Macrocycliczinc(II) Complexes with Tetradentate N2O2 Donor Schiff Base Ligands Incorporating 1,3,4-Thiadiazole Ring

Introduction

Thiadiazoles and its derivatives show interesting biological and pharmacological activities which may be due to the presence of C₂N₂S moiety and its high aromaticity.^1-3. Literature survey reveal that the molecule containing 1,3,4-thiadiazole ring have diverse therapeutic applications such as antimicrobial^4,5, analgesic⁶, antiviral⁷, anti-inflammatory⁸, anticonvulsant^9,10, antidiabetic^11,12, anticancer^13,14, anti-tubercular activity¹⁵. Researchers have used medium to large sized cyclic compounds as a tool in discovery of drugs due to their significant biological and physic-chemical properties^16,17. A number of macrocyclic polyether compounds incorporating a 1,3,4-thiadiazole moiety have been applied to inhibit corrosion of C38 carbon steel in acidic medium¹⁸. The creation of synthetic five to six-membered heterocyclic macrocyclic compounds has wide applications due to their remarkable properties in various fields¹⁹. Host-guest complexation relationships have been studied in such macrocycles depending upon side chain substitution pattern²⁰. A number of macrocyclic metal complexes have been prepared via template synthesis method²¹. Biological and pharmacological properties of 1,3,4-thiadiazole make us confident to synthesize some more bis-Schiff base compounds. In this paper, synthesis, characterization and application of some diazadioxama crocyclicZn(II) derivatives prepared via template synthesis by the reaction of bis-(2-hydrazino-1,3,4-thiadiazole-5-yl)arene/alkanes (2-BHTA) and 3,5-dichlorosalicyldehyde (3,5-DCS)/2-hydroxy-1-naphthaldehyde (2-HN) with 1,4-dibromobutane (1,4-DBB) in presence of Zn(II) acetate salt have been discussed.

Materials and Methods

Chemicals and Instrumentation

The chemicals were purchased from Merck and Aldrich. Elemental analysis was evaluated with a Vario EL III Carlo Erba 1105 CHN analyser. Melting points were determined by Buchi 530 apparatus. Shimadzu 8201 PC model spectrophotometer was used to record IR spectra in KBr; whereas NMR spectra were recorded by Bruker DRX-300 spectrometer in DMSO using tetramethylsilane (TMS) as standard. JOEL model JSM-6390LV scanning electron microscope was used to get SEM micrograph of the Zn(II) complexes. TGA data of the macrocyclic complexes were obtained using a Perkin Elmer-STA 6000 thermal instrument. Bruker AXS DB Advance diffractometer was used to record powder X-ray diffraction dimensions with Cu Kα radiation (λ=1.5406 Å).

Preparation of Schiff Bases

Maffiet al method²² was used to synthesize bis-(2-hydrazino-1,3,4-thiadiazole-5-yl) arene/alkanes. Schiff base was synthesized by the condensation reaction of 2-BHTA and 2-HN/3,5-DCS in 1:2 ratio in absolute ethanol containing concentrated HCl and the mixture was refluxed for 8-9 h. Precipitate thus formed, was filtered and washed with ethanol and ether and recrystallized from alcohol.

Preparation of Macrocycliczinc(II) Derivatives

Macrocycliczinc(II) (MZ-II) complexes was synthesized via template synthesis method for which Zn(CH₃COO)₂.2H₂O (0.02 mol) and 1,4-DBB (0.02 mol) was simultaneously added to the alcoholic solution (30 cm³) of appropriate Schiff base (0.02 mol) and resultant content was refluxed for 11-14 h. The resulting Yellow/brown substances was filtered, washed with ethanol and dried in vacuo. The general reaction scheme is given in Figure 1 and the analytical data of the synthesized complexes is listed in Table 1.

