtration, washed thoroughly with ethanol and then dried in air. Yield: 84%. Anal. Calc. for C26H22N8O2S2Ni2: C, 47.32; H, 3.34; N, 16.99. Found C, 47.21; H, 3.44; N, 17.08%. 1H NMR (CDCl3, 300 MHz)1 d = 5.14 (s, 2NH2), 6.42 (4 bpy-H), 6.54 (t, 2 C5–H, J = 6.3), 6.68 (d, 2 C6–H, J = 6.0), 7.12 (2 C4–H), 7.30 (d, 2 C3–H, J = 6.6), 7.93 (2 azomethine-H), 8.92 (4 bpy-H). [{Ni(L2)}2(bpy)]. Yield: 88%. Anal. Calc. for C28H26N8O2S2Ni2: C, 48.88; H, 3.78; N, 16.29. Found C, 48.96; H, 3.69; N, 16.33%. 1H NMR (CDCl3, 300 MHz) 1 d = 2.21 (s, 2CH3), 5.23 (s, 2NH2), 6.37 (4 bpy-H), 6.43 (2 C5–H), 6.58 (2 C6–H), 6.99 (2 C4–H), 7.23 (2 C3– H), 8.84 (4 bpy-H). [{Ni(L3)}2(bpy)]. Yield: 90%. Anal. Calc. for C34H26N8O2S2Ni2: C, 53.73; H, 3.42; N, 14.75. Found C, 53.67; H, 3.49; N, 14.81%. 1H NMR (CDCl3, 300 MHz) 1 d = 5.18 (s, 2NH2), 6.51 (4 bpy-H), 6.62 (2 C5–H), 6.74 (2 C6–H), 7.19 (2 C7–H), 7.38 (2 C8–H), 7.49 (2 C4–H), 7.62 (2 C3–H), 7.98 (2 azomethine-H), 8.97 (4 bpy-H). 2.3. Physical measurements Microanalyses (C, H, N) were performed using a Heraeus Carlo Erba 1108 elemental analyzer. Mass spectra were recorded with a Micromass LCT electrospray (Qtof Micro YA263) mass spectrometer by electrospray ionization method. IR spectra were obtained on a Shimadzu FTIR-8300 spectrometer with samples prepared as KBr pellets. Electronic spectra were recorded on a JASCO V-570 spectrophotometer. Magnetic susceptibilities were measured using a Sherwood MK-1 balance. 1H NMR spectra were recorded in CDCl3 and DMSO-d6 solutions on a Bruker Avance DPX 300 NMR spectrometer using TMS as the internal standard. Electrochemical measurements were made using a CH Instruments model 600A electrochemical analyzer. A platinum disc working electrode, a platinum wire auxiliary electrode and an aqueous saturated calomel reference electrode (SCE) were used in the cyclic voltammetry experiments. All electrochemical experiments were performed under a dinitrogen atmosphere. All electrochemical data were collected at 298 K and are uncorrected for junction potentials. Optimization of ground-state structures and energy calculations for all the complexes were carried out by density functional theory (DFT) method using the GAUSSIAN 03 (B3LYP/SDD-6-31G) package [39]. 2.4. Crystallography Single crystals of [Ni(L2)(PPh3)] and [Ni(L2)(py)] were obtained by slow evaporation of solvent from ethanolic solutions of the complexes. Single crystals of [{Ni(L1)}2(bpy)] were obtained by careful mixing of the ingredients in ethanol, followed by slow evaporation of the solvent from the solution.2 Selected crystal data and data collection parameters for [Ni(L2)(py)] are given in Table 1, and those for [Ni(L2)(PPh3)] and [{Ni(L1)}2(bpy)] are deposited as Supplementary material (Table S1). Data were collected on a Bruker 2 The single crystals of [{Ni(L1)}2(bpy)] are found to be identical in composition and properties to the amorphous [{Ni(L1)}2(bpy)] complex obtained by Method A or Method B.

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Fig. 1. DFT optimized structure of complex [{Ni(L1)}2].

Table 2 Selected bond distances and bond angles for [{Ni(L1)}2] and [Ni(L2)(py)]. [{Ni(L1)}2]a Bond distances (Å) Ni1–O13 Ni1–N15 Ni1–S2 Ni1–S21 Ni22–O40 Ni22–N27 Ni22–S21 Ni22–S2

1.82 1.87 2.23 2.33 1.79 1.81 2.07 2.15

C3–O13 C14–N15 N15–N41 C19–N41 C19–N16 C19–S2 C32–O40 C28–N27 N27–N42 C23–N42 C23–N24 C23–S21

1.31 1.32 1.39 1.33 1.33 1.75 1.31 1.29 1.47 1.36 1.47 1.78

Bond angles (°) O13–Ni1–N15 N15–Ni1–S2

95.05 87.88

O13–Ni1–S2 N15–Ni1–S21

171.47 158.43

Bond distances (Å) Ni1–N4 Ni1–O1 Ni1–N1 Ni1–S1

1.926(4) 1.834(3) 1.878(3) 2.1351(13)

C1–O1 C7–N1 N1–N2 N2–C9 C9–N3 C9–S1

1.314(5) 1.301(6) 1.413(5) 1.301(6) 1.363(6) 1.728(4)

Bond angles (°) N1–Ni1–N4 O1– Ni1–S1

177.67(15) 176.02(11)

O1–Ni1–N1 N1–Ni1–S1

94.70(15) 88.49(11)

[Ni(L2)(py)]

Fig. 2. View of the complex [Ni(L2)(py)].

SMART Apex CCD area detector using graphite monochromated Mo Ka radiation (k = 0.71073 Å). X-ray data reduction, structure solution and refinement were done using SHELXS-97 and SHELXL-97 programs [40]. The structures were solved by the direct methods.

a

DFT optimized structure.

