Inorganica Chimica Acta 375 (2011) 77–83

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Inorganica Chimica Acta

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Synthesis, crystal structure and DNA interaction studies on mononuclearzinc complexes

Kaushik Ghosh ⇑, Pramod Kumar, Nidhi TyagiDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India

a r t i c l e i n f o

Article history:Received 17 January 2011Received in revised form 5 April 2011Accepted 19 April 2011Available online 27 April 2011

Keywords:Zinc complexesCrystal structure1H and 13C NMR luminescence DNAinteraction

0020-1693/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.ica.2011.04.035

⇑ Corresponding author. Fax: +91 1332 273560.E-mail address: ghoshfcy@iitr.ernet.in (K. Ghosh).

a b s t r a c t

A new family of mononuclear Zn(II) complexes [Zn(Pyimpy)2](ClO4)2 (1), [Zn(Pyimpy)(Cl)2] (2), [Zn(Pyim-py)(SCN)2] (3) and [Zn(Pyimpy)(N3)2] (4) were synthesized using designed tridentate ligand Pyimpyhaving NNN donors (Pyimpy: (2-((2-phenyl-2-(pyridin-2-l)hydazono)methyl)pyridine)). Complexeswere characterized by different spectroscopic studies and it has been found out that all complexesexhibited strong fluorescent emission at room temperature. Molecular structures of [Zn(Pyimpy)2](ClO4)2�C6H5CH3�0.5H2O (1�C6H5CH3�0.5H2O) and [Zn(Pyimpy)(Cl)2]�CH3CN (2�CH3CN) were determinedby X-ray crystallography and ligand coordinated Zn(II) ions was described as distorted octahedral anddistorted square pyramidal, respectively. DNA binding properties of these complexes were investigatedby absorption spectral, fluorescence quenching and circular dichroism spectral studies.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Zinc is an essential element for life and after iron, zinc is thesecond most abundant transition metal present in living system[1]. With a stable 2+ oxidation state zinc does not participate in re-dox activity, however, zinc is important for several cellular pro-cesses [2–4]. Many enzymes need zinc ion as cofactor for theircatalytic activity; moreover zinc complexes are often used as drugs[5–7]. Hence this metal ion exhibit plural roles in biosystem andthere are several applications of zinc complexes. For example,luminescence properties of zinc complexes having d10 configura-tion, in several times used for the preparation of light emittingmaterials for diodes, light harvesting materials for photocatalysisand fluorescent sensors for organic or inorganic analytes [8–10].Excitation and/or emission profile of certain zinc complexes areimportant for zinc ion imaging in living cells [11,12].

Interaction of DNA with transition metal complexes has gainedconsiderable current interest due to their various applications incancer research and nucleic acid chemistry [13–17]. Zinc has be-come one of the best suited metal ion for the development of arti-ficial nucleases because of strong Lewis acidity of Zn(II) ion, rapidligand exchange property and lower toxicity of zinc complexes[18,19]. Oxidative cleavage [19] and hydrolytic cleavage of DNAwere examined and their mechanism was reported for several zinccomplexes [20–24]. The present work stems from our interest inDNA interaction and nuclease activity studies on transition metalcomplexes [25–30]. Recently we have reported nuclease activity

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of a novel copper complex derived from a tridentate ligand Pyimpy(shown in Scheme 1) via self-activation [28] and communicatedother results afforded from this family of complexes [29]. It is wellknown in the literature that analogous complexes of zinc in severaltimes help to interpret the redox, spectroscopic, magnetic andstructural properties of copper complexes [30,31].

Hence we report here the synthesis and characterization of fournew zinc complexes [Zn(Pyimpy)2](ClO4)2 (1), [Zn(Pyimpy)(Cl)2](2), [Zn(Pyimpy)(SCN)2] (3) and [Zn(Pyimpy)(N3)2] (4) derived fromligand Pyimpy. Molecular structures of [Zn(Pyimpy)2](ClO4)2�C6H5CH3�0.5H2O (1�C6H5CH3�0.5H2O) and [Zn(Pyimpy)(Cl)2]�CH3CN(2�CH3CN) were determined by X-ray crystallography. The ligandbinding to zinc center was also supported by 1H and 13C NMR spec-tral studies. Emission properties of these complexes were examinedat room temperature. We investigated the DNA binding propertiesof these complexes by absorption spectral studies, fluorescencequenching and circular dichroism spectral studies and also exam-ined the nuclease activity. We tried to understand the better insightinto the DNA binding events and nuclease activity by comparing thedata obtained from both copper [28,29] and zinc chemistry.

