Novel Synthesis of N–N Azo and Hydrazine Phenyl Ligand Derivatives for Copper(II) Complex Bio-Active Application

Abstract

Copper(II) complexes possess relatively a broad spectrum of medicinal importance with less toxicity. It is important to note that, in this chapter, copper(II) is selected as chelating central metal atom considering its current reputation to design such bio-active compounds due to its; (i) permits in realizing stable coordination compound, (ii) diverse coordinating capability with oxygen (O), nitrogen (N), sulfur (S) and phosphorus (P) donor ligands, and (iii) exhibits potentially better biological activity. Therefore, the presented chapter offers the up-to-date advancement and future perspective of bio-active copper(II) complexes derived from Schiff base of azo- and phenyl hydrazine-based ligands and their derivatives. To showcase the existing trend of these classes of bio-active compounds, due to the wide depth of the literature, selected seminal compounds exhibiting outstanding biological activity are discussed in detail. Recent studies establish that azo- and phenyl hydrazine-based bio-active copper(II) complexes are among the promising candidates that are expected to replace the conventional antibiotics which are suffering from side effects as well as microbial resistance. However, the collaborative efforts of chemists and biotechnologists are still needed to realize their real world application.

1. Introduction

Medicinal inorganic chemistry deals with searching and designing of additional therapeutic alternatives not accessible to the organic compound counterparts, in which metal complexes have been extensively investigated. This is because of the wide range of coordination numbers, oxidation state, coordination geometries, and accessible redox states, which are the intrinsic properties of the Lewis acid central metal ions as well as the different coordination modes of donor atoms (N, O, S, and so on) of the Lewis base ligands offer the medicinal inorganic chemist a large opportunity to extensively exploited [1, 2]. Of the transition metals, copper plays a crucial part in biological processes in mitochondria and cell [3, 4]. For example, Figure 1 demonstrates copper homeostasis i.e. the mode of copper uptake, distribution as well as removal in mammalian cells. Copper can also form stable redox-active biological compounds with a variety of ligands that contain donor atoms like O, S, or N [6]. Moreover, the biological accessible redox potential of copper as Cu(II)/Cu(I) redox pairs have biologically significant implications such as metallo-nucleases [7, 8]. Copper is not only necessary for numerous biological processes but also a powerful anti-microbial tool that can be used to combat invasive diseases [9]. Consequently, copper complexes have been extensively studied for their potential biological activity such as antimicrobial [10], antiviral [11], anti-inflammatory [12], and antifungal [13] properties.

Figure 1.

Copper homeostasis and its mode of uptake, distribution as well as removal in mammalian cells. After reduction of Cu(II) to Cu(I) form, it enters the cell via the copper importer Ctr1 and then passed on to the chaperones COX17, CCS, and ATOX1, which deliver the copper to the cytosolic SOD1, COX in the mitochondria and to ATP7A/B at the TGN, respectively. Moreover, copper binds to cellular the antioxidant (MTs and GSH) that helps to prevent the formation of free catalyzing copper ROS. At the TGN, copper is amalgamated with copper-dependent enzymes (such as ceruloplasmin) that migrate through the secretion pathway. Under intracellular copper elevated condition, its excretion is facilitated via ATP7A as well as ATP7B circulation from the TGN to the plasma membrane, while the transporter Ctr1 is suppressed and is subsequently degraded. Ctr2 can also increase the generation of tCtr1, which facilitate endosomal copper transport to the cytoplasm resulting in the diminishing of the intracellular copper accretion [5]. Abbreviations: Trans-Golgi network = TGN, ATP7A/B = copper transporting ATPase A/B, ATOX1 = antioxidant protein, COX = cytochrome c oxidase, COX17 = cytochrome c oxidase copper chaperone, CCS = copper chaperone for SOD1, ROS = reactive oxygen species, Ctr1/2 = copper transporter 1/2, MT = metallothioneins, SOD1 = Cu/Zn–superoxide dismutase, GSH = glutathione, and tCtr1 = truncated Ctr1.

Researchers have been realized that, compared to the ligands, metal complexes could possess a greater biological activity with lower toxicity [14]. Of the metal complexes, copper complexes mainly depend on the oxidation state i.e. copper(I) and copper(II) as well as nature of the ligands and their donor atoms [9]. Like other transition metal ions, copper has various oxidation states, i.e. I, II, and III, of which the copper(II) with a d⁹ system of electronic configuration is the most favorable and relatively stable oxidation state. Considering its broad spectrum of bioactivity application, copper(II) is relatively vital in medicinal and pharmaceutical fields. Apart from the central metals, a rational designing of the ligands is also a very important step in developing bioactive complexes. For this reason, several ligands have been employed such as, Schiff bases ligands with multi dentate scaffold bearing imine nitrogen (C=N) which can be realized by simple condensation of aldehydes or ketones. Moreover, azo-based ligands possessing the azo functional group (N=N) moiety which can be also prepared by diazotization-condensation [15] as well as hydrazone-based ligands which are another class of Schiff base compounds containing hydrazine moiety (C=N–N) which can be synthesized by hydrazine-aldehyde/ketone condensation or Japp-Klingemann reaction [16, 17] have been also employed. The strategic synthesis of such ligands is well documented and extensively explored. Therefore, due to their straightforward synthesis route of these classes of ligands, their detailed synthesis strategy is omitted to avoid redundancy and only the type of ligands covered in this chapter.

