Synthesis and spectroscopic studies on the Schiff base ligand derived from condensation of 2-furaldehyde and 3,3′-diaminobenzidene, L and its complexes with Co(II), Ni(II), Cu(II) and Zn(II): Comparative DNA binding studies of L and its Cu(II)

Synthesis and spectroscopic studies on the Schiff base ligand derived from condensation of 2-furaldehyde and 3,3′-diaminobenzidene, L and its complexes with Co(II), Ni(II), Cu(II) and Zn(II): Comparative DNA binding studies of L and its Cu(II) and

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  Spectrochimica Acta Part A 78 (2011) 29–35 Contents lists available at ScienceDirect SpectrochimicaActaPartA:MolecularandBiomolecularSpectroscopy  journal homepage: Synthesis and spectroscopic studies on the Schiff base ligand derived fromcondensation of 2-furaldehyde and 3,3 ′ -diaminobenzidene, L and its complexeswith Co(II), Ni(II), Cu(II) and Zn(II): Comparative DNA binding studies of L and itsCu(II) and Zn(II) complexes Mohammad Shakir a , ∗ , Ambreen Abbasi a , Asad U. Khan b , Shahper N. Khan b a Division of Inorganic Chemistry, Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India b Interdisciplinary Biotechnology Unit, Aligarh Muslim University, Aligarh 202002, India a r t i c l e i n f o  Article history: Received 4 September 2009Received in revised form 22 February 2010Accepted 26 February 2010 Keywords: Schiff base condensationSpectroscopic studiesSquare planar geometryCalf thymus DNA a b s t r a c t TheSchiffbaseligand,N,N ′ -bis-(2-furancarboxaldimine)-3,3 ′ -diaminobenzidene(L)obtainedbyconden-sationof2-furaldehydeand3,3 ′ -diaminobenzidene,wasusedtosynthesizethemononuclearcomplexesof the type, [M(L)](NO 3 ) 2  [M=Co(II), Ni(II), Cu(II) and Zn(II)]. The newly synthesized ligand, (L) and itscomplexes have been characterized on the basis of the results of the elemental analysis, molar con-ductance, magnetic susceptibility measurements and spectroscopic studies viz, FT-IR,  1 H and  13 C NMR,mass, UV–vis and EPR. EPR, UV–vis and magnetic moment data revealed a square planar geometry forthe complexes with distortion in Cu(II) complex and conductivity data show a 1:2 electrolytic nature of the complexes. Absorption and fluorescence spectroscopic studies support that Schiff base ligand, L andits Cu(II) and Zn(II) complex exhibit significant binding to calf thymus DNA. The highest binding affinityin case of L may be due to the more open structure as compared to the metal coordinated complexes. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Active and well-defined Schiff base ligands are consideredas ‘privileged ligands’ because they are easily prepared by con-densation of aldehydes or ketones with amines and are able tostabilize different metals in various oxidation states [1]. Schiff  base complexes are extensively studied due to synthetic flexibil-ity, selectivity and sensitivity towards variety of metal ions [2].Schiffbaseshaveapplicationtowards,degradationoforganiccom-pounds [3], radiopharmaceuticals [4] and as corrosion inhibitors in especially acidic environments for various alloys and metalslike steel, aluminium and copper [5]. Complexes of transition and non-transitionmetalswithSchiffbaseligandsarepromisingmate-rialsforoptoelectronicapplicationsduetotheiroutstandingphoto-and electroluminescent properties, and the ease of synthesis thatreadily allows structural modification for optimization of materialproperties [6].Bidentate and tetradentate Schiff base ligands involving N,Odonor sites possess many advantages such as facile approach forsynthesis,relativetolerance,readilyadjustedancillaryligands,andtunable steric and electronic coordination environments on the ∗ Corresponding author. Tel.: +91 5712703515. E-mail addresses:, (M. Shakir). metalcentre.Inviewofabove,tetradentateN 2 O 2  ligandsandtheirtransition metal complexes, act as catalyst [7]. Various transition and inner transition metals complexes with bi-, tri- and