Metal Complexes Derived from Mixed Azo-Linked Schiff-Base Ligandwith Dithiocarbamate Derivative: Formation, Spectral Characterization and Biological Study

Introduction

Azo compounds are an interesting materials that have shown a range of applications including; food technology, analytical chemistry, pharmaceutical application and dyeing or textile industry. Their role in coloring approach has been widely investigated and a range of compounds are fabricated.¹ The biological activity of azo-compounds allowed them to be used in the treatment of textile materials, also azo-compounds are well known for their medicinal importance and have shown a variety of applications as antitumor, antibacterial, antiseptics and antineoplastics.² Variety of ligands type Schiff-base and their metal complexes have been isolated, these compounds have very flexible and diverse structures, therefore their properties have been studied.³ Dithiocarbamates (DTCs) are class of organic compounds that are capable to chelate to metal ions.^4,5 DTCs compounds have a significant role in coordination chemistry. This may due to the metal ion stabilizationability in many oxidation states, and permitting the metal ion to implement its desirable structure.⁶ DTCs have shown asignificant biological activity including their role as antibacterial, antitumor and antifungal agents. They have other potential applications in materials science and supramolecular chemistry.^7,8 In our research, we report here the preparation of mixedazo-linked Schiff-base andDTCs ligands and their metal(II) dithiocarbamate complexes.

Experimental

All chemicals in this work are commercially available and used as received.Solvents were distilled using appropriate protocol before use.

Physical Measurements

Elemental micro-analyses (carbon, hydrogen, nitrogen and sulphur) for ligand and its metal complexes were conducted on a Euro EA 3000.Electrothermal Stuart SMP40 apparatus was used to record melting points. FT-IR spectra were recorded as potassium bromidediscs with a Shimadzu 8300s in the range 4000-400 cm^-1 and as CsI discs in the range 400-200 cm^-1. UV-Vis spectra were obtained with 10^-3 M solutions between 200-1100 nm in dimethylsulfoxide (DMSO) spectroscopic grade solvent at 25°C using a Perkin-Elmer spectrophotometer Lambda. TGA was carried out using a STA PT-1000 Linseis. NMR spectra (¹H-NMR) were acquired in DMSO-d₆ and CDCl₃ using aBrucker-400 MHz and a Brucker-300 with tetramethylsilane (TMS). A Shimadzu (A.A) 680 G atomic absorption spectrophotometer was implemented to determine metal content in complexes. Conductivity measurements were performed using a Jenway 4071 digital conductivity meter with DMSO solutions at room temperature. Chloride was determined using potentiometer titration method on a (686-Titro processor-665Dosimat-Metrohm Swiss). A magnetic susceptibility balance (Sherwood Scientific) was used to determine magnetic moments of complexes.

Synthesis

Preparation of the Precursor

The compound was prepared according to the literature^9,10 1-naphthyl amine (1g, 6.98mmol) was dissolved in H₂O(10ml) and Hydrochloric acid Conc. (2.14ml, 60.98mmol) mixture with stirring, a clear solution was obtained. Temperature of (0–5)°C have been kept, then aqueous solution of sodium nitrite (0.48g, 6.98mmol) dissolved in (5ml) water,was dropwise slowly added, keeping the temperature below 5°C, followed by mixture stirring for 1hr using ice bath, then little amount of urea was added, the pH was  adjusted to (6–7) using sodium acetate [solution(1)]. H₂NC₆H₄OH (0.762g, 6.98mmol) was dissolved in (12ml) KOH(10mmol) aqueous solution, cooled by ice bathto (0–5)°C [solution(2)]. Gradually the last solution was mixed with cooling with (solution1), the mixture resulted was stirredat (0–5)°C continually for 2hrs, the precipitate resulted was then filtered using acidification, cold H₂Oused to wash several times after drying, a brown solid precipitate was obtained, Yield:1.35g (73.7%), melting point132°C.

Synthesis of free ligands

Preparation of [HL¹]

The compound was prepared according to the literature.¹²

A solution of the derivative[2-amino-6-(naphthalen-1-yldiazenyl)phenol] (1g, 3.798mmol) in 25ml ethanol was mixed with(0.566g, 3.798mmol) of (CH₃)₂NC₆H₄CHO dissolved in (10ml) ethanol. After adding glacial acetic acid (3-5) drops, the mixture resulted was refluxed for 2hrs., filtered off, and after washingby ethanol and drying; it was obtained alight brown product. The product solid was recrystallized using EtOH. Yield:1.23g, 82.5%, melting point(190–192°C).

