Polyhedron 50 (2013) 139–145

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Polyhedron

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Synthesis, crystal structure, electrochemical and bioactivitiesof pyridine-2-carboxylato bridged copper(II) complexes

Ram N. Patel ⇑, Vishnu P. Sondhiya, Krishna K. Shukla, Dinesh K. Patel, Yogendra SinghDepartment of Chemistry, A.P.S. University, Rewa, MP 486003, India

a r t i c l e i n f o

Article history:Received 8 June 2012Accepted 14 October 2012Available online 30 October 2012

Keywords:Crystal structureBridged copper(II) complexesHydrogen bondingEprBiological activity

0277-5387/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.poly.2012.10.027

⇑ Corresponding author. Tel.: +91 7662 233683; faxE-mail address: rnp64@ymail.com (R.N. Patel).

a b s t r a c t

Two carboxylato bridged copper(II) complexes have been synthesized and characterized: [Cu4(2-Pca)4-(NH)2(bipy)2](ClO4)2�H2O (1) and [Cu(2-Pca)2]n�3H2O (2), (2-Pca = the pyridine-2-carboxylate ion,NH = nicotinic acid hydrazide and bipy = 2,20-bipyridine). Two types of bridging are present in 1; NHmakes a bridge between the two copper atoms, along with 2-Pca. The crystal structure is stabilized byhydrogen bonds of the O–H. . .O type. Complex 1 belongs to triclinic system, having space group P�1.The copper–copper separation is 5.3220(5) Å for 1. The coordination geometry of 1 is distorted squarepyramidal and significant p. . .p stacking interactions are also present. The electrochemical and epr spec-tral studies at 77 K of complexes 1 and 2 have been investigated. Cleavage activities of both complexeshave been investigated on double stranded pBR322 plasmid DNA by gel electrophoresis. Superoxide dis-mutase and antibacterial activity of both complexes have also been measured and discussed.

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1. Introduction research [22,23]. DNA cleavage may take place via hydrolytic or

Research on carboxylates has always been intriguing in thatthey play an important role in synthetic chemistry, with the es-sence of labile coordination modes [1,2], biological activities [3,4]and physiological effects [5]. Copper(II) carboxylates are structur-ally a very diverse group of coordination compounds due to thevarious coordination modes of the carboxylato ligands [6,7]. Reac-tion centers containing two or more transition metal ions are ofparticular interest to study the cooperative effects of redox activesites [8]. Different coordination modes of carboxylate groups leadto the formation of mono- and polynuclear complexes with transi-tion metal ions. It is well known that the carboxylate group is ableto create hydrogen bonds, leading to the formation of supramolec-ular networks which play an important role in the transmission ofmagnetic interactions. This particular interest has also recentlybeen focused on the development of supramolecular structurescreated by hydrogen bonds. Non-covalent interactions, such ashydrogen bonding, hydrophobic, steric repulsion, aromatic ringstacking and electrostatic interactions play important roles inchemical reactions, molecular recognition and regulating biochem-ical processes [9–11]. Complexation with copper enhances the bio-logical activity of a wide variety of organic ligands [12,13]. DNAcleavage is considered to be an important enzymatic reactioninvolved in a number of biological processes and in the biotechno-logical manipulation of genetic materials [14–21]. Application ofmetal complexes as chemical nucleases is the focus of current

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oxidative pathways [24,25]. Hydrolytic cleavage of DNA causeshydrolysis of the phosphodiester bond, forming DNA fragmentsthat could be rejoined. The oxidative cleavage of DNA results inthe oxidation of the sugar moiety of the base. This process is suit-able for foot printing and therapeutic studies.