tetraden-tateSchiff bases containing nitrogen andoxygen donor atoms playimportantroleinbiologicalsystemsandrepresentinterestingmod-els for metalloenzymes, which efficiently catalyze the reduction of dinitrogen and dioxygen [8].Di- and tetra-Schiff base precursors based on 3,3 ′ -diaminobenzidene have been reported in literature [9,10].Bolos et al. have reported biological studies of Cu(II) coordinationcompounds with Schiff bases of polyamines with heterocyclicaldehydes as ligands [11,12]. Polyamines and heterocyclic alde- hydes (2-thiophene-carboxaldehyde and 2-furaldehyde) wereselected, the former due to its coordinative, conformational andphysicochemical properties, while the latter in order to mimicbiological systems and mechanisms in the process of drug design.The five- or six-membered chelate ring Schiff bases stabilize, inthermodynamic terms of entropy, the compounds synthesized[13].In view of aforesaid importance of Schiff bases and theircomplexes, we have synthesized a novel Schiff base ligand,N,N ′ -bis-(2-furancarboxaldimine)-3,3 ′ -diaminobenzidene derivedfrom condensation reaction of 3,3 ′ -diaminobenzidene with 2-furaldehyde and its complexes with Co(II), Ni(II), Cu(II), Zn(II) andtheir DNA binding studies. 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.02.034  30  M. Shakir et al. / Spectrochimica Acta Part A 78 (2011) 29–35 2. Experimental  2.1. Materials and instrumentation The metal salts, M(NO 3 ) 2 ·  x H 2 O [M=Co, Ni, Zn,  x =6; Cu,  x =3](all E. Merck) were commercially pure samples. The chemicals 2-furaldehyde and 3,3 ′ -diaminobenzidene (both Acros) were usedas received. Solvent acetonitrile (AR) was used without furtherpurification. The DMSO was dried before use by standard proce-dure in order to avoid the effect of water on conductivity [14].Deionized water was used in synthesis. Highly polymerized calf thymus DNA sodium salt (7% Na content) was purchased fromSigma Chemical Co. Other chemicals were of reagent grade andused without further purification. The stock solution of 12.5mMDNA/phosphate calf thymus DNA was prepared by dissolving 0.5%(w/w) in 0.1M sodium phosphate buffer (pH 7.40) at 310K for24h with occasional stirring to ensure homogeneity of solution.The absorption ratio  A 260 /  A 280  in the range of 1.8–1.9 indicatedthat DNA was sufficiently free from protein. The stock solution of N,N ′ bis-(2-furancarboxaldimine)-3,3 ′ -diaminobenzidene and itsCu and Zn complexes with 5mg/ml concentration were prepared.The elemental analysis was obtained from micro-analytical labo-ratory of CDRI, Lucknow using PerkinElmer 2400 CHN ElementalAnalyzer. The FT-IR spectra (4000–400cm − 1 ) were recorded asKBr pellets on Shimadzu 8201 PC spectrophotometer.  1 H and  13 CNMR spectra in DMSO-d 6  at room temperature were recordedusing Bruker Avance II 400 NMR spectrometer. The DART-MSof the Schiff base ligand was recorded on JEOL-AccuTOF JMS-T100LC Mass spectrometer having a DART source. The givensample was subjected as such in front of DART source. Dry heliumwas used with 4 LPM flow rate for ionization at 3500 ◦ C. EPR spectrum at room temperature was recorded on Varian E-112X-band EPR spectrometer using TCNE as standard at RSIC, IIT,Bombay, India. The electronic spectra at room temperature wererecorded with Varian Cary 5E spectrophotometer in 1200–185nmrange using DMSO as solvent. The electrical conductivities of 10 − 3 M solution in DMSO were obtained on a Systronic type302 conductivity bridge equilibrated at 25 ± 0.01 ◦ C. The mag-netic susceptibility measurements were carried out using Faradaybalance at room temperature. Fluorescence measurements wereperformed on a spectrofluorimeter Model RF-5301PC (Shimadzu, Japan) equipped with a 150W Xenon lamp and a slit width of 3nm. A 1.00cm quartz cell was used for measurements. For thedetermination of binding parameters, 50  M of complex solu-tion was taken in a quartz cell and increasing amounts of ctDNA solution was titrated. Fluorescence spectra were recordedat 310K temperatures in the range of 740–880nm upon exci-tation at 280 (  em  was 772nm). The UV measurements of calf thymus DNA were recorded on a Shimadzu double beam spec-trophotometermodel-UV1700usingacuvetteof1cmpathlength.Absorbance value of DNA in the absence and presence of com-plex were made in the range of 220–300nm. DNA concentrationwas fixed at 0.1mM, while the compound was added in increasingconcentration.  