Preparation of [L²]

The compound was prepared according to the literature.¹³

Ethanolic solutionof KOH (1.32g, 23.6mmol, 4eq) was added to solution of diphenyl amine (1g, 5.90mmol) dissolved in(10mL) ethanol. The resulted mixture was stirredkeeping the temperature at (0-5), dropwise with stirring it was added carbon disulphide solution(1.34g, 17.7mmol, 3eq) keeping the temperatureat 0°C for 2 hrs. The solid DTC_S yellow salt was obtained, Yield :1.48g, 88.6%. Melting point (238- 240°C).

Synthesis of complexes

A one pot approach reported in¹⁴ was used to prepare the mixed ligand metal complexes.

Preparation of [Co^II(HL¹)(L²)(H₂O)₂]

To a mixture of HL¹ (0.2g, 0.506mmol) dissolved in (10 ml) was added (0.0568g, 1.0139mmol) of potassium hydroxide dissolved in 10 ml ethanol. While, the solution was allowed to stirring, a mixture of Co^II salt (0.120g, 0.506mmol) in 10ml ethanol with (0.143g, 0.506mmol) of dithocarbamte ligand (L²) was added to the above solution. The reaction mixture was kept stirring for two hrs, during which time agreen product was filtered off. Washed with absolute ethanol and recrystallized from ethanolto give the pure product. Yield: 0.187g, 50.33%, (Dec.over 320).

Preparation of  Ni^II,  Zn^II and Cd^II Complexes

An analogues method to that reported for the synthesis of Co^II complex was implemented to prepare Ni^II, Zn^II, and Cd^IImixed ligands complexes. Table 1 displays the physical properties of the complexes and their reactant amount.

Table 1: Melting points, yields, metal salts quantities and colours of ligands and  thiercomplexes

Compound	Dec. (°C)	Weight of metal salt(g)	Weight of complex(g)	Yield (%)	Colour
[Co(L1)(L2)(H2O)2]	Over 320*	0.123	0.188	51	Light green
[Ni(L1)(L2)]	Over 320*	0.121	0.172	49	Green
[Zn(L1)(L2)(H2O)2]	Over 320*	0.07	0.148	40	Dark brown
K2[Cd(L1)(L2) Cl2]	Over 320*	0.103	0.185	41	light yellow

 

Results and Discussion

Synthesis

The precursor was obtained using a standard azo dye approach. The reaction of naphthyl amine diazonium salt with 2-amino-phenole in a 1:1 mole ratio in water solvent gave the required compound, see Scheme 1. Two sorts of ligands were prepared; (i) Azo-linked Schiff-base ligand, which isolated by reaction of the precursor with (CH₃)₂NC₆H₄CHO and (ii) DTCs ligand that obtained from the reaction of (C₆H₅)₂NH with carbon disulphide using KOH base (Schemes 2 and 3). The complexes were prepared by mixing the ligands with the metal salts in a 1:1:1 mole ratio (Scheme 4).A range of analytical and spectroscopic techniques were used to confirm the entity of compounds including; CHNS, FT-IR, UV-Vis, magnetic susceptibility and ¹H-NMR spectra. The infra-red spectrum of [HL¹], Fig.(1), exhibited bands  at 3423 and 2978cm^–1 attributed to the OH phenolic group^15,16 and ν(C–H)_aliphatic stretching, respectively. Bands related to ν(C=N) imine and ν(C–N) are observed at 1630 and 1257cm^–1, respectively. The formation of Schiff-base ligand  can be indicated by the absence of amine (NH₂) and aldehydic CHO bands and appearance of the new imine (C=N) band in the ligand spectrum.^17-19 Band related to stretching u(N=N) azo group are detected at 1460 cm^–1.^16,20 The FT-IR spectrum of [L²] is displayed in Fig.(2). The ν(C-N) of (N-CS₂) moiety shows a band at 1471 cm^-1.²¹ The spectrum indicates a couple of new bands at (894) and (1047) cm^-1, which designated to ν_s(CS₂) and  ν_as(CS₂) respectively.²² The characteristic bands are summarised in (Table 3). The ¹H-NMR spectrum of [HL¹], Fig.(7), displays peak related to the azomethine group at δ=10.1ppm (H,s,N=C–H).²³ Chemical shift at δ= 10.809 ppm (OH, S, H) correlated to the phenolic proton. The spectrum indicated that the non-equivalent two CH₃ groups appeared at δ= 2.2 and 3.3-3.4ppm (2CH₃, S, 6H). That may probably resulted from the position alternation of the two CH₃ groups in the proposed structure of the molecule. An averaged symmetrical spectrum may resulted from rapid rotation process. On the other hand, two non-equivalent conformations may resulted from the slow rotation. The ¹H-NMR spectrum of [L²] is depicted in (Fig. 8). The spectrum shows peaks at δ= 5.561ppm (2H, m) (C_{1, 1}`, <sub>5, 5</sub>`-H),  δ= 6.789-6.805 ppm (2H, m) (C_{3, 3}`-H) and δ= 7.685-7.694 ppm (2H, m) (C<sub>2, 2</sub>`, _{4, 4}`-H) attributed to the aromatic ring protons. The complexes are air stable solid. The entity of new complexes was confirmed by elemental analysis, FT-IR, electronic spectra and magnetic susceptibility. The analytical data (Table 2) support the proposed formulae. The molar conductance of the complexes in DMSO solvents is indicative of non-electrolyte and 1:2 electrolyte behaviour.^24,25