Pyridine-2-carboxylic acid is an isomer of nicotinic acid and isone of the most important chelating agents in the human body. Dur-ing digestion it is secreted to the intestine and is used as a complex-ing agent (bio-ligand) in the absorption of essential metals [26,27].Nicotinic acid hydrazide is an isomer of isonicotinic acid hydrazide(INH) which is the most important drug of a series of compoundsbased on carboxylic acid hydrazide [28]. Recently some examplesof structurally characterized copper(II) complexes containing2-Pca have been reported in the literature [29–33]. In order to ob-tain mimicry of metalloprotein active sites, we decided to use bio-ligands. Using the bio-ligands pyridine-2-carboxylic acid (2-Pca)and nicotinic acid hydrazide (NH), we have synthesized and charac-terized two polynuclear copper(II) complexes using various physi-cochemical techniques. The different bioactivity properties, suchas superoxide dismutase, antibacterial and DNA cleavage activity,of both complexes were also studied and discussed in detail.

2. Experimental

2.1. Materials and physical measurements

Copper perchlorate hexahydrate (Sigma Aldrich), nicotinic acidhydrazide (Acros Organics), copper carbonate (Acros Organics),2-Pca (S.D. fine) and pBR322 plasmid DNA (Bangalore GeNei). All

140 R.N. Patel et al. / Polyhedron 50 (2013) 139–145

the solvents were purchased from commercial sources and wereused without further purification.

The elemental analyses of the complexes were performed on anElementar Vario ELIII Carlo Erba 1108 Elemental analyzer. FastAtom Bombardment mass spectra of the complexes were recordedon a JEOL SX 102/DA 6000 mass spectrometer using xenon (6 kV,10 mA) as the FAB gas. The accelerating voltage was 10 kV andspectra were recorded at room temperature (RT) with m-nitroben-zyl alcohol as the matrix. UV–Vis spectra were recorded at roomtemperature on a Shimadzu 1601 spectrophotometer. IR spectrawere recorded in KBr medium on a Perkin-Elmer spectrophotome-ter. Cyclic voltammetry was carried out on a BAS-100 Epsilonelectrochemical analyzer, using an electrochemical cell with athree-electrode system. Ag/AgCl was used as the reference elec-trode, glassy carbon as the working electrode and platinum wireas the auxiliary electrode. NaClO4 (0.1 M) was used as supportingelectrolyte and DMSO as the solvent. All measurements were car-ried out at 298 K under a nitrogen atmosphere. Electron paramag-netic resonance (epr) spectra were recorded with a Varian E-lineCentury Series epr spectrometer equipped with a dual cavity andoperating at the X-band of the 100 kHz modulation frequency.Tetracyanoethylene was used as a field marker (g = 2.00, 277).

2.2. Synthesis of [Cu4(2-Pca)4(NH)2(bipy)2](ClO4)2�H2O (1)

In the synthesis of complex 1, Cu(ClO4)2�6H2O (1 mmol, 0.370 g)was dissolved in a 50 ml mixture of ethanol and water (1:1, V/V)then NH (1 mmol, 0.137 g), 2-Pca (1 mmol, 0.123 g) and bipy(1 mmol, 0.156 g) were added one by one with an interval of10 min. The resulting mixture was stirred for 3 h at room temper-ature. After one week, light blue crystals suitable for single crystalX-ray diffraction were obtained. Yield: 65%. Elemental analysis calc.for C56H44Cl2Cu4N14O19 1 (Mr = 1542.12): C, 43.62; H, 2.88; N,12.72. Found: C, 43.05; H, 3.26; N, 12.45%. FAB mass (m/z) calc.:664.61. Found: 664.

2.3. Synthesis of [Cu(2-Pca)2]n�3H2O (2)

Complex 2 was prepared from the reaction of CuCO3�Cu(OH)2

with pyridine-2-carboxylic acid in a 50 ml mixture of ethanoland water (1:1, V/V). CuCO3�Cu(OH)2 (0.5 mmol, 0.110 g) and2-Pca (2 mmol, 0.246 g) were stirred for 4 h at 50 �C. Slow evapo-ration of the solvent led to the formation dark blue block shapedcrystals suitable for single crystal X-ray studies. Yield: 71%. Ele-mental analysis calc. for C12H14CuN2O7 2 (Mr = 361.79): C, 39.84;H, 3.90; N, 7.74. Found: C, 39.10; H, 3.50; N, 7.95%. FAB mass (m/z) calc.: 307.74. Found: 308.