CDCl3 and analyzed by 1H NMR. Percent conversions were determined against the remaining aryl halide [41–44]. 2.6. Cytotoxicity assay

2.5. Catalysis. General procedure for the Heck coupling reactions In a typical run, an oven-dried 10 mL round bottom flask was charged with a known mole percent of catalyst, base (2.4 mmol), n-butyl acrylate (4.0 mmol) and aryl halide (1 mmol) with dimethylformamide (4 mL). The flask was placed in a preheated oil bath at 130 °C. After the specified time the flask was removed from the oil bath and water (20 mL) was added, followed by extraction with ether (4  10 mL). The combined organic layers were washed with water (3  10 mL), dried over anhydrous Na2SO4, and filtered. Solvent was removed under vacuum. The residue was dissolved in

2.6.1. Cell culture Human breast carcinoma cells (MCF-7) were used for our experiments. Cultures were maintained in RPMI-1640 (Himedia Laboratories), supplemented with 10% fetal bovine serum (FBS, Sigma, St. Louis, MO), 100 iu/ml penicillin (Sigma), 100 lg/ml streptomycin (Sigma) and kept at 37 °C in humidified atmosphere containing 5% CO2. 2.6.2. Preparation of test material Stock solutions of the six mononuclear nickel complexes (three [Ni(L)(PPh3)] and three [Ni(L)(py)] complexes) were prepared by

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Fig. 3. Packing pattern showing hydrogen bonding interactions in the lattice of the [Ni(L2)(py)] complex: (a) C–H  O interaction (O  H distance = 2.462 Å, C–H–O angle = 125.65°) and (b) C–H  N interaction (N  H distance = 2.746 Å, C–H–N angle = 146.87°).

dissolving required quantity of each compound in DMSO. All further dilutions were made in RPMI-1640 medium. 2.6.3. MTT assay for cell viability The mitochondrial dehydrogenase activity of the cells after exposure to different complexes was determined by colorimetric assay. For the experiment, 2.5  104 cells per well in a 24-well plate were seeded. After 24 h, they were treated with or without the complexes. MTT assay was carried out after stipulated time interval using standard protocol and optical density was measured at 570 nm using a spectrophotometer [45]. The control value corresponding to untreated cells was taken as 100% and the viability of treated samples were expressed as a percentage of the control. The LD50 values were determined as the concentration that reduced cell viability by 50%. 2.6.4. Annexin V binding assay Staining the cells with Annexin V-FITC [46] (Sigma, USA) and propidium iodide (PI) was used to distinguish between cells undergoing apoptosis (PI negative) and those that are necrotic or dead (PI positive). Cells were incubated with FITC-labeled Annexin V and propidium iodide (PI) at room temperature for 15 min in the dark and analyzed using a FACS Calibur (Becktone Dickinson).

2.6.5. Statistical analysis All data reported are the arithmetic mean from three independent experiments performed in triplicate ±S.D. The unpaired Student’s t-test was used to evaluate the significance differences between groups, accepting p < 0.05 as a level of significance.

3. Results and discussion 3.1. Synthesis and structure Reactions of the selected thiosemicarabazones (H2L, Chart 1) with Ni(ClO4)26H2O in refluxing ethanol in the presence of triethylamine have afforded a family of brown complexes of type [{Ni(L)}2] in decent yields. Preliminary (microanalytical and spectroscopic) characterizations on these complexes indicated the presence of de-protonated (phenolic and hydrazinic proton) thiosemicarbazone in them. Mass spectral data of these complexes are in well agreement with their dimeric composition. As the selected thiosemicarbazones can potentially bind to each metal center in the tridentate ONS-fashion (as in I), formation of the dimeric complexes may occur through sulfur-bridge or through oxygenbridge (as shown, respectively, in IV and V for [{Ni(L1)}2]). The bridging action of the thiosemicarbazone sulfur is well docu-

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S. Datta et al. / Inorganica Chimica Acta 392 (2012) 118–130 Table 3 Electronic spectral and cyclic voltammetric data. Electronic spectral dataa kmax, nm (e, M1 cm1)

Compound 1

b

[{Ni(L )}2] [{Ni(L2)}2] [{Ni(L3)}2] [Ni(L1)(PPh3)] [Ni(L2)(PPh3)] [Ni(L3)(PPh3)] [Ni(L1)(py)] [Ni(L2)(py)] [Ni(L3)(py)] [{Ni(L1)}2(bpy)] [{Ni(L2)}2(bpy)] [{Ni(L3)}2(bpy)]

418 419b 426b 413b 410b 442b 415b 416b 450b 417b 419b 443b

(7800), 372 (16 300), 303 (15200) (7400), 374 (15 000), 307 (17 400) (10 800), 391 (15,400), 308 (19 500) (7700), 370 (15 400), 300b (17 500) (6200), 367 (12 100), 303b (15,800) (8000), 424b (8800), 390 (11 100), 324b (13 300) (6300), 371 (13 100), 303 (13 100) (5200), 371 (10 400), 306 (12 300) (2600), 414b (5000), 389 (6100), 370 (6200), 312 (10 700) (12 000), 374 (24 600), 369 (24 500), 299 (25 800) (11 000), 377 (23 400), 370 (24 200), 298 (26 200) (11 300), 403 (21 200), 382 (26 300), 295 (27 300)

Cyclic voltammetric data E, V vs SCE 0.82c,*, 0.90d,* 0.84c,*, 0.87d,* 0.78c,*, 0.86d,* 0.84c,**, 1.06d,** 0.85c,**, 1.18d,** 0.80c,**, 1.15d,** 0.84c,**, 1.26d,** 0.87c,**, 1.12d,** 0.84c,**, 1.14d,** 0.81c,*, 0.76d,* 0.76c,*, 0.79d,* 0.74c,*, 2x0.82d,*

a

In dimethylsulfoxide. Shoulder. Epa value. d Epc value. * Solvent, dimethylsulfoxide; supporting electrolyte, TBHP; scan rate 50 mV s1. ** Solvent, acetonitrile; supporting electrolyte, TBHP; scan rate 50 mV s1. b

c

mented in the literature [22,28,47–50], and that of the phenolate oxygen is also precedent [51,52]. Structural characterization of these complexes by X-ray crystallography has not been possible since single crystals of these species could not be grown. However, to clear the ambiguity between the two possibilities, viz. sulfurbridging and oxygen-bridging, structures of both the sulfurbridged and oxygen-bridged [{Ni(L1)}2] complexes have been optimized by DFT calculations [39]. The sulfur-bridged structure of [{Ni(L1)}2] has been found to be significantly lower in energy (by 1632 kcal/mol) than the oxygen-bridged structure. Besides, the oxygen-bridged complex is reported to be soluble in dichloromethane [51,52], while the complexes prepared by us are found to be insoluble in dichloromethane (soluble in dimethylsulfoxide or dimethylformamide). Based on these observations it has been concluded that the [{Ni(L)}2] complexes, prepared by us, have the sulfur-bridged structure (as shown in IV). The optimized sulfurbridged structure of [{Ni(L1)}2] is shown in Fig. 1 and some evaluated bond parameters are given in Table 2.

