Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 564–570

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Spectroscopic characterization of Lanthanoid derived from a hexadentate macrocyclic ligand: Study on antifungal capacity of complexes Sulekh Chandra a,⇑, Swati Agrawal b a b

Department of Chemistry, Zakir Husain Delhi College (University of Delhi), JLN-Marg, New Delhi 110 002, India Department of Chemistry, Motilal Nehru College (University of Delhi), Benito Juarez Road, New Delhi 110 021, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 A hexadentate macrocyclic ligand i.e.

Growth inhibition of synthesized compounds against Fusarium oxysporum.

3,5,13,15,21,22-hexaaza-2,6,12,16tetramethyl-4,14-dithiatricyclo[15.3.1.1(7-11)]docosane1(21),2,5,7,9,11(22),12,15,17,19decaene was synthesized and characterized.  Lanthanoid complexes were synthesized and characterized using various spectral techniques.  Spectral parameters suggested formation of metal ligand covalent bonding.  The complexes were found to have coordination number nine.  In vitro antifungal capacity was determined of the synthesized compounds.

a r t i c l e

i n f o

Article history: Received 19 September 2013 Received in revised form 21 December 2013 Accepted 8 January 2014 Available online 21 January 2014 Keywords: Hexadentate ligand Lanthanoid Spectroscopic characterization Antifungal capacity

a b s t r a c t Complexes of Ce(III), Nd(III), Sm(III) and Eu(III) were synthesized with NO-donor macrocyclic ligand, i.e. 3,5,13,15,21,22-hexaaza-2,6,12,16-tetramethyl-4,14-dithia-tricyclo[15.3.1.1(7-11)]docosane-1(21),2,5,7,9,11(22),12,15,17,19-decaene. The ligand was obtained by the condensation of 2,6-diacetylpyridine with thiourea and characterized by elemental analysis, mass, IR and 1H NMR spectral studies. All the complexes were characterized by elemental analyses, molar conductance measurements, magnetic susceptibility measurements, IR, mass, electronic spectral techniques and thermal studies. The ligand acts as a hexadentate and coordinated through four nitrogen atoms of azomethine groups and two nitrogen atoms of pyridine ring. The value of spectral parameters i.e. nephelauxetic effect (b), covalency factor (b1/2), Sinha parameter (d%) and covalency angular overlap parameter (g) account for the covalent nature of the complexes. The macrocyclic ligand and its Lanthanoid were tested in vitro against two plant pathogenic fungi in order to assess their antifungal capacity. Ó 2014 Elsevier B.V. All rights reserved.

Introduction The design and synthesis of complexes of lanthanide metal ions with macrocyclic ligands constitute a fascinating area of research ⇑ Corresponding author. Tel.: +91 1122911267; fax: +91 1123215906. E-mail address: [email protected] (S. Chandra). 1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2014.01.042

because of their importance in basic and applied chemistry [1–3]. They are also useful in industrial and synthetic processes such as catalysis, photochemistry and in biological systems [4]. Macrocyclic ligands are able to recognize the presence of lanthanide metal ions. Therefore they are widely used in the selective extraction of metals [5] and as NMR shift reagents [6]. Lanthanide complexes

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have an increasingly important role in medicine, where they are employed as diagnostic as well as therapeutic agents [7]. A larger number of Lanthanoid complexes with Schiff-base macrocyclic ligands derived from 2,6 diacetylpyridine have already been published [8–11]. The stability of macrocyclic complexes depends upon a number of factors, including the number and types of donor atoms present in the ligand and their relative positions within the macrocyclic skeleton, as well as the number and size of the chelate ring formed on complexation [12]. Schiff bases also play an important role in the development of coordination chemistry related to catalysis, enzymatic reactions, magnetism, and molecular architectures [13]. Schiff base metal complexes have been widely studied because they have industrial, antifungal, antibacterial, anticancer and herbicidal applications [14–16]. In view of these facts, reaction of the Lanthanoid metal ion and macrocyclic ligand has been carried out and structure of the resulting complexes were investigated using spectroscopic techniques. All the synthesized compounds were evaluated against plant pathogenic fungi in order to assess their antimicrobial properties.