2.4. Single crystal X-ray crystallography

Single crystal X-ray data were collected on a CCD detector basedOxford diffractometer using graphite monochromatized Mo Karadiation (k = 0.71073 Å). The diffraction data were solved usingSIR-92 [34] with GUI control and the structure was refined bySHELXL-97 [35] refinement of F2 against all reflections. Non-hydrogen atoms were refined anisotropically and all hydrogenatoms were geometrically fixed and allowed to refine using ariding model. Molecular graphics were generated using differentsoftware, such as Diamond, ORTEP-3v2 for WINDOWS [36],PLATON and Mercury [37].

2.5. Biological activity measurements

The superoxide dismutase activity (SOD) of the present cop-per(II) complexes was evaluated using alkaline DMSO as the sourceof the superoxide radical (O2

�) generating system, in association

with nitro blue tetrazolium chloride (NBT) as a scavenger of super-oxide [38,39]. With certain concentrations of the complex solution,2.1 ml of 0.2 M potassium phosphate buffer (pH 8.6) and 1 ml of56 lM NBT solutions were added. The mixtures were kept in icefor 15 min and then 1.5 ml of alkaline DMSO solution was addedwhile stirring. The absorbance was monitored at 540 nm againsta sample prepared under similar conditions, except NaOH was ab-sent in DMSO. The in vitro antibacterial activities were testedagainst Escherichia (E.) coli, Citrobacter (C.) gillenii and Vibrio (V.)cholera, and comparisons were made with standard antibiotics.The agar disk diffusion method was adopted for determination ofantibacterial activity [40–42]. Autoclaved Nutrient agar mediumwas poured into a sterile Petri-dish and allowed to solidify. Petri-dishes were seeded with bacterial species. A paper disc was placedon the dish after dipping the test compound (DMSO solution). Thewidth of the growth inhibition zone around the disc was measuredafter 24 h incubation at 37 �C. The DNA cleavage experiment wasperformed by the agarose gel electrophoresis method [43–45].The efficiency of DNA cleavage was measured by determining theability of the complexes to form open circular (OC) and nicked cir-cular (NC) DNA from its super coiled (SC) form. The DMSO solution(1 � 10�3 M) containing the metal complexes (5 lL, 250 lM) wastaken in a clean Eppendroff tube and 30 lM of pBR 322 DNA wasadded. The content were incubated for 30 min at 37 �C and loadedon 0.8% agarose gel after mixing 3 ll of loading buffer (0.25% bro-mophenol blue + 0.25% xylene cynaol + 30% glycerol sterilized dis-tilled water). The electrophoresis was performed at constantvoltage (75 V) until the bromophenol blue reached up to 3=4 lengthof the gel. The gel was stained for 10 min by immersing it in ethi-dium bromide solution (5 lg/ml of water) and then de-stained for10 min by keeping it in sterile distilled water. The plasmid bandswere visualized by photographing the gel under a UV transillumi-nator. The reactions were carried out under oxidative and/orhydrolytic conditions.

3. Results and discussions

3.1. Synthesis of the complexes

For the synthesis of polynuclear copper(II) coordination com-plexes, a conventional solution method was adopted and as a resultsingle crystals of 1 suitable for X-ray diffraction analysis were ob-tained. The pyridine-2-carboxylic acid was used in both complexes,and is the main structural unit of the complexes in bridge forma-tion. Both complexes 1 and 2 are deep blue in color, stable in airand soluble in DMF, DMSO and in a mixed CH3OH:CH3CN (1:1, V/V) solution, but they are only partially soluble in other organic sol-vents, such as CHCl3 and CH2Cl2. Both complexes gave satisfactoryelemental analysis and were further characterized by FAB+ massspectrometry.