2. Experimental

2.1. Reagents and materials

All the chemicals were purchased from Sigma, S.D. Fine, AlfaAesar, Acros, Himedia, and Merck. Sodium azide (Sigma–Aldrich,Steinheim, Germany), phenylhydrazine, (S.D. Fine, Mumbai, India),zinc perchlorate hexahydrate (Alfa Aesar, India), ammonium

N

NN N

Pyimpy

Pyimpy

[Zn(Pyimpy)2](ClO4)2

ZnCl21:1

2:1

[Zn(Pyimpy)(Cl)2]

ZnClO4 6H2O

[Zn(Pyimpy)(N3)2]

[Zn(Pyimpy)(SCN)2]

NH4SCN

NaN3

(4)

(2)

(1) (3)

Scheme 1. Synthetic presentation of complexes 1–4.

78 K. Ghosh et al. / Inorganica Chimica Acta 375 (2011) 77–83

thiocyanate, ethylenediamine tetraacetic acid, zinc chloride (MerckLimited, Mumbai, India), agarose (Himedia Laboratories Pvt. Ltd.,Mumbai, India). 2-Chloropyridine, pyridine 2-aldehyde (Acrosorganics, USA) was used as obtained. The supercoiled pBR322DNA and CT DNA were purchased from Bangalore Genei (India).Tris(hydroxymethyl)aminomethane–HCl (Tris–HCl) buffer wasprepared in deionised water. Solvent used for spectroscopic studieswere HPLC grade and purified by standard procedure before use[25].

2.2. Methods and instrumentation

Elemental analysis of ligand and complexes were carried out onElemental model Vario EL-III. Infrared spectra were recorded asKBr pellets on a Nicolet NEXUS Aligent 1100 FT-IR Spectrometer,using 50 scans and were reported in cm�1. Electronic spectra wererecorded in acetonitrile, dichloromethane, DMF and phosphatebuffer solution with Evolution 600, Thermo scientific, UV–Vis spec-trophotometer. Fluorescence spectra were recorded by Varian Cary(Eclipse) fluorescence spectrophotometer. Circular dichroism (CD)spectra were recorded on Chirascan circular dichroism spectrome-ter, Applied photophysics, UK. Molar conductivities were deter-mined in dimethylformamide (DMF) at 10�3 M at 25 �C with aSystronics 304 conductometer.1H and 13C NMR spectra were re-corded on Bruker AVANCE, 500.13 MHz spectrometer, chemicalshift for 1H NMR spectra’s are related to internal Me4Si all residualprotium in the deutrated solvents.

Caution: Perchlorate salt of metal complexes with organic li-gands is potentially explosive. Only a small amount of materialshould be prepared and handled with caution.

2.3. Synthesis of zinc complexes

2.3.1. [Zn(Pyimpy)2](ClO4)2�C6H5CH3�0.5H2O (1�C6H5CH3�0.5H2O)Methanolic solution (5 mL) of Zn(ClO4)2�6H2O (0.186 g,

0.5 mmol) was added dropwise into the 5 mL of methanolic solu-tion of Pyimpy (0.274 g, 1 mmol) with constant stirring. Yellowturbidity appeared and after 2 h of stirring microcrystalline com-pound was filtered off, washed with methanol and dried in vacuo.Slow evaporation of acetonitrile and toluene (3:2) at room temper-ature provided colorless crystals of the compound suitable forX-ray diffraction. Yield: 62%. Anal. Calc. for C41H37N8Cl2O8.5Zn: C,53.87; H, 4.08; N, 12.26. Found C, 53.80; H, 4.12; N, 12.23%. 1HNMR (CD3OD, 500 MHz): d 8.40 [d, J = 5.3 Hz, 1H], 8.11 [d,J = 1.5 Hz, 1H], 8.09 [m, 2H], 7.94 [t, J = 7.5 Hz, 2H], 7.88–7.83 [m,5H], 7.57 [ddd, J = 7.6 Hz, 5.1 Hz, 1.0 Hz, 1H], 7.16 [dt, J = 6.5 Hz,0.7 Hz, 1H], 6.76 [d, J = 8.6 Hz, 1H]. 13C NMR (CD3OD, 500 MHz):154.13, 149.66, 148.63, 146.64, 143.65, 142.87, 137.72, 135.54,133.37, 133.27, 130.83, 129.01, 128.40, 121.34, 112.50. IR (cm�1,KBr disk): 1600(s, mC@N), 1561(m), 1438(s), 1309(m), 1238(m),1090(s, br, mClO4), 775(m), 702(m), 623(m, mClO4). UV�Vis in dichlo-romethane, kmax (nm) (e (M�1 cm�1)): 252 (29 100), 292 (23 240),

299 (22 110), 364 (45 030). Conductivity (KM/X�1 cm2 mol�1) inDMF: 124.