Copper(II) complexes can display a substantial biological activity employing different ligands such as Schiff bases containing azo-, hydrazine-, and hetrocycles- moiety. These types of ligands containing imine moiety can form a stable metal coordination complexes which are responsible for broad spectrum of applications of which biological activity is one application field [18]. The presented chapter offers the current advancement and forwards the future perspective of copper(II) complexes of both azo- and hydrazine-based ligands and their biological importance. Appreciating all efforts that have been done so far, due to the wide depth of the literature, only selected seminal reports of azo- as well as phenyl hydrazine-based copper(II) complexes exhibiting superior biological activity are considered in this chapter. Additionally, more emphasis is also given to the complexes which are structurally well characterized (using single crystal XRD) unless the complexes reported with powder chemistry structural exhibited outstanding biological activity. Finally, the theoretical consideration of some selected seminal works is also incorporated to probe their relationship with the experimental aspect.

2. Bio-active copper(II) compounds

The bio-activity, biocompatibility as well as versatile coordination chemistry nature of the copper metal offer an opportunity for realizing bio-active copper(II) complexes which is usually achieved via enhancing the initial bio-activity of the ligand such as Azo-Schiff base ligands [15] and hydrazone based ligands [19]. Incorporating two or more diverse chromophores into the ligand could potentially enhance the physicochemical as well as the biological properties of the desired ligands and their respective metal complexes [15]. Azo are typically Schiff base ligands possessing a distinct chromophore group (N=N) which is responsible for diverse potential range of applications such as dyes [20] and biological [21] applications. Moreover, hydrazine and/or hydrazine based ligands displaying keto-enol tautomerism are another class of Schiff base ligands where the complexation of copper(II) with such biologically active ligands offers the opportunity of improving the efficacy of anti-microbial activity via evading the drug resistant pathogens [19, 22]. In this chapter, the biological activity of various N–N azo- as well as phenyl hydrazine ligand and/or hydrazone derivatives for copper (II) complex is summarized, including some copper(II) complexes based drugs commonly used as antibiotics, antimicrobials, and anti-inflammatory remedies.

2.1 Azo-based bio-active copper(II) complexes

Apart from industrial applications, dyes mainly azo-dyes possess massive biological applications (Figure 2) confirming the versatility of these class of compounds. Recently, the biological application of azo-based ligands and their metal complexes are getting much attention due to the presence of the optically and biologically active azo- functional group. Azo compounds are commonly prepared by diazotization and coupling i.e. diazotization of an aromatic primary amine followed by coupling with electron-rich nucleophile such as amino and hydroxyl group under alkaline conditions. Due to straight forward synthesis of the azo-based multi dentate ligands and their readily coordination flexibility, azo-based copper(II) bio-active complexes has been extensively explored [15, 21, 23]. For example, more recently Mady et al. reported a series of o-vanillin-azo (OV-AZO) based ligand prepared by condensation of 4-aminoazobenzene and o-vanillin and their transition metal complexes (Mn(II), Ni(II), Co(II), Cu(II), Zn(II), and Zr(IV)) for their anticancer and antimicrobial activity (Figure 3) [23]. The obtained results showed that, in comparison to the free corresponding azo-based ligand, of the other transition metal complexes, the copper(II) complex exhibited the maximum cytotoxic activity of an half maximal inhibitory concentration (IC₅₀) of 18 and 22 μg/mL for human colon and liver cancer cells, respectively as compared to the reference cisplatin drug. This could be due to the copper(II) complex capability to induce oxidative stress as well as generating activated oxygen species in addition to its structural arrangement offering relatively better stability advantage while the other counterparts were suffering from water dehydration and acetate removal. Moreover, similarly, higher antimicrobial activity of metal complexes against the anti-microbial (fungal and bacterial strains) was witnessed as compared to the free corresponding ligand which could be due to the Tweedy’s chelation theory. Normally, IC₅₀ is a quantitative measure of the effectiveness of a compound in inhibiting a specific biochemical or biological function that can be determined by constructing a dose–response curve assay at different concentrations. Usually, IC₅₀ is typically expressed as molar concentration where in this chapter a molar concentration in μg/mL is used as a figure of merit.