3.2. Description of crystal structure

The crystalline structure of complex 1 belongs to the tricliniccrystal system having space group P�1. An ORTEP view of complex1 is shown in Fig. 1 and the crystal data and structure refinementparameters are listed in Table 1. Selected bond angles and bonddistances are listed in Table 2. Complex 1 has three types of ligand,and of these, two ligands make a bridge between two copperatoms. The pyridine-2-carboxylate ion is the main structural unitof the complex. The Cu–NPyridine bond distances (Cu1–N5, Cu1–N4, Cu2–N7, Cu2–N6) are similar in 1, while the Cu–Nhydrazine bonddistance, Cu1–N1 = 2.006(2) Å, is quite a bit longer. The coordina-tion sphere of complex 1 is shown in Fig. 2. The nature of NH incomplex 1 is that of a mono negative ligand, due to keto–enol

Fig. 1. ORTEP view of [Cu4(2-Pca)4(NH)2(bipy)2](ClO4)2�H2O (1).

Table 1Crystal data and structure refinement parameters for complex 1.

Empirical formula C56H44Cl2Cu4N14O19 (1)

Formula weight 1542.12Temperature (K) 150(2)Wavelength (Å) 0.71073Crystal system triclinicSpace group P�1Unit cell dimensionsa (Å) 12.6035(4)b (Å) 12.7783(4)c (Å) 13.4665(4)a (�) 92.665(2)b (�) 117.738(3)c (�) 115.758(3)V (Å3) 1642.50(9)Z 2Calculated density (Mg/m3) 1.778Absorption coefficient (mm�1) 1.540F(000) 888Crystal size (mm) 0.23 � 0.18 � 0.14h (�) 3.45–25.00Limiting indices 14 6 h 6 14, �15 6 k 6 12,

�16 6 l 6 16Reflections collected/unique 12463/5760 (Rint = 0.0198)Completeness to h = 25.00� 99.8%Absorption correction semi-empirical from equivalentsMaximum and minimum transmission 0.8133 and 0.7184Refinement method full-matrix least-squares on F2

Data/restraints/parameters 5760/0/486Goodness-of-fit on F2 1.007Final R indices [I > 2r(I)] R1 = 0.0260, wR2 = 0.0656R indices (all data) R1 = 0.0345, wR2 = 0.0672Largest difference in peak and hole

(e �3)0.559 and �0.378

R.N. Patel et al. / Polyhedron 50 (2013) 139–145 141

tautomerism. In the keto form the C@O bond length should be�1.21 Å, but in the present complex the bond length is 1.255 Å,indicating C–O single bond character [–NH–C@O M –N@C–OH].

The unit cell packing diagram of compound 1 is shown in Sup-plementary Fig. S1. It can be observed that the adjacent units areinterconnected by H-bonding interactions involving the oxygenatom of a carboxylic group and N(1) and N(2) hydrogen atoms ofthe hydrazide groups. Hydrogen bonding data are listed in Table 3.

The H-bonding interactions are observed with donor acceptordistances of 2.846 and 2.753 Å. An inter-molecular aromatic p...pstacking interaction is also present in complex 1 (SupplementaryFig. S2). In the complex 1, the distance between centroids [Cg(I)to Cg(J)] of the pyridine ring [(N(2), C(7), C(8), C(9), C(10) C(11)]is 3.635 Å and the dihedral angle is 0�. The angles and distances be-tween the centroids are present in Supplementary Table 1. Thetrigonality index s is calculated using the equation s = (b � a)/60[46] (for perfect square pyramidal and trigonal bipyramidal geom-etries, the values of s are zero and unity, respectively). The value ofs for 1 is 0.035, indicating a distorted square pyramidal geometry.

3.3. Electronic and IR spectroscopy

Electronic spectra for the complexes were recorded in 100%DMSO (3 � 10�3 M). The complexes show a well resolved d–d band(Fig. 3) at 686 and 646 nm, respectively for 1 and 2. A charge trans-fer band is observed at 300 nm for 2 and its broadness can beexplained as being due to an O ? Cu LMCT transition [47]. The IRspectra of both complexes show symmetric and asymmetricstretching vibrations of the coordinated carboxylate group[48,49], mas (COO) at 1634 cm�1 and ms (COO) at 1384 cm�1. Thepresence of strong, sharp band in the range 1673–1689 cm�1 canbe attributed to a (C@O) vibration [50]. However some new bandswith medium to weak intensities appear in the region 395–405 cm�1 in the complexes, which are tentatively assigned to(M–O)/(M–N) modes [50]. The presence of uncoordinated water,m(OH), in both complexes is indicated by broad bands at 3425and 3430 cm�1 respectively [51]. In complex 1 the presence of

Table 2Selected bond lengths (Å) and bond angles (�) for 1.

