International Journal of Inorganic Chemistry

Volume 2014 (2014), Article ID 902575, 10 pages

http://dx.doi.org/10.1155/2014/902575

Research Article

Complexing Behaviour and Antifungal Activity of N-[(1E)-1-(1H-Benzimidazol-2-yl)ethylidene]morpholine-4-carbothiohydrazide and Related Ligand with Metal Ions

Madhu Bala1,2 and L. K. Mishra³

¹Department of Chemistry, N.I.T, Patna, Bihar 800005, India

²D/O Bansropan Sharma, Moti Chowk Saidpura, Thana Road Khagaul, Patna, Bihar 801105, India

³Department of Chemistry, Patna University, Patna, Bihar 800005, India

Received 6 September 2013; Revised 11 December 2013; Accepted 17 December 2013; Published 22 April 2014

Academic Editor: Alfonso Castiñeiras

Copyright © 2014 Madhu Bala and L. K. Mishra. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The coordination complexes of bivalent metal ions with N-[(1E)-1-(1H-Benzimidazol-2-yl)ethylidene]morpholine-4-carbothiohydrazide (H₂bmctz, H₂L-1) and N-[(1E)-1-(1H-Benzimidazol-2-yl)(phenyl)methylidene] morpholine-4-carbothiohydrazide (H₂bpmctz, H₂L-2) were prepared and their neutral, monoanionic, and dianionic forms of ligands of compositions [M(H₂L)X₂] (M=, , , , , or , X=Cl⁻ or Br⁻, and H₂L=H₂L-1 or H₂L-2), [M(HL)₂]nH₂O where (M=, , , , or , H₂L=H₂L-1 or H₂L-2, and n = 0 or 2), and [MLB]nB (M=, , , , or , B=H₂O, Py, or -pic, n = 0 and n = 1 if B=H₂O for Ni(II) and H₂L=H₂L-1 or H₂L-2) have been characterised by magnetic susceptibility measurements, electrical conductance values, and spectral properties. The magnetic moment value of [M(HL₂)] (M=, , or ) type complexes is consistent with high spin octahedral structure while those of [M(H₂L)X₂] (M=, , , , or , X=Cl⁻ or Br⁻) possess five coordinated trigonal bipyramidal geometry. The adduct complexes [MLB]·nB (M=, or , B=H₂O, Py, or -pic) are four coordinated planar and those of and complexes [MLB], (H₂L=H₂L-1 or H₂L-2, B=H₂O, Py or -pic) are tetrahedral. These ligands have been suggested to coordinate as tridentate (N N S) donor molecule in complexes of type [M(H₂L)X₂], [M(HL)₂], and [MLB]. The antifungal activity of ligands and some of their metal complexes were studied and it was observed that metal complexes show higher activity than free ligand.

1. Introduction

Preparation and structural aspects of various metal ions with a number of benzimidazole derivatives have been reported by one of us and a number of chemists [1–10]. Medicinal properties of benzimidazole derivatives have established that benzimidazoles have privileged substructures for drug design [11, 12]. The most vital benzimidazole derivatives is N-ribosyl-5,6-dimethylbenzimidazole which is axial ligand in vitamin B₁₂ possessing selective neuropeptide YY1 receptor antagonists [13], 5-lipo xyginase inhibitor [14] and poly (ADP-ribose) polymerase inhibitors [14]. Benzimidazole derivatives are of immense interest because of their wide spectrum of biological activity such as anticancer [15], antiviral [16, 17], antihistaminic [18], antifungal [19], anti-HBV [20], antibacterial [21], antitumoral [22], antiparasitic [22], antihelminitic [23], anti-inflammatory, local analgesic, hypotensive [24], antiulcer [25], and neuro leptic cardiotonic [26]. Extensive biochemical and pharmacological studies have confirmed that various benzimidazole derivatives are effective against various strains of microorganisms [25]. The wide ranging biological and industrial interest intrigued chemists to explore new benzimidazole derivatives and their metal complexes which might have novel mechanisms of action and drug activity [15–25]. The metal ions selected for present studies Mn(II), Co(II), Cu(II), Fe(II), Ni(II), Zn(II), and Cd(II) are versatile essential micronutritents in biological systems [27–32]. Copper is crucial for biological function of several enzymes and proteins, in energy metabolism, antioxidant, and mitochondrial respiration [27]. Zinc(II) is essential for nucleic acid synthesis, cell growth division, and differentiation and its deficiency causes abnormalities in function of cell carboxypeptidase, amino peptidase, carbonic anhydrase, DNA and RNA polymerase. Zinc is also essential for life and it regulates the function of genes in the nuclei of thecell (Lipsocomb and Starter, 1996). Iron is the most vital element for life and iron-sulphur proteins formation (Richardson, 2002). Manganese is a component of nucleic acid and it accelerates the synthesis of cholesterol. Manganese enzymes are involved in metabolic pathway of DNA synthesis, sugar metabolism and protein modification. Cobalt is a component of vitamin B₁₂ and enzyme nutritional cofactor necessary for the formation of red blood cell [31]. Nickel is remarkably useful metal in biological chemistry. It modifies antioxidant system and is associated with chromosomal aberrations and mutation [31]. In continuation of our work on coordinating behaviour of benzimidazole and imidazole derivatives with various metal ions [2–6], and biological significance of above metal ions, we report in present communication, the preparation, characterization, and antifungal activity of complexes of these metals with of N-[(1E)-1-(1H-benzimidazol-2-yl)ethylidene]morpholine-4-carbothiohydrazide (H₂L-1) and N-[(1E)-1-(1H-benzimidazol-2-yl)(phenyl)methylidene]morpholine-4-carbothiohydrazide (H₂L-2).