2.3.2. [Zn(Pyimpy)(Cl)2]�CH3CN (2�CH3CN)A solution of anhydrous ZnCl2 (0.136 g, 1 mmol) in 5 mL meth-

anol was added dropwise to a 5 mL methanolic solution of Pyimpy(0.273 g, 1 mmol) with stirring. After 2 h of stirring the yellow pre-cipitate was filtered off and washed with methanol. Slow evapora-tion of acetonitrile solution at room temperature providedcolourless crystals suitable for X-ray diffraction. Yield: 73%. Anal.Calc. for C19H17N5Cl2Zn: C, 50.52; H, 3.79; N, 15.51. Found: C,50.44; H, 3.74; N, 15.53%. 1H NMR (CDCl3, 500 MHz): d 8.95 [d,J = 4.8 Hz, 1H], 8.72 [d, J = 5.6 Hz, 1H] 7.88 [dt, J = 7.7 Hz, 1.6 Hz,1H], 7.75 [tt, J = 7.3 Hz, 1.6 Hz, 2H], 7.70 [tt, J = 7.4 Hz, 1.3 Hz,1H], 7.66 [dt, J = 7.9 Hz, 1.8 Hz, 1H], 7.53 [ddd, J = 7.9 Hz, 5.1 Hz,1.0 Hz, 1H], 7.41 [td, J = 7.1 Hz, 1.6 Hz, 2H], 7.31 [td, J = 7.7 Hz,1.2 Hz, 1H], 7.28 [s, 1H], 7.16 [ddd, 7.5 Hz, 5.3 Hz, 0.7 Hz, 1H],6.43 [d, J = 8.4, 1H]. 13C NMR (CDCl3, 500 MHz): 151.81, 150.34,147.98, 146.55, 140.83, 139.92, 134.80, 132.14, 131.86, 132.00,131.74, 129.16, 126.68, 124.59, 119.54, 109.66. IR (cm�1, KBr disk):1596(s,mC@N), 1560(m), 1439(s), 1303(m), 1241(m), 1126(m),774(m), 706(m). UV�Vis in dichloromethane, kmax (nm) (e(M�1 cm�1)): 249 (14 960), 292 (11 860), 300 (11 530), 361(25 140). Conductivity (KM/X�1 cm2 mol�1) in DMF: 8.

2.3.3. [Zn(Pyimpy)(SCN)2] (3)The methanolic solution (5 mL) of anhydrous ZnCl2 (0.136 g,

1 mmol) was added slowly to a magnetically stirred 5 ml methano-lic solution of the ligand Pyimpy (0.273 g, 1 mmol). The mixturewas stirred for 30 min and ammonium thiocyanate (0.152 g,2 mmol) was added to the above reaction mixture. Yellow precip-itate, different to that of complex 2 was obtained after 3 h of stir-ring. The precipitate was filtered off, washed with methanol anddried in vacuo. Yield: 76%. Anal. Calc. for C19H14N6S2Zn: C, 50.06;H, 3.10; N, 18.43; S 14.34. Found: C, 50.10; H, 3.18; N, 18.44; S14.21%. 1H NMR (CDCl3, 500 MHz): d 8.74 [d, J = 4.8 Hz, 1H], 8.47[d, J = 4.5 Hz, 1H], 7.98 [dt, J = 7.7 Hz, 1.6 Hz, 1H], 7.80–7.72 [m,4H], 7.61 [ddd, J = 7.9 Hz, 5.0 Hz, 1.0 Hz, 1H], 7.43–7.38 [m, 3H],7.30 [s, 1H], 7.23 [ddd, J = 7.5 Hz, 5.4 Hz, 0.6 Hz, 1H], 6.50 [d,J = 8.5 Hz, 1H]. 13C NMR (CDCl3, 500 MHz): 151.76, 149.63,146.97, 146.38, 141.45, 140.57, 136.39, 134.12, 132.42, 132.19,131.96, 128.87, 127.10, 125.00, 119.84, 110.00. IR (cm�1, KBr disk):2093, 2072(d, s, mSCN), 1599(s,mC@N), 1560(m), 1469(m), 1439(s),1306(m), 1239(m), 773(m), 699(m). UV�Vis in dichloromethane,kmax (nm) (e (M�1 cm�1)): 251 (22 630), 291 (11 960), 298(11 580), 363 (22 970). Conductivity (KM/X�1 cm2 mol�1) inDMF: 12.

2.3.4. [Zn(Pyimpy)(N3)2] (4)Complex 4 was prepared following the same procedure as for 3,

