Structures, redox behavior, antibacterial activity and correlation with electronic structure of the complexes of nickel triad with 3-(2-(alkylthio)phenylazo)-2,4-pentanedione

Inorganica Chimica Acta 370 (2011) 175–186

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Inorganica Chimica Acta

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Structures, redox behavior, antibacterial activity and correlation with electronicstructure of the complexes of nickel triad with 3-(2-(alkylthio)phenylazo)-2,4-pentanedione

Mrinal Kanti Paira a, Tapan Kumar Mondal a, Durbadal Ojha b, Alexandra M.Z. Slawin c, Edward R.T. Tiekink d,Amalesh Samanta b, Chittaranjan Sinha a,⇑a Inorganic Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata 700032, Indiab Department of Pharmaceutical Technology, Jadavpur University, Kolkata 700032, Indiac Department of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, UKd Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysia

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 May 2010Received in revised form 12 January 2011Accepted 17 January 2011Available online 31 January 2011

Keywords:3-(2-(Alkylthio)phenylazo)-2,4-pentanedioneGroup 10 metalsStructureRedoxAntibacterial activityDFT computation

0020-1693/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.ica.2011.01.049

⇑ Corresponding author. Fax: +91 033 2413 7121.E-mail address: [email protected] (C. Sinha).

3-(2-(Alkylthio)phenylazo)-2,4-pentanedione (HL), an O, N, S donor ligand, is used for the synthesis ofNi(II), Pd(II) and Pt(II) complexes. The spectroscopic (IR, UV–Vis, and NMR) data determine the structure.The single crystal X-ray diffraction measurement of [Ni(L)2] and [Pt(L)Cl] has confirmed the structures.Coulometric oxidation of [Ni(L)2] and EPR spectra thereof show formation of Ni(III) state. DFT computa-tion has calculated the electronic configuration and has explained the spectral and redox properties of thecomplexes. The compounds are screened for their in vitro anti-bacterial activity using Gram-positive andGram-negative bacteria (Bacillus subtilis UC564, Escherichia coli TG1, Staphylococcus aureus Bang25,Pseudomonas aeruginosa C/1/7, Salmonella typhi NCTC62, Salmonella paratyphi NCTC A2, Shigella dysenteriae8NCTC599/52, Streptococcus faecalis S2, Vibrio cholerae DN7 and Mricococcus luteus AGD1). The minimuminhibitory concentration is determined for the compounds. The effect of the structure of the investigatedcompounds on the antibacterial activity is discussed.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

The coordination behavior of transition metals with azo(–N@N–) ligands is of interest for their p-acidity, metal bindingability, dyes and pigmenting behavior, redox, photo-physical, cata-lytic and biological properties [1–8]. Thus, the synthesis of ligandsincorporating –N@N– group is an important field of research. Nota-ble examples of these ligands are arylazobenzene [9], arylazooxime[10], arylazophenol [11], arylazopyridine [7,8,12], arylazoimidaz-ole [13], arylazopyrimidine [14], and arylazoaniline [15]. Theligands containing oxygen, nitrogen and sulfur donor centers havebeen effectively used in modeling biomolecule, in exploring chem-ical, electrochemical, catalytic activities and magnetic behaviors[16–20]. In this work we report the synthesis of a ligand containingO, N, and S donor centers with incorporation of the thioarylazogroup into acetylacetone. Acetylacetone and its derivatives havemany applications. They can be used for synthesizing metal com-plexes either directly/functionalized molecules (condensation orcoupling with other molecules) [21–30]. Thioarylazo (R–S–C6H4–

ll rights reserved.

N@N–) is appended into acetylacetone to synthesize 3-(2-(alkyl-thio)phenylazo)-2,4-pentanedione. We have also prepared Gr 10(nickel(II), palladium(II), and platinum(II)) metal complexes withthis ligand. Two of these complexes ([Ni(L)2] and [Pt(L)Cl]) havebeen structurally confirmed by X-ray diffraction studies. The struc-ture of molecule plays significant role in determining their chem-ical properties. DFT calculation using optimized geometry of themolecules provides a competent method to correlate the electronicstructure and properties of the compounds. The electronic spectraand redox properties of the complexes have been explained withDFT computation results of the complexes. Antibacterial activityof the ligands and the complexes has been studied against somebacteria. The structure activity relation (SAR) of the compoundshas also been discussed in this work.

2. Results and discussion

2.1. Synthesis and formulation

The ligands 3-(2-(alkylthio)-2-phenylazo)-2,4-pentanedione,(alkyl (R) = Me, HL1; Et, HL2) are synthesized by coupling 2-(alkyl-thio)phenyldiazonium chloride with acetylacetone in sodium

S

N

N

O

Me

MeO

RN

O Me

Me

O

NS

R

Ni

S

N

R

N M

O

Me

Me

O

Cl

1 M = Pd(II) (2), Pt(II) (3)

R = Me (a), Et (b)

SH

NH2

S

NH2

S

NH2

R

S

N N

R

Me Me

O O

N S RN

O Me

OHMe

Metalic Na

Dry MeOH

-Na+

RI

i) 1:1 HCl

ii) NaNO2 +Cl-in Na2CO3

HL

R = Me (HL1); Et (HL2)

3

4 5

6

10

11

Scheme 1. The ligands and the complexes.

176 M.K. Paira et al. / Inorganica Chimica Acta 370 (2011) 175–186

carbonate solution. The precipitate so obtained is purified by re-peated crystallization from aqueous ethanol (1:1, v/v) mixture(Scheme 1). The purity of the products is checked by TLC and ele-mental analysis.

The reaction between Ni(OAc)2, 4H2O and HL is carried out in1:2 mol ratio in methanol. Upon slow evaporation of the solutionaffords shining dark green crystals of [Ni(L)2] (1). The reaction ofPd(MeCN)2Cl2 or K2[PtCl4] with HL in 1:1 mol ratio in acetonitrilesolution has synthesised complexes, [M(L)Cl] (M = Pd(II), (2); Pt(II),(3)). The composition of the complexes is supported by the micro-analytical data. The complexes are soluble in chloroform, dichloro-methane, acetonitrile, DMF, DMSO but insoluble in hexane,benzene, toluene. Their solutions are non-conducting. At roomtemperature the effective magnetic moment (l) of [Ni(L1)2] (1a)and [Ni(L2)2] (1b) are 2.7 and 3.1 BM, respectively. This supportsd8 (t6

2ge2g ) electronic configuration of a mononuclear Ni(II) system

in octahedral symmetry. The complexes 2 and 3 are diamagnetic.

Fig. 1. UV–Vis spectra of HL2 (⎯⎯), [Ni(L2)2] (1b) (—), [Pd(L2)Cl] (2b) (⎯⎯) and[Pt(L2)Cl] (3b) (⎯⎯) and. The inset picture shows the higher wavelength spectra of[Ni(L2)2] (1b) complex in CHCl3.