2. Material and Methods

The metal salts used were usually BDH Anal-R chemicals or E. Merk Extra pure reagent. Palladium(II) chloride was obtained from Johnson Matthey London. Solvents and organic reagent, hydrazine, morpholine, carbon disulphide, Chloroacetic acid were obtained from E. Merk, Nice, Sd. Fine and Ranbaxy. The metal contents of complexes were estimated by standard method sulphur content was determined as sulphate, and nitrogen content was estimated by Nitrometer (Duma,s method) in the laboratory. The infrared spectra of complexes were recorded on Shimadzu-FTIR spectrometer at IIT patna. The results of C, H, and N were obtained from CDRI Lucknow. The results of mass spectrum and U-V absorption spectrum of ligand and some complexes were obtained from IIT patna. The ¹HNMR spectrums were recorded at CDRI Lucknow. The magnetic susceptibilities of complexes were determined by Gouy method at room temperature. The electrical conductance values of complexes were determined in DMF on Systonic conductivity Meter Bridge at 30–31°C. The results of measurements are given in Tables 1 and 2.

Table 1: Analytical results and physical data of complexes.

Compound
Colour
% analysis found (Calc)
 B.M (305°K)
 mol⁻¹ ohm⁻¹ cm², 30°C

M
N
Halogen

Cu(H₂L-1)Cl₂
Green ash
14.31 (14.52)
15.73 (16.00)
16.01 (16.25)
1.81
12

Cu(H₂L-1)Br₂
Brown
11.91 (12.07)
13.31 (13.48)
30.11 (30.44)
1.84
8

Zn(H₂L-1)Cl₂
White
14.61 (14.88)
15.91 (16.18)
15.01 (15.12)
Dia
10

Zn(H₂L-1)Br₂
White
12.31 (12.52)
13.01 (13.25)
30.11 (30.25)
Dia
8

Cd(H₂L-1)Cl₂
Cream
22.86 (23.13)
14.11 (14.29)
14.36 (14.53)
Dia
11

Cd(H₂L-1)Br₂
Cream
19.46 (19.54)
12.11 (12.32)
27.38 (27.67)
Dia
10

Ni(H₂L-1)Cl₂
Greenish yellow
13.32 (13.54)
16.31 (16.48)
16.57 (16.70)
3.18
10

Co(H₂L-1)Cl₂
Brown
13.41 (13.58)
16.28 (16.49)
16.51 (16.71)
3.25
11

Ni(H₂L-1)Br₂
Bottle green
11.10 (11.21)
13.50 (13.61)
30.17 (30.64)
3.21
8

Cu(H₂L-2)Cl₂
Ash green
12.53 (12.71)
14.21 (14.02)
13.87 (14.19)
1.81
11

Cu(H₂L-2)Br₂
Brown
10.61 (10.79)
11.63 (11.81)
26.93 (27.13)
1.83
8

Cd(H₂L-2)Cl₂
Cream
20.49 (21.67)
12.76 (13.54)
12.69 (12.93)
Dia
11

Zn(H₂L-2)Cl₂
Cream
13.31 (13.45)
13.21 (13.96)
14.00 (14.15)
Dia
10

Cd(H₂L-2)Br₂
Cream
17.38 (17.64)
10.71 (10.98)
23.01 (23.30)
Dia
9

Zn(H₂L-2)Br₂
Cream
10.91 (11.08)
11.61 (11.84)
26.81 (27.06)
Dia
11

Ni(H₂L-2)Cl₂
Greenish yellow
11.61 (11.86)
13.96 (14.15)
14.16 (14.35)
3.20
12

Co(H₂L-2)Cl₂
Brown
11.68 (11.89)
13.89 (14.16)
14.23 (14.36)
3.32
13

Ni(H₂L-2)Br₂
Bottle green
9.92 (10.05)
11.71 (11.99)
27.01 (27.38)
3.18
14

(H₂L-1)=H₂bmctz and (H₂L-2)=H₂bpmctz.

Table 2: Analytical results and physical data of complexes.