Figure 2.

Schematic illustration of azo-dyes and their biological as well as industrial applications.

Figure 3.

Proposed structures of the azo-based ligand (OV-AZO) and its transition metal complexes [23].

Kurtoglu and coworkers reported azo-azomethine based ligands of Cu(II) complexes and their antioxidant and cytotoxicity against human uterus carcinoma cells lines (Figure 4) [24]. Interestingly, all the ligands and the complexes exhibited better antiproliferative activities at 25, 50, and 100 μgmL⁻¹ concentrations where complexation of the ligands with the copper(II) ion slightly declined their anticancer properties which could be due to the hindering of the imine, phenolic and methoxy groups. Similarly, the antioxidant properties of the ligands were slightly better than the copper(II) complexes. Moreover, Krishnamurthy and coworkers reported an isonicotine hydrazide based azo- ligand and its transition (Cu(II), Co(II), and Ni(II)) metal complexes with octahydral geometry displaying biological activity such as, human breast cancer cells (MDA-MB 231) where MDA stands for M.D. Anderson and MB stands for mammalian breast (Figure 5), as well as deoxyribonucleic acid (DNA) cleavage activity [25]. Scanning electron microscope (SEM) images revealed that the presence of variation in ligand and metal complexes surface morphology which could be because of the change of metal ion i.e. broken rock like, and ice block like structure for ligand and copper(II) complex, respectively as well as Cotton-like filamentary morphology for both cobalt(II) and nickel(II) complexes (Figure 5, bottom). Of the transition metal complexes, copper(II) complex exhibited the highest biological activity against all the investigated cell lines confirming that biological activities depends on the type of the central metal ion.

Figure 4.

Crystal structure of the azo-azomethine based ligands, their Cu(II) complexes and their antiproliferative activities where 1 is the azo-aldehyde, 2-4 are the ligands (R = 3-ethylaniline or 3-CH₂CH₃; 4-ethylaniline or 4-CH₂CH₃, and 4-isopropylaniline or 4-CH(CH₃)₂), 5-7 are the complexes and 5-Fu is fluorouracil [24].

Figure 5.

Schematic representation of the metal complexes and their cytotoxicity effect by methyl thiazolyl tetrazolium (MTT) assay of ligand as well as the complexes on breast cancer cells (MDA-MB-231) cells (top) as well as their SEM morphological structure of the ligand (HL) and the corresponding metal complexes (bottom) [25].

Keshavayy and coworkers reported benzothiazole based azo- ligand with indol wing and its transition metal complexes (Cu(II), and Ni(II) with distorted square planar, Co(II), and Fe(III) with octahedral as well as Zn(II) with tetrahedral geometry) for their potential antimicrobial, antitubercular as well as DNA cleavage activities (Figure 6) [26]. Apart from the investigated biological activities, all the formulated compounds demonstrated a promising anti-tubercular activity i.e. against M. tuberculosis (Figure 6, right). Moreover, in their extended work, i.e. benzothiazole based azo- ligand possessing a pyrazole ring and its transition metal complexes (Cu(II) with distorted tetrahedral, Ni(II), and Co(II) with octahedral geometry) for their anti-bacterial, anticancer and DNA cleavage activity (Figure 7) [27]. Apart from the promising anti-bacterial, and anticancer properties, copper(II) and nickel(II) complexes exhibited complete cleavage of all forms of DNA which could be due to the effective coordination role of oxygen and nitrogen, while ligand as well as cobalt(II) complex displayed partial DNA cleavage compared to the control DNA. Moreover, Keshavayy and coworkers also reported another benzothiazole based azo- ligand with pyridone moiety and its transition metal complexes (Cu(II), Ni(II), Co(II), Cd(II), and Zn(II)) for their potential biological activity (Figure 8) [28]. All the synthesized compounds (the azo- ligand and its respective metal complexes) displayed significant DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging potential compared with the standard ascorbic acid as well as good anticancer activity against Jurkat, K562, and A549 cell lines (Figure 8).

Figure 6.

Proposed schematic structures of the metal complexes and the anti-tubercular activity of the ligand and its metal complexes [26].

Figure 7.

The proposed molecular geometries for the metal complexes and their DNA (plasmid pUC18) cleavage activity; M = standard DNA, C = control DNA (untreated pUC 18), 1 = azo- based ligand, and its complexes (1a = Cu(II), 1b = Co(II) and 1c = Ni(II), respectively) (top). The molecular geometries (calculated) of the ligand (L) as well as its complexes (bottom) [27].

Figure 8.

The proposed molecular geometries for the metal complexes and their biological activity [28].