2.2. Spectral studies

Infrared spectra of the ligands (HL) exhibit a large number ofvibration and significant frequencies are l(N@N) and l(C@O) at1410 and 1670 cm�1, respectively. In the complexes the bandsare shifted to lower frequency and bands at 1380–1400 and1640–1650 cm�1 are assigned to l(N@N) and l(C@O), respec-tively. The presence of stretching at 335–340 cm�1 correspondsto l(M–Cl) and confirms the composition of [M(L)Cl] stoichiometryin Pd(II) and Pt(II) complexes.

The electronic spectra of HL in chloroform show two significanttransitions at 250–255 and 370–380 nm with two shoulders at275–280 and 390–400 nm which are assigned to p–p⁄ and n–p⁄

transitions, respectively. The complexes show bands in the region>400 nm along with intraligand charge transfer transitions(Fig. 1). The Ni(II) complexes, [Ni(L)2] (1) show an intense (e,104 M�1 cm�1) band at 422 nm and weak (e, 102–103 M�1 cm�1)transitions at 520, 570 and 850 nm. The weak band at 850 nm isattributed to symmetry forbidden d–d transition while the transi-tions at 520 and 570 nm, are more intense than former which maybe due to metal-to-ligand charge transfer (MLCT) (vide DFT, TD-DFTcomputation). Thioalkyl (–SR) group of the ligand is the reducing

Table 11H NMR data of HL and [M(L)Cl] in CDCl3.

Compounds d (ppm) (J, Hz)

3-Ha 4-Hb 5-Hb 6-Ha 10-Hc 11-Hs 12-H 13-Hb S-CH3c Enolic O-Hc

HL1 7.78 (8.1) 7.16 (7.2) 7.35 (7.7). 7.49 (7.2) 2.51 2.63 – – 2.47 15.05HL2 7.80 (8.2) 7.15 (7.5) 7.40 (7.7) 7.53 (7.6) 2.52 2.63 2.8d (14.6) 1.25 (7.3) – 15.07[Pd(L1)Cl] (2a) 8.10 (8.4) 7.40 (7.4) 7.52 (7.7) 7.64 (7.9) 2.62 2.81 – – 2.96 –[Pd(L2)Cl] (2b) 8.10 (8.4). 7.40 (7.3) 7.52 (7.6) 7.59 (7.6) 2.63 2.81 3.13e, 3.43e (55.0) 1.49 (7.2) – –[Pt(L1)Cl] (3a) 8.19 (8.3) 7.39 (7.5) 7.52 (7.6) 7.68 (7.4) 2.54 2.63 – – 2.90 –[Pt(L2)Cl] (3b) 8.20 (8.3) 7.40 (7.5) 7.50 (7.5) 7.65 (7.2) 2.54 2.64 3.19e, 3.36e (57.0) 1.31 (7.0) – –

a Doublet.b Triplet.c Singlet.d Quatret.e Sextet.

M.K. Paira et al. / Inorganica Chimica Acta 370 (2011) 175–186 177

center which enhances the probability of charge transfer by MLCTpath [17]. The complexes [Pd(L)Cl] (2) show metal assisted transi-tions at 420 and 445 nm while [Pt(L)Cl] (3) give absorption bandsat 443 and 464 nm.

The 1H NMR spectral data of the ligands, [Pd(L)Cl] (2) and[Pt(L)Cl] (3), are given in Table 1 Free ligand, HL may exist inketo-enol tautomeric form; a sharp resonance is observed at15.05 ppm that is assigned to –OH frequency. On comparison withintensity of proton signal the presence of only enolic, –(C@C–)–OHis recommended. In the complexes, [Pd(L)Cl] (2) and [Pt(L)Cl] (3),d(OH) is not observed which supports dissociation of the enolicO–H and formation of the M–O bond; the –S–Me resonance isdownfield shifted by �0.5 ppm which accounts the coordinationof thioether, –S–Me, to Pd(II) (2) and Pt(II) (3). Similar observationis also recorded for HL2. The –Me signals of acetylacetonato groupand aromatic protons of phenyl group are also downfield shifted.This supports drifting of electron density from the metal ion (Pd(II)and Pt(II)) upon coordination to the ligands [31].

2.3. Molecular structures

2.3.1. Bis-{3-(2-(ethylthio)phenylazo)-2,4-pentanedionato}nickel(II),[Ni(L2)2] (1b)

The X-ray structure of [Ni(L2)2] (1b) is shown in Fig. 2a. Selectedbond lengths and bond angles are given in Table 2. The coordina-tion environment around nickel is distorted octahedral, cis,cis-O;trans,trans-N and cis,cis-S to NiN2O2S2 coordination sphere. The li-

Fig. 2a. ORTEP view of Ni(L2)2 (1b) (40% probability ellipsoids).

gand HL2 acts as tridentate monoanionic O, N, S chelator (O, N andS refer to enol-O, azo-N and thioether S–Et donor centers). The Ni–N(azo) distance is comparable with reported data [32]. The Ni–Sdistance is 2.4565(8) Å. The N@N distance is 1.290(3) Å. We donot have free ligand bond length data to compare with the elec-tronic delocalization in the complexes. However, on comparingwith free ligand data of 1-alkyl-2-(arylazo)imidazole (free liganddata, 1.26 Å) [33] we believe that the N@N distance is significantlylong which may be due to charge delocalization from Ni(II) to p-acidic azo group. Inter- and intramolecular hydrogen bonds leadto the formation of supramolecular chain structure (Fig. 2b). Thependant –CO–(CH3) and azo-N (N(8)) are hydrogen bond acceptorsand coordinated –S–CH2–CH3 are hydrogen donors and they form ahydrogen bonded 1-D chain.

2.3.2. [Pt(L2)Cl] (3b)A view of the molecular structure of [Pt(L2)Cl] (3b) is shown in

Fig. 3a and the selected bond parameters are listed in Table 2. Theligand forms two essentially planar five- and six-member chelaterings with the bite angles 91.66(15)� and 87.77(12)� for N(1)–Pt–O(2) and N(1)–Pt–S(1), respectively. The N@N bond length is1.286(6) Å, which is shorter than in Ni(L2)2 (1.290(3)) (Fig. 2a,Table 2). This implies weaker charge delocalization in dp(Pt)–p⁄(azo group) compared to dp(Ni)–p⁄(azo group). This may bedue to a higher energy difference between interacting orbitals in3b compared to 1b (see DFT–TD–DFT computation, discussed below).The most prominent intermolecular interaction in the crystalstructure appears to be of the type C–H���Cl [C(5)–H���Cl(1)i = 2.66 Å, C(5)���Cl(1)i = 3.579(6) Å with an angle at 164�for symmetry operation i: x, �1 + y, z] which result the formationof linear supramolecular chains (Fig. 3b) aligned along the b-axis.

2.4. Redox behavior of the complexes and electro-generation of Ni(III)species

The complexes [Ni(L)2] (1) display a quasireversible (0.82 V (1a)and 0.76 V (1b) and peak-to-peak separation >200 mV) one-elec-tron redox response. Representative voltammogram is shown inFig. 4. The potential (E) values refer to one-electron stoichiometryand are listed in Table 3 The redox behavior at positive to SCE ismetal centric and can be represented as in Eq. (1). DFT calculationof [Ni(L2)2] (1b) shows that the higher energy singly occupiedmolecular orbitals (SOMOs) are mainly dx2 � y2 and dz2 (138,83%, dx2 � y2 and 139, 77%, dz2) of central metal ion, Ni(II)(Fig. 5). Oxidation is electron extraction from highest level occu-pied MO and thus, SOMO, 139 may participate in the redoxprocess.