Compound H₂L=H₂bmctz H₂L′ H₂bpmctz
Colour
% analysis found/(Calc)
 B.M 305°K
 mol⁻¹ Ohm⁻¹ cm², 30°C

M
N
Sulphur

Co(HL-1)₂2H₂O
Brown
8.61 (8.88)
21.01 (21.12)
9.50 (9.65)
3.26
8

Cu(HL-1)₂
Ash colour
9.31 (9.52)
20.87 (21.09)
9.31 (9.59)
1.86
7

Mn(HL-1)₂
Dull cream
8.14 (8.33)
21.22 (21.24)
9.41 (9.78)
5.88
8

Fe(HL-1)₂
Light brown
8.31 (8.47)
21.01 (21.20)
9.48 (9.64)
4.91
6

Cu(HL-2)₂
Ash green
7.91 (8.01)
17.61 (17.79)
8.00 (8.08)
1.88
8

Mn(HL-2)₂
Dull cream
6.88 (7.01)
17.61 (17.88)
9.11 (9.26)
5.92
8

Fe(HL-2)₂
Light brown
7.01 (7.12)
17.15 (17.86)
9.11 (9.24)
4.86
6

Co(HL-2)₂2H₂O
Brown
7.02 (7.16)
17.00 (17.01)
7.61 (7.77)
3.68
10

Ni(HL-1)₂2H₂O
Reddish brown
8.37 (8.40)
19.81 (20.02)
9.00 (9.22)
3.13
8

Ni(HL-2)₂2H₂O
Reddish brown
7.10 (7.13)
16.81 (17.01)
7.57 (7.78)
2.98
7

Zn(HL-1)₂
Yellow
9.82 (9.76)
20.73 (20.90)
9.09 (9.10)
Dia
10

Zn(HL-2)₂
Yellow
8.01 (8.29)
17.51 (17.65)
8.01 (8.06)
Dia
8

Pd(HL-1)₂
Reddish brown
14.61 (14.98)
19.56 (19.71)
8.90 (9.01)
Dia
9

Pd(HL-2)₂
Reddish brown
12.61 (12.75)
16.66 (16.87)
7.51 (7.67)
Dia
7

Pd(L-1)Py
Orange
21.63 (21.75)
17.01 (17.27)
6.51 (6.58)
Dia
6

Pd(L-2)Py
Orange
19.31 (19.40)
15.01 (15.32)
5.69 (5.83)
Dia
8

Pd(L-1)(Y-pic)
Orange
21.11 (21.27)
16.68 (16.86)
6.30 (6.39)
Dia
8

Pd(L-2)(Y-pic)
Orange
18.68 (18.98)
14.91 (15.01)
5.68 (5.73)
Dia
7

Ni(L-1)Py
Yellowish red
13.31 (13.38)
19.11 (19.21)
7.10 (7.29)
Dia
8

Ni(L-1)(Y-pic)
Yellowish red
12.78 (12.99)
18.48 (18.59)
6.87 (7.08)
Dia
7

Ni(L-1)(H₂O)H₂O
Yellowish orange
14.63 (14.83)
17.51 (17.69)
8.11 (8.06)
Dia
9

Ni(L-2)Py
Orange yellow
11.43 (11.72)
16.71 (16.77)
6.11 (6.39)
Dia
10

Ni(L-2)(Y-pic)
Orange yellow
11.14 (11.42)
16.21 (16.32)
6.01 (6.23)
Dia
8

Ni(L-2)(H₂O)H₂O
Orange yellow
12.78 (12.82)
15.11 (15.29)
6.81 (6.99)
Dia
7

Cd(L-1)(Y-pic)
Orange yellow
22.01 (22.22)
16.51 (16.62)
6.11 (6.33)
Dia
10

Cd(L-1)(H₂O)
Orange yellow
26.15 (26.05)
16.02 (16.23)
7.31 (7.42)
Dia
8

Cd(L-1)Py
Orange yellow
22.61 (22.81)
17.34 (17.51)
6.41 (6.49)
Dia
9

Cd(L-2)Py
Orange yellow
20.11 (20.21)
15.01 (15.15)
5.70 (5.77)
Dia
11

Cd(L-2)(Y-pic)
Orange yellow
19.61 (19.88)
14.92 (14.88)
5.41 (5.64)
Dia
10

Cd(L-2)(H₂O)
Yellow
22.48 (22.78)
14.21 (14.19)
6.51 (6.48)
Dia
9

Zn(L-2)Py
Orange yellow
14.51 (14.62)
18.76 (18.86)
7.00 (7.18)
Dia
10

Zn(L-1)(H₂O)
Yellow
17.14 (17.01)
18.41 (18.23)
8.10 (8.32)
Dia
8

Zn(L-2)Py
Yellow
12.81 (12.88)
16.47 (16.55)
6.03 (6.36)
Dia
6

Zn(L-2)(Y-pic)
Orange yellow
12.41 (12.54)
15.91 (16.11)
5.91 (6.14)
Dia
9

Zn(L-2)(H₂O)
Yellow
14.61 (14.72)
15.58 (15.78)
6.91 (7.17)
Dia
8

Zn(L-1)(Y-pic)
Yellow
14.21 (14.23)
18.41 (18.59)
6.91 (7.08)
Dia
11

Cu(L-1)Py
Bottle green
14.01 (14.32)
18.32 (18.54)
7.03 (7.21)
1.86
8

Cu(L-2)Py
Bottle green
13.81 (13.95)
18.71 (18.93)
7.11 (7.21)
1.84
9

Cu(L-1)(Y-pic)
Ash colour
13.61 (13.88)
18.17 (18.36)
6.81 (6.99)
1.89
6

Cu(L-2)(Y-pic)
Ash colour
12.31 (12.25)
16.01 (16.20)
6.01 (6.16)
1.84
5

Cu(L-2)(H₂O)
Brown
13.61 (13.73)
15.00 (15.14)
6.71 (6.91)
1.79
8

Cu(L-1)(H₂O)
Brown
15.48 (15.74)
17.13 (17.35)
7.76 (7.93)
1.78
10