Scheme 1: Preparation path for Precursor. Click here to View scheme

 

Scheme 2: Preparation route for ligand [HL1] Click here to View scheme

 

Scheme 3: Preparation route of ligand [L2] Click here to View scheme

 

Scheme 4: Structure of mixed ligand complexes Click here to View figure

 

Figure 1: FT-IR spectrum of ligand [HL1] Click here to View figure

 

Figure 2: FT-IR spectrum of ligand [L2] Click here to View figure

 

Figure 3: FT-IR spectrum of [Co(L1)(L2) (H2O)2]complex Click here to View figure

 

Figure 4: Electronic spectrum of [HL1] Click here to View figure

 

Figure 5: Electronic spectrum of [L2] Click here to View figure

 

Table 2: Colours, Yields, (C, H, N, S) analysis and chloride content values for ligands and complexes.

Compound	M.wt		colour	Found (calc.) %
		Yield %		M%	C	H	N	S	Cl
HL1	394.47	82.5	Light brown	–	75.74	5.38	14.04
					-76.12	-5.62	-14.2
L2	283.46	88.6	yellow	–	53.38	3.38	4.65	22.34
					-55.08	-3.56	-4.94	-22.63
[Co(L1)(L2) (H2O) 2]	732.78	51	Light green	7.12	–	–	–	–	–
				(8.04.)
[Ni(L1)(L2)]	696.51	49	Light green	7.55	–	–	–	–	–
				(8.42.)
[Zn(L1)(L2) (H2O) 2]		40	Dark brown	8	–	–	–	–	–
	739.23			(8.84.)
K2 [Cd (L1)(L2)Cl2]	898.33	41	Light yellow	12.01	50	3.11	6.89	7.44	7
				-12.51	-50.76	-3.45	-7.79	-7.21	-7.89

 

Table 3: FT-IR spectral data for ligands and complexes.

Compound	ν(C-H)ar	ν(C-H)ali	ν(C=N)im	ν(N-CS2)di	ν(N=N)azo	ν(CS2)as	ν(M-N)	ν(M -S)	ν(M -Cl)
						ν(CS2)s	ν(M-O)
HL1	3097,3049,	2906	1642	–	1437	–	–	–	–
	3008
L2	2968	–	–	1471	–	1,047,894	–	–	–
[Co(L1)(L2)(H2O)2]	2980	2927	1606	1500	1469	1,053,966	665,609	385,320	–
							489,420
[Ni (L1)(L2)	3035	2983,2891	1639	1510	1462	1,051,993	673,619	387,302	–
[Zn(L1)(L2)(H2O)2]	2981	2895	1622	1506	1454	1,043,997	671	385,302
K2[Cd(L1)(L2)Cl2]	3035	2983,2889	1624	1504	1458	1,050,948	621 457416	393,304	298,268

 