½NiðLÞ2�þ þ e ! ½NiðLÞ2� ð1Þ

Table 2Bond lengths and angles of [Ni(L2)2] (1b), [Pt(L2)Cl] (3b) and DFT generated data for 1b, [Pd(L2)Cl] (2b) and (3b).

[Ni(L2)2] (1b) [Pt(L2)Cl] (3b) [Pd(L2)Cl] (2b)

X-ray DFT X-ray DFT DFT

Ni(1)–N(7) 2.012(2) 2.046 M–Cl(1) 2.3019(15) 2.354 2.333Ni(1)–O(10) 2.021(19) 2.020 M–S(1) 2.2258(13) 2.273 2.281Ni(1)–S(1) 2.4565(8) 2.598 M–O(2) 2.023(3) 2.029 2.029N(7)–N(8) 1.290(3) 1.303 M–N(1) 1.960(4) 2.004 2.007N(7)i–Ni(1)–O(10) 90.60(8) 90.79 S(1)–C(1) 1.779(5) 1.790 1.789N(7)–Ni(1)–O(10) 87.34(8) 87.93 O(1)–C(8) 1.206(7) 1.224 1.224N(7)–Ni(1)–O(10)i 90.60(8) 91.24 O(2)–C(9) 1.283(6) 1.269 1.261N(7)i–Ni(1)–S(1) 98.78(7) 98.65 N(1)–N(2) 1.286(6) 1.282 1.280N(7)–Ni(1)–S(1) 83.38(7) 82.66 C(7)–C(9) 1.423(8) 1.449 1.455O(10)–Ni(1)–S(1) 170.23(5) 170.11 Cl(1)–M–S(1) 92.86(5) 93.46 92.29O(10)i–Ni(1)–S(1) 93.26(6) 93.04 Cl(1)–M–O(2) 87.69(11) 88.81 90.55N(7)i–Ni(1)–S(1)i 83.38(7) 82.53 Cl(1)–M–N(1) 178.87(13) 179.3 179.3N(7)–Ni(1)–S(1)i 98.78(7) 98.22 S(1)–M–O(2) 178.91(12) 177.6 177.1O(10)–Ni(1)–S(1)i 93.26(6) 92.99 S(1)–M–N(1) 87.79(13) 87.03 87.03O(10)i–Ni(1)–S(1)i 170.23(5) 170.12 O(2)–M–N(1) 91.65(16) 90.67 90.11

Symmetry: i�x, y, 1/2 � z.

Fig. 2b. Inter and intra molecular H-bonding structure of Ni(L2)2 (1b).

Fig. 3a. ORTEP structure of [Pt(L2)Cl] (3b) H atoms are omitted for clarity (30%probability ellipsoids).

Fig. 3b. H-bonded structure of C13H15ClN2O2PtS (3b) where the H-bond is C(10)–H–Cl_b.

Fig. 4. Cyclic voltammogram of [Ni(L2)2] in CH3CN–CH2Cl2 solution mixture usingPt-working electrode, Pt-auxiliary electrode and SCE reference electrode in pres-ence of [n-But

4N](ClO4) as supporting electrolyte.

178 M.K. Paira et al. / Inorganica Chimica Acta 370 (2011) 175–186

Ni(III)–Ni(II) reduction potential data are known for few com-plexes involving thioether co-ordination [17]. The peak-to-peakseparation of the couple is largely dependent on scan rate and in-creases from 130 mV at 50 mV s�1 to 400 mV at 1000 mV s�1. At

slow scan rate (20–100 mV s�1) DEP remains almost constant andalso the EPa and the EPc values. This observation suggests low het-erogeneous electron-transfer rate constant which has been influ-enced by the applied potential. This observation may reflect the

Table 3Cyclic voltammetric data for the complexes.

Compound Cyclic Voltammetry dataa

E (V), (DE, mV)

EM EL

[Ni(L1)2] (1a) 0.82 (220) �0.38 (120) �0.76 (140) �1.28b

[Ni(L2)2] (1b) 0.76 (230) �0.43 (110) �0.80 (160) �1.22b

[Pd(L1)Cl] (2a) �0.52 (120) �1.28b

[Pd(L2)Cl] (2b) �0.50 (120) �1.20b

[Pt(L1)Cl] (3a) �0.38 (100) �1.28b

[Pt(L2)Cl] (3b) �0.34 (150) �1.35b

a Solvent MeCN–CH2Cl2 (3:1, v/v), supporting electrolyte [Bu4N](ClO4), Pt-diskmilli working electrode, Pt-wire auxiliary electrode, Reference electrode SCE, at298 K, EM = metal oxidation Ni(III)/Ni(II) couple, EL = ligand reductions [–N@N–]/½—N ——- - N—�� and ½—N ——- - N—��/[–N–N–] = E = 0.5 (Epa + Epc) where Epa is anodicpeak potential and Epc is cathodic peak potential, DEp = |Epa � Epc|.

b Epc.

Fig. 5. X-Band EPR of [Ni(L2)2]+ (1b+) (generated by coulometric oxidation) infrozen acetonitrile-CH2Cl2 solution (G = 10�4 T).

M.K. Paira et al. / Inorganica Chimica Acta 370 (2011) 175–186 179

change in geometry of the complex during redox transformation.Ni(III), being a hard acid center, may develop strong covalent inter-action with N(azo) and O(enolic) centers while Ni(II), a borderlineion may interact with S(thioether). Thus, there will be a structuraldistortion on going from [Ni(L)2] to [Ni(L)2]+ in the electrochemicaltime scale. Reductive responses are observed at negative potentialto SCE; quasireversible couple at �0.3 to �0.5 V and �0.75 to�0.90 V may be the description of [–N@N–]/½—N ——- - N—�� and anirreversible response at �1.3 V is assigned to ½—N ——- - N—��/[–N–N–] (Table 3). Pd(II) (2) and Pt(II) (3) complexes only show reduc-tive responses at negative to reference potential but no oxidativeresponse is observed.

Upon coulometric oxidation of [Ni(L)2] in acetonitrile-CH2Cl2

the solution turns to bluish violet due to the formation of Ni(III)species. The tridentate nickel(III) complexes are thermally less sta-ble and for these coulometry are performed at 258 K. Upon one-electron reduction of the oxidized solutions Ni(II) species areregenerated. However, we could not isolate Ni(III) complexes inthe solid state. Frozen solutions (77 K) of the Ni(III) complexes dis-play axial EPR spectra with g\ � 2.10 and g|| � 2.02. A representa-tive spectrum is shown in Fig. 5.