except by adding sodium azide instead of sodium thiocyanate.Yield: 71%. Anal. Calc. for C17H14N10Zn: C, 48.18; H, 3.33; N,33.05. Found: C, 48.22; H, 3.36; N, 33.09%. 1H NMR (CDCl3,500 MHz): d 8.85 [d, J = 5.3 Hz, 1H], 8.59 [d, J = 2.8 Hz, 1H], 7.92[t, J = 5.2 Hz, 1H], 7.78–7.68 [m, 4H], 7.56 [dd, J = 7.7 Hz, 5.0 Hz,1H], 7.44 [d, J = 7.4 Hz, 2H], 7.37 [d, J = 7.6 Hz, 1H], 7.32 [s, 1H],7.19 [dd, J = 7.5 Hz, 5.4 Hz, 1H], 6.48 [d, J = 9.0 Hz, 1H]. 13C NMR(CDCl3, 500 MHz): 151.87, 150.03, 147.46, 146.61, 140.96, 140.05,134.44, 132.14, 132.05, 131.73, 129.04, 126.55, 124.84, 119.37,109.83. IR (cm�1,KBr disk): 2055(s, mN3), 1598(s, mC=N), 1561(m),1468(s), 1440(s), 1306(m), 1242(m), 777(m), 702(m). UV�Vis indichloromethane, kmax (nm) (e (M�1 cm�1)): 249 (13 480), 292(11 830), 299 (11 270), 361 (23 600). Conductivity (KM/X�1 cm2 mol�1) in DMF: 9.

K. Ghosh et al. / Inorganica Chimica Acta 375 (2011) 77–83 79

2.4. DNA binding experiments

The DNA binding experiments were carried out in 0.1 M phos-phate buffer (pH 7.2) using a solution of calf thymus (CT) DNAwhich gave a ratio of UV–Vis absorbance at 260 and 280 nm(A260/A280) of ca. 1.8, indicating that the CT DNA was sufficientlyprotein free [26]. The concentration of DNA solution was deter-mined by UV absorbance at 260 nm and the extinction coefficiente260 was taken 6600 cm�1 as reported in the literature [26]. Absor-bance titration experiment were carried out with complex concen-tration of 40 lM varying the CT DNA concentration from 0 to90 lM in 0.1 M phosphate buffer (pH 7.2) containing 5% DMF.The binding constant Kb were determined from a plot of [DNA]/(ea � ef) versus [DNA] using the equation: [DNA]/(ea � ef) = [DNA]/(eb � ef) + [Kb(eb � ef)]�1, where [DNA] is the concentration ofDNA in base pairs. The apparent absorption coefficients ea, ef andeb correspond to Aobs/[Complex], the extinction coefficient of thefree complex and the extinction coefficient of the copper complexin the fully bound form, respectively.

Fluorescence quenching experiments were carried out by thesuccessive addition of complexes 1 and 2 to the DNA (25 lM) solu-tions containing 5 lM ethidium bromide (EB) in 0.1 M phosphatebuffer (pH 7.2). For better solubility of complexes 1 and 2, weuse 5% DMF. These samples were excited at 250 nm and emissionswere observed between 500 and 700 nm. Stern–Volmer quenchingconstants were calculated using the given equation

I0=I ¼ 1þ KsvQ ;

where I0 and I are the fluorescence intensities in the absence andpresence of complex and Q is the concentration of quencher(complexes 1 and 2). Ksv is a linear Stern–Volmer constant and givenby the ratio of slope to intercept in the plot of I0 /I versus Q.

Circular dichroism (CD) spectra of CT-DNA in absence and pres-ence of the zinc complexes were recorded with a 0.1 cm path-length cuvette after 10 min incubation at 25 �C. The concentrationof the complexes and CT-DNA were 50 and 200 lM, respectively.Details of nuclease activity studies were described in supportinginformation.

2.5. X-ray crystallography

Crystals for single-crystal X-ray diffraction study were chosenand mounted on glass fibers and transferred to an Bruker AXS sin-gle crystal X-ray diffractometers equipped with a graphite–mono-chromatic Mo Ka X-ray source (k = 0.71073 Å). Data were collectedat 293 K using x\2h scan technique. Structure solution, refine-ment and data output were carried out with the SHELXTL program

Fig. 1. Molecular structure of the cation [Zn(Pyimpy)2]2+ of 1.

[32]. All non-hydrogen atoms were refined anisotropically. Hydro-gen atoms were placed in geometrically calculated positions andrefined using a riding model. Images were created with the DIA-

MOND program [33].

3. Results and discussion

3.1. Synthesis

The tridentate ligand Pyimpy synthesized by the reported pro-cedure [27]. Reaction of the ligand Pyimpy with Zn(ClO4)2�6H2Oafforded [Zn(Pyimpy)2](ClO4)2] (1) whereas starting salt anhydrousZnCl2 gave rise to complexes [Zn(Pyimpy)(Cl)2] (2). One equivalentof anhydrous ZnCl2 when reacted with one equivalent of Pyimpyand two equivalents of NH4SCN or NaN3 and complexes [Zn(Pyim-py)(SCN)2] (3) and [Zn(Pyimpy)(N3)2] (4) were obtained respec-tively. We want to mention here that one equivalent of NH4SCNor NaN3 also afforded complexes 3 and 4, respectively. Complexes3 and 4 can also be prepared by Zn(ClO4)2�6H2O however, the yieldof the desired product is low due to the concomitant formation ofcomplex 1. Details of the syntheses are summarized in Scheme 1.

