Some new four coordinated Hg (II) complexes: Synthesis, characterization, electrochemical behavior, antibacterial/antifungal properties and theoretical investigation

Some New Four Coordinated Hg(II) Complexes: Synthesis,Characterization, Electrochemical Behavior, Antibacterial/Antifungal Properties, and Theoretical Investigation

MORTEZAMONTAZEROZOHORI1, SAHAR YADEGARI1, ASGHAR NAGHIHA2, and AHMADHOJJATI1

1Department of Chemistry, Yasouj University, Yasouj, I. R. Iran2Department of Animal Sciences, Faculty of Agriculture, Yasouj University, Yasouj, I. R. Iran

Received 9 May 2013; accepted 11 June 2013

Five mercury(II) complexes of a recently synthesized bidentate Schiff base ligand were prepared. All mercury(II) coordinationcompounds were characterized perfectly with the aid of elemental analysis, FTIR, 1H NMR, 13C NMR, UV-Visible, and molarconductance. Morphology of mercury bromide particles as typical one in the view point of shape and size were qualified by SEM.Electrochemical behaviors of Hg(II) complexes as compared with ligand were evaluated by using cyclic voltammetry method. Theresults showed a similar redox behavior for coordinated ligand in complexes with respect to free ligand. Antibacterial/antifungalproperties of the compounds were checked against three Gram-negative bacteria Escherichia coli (ATCC 25922), Pseudomonasaeruginosa (ATCC 9027), Salmonella spp., and two Gram-positive bacteria Staphylococcus aureus (ATCC 6538) and Coryne-bacterium renale, and three fungal strains (Aspergillus niger, Penicillium chrysogenum, and Candida albicans). The results revealedacceptable antibacterial/antifungal activities for most of the compounds. HgL(N3)2 showed remarkable activity against Penicilliumchrysogenum. Furthermore theoretical investigation of ligand and mercury complexes was performed by Gaussian 98 at theUB3LYP/LANL2MB level of theory. Some structural and energetic data such as bond length, bond angle, torsional angle, dipolemoment of compounds, Gibbs free energy, and enthalpy and entropy of complex formation were evaluated.

Keywords: antibacterial, antifungal, complex, mercury(II), Schiff base

Introduction

Ordinary, Schiff base synthesis is accomplished by condensa-tion between primary amines and aldehydes or ketones assubstances. The products of these kind of reactions, alsoknown as azomethine compounds, have the characteristicbond of CHN. Schiff base ligands are able to be coordinatedto different metal ions through azomethine nitrogen. Schiffbase ligands have been synthesized in a variety of methodsand they bind to various metal ions by a number of donoratoms. Synthesis of Schiff bases has been increased from thefirst one by Hugo Schiff[1] up to now due to wide and usefulapplications of them in many fields such as dyes and pig-ments,[2–4] catalyst for polymerization, oxidation and epoxi-dation reactions,[5–7] application in nano-chemistry,[8–10] andespecially with the development of biological active agentssuch as anticancer,[11] antitumor,[12] antiviral,[13] antibacte-rial,[14] antifungal,[15] and antimalarial.[16] Mercury(II) forms

complexes with different coordination numbers on accountof 5d10 configurations.[17–19] A literature survey illustrates thesynthesis and characterization of somemercury Schiff base com-plexes with ligands such as N,N0-bis (pyridin-2-yl)benzylidene)-ethane-1,2-diamine, N-((1-pyridin-2-yl)formylidene)-N0-[2-(4-{2-[((1-pyridin-2-yl)formylidene)amino]ethyl}piperazin-1-yl)ethyl] amine,[20] benzaldehyde-N,N-dimethylthiosemicar-bazone,[21] and N,N0-bis(a-methylcinamaldehydene)propane-1,2-diimine.[22] In some reports, mercury Schiff base com-plexes have been used as catalyst.[23] Although some mer-cury compounds influence some organs in body but recentlyseveral useful applications such as the antimicrobial activityof some mercury complexes have been found of interest forbiochemist and bioinorganic chemists. For example, mer-cury complex of 2-formylpyridine thiosemicarbazone,[24] gly-cine and histidine complexes of Hg(SeCN)2,

[25] hydrazonecomplexes of Hg(II),[26] Hg(II) complexes of quercetin,[27]

mercury(II) complexes of sulfonium ylides,[28] and mercury(II) cystine complex[29] have been found as antimicrobial,antibacterial, and antineoplastic materials. With regard tothe previous reports on biological activity of mercury com-plexes and in continuation of our previous studies,[30–33]

herein we wish to report the synthesis, characterization, andelectrochemical behavior of some new four coordinatedSchiff base mercury complexes and then their anitibacterial/

Address correspondence to Morteza Montazerozohori, Depart-ment of Chemistry, Yasouj University, Yasouj 75918-74831,I. R. Iran. E-mail:[email protected] versions of one or more figures in this article can be foundonline at www.tandfonline.com/lsrt.

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, (2015) 45, 48–57Copyright © Taylor & Francis Group, LLCISSN: 1553-3174 print / 1553-3182 onlineDOI: 10.1080/15533174.2013.818029

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antifungal activity against three Gram-negative bacteria,Escherichia coli (ATCC 25922), Pseudomonas aeruginosa(ATCC 9027), and Salmonella spp., and two Gram-positivebacteria, Staphylococcus aureus (ATCC 6538) andCorynebac-terium renale, and three fungal strains (Aspergillus niger, Peni-cillium chrysogenum, andCandida albicans) are investigated.