FTIR and NMR Spectra for Complexes

The FTIR spectra of complexes show bands at rang (1606-1639) cm^–1 were assigned to of imine v(C=N) group, with alower frequency shift. This may be related to the engagement of the nitrogen atom of the iminic moiety in the coordination reaction.^26-28 The shift also may explained by delocalisation process of the d^-10 (metal electron density) to the ligand (psystem).^29,30 Spectra for complexes appeared bands at rang (1500-1510) cm^–1 that attributed to v(N-CS₂).²¹ This confirms that the v(N-C) double bond character may increase as a consequence of the moving of electrons to the metal centre as a result of coordination to the DTCs.²² Band located in the range (1454-1469) cm^–1 is assigned to ѵ(N=N) azo. Finally, the spectra showed new bands in the range (609-673) and (416-489) cm^–1 that attributed to v(M–N) and v(M–O), respectively. The appearance of these bands supported the involvement of the nitrogen of imin and oxygen phenolic atoms in the coordination of the ligand to the metal centre. These results are in accordance with that reported in literature.^31,32 Bands detected at (1043-1053) and (948-997) cm^-1are due to v_as(CS₂) asymmetric  and v_s(CS₂) symmetric mode of the DTCs moiety, respectively. This is in agreement with an anisobidentate chelation mode of the ligand to the metal ion.²² The anisobidentate mode of chelation of the ligand, may confirmed by the bands observed in the range (302-393) cm^-1 which may attributed to v(M-S) bond.33 The spectra of Co, see Fig. (3), and Zn complexes exhibited a broad band that assigned to ѵ(OH) of the hydrated water molecule.¹⁶ Table (4) includes the prominent FTIR bands of complexes. The ¹H-NMR spectrum of K₂ [Cd(L¹)(L²)Cl₂], (Fig.9), displays peak at δ=10.9ppm related to the proton of the iminicmoiety (1H, s, N=C-H).²³ The chemical shift at δ= 2.2 ppm that equivalent to six protons assigned to the methyl groups (2CH₃,s, 6H ).

Figure 6: Electronic spectrum of [Co(L1L2)(H2O)2] Click here to View figure

 

Figure 7:1H-NMR spectrum of ligand HL1 in CDCl3 Click here to View figure

 

Figure 8: 1H-NMR spectrum of ligand L2 in DMSO-d6 Click here to View figure

 

Table 4: Electronic spectral data of complexes in DMSO solutions

Compound	Wave number	Wave number	emaxmolar1 cm1	Assignment	Suggested structure
	λnm	(cm-1)
[Co(L1) (L2) (H2O)2]	287	34843	828	n→π*, π→π*	Distorted octahedral
	348	28735	559	C.T
	528	19047	107	4T1g(F)→4T1g(P)
	886	11286	16	4T1g(F)→4T2g(F)
[Ni(L1) (L2)]	295	33898	351	n→π*, π→π*	Square planar
	348	28735	551	C.T
	617	16207	26	1A1g(F)→ 1A2g(F)
[Zn(L1) (L2)(H2O)2]	262	38167	33	n→π*, π→π*	Distorted octahedral
	349	28653	124	C.T
K2 [Cd(L1) (L2) Cl2]	305	32786	811	n→π*, π→π*	Distorted octahedral
	346	28901	1113	C.T

 

UV-Vis Spectral Data and Magnetic Susceptibility for the Complexes

The electronic spectra of the complexes exhibited peaks in the range 262-305 nm attributed to π→π^* transition of the aromatic rings.³⁴ The spectra of complexes revealed peaks around 349 nm related to (n→π^*) transition of azo moiety. The blue shift recorded may attribute to the energy change of the conjugated chromophore (n→π^*) and (π→π^*electronic transitions, due to the chelation between metal ions and azo ligand.³⁵ The spectrum of Co^II complex, see (Figure.6), reveals peaks at 886 and 528 nm assignable to⁴T₁g^(F)→⁴T₂g^{(F) 4}T₁g^(F)→ ⁴T₁g^(P) , transitions respectively whichmay confirm a distorted octahedral structurefor Co^IIcomplex,^36-39 this result was confirmed by the magnetic moment value μ_eff of 5.07 B.M for the Co^II-complex.⁴⁰ The Ni^II complex shows a peak at 617 nm attributed to ¹A₁g^(f)→ ¹A₂g^(F), revealing a distorted square planar arrangement about Ni atom. This result was confirmed by the diamagnetic moment behavior of the Ni^II-complex.⁴⁰ The spectra of Zn^II and Cd^II compounds revealed peaks attributed to ligand field π→π^* and M→L charge transfer.⁴¹ These data along with other analytical results indicated that the Zn^II and Cd^II complexes adopt octahedral arrangement about metal centre.⁴² The six-coordinate number for the Zn^II and Cd^II compounds may be due to sort of ligands that surrounding metal centre and their steric and electronic interaction that   occurred upon complex formation.⁴³ The electronic data of the complexes are tabulated in (Table 5).