3.2. Description of crystal structure

Molecular structures of [Zn(Pyimpy)2](ClO4)2�C6H5CH3�0.5H2O(1�C6H5CH3�0.5H2O) and [Zn(Pyimpy)(Cl)2]�CH3CN (2�CH3CN) aredepicted in Figs. 1(a) and 2(a), respectively. The matrix parametersare described in Table 1 and selected bond distances and bond an-gles are described in Table 2. In the structure of 1�C6H5CH3�0.5H2O,we found out disorder in toluene molecules in the crystal lattice. Inboth complexes ligand Pyimpy was found to coordinate with twopyridine nitrogen (NPy) and azomethine nitrogen (NIm) to the metalcentre in meridional fashion. The ligand has three six-memberedrings; among them, two pyridine rings are in the same planewhereas the other phenyl ring is roughly perpendicular (81.48�for 1�C6H5CH3�0.5H2O and 86.55� for 2�CH3CN) to the ligand bind-ing plane.

The geometry around metal centers in 1�C6H5CH3�0.5H2O is de-scribed as distorted octahedral. The equatorial plane consists oftwo NPy (trans to each other) and two NIm donors along with twoNPy donors as axial ligands. Complex 2�CH3CN afforded a distortedsquare-pyramidal geometry. From structural index parameter cal-culation (s) [34], the s value for 2�CH3CN is 0.076 (for a perfectlytrigonal bipyramidal geometry s is equal to unity) which also indi-cated the square pyramidal geometry. In complex 2�CH3CN theequatorial plane consists of two NPy and one NIm donors along witha Cl� ligand and another Cl� occupied axial position. All the Zn–NPy

Solvent molecules and H-atoms are not shown for clarity.

Fig. 2. Molecular structure of [Zn(Pyimpy)(Cl)2] (2). Solvent molecules and H-atoms are not shown for clarity.

Table 1Summary of crystal data and data collection parameters for 1�C6H5CH3�0.5H2O and 2�CH3CN.

Empirical formula C82H72N16O17Cl4Zn2 C19H17N5Cl2ZnFormula weight (g mol�1) 1826.10 451.65T (K) 293(2) 293(2)k (Å) (Mo Ka) 0.71073 0.71073Crystal system (mm) monoclinic monoclinicSpace group P(1)2(1)/c1 P(1)2(1)/c1a (Å) 10.222(5) 13.440(3)b (Å) 39.193(5) 14.232(4)c (Å) 21.372(5) 10.793(3)b (�) 100.969(5) 100.799(9)V (Å3) 8406(5) 2028(9)Z 4 4qcalc (g cm�3) 1.443 1.479Crystal size (mm) 0.22 � 0.20 � 0.16 0.24 � 0.20 � 0.16F(0 0 0) 3760 920h (�) 1.94–26.86 0.953–26.86Index ranges �9 < h < 9, �34 < k < 34, �19 < l < 19 �17 < h < 17, 0 < k < 18, 0 < l < 14Refinement method Full matrix least-squares on F2 Full matrix least-squares on F2

Data/restraints/parameters 6255/0/1045 4714/0/245Goodness-of-fit (GOF)a on F2 0.975 1.034R1

b [I > 2r(I)] 0.0854 0.1006R1 [all data] 0.0610 0.0628wR2

c [I > 2r(I)] 0.1872 0.2057wR2 [all data] 0.1616 0.1653

a GOF = ½P½wðF2

o � F2c Þ

2�=M � N�1=2 (M = number of reflections, N = number of parameters refined).b R1 ¼

PjjFoj � jFc jj=

PjFoj.

c wR2 ¼ ½P½wðF2

o � F2c Þ

2�=P½ðFo2Þ2��1=2.

80 K. Ghosh et al. / Inorganica Chimica Acta 375 (2011) 77–83

distances (range 2.13–2.17 Å) in 1�C6H5CH3�0.5H2O and 2�CH3CNare higher than the reported values [35]. Zn–Nim distances for com-plexes 1�C6H5CH3�0.5H2O and 2�CH3CN were found to be in therange 2.15–2.16 Å. These distances are lower than the literaturevalues (�2.26 Å) [36] but higher than the Zn–NIm (�2.02 Å) dis-tances reported by Keypour et al. [37]. In complex 2�CH3CN theZn–Cl distances are 2.250 and 2.264 Å, respectively and higherthan the reported values in the literature [38]. These axial andequatorial Zn–Cl distances were found to be similar.