Experimental

Materials and Methods

2,2-dimethyl-1,3-propylenediamine, (E)-3-(2-nitrophenyl)acryl-aldehyde, and the other chemicals were purchased fromAldrich,Merck, or BDH companies. Mercury thiocynate and azide wereprovided according to our previous work.[30] Elemental micro-analysis of compounds was done with a CHN analyzer. FT/IRspectra were recorded by JASCO-FT/IR680 instrument at therange of 4000–400 cm¡1 as KBr tablets. 1H and 13CNMR spec-tra were recorded using an NMR spectrometer 400 MHz,Avance III 400 in (CD3)2SO, and/or CDCl3 as solvents. UV-Visible spectra were scanned in chloroform and DMF on aJASCO-V570 spectrometer. Molar conductance of compoundswas determined by a Metrohm-712 conductometer on samplesolution of 10¡3M in chloroform and/orDMF at room temper-ature. Electrochemical redox behavior of each sample was evalu-ated in dry acetonitrile (10¡3M) by SAMA500Electro-Analyzercontaining three available electrodes system (glassy carbon asworking, Pt-disk as supporting, and wire of silver as referenceelectrodes) on deoxygenated medium (by argon saturation) atroom temperature with a scan rate of 0.1 V/S. (n-Bu)4NPF6(T-BAHP)was used as supporting electrolyte.

Synthesis of Ligand (L)

Synthesis of N,N-bis ((E)3-(2nitrophenyl)acrylaldehydene)-2,2-dimethyl-1,3–diaminopropane was performed based on ourprevious report.[33] Accordingly, addition of 2,2-dimethyl-1,3-diaminopropane to (E)-3-(2-nitrophenyl)acrylaldehyde in1:2 ratio in pure ethanol under vigorous stirring for about 5 hand then isolation and recrystallization from ethanol led toproduct in 83% yield.

Synthesis of Mercury(II) Complexes

Hg(II) complexes were synthesized with drop wise addition ofethanolic solution (10 mL) of ligand (1 mmol) to mercury

salts (1 mmol) in pure ethanol (10 mL) with 1:1 molar ratiounder severe stirring for about 2–3 h at room temperature.Purification of the complexes was accomplished by recrystal-lization from dichloromethane/ethanol mixture (1:1). Char-acteristic data such as IR and UV-Visible data of mercurycomplexes have been summarized in Tables 1 and 2 and their1H and 13C NMR data are listed in the following:

[HgLCl2]:1H-NMR (CDCl3): 8.33 (d, 2Hc,c0, JD 8.72 Hz),

8.05(d, 2Hi,i0, J D 8.12 Hz), 7.95(d, 2Hf,f0, J D 7.76 Hz),7.80(t,2Hg,g0, J D 7.68, 7.52 Hz), 7.65(t, 2Hh,h0, J D 7.92,7.52 Hz), 7.54(d, 2He,e0, JD 15.76 Hz), 7.27(dd, 2Hd,d0, JD15.64 Hz, 8.8 Hz, 8.72 Hz), 3.35(s,4Hb,b0), 0.93(s,6Ha,a0)ppm.13C-NMR(CDCl3): 165.50(C4,40), 148.06(C8,80), 138.09(C6,60), 133.72(C9,90), 130.88(C7,70), 130.42(C5,50), 129.84(C12,120), 128.35(C11,110), 124.67(10,100), 70.18(C3,30), 36.77(C1), 24.15(C2,20) ppm.[HgLBr2]:

1H-NMR spectra (in CDCl3): 8.23 (d, 2Hc,c0,J D 8.68 Hz), 8.04(dd,2Hi,i0, J D 8.2 Hz, 1.2 Hz), 8.8(dd,2Hf,f0, J D 7.92 Hz, 1.12 Hz), 7.80(dd,2Hd,d0, J D15.60 Hz, 8.72 Hz), 7.73(dt,2Hg,g0, J D 7.62 Hz, 7.56 Hz,1.08 Hz), 7.70(d, 2He,e0, J D 15.64 Hz), 7.57(dt, 2Hh,h0, J D7.92 Hz, 7.68 Hz, 1.28 Hz), 3.78(s,4Hb,b0), 1.01(s, 6Ha,a0)ppm. 13C-NMR(CDCl3): 167.37(C4,40), 148.15(C8,80),141.56(C6,60), 133.85(C9,90), 130.61(C5,50), 130.05(C7,70),129.10(C12,120), 129.03(C11,110), 124.99(C10,100), 71.34(C3,30), 37.70(C1), 24.90(C2,20) ppm.[HgLI2]:

1H-NMR(in CDCl3): 8.25 (d, 2Hc,c0, JD 8.24 Hz),8.05(dd, 2Hi,i0, J D 8.16 Hz, 1.2 Hz), 7.97(dd, 2Hf,f0, J D7.82 Hz, 1.00 Hz), 7.76(dd,2Hd,d0, J D 15.74 Hz, 8.16 Hz),7.73(t, 2Hg,g0, J D 8.12 Hz, 7.44 Hz), 7.69(d,2He,e0, J D15.76 Hz), 7.58(dt, 2Hh,h0, J D 7.96 Hz, 7.64 Hz, J D1.36 Hz), 3.67(s,4Hb,b0), 1.02(s,6Ha,a0) ppm. 13C-NMR-(CDCl3): 166.91(C4,40), 148.09(C8,80), 141.10(C6,60), 133.85(C9,90), 130.53(C5,50), 130.23(C7,70), 129.12(C12,120), 129.07(C11,110), 125.01(C10,100), 69.97(C3,30), 37.58(C1), 24.90(C2,20) ppm.[HgL(SCN)2]:

1H-NMR(in CDCl3): 8.42 (d, 2Hc,c0, J D8.56 Hz), 8.12(dd, 2Hi,i0, J D 8.18 Hz, 1.16 Hz), 7.95(dd,2Hf,f0, J D 7.88 Hz, 1.12 Hz), 7.89(d, 2He,e0, J D 15.48 Hz),7.78(dt, 2Hg,g0, J D 7.52 Hz, J D 0.84), 7.62(dt,2Hh,h0, J D7.94 Hz, 7.70 Hz, 1.32 Hz), 7.37(dd,2Hd,d0, J D 15.4 Hz,9.16 Hz), 3.83(s,4Hb,b0), 1.08(s,6Ha,a0) ppm. 13C-NMR(CDCl3): 168.81(C4,40), 147.99(C8,80), 144.27(C6,60), 134.16(C9,90), 131.02(C7,70), 130.09(C5,50), 129.45(C12,120), 128.00

Table 1. Analytical and physical data of the Schiff base ligand (L) and its Hg(II) complexes.

Compound ColorDecomposedpoint. (�C) Yield (%)

Found(Calcd.) (%)

ɅM (ms.cm¡1)C H N

Ligand Honey colored 85 83.2 65.5(65.70) 5.6(5.75) 13.4(13.33) 0.011HgLCl2 Milky 180 82.0 39.7(39.92) 3.7(3.50) 8.3(8.10) 0.092HgLBr2 Milky 178 98.2 35.4(35.2) 3.1(3.2) 7.2(7.9) 0.024HgLI2 Light olive 164 94.4 31.3(31.58) 2.9(2.77) 6.6(6.40) 0.027HgL(SCN)2 Gray 169 44.3 40.7(40.5) 3.3(3.4) 11.4(11.3) 0.018HgL(N3)2 Dark cream 153 10.2 39.1(39.18) 3.7(3.43) 19.6(19.87) 12.70

Some New Four Coordinated Hg(II) Complexes 49

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(C11,110), 125.17(C10,100), 117.43(CSCN), 71.94(C3,30), 37.51(C1), 24.99(C2,20) ppm.[HgL(N3)2]:

1H-NMR(DMSO): 8.15 (d, 2Hc,c0, JD 8.72 Hz),8.02(d,2Hi,i0, J D 7.72 Hz), 8.00(d,2Hf,f0, J D 7.44 Hz), 7.74(t,2Hg,g0, J D 7.64 Hz, 7.48 Hz), 7.60(t,2Hh,h0, J D 7.92 Hz,7.80 Hz), 7.38(d,2He,e0, J D 15.84 Hz), 7.03(dd,2Hd,d0, J D15.80 Hz, 8.72 Hz, 8.68 Hz), 3.35(s,4Hb,b0), 0.93(s,6Ha,a0)ppm. 13C-NMR(DMSO): 162.83(C4,40), 147.98(C8,80), 135.32(C6,60), 133.54(C9,90), 132.42(C5,50), 130.11(C7,70), 129.85(C12,120), 128.43(C11,110), 124.48(C10,100), 69.71(C3,30), 36.51(C1), 24.25(C2,20) ppm.

Antibacterial Activity of Compounds

Antibacterial activity of the mercury complexes in compari-son with ligand was examined in vitro by agar diffusionmethod against two Gram-positive bacteria such as Staphylo-coccus aureus (ATCC 6538) and Corynebacterium renale andalso three Gram-negative bacteria including Escherichia coli(ATCC 25922), Salmonella spp., and Pseudomonas aerugi-nosa (ATCC 9027).[34] In this procedure, Muller Hinton agar(Merck, Germany) was used as medium and each plate waspoured with about 15 mL of prepared medium under sterilecondition. 0.1 mL of particular bacterium containing 0.5 £106 (CFU/mL) equivalent to 0.5 McFarland standards,which was inoculated for 24 h (old), was injected on the sur-faces of plates and then was dispread by cotton swab.[35] Vari-ous concentrations of 25, 50, and 100 ppm of the Schiff baseligand and its Hg(II) complexes in DMSO were used for pro-viding the disks containing 0.5, 1.25, and 2.5 mg compounds.For this mean, sterile disks (6 mm in diameter) were soakedin sample solutions and then were placed on distinctive posi-tion of the plates. Afterward plates with located disks wereincubated at 37�C for 24 h. Inhibition diameter of zone (mm)

for each compound was appeared after passing the time dura-tion and it is known as antibacterial activity(Table 4). In thisstudy, a disk containing absolute DMSO (Table 5) was alsoapplied as the blank at the same condition. Some kinds ofantibiotics, for instance, amoxicillin, penicillin, and cepha-lexin, were used as reference bactericidal drugs (Table 5).