2.5. Theoretical calculation of electronic structure and correlationelectronic spectra, redox and magnetic properties of the complexes

Theoretical calculation using DFT computation technique is cur-rently being used to define electronic configuration and to explain

electronic properties of the molecules. The structures of 1b, 2b and3b have been optimized by using DFT computation technique. Thecalculated structures reproduce the experimental structures for 1band 3b (Table 3). The theoretical metric parameters (bond lengthsand angles) are slightly longer in the optimized structures com-pared to the X-ray crystallographic structures. Most significantbond length distortion is observed for Ni–S distance (elongatedby 0.14 Å compared to the X-ray structure of 1b). The N@N dis-tance is elongated by 0.05 Å in 3b while it is 0.01 Å longer in 1bin calculated structures than that of X-ray structures. The orbitalenergies along with contributions from the ligands and metal aregiven in Supplementary Materials (Tables S1–3). The features ofsome selected occupied and unoccupied frontier orbitals areshown in Fig. 6. Energy level correlations of M(O, N, S) motif is gi-ven in Fig. 7. Two unpaired electrons of Ni(II) reside at two MOs(abbreviated SOMO) 138 (83%, dx2 � y2; energy, �2.59 eV) and139 (77%, dz2; energy, �2.17 eV) where orbital designation inparenthesis represents the percentage contribution of d-functionand energy (Fig. 6). Thus these two functions are unusually nonde-generate [34]. HOMO-2 (137) and HOMO-3 (136) are degenerate(energy = �5.63 eV) and constituted by ligand functions (>95%).Other filled MOs are also constituted by ligand contribution. TheLUMOs (orbitals 140, 141, etc.) are of ligand restricted functions(contribution >95%). Mulliken spin densities around Ni(O, N, S) mo-tif are Ni, 1.574; S, 0.145; O, 0.137; N@Nazo, 0.146e (Fig. 8). In 2band 3b the HOMO (energy: �6.21 eV for 2b and �6.16 eV for 3b)is constituted by metal, chelated ligand and Cl. The contributionof 2b: Pd, 18%; L2, 57%; Cl, 25% and 3b: Pt, 28%; L2, 37%, Cl, 35%.The HOMO-1 (energy: �6.64 eV for 2b and �6.61 eV for 3b) alsocarries both metal (12% in 2b and 18% in 3b) and ligand (2b: L2,21%; Cl, 66% and 3b: L2, 10%, Cl, 72%). Other occupied MOs aremainly composed of contribution from ligand (L2) and Cl exceptHOMO-4 who carries >80% metal character. The LUMO (energy:�2.67 eV (2b); �2.87 eV (3b)) has mainly ligand characteristics(97%) of which 40% comes from azo (–N@N–) function. LUMO+1has significant metal (>45%) and ligand contribution (L2, 40% andCl, 12%). Other unoccupied MOs carry mainly properties of L2.

The electronic absorption spectra have been assigned using thetime-dependent DFT method. Experimental absorption spectra ofthe complexes are shown in Fig. 1 and calculated spectra are givenin Supplementary material (Figs. S1–3). The calculated excitationwavelength and their assignment are given in Table 4. As seen,TDDFT calculations well reproduce the absorption spectrum ofthe complexes measured in chloroform.

DFT and TD-DFT computation of optimized geometry of 1b hascalculated a weak transition at 758.2 nm (f, 0.0002) which is a d–d band and in experiment the transition appears at 854 nm inCHCl3. The calculated transition at 640.3 nm is the admixture ofMLCT, Ni(dp) ? N@N(p⁄) and SMCT, S(p) ? Ni(dp) which appearsin experimental spectrum at 571 nm. Transition of 547.6 nm isassigned to the mixture of Ni(dp) ? N@N(p⁄) and L(p) ?N@N(p⁄) which appears in the experimental spectrum at520 nm. A strong band is calculated at 425.5 nm while experi-mental transition is observed at 422 nm. This is assigned toL(p) ? Ni(dp) transition. Transitions calculated <400 nm are dueto ligand centerd transitions (p–p⁄). In Pd(II), 2b and Pt(II), 3bthe transitions at longer wavelength (500–400 nm) may be con-sidered as the combination of MLCT and XLCT [(X = Cl) ? p⁄(L)]transitions (abbreviated in Table 4 as XLCT) along with a seriesof M-to-ligand (MLCT), intraligand charge transfer (ILCT) transi-tions, etc. They are predicted in the range between 500 and300 nm. High intense absorption (f > 0.1) generally observes forXLCT and ILCT transitions. In solution phase (CHCl3) the transitionenergies are shifted to higher energy side which signifies prefer-ential stabilization of occupied MOs than unoccupied MOs andthus energy separation increases.

Fig. 6. Surface plots of some MOs of 1b, 2b and 3b along with their energy and composition.

Fig. 7. Energy correlation, composition and transition energies among MOs of 1b, 2b and 3b.

180 M.K. Paira et al. / Inorganica Chimica Acta 370 (2011) 175–186

2.6. Antibacterial activity

Antibacterial screening test of the ligands HL1/HL2 and theirmetal chelates against the microorganisms initially employed the

disk diffusion method [35]. The complexes 2a and 2b are demon-strated to be good in vitro inhibitory activity against 7 out of 10bacterial strains used in this work. The results of the antimicrobialactivity of the compounds are shown in Table 5. Data reveal that

Fig. 8. Spin density plot in triplet state (isosurface cutoff value 0.003) of [Ni(L2)2].Mulliken spin densities: Ni, 1.574; Sthio, 0.145; Oenolate, 0.137; N@Nazo, 0.146e.

M.K. Paira et al. / Inorganica Chimica Acta 370 (2011) 175–186 181

the complex, 2a inhibits the growth of two organisms (Bacillus sub-tilis and Micrococcus luteus), as the MIC is 25 lg/ml (6.4 � 10�8

mol), and 2b inhibits the growth of three organisms (Shigelladysenteriae, Salmonella paratyphi and M. luteus), as MIC 10 lg/ml(2.4 � 10�8 mol). We have tested 6% DMSO as control and no anti-microbial activity has been found against these bacterial strains.

The studies on the action of 2a and 2b on two susceptible bac-terial species (Figs. 9 and 10, respectively) at different concentra-tions show that the growth of these organisms decreases withincreasing concentration of the complexes and are completelyinhibited at their MIC values. The minimum bactericidal concen-tration (MBC) is found to be 4- to 8-times higher than MIC values.