Ligand molecules (H₂bmctz, H₂L-1 or H₂bpmctz, H₂L-2) were prepared by condensing 2-acetylbenzimidazole or 2-benzoylbenzimidazole [33] with morpholine-N-thiohydrazide [34] in aqueous ethanol containing a little amount of acetic acid. 2-Acetylbenzimidazole or 2- benzoylbenzimidazole in situ was prepared by oxidation of 1-(1H-benzimidazol-2-yl)ethanol and 1-(1H-benzimidazol-2-yl)phenylmethanol [35] with chromic acid [33]. The 0.1 mole of 2-acetylbenzimidazole in 100 mL aqueous ethanol containing 2 mL acetic acetic acid was refluxed with 0.12 mole of aqueous ethanolic solution of morpholine-N-thiohydrazide(morpholine-4-carbothiohydrazide) on a steam bath for two hours. The ligand separated as cream of yellow product. The ligands were analysed. For H₂bmctz (H₂L-1) (C₁₄H₁₇N₅SO) nitrogen was found 23.06% calculated value is 23.11%. M.pt recorded, 197°C (uncorrected). The value of nitrogen found for H₂bpmctz (H₂L-2) is 19.01%, and that calculated for H₂L-2 (C₁₉H₁₉N₅SO) is 19.18%, M.pt recorded 242°C (uncorrected).

2.1. Preparation of Complexes [M(H₂L)X₂] (M=CuII, CoII, NiII, ZnII or CdII X=Cl⁻ or Br⁻ and H₂L=L-1 or L-2)

About 0.01 mole of metal halide was dissolved in 40–50 mL ethanol and treated with 0.01 mole of appropriate ligand in dry ethanol (30–40 mL) slowly with constant stirring. The resulting solution was refluxed on a steam bath for half an hour and concentrated to 20–30 mL. The concentrated solution was chilled when crystalline precipitate separated. In case of Cu (II) the product were obtained immediately. The product was filtered, washed with cold dry ethanol, and dried over CaCl₂. Yield 80–85%.

2.2. Preparation of Bis Ligated Complexes [M(HL)₂], nH₂O (M=MnII, FeII, CoII, NiII, CuII, ZnII, CdII or PdII, H₂L=H₂L-1 or H₂L-2 and but 2 for NiII or CoII)

About 20 millimoles of ligand was dissolved in 80–90 mL hot methanol and treated slowly with aqueous methanolic solution of 10 millimoles of appropriate metal acetate {chloride in case of Pd(II) and sulphate in case of Fe(II)} and refluxed on steam bath for half an hour. In some cases a little aqueous solution of sodium acetate was added to adjust the pH 6–8. In most of the cases complexes were obtained immediately on diluting them with water. The products were digested on a steam bath and collected on a filter, washed with aqueous methanol, and dried over CaCl₂. Yield is 95–98%.

2.3. Preparation of [MLB]·nB (M=CuII, PdII, NiII, ZnII or CdII B=H₂O, py or  -pic and , H₂L=H₂L-1 or H₂L-2, if B=H₂O, for NiII and H₂L-1 or H₂L-2)

About 5 or 10 millimole of metal chloride was dissolved in 30–40 mL aqueous methanol and treated with 5-6 mL pyridine or -pic. The resulting solution was treated with 5 or 10 millimole of ligand dissolved in hot methanol (20–25 mL).The pH of the solution was raised by adding dilute ammonia when appropriate complexes separated. The aqua complexes were obtained on refluxing [MLPy] in hot water in presence of a little ammonia. The products were collected on a filter, washed with cold aqueous methanol, and dried over CaCl₂. The results of elemental analysis and physical data of complexes are presented in Tables 1 and 2.