More recently, copper(II) and cobalt(II) complexes of azo- ligand obtained via the diazotization reaction of 2-amino pyridine with pyrazolone and their DNA cleavage and antibacterial (Klebsiella pneumonia, and Bacillus subtills) activity was reported by Channabasappa and coworkers (Figure 9) [29]. The show that Copper(II) and Cobalt(II) complexes exhibited promising DNA Cleavage activity as well as antibacterial against K. pneumonia and B. subtills as compared to the standard ampicillin properties. Moreover, the azo-ligand and its respective metal complexes displayed well-established bonds with the target amino acids (Figure 9, right).

Figure 9.

The proposed molecular geometries for the metal complexes, DNA cleavage of by the Cu(II), and Co(II) complexes at body temperature where lane 1 DNA alone; lane 2 = DNA with 40 μl of copper(II) complex; lane 3 = DNA with 60 μl of copper(II) complex; lane 4 = DNA with 40 μl of cobalt(II) complex; lane 5 = DNA with 60 μl of cobalt(II) complex; form I = supercoiled and form II = nicked circular DNA, respectively as well as zone of inhibition of the azo- ligand and their metal complexes against (A) Klebsiella pneumonia and (B) bacillus subtills; where u-1 = azo- ligand, u-2-1 = Co(II), and u-2-2 = Cu(II) complexes, respectively (left). In silico molecular docking investigations of the azo- ligand as well as its respective metal complexes (right) [29].

More recently, our group also reported an azo- ligands possessing pyridone moiety with long aliphatic chain and their respective nitro and methyl group substituent as well as solvent dependent copper(II) complexes (elongated octahedral geometry) targeting its potential inhibitory against Alzheimer’s disease in comparison with Donepezil drug [21]. Our findings revealed that the binding affinity of the ligand as well as its copper(II) complexes were found with 1EVE ranges from −9.6 to −8.3 kcal/mol while −10.9 kcal/mol is for Donepezil drug suggesting our complexes could be promising drug candidates for Alzheimer’s disease. Moreover, comparing to both complexes, the Donepezil drug and ligands were found functioning as carcinogenic.

Recently, in continuation of our work on azo- ligands owning pyridone moiety, we report a diazenyl pyridinone based heterocyclic ligands and their Cu(II) complexes (octahedral geometry) for their potential antimicrobial, anticancer, and antioxidant properties (Figure 10) [30]. Our results revealed that the complexes showed better antitumor activity as well as selectivity index against the human primary hepatocytes THLE-2 and breast adenocarcinoma cells MCF-7 than the reference doxorubicin. Moreover, the prepared compounds exhibited significant anti-bacterial and anti-fungal properties (Figure 11, right).

Figure 10.

Schematic illustration of the synthetic route of the copper(II) complexes employing the azo- ligand (top); the molecular structure of complexes of CuL1 (CuO₂N₄-octahedron) and CuL2 (CuO₄N₂-octahedron) (middle); as well as (a) simultaneous docking into the binding pocket of the 1EVE under similar conditions for the the superpositions of azo- ligands (L1H and L2H), complexes (CuL1 and CuL2) and donepezil drug (orange color) appeared in each window in comparison to the appearance of each ligand, (b) CuL2 and donepezil; (c) all molecules and donepezil; (b) CuL2 and donepezil; (d) CuL1 and donepezil; (e) L1H and donepezil; and (f) L2H and donepezil [21].

Figure 11.

Schematic illustration and the optimized molecular structure of the copper(II) complexes (left); cell viability results of copper(II) complexes against the human breast adenocarcinoma MCF-7 cells at different (0–100 μM) concentrations, anti-bacterial and anti-fungal activities [30].

More interestingly, the azo- pyridone moiety containing ligands and their copper(II) complexes are becoming biologically promising where Lađarević and coworkers also reported on a similar work by ligand substituent tuning strategy [31]. In this work, both complexes, the chloride substituted (Cu(II)-Cl) and methoxy substituted (Cu(II)-OCH₃) was found to be promising compounds for the treatment of various types of cancer which could be considered as potential therapeutic candidates. Among the two complexes, The most favorable structure exhibiting the best docking potential toward both proteins was the complex Cu(II)-OCH₃ bearing methoxy group. Moreover, the Cu(II)-OCH₃ complexes exhibited the highest antioxidative activity compared with its parent ligand and the standard ascorbic acid antioxidant which could be due to the presence of an electron–donor methoxy group making this complex as a favorable antioxidant agent. This findings revealed that both anticancer and antioxidant activities exhibited substituent dependent biological activity where for the azo- pyridone moiety containing ligands an electron donating methoxy group was found favorable for designing copper(II) bio-active compound (Figure 12).

Figure 12.