 

Figure 1: Reaction route for the synthesis of MZ-II derivatives Click here to View figure

Table 1: Analytical data of MZ-II derivatives

Compound	Molecular formula	% Yield	% Analysis  Found (Calc.)
			C	H	N	S	Cl	Zn
[Zn(M1)(H2O)2](OAc)2 [Zn(M2)(H2O)2](OAc)2 [Zn(M3)(H2O)2](OAc)2 [Zn(M4)(H2O)2](OAc)2 [Zn(M5)(H2O)2](OAc)2 [Zn(M6)(H2O)2](OAc)2	C36H38N8O8S2Zn C38H42N8O8S2Zn C40H38N8O8S2Zn C28H30N8O8S2Cl4Zn C30H34N8O8S2Cl4Zn C32H30N8O8S2Cl4Zn	65 56 75 71 69 78	51.31 (51.46) 52.49 (52.56) 53.98 (54.09) 38.26 (38.31) 39.67 (39.77) 41.46 (41.50)	4.51 (4.56) 4.85 (4.88) 4.24 (4.31) 3.41 (3.44) 3.72 (3.78) 3.21 (3.27)	13.29 (13.36) 12.81 (12.91) 12.51 (12.62) 12.69 (12.76) 12.30 (12.37) 12.02 (12.10)	7.49 (7.63) 7.31 (7.39) 7.10 (7.22) 7.22 (7.31) 7.01 (7.08) 6.84 (6.93)	– – – 16.01 (16.15) 15.59 (15.65) 15.21 (15.32)	7.68 (7.78) 7.46 (7.53) 7.27 (7.36) 7.34 (7.45) 7.10 (7.22) 6.93 (7.06)

Antimicrobial Properties

Antifungal effect of MZ-II complexes were evaluated against two pathogenic fungal strains (A. niger and A. alternata) by agar plate technique method reported in litrature²³; the outcomes were recorded as percentage of inhibtion and compared with fluconazole which was used as standard drug.Antibacterial effect of MZ-II complexes were screened against Gram-positive B. subtilis and Gram-negative E. coli by agar well diffusion method²⁴; and the results were recorded by measuring zone of inhibition (mm) and compared with tetracyclin.

Results and Discussion

The synthesized MZ-II complexes were air stable and soluble in DMSO and DMF solvents. The analytical analyses of the synthesized MZ-II complexes agree well withthe proposed macrocyclic frame. The electrical conductance measurements have been evaluated in DMF which indicate that the MZ-II derivatives are electrolytic nature. The presence of acetate ion has also been confirmed by neutral FeCl₃ solution test which gives blood red color with the complexes. TGA studies confirm the presence of coordinated water molecules in the MZ-II complexes, as it shows weight loss in the range of 135-170°C.

Spectral Interpretation

Table 2 contains IR spectral data of the synthesized MZ-II derivatives.A band at ca. 3232-3192 cm^-1 in the acyclic ligands and their corresponding MZ-II derivatives is due to ѵ(N-H). A medium band at ca. 1638 cm^-1 is assignable to ѵ(C=N) group in acyclic ligands which appears at lower frequency (~15-25 cm^-1) in the related MZ-II complexes^25,26, and this change assures the involvement of azomethine nitrogen in bonding with zinc ion, which is also confirmed by a new band at ca. 418-462 cm^-1 attributable to ѵ(Zn-N) in the MZ-II complexes. Spectra of acyclic ligands show intramolecular H-bonded phenolic –OH bands²⁷ at nearly 2900 cm^-1 which vanishes in their respective MZ-II derivatives, and a new band appears at ca. 509-528 cm^-1 attributable to ѵ(Zn-O)²⁸. Acyclic ligands show a band at ca. 1101 cm^-1 because of ν(C-S-C) vibration, which remains unaltered in corresponding MZ-II derivatives, suggesting the non-involvement of thiadiazole ring sulphur. A broad band in the region ca. 3409-3452 cm^-1 is attributable to coordinated water molecules²⁹ in the MZ-II complexes. Presence of acetate ions in the MZ-II complexes were confirmed by asymmetrical and symmetrical stretching band at ca. 1625-1640 and ca. 1420 cm^-1 respectively³⁰.