and [{Ni(L3)}2], have also been optimized by DFT calculations [39]. These two optimized structures, along with some relevant bond parameters, are also deposited as supporting information (Figs. S2, S3 and Table S2). The computed bond parameters are found to be comparable with those observed in related complexes [53–59]. In order to explore the possibility of forming mononuclear complexes by splitting the sulfur bridge in the [{Ni(L)}2] complexes, their reaction has been carried out with two monodentate ligands, viz. triphenylphosphine (PPh3) and pyridine (py). From these reactions two series of mononuclear complexes of type [Ni(L)(PPh3)]

H2N N

H

S O

Ni

Ni N H

N O

S N NH2 IV

H N H2N

N

O

S

NH2

S

Ni

Ni O

N

N H

V

The optimized oxygen-bridged structure of [{Ni(L1)}2] is deposited as supporting information (Fig. S1). The sulfur-bridged structures of the other two complexes of this family, viz. [{Ni(L2)}2]

Fig. 4. Contour plot of complex [{Ni(L1)}2].

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osemicarbazone is coordinated to nickel in the expected ONS-fashion and a triphenylphosphine is also coordinated to the metal center (see VI). The structure of [Ni(L2)(py)] is shown in Fig. 2 and selected bond parameters are listed in Table 2. This structure shows that the thiosemicarbazone is coordinated to nickel in the same ONS-fashion, as before and a pyridine is also coordinated to the metal center, which is trans to the central imine-nitrogen of the coordinated thiosemicarbazone. Nickel is thus nested in an ONSN core, which is slightly distorted from ideal square planar geometry, as reflected in the bond parameters around the metal center. The bond distances centering nickel, as well as those within the coordinated thiosemicarbazone, are all quite normal, [53–59]. The absence of any solvent of crystallization in the lattice of [Ni(L2)(py)] indicates existence of non-covalent interactions between individual complex molecules. To scrutinize these interactions packing pattern in the lattice has been probed, which (Fig. 3) shows that the NH2 fragments of the coordinated thiosemicarbazone are found to be involved in intermolecular N–H  O hydrogen-bonding with the coordinated phenolate oxygen. Besides, intermolecular C–H  N hydrogen bonding involving phenyl C–H and imine-nitrogen belonging to neighboring molecules are also active in the lattice. In view of the similarity in the method of synthesis, composition and properties (vide infra), the [Ni(L1)(PPh3)] and [Ni(L3)(PPh3)] complexes are assumed to have similar structures as [Ni(L2)(PPh3)]. Likewise the [Ni(L1)(py)] and [Ni(L3)(py)] complexes are assumed to have similar structures as [Ni(L2)(py)]. PPh3

O Ni N H3C

S N NH2 VI

H2N N

H N

S Ni N

O

N

O Ni

S

N Fig. 5. Contour plot of complex [Ni(L2)(py)].

H

N NH2 VII

and [Ni(L)(py)], respectively, were obtained. The same mononuclear complexes have also been synthesized by direct reaction of the thiosemicarbazones (H2L1, H2L2, H2L3) with Ni(ClO4)26H2O and triphenylphosphine or pyridine (in 1:1:1 mol ratio) in refluxing ethanol in the presence of triethylamine. To authenticate the identity of these mononuclear complexes, with particular reference to coordination mode of the thiosemicarbazone in them, the structure of a selected member from each of the two families, viz. [Ni(L2)(PPh3)] and [Ni(L2)(py)], has been determined by X-ray crystallography. It needs to be mentioned here that syntheses of two complexes of [Ni(L)(PPh3)] type (L = L1 and L2) by entirely different routes, as well as their crystal structures, are precedent in the literature [60,61]. The crystal structure of [Ni(L2)(PPh3)] solved by us (Fig. S4 and Table S3; Supporting information), which is found to be similar to that reported earlier [61], shows that the thi-

From the foregoing discussions, it becomes quite apparent that interaction of the thiosemicarbazones (H2L) with Ni(ClO4)26H2O leads to the formation of a NiL fragment in which three coordination sites on the metal center are taken up by the ONS-coordinated thiosemicarbazone and the fourth site remains virtually vacant. With an aim to synthesize dinuclear complexes of nickel by utilizing this vacant coordination site of nickel in the NiL fragment, Ni(ClO4)26H2O was allowed to react with the thiosemicarbazones and 4,40 -bipyridine (in 2:2:1 mol ratio) in refluxing ethanol in the presence of triethylamine. A family of three reddish-brown complexes of type [{Ni(L)}2(bpy)] were indeed obtained in excellent yields. The same dinuclear complexes have also been synthesized from the parent [{Ni(L)}2] complexes by splitting the sulfur bridge by 4,40 -bipyridine. The structure of a selected member of this family,