2.2. Synthesis of N,N  ′ -bis-(2-furancarboxaldiamine)-3,3 ′ -diaminobenzidene (L) A hot solution of 3,3 ′ -diaminobenzidene (1mmol, 0.214g)in 1:1 acetonitrile to water mixture (25ml) was slowly addedto a solution of 2-furaldehyde (2mmol, 0.167ml) dissolvedin acetonitrile (5ml). The reaction mixture was magneticallystirred for 24h at room temperature leading to the isola-tion of yellow solid product which was washed with waterand acetonitrile and dried in vaccum over anhydrous calciumchloride. Fig.1.  FluorescenceemissionspectraofLintheabsenceandpresenceofincreasingamount of DNA from (a) to (i): pH 7.4,  T  =298K and (x) represents the DNA alone.  2.3. Synthesis of metal complexes A solution of hydrated metal nitrate (.085mmol) in acetonitrile(10ml) was added slowly to a hot solution of L (.085mmol) in ace-tonitrile (60ml). The resulting solution was stirred under refluxfor 6h resulting in the isolation of solid product. The product thusformedwasfiltered,washedwithacetonitrileanddriedinvaccumover anhydrous calcium chloride.  2.4. Binding data analysis of L and its Cu and Zn complex To elaborate the fluorescence quenching mechanism theStern–Volmer equation (1) was used for data analysis [15]. F  0 F   = 1 + K  SV [ Q  ] (1)where  F  0  and  F   are the steady-state fluorescence intensities inthe absence and presence of quencher, respectively.  K  SV  theStern–Volmer quenching constant and [ Q  ] is the concentration of quencher (DNA). The  K  SV  for L and its Cu and Zn complex werecalculatedtobe1.79 × 10 4 ,1.42 × 10 4 and1.36 × 10 4 LM − 1 ,respec-tively. A higher  K  SV  value of L as compared to its Cu(II) and Zn(II)complexes suggests its stronger quenching ability. Further impli-catesitshigherbindingaffinitytowardtheDNAthenitscomplexes.The linearity of the  F  0 / F   versus [ Q  ] (Stern–Volmer) plots for DNA-L anditsCu(II)andZn(II)complexes(Fig.1)depictsthatthequench- ingmaybestaticordynamic,sincethecharacteristicStern–Volmerplotofcombinedquenching(bothstaticanddynamic)isanupwardcurvature. When ligand molecules bind independently to a set of equivalentsitesonamacromolecule,theequilibriumbetweenfreeand bound molecules is given by the equation [16]:log  F  0 − F F   = log K   + n log[ Q  ] (2)where  K   and  n  are the binding constant and the number of bind-ing sites, respectively. Thus, a plot of log( F  0 − F  )/ F   versus log[ Q  ](Fig. 2) can be used to determine  K   as well as  n . The bindingconstant and the number of binding sites for L and its Cu andZn complex were found to be  K  =2.75 ± 0.22 × 10 4 M − 1 ;  n =1.07, K  =0.56 ± 0.41 × 10 4 M − 1 ;  n =0.91 and  K  =0.91 ± 0.34 × 10 4 M − 1 ; n =0.94 respectively. The results suggest the compounds exhibitdifferent degree of affinity toward the DNA molecule. The highestbinding affinity in case of L may be due to the more open structureas compared to the metal coordinated complexes.  M. Shakir et al. / Spectrochimica Acta Part A 78 (2011) 29–35 31 Scheme 1. 3. Results and discussion Schiff base ligand, L was synthesized by condensation of 3,3 ′ -diaminobenzidene and 2-furaldehyde in 1:2 molar ratio dissolvedin water and acetonitrile. The complexes of type, [M(L)](NO 3 ) 2 were synthesized by the reaction of L and metal salt in 1:1 molarratioinacetonitrile(Scheme1).Allcomplexeswerestableatroom Fig. 2.  