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peak at 346 is 70% because it corresponds to the macrocyclic moiety. A peak of very high intensity (100%) is also present at m/ z = 79 which is the base peak due to the pyridine ring. Some other peaks are also present in the spectrum at 330, 276, 203, 180, 126, 44, 27 and 15 due to the other fragmented ions [18]. The peaks area provides an idea of abundance of these ions. The fragmentation path of the ligand is given in Fig. 4. Synthesis of complexes The Lanthanoid of ligand (L) were prepared by the mixing of hot ethanolic solution (20 mL) of the corresponding metal salts (0.001 mol), to a hot ethanolic solution (20 mL) of the ligand L (0.001 mol) with constant stirring on a magnetic stirrer. The reaction mixture was continuously stirred and refluxed on a waterbath for 30–35 h at 75–80 °C. On cooling the reaction mixture at 0 °C for 24 h, complex was precipitated out, washed with cold EtOH and dried under vacuum over silica gel. Physical measurements

Experimental Materials and methods All the chemicals used were of Anala R grade and received from Sigma–Aldrich and Fluka. Lanthanoide salts were purchased from E. Merck and were used as received. Synthesis of ligand A hot ethanolic solution (20 mL) of 2,6-diacetylpyridine (3.26 g, 0.02 mol) was added to an ethanolic (20 mL) solution of thiourea (1.52 g, 0.02 mol). This mixture was refluxed at 80 °C for 8 h in the presence of few drops of concentrated HCl (pH4) over a water condenser. On cooling the reaction mixture, a solid white product was precipitated out. It was filtered, washed with cold EtOH and dried under vacuum over P4O10. Yield 75%, mp 250 °C. Elemental analyses found (Calcd.) for C20H18 N6S2: C, 59.0 (59.11); H, 4.47 (4.43); N, 20.50 (20.68)%. Scheme of synthesis of ligand is given in Fig. 1. 1H NMR spectrum of ligand (L) was recorded in dueterated acetonitrile (CD3CN) and is depicted in Fig. 2. The spectrum exhibits a singlet at ca. d 2.0 ppm (s, 12H, 4CHa3 ) due to four methyl groups. The spectrum exhibits another multiplet in the range ca. d 7.16–7.35 ppm which may be due to the protons of aromatic rings [17]. The mass spectrum of the ligand (Fig. 3) shows the peaks at m/z = 406 and 407. These peaks correspond to the M+ (parental ion) and M+ + 1, 13C isotope. The peak at m/z = 391, 376, 361 and 346 are due to the stepwise removal of the four methyl groups from the macrocyclic ligand. The intensity of the

H3C

Carbon, hydrogen and nitrogen were carried out on a Carbo-Erba EA 1106 analyzer. Metal ion in the complexes were determined by volumetric method using EDTA solution. Molar conductivity was measured on an ELICO (CM 82T) conductivity bridge with a cell having a cell constant of 0.51 cm1. The magnetic moment was determined at room temperature on a Gouy balance using Hg[Co(SCN)4] as a callibrant. The electronic spectra of the complexes were recorded on a ShimazduUVmini-1240 spectrophotometer using DMSO as a solvent. IR spectra were recorded as CsI pellets on a FT-IR spectrum BX-II spectrometer in the region 200–4000 cm1. The 1H NMR spectrum of macrocyclic ligand was recorded at room temperature on a model Bruker Advance DPX-300 spectrometer operating at 300 MHz using CDCl3 as solvent and tetramethyl silane as the internal standard. Electron impact mass spectrum was recorded on JEOL. JMS, DX-303 mass spectrometer. Thermogravimetric analysis (TGA) was carried out in dynamic nitrogen atmosphere (30 ml/min) with a heating rate of 10 C/min using a Schimadzu TGA-50H thermal analyzer. Antifungal screening The synthesized compounds were assayed against the fungi, i.e. Aspergillus niger and Fusarium oxysporum for their fungicidal behavior by employing Poison Food Technique [19,20]. The stock solutions of the compounds were prepared in DMSO solvent. The diluted solution was directly added to the PDA (Potato Dextrose Agar) medium and the mixture was poured into the petri plate. The petri plates were kept for a day to check the sterility. A disk

H3C

CH3

CH3 N

N O

O S H2N

reflux 8 h at 80 oC

S

N

N

N

N

C

few drops of conc. HCl

C

N

C NH2

H3C

Fig. 1. Synthesis and structure of the ligand.