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3.2. Spectral properties

[{Ni(L)}2] complexes have been recorded in DMSO-d6 solution. In the spectra of [{Ni(L1)}2] and [{Ni(L3)}2], the azomethine proton signal is observed at 7.91 and 8.09 ppm, respectively. The methyl signal from the coordinated thiosemicarbazone in [{Ni(L2)}2] is observed at 2.30 ppm. In all the three [{Ni(L)}2] complexes the NH2 proton signal is observed around 4.4 ppm. All the expected signals from the aryl fragment of the coordinated thiosemicarbazone are clearly observed in the aromatic region. Besides appearance of broad peaks within 7.3–7.9 ppm for the coordinated triphenylphosphine, 1H NMR spectra of the [Ni(L)(PPh3)] complexes are found to be similar to those of the corresponding parent [{Ni(L)}2] complexes. Similarly in the [Ni(L)(py)] and [{Ni(L)}2(bpy)] complexes signals from the pyridine and 4,40 -bipyridine, respectively, are observed in addition to those arising from the Ni(L) fragment. Infrared spectra of all the complexes show many vibrations of different intensities in the 1600–400 cm1 region. Assignment of each individual band to a specific vibration has not been attempted. However, a sharp band displayed near 736 cm1 by the [{Ni(L)}2] complexes is attributable to the C–Sbridging stretching [63]. Similarly, a sharp band shown near 755 cm1 by the triphenylphosphine, pyridine and bipyridine complexes, is believed to be due to the C–Snon-bridging stretching [63]. In the spectra of the [Ni(L)(PPh3)] complexes, three strong bands are observed near 530, 692 and 746 cm1, which are attributed to the coordinated PPh3 ligand. The 1H NMR and IR spectral data of the mononuclear and dinuclear complexes are therefore consistent with their composition. Electronic spectra of all the complexes have been recorded in dimethylsulfoxide solution. Spectral data are presented in Table 3. All the complexes show several intense absorptions in the visible and ultraviolet regions. The absorptions in the ultraviolet region are attributable to transitions within the ligand orbitals. To have an insight into the nature of absorptions in the visible region, electronic structures of all the complexes have been probed with the help of DFT calculations [39].3 Compositions of the selected molecular orbitals are given in Table 4. Contour plots of the molecular orbitals for a parent dinuclear complex, viz. [{Ni(L1)}2], and a mononuclear derivative, viz. [Ni(L2)(py)], are shown, respectively, in Figs. 4 and 5, and those for [Ni(L2)(PPh3)] and [{Ni(L1)}2(bpy)] are deposited as Supplementary material (Figs. S6 and S7). In the parent [{Ni(L)}2] complexes, the highest occupied molecular orbital (HOMO) is mostly spread over one Ni(L) fragment where major contribution (>53%) comes from the thiosemicarbazone ligand and relatively less, but significant, contribution (>22%) from the nickel center. The other Ni(L) fragment collectively makes about 25% contribution to the HOMO. The lowest unoccupied molecular orbital (LUMO) is primarily (>73%), and almost equally, delocalized over the two thiosemicarbazone ligands (Fig. 4). The intense absorption near 420 nm displayed by the [{Ni(L)}2] complexes is hence assignable to a transition from a filled orbital (HOMO) having mixed metal and ligand character to a vacant p⁄-orbital (LUMO) spread over the coordinated thiosemicarbazones. In the [Ni(L)(PPh3)] complexes the HOMO has major (>83%) contribution from coordinated thiosemicarbazone ligand, with a significant (>14%) contribution from the metal center, whereas the LUMO is distributed mostly (>90%) over the coordinated thiosemicarbazone ligand (Fig. S6). Therefore the lowest energy absorption (near 410 nm)4 is basically due to a p–p⁄ transition localized in the thiosemicrbazone ligand. For both the [Ni(L)(py)] and [{Ni(L)}2(bpy)] complexes, the HOMO has major (>72%) contribution from the coordinated thiosemicarbazone ligand, with a significant contribution (>20%) from metal center, whereas, the LUMO is distributed mostly (>93%) over the coordinated pyridine fragment

All the dinuclear and mononuclear nickel complexes are diamagnetic, which corresponds to the bivalent states of nickel (square-planar d8, S = 0) in them. 1H NMR spectra of the

3 Phenyl rings of the triphenylphosphines in the [Ni(L)(PPh3)] complexes have been replaced by hydrogens. 4 In [Ni(L3)(PPh3)] the absorption occurs at longer wavelength.

Table 4 Composition of selected molecular orbitals. Compound

Contributing fragments

% Contribution of fragments to HOMO

LUMO

[{Ni(L1)}2]

Ni1, a L1, b Ni22, a L1, c

10.44 9.62 22.35 57.59

10.24 35.55 11.89 42.32

[{Ni(L2)}2]

Ni1, a L2, b Ni21, a L2, c

11.13 10.04 24.86 53.97

9.81 35.34 13.25 41.60

[{Ni(L3)}2]

Ni1, a L3, b Ni22, a L3, c

9.78 11.73 23.90 54.59

10.99 34.85 14.22 39.94

[Ni(L1)(PPh3)]

Ni L1 PPh3

16.40 83.41 0.19

6.92 92.82 0.26

[Ni(L2)(PPh3)]

Ni L2 PPh3

15.26 84.60 0.14

7.44 92.22 0.34

[Ni(L3)(PPh3)]

Ni L3 PPh3

14.70 85.14 0.16

7.13 92.68 0.19

[Ni(L1)(py)]

Ni L1 py

20.89 79.00 0.11

4.97 1.03 94.00

[Ni(L2)(py)]

Ni L2 Py

21.18 78.67 0.15

5.12 1.07 93.95

[Ni(L3)(py)]

Ni L3 py

22.02 77.90 0.08

5.63 0.98 93.39

[{Ni(L1)}2(bpy)]

Ni L1 bpy

25.90 72.78 1.32

4.60 1.03 94.37

[{Ni(L2)}2(bpy)]

Ni L2 bpy

24.84 74.25 0.91

4.12 1.11 94.77

[{Ni(L3)}2(bpy)]

Ni L3 bpy

26.05 72.19 1.76

4.02 2.54 93.44

a The two non-equivalent nickel centers are labeled as in their corresponding DFT optimized structures (see Figs. 1, S2 and S3). b Thiosemicarbazone coordinated to Ni1 center. c Thiosemicarbazone coordinated to the other nickel center.

viz. [{Ni(L1)}2(bpy)], has been determined by X-ray crystallography. It needs to be mentioned here that synthesis of [{Ni(L1)}2(bpy)] by an entirely different route, as well as its crystal structure, is precedent in the literature [62]. Similarly, a sharp band shown of [{Ni(L1)}2(bpy)] solved by us (Fig. S5 and Table S3; Supporting information), which is found to be similar to that reported earlier [62], shows that the thiosemicarbazone ligand is coordinated to nickel as ONS-donor and two such NiL fragments are bridged by the 4,40 -bipyridine (see VII). Since all these three [{Ni(L)}2(bpy)] complexes have been synthesized similarly and they show similar properties (vide infra), the [{Ni(L2)}2(bpy)] and [{Ni(L3)}2(bpy)] complexes are assumed to have similar structures as [{Ni(L1)}2(bpy)].