Stern–Volmer plot for the binding of L and its Cu(II) and Zn(II) complex. temperature and dissolve in DMSO. The molar conductivity datafor 1mM solutions of complexes (Table 1) suggest that complexes have 1:2 electrolytic nature. The analytical data along with somephysicalpropertiesofSchiffbaseligandanditscomplexesaresum-marized in Table 1. The formation of Schiff base ligand L and its complexes and bonding modes were inferred from characteristicband positions in FT-IR spectra and resonance signals in  1 H NMR and 13 CNMRspectracorrespondingtocoordinatedSchiffbasemoi-ety. The geometry around Co(II),Ni(II), Cu(II) ions in the complexeswere deduced from the positions of absorption bands observedin the UV–vis spectra and magnetic moment values. The bindingparameters were found to be  K  =2.75 ± 0.22 × 10 4 M − 1 ;  n =1.07, K  =0.56 ± 0.41 × 10 4 M − 1 ;  n =0.91 and  K  =0.91 ± 0.34 × 10 4 M − 1 ; n =0.94forLanditsCu(II)andZn(II)complexes,respectively.Theseparameters suggested that the Schiff base ligand L and its Cu andZn complexes have good binding affinity toward DNA molecule.  3.1. IR spectra The prominent bands observed in the IR spectra of the Schiff base ligand (L) and its complexes are listed in Table 2. The IR  spectra of complexes were compared with that of free ligand inorder to determine the coordination sites. Spectra of ligand aswell as its complexes showed absence of band at 1735cm − 1 dueto   (C O) stretching frequency [17]. The appearance of bands in the region 3150–3450cm − 1 in the spectrum of ligand, L corre-  32  M. Shakir et al. / Spectrochimica Acta Part A 78 (2011) 29–35  Table 1 Analytical data, % yield, color, melting point and molar conductivity of Schiff base ligand L and its metal complexes.Compound Analysis found (calcd)% Yield (%) Color mp ( ◦ C) Molar conductivity (  − 1 cm 2 mol − 1 )C H NL 69.05 4.96 15.43 50 Yellow 220–224 –C 22 H 18 N 4 O 2  (68.93) (4.89) (15.12)[Co(L)](NO 3 ) 2  48.99 3.42 15.37 15 Green >300 120C 22 H 18 CoN 6 O 8  (48.96) (3.38) (15.34)[Ni(L)](NO 3 ) 2  47.97 3.30 15.20 30 Red brown >300 113C 22 H 18 NiN 6 O 8  (47.77) (3.28) (15.20)[Cu(L)](NO 3 ) 2  52.07 3.74 15.48 35 Dark brown >300 117C 22 H 18 CuN 6 O 8  (51.94) (3.85) (15.30)[Zn(L)](NO 3 ) 2  42.20 2.86 14.63 20 Light brown >300 114C 22 H 18 ZnN 6 O 8  (42.37) (3.24) (15.01) spondedtoN–Hstretchingfrequencyoftheuncondensed1 ◦ aminogroup of diaminobenzidene [18]. However, the position of these groupsremainedunchangedinthecomplexesindicatingthat–NH 2 groups were not involved in coordination to metal ion. The bandat 1621cm − 1 in the free ligand was assigned to   (C O) stretching[19]. However in the complexes this band was shifted to higher(2–27cm − 1 ) or lower (5cm − 1 ) wavenumbers indicating the par-ticipation of azomethine nitrogen in the coordination to metal ion[20]. A medium intensity band due to   (C–O–C) stretching vibra-tion of furan appeared at 1247cm − 1 in the ligand L  [21]. This band disappeared in Zn(II) complex while shifted to lower frequency at1231cm − 1 , 1230cm − 1 and 1236cm − 1 in Co(II), Ni(II) and Cu(II)metal complexes, respectively suggesting a coordination throughoxygenoffuranmoiety[22].Appearanceofnewbandsinthespec- tra of complexes in the regions 552–596cm − 1 and 424–468cm − 1 wereassignedto  (M–O)and  (M–N)stretchingvibration[21].The bands appearing in the region 1480–1440cm − 1 , 1100–1064cm − 1 and 805–740cm − 1 were usual modes of phenyl ring vibration. Inall complexes a very strong band corresponding to   (N–O) of freenitrate anion was observed around 1383cm − 1 [23].  3.2.  1 H NMR and  13 C NMR spectra The 1 HNMRspectraoftheligandLanditsZn(II)complexshownin Figs. 3 and 4. The spectrum of the ligand shows a singlet at8.5ppm [24] assigned to azomethine protons (2H) which undergo upfield shift in the Zn(II) complex and appears at 7.98ppm [21]suggesting the coordination of azomethine nitrogen to Zn(II) ion.Theupfieldshiftinthechemicalshiftvaluesofaromaticprotonsaswell as furan ring protons in complex 5.69–7.74ppm (m,12H,6Arand6furanH)againstthefreeligand6.61–7.88ppmagainindicatethe involvement of azomethine nitrogen in the coordination withZn(II) ion [21]. However a broad singlet around 4ppm is indicative of uncondensed NH 2  protons (4H) on the other side of benzidenemoiety [9] in the ligand which remain unchanged in the  1 H NMR spectrum of Zn(II) complex. 