Complex 1

Cu(1)–N(5) 1.9685(19) Cu(1)–O(1) 1.9724(14)Cu(1)–N(4) 1.9904(18) Cu(1)–N(1) 2.006(2)Cu(1)–O(2) 2.1766(17) Cu(2)–N(6) 1.9612(19)Cu(2)–N(7) 1.9663(19) Cu(2)–O(4) 1.9713(15)Cu(2)–O(3) 1.9748(16) Cu(2)–N(3)#1 2.2585(18)N(5)–Cu(1)–O(1) 93.29(7) N(5)–Cu(1)–N(4) 81.99(8)O(1)–Cu(1)–N(4) 166.78(7) N(5)–Cu(1)–N(1) 166.71(9)O(1)–Cu(1)–N(1) 82.14(7) N(4)–Cu(1)–N(1) 99.69(7)N(5)–Cu(1)–O(2) 101.77(7) O(1)–Cu(1)–O(2) 89.97(7)N(4)–Cu(1)–O(2) 103.06(7) N(1)–Cu(1)–O(2) 90.74(8)N(6)–Cu(2)–N(7) 168.97(8) N(6)–Cu(2)–O(4) 95.98(7N(7)–Cu(2)–O(4) 82.78(7) N(6)–Cu(2)–O(3) 83.17(7)N(7)–Cu(2)–O(3) 94.97(7) O(4)–Cu(2)–O(3) 163.93(6)

Symmetry transformations used to generate equivalent atoms: #1 x+2, y, z+1.

Table 3Hydrogen bonding interactions for 1 (Å and �).

D–H. . .A (D–H) (H. . .A) <DHA (D. . .A)

Complex 1N2–H2. . .O5 0.880 1.885 168.54 2.753N1–H1N. . .O5 0.788 2.059 176.31 2.846 [x + 1, y + 1, z]N1–H2N. . .O333 0.883 2.248 155.09 3.071 [�x + 1, �y, �z + 1]N1–H2N. . .O555 0.883 2.514 124.50 3.100 [�x + 1, �y, �z]

142 R.N. Patel et al. / Polyhedron 50 (2013) 139–145

bands at 1089 and 622 cm�1 indicate a Td symmetry of ClO4�. This

therefore suggests the presence of ClO4� outside the coordination

sphere of the complex [52–54].

3.4. Electron paramagnetic resonance spectra

The epr spectra of both complexes, recorded in DMSO at 77 K,showed four well resolved hyperfine lines corresponding to cou-pling of the electron spin with the nuclear spin (63,65Cu, I = 3/2), ob-tained in the parallel region. The epr parameters of complexes 1and 2 are presented in Table 4. The epr spectrum of complex 1(Fig. 4) showed five nitrogen superhyperfine lines in the perpen-dicular region, which arise from coupling of the electron spin withthe nuclear spin of the coordinating nitrogen atoms, while complex2 showed four superhyperfine lines (Fig. 5).

Th epr parameters and d–d transition energies were used toevaluate a2, b2 and c2, which may be regarded as measures ofthe covalency of the in-plane r bonding, in-plane p and out-of-plane p-bonding respectively. The in-plane r bonding parametera2 was calculated [55] from the expression.

a2 ¼ Ak0:036

� �þ ðgk � 2:0023Þ þ 3=7ðg? � 2:0023Þ þ 0:04

The orbital reduction factors K2k and K2

? were calculated fromthe expressions below [56]. Significant information about the nat-ure of the bonding in the copper(II) complexes can be derived fromthe magnitude of K|| and K\.