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Table 5 Heck cross-coupling of aryl halides with butyl acrylate.a

R

X

+

Ni catalyst

CO 2Bu

Cs2CO 3,

R CO 2Bu

Dimethylformamide, 130 °C

a b c d

Entry

R

X

Catalyst

Time (h)

Amt. of cat. (mol%)

Conversion%b

TONc

TOFd [s1]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

COCH3 COCH3 COCH3 CHO CN COCH3 COCH3 COCH3 COCH3 COCH3 COCH3 CHO CN COCH3 COCH3 COCH3 COCH3 COCH3 CHO CN COCH3 COCH3

Br Br Br Br Br I Cl Br Br Br Br Br Br I Cl Br Br Br Br Br I Cl

[Ni(L1)(PPh3)] [Ni(L2)(PPh3)] [Ni(L3)(PPh3)] [Ni(L2)(PPh3)] [Ni(L2)(PPh3)] [Ni(L2)(PPh3)] [Ni(L2)(PPh3)] [Pd(L3)(PPh3)] [Ni(L1)(py)] [Ni(L2)(py)] [Ni(L3)(py)] [Ni(L1)(py)] [Ni(L1)(py)] [Ni(L1)(py)] [Ni(L1)(py)] [{Ni(L1)}2(bpy)] [{Ni(L2)}2(bpy)] [{Ni(L3)}2(bpy)] [{Ni(L2)}2(bpy)] [{Ni(L2)}2(bpy)] [{Ni(L2)}2(bpy)] [{Ni(L2)}2(bpy)]

24 24 24 24 24 18 24 6 24 24 24 24 24 24 24 24 24 24 24 24 11 24

2 2 2 2 2 2 2 0.01 2 2 2 2 2 2 2 2 2 2 2 2 2 2

72 74 67 55 40 >99 52 80 56 53 51 42 36 >99 39 >99 >99 92 73 58 >99 70

36 37 33.5 27.5 20 50 26 8  103 28 26.5 25.5 21 18 50 19.5 50 50 46 36.5 29 50 35

4.2  104 4.3  104 3.9  104 4.3  104 3.2  104 7.7  104 3.0  104 3.7  101 3.2  104 3.1  104 3.0  104 2.4  104 2.1  104 5.8  104 2.3  104 5.8  104 5.8  104 5.3  104 4.2  104 3.4  104 1.3  103 4.1  104

Reaction conditions: aryl halide (1.0 mmol), butyl acrylate (4.0 mmol), Cs2CO3 (2.4 mmol), Ni catalyst, dimethylformamide (4 mL), 130 °C. Determined by 1H NMR on the basis of residual aryl halide [41–44]. TON, turnover number {(mol of product)/(mol of catalyst)}. TOF, turnover frequency (TON per unit time).

negative side. For the parent [{Ni(L)}2] complexes, in view of composition of the HOMO, the oxidative response is assigned to oxidation of the coordinated thiosemicarbazone. Similarly in view of the composition of the LUMO the reductive response is attributable to reduction of the coordinated thiosemicarbazone ligand. In the [Ni(L)(PPh3)] complexes too, the oxidation and reduction occurs on the thiosemicarbazone ligand. For the [Ni(L)(py)] and [{Ni(L)}2(bpy)] complexes, the oxidative response is assigned to oxidation of coordinated thiosemicarbazone and the reductive response is assigned to reduction of the coordinated pyridine and bipyridine, respectively. 3.4. Catalysis Fig. 6. Comparison of the cytotoxic effects of six mononuclear nickel complexes ([Ni(L)(PPh3)] and [Ni(L)(py)]) on MCF-7 cells, as assessed by MTT assay. Cells were seeded into 24-well plates and allowed to overnight incubation at 37 °C. Next day, these cells were treated with stipulated concentrations of the complexes for 72 h incubation. Results were expressed as percentage viability of cells (Y-axis) against varying concentrations (DM) (X-axis). Mean ± S.D. were given.

(Figs. 5 and S7). The lowest energy absorption for those complexes is hence assignable to a transition from a filled orbital (HOMO) having mixed metal and thiosemicrbazone character to a vacant orbital of the coordinated pyridine fragment.

3.3. Electrochemical properties Electrochemical properties of the complexes have been studied by cyclic voltammetry in acetonitrile and dimethylsulfoxide solution (0.1 M TBHP). Voltammetric data are given in Table 3. All the complexes show one irreversible oxidative response on the positive side of SCE and one irreversible reductive response on the

The fact, that nickel complexes are becoming increasingly popular as catalyst in bringing about C–C cross coupling reactions of different types [32–34], has led us to explore such catalytic properties in the present series of [Ni(L)(PPh3)]), [Ni(L)(py)] and [{Ni(L)}2(bpy)] complexes. The catalytic activity of all these complexes has been examined for Heck type coupling reactions, which is a popular method for C–C bond-formation [64–66]. The targeted coupling reactions between butyl acrylate and aryl bromides/iodides could be achieved in reasonably high yield, and the results are summarized in Table 5. The [Ni(L)(PPh3)] complexes were first tested as catalysts in the coupling of butyl acrylate and p-bromoacetophenone, and all the three complexes showed comparable catalytic efficiency. For example, using [Ni(L1)(PPh3)] or [Ni(L2)(PPh3)] as catalyst the Heck coupling was achieved in moderate yield (entries 1 and 2). The same coupling reaction, when carried out using [Ni(L3)(PPh3)] as the catalyst, showed slightly lower yield (entry 3). Thus, among these three [Ni(L)(PPh3)] complexes, [Ni(L2)(PPh3)] showed maximum efficiency, and hence only results obtained with [Ni(L2)(PPh3)] as the catalyst are highlighted here.

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127

Fig. 7. Flow cytometric analysis of Annexin V-FITC binding and plasma membrane integrity (propidium iodide) in MCF-7 cells. Cells were incubated with six different complexes for the indicated time and then stained with Annexin V-FITC, which specifically detects exposed phosphatidyl serine residues at the cell surface. The number of annexin-V-positive cells was determined using a flow cytometer and the percentage was represented in each panel.

Use of p-bromobenzaldehyde or p-bromobenzonitrile instead of pbromoacetophenone has been found to lower the yield appreciably (entries 4 and 5). Use of aryliodides instead of arylbromides in the similar coupling reactions has improved the effectiveness of catalysts appreciably (entries 2 and 6), while use of arylchlorides instead of arylbromides has decreased the effectiveness of catalysts very much significantly (entries 2 and 7). As the palladium compounds are the standard catalysts for these type of reactions, in or-

der to compare the efficiency of the present nickel complexes with analogous palladium complexes, we have prepared a mononuclear palladium complex [Pd(L3)(PPh3)] [23], in which the palladium center has a very similar coordination environment as in the [Ni(L3)(PPh3)] complex. The catalytic efficiency of this [Pd(L3)(PPh3)] complex has been examined towards Suzuki coupling reaction between p-bromoacetophenone and phenylboronic acid, and the complex has been found to successfully catalyze

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Table 6 Comparative study of cell viability after treatment with six different complexes, as measured by MTT assay and calculation of LD50 values of the complexes and cisplatine.a

a

Name of complex

LD50 (lM)

Concentration (lM) 1

3

5

10

[Ni(L1)(PPh3)] [Ni(L2)(PPh3)] [Ni(L3)(PPh3)] [Ni(L1)(py)] [Ni(L2)(py)] [Ni(L3)(py)]

25 12 22.4 10.6 18.5 7.6

93.24 85.14 91.9 89.77 82.25 75.62

80.34 78.22 80.65 72.73 78.34 61.55

77.73 63.88 79.94 67.33 71.26 54.67

68.29 57.43 65.79 53.96 65.81 45.06

After 72 h in MCF-7 cells.