CH3

S

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Fig. 2. 1H NMR spectrum of ligand.

Fig. 3. Mass spectrum of ligand.

of 5 mm of test fungal culture was placed at the center of the petri plate with the help of inoculums needle. The plates were sealed with parafilm and incubated at 29 ± 2 °C for a week. All determinations were performed in duplicate. DMSO and Captan were employed as the control and the standard fungicide, respectively. The fungicidal capacity of the compounds was determined in percentage terms from the growth of the fungus in the test plate to the respective control plate.

Result and discussion Composition and coordination geometry The analytical data of complexes are given in Table 1, which indicate that all the complexes have the general composition [Ln(L)Cl2H2O]Cl, where Ln@Ce(III), N,d(III), Sm(III) and Eu(III). All the complexes are sparingly soluble in common organic solvents such as ethanol and methanol but highly soluble in DMF and

DMSO. Molar conductance values of all the complexes in DMSO (Table 1) fall in the expected range of 1:1 electrolytes.

Mass spectra of the complexes The fragment of the mass spectrum (Fig. 5) of [Ce(L)Cl2H2O]Cl: 672 [(CeC20H20N6S2OCl3 + 2H)+, 52%], 636 [(CeC20H20N6S2OCl2 + 2H)+, 45%], 565 [(CeC20H20N6S2O + 2H)+, 59%], 549[(CeC20H20N6S2 + 2H)+, 40%], 505 [(CeC19H20N6S + 2H)+, 61%], 461[(CeC18H20N6 + 2H)+, 30%], 381[(CeC14H16N4 + 2H)+, 37%], 239[(C14N16N4)+, 44%], 160[(C9H11N3)+, 57%], 140 [(Ce)3+, 67%], 132[(C8H9N2)+, 29%], 93[(C6H5N)+, 34%] 79 [(C5H5N)+, 100%]. The fragment of the mass spectrum of [Nd(L)Cl2H2O]Cl: 678 [(CeC20H20N6S2OCl3 + 4H)+, 55%], 642[(NdC20H20N6S2OCl2 + 4H)+, 39%], 626[(NdC20H20N6S2Cl2 + 4H)+, 25%], 555[(NdC20H20N6S2 + 4H)+, 55%], 511 [(Nd19H20N6S + 4H)+, 32%], 471[(NdC17H18N5S + 4H)+, 23% ], 427[(NdC16H18N5 + 4H)+, 45%], 387[(NdC14H16N4 + 4H)+, 49%], 239[(C14N16N4)+, 52%], 160[(C9H11N3)+, 40%], 144[(Nd)3+, 64%],

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CH3

H 3C

H3 C

N

N N

N S

C

C N

-CH3

S

S

N

N

N

N

C

C

N N

H3 C

S

N

CH3

H3 C

CH3

m/z = 391

m/z = 406

-CH3

H3 C

H 3C

N

N

N S

N

C

C N

-CH3

S

S

N

N

N

N

C

N

C

N

S

N H3 C

m/z = 361

m/z =376

-CH3

N

S

N

N

N

N

C

C

S

N

m/z =346 Fig. 4. The proposed fragmentation path of the ligand.