13 C NMR spectral data were consistent with  1 H NMR spectraldata. A strong NMR signal appearing at 168ppm may reason-ably be assigned to azomethine carbon [25]. The resonance signals observedintheregion119–141.8ppmwereassignedtophenylandfuranyl moiety of the ligand [25,26]. These signals showed down- fieldshift(1–2ppm)intheZn(II)complexinvokingcoordinationof the ligand to Zn(II) ion through its azomethine N and furan O.  3.3. Electronic spectra and magnetic moments The electronic spectrum of Co(II) complex showed two bandsat 17,055cm − 1 and 14,960cm − 1 assignable to  4 A 2g → 4 T 1g  (P)and  4 A 2g → 4 T 1g  (F) transitions, respectively. However, a band at22,730cm − 1 may be assigned to   –  * transition supporting asquare planar geometry [27]. The observed magnetic moment of  3.12B.M. further corroborated the electronic spectral finding.The electronic spectrum of Ni(II) complex exhibited threeabsorption bands at 13,720cm − 1 , 21,207cm − 1 and 25,070cm − 1 which may be assigned to three spin allowed transitions, 1 A 1g → 1 A 2g ,  1 A 1g → 1 B 2g  and  1 A 1g → 1 E g , respectively character-istic of square planar geometry around Ni(II) ion. The diamagneticnature revealed by magnetic moment studies further confirm thesquare planar environment around the Ni(II) ion [28].However, the electronic spectrum of Cu(II) complex showedtwo bands at 17,827 and 22,275cm − 1 assignable to  2 B 1g → 2 A 1g and  2 B 1g → 2 E g  transitions, respectively which corresponded to asquare planar geometry around Cu(II) ion [27]. The observed mag- neticmomentof1.72B.M.furthersupportedtheelectronicresults.  3.4. EPR spectra EPR spectrum of Cu(II) complex (Fig. 5) shows two regions of  absorption centered at  g  ||  and  g  ⊥ with well resolved nuclear hyper-finesplittingof   g  || signalduetocoppernucleiinparallelorientation.There is no observable splitting of   g  ⊥  signal. The analysis of spec-trum gives  g  || =2.23,  g  ⊥ =2.05,  A || =167 × 10 − 4 cm − 1 which agreewell with the values reported for distorted square planar geom-etry around Cu(II) ion [29,30]. The spectrum shows the relation  g  || >  g  ⊥ >2.0023 which is typical of axially symmetric d 9 Cu(II) hav-ing one unpaired electron in  dx 2 −  y 2 orbital [31]. The absence of  signal corresponding to (  M  s = ± 2) in the half field indicates theabsence of any Cu–Cu interaction thus ruling out possibility of dimeric structure. In axial symmetry, the  G -parameter defined as G =  g  || − 2/  g  ⊥ − 2, reflects the spin interaction between Cu(II) cen-  Table 2 IR spectral data of Schiff base ligand (L) and its metal complexes (cm − 1 ).Compound   (C N)   (N–H)(str)   (C–O–C)   (M–O)   (M–N)  v  (N–O) Phenyl ring vibrationL 1621 3474 1247 – – – 1480, 1146, 753[Co(L)](NO 3 ) 2  1616 3470 1231 585 430 1384 1461, 1069, 668[Ni(L)](NO 3 ) 2  1648 3471 1228 591 434 1384 1420, 1073, 745[Cu(L)](NO 3 ) 2  1646 3472 1236 590 436 1383 1440, 1078, 765[Zn(L)](N0 3 ) 2  1623 3470 – 590 438 1383 1471, 1105, 750  M. Shakir et al. / Spectrochimica Acta Part A 78 (2011) 29–35 33 Fig. 3.  1 H NMR spectrum of the ligand. ters in solid polycrystalline complexes. According to Hathawayand Billing [32], if   G >4, the spin-exchange interaction is negligi-ble and if it is less than 4 considerable spin-exchange interactionprevails. In the present case this value comes out to be 4.6 whichagain indicates absence of Cu–Cu interaction, thus supporting pro-posed monomeric structure. Kivelson and Neiman have reportedthat  g  ||  values less than 2.3 indicates considerable covalent char-acter of M–L bond and greater than 2.3 indicates ionic character. Fig. 4.  1 H NMR spectrum of the Zn(II) complex.
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