Figure 9: 1H-NMR spectrum of K2[Cd(L1)(L2)Cl2] in CDCl3 Click here to View figure

 

Figure 10: (TGA/ DTA and DSC) thermogram of [Co(L1) (L2) (H2O)2]in an argon atmosphere Click here to View figure

 

Figure 11: The effect of L1 L2 and its complexes on Escherichia coli. Click here to View figure

 

Figure 12: The effect ofL1 L2and its complexes on Enterobacter. Click here to View figure

 

Figure 13: The effect of L1L2andits complexes on Bacillus stubtilis. Click here to View figure

 

Table 5: Biological activity of  compounds

	Gram negative (G+)		Gram negative (G-)
Compounds	Bacillus stubtilis	Staphylococcus aureus (G+)	Escherichia coli(G−)	Enterobacter(G−)
Control	_	_	_	_
HL1	_	_	–	_
L2	22	12	–	–
[Co(L1)(L2)(H2O)2]	12	_	–	_
[Ni(L1)(L2)]	_	_	_	_
[Zn(L1)(L2)(H2O)2]	13	_	–	_
K2[Cd(L1) (L2) Cl2]	14	_	–	_

 

Thermal Analysis

The thermogram chart indicated the stability of [Co(L²)(L⁴)(H₂O)₂] up to 97ºC, see(Figure 10). peak detected at 297ºC may due to the loss of (HCN, 2H₂O, NO and CS₂) moieties, (obs. = 4.86mg, 23.15 %; calc. = 4.85mg. 23.1%. While second step at (594)ºC may refers to the loss of (C₆H₆ and N₂) fragment, (det.=3.06mg, 14.56%; calc.= 3.07mg.14.46%). The final residue from the compound which may assigned to the (Co, CH₃NCH₃, 2C₆H₄, C₆H₃, and C₁₀H₇), calc. =13.1 mg. 62.4%). The dithiocarbamates analysis thermogram showed peaks at 97, 310, 320, 398 and 594 may assigned to an exo and endo thermal decomposition process. The peak at 320 (exothermic) may due to combustion process of the organic ligand part(in an argon atmosphere). While metal-ligand bond breaking maysignified by endothermic peaks at 97, 310, 398 and 594ºC.^44,45

Figure 14: The effect of L1 L2and its complexes on Staphylococcus aureus. Click here to View figure

 

Biologicalactivity

The synthesized ligands and its complexes were screened for their biological activity against some bacterial strains (Escherichia coli and Enterobacter(G−) Bacillus stubtilis and Staphylococcus aureus (G+)).The involvement of dimethylsulphoxide in the bacterial test was confirmed by individual tests that conducted with the DMSO alone that indicated no activity towards any bacterial species.⁴⁶ The measured size of inhibition zones against growth of different microorganisms are summarised in (Table 6) that displays the effect of the prepared compounds on bacterial species. From collected data, it is clear that, compared with the free ligands,the ligands (HL¹ and L²) showed no antimicrobial activity against Escherichia coli and Enterobacter. Hence, formation of complexes enhances the antimicrobial activity. Such increased activity of complexes may be related to the chelation theory.³³ Therefore, the chelation decreases the polarity of the metal atom that resulted in the partial sharing of its positive charge with donor group and possible π-electron delocalisation over the whole ring. Zinc and cadmium complexes showed almost the higher antibacterial activity, compared with other compounds. This due to to their molecular weight and their electronic configuration (d¹⁰ system), compared with other metal complexes (Figures.^{11-14,47, 48}

Conclusion

The preparation and characterisation of mixed ligands and their complexes are described. This was based on the preparation of two sorts of ligands; (i) the azo-linked Schiff-base ligand that obtained by reaction of the precursor with (CH₃)₂NC₆H₄CHOand (ii) the DTC_S ligand that fabricated by reaction of (C₆H₅)₂NH with carbon disulphide. The mixed ligand complexes were achieved by adding the HL¹and  L² with the appropriate metal salt in a 1:1:1 mole ratio. Physico-chemical and spectroscopic methods were implemented to confirm mode of bonding and over all structure of the complexes. These results lead to the preparation of four and six coordinate complexes.

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