Table 7 Study of apoptosis as induced by [Ni(L)(PPh3)], [Ni(L)(py)] and cisplatine complexes in MCF-7 cells after flow cytometric analysis of annexin V-FITC binding. Name of complex

Concentration used (ll)

Control [Ni(L1)(PPh3)] [Ni(L2)(Ph3)] [Ni(L3)(PPh3)] [Ni(L1)(py)] [Ni(L2)(py)] [Ni(L3)(py)]

0 25 15 25 10 20 10

Percentage of cells in apoptosis Early

Late

0 0 0 0 0 0.01 0.01

0.16 3.12 4.47 5.39 5.57 4.81 6.81

the coupling reaction (entry 8) with very high efficiency compared to the analogous nickel complexes (entry 3). Catalytic efficiency of the [Ni(L)(py)] complexes in similar Heck coupling reactions have been investigated (entries 9–15). The observed activity was found to be comparable for all the [Ni(L)(py)] complexes. When the dinuclear [{Ni(L)}2(bpy)] complexes were used as catalysts in the similar coupling reactions, yields were found to increase to a great extent under similar experimental conditions (entries 16–20). When p-iodoacetophenone was used instead of p-bromoacetophenone, full conversion was obtained at significantly shorter time (entry 21). Even, using p-chloroacetophenone instead of p-bromoacetophenone, an appreciable yield has been obtained (entry 22). While both the [Ni(L)(PPh3)] and [Ni(L)(py)] complexes could successfully catalyze the Heck coupling reactions, the catalytic efficiency of the phosphine complexes are found to be much better than the corresponding pyridine complexes, and the observed enhancement in catalytic activity is attributable to presence of the soft PPh3 ligand in the former complexes. The dinuclear [{Ni(L)}2(bpy)] complexes are found to be much better catalysts than the corresponding mononuclear phosphine or pyridine complexes. The observed enhancement in catalytic activity is attributable to presence of two nickel centers in a single complex molecule. Another notable aspect of the observed catalysis by the present nickel complexes is that no additional ligands were necessary, and also, they could successfully catalyze even the coupling of aryl chlorides with moderate yields. The observed catalytic activity of the nickel complexes, though not remarkably high, is found to be comparable to that of some recently reported species [67–70]. 3.5. Cytotoxicity assay As already delineated in the introduction, thiosemicarbazone complexes of transition metal ions are well known to exhibit cytotoxic properties, and we also wanted to explore cytotoxic properties of the present group of nickel thiosemicarbazone complexes. This study was restricted to the mononuclear [Ni(L)(PPh3)] and

[Ni(L)(py)] complexes having different ancillary ligands, viz. triphenylphosphine and pyridine.5 The efficacy of cell killing by six mononuclear nickel complexes ([Ni(L)(PPh3)] and [Ni(L)(py)]) depends upon the concentration and time of treatment. We have checked the cell survival for a wide range of concentrations of all the complexes for a duration of 24–72 h s in MCF-7 cells by MTT assay. Cells were treated with stipulated doses of the complexes separately, followed by MTT assay. Untreated cells were taken as respective control. Prior to investigating the cytotoxic action of the complexes, an important standardization experiment was carried out with MTT assay. Initial seeding concentration was calculated in such a way that even after 72 h s, cells should be semi-confluent so that the decrease in OD570 value (a measure of the viability) could be attributed to the complex rather than to the greater number of cells in the wells leading to scarcity of nutrients and area of proliferation. It was also found that after 72 h, there was nearly 95% cell survival and the data was normalized taking this as control set. The percentage of cell survival decreased with the increasing concentrations for all the complexes we used and the results are shown in Fig. 6. At lower concentration (up to 10 lM) of [Ni(L3) (py)], the survivability of the cells decreased significantly but at higher concentration all the complexes show almost equal toxicity. The data of MTT assay for lower concentrations of the complexes are shown in Table 6. The OD570 value decreased significantly when the cells were treated with [Ni(L3)(py)], compared to other complexes. It was observed that the later was decreased by almost 55% (for 5 lM of [Ni(L3)(py)]), while for [Ni(L1)(PPh3)] at the same concentration range, it is nearly 80% (for 5 lM of [Ni(L1)(PPh3)]). The LD50 value for [Ni(L3)(py)] was detected to be the lowest (7.6 lM). However, in a similar experiment, the LD50 value for Doxorubicin, a well known anti-neoplastic agent was found to be 50 lM (data not shown). For Cisplatin, another widely used chemotherapeutic drug, LD50 value was reported to be 18 lM [71]. Now apoptosis was the most desired event in drug induced cell killing for therapeutic purposes. Ideally extent of cell death should be checked with Annexin V-FITC binding assay which clearly differentiates between early and late apoptotic cells along with their living counterparts. So we opted for FACS calibur in order to carry out Annexin V-FITC binding assay. The results obtained for the complexes are shown in Fig. 7. As depicted from this figure, there was a clear indication of fewer amounts of apoptosis (both early and late) in contrast to percentage of dead cells. Table 7 shows the percentage of early and late apoptotic cells for different complexes as compared to the control set. The doses of treatment of different complexes were ascertained according to their respective LD50 values. Percentage of necrotic cells were higher because we have incubated the cells for 72 h and in vitro most of the apoptotic cells would behave like necrotic cells due to lack of phagocytosis. A detailed time course experiments were done by simple DAPI staining of the cells followed by visualization under a fluorescence microscope. The amount of apoptotic cells increased with the concentration and time of incubation of the complex (data not shown). Further after 72 h of incubation EtBr staining clearly showed necrotic cell death. Among all the complexes, although the amount of apoptosis induction was less (Table 7), complex [Ni(L3)(py)] was most effective in response to the induction of apoptosis. However, for a more practical approach towards therapeutic application of these complexes, the signaling events associated with apoptosis and/or necrosis should be elucidated to get a detailed insight of the mechanism involved in the cellular response. Though nickel compounds are often found to be toxic and serious