Table 1 Analytical data of the Ln(III) complexes. Compounds

Color

Yield(%)

m.p. (°C)

leff BM

Molar conductance (X1 cm2 mol1)

Elemental analysis found (calcd.)%

[Ce(L)Cl2H2O]Cl [Nd(L)Cl2H2O]Cl [Sm(L)Cl2H2O]Cl [Eu(L)Cl2H2O]Cl

Green Brown Brown Brown

62 67 60 65

>300 287 >300 295

3.44 3.52 1.45 3.59

116 105 98 112

35.72 35.48 35.23 35.27

106[(C7H9N)+, 25%], 79[(C5H5N)+, 30%], 54[(C3H4N)+, 22%] and 14 [(CH2)+, 53%]. The fragment of the mass spectrum of [Sm(L)Cl2H2O]Cl: 683 [(SmC20H20N6S2OCl3 + 3H)+, 52%], 648[(SmC20H20N6S2OCl2 +3H)+, 45%], 577[(SmC20H20N6S2O + 3H)+, 59%], 561[(SmC20H20N6S2 + 2H)+, 40%], 517[(SmC19H20N6S + 2H)+, 61%], 473[(SmC18H20N6 + 3H)+, 30%], 393[(SmC14H16N4 + 3H)+, 37%], 240[(C14N16N4)+, 44%],

C

H (35.80) (35.60) (35.29) (35.19)

2.92 3.02 2.90 2.98

N (2.98) (2.96) (2.94) (2.93)

12.60 12.55 12.41 12.28

M (12.53) (12.46) (12.35) (12.31)

20.83 21.30 22.00 22.28

(20.89) (21.36) (22.05) (22.20)

161[(C9H11N3)+, 57%], 150 [(Sm)3+, 67%], 133[(C8H9N2)+, 29%] 93[(C6H7N)+, 46% ] 79[(C5H5N)+, 100%]. The fragment of the mass spectrum of [Eu(L)Cl2H2O]Cl: 684[(EuC20H20N6S2OCl3 + 2H)+, 56%], 649[(EuC20H20N6S2OCl2 + 2H)+, 23%], 578[(EuC20H20N6S2O + 2H)+, 562[(EuC20H20N6S2 + 2H)+, 48%], 474[(EuC18H20N6 + 2H)+, 45%], 446[(EuC17H18N5 + 2H)+, 20%], 418[(EuC15N16N4 + 2H+, 30%], 339[(EuC10H11N3) + 2H+, 34%],

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Fig. 6. Electronic spectrum of complex [Sm(L)Cl2H2O]Cl .

Fig. 5. Mass spectrum of complex [Ce(L)Cl2H2O]Cl.

311[(EuC9H9N3)+, 41%] 152[(Eu)3+, 49%], 119[(C7H7N2)+, 20%] and 79[(C5H5N)+, 100%] [21]. IR spectra of the complexes The IR absorption bands, which provide information about the formation of macrocyclic ligand and the mode of coordination in its complexes are given in Table 2. The spectrum displays a strong band at 1631 cm1 corresponding to m(C@N) stretching vibration on complex formation, the m(C@N) band shows the negative shift which indicates that the nitrogen atom of azomethine group is coordinated to metal ion [22,23]. The spectrum also shows the bands at 1480 cm1, 660 cm1 and 425 cm1 due to the pyridine ring deformation, in-plane-ring-deformation and out-of plane-ring deformation, respectively. These absorption bands show the positive shift in complexes which indicates that the nitrogen of pyridine ring is involved in coordination [24]. This mode of coordination of ligand is also supported by appearance of band corresponding to m(MAN) in the range 420–480 cm1 [25]. The chloro complexes show the bands corresponding _(LnA Cl) in the region 315–370 cm1 [26].

Fig. 7. Electronic spectrum of complex [Nd(L)Cl2H2O]Cl.