Table 2: IR spectral data of MZ-II derivatives

Compound	IR data (cm-1)
	O-H	N-H	C=N	Aromatic C-O	Thiadiazole ring	-OOCCH3	Zn-O	Zn-N
[Zn(M1)(H2O)2](OAc)2 [Zn(M2)(H2O)2](OAc)2 [Zn(M3)(H2O)2](OAc)2 [Zn(M4)(H2O)2](OAc)2 [Zn(M5)(H2O)2](OAc)2 [Zn(M6)(H2O)2](OAc)2	3421mb 3452mb 3425mb 3412mb 3409mb 3412mb	3234m 3232m 3235m 3191m 3185m 3192m	1621s 1624s 1626s 1615s 1612s 1618s	1295s 1281s 1305s 1289s 1282s 1298s	1106s 1101s 1107s 1104s 1102s 1105s	1638s 1631s 1640s 1628s 1625s 1632s	518m 509m 528m 512m 511m 515m	457m 454m 462m 425m 418m 429m

Table 3containschemical shifts for MZ-II complexes protons which were recorded in dissimilar environments using deuterated DMSO solvent. A signal at ca. 11.58 ppm is observed in acyclic ligands due to phenolic protons which vanish in the corresponding MZ-II derivatives. Naphthyl and aromatic protons appear as a multiplet at ca. 6.99-7.85 ppm. Signals at ca. 10.16 and 8.28 ppm is because of hydrazino NH and azomethine protons correspondingly in Schiff bases. Among the two signals, first one remains unchanged whereas the second signal shifts downfield and observed at ca. 8.48 ppm in the corresponding MZ-II derivatives and it confirms the coordination of the azomethine nitrogen to the zinc(II) ion. A signal at ca. 2.35 ppm indicates the presence of acetate ion in the MZ-II complexes. The spectra of all MZ-II derivatives show a new signal at ca. 5.5 ppm assignable to water protons.

Table 3: ¹H-NMR spectral data of MZ-II derivatives

Compound	1H-NMR data (δ, ppm)
	-CH2–	-O(CH2)4O-	-NH	Aromatic rings	-HC=N-	-OOCCH3	H2O
[Zn(M1)(H2O)2](OAc)2 [Zn(M2)(H2O)2](OAc)2 [Zn(M3)(H2O)2](OAc)2 [Zn(M4)(H2O)2](OAc)2 [Zn(M5)(H2O)2](OAc)2 [Zn(M6)(H2O)2](OAc)2	2.52 t 2.77 t,1.91 m – 2.55 t 2.69 t,1.94 m –	3.62 t,2.13 m 3.57 t,2.11 m 3.65 t,2.18 m 3.56 t,2.17 m 3.54 t,2.14 m 3.59 t,2.18 m	10.25 s 10.16 s 10.31 s 10.51 s 10.41 s 10.58 s	7.12-8.00 m 6.94-7.85 m 7.18-8.05 m 7.51, 7.75 s 7.50, 7.75 s 7.52 m, 7.56, 7.79 s	8.54 s 8.48 s 8.61 s 8.72 s 8.65 s 8.78 s	2.37 s 2.35 s 2.39 s 2.41 s 2.38 s 2.45 s	5.38 s 5.31s 5.49 s 5.47 s 5.42 s 5.57 s

SEM

Surface morphology of one of the representative complex [Zn(M²)(H₂O)₂](OAc)₂ was evaluated and the micrograph show irregular arrangement of nano-ranged particles with globular morphology (Figure 2).

Figure 2: SEM image of [Zn(M2)(H2O)2](OAc)2 Click here to View figure

X-Ray Diffraction

The XRD studies of one of the complex [Zn(M²)(H₂O)₂](OAc)₂, indicates the nano-crystal (Figure 3) formation. The particles size of the selected macrocyclicZn(II) complex have been determined by Debye-Scherer formula³¹ (D=0.94λ/βCosθ). Calculated particles size falls in range of 83.02-83.65 nm.

Figure 3: XRD image of [Zn(M2)(H2O)2](OAc)2 Click here to View figure

Biological Activity Results

MZ-II complexes are more toxic than the parent ligands. The cause behind the better toxicity of the MZ-II derivatives can be understood by chelation theory, which explains that chelation reduces the polarity of the metal ion, thus it allows the permeation of the compounds via cell membranes³². In vitro antifungal studies of all the compounds was evaluated against A. niger and A. alternata, using Fluconazole as standard drug and the results were noted at 10, 100 and 1000 ppm concentration. Antifungal results (Fig. 4 and Table 4) show that all MZ-II complexes were more active against A. niger while complexes 4, 5 and 6 were more toxic which may be due to chloro group.