Fig. 2. Coordination sphere of [Cu4(2-P

K2k ¼ðgk � 2:0023ÞEd—d

8k0

K2? ¼ðg? � 2:0023ÞEd—d

2k0

K|| = a2b2, K\ = a2c2 and k0 represents the one electron spin–orbitcoupling constant for the free ion, and is equal to �828 cm�1. Hath-away [57] has pointed out that for pure sigma bondingK|| � K\ � 0.77 and for in plane p-bonding K|| < K\, while for out-of-plane p-bonding K|| > K\.

The polycrystalline solution spectra of 1 and 2 were axiallysymmetric (g|| = 2.23 and g\ = 2.06), having a quartet hyperfinestructure on the parallel component arising from the interactionof an unpaired electron. In both complexes, it is observed thatK|| < K\, indicating the presence of in-plane p-bonding. Moreover,the a2, b2 and c2 values of complex 1 are found to be less than1.0, indicating a covalent character of the bonds. The g|| valuesare nearly the same for both complexes, indicating that the bond-ing is dominated by the pyridine-2-carboxylato moiety. The g|| > g\values accounts for the distorted square pyramidal structure of 1and rules out the possibility of a trigonal bi pyramidal structure,which would be expected to have g\ > g|| [58,59].

3.5. Electrochemical studies

The electrochemical properties of both complexes have beenstudied by cyclic voltammetry (CV) under a nitrogen atmospherein DMSO solution in the potential range +1.2 to �1.2 V versusthe Ag/AgCl reference electrode. The electrochemical propertiesof the metal complexes have been particularly studied in orderto monitor any spectral and structural changes accompanyingthe electron transfer [60]. A representative cyclic voltammogram

ca)4(NH)2(bipy)2](ClO4)2�H2O (1).

450 600 700 800 Wavelength (nm)

0.590

0.400

0.300

0.000

Abs

orba

nce

500 840

Fig. 3. UV–Vis spectra of complexes 1 and 2 (0.003 mol�1 dm�3) in DMSO.

Table 4Epr and UV–Vis parameters of the copper(II) complexes.

Parameter DMSO (77 K) 1 2

g|| 2.232 2.230g\ 2.068 2.069A|| (G) 180 180G 3.496 3.413a2 0.763 0.761b2 0.922 0.958c2 0.986 1.037K|| 0.703 0.729K\ 0.752 0.789f (cm�1) 132.7 132.5kmax (nm) 686 646

Fig. 4. Epr of [Cu4(2-Pca)4(NH)2(bipy)2](ClO4)2�H2O (1) at LNT, (0.003 mol�1 dm�3)in DMSO.

Fig. 5. Epr of [Cu(2-Pca)2]n�3H2O (2) at LNT, (0.003 mol�1 dm�3) in DMSO.

Fig. 6. Cyclic voltammogram of [Cu4(2-Pca)4(NH)2(bipy)2](ClO4)2�H2O (1) in DMSO(0.1 M NaClO4 as the supporting electrolyte, scan rate: 100 mV/s).

R.N. Patel et al. / Polyhedron 50 (2013) 139–145 143

is shown in Fig. 6. Two one electron reductive responses are ob-served, presumably due to the following electrode reactions:

CuII—CuII þ e� ! CuII—CuI

CuII—CuI þ e� ! CuI—CuI

These indicate significant metal coupling in both complexes[61–63]. These redox couples seem to be totally irreversible, withan average peak separation between the cathodic and anodic wavesof 0.45 V. Cyclic voltametric data along with the conproportionationequilibrium constant (Kcon) of 1 and 2 are given in Table 5. The sta-bility of the mixed valance form can be quantified by the conpro-portionation equilibrium constant (Kcon). It is observed that largerthe value of Kcon is, the greater the electronic coupling. Kcon isusually estimated from the difference in the half-wave potentialsbetween metal center couples, log Kcon = 16.9(DE1/2). It is knownthat an adequate Cu(II)/Cu(I) redox potential for the effective catal-ysis of the superoxide radical must be between E� = �0.16 V versusNHE (�0.405 V versus SCE) for O2/O2

� and E� = 0.89 V NHE(0.645 mV versus SCE) for O2

�/H2O2. Though it is inappropriate tocompare the E values with the reduction potentials of the couplesO2/O2

� and O2�/H2O2, the redox potentials of the two complexes

in DMSO should be in the allowed ranges of an SOD mimic, sincethe obtained results of the SOD activity assay showed that bothcomplexes have relatively less activity.