Optimized molecular structure of the copper(II) complexes of Cu(II)-Cl (a) and Cu(II)-OCH₃ (b). Best docking poses with interacting residues, as well as intermolecular interactions of CuL-OCH₃complex with VEGFR-2 (c) and Aurora kinase A (d) [31].

It have been witnessed that azo- based ligands (derived from o-vanillin, salicylic, isonicotinic, thiazole, indole, pyrazole, pyridinone, and their derivatives, for details see Section 2.1) and their copper(II) complexes are found to be a favorable candidates for realizing potentially promising bio-active compounds. Such compounds are expected to replace the conventional drags which are suffering from side effect and biological pathogens resistance. Due to the vast depth of the literature with regard to azo- based copper(II) bio-active complexes, we attempt to discuss some selected promising compounds in the above. Therefore, Table 1 below attempts to incorporate some other works which are discussed in the above in detail. Thus, we encourage readers to cross refer the compounds presented in Table 1 for further detailed structural insights as well as to refer recent comprehensive review detailing Azo- based transition metal complexes and their biological activity reported by Younas and coworkers [15].

Chemical formula*	Geometry	Characterization techniques	Biological activity	Refs.
[Cu(LOV-AZO)2]	Square planar	FTIR, UV–Vis, TGA	Antibacterial	[32]
[Cu(L1OV-AZO)].3H2O	Square planar	UV–Vis, FTIR, XRD	Anticancer	[33]
[Cu(L2OV-AZO)Cl].2H2O	Square planar	FTIR, UV–Vis, TGA, EPR, XRD	Antibacterial and Antifungal	[34]
[Cu(LSAL-AZO)].H2O	Square planar (distorted)	UV–Vis, FTIR, MS	Molluscicidl	[35]
[Cu(L1SAL-AZO)H2O]	Square planar (distorted)	FTIR, UV–Vis, MS, ESR, XRD	Antibacterial and Antifungal	[36]
[Cu(L2SAL-AZO) H2O](OAc)2	Tetrahedral/Square planar	FTIR, UV–Vis, TGA	Antibacterial	[37]
[Cu(L3SAL-AZO)2(H2O)2]	Octahedral	UV–Vis, FTIR, ESI-MS	Antibacterial and Antifungal	[38]
[Cu(LTSC-AZO)H2O]		FTIR, UV–Vis, TGA	Antibacterial	[39]
[Cu(LPYRI-AZO)DMF]	Square-pyramidal	ICP-OES, UV–Vis, FTIR, TGA, Single crystal X-ray	Antioxidant, and Anticancer	[31]
[Cu(LPYRA-AZO)].nH2O	Octahedral	UV-Visible, FT-IR, NMR, MS, XRD	DNA cleavage and Antibacterial	[29]
[Cu(LTHAI-AZO)]	Octahedral	FTIR, UV–Vis, TGA, ESR, XRD, MS, EDAX, SEM	Anticancer	[28]
[Cu(LINDO-AZO)]	Square planar (distorted)	FTIR, UV–Vis, MS, ESR, XRD	DNA cleavage and Antibacterial	[26]
[Cu(LNICO-AZO)].3H2O	Octahedral	FTIR, UV–Vis, MS, ESR, XRD	DNA cleavage and Cytotoxicity	[25]

Table 1.

Copper(II) complexes of azo- based ligands and their biological activities.

*

The abbreviated expressions in subscript position; OV, SAL, TSC, PYRI, PYRA, THAI, INDO, and NICO stands for o-vanillin, salicylic, thiosemicarbazide, pyridinone, pyrazole, thiazole, indole, nicotinic, and their derivatives respectively, for more structural details of the ligands, please refer the original article from the provided references.

2.2 Phenyl hydrazine- and hydrazone-based bio-active copper(II) complexes

Hydrozones are very essential intermediates for the synthesis of different heterocyclic compounds. Acid hydrazides as well as their derivatives are also another class of hydrazine based compounds which useful chemical synthons for synthesis of numerous heterocyclic compounds that with pronounced biological as well as pharmacological applications [40]. Apart from biological applications, Brady and Elsmie reported that, of the nitro-substituted phenylhydrazine, dinitrophenylhydrazine is a reddish yellow compound which has been used as an analytical reagent for the identification of carbonyl functional groups quantitatively [41] e.g. in biological samples [42]. This confirms that phenylhydrazine and their derivatives are good nucleophile for carbonyl ketones as well as aldehydes in the formation multi-functional hydrazone based ligands (Figure 13) and their metal complexes for different applications such as biological activity [16, 17]. Currently, hydrazone ligands possessing oxygen and nitrogen are under extensive exploration due to their ease of synthesis, chelating ability, potential biological as well as pharmacological applications [19, 40]. Amid the different metal ions, the divalent Cu(II), Ni(II), Co(II), and Zn(II) are particularly favored as they play a dynamic role in biological activities [43, 44]. Due to the vast reports available in the literature, below only some selected phenyl hydrazine as well as hydrazone based copper(II) exhibiting potentially better biological activity are discussed in detail.