Table 4: Antifungal screening data of MZ-II derivatives

Compound	Percentage Inhibition
	A. niger				A. alternata
	10	100		1000	10		100	1000
[Zn(M1)(H2O)2](OAc)2	31		48	62		–	24	39
[Zn(M2)(H2O)2](OAc)2	28		45	58		–	–	35
[Zn(M3)(H2O)2](OAc)2	45		51	65		21	42	51
[Zn(M4)(H2O)2](OAc)2	49		56	69		35	45	60
[Zn(M5)(H2O)2](OAc)2	45		51	65		31	42	58
[Zn(M6)(H2O)2](OAc)2	55		64	72		39	48	61
Fluconazole	100		100	100		100	100	100

Figure 4: Antifungal activities of MZ-II complexes at 1000 ppm Click here to View figure

The antibacterial activities of the compounds were studied against E. coli and B. subtilis, using Tetracyclin as standard and the results were listed by measuring the diameter complete inhibition zone (mm). The antibacterial studies (Fig. 5 and Table 5) reveal that MZ-II derivatives were more toxic to B.substilis, while complexes containing chloro group were more effective.

Table 5: Antibacterial screening data of MZ-II derivatives

Compound	Zone of Inhibition
	E. coli			B.subtilis
[Zn(M1)(H2O)2](OAc)2		–				–
[Zn(M2)(H2O)2](OAc)2		–				–
[Zn(M3)(H2O)2](OAc)2		13				15
[Zn(M4)(H2O)2](OAc)2		11				19
[Zn(M5)(H2O)2](OAc)2		10				18
[Zn(M6)(H2O)2](OAc)2		14				21
Tetracyclin		28				25

Figure 5: Antibacterial activities of MZ-II complexes Click here to View figure

Conclusions

The tetradenatae N₂O₂ type ligands were used to prepare stable macrocyclic derivatives with Zn²⁺ ion and an octahedral geometry of MZ-II complexes has been estimated with the help of spectral data. The TG analysis study reveals the presence of water molecules that are coordinated to the Zn(II) ion in the MZ-II complexes. Nano-range structure of the MZ-II complexes was confirmed By XRD pattern, whereas SEM studies reveal irregular globular morphology of the MZ-II derivatives. Good antifungal and antibacterial behavior is shown by MZ-II derivatives which are due to chelation effect.

Acknowledgements

The authors are thankful to the Head, SAIF, Cochin University for providing spectral data and Chemistry Department, DDUGU Gorakhpur for providing necessary research facilities; whereas AV is grateful to the UGC, New Delhi, for financial support.

Conflict of Interest

Authors declare no conflict of interest.