Table 3 Electronic spectral data (cm1) and related bonding parameters of Ln(III)complexes. Complexes

Bond positions

Assignments

Calculated parameters

[Nd(L)Cl2H2O]Cl

11,173 12,453 13,333 17,182

4

I9/2 ? 4H3/2 I9/2 ? 4F5/2, 4H9/2 4 I9/2 ? 4F7/2 4 I9/2 ? 4G5/2, 2G7/2

b = 0.9972 b1/2 = 0.0374 d% = 0.2807 g = 0.0015

9245 11,135 18,589 23,145

6

b = 0.9849 b1/2 = 0.0614 d% = 1.5331 g = 0.0076

[Sm(L)Cl2H2O]Cl

4

H5/2 ? 6F7/2 H5/2 ? 6F9/2 6 H5/2 ? 4F3/2 6 H5/2 ? 6P5/2 6

Electronic spectra of the complexes The electronic spectra of the complexes were recorded in DMSO solution. Nd(III) and Sm(III) (Figs. 6 and 7) complexes display four electronic spectral bands in the range 9245–1173 cm1, 11, 305–12,453 cm1, 13,333–18,589 cm1 and 17,182–23,145 cm1 [27–29], whereas in other complexes bands are appeared only in the ultra violet region. The assignments of these bands are given in Table 3. The complexes exhibit effective enhancement in the intensity of ‘‘hypersensitive’’ bands from which nephelauxetic effect (b) is calculated by using the following expression:

b¼

n1 1X mi complex n mi aqua

b is the measure of the metal–ligand covalent bonding. The value of b is utilized to estimate the covalency factor (b1/2), Sinha parameter

Table 2 Important infrared spectral (cm1) bands and their assignments. Compounds

m(C@N)

m(MAN)

pr(H2O)

pr(H2O)

m(MACl)

Ligand [Ce(L)Cl2H2O]Cl [Nd(L)Cl2H2O]Cl [Sm(L)Cl2H2O]Cl [Eu(L)Cl2H2O]Cl

1631 1615 1600 1592 1607

– 480 455 420 435

– 660 790 702 649

– 592 528 554 521

– 350 315 360 340

i.e. degree of metal–ligand covalency (d%) and the covalency angular overlap parameter (g). Thermal studies The thermogravimetric (TG) analyses of the complexes were carried in the temperature range 30–60 °C in a static nitrogen atmosphere. All the complexes undergo first step decomposition with weight loss exp. 2.72–2.93% (ca. 2.63–2.68%), between 90 and 135 °C due to the loss of the one coordinated water molecule (Fig. 8). In the second step decomposition of the complexes show weight loss exp. 17.15–27.04% (ca. 15.61–15.89%) in the temperature range 335–400 °C due to the removal of the coordinated and uncoordinated anions. The third step decomposition is due to the loss of the ligand exp. 49.91–56.79% (ca. 59.53–60.59%) in the temperature range 500–580 °C . The final residues was analyzed by IR spectra and identified as metal oxide which is corresponding to the calculated value. On the basis of above spectral studies the following structure may be suggested for the complexes (Fig. 9). Mode of action of fungus on compounds Metal ions are adsorbed on the cell walls of the microorganisms, disturbing the respiration processes of the cells and thus

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S. Chandra, S. Agrawal / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 564–570 Table 4 Antifungal screening data of the ligand and its Ln(III) complexes. Compounds

Fungal inhibition (%) (conc. in lgml1) Aspergillus niger

Ligand [Ce(L)Cl2H2O]Cl [Nd(L)Cl2H2O]Cl [Sm(L)Cl2H2O]Cl [Eu(L)Cl2H2O]Cl Bavistin DMSO

Fig. 8. Mass spectrum of complex [Eu(L)Cl2H2O]Cl.

+

CH3

H3 C N N

N S

Cl

C

Cl

Ln

N

H2O

C

S

Cl

N

N H3 C

CH3

Fig. 9. Suggested structure of the complexes [Ln = Ce3+, Nd3+, Sm3+ and Eu3+].

blocking the protein synthesis that is required for further growth of the organisms. Hence, metal ions are essential for the growth-inhibitory effects [30]. According to Overtone’s concept of cell permeability, the lipid membrane that surrounds the cell favors the passage of only lipid-soluble materials, so lipophilicity is an important factor controlling the antifungal activity. Upon chelation, the polarity of the metal ion will be reduced due to the