3. Results and Discussion

The ligands N-[(1E)-1-(1H-benzimidazol-2-yl)ethylidene]morpholine-4-carbothiohydrazide (H₂L-1) and N-[(1E)-1-(1H-benzimidazol-2-yl)phenyl(methylidene)]morpholine-4-carbothiohydrazide (H₂L-2) are potential N N S donor coordinating molecule capable of forming five membered strong chelates with The elemental analysis of complexes of , , , , and halide formed in dry ethanol corresponds to compositions [M(H₂L)X₂ ], and (H₂L=H₂L-1 or H₂L-2, X=Cl or Br) metal ions. Depending on pH of medium, these ligands coordinate as neutral, monoanionic, and dianionic donor molecules. The elemental analysis of complexes of , , , , , , , and corresponds to compositions [M(H₂L)X₂] (=, , or and X= or ) [M(HL)₂]H₂O (=, , , , , , or , H₂L=H₂L-1 or H₂L-2, but 2 for and ) and [MLB]B (=, , , or and = or H₂L-2, B=Py, -pic or H₂O and or 1). These complexes are quite stable at room temperature and do not lose texture in desiccator. On heating the complexes [M(HL)₂·H₂O (=, or , and HL=HL-1 or HL-2) in air oven between 60 and 80°C for one hour, is loss H₂O and loss in weight incurred in complexes correspond to loss of required for 2H₂O, and no change in colour of the complexes that was noticed which indicated that H₂O molecules are not coordinated to metal atom rather they are held up in crystal lattices. The weight loss incurred in complexes Ni(L-1)2H₂O and Ni(L-2)2H₂O at 120°C corresponds to loss of only one H₂O, indicating that one H₂O is coordinated to nickel (II) in complex [NiL(H₂O)]H₂O and the second H₂O is held up in crystal lattices. The dihalo complexes are formed in dry ethanol by interacting solutions of ligand with metal halides. The bis ligated complexes [M(HL)₂]H₂O were formed in neutral or weakly basic medium (pH 6–8) by interacting aqueous methanolic or ethanolic solutions of metal acetate or metal chloride and appropriate ligand in 1 : 2 molar proportion. The ligands coordinate as dianionic tridentate molecule in much basic medium (pH 9–12) and form complexes of stoichiometry, [MLB]·nB (=, , , or and H₂L=H₂L-1 or H₂L-2, B=H₂O, py or -pic and or 1). The complexes are all insoluble in water but aqueous ethanolic suspension of [M(H₂L)X₂] gradually dissociates into M(HL)₂ and MX₂ giving conducting solution. Freshly prepared DMF solution of complexes, however shows negligible electric conductance value indicating them to be nonionic and even halogens are coordinated to metal atom [36]. Zinc(II), cadmium(II), and nickel(II) complexes [NiLB]·nB are diamagnetic. At room temperature all copper(II) complexes show normal magnetic moment value (1.81–1.89 B.M) and magnetic moment value of Mn(II) complexes [Mn(HL)₂] (5.88–5.92 B.M), Ni(II) complexes [Ni(HL)₂] (2.93–3.01 B.M), and Fe(II) complexes Fe(HL)₂ (4.89–4.91 B.M) corresponds to spin free octahedral geometry of metal ions [37]. The moment value of cobalt(II) complexes [Co(HL)₂] (3.26–3.68 B.M) is lower than spin free octahedral geometry and anomalous probably due to spin-exchange interactions [38]. The dihalo complexes [Co(HL)X₂] show magnetic moment value (4.54–4.71 B.M) similar to five coordinated TBP structure [37, 38]. As expected zinc(II) or Cd(II) complexes M(HL)₂ and [M(H₂L)X₂] (X=  or ) are diamagnetic. The electronic absorption spectra of ligand (H₂L-1 or H₂L-2) and their complexes were determined in ethanol or ethanol DMF (50%) mixed solvent in the range 200–850 nm. The ligand H₂L-1 displays strong bands at 214, 252, and 268 and a medium band at 295 nm attributed from , , , and transitions of ligand molecule [39]. The electronic absorption bands of H₂L-2 are observed as strong band at 212, 245, 270, and 290 nm attributed from , , , and (C=S) of ligand. In complexes these transition are mostly enveloped by strong charge transfer from and electronic transitions. The metal complexes form very deep coloured solutions in DMF-ethanol mixture and distinct d–d electronic transitions could not be located below 380–390 nm. The Zn(II) and Cd(II) complexes as expected do not have d–d electronic transition but due to charge transfer band show strong absorption near 420–400 nm. The medium band at 460–480 nm for [NiLB]·nB (H₂L=H₂L-1 or H₂L-2, B=py or -pic) is attributed to transition in planar field [39]. In DMF-ethanol mixture the complexes [Ni(HL)₂] as well as [Ni(H₂L)Cl₂] have same colour and similar electronic absorption band near 460–480 nm. The complexes probably acquire planar structure in basic DMF medium. The cobalt (II) complexes display weak electronic absorption near 430–440 nm and 560–580 nm assignable to and , transitions in octahedral field [39]. The Cu(II) complexes [Cu(HL)₂] show strong absorption below 420 assignable to charge transfer transition. The complexes display d–d transitions at 450 nm (medium) and 650–680 nm as broad weak band assignable to , , and transition in distorted octahedral field [39]. In case of Mn(II) no distinct d–d transitions could be located definitely, since all d–d transitions of Mn(II) complexes are of spin forbidden type. Iron (II) complexes display a shoulder near 440 and 450 nm attributable to transitions in octahedral field [37–39] (See Figures 1 and 2).