Funding sourses

There are no funding soures

References

1. Foks, D. P.-K. H.; Gobis, K.; J. Heterocycl. Chem.,2014, 51, 507-512.
   CrossRef
2. Hu, Y.; Li, C.-Y.; Wang, X.-M.; Yang, Y.-H.; Zhu, H.-L.; Chem. Rev., 2014, 114 (10), 5572-5610.
   CrossRef
3. Shivakumara, N.; Krishna, P. M.; J. Mol. Struc.,2020, 1199, 126999.
   CrossRef
4. Kamal, M.; Shakya, A. K.; Jawaid, T.; Int. J. Biomed. Res.,2011, 2 (1), 41-61.
   CrossRef
5. Serban, G.; Stanasel, O.; Serban, E.; Bota, S.; Drug Des. Dev. Ther., 2018, 12, 1545-1566.
   CrossRef
6. Mathew, V.; Keshavayya, J.; Vaidya, V.P.; Giles, D.; Eur. J. Med. Chem.,2007, 42, 823-840.
   CrossRef
7. Sidwell, R.W.; Robins, R.K.; Hillyard, I.W.; Pharmacol. Ther. Drugs, 1978, 6, 123-146.
   CrossRef
8. Mullican, M. D.; Wilson, M. W.; Conner, D. T.; Kostlan, C. R.; Schrier, D. J.; Dyer, R. D.; J. Med. Chem.,1993, 36 (8), 1090-1099.
   CrossRef
9. Stillings, M. R.; Welbourn, A. P.; Walter, D. S.; J. Med. Chem.,1986, 29 (11), 2280-2284.
   CrossRef
10. Yar, M. S.; Akhter, M.W.; Acta Pol. Pharm., 66 (2009) 393-397.
    CrossRef
11. Pattan, S. R.; Kittur, B. S.; Sastry, B. S.; Jaday, S. G.; Thakur, D. K.; Madamwar, S. A.; Shinde, H.V.; Indian J. Chem.,2011, 50B, 615-618.
12. Datar, P.A.; Deokule, T.A.; Med. Chem.,2014, 4 (4), 390-399.
13. Gomha, S. M.; Edrees, M. M.; Muhammad, Z. A.; El-Reedy, A. AM.; Drug Des. Dev. Ther., 2018, 12, 1511-1523.
14. Gomha, S. M.; Abdel-Aziz, H.M.; Heterocycles,2015, 91, 583-592.
    CrossRef
15. Foroumadi, A.; Kargar, Z.; Sakhteman, A.; Sharifzadeh, Z.;
    Feyzmohammadi, R.; Kazemi, M.; Shafiee, A.; Bioorg. Med. Chem. Lett,2006, 16, 1164-1167.
    CrossRef
16. Veber, D. F.; Holly, F. W.; Nutt, R. F.; Bergstrand, S. J.; Brady, S. F.; Hirschmann, R.; Glitzer, M. S.; Saperstein, R.; Nature, 1979, 280, 512-514.
    CrossRef
17. Abdelraheem, E. M. M.; Shaabani, S.; Domling, A.; Drug Discovery Today: Tech.,2018, 29, 11-17.
    CrossRef
18. Bentiss, F.; Lebrini, M.; Vezin, H.; Chai, F.; Traisnel, M.; Lagrené, M.; CorrosionScience, 2009, 51, 2165-2173.
    CrossRef
19. Foroughifar, N.; Mobinikhaledi, A.; Ebrahimi, S.; Moghanian, H.; Fard, M. A. B.; Kalhor, M.; Tetrahedron Lett.,2009, 50, 836-839.
    CrossRef
20. Lv, J.; Liu, T.; Wang, Y.; Eur. J. Med. Chem., 2008, 43, 19-24.
    CrossRef
21. Beattie, J. W.; SantaLucia , D. J.; White, D. S.; Groysman, S.; Inorg. Chim. Acta,2017, 460, 8-16.
    CrossRef
22. Maffii, G.; Testa E.; Ettorre, R.; Chem. Abstr.,1959, 53, 2211a.
23. Grower, R. K.; Moore, J. D.; Phytopathology, 1962, 52, 876-880.
    CrossRef
24. Rahman, A. U.; Choudhary, M. I.; Thomsen, W. J.; Harwood Academic Publishers, The Netherlands, 2001.
25. Foroughifar, N.; Mobinikhaledi, A.; Ebrahimi, S.; Moghanian, H.; Fard, M. A. B.; Kalhor,M.; TetrahedronLett. , 2009, 50, 836-839.
    CrossRef
26. Gull, P.; Malik, M. A.; Dar, O. A.; Hashmi, A. A.; Microb. Pathogenesis, 2017, 104, 212-216.
    CrossRef
27. Filarowski, A.; Koll, A.; Kochel, A.; Kalenik, J.; Hansen, P. E.; J. Mol. Struc.,2004, 700(1-3), 67-72.
28. Koch, A.; Phukan, A.; Chanu, O. B.; Kumar, A.; Lal, R. A.; J. Mol. Struc.,2014, 1060, 119-130.
    CrossRef
29. Shukla, P. R.; Singh, V. K.; Jaiswal, A. M.; Narain, J.; J. Indian Chem. Soc.,1983, 60, 321-324.
    CrossRef
30. Ferrari, A.; Braibanti, A.; Bigliardi, G.; Lanfredi, A. M.; ActaCrystallogr., 1965, 19(4), 548-555.
    CrossRef
31. Langford, J. I.; Wilson, A. J. C.; J. Appl. Cryst.,1978, 11, 102-113.
    CrossRef
32. Mahiwal, K.; Kumar, P.; Narasimhan, B.; Med. Chem. Res.,2012, 21, 293-307.
    CrossRef

---

This work is licensed under a Creative Commons Attribution 4.0 International License.