3.6. Superoxide dismutase (SOD) activity

The SOD activity of complexes 1 and 2 were investigated by theNBT assay method and the catalytic activity towards the dismuta-tion of the superoxide anion was measured by the literature meth-od [64,65]. Superoxide dismutation is a redox process. The IC50

value for SOD activity has been defined as the chromophore con-centration required to yield 50% inhibition of the reduction ofNBT. The IC50 values of complexes 1 and 2 are 12 ± 5 and33 ± 5 lM, respectively. The present complexes showed SOD activ-ity, but are less active than the native SOD (IC50 = 0.04 lM). The ki-netic catalytic constants (KMcCF) [66,67] of complexes 1 and 2 havealso been estimated and are given in Table 6. The activities of thesecomplexes may be attributed to the flexible nature of the ligandused, which is able to accommodate the geometrical change fromcopper(II) to copper(I) [66] in the catalytic process, just like O2

�

in place of the water molecule bound to the copper site in themechanism of dismutation of O2

� by native SOD. The SOD graphof 1 and 2 is presented in Fig. 7. The active concentration (IC50)of complex 1 is lower than that of complex 2. In other words, com-plex 1 is actually more active than complex 2. The difference in theIC50 values for the two complexes should be ascribed to the evidentdiscrepancy in their structures. A distorted geometry of complexesmay favor a geometrical change, which is essential for the catalytic

Table 5Cyclic voltammetric data for a 0.003 M solution of the Cu(II) complexes in DMSO containing 0.1 M NaClO4 as a supporting electrolyte.

Complex Epc1 (V) Epa1 (V) Epc2 (V) Epa2 (V) E11/2 (V) E2

1/2 (V) Kcon

1 �0.010 0.700 �0.467 0.225 0.345 �0.121 1.3 � 10�8

2 �0.395 0.483 �0.781 0.199 0.439 �0.632 3.5 � 10�9

Table 6IC50 values and kinetic constant for complexes 1 and 2.

Complex IC50 (lmol) KMcCF (M�1 s�1) � 104

1 12 ± 5 27.722 33 ± 5 10.08

KMcCF values were calculated by K = kNBT � [NBT]/IC50, kNBT (pH 7.8) = 5.94 � 104 -M�1 s�1 (Ref. [66,67]).

Fig. 7. SOD activity of copper(II) complexes of 1 and 2.

Fig. 8. Antibacterial activity of [Cu4(2-Pca)4(NH)2(bipy)2](ClO4)2�H2O (1); 1, 2, 3, 4and 5 = 250, 200, 150, 100 and 50 lg/ml, respectively, (a) Citrobacter and (b) Vibrio.

1 2 3 4 5 6

Form-II

Form-I

Fig. 9. Gel electrophoresis showing the chemical nuclease activity of the copper(II)complexes on pBR 322 plasmid DNA, incubation at 37 �C for 1 h in the presence andabsence of H O . Lane 1 – control DNA (30 lM); Lane 2 – DNA (30 lM) + H O

144 R.N. Patel et al. / Polyhedron 50 (2013) 139–145

property of the complexes. Similar results have been reported inthe literature [68] and by our group [65,69].

2 2 2 2

(50 lM); Lane 3 – DNA (30 lM) + complex 1 (250 lM); Lane 4 – DNA(30 lM) + complex 1 (250 lM) + H2O2 (50 lM); Lane 5 – DNA (30 lM) + complex2 (250 lM); Lane 6 – DNA (30 lM) + complex 2 (250 lM) + H2O2 (50 lM).