Figure 1: Figure of thione where R=CH₃=H₂bmctz and R=C₆H₅=H₂bpmctz.

Figure 2: Figure of thiol where R=CH₃=H₂bmctz and R=C₆H₅=H₂bpmctz.

The IR spectra of ligands and their complexes were recorded as KBr optics in range 4000–400 cm⁻¹. The diagnostic I.R. bands of ligands and some of their complexes are recorded in Table 3. The ligand H₂L-1 displays medium I.R. bands at 3350, 3240, 3120, 3040, 2949, 2860, and 2840 cm⁻¹ attributed from ν(NH), phenyl (C–H) stretches, and morpholine ring ν(CH₂) vibrations and most of them are retained in complexes. The ν(C=N), ν(N–N) of imidazole ring and azomethene part of ligand were observed at 1653, 1610 and 1585 cm⁻¹. The  (NH) of ligand was located at 1515 and 1485 cm⁻¹. The IR spectrum of Cu(H₂L-1)Cl₂ displays azomethene ν(C=N) at 1622 cm⁻¹ and imidazole ring ν(C=N) at 1590 which is lower than free ligand molecule that indicated the coordination of both (C=N) nitrogen to metal atom. The thione ν(C=S) of ligand shifted from 912 cm⁻¹ to 892 cm⁻¹ in complex [Cu(H₂L-1)Cl₂] indicated the involvement of thiol sulphur in coordination. Thus the ligand is bonded as tridentate N, N, S donor molecule in complexes. The (NH) at 1515 and ν(N–H) at 3240 cm⁻¹ of ligand were absent in complexes with dianionic ligand [M(L-1)B]. The complexes containing pyridine or -picoline molecule display a prominent band at 1590–1595 and 1015–1025 cm⁻¹ due to ν(C=N) of coordinated pyridine ring and pyridine ring breathing mode of vibration [40]. The imidazole part ν(C=N) and azomethene part ν(C=N) stretches of ligand shift to lower frequency in complexes by 10–15 cm⁻¹ that suggested the coordination of both (CN) nitrogen. The thioamide ν(C=S) of ligand was observed at 912 cm⁻¹, which is shifted by 20–25 cm⁻¹ in complexes of type [M(H₂L-1)X₂] indicating that (C=S) is bonded through thione sulphur. The ν(C=S) of ligand shifts to 760–780 cm⁻¹ in complexes [M(HL-1)₂] and [M(L-1)B] type of complexes indicating the coordination of thioamide (C=S) through deprotonated thiol sulphur. The ligand H₂L-2 shows ν(N–H), ν(C–H) phenyl ring and (CH₂) ring vibration morpholine at 3238, 3190, 3060, 2950, and 2832 cm⁻¹. These are retained with some change in intensity and band position in complexes. The complexes [M(HL)₂]2H₂O,(HL=HL-2 or HL-1) show broad IR band near 3180–3450 cm⁻¹ attributed to hydrogen bonded ν(OH) of H2O. The ν(C=N) of azomethene part and benzimidazole located at 1628, 1605, and 1585 of (H₂L-1) shift to lower frequencies that indicated their bonding to metal atom. The thioamide ν(C=S) of H₂L-2 located at 938 cm⁻¹ shifted by 20–30 cm⁻¹ in complexes of types [M(H₂L-2)X₂] indicating bonding of thione sulphur (Table 3). The ν(C=S) shifts to 770–790 cm⁻¹ in complexes of types [M(HL-2)₂] and [M(L-2)B] supporting bonding through deprotonated thiol sulphur of thioamide group [40, 41]. The I.R. spectral band of both H₂L-1 and H₂L-2 shows similar trend of bonding of donor atoms. Infar I.R. range, the I.R. spectra of complexes were recorded above 400 cm⁻¹, therefore ν(M–S) and ν(M–N) stretches could not be located definitely. However, the new I.R. bands in complexes at 420–435 cm⁻¹ are tentatively suggested to ν(M–N) and ν(M–S) vibrations (Table 3). In Finger print region H₂L-1, H₂L-2, and their complexes display a number of I.R. bands due to phenyl ring skeletal vibrations and (CH₂), (CH₃), ν(N–N), ν(C–N), and ν(C–C) vibrations as well as ring deformation vibrations [40, 41]. In general, almost all complexes display very strong band near 735–745 cm⁻¹ attributed to phenyl ring out of plane bending of substituted phenyl ring. The Finger print region, I.R. stretches of H₂L-1 are located at 1545, 1515, 1480, 1441, 1363, 1271, 1235, 1155, 1110, 1004, and 744 cm⁻¹. These are retained in complexes with slight change in intensity and slight change in position [40–42]. The I.R. bands of H₂L-2 were observed at 1540, 1505, 1482, 1442, 1336, 1280, 1240, 1150, 1105, 1030, 845 and 742 cm⁻¹. From the I.R. studies, it is concluded that the ligands are tridentate and dianionic in [MLB] type of complexes.