Table 4Calculated transitions obtained from TD-DFT computation of optimized geometry of [Ni(L

Excited state k (nm) (f � 103) Energy (eV) Transition

[Ni(L2)2] (1b)1 758.7 (0.2) 1.634 (65%)125(b3 640.3 (1.2) 1.936 (33%)122(b

(26%)123(b7 547.6 (1.0) 2.264 (42%)138(a

(17%)137(a11 425.3 (110) 2.915 (44%)135(b17 388.4 (11.8) 3.192 (53%)136(a22 364.5 (179.2) 3.402 (27%)136(b

(17%)137(a26 349.7 (189) 3.544 (50%)136(b

[Pd(L2)Cl] (2b)1 496.5(0.0) 2.4968 (85%)HOM2 435.3(12.5) 2.8481 (60%)HOM3 418.1(58.4) 2.9651 (33%)HOM

(29%)HOM5 395.6(33.3) 3.1340 (43%)HOM

(30%)HOM10 336.6(145.7) 3.6833 (43%)HOM

(23%)HOM15 291.8(71.9) 4.2487 (23%)HOM

(23%)HOM18 275.2(38.4) 4.5046 (40%)HOM23 260.0(115.6) 4.7681 (39%)HOM

[Pt(L2)Cl] (3b)1 462.0(63.9) 2.6834 (84%)HOM2 422.9(0.5) 2.9317 (80%)HOM6 348.9(46.5) 3.5540 (42%)HOM

(41%)HOM7 343.2(52.8) 3.6116 (43%)HOM

(26%)HOM9 330.0(35.5) 3.7570 (73%)HOM14 285.7(60.5) 4.3395 (87%)HOM18 270.9(49.3) 4.5763 (73%)HOM

The results reveal that the drugs exhibit bacteriostatic but bacteri-cidal at higher concentrations, probably due to interference by theactive principle(s) of the drugs.

The complexes 2a (12.8 � 10�8 mol) and 2b (24.7 � 10�8 mol)exhibit separate zone of inhibition in different bacterial stains withrespect to concentration of streptomycin (50 lg/ml and 100 lg/ml)and data are listed in Tables 6 and 7, respectively. In case of bacte-rial stain S. paratyphi 2a does not show any zone of inhibition instreptomycin but 2b exhibits different zone (in 50 and 100 lg/mlexhibited 9 mm and 12 mm). The ligands do not show antibacterialactivity. The activity of metal complexes may be due to chelation ofthe ligands with metal ions which reduce the polarity of the metalion mainly by partial sharing of positive charge with the donorgroups and possible p-electron delocalization over the whole che-late ring enhancing the lipophilicity of the complexes. This in-creased lipophilicity may lead to breakdown of the permeabilitybarrier of the cell and thus retards the normal cell processes[36,37]. Antimicrobial activity of the complexes is compared withstandard data of Gentamicin (Table 5). Ni(II) complexes do not ex-hibit any antimicrobial activity up to 500 lg/ml and some of themicrobes become resistant to other complexes also. [Pd(L2)Cl]exhibits promising activity againstS. dysenteriae, S. paratyphi, M. lu-teus and MIC is 10 lg/ml (Tables 6 and 7). Pt(II) complexes are notpromising towards antimicrobial activity. The structures and thelability of M–Cl bond explain the trends. [Ni(L)2] is octahedral(Figs. 2a and 2b) and does not have any vacant coordination siteto bind with bio-molecules of bacterial cell wall and requires highenergy to cleave any one of the Ni–O/N/S bonds. [M(L)Cl] aresquare planar (Figs. 3a and 3b); two axial coordination sites are va-cant and these complexes may interact with donor centers avail-able in bio-molecules and are responsible for better anti-bacterial

2)2] (1b), [Pd(L2)Cl] (2b) and [Pt(L2)Cl] (3b).

Character Experimental (nm)

) ? 139(b) Ni(dp) ? Ni(dp) 854) ? 141(b)) ? 140(b)

Ni(dp)/S(p) ? N@N(p⁄)Ni(dp)/S(p) ? Ni(dp)

571

) ? 141(a)) ? 141(a)

Ni(dp) ? N@N(p⁄)L(p) ? N@N(p⁄)

520

) ? 138(b) L(p) ? Ni(dp) 422) ? 141(a) L(p) ? N@N(p⁄) 382) ? 140(b)) ? 141(a)

L(p) ? N@N(p⁄)

) ? 140(b) L(p) ? N@N(p⁄)

O ? LUMO+1 L/Cl ? Pd/L(p⁄)O-1 ? LUMO+1 Cl ? Pd/L(p⁄) 441O-4 ? LUMO+1O ? LUMO

d ? d, Pd ? L(p⁄)L(p) ? L(p⁄)

420

O-4 ? LUMO+1O ? LUMO

d ? d, Pd ? L(p⁄)L(p) ? L(p⁄)

O-3 ? LUMOO-2 ? LUMO+1

Cl/L(p) ? L(p⁄)L(p) ? Pd/L(p⁄)

354

O-6 ? LUMOO-5 ? LUMO+1

L(p) ? L(p⁄)L(p) ? Pd/L(p⁄)

O-7 ? LUMO Pd(dp)/L(p) ? L(p⁄) 269O ? LUMO+3 L(p) ? L(p⁄)

O ? LUMO Pt(dp)/Cl/L(p) ? L(p⁄) 464O-1 ? LUMO Cl ? L(p⁄) 443O-1 ? LUMO+1O-3 ? LUMO

Cl ? Pt/L(p⁄)Cl/L(p) ? L(p⁄)

363

O-1 ? LUMO+1O-3 ? LUMO

Cl ? Pt/L(p⁄)Cl/L(p) ? L(p⁄)

O-4 ? LUMO+1 d ? d, Pt(dp) ? L(p⁄)O ? LUMO+3 Pt(dp)/Cl/L(p) ? L(p⁄) 278O-2 ? LUMO+1 L(p) ? L(p⁄)

Table 5Antibacterial activities of the compounds.*

Name of bacteria Minimum inhibitory concentration (MIC) (lg/ml)

HL1 HL2 [Ni(L1)2] (1a) [Ni(L2)2] (1b) [Pd(L1)Cl] (2a) [Pd(L2)Cl] (2b) [Pt(L1)Cl] (3a) [Pt(L2)Cl] (3b) Gentamicin

Shigella dysenteriae 500 _ _ _ 50 10 _ _ 2Escherichia coli _ 500 _ _ 400 _ 500 _ 1Salmonella typhi _ _ _ _ 500 500 _ _ 1Salmonella paratyphi _ 500 _ _ 50 10 _ _ 1Bacillus subtilis _ _ _ _ 25 25 _ _ 1Pseudomonas aeruginosa _ _ _ _ _ _ _ _ 64Streptococcus faecalis _ _ _ _ _ 500 _ _ 1Micrococcus luteus _ 300 _ _ 25 10 _ _ 4Staphylococcus aureus _ _ _ _ 50 25 300 300 1Vibrio cholerae _ _ _ _ 500 300 _ _ 1

‘–’ Shows no antimicrobial activity upto 500 lg/ml.* No inhibition was found even upto 6% DMSO that was used as control. While complex solution were prepared in 4% DMSO solution.

Fig. 9. Effect of 2a on two bacteria at different concentrations.

Fig. 10. Effect of 2b on two bacteria at different concentrations.

Table 6Different concentration of drug 2a exhibit different zone of inhibition in differentbacterial strains with respect to streptomycin.*

Name of Organisms Diameter of zone ofinhibition (in mm) ofdrug in differentconcentration

Diameter of zone ofinhibition (in mm) ofstreptomycin in differentconcentration

50 (lg/ml) 100 (lg/ml) 50 (lg/ml) 100 (lg/ml)

Bacillus subtilis 11 ± 0.33 13 ± 0.16 18 ± 0.33 20.5 ± 0.28Micrococcus luteus 14.5 ± 0.28 16 ± 0.33 21 ± 0.66 26 ± 0.33

* Values are in terms of mean ± SEM of results done in triplicate.