5 The dinuclear complexes are found unsuitable for this study due to their solubility problem.

S. Datta et al. / Inorganica Chimica Acta 392 (2012) 118–130

allergens for some people, we are still motivated by the fact that a more complete understanding of the pathways may lead to the development of effective cancer therapies. 4. Conclusions

[10] [11] [12] [13] [14] [15]

The present study shows that the thiosemicarbazones of salicylaldehyde, 2-hydroxyacetophenone and 2-hydroxynaphthaldehyde can readily bind to nickel as dinegative tridentate ONS-donors, and in combination with monodentate (viz. triphenylphosphine and pyridine) and bridging bidentate (viz. 4,40 -bipyridine) ancillary ligands, afford stable mononuclear and dinuclear mixed-ligand complexes, respectively. This study has also demonstrated that all these mixed-ligand complexes of nickel can successfully catalyze Heck coupling reactions. The mononuclear complexes are also found to be cytotoxic towards human breast carcinoma cell line (MCF-7), which is manifested through the LD50 values and has been further corroborated by the apoptosis study. Acknowledgments

[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

The authors thank the reviewers for their constructive comments, which have been helpful in preparing the revised manuscript. Financial assistance received from the Department of Science and Technology, New Delhi, India [Grant No. SR/S1/IC-29/ 2009] is gratefully acknowledged. The authors thank Prof. Gopal Das of Department of Chemistry, IIT Guwahati, Assam 781039, India, for his help with the crystal structure determination of [Ni(L2)(PPh3)]. Sayanti Datta and Dipravath Kumar Seth thank the Council of Scientific and Industrial Research, New Delhi, India, for their fellowship [Grant No. 09/096(0563)/2008-EMR-I and 09/ 096(0511)/2006-EMR-I, respectively].

[27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]

Appendix A. Supplementary data CCDC 829499–829501 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. DFT optimized structures of complex [{Ni(L1)}2] containing oxygen bridge (Fig. S1), complex [{Ni(L2)}2] (Fig. S2) and complex [{Ni(L3)}2] (Fig. S3), crystal structures of complex [Ni(L2)(PPh3)] and [{Ni(L1)}2(bpy)] (Fig. S4 and Fig. S5, respectively), contour plots of HOMO and LUMO of complexes [Ni(L2)(PPh3)] and [{Ni(L1)}2(bpy)] (Fig. S6 and Fig. S7, respectively), 21 Selected crystal data and data collection parameters for [Ni(L2)(PPh3)] and [{Ni(L1)}2(bpy)] (Table S1), selected bond parameters for DFT optimized structures of complexes [{Ni(L2)}2] and [{Ni(L3)}2] (Table S2), selected bond parameters for crystal structures of complexes [Ni(L2)(PPh3)] and [{Ni(L1)}2(bpy)] (Table S3) have been deposited. Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.ica.2012.05.034. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2012.05.034.

[40] [41] [42] [43] [44] [45] [46] [47] [48] [49]

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

A. Garoufis, S.K. Hadjikakou, N. Hadjiliadis, Coord. Chem. Rev. 253 (2009) 1384. T.S. Lobana, R. Sharma, G. Bawa, S. Khanna, Coord. Chem. Rev. 253 (2009) 977. A.G. Quiroga, C.N. Ranninger, Coord. Chem. Rev. 248 (2004) 119. J.R. Dilworth, P. Arnold, D. Morales, Y.L. Wong, Y. Zheng, Modern Coord. Chem. (2002) 217. D.X. West, A.E. Liberta, S.B. Padhye, R.C. Chikate, P.B. Sonawane, A.S. Kumbhar, R.G. Yerande, Coord. Chem. Rev. 123 (1993) 49. D.X. West, S.B. Padhye, P.B. Sonawane, Struct. Bonding 76 (1992) 1. I. Haiduc, C. Silvestru, Coord. Chem. Rev. 99 (1990) 253. S.B. Padhye, G.B. Kaffman, Coord. Chem. Rev. 63 (1985) 127. M.J.M. Campbell, Coord. Chem. Rev. 15 (1975) 279.

[50] [51] [52] [53] [54] [55] [56] [57]