Table 3: Prominent I.R. bands of ligands and their complexes in cm⁻¹.

Compound
(NH), (OH), (CH₂), +(C–H)
(C=N)imidazole, (C=N)azomethene
Ring breathing mode
(C=S)
(M–N)

(M–S)

H₂L-1
3350, 3240, 3120, 3040, 2949, 2860
1653, 1610, 1585
1040
912
435, 415

H₂L-2
3238, 3190, 3060, 2950, 2832
1628, 1605, 1585
1035
938
431, 418

Cu(H₂L-1)Cl₂
3280, 3235, 3125, 3040, 2950, 2850
1622, 1590, 1580
1055
892
427

Zn(H₂L-1)Cl₂
3320, 3235, 3140, 3035, 2960, 2845
1615, 1595, 1580
1024
896
429

Cu(H₂L-2)Cl₂
3340, 3230, 3145, 3045, 2960, 2840
1621, 1610, 1590
1025
908
436, 413

Ni(H₂L-2)Cl₂
3340, 3240, 3140, 3045, 2950, 2840
1618, 1595, 1584
1040
912
428, 410

Co(HL-1)₂2H₂O
3320, 3245, 3140, 3050, 2960, 2840
1618, 1602, 1595
1035
768
441, 418

Zn(HL-2)₂
3340–3140, 3050, 2960, 2850
1622, 1605, 1592
1030
772
438, 416

Ni(HL-2)₂2H₂O
3380–3100br, 2960, 2840
1615, 1600, 1595
1040
782
435, 418

Cd(HL-2)₂
3320, 3240, 3130, 2960, 2850
1612, 1595, 1580
1038
788
436, 417

Co(HL-2)₂2H₂O
3400–3120br, 2960, 2840
1618, 1605, 1585
1028
792
438, 422

Ni(L-1)Py
3320–3180, 3040, 2980, 2845
1612, 1595, 1575
1024
760
443, 418

Cu(L-2)Y-pic
3340, 3260, 3140, 3050, 2840
1620, 1590, 1580
1025
772
434, 415

Cu(L-2)2H₂O
3350–3060, 2950, 2840
1620, 1600, 1580
1025
781
438, 410

Zn(L-1)Py
3320, 3270, 3130, 3040, 2955, 2845
1615, 1600 1590
1030
776
436, 413

¹H NMR spectra of ligands H₂L-1 and H₂L-2 were recorded in DMSO at field strength 300 MHz. The ligand H2L-1 shows (CH₂) proton. Signals of morpholine ring as two triplets for 8H at = 4.016, 4.030, 4.047and 4.154, 4.170, 4.184 ppm, the CH₃, proton signals at = 1.787 ppm (Singlet), the phenyl ring signal at =  ppm as multiplet (4H). The benzimidazole ring N–H and hydrazide part NH signals are located at = 7.262 and = 8.6 ppm. The ligand H₂L-2 displays (CH₂) proton signal of morpholine ring at 3.781–4.145 ppm as two triplets (8H, t) and phenyl ring (C–H) proton signal at = as multiplet (9H). The benzimidazole ring (NH) and hydrazide (N-H) proton signals were located at = 7.415 and 8.215 ppm. The mass spectrum of H₂L-1 gave at 303(3%), 288(3.8%), 217(40%), 117(100%), and 86(30%). The calculated value of mass was 303. The mass spectral results of H₂L-2 indicated values 365(5%), 288(3%), 279(8%), 86(30%), and 117(100%) as base peak. The mass calculated for H₂L-2 was 365. The ¹HNMR spectra of complexes could not be recorded due to their poor solubility (See Figure 3).

Figure 3: Graph of I.R. spectra of morpholine-N-carbothiohydrazide of 2-acetylbenzimidazole (H₂bmctz).

3.1. Antifungal Studies

The antifungal activity of ligands and some of their complexes have been evaluated by radial growth method [43, 44]. Czapek agar medium was prepared by dissolving 20 g starch, 20 g agar agar and 20 g glucose in one litre distilled water. The resulting medium was added requisite amount of test compound to get 100 and 200 ppm of solution. The resulting medium was then poured into Petri plates and the spores of fungi were placed on medium with the help of inoculum needle. The petri plates were wrapped in polythene bags containing two to three drops of ethanol and then placed in incubator at °C. The linear growth of fungus was evaluated by measuring the fungal zone diameter after five days. The percentage inhibition was calculated from the relation where and are the diameter of the fungus colony and control test plate, respectively. The fungi used in present investigation for screening against the fungi, Aspergillus niger, Aspergillus flavus, F. oxysporum, Rhizoctonia bataticola, and R. phaseoli. The control solution was mycostatin. The result of activity is shown in Table 4. It has been found that the ligand containing phenyl substituent is more active than methyl substituent. The Cu(II) and Zn(II) complexes in general show larger activity than Mn(II) and Fe(II) complexes. The bromo complexes show larger activity than that of chloro or pydridine or -picoline adduct. The chemical Mancozeb fungicides were used as standard for this process. In general the antifungal activities observed for Cu(II) and Zn(II) halo complexes were much larger than other metal complexes and ligand. The antifungal activity observed for R. phaseoli is larger compared to other fungi, indicating selectivity of complexes for particular fungi. The enlarged activity of complexes may be attributed to increased delocalisation of π-electrons of ligand over the whole chelate ring which enhances the lipophilicity of the complexes. The increase lipophilicity enhances the penetration of complexes into fungi membrane and blocking the active binding sites in the microorganism. In addition the complexes disturb the respiration process of the fungi cell thus blocking the synthesis of proteins essential for fungi growth and restrict the further growth of the organism. The activity of complexes of phenyl substituted ligand is larger than methyl derivatives due to larger π-delocalised electron in phenyl substituted ligand than that of methyl derivatives and free ligand as well.