182 M.K. Paira et al. / Inorganica Chimica Acta 370 (2011) 175–186

activity. Both palladium(II) and platinum(II) ions strongly prefernitrogen and oxygen donor atoms [38] available in the bio mole-cules. Due to very low reacting equilibrium constants for Pt(II)complexes and their kinetic inertness compared to Pd(II) com-plexes (105 times slow) [39], the interaction of Pt(II) complexesis much slower than the isostructural Pd(II) complexes.

3. Conclusion

3-(2-(Alkylthio)phenylazo)-2,4-pentanedione (HL) is used forthe synthesis of [Ni(L)2], [Pd(L)Cl] and [Pt(L)Cl]. The complexesare characterized by spectroscopic techniques and single crystalX-ray diffraction measurement in case of nickel(II) and plati-num(II) complexes. Ni(II) complexes show a high potentialNi(III)/Ni(II) redox couple. The electronic properties are explainedby DFT computation. Antibacterial properties of the ligands andthe complexes have been examined. The ligands are inactive and[Pd(L)Cl] show the highest activity in the family of nickel triad.The structural difference of octahedral [Ni(L)2] and square planar,[M(L)Cl] account for the antibacterial efficiency of the compounds.

Table 7Different concentration of drug 2b exhibit different zone of inhibition in differentbacterial strains with respect to streptomycin.*

Name of Organisms Diameter of theinhibition zone (in mm)of drug in differentconcentration

Diameter of theinhibition zone (in mm)of streptomycin indifferent concentration

50 (lg/ml)

100 (lg/ml)

50 (lg/ml) 100 (lg/ml)

Shigella dysenteriae 13 ± 0.16 16 ± 0.16 12 ± 0.57 13.5 ± 0.28Salmonella paratyphi 9 ± 0.44 12 ± 0.88 5 5Micrococcus luteus 16 ± 0.28 21 ± 0.33 21 ± 0.16 26Bacillus subtilis 9.5 ± 0.28 13 ± 0.44 15.5 ± 0.28 20 ± 0.16Staphylococcus

aureus8 ± 0.76 9 ± 0.16 12 ± 0.44 16

* Values are in terms of mean ± SEM of results done in triplicate.

M.K. Paira et al. / Inorganica Chimica Acta 370 (2011) 175–186 183

The M–Cl bond lability, Pd–Cl� Pt–Cl, may explain the highestantibacterial efficiency of [Pd(L)Cl]. Among the organisms testedM. luteus is most susceptible to the drugs. Further pharmacologicaland clinical studies are in progress to understand the mechanismand the actual efficacy of these compounds in treating variousinfections and skin diseases.

4. Experimental section

4.1. Materials

Acetylacetone (Hacac), 2-aminothiophenol, iodomethane (MeI),iodoethane (EtI), Ni(OAc)2�4H2O were purchased from E. Merck, In-dia. PdCl2 and K2[PtCl4] were purchased from Arrora Mathey, Kolk-ata, India. All other chemicals used were of A.R. quality and wereused as received. The organic solvents were purified and dried bystandard methods [40]. 2-(Alkylthio)aniline was prepared by thereported procedure [41].

4.2. Physical measurements

UV–Vis spectra were recorded using a Perkin–Elmer Lambda 25UV–Vis spectrophotometer and infrared spectra (4000–200 cm�1)were obtained from a Perkin–Elmer Spectrum RX1 instrument.Microanalyses were collected on a Perkin–Elmer 2400 CHN ele-mental analyzer. 1H NMR spectra were recorded in a Bruker300 MHz FT-NMR. Room temperature magnetic moment was mea-sured using a Magnetic Susceptibility Balance, Sherwood ScientificCambridge, UK. The experimental susceptibilities were correctedfor the diamagnetism of the constituent atoms (Pascal tables). Mo-lar conductance (KM) was measured in a Systronics conductivitymeter 304 model using ca. 10�3 M solutions in acetonitrile. Electro-chemical measurements were performed using computer-con-trolled PAR model 250 VersaStat electrochemical instrumentswith Pt-disk electrodes. All measurements were carried out undernitrogen environment at 298 K with reference to SCE in acetonitrileusing [nBu4N][ClO4] as supporting electrolyte. The reported poten-tials are uncorrected for junction potential. EPR spectra were mea-sured in MeCN–CH2Cl2 solution at room temperature (298 K) andliquid nitrogen temperature (77 K) using a Bruker EPR spectrome-ter model EMX 10/12, X-band ER 4119 HS cylindrical resonator.

4.3. Synthesis

4.3.1. 3-(2-(Methylthio)phenylazo)-2,4-pentanedione, HL1

To a cold solution of 2-aminothiophenol (2.9 g, 23 mmol) inethanol (50 ml) sodium (0.54 g, 23 mmol) was slowly added withstirring for 1 h. The color changed to orange from yellow. Then

MeI (1.45 ml, 23 mmol) was added at cold condition and was stir-red for another 30 min. The mixture was then refluxed for 2 h; thecolor of the solution changed to red. The mixture was then pouredinto large volume of water, a gummy mass separated and it wasextracted with benzene (15 ml � 3) and washed with water(25 ml � 4). Benzene was then removed using a rotary evaporator.The gummy mass was dissolved in 1:1 HCl (10 ml) and cold solu-tion of NaNO2 (1.8 g in 10 ml water) was added in drops at 0 �C(ice bath). The resulting mixture was then added in drops to thesolution of Na2CO3 (6 g in 30 ml water) and acetylacetone(2.34 ml, 0.023 mol). A yellow-orange precipitate was obtained;it was filtered and washed with cold water and dried over CaCl2.Recrystallization from aqueous-ethanol solution (1:3 v/v) gavethe desired product 4.75 g (81.9%), m.p. 121 �C. Microanalyticaldata: Anal. Calc. for C12H14N2O2S: C, 57.58; H, 5.64; N, 11.19.Found: C, 57.30; H, 5.67; N, 11.06%. IR data (KBr disk) (l, cm�1):1671(s), 1626(m), 1506, 1357, 1319. kmax (nm, e � 10�3,mol�1 cm�1): 397(18.3); 380(21.3); 275(8.82); 253(15.0).

4.3.2. 3-(2-(Ethylthio)phenylazo)-2,4-pentanedione, HL2

HL2 was also prepared by the same procedure. Yield was 4.37 g(85%), m.p. 101 �C. Microanalytical data: Anal. Calc. C13H16N2O2S:C, 59.05; H, 6.10; N, 10.59. Found: C, 60.09; H, 6.08; N, 10.33%. IRdata (KBr disk) (l, cm�1): 1673(s), 1627(m), 1507, 1358, 1321. kmax

(e � 10�3, mol�1 cm�1): 396(14.3); 374(17.6); 277(5.63);253(12.6).