129

A.C.F. Caires, Med. Chem. 7 (2007) 484. T. Wang, Z. Guo, Curr. Med. Chem. 13 (2006) 525. H. Beraldo, D. Gambino, Min. Rev. Med. Chem. 4 (2004) 31. E.M. Jouad, X.D. Thanh, G. Bouet, S. Bonneau, M.A. Khan, Anticancer Res. 22 (2002) 1713. M.B. Ferrari, F. Bisceglie, G. Pelosi, M. Sassi, P. Tarasconi, M. Cornia, S. Capacchi, R. Albertini, S. Pinelli, J. Inorg. Biochem. 90 (2002) 113. A.R. Cowly, J.R. Dilworth, P.S. Donnely, E. Labisbal, A. Sousa, J. Am. Chem. Soc. 124 (2002) 5270. R.I. Maurer, P.J. Blower, J.R. Dilworth, C.A. Reynolds, Y. Zheng, G.E.D. Mullen, J. Med. Chem. 45 (2002) 1420. J. Patole, S. Dutta, S.B. Padhye, E. Sinn, Inorg. Chim. Acta 318 (2001) 207. Z. Iakovidou, A. Papageorgiou, M.A. Demertzis, E. Mioglou, D. Mourelatos, A. Kotsis, P.N. Yadav, D. Kovala-Demertzi, Anticancer Drugs. 12 (2001) 65. S. Datta, D.K. Seth, R.J. Butcher, S. Bhattacharya, Inorg. Chim. Acta 377 (2011) 120. N.S. Chowdhury, D.K. Seth, M.G.B. Drew, S. Bhattacharya, Inorg. Chim. Acta 372 (2011) 183. S. Basu, R. Acharyya, F. Basuli, S.M. Peng, G.H. Lee, G. Mostafa, S. Bhattacharya, Inorg. Chim. Acta 363 (2010) 2848. S. Dutta, F. Basuli, A. Castineiras, S.M. Peng, G.H. Lee, S. Bhattacharya, Eur. J. Inorg. Chem. (2008) 4538. S. Halder, S.M. Peng, G.H. Lee, T. Chatterjee, A. Mukherjee, S. Dutta, U. Sanyal, S. Bhattacharya, New J. Chem. 32 (2008) 105. S. Halder, R.J. Butcher, S. Bhattacharya, Polyhedron 26 (2007) 2741. S. Basu, R. Acharyya, W.S. Sheldrick, H.M. Figge, S. Bhattacharya, Struct. Chem. 18 (2007) 209. R. Acharyya, S. Dutta, F. Basuli, S.M. Peng, G.H. Lee, L.R. Falvello, S. Bhattacharya, Inorg. Chem. 45 (2006) 1252. S. Dutta, F. Basuli, S.M. Peng, G.H. Lee, S. Bhattacharya, New. J. Chem. 26 (2002) 1607. I. Pal, F. Basuli, T.C.W. Mak, S. Bhattacharya, Angew. Chem., Int. Ed. 40 (2001) 2923. F. Basuli, S.M. Peng, S. Bhattacharya, Inorg. Chem. 39 (2000) 1120. F. Basuli, M. Ruf, C.G. Pierpont, S. Bhattacharya, Inorg. Chem. 37 (1998) 6113. F. Basuli, S.M. Peng, S. Bhattacharya, Inorg. Chem. 36 (1997) 5645. A. Molnar, Chem. Rev. 111 (2011) 2251. S.D. Gonzalez, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009) 3612. A. Fihri, P. Meunier, J.C. Hierso, Coord. Chem. Rev. 251 (2007) 2017. C.J. Elsevier, Coord. Chem. Rev. 185 (1999) 809. R.B. Bedford, C.S.J. Cazin, D. Holder, Coord. Chem. Rev. 248 (2004) 2283. C. Barnard, Platinum Metals Rev. 52 (2008) 38. V. Polshettiwar, C. Len, A. Fihri, Coord. Chem. Rev. 253 (2009) 2599. M.J. Frisch, G.W. Tracks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Calamani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Ukuda, I. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskroz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, GAUSSIAN 03, revision D01; GAUSSIAN Inc.: Pittsburgh, PA, 2003. G.M. Sheldrick, SHELXS-97 and SHELXL-97, FORTRAN programs for crystal structure solution and refinement, University of Gottingen, 1997. P. Evans, P. Hogg, R. Grigg, M. Nurnabi, J. Hinsley, V. Sridharan, S. Suganthan, S. Korn, S. Collard, J.E. Muir, Tetrahedron 61 (2005) 9696. C.L. Chen, Y.H. Liu, S.H. Peng, S.T. Liu, Organometallics 24 (2005) 1075. E. Peris, J. Mata, J.A. Loch, R.H. Crabtree, Chem. Commun. (2001) 201. D.A. Albisson, R.B. Bedford, S.P. Noelle, S.E. Lawrence, Chem. Commun. 19 (1998) 2095. S. Gangopadhyay, P. Karmakar, U. Dasgupta, A. Chakraborty, Mutat Res. 633 (2007) 117. I. Vermes, C. Hannen, H. Steffens-Nakken, C. Reutelingsperger, J. Immunol. Methods 184 (1995) 39. A. Montenegro, R. Carballo, E.M. Vázquez-López, Polyhedron 28 (2009) 3915. W. Hong, C. Wu, C. Lee, W. Hwang, M.Y. Chiang, J. Organomet. Chem. 689 (2004) 277. E.L. Torres, M.A. Mendiola, C.J. Pastor, B.S. Pe´rez, Inorg. Chem. 43 (2004) 5222. P. Gómez-Saiz, J. García-Tojal, A. Mendia, B. Donnadieu, L. Lezama, J.L. Pizarro, M.I. Arriortua, T. Rojo, Eur. J. Inorg. Chem. (2003) 518. X.W. Douglas, Y. Yonghong, L.K. Tracey, I.G. Karen, L. Anthony, V.M. Jesus, H.O. Simon, Polyhedron 14 (1995) 3051. B.Z. Lu, C. White, A.L. Rheingold, R.H. Crabtree, Angew. Chem., Int. Ed. 32 (1993) 92. L. Latheef, E.B. Seena, M.R.P. Kurup, Inorg. Chim. Acta 362 (2009) 2515. A. Roth, E.T. Speilberg, W. Plass, Inorg. Chem. 46 (2007) 4362. C.Q. Ma, X.N. Wang, W.X. Zhang, Z.G. Yu, D.H. Jiang, S.L. Dong, Acta Chim. Sinica 54 (1996) 562. X.G. Cui, Q.P. Hu, Chin. J. Struct. Chem. 13 (1994) 340. B.F. Hoskins, R. Robson, H. Schaap, Inorg. Nucl. Chem. Lett. 8 (1972) 759.

130

S. Datta et al. / Inorganica Chimica Acta 392 (2012) 118–130

[58] E. Gyepes, T. Glowiak, Sect. C: Cryst. Struct. Commun. 45 (1989) 391. [59] Y.T. Wang, H.L. Li, J.G. Wang, Z. Kristallogr, New Cryst. Struct. 225 (2010) 79. [60] L.E. Nikolaeva, T.N. Tarkhova, N.V. Belov, Crystallogr. Rep. (Kristallografiya, Russ.) 19 (1974) 746. [61] S. Guveli, N. Ozdemir, T.B. Demirci, B. Ulkuseven, M. Dincer, O. Andac, Polyhedron 29 (2010) 2393. [62] S.V. Kolotilov, O. Cador, S. Golhen, O. Shvets, V.G. Ilyin, V.V. Pavlishchuk, L. Ouahab, Inorg. Chim. Acta 360 (2007) 1883. [63] A.S.A. Zidan, A.I. El-Said, Phosphorus Sulfur Silicon 179 (2004) 1293. [64] A.M. Trzeciak, J.J. Ziółkowski, Coord. Chem. Rev. 249 (2005) 2308.

[65] [66] [67] [68]

A.M. Trzeciak, J.J. Ziółkowski, Coord. Chem. Rev. 251 (2007) 1281. T.T.L.A. Yeung, A.S.C. Chan, Coord. Chem. Rev. 248 (2004) 2151. K. Mitsudo, T. Imura, T. Yamaguchi, H. Tanaka, Tet. Lett. 49 (2008) 7287. K. Inamoto, J.I. Kuroda, K. Hiroya, Y. Noda, M. Watanabe, T. Sakamoto, Organometallics 25 (2006) 3095. [69] S. Ma, H. Wang, K. Gao, F. Zhao, J. Mol. Cat. 248 (2006) 17. [70] A. Houdayer, R. Schneider, D. Billaud, J. Ghanbaja, J. Lambert, Syn. Metals 151 (2005) 165. [71] K.I. Ansari, J.D. Grant, S. Kasiri, G. Woldemariam, B. Shrestha, S.S. Mandal, J. Inorg. Biochem. 103 (2009) 818.