Table 4: Results of fungal activity (conc. of compound in ppm).

Compound
Conc. in ppm
A. niger
A. flavus
F. oxisporum
R. bataticola
R. phaseoli

H₂bmctz = H₂L-1
100
40
35
30
45
50

200
55
50
52
68
75

H₂bpmctz = H₂L-2
100
46
36
40
48
55

200
68
52
65
72
80

Cu(H₂L-1)Cl₂
100
48
55
32
50
55

200
65
70
55
75
85

Cu(H₂L-2)Cl₂
100
50
60
42
52
56

200
70
78
70
78
86

Cu(H₂L-1)Br₂
100
50
56
42
52
57

200
68
72
68
76
87

Cu(HL-1)₂
100
48
50
35
50
54

200
62
65
60
70
80

Zn(H₂L-1)2 Cl₂
100
48
42
40
48
54

200
64
60
60
70
80

Zn(H₂L-2)2 Br₂
100
50
45
42
50
55

200
68
62
65
74
82

Zn(H₂L-2)Br₂
100
52
42
45
52
60

200
72
60
70
76
85

Mn(HL-1)₂
100
42
40
35
46
50

200
58
53
55
70
75

Fe(HL-2)₂
100
41
42
36
48
52

200
57
55
55
74
78

Pd(L-1)Py
100
52
45
40
50
52

200
68
68
62
72
78

Ni(H₂L-1)Cl₂
100
48
42
45
48
54

200
56
56
55
72
80

Ni(L-1)Py
100
46
40
32
45
52

200
56
52
55
70
76

Cd(HL-1)₂
100
46
43
35
48
54

200
60
58
60
72
80

Cd(HL-2)₂
100
48
42
43
50
60

200
66
60
65
74
85

Ref. Mancozeb
100
75
80
78
81
82

200
99
98
97
99
100

In addition to above facts, the chelate formation reduces the polarity of metal ions, increases the lipophilic character of the chelate, and increases the interaction between metal ion and fungi cell membrane. Consequently it blocks the cell growth of microorganism. According to overtone’s concept the lipophilicity of complexes is major factor which favours antimicrobial activity [45, 46]. The hydrogen bond formation between fungi and complex molecule through azomethene (C=N) and heteroatoms S and O and halogens of complexes interacts with cell constituents resulting in interference with the normal cell process. These factors are tentatively suggested for enhanced antimicrobial activities of metal chelates complexes of N and S donor.

From the results of I.R. and physical data, the probable structures of complexes are shown in Figures 4, 5, 6, and 7.

Figure 4: Proposed structure of [M(HL)₂] where M=(, HL=Hbmctz or HL=Hbpmctz.

Figure 5: Proposed structure of [M(H₂L)X₂] type of complexes where =, , , , and X= or .

Figure 6: Tetrahedral structure for or , square planar for or .

Figure 7: Structure of [MLB] where , are planar but tetrahedral for , H₂L=H₂bmctz or H₂bpmctz, B=Py, H₂O and -Pic.

4. Conclusion

The physicochemical data suggested octahedral structure for [M(HL)₂] type of complexes of Ni(II), Co(II), and Cu(II) while four coordinated planar to Pd(II) complexes. The monoligated Ni(II) and Pd(II) complexes [M(L-1)/(l-2)B] show characteristics of planar geometry. The complexes of [M(H₂L)Cl₂], (=, , , or ) have been suggested to possess trigonal bipyramidal structures. The ligand H₂L-1 and H₂L-2 coordinate as neutral, monoanionics or dianionic tridentate (N, N, S) donor molecules. The biological activity of phenyl substituted ligand and its complexes are larger than those of methyl substituted products probably due to excessive delocalized π-electron system present in phenyl ring.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

Thanks are due to authority of IIT Patna for recording IR, U-V, and mass spectra, BIT Mesra Ranchi for C, H, or N analysis, and CDRI Lucknow for ¹H NMR spectra, and the Head of the Chemistry Department, Patna for magnetic susceptibility measurements. The authority of NIT Patna is thankful for providing necessary laboratory facilities. We thankfully acknowledge the help of Biotechnology Department, Science College Patna, for screening antifungal activity.

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