Minimum inhibitory concentration

Minimum inhibitory concentration (MIC) was the othermethod that was used for determining the antibacterial activ-ity based on broth medium under serial dilution.[36] For thismean a serial dilution of compounds in the range of 16000 to0.97 mg/mL was performed. Each sterile sample tube con-tains 0.65 mL of Muller Hinton broth medium (Scharlab)and 0.1 mL of specific bacterium. Then entire tubes wereincubated at 37�C for 24 h. Growth of bacteria in the tubeswas considered after incubation and lowest concentration,which inhibited visible growth reported as MIC (Table 5).

Minimum bactericidal concentration

Sometimes, observation of bacteria growth would becomeimpossible by MIC so that minimum bactericidal concentra-tion (MBC) is used as the alternative method for evaluatingof antibacterial activities and therefore a loop full of MICsolution in each test tubes was subcultured on the plates ofMuller Hinton agar medium (Merck, Germany) and thenwas incubated at 37�C for 24 h (Table 6).[37]

Investigation of Antifungal Activity

Examination of antifungal activities of mercury complexes ascompared with ligand was carried out with disk diffusionmethod against three fungal strains (Aspergillus niger, Peni-cillium chrysogenum, and Candida albicans [local isolates]).For this aim, sterile disks were drenched into the preparedsolution of the compounds in DMSO and then disks contain-ing compounds (1.25, 2.5, and 5 mg/disk) were put on dis-tinctive place of Petri plates, which were poured withSabouraud dextrose agar (SDA) medium (Oxoid, Hamp-shire, England) and also have been fruitful with 100 mL (105

CFU/mL) of fungal spore suspensions. Then, all the plateswere incubated at 32�C for seven days in the cases of Asper-gillus niger and Penicillium chrysogenum and at 37�C for 24 hfor Candida albicans (Table 7).[37]

Table 2. Vibrational frequencies (cm¡1) and electronic transition spectral data (nm) of the Schiff base and its complexes.

Compound nCHN nCHimine nNO2 nSCN nN3 λmax (e; cm¡1 M¡1)

L 1636(s), 1614(s) 2869(w) 1529(vs), 1338(vs) — — 295(19876), 328(sh)(11045)HgLCl2 1635(vs) 2854(m) 1521(vs), 1345(vs) — — 273(sh), 291, 324(sh)HgLBr2 1635(vs) 2853(w) 1523(vs), 1348(vs) — — 302, 324(sh)HgLI2 1633(vs) 2851(w) 1521(vs), 1348(vs) — — 291, 323(sh)HgL(SCN)2 1624(vs), 1610(sh) 2872(w) 1520(vs), 1342(vs) 2114 — 290, 320(sh)HgL(N3)2 1631(vs), 1611(sh) 2867(w) 1523(vs), 1340(vs) — 2023 296, 330(sh)

Table 3. Electrochemical data of ligand and its mercurycomplexes.

Compound EPa1, EPa2 EPC1, EPC2, EPC3

Ligand ¡1.53, ¡1.02 ¡1.10, ¡1.77HgLCl2 ¡1.17, ¡0.32 ¡0.45, ¡1.25, ¡1.92HgLBr2 ¡1.17, ¡0.34 ¡0.39, ¡1.16, ¡1.79HgLI2 ¡1.15, ¡0.38 ¡0.53, ¡1.23, ¡1.90HgL(SCN)2 ¡1.20, ¡0.05 ¡0.38, ¡1.27, ¡1.89HgL(N3)2 ¡1.03, 0.0 ¡0.20, ¡1.12, ¡1.78

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Results and Discussion

Physical and Analytical Data

The four coordinated mercury complexes were provided byaddition of ligand to mercury halide or pseudo-halide salts in1:1 molar ratio. Elemental analysis confirmed perfectly 1:1ratio of ligand to metal so that HgLX2 (in which X ion washalide or pseudo-halides of thiocyanate and azide) is sug-gested formula for the complexes (Scheme 1). Ligand and allHg(II) complexes are stable at room temperature for longtime. All complexes were soluble in DMSO, DMF, THF,and insoluble in alcohols. Physical characteristics and somespectral data of the mercury coordination compounds havebeen tabulated in Tables 1 and 2. Mercury chloride and bro-mide complexes were milky colored powder while mercuryiodide, thiocyanate, and azide complexes were obtained aslight olive, gray, and dark cream precipitate, respectively.The SEM image of mercury bromide complex shows a

nanostructure size (100–200 nm) in the solid state (Figure 1).The mercury complexes are decomposed in the temperaturerange of 164–180�C. Molar conductivities of the complexeswere examined in chloroform except for HgL(N3)2 (measuredin DMF) and the values in the range of 0.018–12.70 ms.cm¡1

proposed nonelectrolytic nature for entire compounds, con-firming coordination of Schiff base ligand and X ions tomercury ion in a molecular structure.[41]

Spectral Investigation

Infrared and electronic spectra

IR spectra provide preliminary and facile characterization.Some characteristic IR data of the ligand were reported pre-viously while the IR data of its mercury coordination com-pounds, which were assigned to functional groups and arelisted in Table 2. Two medium signals at 1636 and1614 cm¡1 assigning to asymmetric and symmetric vibration

Table 4. Antibacterial activities of constructed disks containing 2.5, 1.25, and 0.5 mg/disk of Schiff base and its mercury complexesin diameter zone (mm) on various bacterial strains.