3.7. Antibacterial activity

The in-vitro antibacterial activities of the complexes were testedagainst E. coli, Citrobacter and Vibrio sp. bacteria as test organisms.The susceptibility of certain strains of bacteria towards the presentcopper(II) complexes were determined by measuring the zone ofinhibition diameter. The antibacterial activities of the complexes1 and 2 were measured at different concentrations and were com-pared with lower concentrations of standard antibiotics (chloram-phenicol). It is observed that both complexes are moderately activeat higher concentrations, but are less active at lower concentra-tions. A maximum 15 mm growth of inhibition was accountedfor by complex 2 against Vibrio species, whereas 14 mm growthof inhibition was obtained against E. coli and Vibrio species bythe action of complex 1 at the same 250 lg/ml concentration(Table 7). Baseline ranges of 9–6 mm as the minimum growth ofinhibition zone were recorded against all three test species. Similar

Table 7Antibacterial activity data of 1 and 2.

Inhibition zone in mm

250 lg/ml 200 lg/ml 150 lg/ml

1 2 1 2 1 2

E. coli 14 12 11 10 9Citrobacter 12 13 10 11 9Vibrio 14 15 12 13 10 1

Control DMSO: no inhibition.#, no inhibition; each value is the average of three replicates.

results have been previously reported by our school [70–72]. Bothcomplexes are less active than standard antibiotics. Antibacterialactivity photographs are presented in Fig. 8.

3.8. DNA cleavage activity

The copper(II) complexes were studied for their DNA cleavageactivity by the gel electrophoresis method (Fig. 9). The gel afterelectrophoresis clearly revealed that the intensity of the treatedDNA sample diminished, possibly because of the cleavage ofDNA. The complete cleavage of DNA was observed by both cop-per(II) complexes. A difference was observed in the bands withthe complexes compared to that of the control DNA. The reactionis modulated by a metallo-complex bound hydroxyl radical,

100 lg/ml 50 lg/ml 50 lg/ml

1 2 1 2 Chloramphenicol

9 8 7 6 6 257 8 7 6 # 240 7 9 # 7 24

R.N. Patel et al. / Polyhedron 50 (2013) 139–145 145

generated from the co-reactant H2O2. The results indicate that thecopper(II) complexes play an important role in the cleavage ofDNA, as the complexes were observed to cleave the DNA [73,74].A cleavage activity difference was observed in the bands of thecomplexes (lanes: 3–6) compared to the control (lanes: 1 and 2)pBR 322 DNA.

4. Conclusion

We have synthesized two copper(II) complexes, viz.[Cu4(2-Pca)4(NH)2(bipy)2](ClO4)2�H2O (1) and [Cu(2-Pca)2]n�3H2O(2), and they were characterized by various physicochemical tech-niques. Complex 1 has a combination of coordination bonds,hydrogen bonds and p. . .p stacking interactions. The present studyhighlights that the spectroscopic data together with redox behav-ior may be regarded as an index for constructing a potentially fea-sible structural model of copper containing metallo-proteins. Boththese complexes showed superoxide dismutase activity, but 1 ismore active than complex 2. The DNA cleavage activity was mea-sured and it was observed that the reaction was modulated bythe metallo-complexes. Both complexes were also tested againstsome bacterial sp. and showed appreciable antibacterial activityat higher concentrations.

Acknowledgements

Our grateful thanks are due to the School of Chemistry,University of Hyderabad, Hyderabad for single crystal X-ray datacollection, RSIC (SAIF) IIT, Bombay for epr measurements and SAIF,Central Drug Research Institute, Lucknow for providing elementaland IR spectral analysis. Financial assistance from CSIR [SchemeNo. 01(2451)/11/EMR-II] and UGC [Scheme No. 36-28/2008 (SR)]New Delhi are also thankfully acknowledged.

Appendix A. Supplementary data

CCDC 862604 contains the supplementary crystallographic datafor [Cu4(2-Pca)4(NH)2(bipy)2](ClO4)2�H2O (1). These data can be ob-tained free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-ing.html, or from the Cambridge Crystallographic Data Centre, 12Union Road, Cambridge, CB2 1EZ, UK; fax: (+44) 1223-336-033;or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associatedwith this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2012.10.027.

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