In complex 1�C6H5CH3�0.5H2O, the N4–Zn1–N5 and N1–Zn1–N8angles are nearly 148� whereas N2–Zn1–N6 angle was found to be177.5�. These data indicated the distortion along N4–Zn1–N5 andN1–Zn1–N8 axes however, lesser deviation was observed alongN2–Zn1–N6 axis. The mean plane generated by pyridine donoratoms namely N1, N4, N5 and N6 passes through zinc and thesedonor atoms are nearly 0.60 Å away from above plane (Fig. 1b).In distorted square pyramidal complex 2�CH3CN, zinc is 0.57 Åaway from the mean plane generated by N1, N2, N4 and Cl1. Inter-estingly, axial Cl2 makes equal angles at the metal centre (101�)with all equatorial nitrogen donors however, angle Cl2–Zn1–Cl1

with equatorial Cl1 is 112.93�. These data show that equatorialCl1 have more contribution in distortion as compared to nitrogendonors (Fig. 2b).

p-Stacking interaction with aryl hydrogen and hydrogen bond-ing network is important in crystal engineering and supramolecu-lar chemistry [39]. The non-covalent interaction found in 2�CH3CNwere hydrogen bonding and are described in Fig. S5. These hydro-gen bonding interaction generated a polymeric structure in thepacking diagram (Figs. S6 and S7). The overall crystal structureand position of CH3CN molecules in 2�CH3CN are reperesented inFig. S8. The three dimensional interaction indicated that the CH3CNmolecules are staying between two layers of [Zn(Pyimpy)(Cl)2]. Wewould like to mention here that similar complexes of copper wereisolated and molecular structures were determined by X-ray crys-tallography [29].

3.3. General properties

For all complexes coordination of azomethine nitrogen was sup-ported by shifting of mC@N as compared to free ligand in IR spectra

Table 2Selected bond lengths (Å) and angles (�) for 1�C6H5CH3�0.5H2O and 2�CH3CN.

1�C6H5CH3�0.5H2O 2�CH3CN

Bond length (Å)Zn(1)�N(1) 2.170(8) Zn(1)�N(1) 2.164(4)Zn(1)�N(2) 2.157(7) Zn(1)�N(2) 2.151(4)Zn(1)�N(4) 2.133(8) Zn(1)�N(4) 2.149(4)Zn(1)�N(5) 2.140(8) Zn(1)�Cl(1) 2.2501(13)Zn(1)�N(6) 2.133(7) Zn(1)�Cl(2) 2.2639(15)Zn(1)�N(8) 2.163(8)

Bond angles (�)N(1)�Zn(1)�N(2) 74.5(4) N(1)�Zn(1)�N(2) 73.45(13)N(1)�Zn(1)�N(4) 148.0(4) N(1)�Zn(1)�N(4) 141.42(13)N(1)�Zn(1)�N(5) 91.4(3) N(2)�Zn(1)�N(4) 72.18(13)N(1)�Zn(1)�N(6) 103.0(3) N(1)�Zn(1)�Cl(1) 98.77(9)N(1)�Zn(1)�N(8) 96.8(3) N(1)�Zn(1)�Cl(2) 101.64(10)N(2)�Zn(1)�N(4) 73.7(4) N(2)�Zn(1)�Cl(1) 145.96(12)N(2)�Zn(1)�N(5) 104.9(3) N(2)�Zn(1)�Cl(2) 101.11(11)N(2)�Zn(1)�N(6) 177.5(4) N(4)�Zn(1)�Cl(1) 99.89(10)N(2)�Zn(1)�N(8) 107.3(4) N(4)�Zn(1)�Cl(2) 101.66(11)N(4)�Zn(1)�N(5) 100.1(3) Cl(1)�Zn(1)�Cl(2) 112.93(6)N(4)�Zn(1)�N(6) 108.8(4)N(4)�Zn(1)�N(8) 89.2(3)

400 450 500 550 600 6500

50

100

150

200 1 2 3 4

Inte

nsi

ty

Wavelength (nm)

Fig. 3. Emission spectra of complexes 1–4 in toluene at kex = 370 nm.

K. Ghosh et al. / Inorganica Chimica Acta 375 (2011) 77–83 81

[40]. Complex 1 showed IR bands near 1090 cm�1 together with aband at 623 cm�1 suggesting the presence of non-coordinated per-chlorate ion [26,30]. N-bonded thiocyanate ligand to the metal cen-tre could be identified by two bands near 2070 and 2090 cm�1 in IRspectra. In general, stretching frequencies of mSCN fragment of thio-cyanate moiety are lower in N-bonded complexes (near or below2050 cm�1) than S-bonded complexes (near 2100 cm�1). In com-plex 3, the strong bands at 2091 and 2072 cm�1 for mSCN showedthe presence of the N-coordinated thiocyanate (Fig. S1) [41]. Thestrong bands at 2055 cm�1 for complex 4 indicated the terminalcoordination of azide to the zinc centre (Fig. S2) [42]. All the com-plexes exhibited a charge transfer band near 360 nm in UV–Visspectra. The molar conductivity measurement for complexes 2–4in DMF solution (ca. 10�3 M) gave rise to conductivity values inthe range 9–32 X�1 cm2 mol�1 at 25 �C indicating neutral electro-lytic behavior, whereas complex 1 afforded 124 X�1 cm2 mol�1 at25 �C indicating bi-univalent electrolytic behavior [43].