Gram Negative bacteria Gram-positive bacteria

Compound

Escherichiacoli

Pseudomonasaeruginosa

Salmonellaspp.

Corynebacteriumrenale

Staphylococcusaureus

2.5 1.25 0.5 2.5 1.25 0.5 2.5 1.25 0.5 2.5 1.25 0.5 2.5 1.25 0.5

Ligand 9 7.8 7 17.1 13.7 7.3 9.3 9.0 8.6 14.4 12 10 16 14 11HgLCl2 22.1 20.9 18.6 14.1 13.6 11.2 15.1 13.3 11.2 75.8 41.7 32.7 30.6 23.6 22HgLBr2 20 19 16.2 18.1 16.4 14.2 18.5 17 15.3 51.8 36.6 21.5 24.3 22.6 21.7HgLI2 21 20.3 18.8 12.9 12.2 10.4 14.3 13.4 10.6 38.6 30.6 18.9 28.1 27.7 21.4HgL(SCN)2 22.1 21.9 17.4 17.5 15.3 11.7 20.7 18.7 17.9 9.1 9.0 8.3 32.8 26.5 21.5HgL(N3)2 17.6 16.3 14.6 27.6 22 21.6 32.2 21 15.8 38.4 36.7 30.5 27.2 25.4 21

Table 5. Antibacterial activities of some antibiotic and DMSO(alone).

AntibioticCorynebacterium

renaleSalmonella

spp.Pseudomonasaeruginosa

Staphylococcusaureus

Escherichiacoli

Amoxicillin (25 mg/disk) 20.80 41.60 — 18.54 36.28Penicillin (10 mg/disk) 9.70 47.40 — 11.00 36.28Cephalexin (30 mg/disk) 22.56 41.60 — 24.50 45.80DMSO

Table 6.MIC and MBC data of Schiff base ligand and its mercury complexes for inhibition of growth in mg/mL.

Escherichia coli Pseudomonas aeruginosa Salmonella spp. Corynebacterium renale Staphylococcus aureus

Compound MIC MBC MIC MBC MIC MBC MIC MBC MIC MBC

Ligand N.D 31.2 N.D 1000 N.D 16000 N.D 500 N.D 125HgLCl2 N.D 7.8 N.D 3.9 N.D 250 N.D 31.2 N.D 125HgLBr2 N.D 3.9 N.D 15.6 N.D 250 N.D 3.9 N.D 125HgLI2 N.D 7.8 15.6 15.6 N.D 250 N.D 1.95 N.D 125HgL(SCN)2 N.D 3.9 N.D 7.8 N.D 1000 N.D 3.9 N.D 125HgL(N3)2 N.D 3.9 N.D 15.6 N.D 250 N.D 0.97 N.D 125

N. D D not possible for detection.

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of CHN group of ligand are converged in complexes spectraand a very strong peak emerges at 1624–1635 cm¡1 confirm-ing coordination of ligand to metal through iminic link-age.[30–33,39] The weak iminic C-H vibrations were appearedat wave numbers of 2851–2872 cm¡1 after coordination. Thevibrations at 1529 and 1338 cm¡1 allocating to asymmetricand symmetric vibrations of nitro-groups were shifted tolower wave numbers (by »9 cm¡1) and to higher wave num-bers (»10 cm¡1) in the complexes, respectively.[40] A newsharp vibration peak at 2023 cm¡1 in HgL(N3) and also at2114 cm¡1 in HgL(SCN)2 spectra were appeared in additionto other characteristic vibrations in other complexes thatwere assigned to coordinated N3

¡ as terminal azido and coor-dinated ¡NCS as S-thiocyanato.[42,43]

Electronic spectra of the complexes were recorded at roomtemperature in chloroform (1 £ 10¡3 M) except for HgL(N3)2 because of insolubility in CHCl3. Therefore, its spec-trum was recorded in DMF. Absorption wavelengths of com-plexes in UV-Visible region have been summarized inTable 2. According to electronic spectrum of the ligand, twoabsorption bands arise at 295 nm and 328 nm (as a shoulder)that may be assigned to intraligand charge transfer (p-p*)within existing p-systems (aromatic, olefinic, and azome-thine). A comparison between electronic spectra of the Hg(II)complexes and ligand demonstrated a blue or red shift in themercury complexes indicating coordination of azomethinegroups of ligand to metal center.

1H and 13C NMR spectra

The 1H and 13C NMR spectra of Schiff base ligand[33] and itsHg(II) complexes provide some accessible data confirmingthe suggested structures in Scheme 1. The 1H NMR and 13CNMR spectra of the mercury chloride and azide complexeshave been exhibited as typical spectra in Figure 2. A doublet(d) signal at 8.09 ppm in the 1H NMR spectrum of the ligandwith coupling constants of 8.76 Hz was attributed to protonsof azomethine (Hcc0).