Figure 13.

Multi-functionality of hydrazone (red color) and its general synthesis route [17].

Idemudia and coworkers reported air-stable keto imine form of acylpyrazolone phenylhydrazones based ligands prepared by condensation of 4-acetyl and 4-benzoyl pyrazolone with dinitrophenylhydrazine as well as phenylhydrazine namely; benzoylphenylhydrazone (Bmpp-Ph), acetyldinitrophenylhydrazone (Ampp-Dh), as well as benzoyldinitrophenylhydrazone (Bmpp-Dh) and their transition metal complexes (Cu(II), Mn(II), Co(II), and Ni(II)) (Figure 14) [43]. All the complexes are under octahedral geometry and exhibited better biological activity than their respective ligands. Moreover in their extended work on salicylic hydrazide based transition metal complexes (Cu(II), Ni(II), Co(II), and Zn(II)) in a 1:1 molar ratio and their biological activities, where both ligand substituent and central metal atom dependent biological activity in which the efficacy of the metal complexes (Cu(II) > Zn(II) > Co(II) > Ni(II)) was higher than to that of the ligands (Figure 15) [44].

Figure 14.

Proposed schematic structural representation of the metal complexes and crystal structure of acetyldinitrophenylhydrazone (Ampp-Dh) (top). Antioxidant activities of phenylhydrazones-based ligands and their respective metal complexes (bottom).

Figure 15.

Structure (proposed) and activity relationship of the complexes (top). Graphical illustration of IC₅₀ value of all the compounds (bottom).

Raman and coworkers reported an isatin-based hydrazone ligand prepared by condensation of 2,3,5-trichlorobenzaldehyde with isatin monohydrazone and their transition metal complexes (Cu(II), Ni(II), Co(II), and Zn(II)) in a 1:2 molar ratio for their potential biological activity (Figure 16) [45]. The results showed that all complexes exhibited effective oxidative DNA cleavage as well as promising antimicrobial activity where particularly copper(II) and zinc(II) complexes showed higher antimicrobial activity than the nickel(II) and cobalt(II) complexes.

Figure 16.

Proposed structure and SC pBR322 DNA (0.3 μg) cleavage of by metal(II) complexes (0.30 mmol L⁻¹) in the presence of H₂O₂ (100 μmol L⁻¹) at pH 7.2. DNA control; DNA and H₂O₂; DNA + [CuL₂Cl₂]; DNA + ethanol (4 μL); are lanes 1–4 respectively; DNA + H₂O₂ + [CoL₂Cl₂], [CuL₂Cl₂], [ZnL₂Cl₂], and [NiL₂Cl₂], are lanes 5–8, respectively; DNA + H₂O₂ + ethanol + [CuL₂Cl₂]; DNA + H₂O₂ + SOD + [CuL₂Cl₂] are lane 9 and lane 10 respectively (top). Antibacterial and antifungal activities of ligands, complexes and control drugs (bottom) [45].

Lodyga-Chruscinska and coworkers reported a good chelating hesperetin based hydrazine ligand (HHSB) and its copper(II) complex exhibiting promising cytotoxic activity against cancer cells (normal (LLC-PK1) cell lines and tumor (HeLa and Caco-2)) [46, 47]. Moreover, further coordinating atom (changing O₂N to ONS) and substituent changes using thiosemicarbazide (TSC) also exhibited similar cytotoxic activity confirming the versatility of the hesperetin based hydrazine ligand for chemical substituent modification (Figure 17).

Figure 17.

Chelating ability of hesperetin based hydrazine ligand (HHSB) and its copper(II) complex proposed structure (top) [46]. Crystal structure of the hesperetin based hydrazine ligand with hydrogen bonding [46] and DNA damage of 4′,6-diamidino-2-phenylindole (DAPI) stained comets in the HeLa cells: (A) 50 μM cisplatin; (B) negative control; (C) 50 μM HHSB in Caco-2 cells; (D) 10 μM CuHHSB in LLC-PK1 cells; (E) 50 μM CuHHSB, and (F) 50 μM HHSB (bottom) [47].