4.3.3. Preparation of bis-{3-(2-(methylthio)phenylazo)-2,4-pentanedionato}nickel(II), [Ni(L1)2] (1a)

To Ni(OAc)2,4H2O (0.060 g, 0.240 mmol) solution in methanol(15 ml) the ligand, HL1 (0.120 g, 0.480 mmol) was added. Stirringwas continued for half an hour, the color changed to bluish green.The solution was filtered and left undisturbed for a week; bluishgreen crystals deposited at the bottom of the container. These werecollected by filtration, washed with MeOH–water (1:1 v/v) and fi-nally with hexane. Yield, 0.085 g (63%). [Ni(L2)2] (1b) was preparedby the same procedure. Yield, 0.088 g (59%).

Microanalytical data: Anal. Calc. for C24H26N4O4S2Ni (1a): C,51.32; H, 4.70; N, 10.05. Found: C, 51.42; H, 4.72; N, 9.94%. IR data(KBr disk) (l, cm�1): 1650, 1538, 1355, 1290. kmax (e � 10�3,mol�1 cm�1): 852(0.038); 576(0.159); 519(0.295); 422(16.99);382(14.33); 278(21.02). Anal. Calc. for C26H30N4O4S2Ni (1b): C,53.55; H, 5.17; N, 9.57. Found: C, 53.60; H, 5.20; N, 9.50%. IR data(KBr disk) (l, cm�1): 1649, 1540, 1358, 1294. kmax (e � 10�3,mol�1 cm�1): 854(0.084); 571(0.367); 520(0.719); 422(33.84);382(28.38); 285(34.78).

4.3.4. Preparation of chloro-{3-(2-(methylthio)phenylazo)-2,4-pentanedionato}palladium(II), [Pd(L1)Cl] (2a)

PdCl2 (0.065 g, 0.368 mmol) was dissolved by refluxing in 20 mlacetonitrile, an orange color solution resulted. To this solution HL1

(0.092 g, 0.370 mmol) in acetonitrile (10 ml) was added in dropswith stirring. The mixture was refluxed for 4 h. A silky yellow pre-cipitate appeared on cooling. The solution was concentrated on arotary vacuum evaporator to 10 ml; the solid was filtered off,washed with cold acetonitrile and then n-hexane and dried. Yield,0.092 g (64%). The other complex was prepared by the same proce-dure. The yield, 0.087 g (67%).

Microanalytical data: Anal. Calc. for C12H13N2O2SClPd (2a): C,36.85; H, 3.35; N, 7.16. Found: C, 36.90; H, 3.42; N, 7.21%. IR data(KBr disk) (l, cm�1): 1667, 1532, 1369, 1327, 1301, 334. kmax

(e � 10�3, mol�1 cm�1): 442(9.84); 420(12.45); 360(5.93);269(22.90); 246(17.66). Microanalytical data: Anal. Calc. forC13H15N2O2SClPd (2b): C, 38.54; H, 3.73; N, 6.91. Found: C,38.49; H, 3.80; N, 6.83%. IR data (KBr disk) (l, cm�1): 1367, 1329,

184 M.K. Paira et al. / Inorganica Chimica Acta 370 (2011) 175–186

1301, 335. kmax (e � 10�3, mol�1 cm�1): 441(14.97); 420(18.20);354(8.74); 269(34.08); 246(31.33).

4.3.5. Preparation of chloro-{3-(2-(methylthio)phenylazo)-2,4-pentanedionato}platinum(II), [Pt(L1)Cl] (3a)

K2PtCl4 (0.11 g, 0.25 mmol) was dissolved in acetonitrile–water(15 ml, 1:2, v/v) by refluxing, a light orange color developed. HL1

(0.066 g, 0.26 mmol) in acetonitrile (10 ml) was added to this solu-tion and refluxed for another 8 h. The resultant yellow color solu-tion was filtered and kept undisturbed. After a week, yellowcrystals suitable for X-ray diffraction were collected. Yield,0.061 g (48%). The other complex was prepared by the same proce-dure. Yield, 0.062 g (53%).

Microanalytical data: Anal. Calc. for C12H13N2O2SClPt (3a): C,30.04; H, 2.73; N, 5.84. Found: C, 29.92; H, 2.84; N, 5.95%. IR data(KBr disk) (l, cm�1): 1668, 1324, 1295, 338. kmax (e � 10�3,mol�1 cm�1): 463(3.51); 443(3.99); 349(4.96); 272(6.54);246(7.84). Microanalytical data: Anal. Calc. for C13H15N2O2SClPt(3b): C, 31.62; H, 3.06; N, 5.67. Found: C, 31.58; H, 3.03; N,5.73%. IR data (KBr disk) (l, cm�1): 1668, 1499, 1371, 1354,1289, 340. kmax (e � 10�3, mol�1 cm�1): 464(5.45); 443(6.09);363(13.65); 251(16.65).

4.4. Crystal structure analysis of bis-{3-(2-(ethylthio)phenylazo)-2,4-pentanedionato}nickel(II), [Ni(L2)2] (1b) and chloro-{3-(2-(ethylthio)phenylazo)-2,4-pentanedionato}platinum(II), [Pt(L2)Cl](3b)

The crystals were grown by slow evaporation of the reactionmixture over a week. Data were collected in 2h range, 6.4 to50.7� with a Bruker CCD diffractometer using fine-focus sealedgraphite-monochromatized Mo Ka radiation (k = 0.70930 Å) at125(2) K for a crystal of [Ni(L2)2] (1b) with dimensions0.30 � 0.30 � 0.29 mm3. In case of [Pt(L2)Cl] (3b) intensity datawere measured in 2h range 5.4 to 53.0� at 153(2) K on a RigakuAFC12/Saturn724 CCD fitted with graphite-monochromatized MoKa radiation (k = 0.71070 Å) for an olive-green crystal with dimen-sions 0.10 � 0.20 � 0.30 mm3. Crystallographic data and structurerefinement parameters are given in Table 8. Omega scans wereused for 3b. For 1b, data reduction was carried out using Crystal

Table 8Crystallographic data for [Ni(L2)2] (1b) and [Pt(L2)Cl] (3b).

Crystal parameters [Ni(L2)2] (1b) [Pt(L2)Cl] (3b)

Empirical formula C26H30N4NiO4S2 C13H15ClN2O2PtSFormula weight 585.37 493.87Crystal system monoclinic monoclinicSpace group C2/c C2/cUnit cell dimensionsa (Å) 24.355(2) 19.367(5)b (Å) 8.5168(8) 10.588(3)c (Å) 15.1757(14) 14.753(3)b (�) 123.416(2) 100.198(6)V (Å3) 2627.5(4) 2977.4(13)Z 4 8Dcalc (Mg/m3) 1.480 2.204l (Mo Ka) (mm�1) 0.938 9.746F(0 0 0) 1224 1872T (K) 125(2) 153(2)Total data 11193 16123Unique data 2410 3084Parameters 171 183Ra 0.045 0.029wR2

b 0.103 0.070Goodness-of-fit (GOF) 1.11 1.20

a R = R||Fo| � |Fc||/R|Fo|.b wR2 = [Rw(F2

o � F2c )2/Rw(F2

o )2]1/2, w = 1/[r2(Fo)2 + (0.049P)2 + 4.025P] for[Ni(L2)2]; w = 1/[r2(Fo)2 + (0.022P)2 + 30.836P] for [Pt(L2)Cl]; where P = ((F2

o þ 2F2c )/

3.