[30–32] At 7.99 and 7.74 ppm, two dis-tinctive doublets of doublets (dd) with coupling constants of8.2, 1.12 Hz and 7.86, 1 Hz respectively appeared that wereassigned to Hii0 and Hff0. Hydrogens of gg0 and hh0 demon-strated two doublet of triplet (dt) at 7.64 and 7.48 ppm,respectively. Strong doublet signal with a notable couplingconstant of 15.84 Hz at 7.48 ppm was ascribed to Hee0. Obvi-ous doublet of doublet (dd) signal at 6.94 ppm possessingcoupling constants of 15.86 and 8.76 Hz was assigned to dd0hydrogens. Two singlet peaks at 3.46 and 1.02 ppm wereascribed to methylene and methyl hydrogens (bb0 and aa0),respectively. All protons of the ligand are present in1H NMR spectra of mercury complexes but with differentchemical shifts and models. After complexation, 1H NMRspectra of complexes show some changes with respect to thefree ligand that some of these changes are expressed in below.

Table 7.Antifungal activities of constructed disks containing 5, 2.5, and 1.25 mg/disk of Schiff base and its mercury complexes basedon diameter zone (mm) against various fungi.

Aspergillus niger Penicillium chrysogenum Candida albicans

Compound 5 2.5 1.25 5 2.5 1.25 5 2.5 1.25

Ligand 0.9 0.8 0.65 16.7 15 14 21 19.2 18HgLCl2 15.6 14.2 10.5 31.3 30.4 21.3 27.9 22.4 19HgLBr2 25 22.5 19 18.3 17.6 14.6 20.5 18.3 17HgLI2 23 22.4 22 29.6 23.2 22 25.3 23.8 20.8HgL(SCN)2 21.6 16.4 15.9 19 16.5 15.5 20 17.7 16HgL(N3)2 22 20 16 87.9 70 48 46.3 33.7 26.8

Sch. 1. Structural formula of ligand and its mercury complexes. Fig. 1. The SEM of HgLBr2.

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Doublet signal of iminic hydrogens (Hcc0) in mercury com-plexes is considerably shifted to downfield with respect to freeligand up to 0.33 ppm. These shifts also happen in the chemi-cal shifts of Hii0, Hff0, Hgg0, Hhh0, and Hdd0 of ligand protonsafter mercury complexes formation. Hee0 hydrogens inall mercury complexes shifted to downfield except for themercury azide complex that has been upfielded. Among themercury complexes, maximum downfield in chemical shiftsof proton signals is observed in HgL(SCN)2 indicating stron-ger binding the ligand to mercury center with respect to othercases. This stronger binding may be due to suitable soft-softinteraction between mercury ion and sulfur atom of thio-cyante effecting also on improvement of mercury-azomethine

bounding force. This factor causes higher deshielding ofiminic proton and carbon in NMR spectra. The 13C NMRspectrum of the ligand exhibited a signal at 162.69 ppm thatassigned to azomethine carbons (4,40). This signal notablyshifts to downfield positions at 162.83–168.81 ppm, provingthe ligand coordination via azomethine nitrogens. The carbonNMR spectra of all complexes demonstrated downfield shiftfor C(8,80) while C(7,70) shifted to upfield in comparison withthe free ligand. C(1), C(2,20), C(6,60), and C(10,100) in allcomplexes shifted to downfield positions except for HgL(N3)2 but for the C(5,50) a reverse trend is observed. The sig-nals of C(9,90) in all mercury compounds shifted to downfieldexcept for mercury chloride. Specified signals for C(11,110)

Fig. 2. 1H NMR and 13C NMR of mercury chloride and azide complex in DMSO-d6.

Some New Four Coordinated Hg(II) Complexes 53

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indicated downfield shift except for mercury thiocyanatecomplex. Related signals for C(3,30) have moved to upfieldafter complexation except for mercury thiocyanate and bro-mide complexes. Observed signals for C(12,120) of ligand incoordination compounds were shifted to downfield except formercury iodide and bromide complexes. Finally, one moresignal at 117.43 ppm that may be assigned to carbon of coor-dinated thiocyanate is appeared in the 13C NMR spectrum ofmercury thiocyanate complex. It is to be noted that allchanges in chemical shifts of the complexes certainly confirmcoordination of ligand to mercury ion.

Electrochemical Study

Electrochemical behavior of compounds was followed bycyclic voltammetry method. All voltammograms wererecorded in DMF at deoxygenized atmosphere by argon satu-ration with sweeping rate of 0.1 V/S and shown in Figure 3.All the electrochemical data have been tabulated in Table 3.TBAH was used as supporting electrolyte. As mentioned inour previous work, the voltammogram of the ligand shows aquasi- reversible and a reversible signals in the span of –1.0 to–1.1 V and –1.5 to –1.8 V.[33] The first redox wave may beattributed to one-electron redox of nitro group as nitro/nitroradical anion couple and the second redox wave may beascribed to three-electron redox by hydroxylamine/nitrosocouple.[42] According to Figure 3, all complexes were foundto be redox active on defined conditions similar to the ligand.In the complexes voltammograms, the second redox arose asirreversible meanwhile other redox waves appeared with anegative shift with respect to the free ligand. This observednegative shift may be due to an increase of the electron den-sity on ligand p* orbitals via p-back bonding from metal toligand after coordination of ligand’s azomethine groups tomercury ion lowering tendency of coordinated ligand towardany reduction while it facilities its oxidation. Furthermore, involtammogram of mercury complexes, a new reversible orquasireversible wave emerged in –0.5 to 0.0 V that may berelated to the redox behavior of the mercury(II)/(I) couple.