Gatto and coworkers described pyridoxal-thiosemicarbazone and pyridoxal-S-allyldithiocarbazate hydrazone hydrazone ligands bearing ONS-donor atoms and their copper(II) complexes demonstrating promising potential anticancer activity (Figure 18) [48]. Interestingly, copper(II) complexes cytotoxic tests showed effective growth inhibition of both S-180 as well as Ehrlich tumor cells where enhancement of the cytotoxicity was also observed up on complexation of the ligands with copper(II) metal ions. Moreover, copper(II) complex (compound 5) demonstrated the death of the tumor cells confirming that pyridoxal derivative copper(II) complexes could be considered as great potential candidates for designing antitumor drugs which could be due to the presence of thiosemicarbazide pendant.

Figure 18.

Effect of each copper (II) complex (1–5 with coordinated atoms color code: Blue N, red O, gold S, green Cl, maroon Br and cyan Cu) on the viability of the murine S-180 and Ehrlich cells treated with different concentrations with statistical p-values (*p < 0.05; **p < 0.01; ***p < 0.001) (bottom) [48].

Naik and coworkers also reported bi-dentate indol based hydrazone ligand prepared by condensation of nicotinic acid hydrazide and indole-3-carboxaldehyde and their transition metal complexes (Cu(II), Co(II), Mn(II), and Ni(II)) under octahedral geometry for their potential biological activity (Figure 19) [49]. Of the transition metal complexes, copper(II) and cobalt (II) complexes exhibited better antibacterial as well as effective oxidative DNA cleavage activity. It is worthy to mention the importance of indol and nicotinic based hydrazone based ligands and their complexes suitability for designing bio-active compounds. Moreover, a similar trend where copper(II) and cobalt (II) complexes exhibited better antibacterial as well as effective DNA cleavage activity reported by Velladurai and coworkers using triazine based tridentate ligand prepared from 1,2,4-triazine and 1,4-Naphthoquinon pendants Figure 20 [50].

Figure 19.

Proposed structure of copper(II) complexes and percentage LDL oxidation inhibition by ligand and their metal complexes (4a–d) at 10 μM concentration (top). Cleavage of DNA (supercoiled pBR322 of 0.5 μg) by the Co(II), Ni(II) and Cu(III) complexes in a buffer at 37°C where DNA alone; DNA + 40 μl of cobalt complex + H₂O₂; DNA + 40 μl of Co(II); DNA + 60 μl of Co(II); DNA+ 40 μl of Ni(II); DNA + 60 μl of Ni(II); DNA + 40 μl of Cu(II); DNA + 60 μl of Cu(II) are lane 1–8 respectively while forms I-II are supercoiled as well as nicked circular DNA, respectively [49].

Figure 20.

Proposed structure of transition metal complexes and their antibacterial activity [50].

Adimule and coworkers also reported benzimidazol based hydrazone bi-dentate ligand bearing NS donor atoms and its transition metal complexes (Cu(II), Ni(II), and Co(II)) for their DNA binding and antibacterial activity (Figure 21) [51]. In addition to antibacterial activity of all the complexes, among the transition metal complexes, copper (II) complex showed that an intercalative DNA binding and DNA cleavage in presence as well as absence of oxidant H₂O₂. was also observed.

Figure 21.

Benzimidazol based hydrazone ligand and its proposed structure of transition metal complexes with inset: DNA binding where DNA control; DNA + H₂O₂; ligand + DNA; ligand + DNA + H₂O₂; Ni(II) + DNA; Ni(II) + DNA+ H₂O₂; Co(II) + DNA; Co(II) + DNA + H₂O₂; Cu(II) + DNA; Cu(II) + DNA+ H₂O₂ are lane 1-10 respectively (top). Antibacterial activity of transition metal(II) complexes with the control ciprofloxacin drug (a); and their Bar diagram representation of their antibacterial activity including the ligand (b) [51].

Moreover, a mixed-ligand copper(II) complexes of hydrazone-based ligands displaying efficient biological activities were also reported by Annaraj and coworkers where the vital role of co-ligands on cytotoxicity and DNA/protein interactions has been described in detail (Figure 22) [52]. The results of this work showed that the copper(II) complexes showed multi-functionality including anti-inflammatory, free-radical scavenging, antibacterial and DNA/protein interaction properties. More recently, Guo and coworkers also reported similar strategy, i.e. mixed-ligand hydrazone based copper(II) complexes and examined their potential anti-lung cancer activity (Figure 23) [53]. Results of this study revealed that the investigated mixed-ligand copper(II) complexes especially the complex with furan pendant was found playing a good role in inducing ROS-mediated mitochondrial pathway regulated A549 cancer cell apoptosis. In both mixed ligand copper(II), the observed multi-functionality could be due to the parent complex as well as the synergistic combination of the parent hydrazone ligand as well as the co-ligands which was also observed in other works [54].

Figure 22.