The structures of complexes 3 and 4 were established by 1H and13C NMR spectral studies as shown in Figs. S3 and S4, respectively.The spectra were similar to the structurally characterized 1 and 2.The formation of S-coordinated and N-coordinated complexes caneasily be identified by the characteristic 13C signal. S-coordinatedand N-coordinated thiocyanate ligands afford 13C signals near 111and 145 ppm, respectively [41]. In case of complex 3, excluding thepeaks for the ligand frame, one extra peak at 136.4 ppm in 13C NMRspectra, confirms N-bonded thiocyanate to the zinc centre [44]. Theformation of mononuclear complexes without bridging SCN� orN3�was also supported by IR and conductivity measurements.

3.4. Photophysical properties

All these compounds (1–4) showed emission properties in tolu-ene solution at room temperature upon excitation of the chargetransfer band near 350 nm and are depicted in Fig. 3. There is noemission of the complexes 1–4 in methanol, acetonitrile, dichloro-methane and water. The free ligand Pyimpy displays no fluores-cence in the same experimental condition. Absence of emissionproperties in water restricted us in applying this spectral propertyin DNA interaction studies and detail work is under progress. It isimportant to note here that fluorescence enhancement due to com-plexation is useful for the applications of such complexes in photo-chemical reactions [45]. The fluorescence intensity may be greatlyenhanced by the coordination of Zn(II) where zinc coordination in-

creases the rigidity of the ligand and thus reduces the loss of en-ergy by thermal vibrational decay [46].

3.5. DNA interaction studies

3.5.1. Stability of complexes in bufferWe have performed all the DNA interaction studies in phos-

phate buffer (0.1 M, pH 7.2) so it was necessary to check the stabil-ity of these complexes (1 and 2) in the same experimentalcondition. We have chosen mainly theses two complexes becausesimilar complexes of copper were synthesized and characterized[29]. Fig. S9 showed that there was a very little change in absor-bance however no change in peak position was observed for com-plexes 1 and 2 within 10 days. A small change in absorbancewithout considerable k shifting predicted the stability of these zinccomplexes in the above buffer solution.

3.5.2. Absorption spectral studiesElectronic absorption spectroscopic techniques were used to

investigate the DNA binding of zinc complexes. To achieve this,the absorption spectra of complexes in the absence and presenceof calf thymus DNA (CT DNA) at different concentrations weremeasured. A blue shift of �4 nm and hyperchromic shift of theabsorption band were observed in case of complex 1. These spec-tral changes afforded an isosbestic point at 353 nm (Fig. S10). Onthe other hand, similar spectral changes were found for complexes2 and 3 and isosbestic points near 350 nm were observed for boththe complexes (Figs. 4 and S11, respectively). To the best of ourknowledge, we found only one report by Arslantas et al. [47] wheresuch type of spectral changes were observed and they describedcovalent binding of zinc complex with DNA. However, investiga-tion of literature afforded few reports where new species genera-tion and/or covalent binding of copper with DNA was described[48,49]. Our data are similar to the data obtained for copper com-plexes [29], hence we speculate that zinc complexes in presence ofDNA got attached to the nucleic acid base(s) and a new species wasgenerated during DNA interaction. The intrinsic binding constantsKb for complexes 1, 2 and 3 have been determined from the plot of[DNA]/(ea � eb) versus [DNA] and were found to be 2.86 � 103,1.22 � 103 and 1.58 � 103 M�1, respectively. The observed bindingconstants are smaller than the classical intercalators and metallo-intercalators where binding constant was reported to be in the or-der of 107 M�1 [26].

We have performed the reverse titration experiments for com-plexes 1 and 2 (Fig. S12). We added complex solution into DNA

300 350 400 450

0.0

0.4

0.8

1.2

Ab

sorb

ance

Wavelength (nm)

Fig. 4. Absorption spectra of complex 2 in 0.1 M phosphate buffer (pH 7.2)containing 5% DMF in the presence of increasing amounts of DNA. [Com-plex] = 40 lM, [DNA] = 0–90 lM. Arrows show the absorbance changes uponincreasing DNA concentration.

550 600 650 700 7500

30

60

90

120

Inte

nsi

ty

Wavelength (nm)

10 20 30

1.0

1.5

2.0

2.5

I 0/I

[Complex] (μM)

Fig. 5. Fluorescence emission spectra of the EB–DNA in 0.1 M phosphate buffer (pH7.2) containing 5% DMF in presence of complex 1. [EB] = 5 lM, [DNA] = 25 lM,[Zn(Pyimpy)2](ClO4)2 = 0–31.50 lM, kex = 250 nm and kem = 602 nm. Inset: Stern–Volmer plots for 1.