Antibacterial Bioassay (In Vitro)

Inhibitory effects on the growth of Gram-negative bacteria,Escherichia coli (ATCC 25922), Salmonella spp., and Pseudo-monas aeruginosa (ATCC 9027), and Gram-positive bacteria,Staphylococcus aureus (ATCC 6538) and Corynebacteriumrenale were examined by mercury Schiff base complexes andcompared with free ligand at the same conditions. Antibacte-rial activity of compounds in different concentrations (mg/disk) has been tabulated in Table 4 and it also graphicallyhas been exhibited in Figure 4 for 2.5 mg/disk of com-pounds. Antimicrobial activity of DMSO (alone) as solventwas checked (Table 5) and we found no effect, similar tomany previous reports.[44] Some standard antibiotics (amoxi-cillin, penicillin, and cephalexin) were used in the same condi-tion as references in this study. As shown in Table 5,amoxicillin and penicillin with lower effective amounts (25and 10 mg/disk) showed higher activity than the ligand andits mercury complexes against tested bacteria except for Pseu-domonas aeruginosa. Schiff base ligand and its mercury com-plexes were evaluated as more suitable antibacterial activityagainst bacterial strains with respect to cephalexin becausewith lower effective amount showed comparable activity. Onthe other hand, the results showed acceptable antibacterialactivity against Pseudomonas aeruginosa while the tested anti-biotics exhibited no effect on it. A comparison between anti-bacterial activities of compounds can be found as it follows.The most activity against Escherichia coli bacterium wasbelonged to HgLCl2 and HgL(SCN)2. Significant effectagainst Salmonella spp. and Pseudomonas aeruginosa wasobserved by HgL(N3)2. Average results of inhibitory zoneabout Corynebacterium renale were higher than the otherbacteria and the maximum effect was evaluated for HgLCL2.Minimum activity against Corynebacterium renale wasrevealed by HgL(SCN)2; however, it was the effective com-pound against Staphylococcus aureus. Schiff base ligandshowed lowest activities against Escherichia coli, Salmonellaspp., and Staphylococcus aureus. MIC and MBC were usedas alternative methods to investigate the antibacterial activi-ties and the results were obtained in mg/mL. MIC and MBCData have been summarized in Table 6. Impossibility to reor-ganization of turbidity caused by growth of bacteria in mostcases due to inherent turbidity of the compounds revealed

Fig. 4. Inhibited zone of the growth of bacteria by disks contain-ing 2.5 mg/disk of mercury complexes as compared with ligand.

Fig. 3. Cyclic voltammograms of L(A), HgLI2(B), HgLCl2(C),HgLBr2(D), HgL(N3)2(E), and HgL(SCN)2(F).

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only one result for MIC (15.6 mg/mL against Pseudomonasaeruginosa by HgLI2) so that MBC was considered as thereplacement method. MBC results revealed that the lowesteffect against all bacteria pertained to the ligand. MBCresults against Staphylococcus aureus by all compounds werefound to be equal to 125 mg/mL. Minimum values of MBC,0.97 and 3.9 mg/mL, declared the maximum activity by HgL(N3)2 against Corynebacterium renale and Escherichia coli.HgLBr2 and HgL(SCN)2 also exhibited the same value asHgL(N3)2 against Escherichia coli. On the other hand in thecase of Pseudomonas aeruginosa, maximum of activity(MBC: 3.9 mg/mL) was achieved by use of HgLCl2. Mercurycomplexes except for HgL(SCN)2 revealed MBC value of250 mg/mL against Salmonella spp. bacterium.

Antifungal Bioassay (In Vitro)

Hg(II) complexes were scrutinized against Aspergillus niger,Penicillium chrysogenum, and Candida albicans fungal strainsto determine their antifungal activities with respect to free

Schiff base ligand. Inhibition diameters of zone (mm) as theantifungal activities have been collected in Table 7. Two typi-cal photographs of inhibited zone around the disks contain-ing mercury azide and iodide complexes disks are shown inFigure 5. According to the results, mercury azide with a nota-ble inhibition diameter zone of 70 mm revealed high activityagainst Penicillium chrysogenum. This complex also showedmaximum activity against Candida albicans. Mercury bro-mide and iodide were evaluated as the effective compoundagainst Aspergillus niger. The least activities against Aspergil-lus niger and Penicillium chrysogenum were found for Schiffbase ligand while HgLBr2 and HgL(SCN)2 exhibited loweractivities than the ligand against Candida albicans.

Molecular Modeling and Analysis

Theoretical investigation such as geometry and structuraloptimization of mercury complexes and ligand was per-formed based on definition of C1 point group for them byGaussian 98 at the UB3LYP/LANL2MB level of theory forligand (L) and all mercury complexes. For example, the opti-mized structures of ligand and mercury thiocyanate complexare exhibited in Figure 6. Some structural data such as bondlength, bond angle and torsion angle were extracted fromoptimized structure for all compounds based on Scheme 1

Fig. 6. Optimized structure of ligand (left) and HgL(SCN)2(right).