Fusarium oxysporum

500

750

1000

500

750

1000

– 30 45 41 34 0 0

45 52 70 67 56 52 0

62 76 95 89 81 100 0

– 42 33 48 38 0 0

40 61 56 65 50 100 0

58 78 73 89 69 100 0

overlap of the ligand orbitals and partial sharing of the positive charge of the metal ion with donor groups. In addition, chelation allows for the delocalization of p-electrons over the entire chelate ring and enhances the lipophilicity of the complexes. This increased lipophilicity facilitates the penetration of the complexes into lipid membranes, further restricting proliferation of the microorganisms. The variation in the effectiveness of different compounds against different organisms depends either on the impermeability of the microbial cells or on differences in the ribosomes of the cells [31]. All of the metal complexes possess higher antifungal activity than the ligand [32]. Although the exact biochemical mechanism is not completely understood the mode of action of antimicrobials may involve various targets in the microorganisms. These targets include the following. (i) The higher activity of the metal complexes may be due to the different properties of the metal ions upon chelation. The polarity of the metal ions will be reduced due to the overlap of the ligand orbitals and partial sharing of the positive charge of the metal ion with donor groups. Thus, chelation enhances the penetration of the complexes into lipid membranes and the blockage of metal binding sites in the enzymes of the microorganisms [33]. (ii) Tweedy’s chelation theory predicts that chelation reduces the polarity of the metal atom mainly because of partial sharing of its positive charge with donor groups and possible electron delocalization over the entire ring. This consequently increases the lipophilic character of the chelates, favoring their permeation through the lipid layers of the bacterial membrane [34]. (iii) Formation of a hydrogen bond through the azomethine group with the active centers of various cellular constituents, resulting in interference with normal cellular processes [35].

Fig. 10. Growth inhibition of synthesized compounds against Aspergillus niger.

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Fig. 11. Growth inhibition of synthesized compounds against Fusarium oxysporum.

In vitro antifungal effects of the investigated compounds were tested against two fungal species (Aspergillus niger and Fusarium oxysporum). The results showed that the ligand itself does not exhibit any antifungal activity, but all metal–ligand complexes exhibit good activities (Table 4). The [Nd(L)Cl2H2O]Cl shows more activity against Asperigillus niger while [Sm(L)Cl2H2O]Cl found to be more active against Fusarium oxysporum than other Lanthanoid. The experimental findings reveal that the antifungal capacity of the compounds increases [36–39] with concentration (Figs. 10 and 11). Conclusion In this paper, we describe the synthesis and characterization of a hexadentate macrocyclic ligand and its Ce3+, Nd3+, Sm3+ and Eu3+ complexes. On the basis of absorption spectra we calculate the different parameters i.e. nephelauxetic ratio (b), covalency factor(b1/2), Sinha parameter (d%) and covalency angular overlap parameter(g) for Nd(III) and Sm(III). The values of these parameters suggest the metal–ligand covalent bonding. All the complexes were found to be paramagnetic in nature. The thermal studies reveal the presence of coordinated water molecule. The administration of the compound as the metal ion derivative exhibits the moderate antifungal behavior. This accounts that the efficacy of the organic compound is positively modified on association with metal ion. Acknowledgement Authors are thankful to UGC, New Delhi for financial assistance. References [1] R.V. Singh, P. Chaudhary, S. Chauhan, M. Swami, Spectrochim. Acta A 72 (2009) 260. [2] D.P. Singh, R. Kumar, V. Malik, P. Tyagi, Trans Met. Chem. 32 (2007) 1051. [3] H. Kaur, K. Dhir, B. Mittu, N. Preet, K. Dhanjals, Int. J. Fundam. Appl. Sci. 1 (2012) 30. [4] Q. Wang, K.Z. Tang, W.S. Liu, Y. Tang, M.Y. Tan, J. Solid State Chem. 182 (2009) 3118. [5] J. Yao, W. Dou, W. Liu, J. Zheng, Inorg. Chem. Commun. 12 (2009) 430.

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