Structure (Rigaku Corp., 2004). The structure was solved by the di-rect-methods using SHELXS-97 [42] and successive difference Fouriersyntheses. Data processing and empirical absorption correction for3b were accomplished with the programs CRYSTAL CLEAR [43] and AB-

SCOR [44], respectively. The structure was solved by heavy-atommethods [45], the non-hydrogen atoms were refined with aniso-tropic displacement parameters, and the hydrogen atoms werefixed geometrically and refined using the riding model approxima-tion. The molecular graphics were carried out using ORTEP-3 forWindows [46] and PLATON-99 [47] programs. The residual electrondensity is in the range 0.92 and �0.69 e �3 for 1b and 1.40 to�1.36 e �3 for 3b.

4.5. Computation

Full geometry optimizations of the complexes were carried outusing the density functional theory method at the B3LYP level [48].All elements except Ni, Pd and Pt were assigned the 6-31G(d) basisset. The SDD basis set with effective core potential was employedfor the Pd and Pt atoms [49] and LanL2DZ with effective core po-tential for Ni atom. The vibration frequencies were calculated toensure that the optimized geometries represent the local minimaand there are only positive eigen values. All calculations were per-formed with GAUSSIAN03 program package [50] with the aid of theGAUSSVIEW visualization program [51]. Vertical electronic excitationsbased on B3LYP optimized geometries were computed usingthe time-dependent density functional theory (TD–DFT) formalism[52] in chloroform using conductor-like polarizable continuummodel (CPCM) [53]. GAUSSSUM [54] was used to calculate thefractional contributions of various groups to each molecularorbital.

4.6. Antibacterial assays

4.6.1. MicroorganismsThe microorganisms used in this study included B. subtilis

UC564, Escherichia coli TG1, Staphylococcus aureus Bang25, Pseudo-monas aeruginosa C/1/7, S. typhi NCTC62, S. paratyphi NCTC A2,S. dysenteriae 8NCTC599/52, Streptococcus faecalis S2, Vibrio chol-erae DN7 and M. luteus AGD1. They were obtained from Divisionof Microbiology, Dept. of Pharmaceutical Technology, JadavpurUniversity, Kolkata-700 032, and West Bengal, India. The bacterialstrains were grown in blood agar or MacConkey agar plates at 37 �Cand maintained on nutrient agar slants.

4.6.2. Preparation of inoculumsSuspensions of organisms were prepared as per McFarland

Nephelometer standard [48]. A 24 h old culture was used for thepreparation of bacterial suspension in a sterile isotonic solutionof sodium chloride (0.9% w/v) and the concentration wasapproximately 1.5 � 108 cells/ml. It was obtained by adjustingthe optical density of the bacterial suspension to that of a solutionof 0.05 ml of 1.175% of barium chloride and 9.95 ml of 1% sulfuricacid.

4.6.3. Drug solutionThe compounds (10 mg) HL1 (0.04 mmol), HL2 (0.04 mmol),

[Ni(L1)2] (1a) (0.02 mmol), [Ni(L2)2] (1b) (0.02 mmol), [Pd(L1)Cl](2a) (0.027 mmol), [Pd(L2)Cl] (2b) (0.024 mmol), [Pt(L1)Cl] (3a)(0.02 mmol), [Pt(L2)Cl] (3b) (0.02 mmol) were screened for theirantibacterial activity. All the drugs were dissolved in 4% of DMSOto get the concentration of 1 mg/ml, which were used as stocksolution. Evaluation of the activity was carried out by agar dilutiontechnique using nutrient agar medium.

M.K. Paira et al. / Inorganica Chimica Acta 370 (2011) 175–186 185

4.6.4. AssaySensitivity tests were performed by disk diffusion method, as

per NCCLS [55] protocol. The Mueller Hinton agar plates, contain-ing an inoculum size of 106 cfu/ml of bacteria were used. Prepareddrug solution impregnated disks at concentrations of 0–500 lg/mlwere placed aseptically on sensitivity plates with appropriate con-trols [56]. All the plates were then incubated at 37 �C overnight.The sensitivity was recorded by measuring the clear zone ofgrowth inhibition on agar surface around the disks.

4.6.5. Determination of minimum inhibitory concentration (MIC)MIC was determined by agar dilution methods [57]. Previously

prepared drug dilutions (0–500 lg/ml) of the crude drug, withappropriate antibiotic control were prepared in Mueller HintonAgar [58]. For agar dilution assay previously prepared sensitivityplates, using serial 2-fold dilutions of the drug and control antibi-otics as above, were spot inoculated (2 � 106 cfu/spot). The inocu-lated plates were then incubated at 37 �C for 24 h. The lowestconcentration of plate which did not show any visible growth aftermacroscopic evaluation was considered as the MIC.

4.6.6. Determination of minimal bactericidal concentration (MBC)MBC was determined by broth dilution methods. Previously

prepared drug dilutions (0–200 lg/ml) of the crude drug 2a and2b in Mueller Hinton broth were used. The mixtures were thenincubated at 37 �C for 18 h with shaking on a platform shaker at200 rpm. The drug concentration (10 lg/ml for S. dysenteriae,S. paratyphi and M. luteus for 2b but in 2a 25 lg/ml for B. subtilisand M. luteus) was added to the mid-logarithmic phase of growthand aliquots of 1.0 ml were withdrawn at intervals for the determi-nation of optical density at 540 nm and colony count [57]. The low-est concentration of the tube which did not show any visiblegrowth after colony count was considered as the MBC [59].

4.6.7. Determination of zone of inhibitionThe medium was prepared by dissolving all the ingredients in

distilled water and subjected to sterilization in an autoclave at121 �C for 15 min. The Petri plates were washed thoroughly andsterilized in hot air oven at 160 �C for 1½ h 30 ml of sterile moltenagar medium was seeded by organisms (about 2 ml according toMc Farland’s standard [60]), in semi hot conditions (40 �C) waspoured aseptically in sterile Petri plate and allowed to solidify atroom temperature. Bores were made on the medium using sterileborer and 0.1 ml of the diluted drugs (50 lg/ml and 100 lg/ml)were added to respective bore and 0.1 ml of the standard Strepto-mycin at a concentration of 50 lg/ml and 100 lg/ml were taken asstandard. The Petri plates seeded with organisms, containing ex-tracts and the standard were kept in refrigerator at 4 �C for 1 h tofacilitate the diffusion of the extracts and the standard into themedia. After diffusion the Petri plates were incubated at 37 ± 1 �Cfor 24 h in an incubator and zone of inhibition was observed andmeasured using a scale [61].

Acknowledgments

Financial support from the UGC-CAS programme, UniversityGrants Commission and Department of Science and Technology,New Delhi are gratefully acknowledged.

Appendix A. Supplementary material

CCDC 747642 and 747643 contain the supplementary crystallo-graphic data for compounds [Ni(L2)2] (1b) and [Pt(L2)Cl] (3b),respectively. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/

data_request/cif. Supplementary data associated with this articlecan be found, in the online version, at doi:10.1016/j.ica.2011.01.049.

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