Fig. 5. Inhibited zone of the growth by disks containing 2.5 mg/disk of HgLI2 (left) and HgL(N3)2 (right) against Aspergillusniger and Penicillium chrysogenum.

Table 8. Some structural data of optimized structure of ligand and its mercury complexes.

Bond length (A�) Ligand HgLCl2 HgLBr2 HgLI2 HgL(SCN)2 HgL(N3)2

M-N — 2.373 2.375 2.378 2.363 2.501M-N0 — 2.372 2.373 2.372 2.369 2.496M-X — 2.644 2.812 2.981 2.786 2.263M-X0 — 2.621 2.784 3.013 2.749 2.269C4 DN 1.324 1.328 1.328 1.330 1.330 1.328C40 DN0 1.324 1.328 1.328 1.329 1.329 1.326

Bond angle (�)N0-M-N — 81.62 81.87 82.23 82.56 78.06N0-M-X — 98.37 97.68 96.89 102.04 93.99N0-M-X0 — 109.71 112.52 115.53 112.30 93.89N-M-X — 108.82 111.86 114.95 118.33 92.02N-M-X0 — 111.80 108.99 107.28 108.68 93.30X-M-X0 — 133.08 131.79 129.50 124.35 171.27

Torsion angle (�)N-C4-C5-C6 1.941 –35.590 –33.796 –31.039 –30.545 ¡20.510N0-C40-C50-C60 179.927 179.100 179.354 179.871 179.808 ¡179.538C5-C6-C7-C8 17.337 16.848 17.054 17.044 16.431 19.526C50-C60-C70-C80 –179.605 178.895 178.915 179.441 178.189 179.825

Some New Four Coordinated Hg(II) Complexes 55

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and tabulated in Table 8. Lengthening the C4 D N, C40 D N0

(1.32367 A�and 1.32408 A

�) to longer values than the free

ligand is one evidence for confirming the coordination ofnitrogen atoms of azomethine groups to mercury ion. M-Nand M-N0 bond lengths of complexes were increased frommercury chloride to mercury azide complex so that HgL(N3)2showed the maximum bond lengths (2.50065 A

�and 2.49605

A�) for M-N and M-N0, respectively. M-X and M-X0 bonds

were lengthened from mercury chloride to mercury thiocya-nate and then shortened in HgL(N3)2 to 2.26273 A

�and

2.26912 A�

values, respectively. The geometry around themercury center in all complexes is found as pseudotetrahe-dral. In accordance with related bond angles in Table 8, theN0-M-N angle is evaluated in the range of 78.06–82.56� withthe maximum and minimum values assigned to mercury thio-cyanate and azide complexes, respectively. The X-M-X0angles are set in the span of 124.35–171.27� with maximumvalue for mercury azide complex. Other surrounded angles ofmercury ion in pseudotetrahedral geometry (N0-M-X, N-M-X,N-M-X0, N0-M-N, N0-M-X0, X-M-X0) were found in therange of 92.02–118.33�. Also some torsion angles includingN-C4-C5-C6, N0-C40-C50-C60, C5-C6-C7-C8, and C50-C60-C70-C80

were extracted from optimized structures that are observable inTable 8, suggesting nonplane status around the C4-C5, C40-C50,C6-C7, and C60-C70 bonds. As shown in Table 9, dipole momentof ligand was found to be 2.26 D value. Maximum value of7.35Dwas achieved for HgLI2 indicating this compound as themost polar complex in the series. In addition to the previousstructural parameters, some energetic parameters including,Gibbs free energy (DG�), enthalpy (DH�), and entropy (DS�) ofcomplex formation at 298 K were calculated as listed inTable 10. All obtained parameters have negative value indicat-ing spontaneous formation of the complexes at room tempera-ture. The DG� value of the complexes is decreased frommercury chloride with –70.084 kcal/mol to mercury azide com-plex with –33.538 kcal/mol. The DH� value is decreased frommercury chloride to mercury thiocyanate complex and then

increased in mercury azide complex. The entropy change of thecomplexes formation is also negative and has an increase frommercury chloride tomercury thiocyanate complex and then it isdecreased inmercury azide compound.

Conclusion

In this article, the synthesis, characterization, electrochemicalbehavior, biological properties of some new mercury(II) com-plexes of a bidentate Schiff base ligand and then theoreticalinvestigation of them were presented. Four coordinated Hg(II) complexes were stable at room temperature for longtime. Elemental analysis confirmed a perfectly 1:1 molar ratioof ligand to metal salts. Molar conductance revealed nonelec-trolytic character for all the complexes. Complexation causedsome changes in IR, UV-Vis, 1H NMR, and 13C NMR spec-tra and also in electrochemical behavior with respect to thefree ligand. SEM image suggested a nanostructure size forHgLBr2 complex in solid state. Investigation of antimicrobialactivities expressed that these compounds have suitable anti-bacterial and antifungal properties. In the view of antimicro-bial testing, the ligand demonstrated lower activity than themercury complexes. Notable antibacterial activity wasrelated to HgLCl2 against Corynebacterium renale andremarkable antifungal activity was achieved by HgL(N3)2against Penicillium chrysogenum. In accordance with spectraland theoretical investigation and evaluated angles aroundmercury ion, a pseudotetrahedral geometry was suggested forall complexes.

Funding

Partial support of this research by Yasouj University isacknowledged.

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