-1

0

1

2θ

(m d

eg)

82 K. Ghosh et al. / Inorganica Chimica Acta 375 (2011) 77–83

solution in these titration experiments. During these experiments,initially (till �20 lM for 1 and �40 lM for 2) we did not see thespectra of complexes 1 and 2 but further addition of complexesafforded the spectra of 1 and 2. This is happening may be due toformation of new species when the concentration of metal com-plexes is low as compared to DNA concentration. At higher concen-tration of the complexes there is no more formation of new speciesand we see the spectra of the metal complexes. These data indicatethe formation of new species consist of DNA and metal complex.

240 260 280 300-3

-2 DNA21

Wavelength (nm)

Fig. 6. Circular dichroism spectra of CT-DNA and its interaction with complexes 1and 2 in 0.1 M phosphate buffer (pH 7.2) after 10 min incubation at 37 �C.

3.5.3. Ethidium bromide displacement assayWe have examined the competitive binding of ethidium bro-

mide versus our complexes with DNA using fluorescence spectralstudies to get better insight into DNA binding events. The fluores-cence quenching curves of ethidium bromide bound to DNA bycomplexes 1 and 2 are shown in Figs. 5 and S13, respectively.The Stern–Volmer quenching constant Ksv for 1 and 2 were ob-tained by Stern–Volmer plot. The Ksv values for 1 and 2 were foundto be 5.16 � 104 M�1and 3.46 � 104 M�1, respectively. These Ksv

values are close to the value obtained from similar copper com-plexes [29] however they are less than the values reported byKessissoglou and coworkers [50].

3.5.4. Circular dichroismCircular dichroism (CD) spectroscopic technique is useful in

monitoring the conformational change of DNA in a solution. TheCD spectrum of CT-DNA was recorded in the range 225–300 nmand it has been found that there were one positive band at278 nm due to base stacking and one negative band at 246 nmdue to helicity. The examination of Fig. 6 indicated small changein negative and the positive bands with a negligible shift in kmax

for complex 2, however there is a change in both the negativeand positive bands for complex 1.

It has been found that complex 1 was more effective in DNAbinding as compared to other zinc complexes because of higherKb and Ksv values and hence higher change in circular dichroismspectra was observed. Examination of DNA binding studies viaUV–Vis absorption spectral data, fluorescence quenching studiesand circular dichroism indicated that zinc and copper complexesinteracted with CT DNA in a similar fashion [28,29]. The ligand

Pyimpy itself does not show such type of spectral changes in anyof the above investigation. We have examined the nuclease activityof our zinc complexes using pBR322 DNA without success(Fig. S14). It is important to note here that the only difference be-tween zinc and copper was the redox property. Due to the lack ofredox property zinc was unable to generate reactive oxygen spe-cies which were found to be probable species for nuclease activity[28,29]. All these data clearly indicated that the lack of nucleaseactivity of the zinc complexes was due to redox inactive zinccentre.

4. Conclusion

Mononuclear zinc complexes derived from tridentate ligandPyimpy, [Zn(Pyimpy)2](ClO4)2 (1), [Zn(Pyimpy)(Cl)2] (2), [Zn(Pyim-py)(SCN)2] (3) and [Zn(Pyimpy)(N3)2] (4) were synthesized andcharacterized by UV–Vis, IR and NMR (1H and 13C) spectral studies.Molecular structures of [Zn(Pyimpy)2](ClO4)2�C6H5CH3�0.5H2O(1�C6H5CH3�0.5H2O) and [Zn(Pyimpy)(Cl)2]�CH3CN (2�CH3CN) weredetermined by X-ray crystallography. The stereochemistry aroundmetal centers in 1�C6H5CH3�0.5H2O was described as distorted

K. Ghosh et al. / Inorganica Chimica Acta 375 (2011) 77–83 83

octahedral whereas complex 2�CH3CN afforded a distorted square–pyramidal geometry. All these complexes show emission proper-ties upon excitation with UV irradiation in toluene at roomtemperature however no luminescence was observed for ligandPyimpy. DNA interaction studies were investigated by absorptionspectral studies, fluorescence quenching and circular dichroismspectral studies and we observed a nice similarity in DNA bindingby zinc and copper complexes. However, zinc complexes did notexhibit nuclease activity although we found the same for the cor-responding copper complexes. Hence we conclude that redoxproperty of the metal centre is important for DNA cleavage activityby complexes having Pyimpy ligand frame.

Acknowledgements

K.G. is thankful to DST, New Delhi, India for SERC FAST TrackProject (SR/FTP/CS-44/2006) for financial support. N.T. and P.K.are thankful to CSIR, India for financial assistance. We are thankfulto SAIF IIT Madras, Dr. Babu Varghese for X-ray crystallographicdata collection and help.

Appendix A. Supplementary material

CCDCs 771773 and 771774 for Complexes 1 and 2 contain thesupplementary crystallographic data for this paper. These datacan be obtained free of charge from The Cambridge Crystallo-graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.ica.2011.04.035.

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