Schematic representation of copper(II) complexes (1–4), the hydrazone ligand as well as the co-ligands (a). DNA cleavage (supercoiled pUC18 DNA) by copper(II) complexes 1–4 with buffer of pH = 7.14 at 37°C in the presence of ascorbic acid (ASC) as a reducing agent, where DNA control; DNA + ASC; lane 3: DNA + ASC + complex 1 (50 μM); DNA + ASC + complex 2 (200 μM); DNA + ASC + complex 3 (150 μM); DNA + ASC + complex 4 (300 μM) are lane 1–6, respectively (b). Antibacterial activity of complex 1 observed under the fluorescence microscope as well as stained using the live/dead BacLight kit against Pseudomonas aeruginosa and Staphylococcus aureus [where (a) and (c) are control and (b) and (d) are treated] (c). Hoechst 33342/PI staining of AGS human gastric cells exhibiting changes in the nuclear morphology after treating with the complexes 1–4 at concentration of 25 μM (d) [52].

Figure 23.

Single crystal structure of the copper(II) complex and its schematic illustration of ROS-mediated mitochondrial pathway regulated A549 cancer cell apoptosis; (top) the 3D morphological changes on the A549 cells treated with the complex for 6 days with scale bar: 200 μm (middle) and wound healing results exhibited by the complex through inhibition of the A549 cells migration where pictures with scale bar: 200 μm taken at 0 h as well as 24 h, respectively (bottom) [53].

In general it have been demonstrated that hydrazone based copper (II) complexes have been extensively explored for their biological activities. Therefore, due to the vast literature report on hydrazone-based copper(II) complexes, only selected representative works are discussed in detail. From the above seminal representative works discussed, it is interesting to note that similar to the azo- based copper(II) complexes, hydrazone-based ligands processing indol, pyrazole, nicotinic, salicylic, isatin, thiozole, quinoline and pyridoxal moieties are relatively promising for designing copper(II) complexes with pronounced biological activity (Table 2).

Chemical formula*	Geometry	Characterization techniques	Biological activity	Refs.
[Cu(LNICO)2]	Octahedral	FTIR, UV–Vis, TGA, MS, XRD, SEM	Antibacterial	[55]
[Cu(LINDO)(Cl)(H2O)2]	Octahedral	FTIR, UV–Vis, TGA, MS, XRD, SEM, ESR	Antibacterial; Antifungal and Antioxidant	[56]
[Cu(LSAL)2]	Octahedral	FTIR, UV–Vis, MS	Antibacterial; Antifungal and Antioxidant	[57]
[Cu(L2QUIN)]	Octahedral	FTIR, UV–Vis, MS, single crystal XRD	Anticancer	[58]
[CuLPYRA(phen)(CH3OH)][CuLPYRA(phen)]·CH3CH2OH·CH3OH	Octahedral and square pyramidal	FTIR, UV–Vis, MS, single crystal XRD	Anticancer	[59]
[Cu(LISAT)2Cl2]	Octahedral	FTIR, UV–Vis, TGA, MS, EPR	Antibacterial; Antifungal and DNA cleavage	[45]

Table 2.

Copper(II) complexes of hydrazine-based ligands and their biological activities.

*

The abbreviated expressions in subscript position; NICO, SAL, INDO, QUI, PYRA, and stands for nicotinic, salicylic, indole, quinoline, pyrazole, isatin, and their derivatives respectively, for more structural details of the ligands, please refer the original article from the provided references.

3. Conclusions and future outlooks

The presented chapter provides an over view of the up-to-date advances and perspectives of azo- and hydrazine-based copper(II) complexes and their biological activities. These complexes have been proved to be indispensable class of bio-active compounds with broad spectrum of biological activities such as anticancer, antibacterial, antifungal, DNA cleavage, etc. Moreover, azo- and hydrazine-based ligands bearing salicylic, thiosemicarbazide, pyridinone, pyrazole, thiazole, indole, isatin, quinoline, nicotinic, and their derivatives are found to be promising to design bio-active copper(II) compounds. Therefore, future research works are expected to explore on novel azo- and hydrazine-based copper(II) complexes owing broad spectrum biological activities accompanied with less toxicity. Biologically active starting materials such as hesperetin, curcumin and the like are recommended to rationally design the chelating azo- and hydrazine-based ligands. Additionally, chromophore tagged azo- and hydrazine-based ligands are also expected to expand the multifunctionality of their respective copper(II) bio-active complexes. Finally, the prospect of copper(II) complexes of azo- and hydrazine-based bio-active copper(II) complexes is a bright future which are expected to replace the conventional drugs which are suffering from side effects as well as pathogenic resistance. However, a collaborative work between Chemists, Biochemists as well as molecular Biologists is still needed to realize the promising biological activities of such compounds to the commercialization level. Although the research works undergoing in academia are encouraging, a collaborative work from multidisciplinary experts is a must to realize the industrialization of such auspicious compounds.