Synthesis, electrochemical, fluorescence and antimicrobial studies of 2-chloro-3-amino-1,4-naphthoquinone bearing mononuclear transition metal dithiocarbamate complexes [M{κ²S,S-S₂C–piperazine–C₂H₄N(H)ClNQ}_n]

Abstract

A novel multifunctional ligand precursor 2-chloro-3-{2-(piperazinyl)ethyl}-amino-1,4-naphthoquinone (L) was synthesized by chemo-selective reaction of 2,3-dichloro-1,4-naphthoquinone and 2-(piperazin-1-yl)ethanamine and characterized, prior to use. A series of mononuclear transition metal dithiocarbamate complexes [M{κ²S,S-S₂C–piperazine–C₂H₄N(H)ClNQ}_n] {M = Mn(III) 1, Co(III) 2, Ni(II) 3, Cu(II) 4, Zn(II) 5; ClNQ = 2-chloro-1,4-naphthoquinone; n = 3 for 1–2 and n = 2 for 3–5} bearing pendant 2-chloro-3-amino-1,4-naphthoquinone groups were efficiently synthesized through a self-assembly process involving L, CS₂ and corresponding metal acetates. 1–5 were characterized by microanalysis and standard spectroscopic methods such as ESI-MS, IR, ¹H and ¹³C NMR and UV-visible absorption spectroscopy. Significantly, all compounds exhibit medium to very strong fluorescence emission bands in the visible region with concomitant Stokes shifts of ≈280 nm upon excitation at their respective λ_max values. Thermal stability of complexes 1–5 has been investigated by thermogravimetric analysis. Cyclic voltammetric study clearly reveals that L and 1–5 are electro active with respect to the dominant redox-active naphthoquinone moiety. Further, L and 1–5 were tested against six important pathogen microorganisms viz. S. aureus, B. subtilis, E. coli, P. aeruginosa, C. albicans and A. niger by using the Broth dilution method. A broad spectrum of the strongest antimicrobial activity was determined for the copper(II) complex 4 and L. The enhanced antibacterial activity of L and copper(II) complex 4 against S. aureus apparently reveals them to be a superior antibacterial agent than the reference drug ciprofloxacin.

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1. Introduction

Over the decades, transition metal complexes derived from sulphur rich ligands have received a great deal of attention because of their conducting properties, molecular magnetism, electrochemical, optoelectronic properties and biological applications.^1–8 In particular, dithiocarbamates have emerged as versatile ligands to construct a huge variety of inorganic–organic complex materials with fascinating physico-chemical properties. The ease of synthesis and ability to bind to a wide range of elements in different oxidation states^9–11 add further merit to this ligand system. The dithiocarbamate group is considered as a versatile pharmacophore since the discovery of the first dithiocarbamate derivative i.e. tetramethylthiuram disulfide (thiram),¹² which has prominent fungicidal properties and hence, it is used in compounds of biological interest.¹³ Thermogravimetric studies have been utilized to demonstrate the widespread industrial applications of dithiocarbamate compounds such as foam rubber, fungicides, effective heat stabilizers, suitability as a single source precursor for the preparation of metal sulphide nano-particles, antioxidant action, reprocessing of polymers etc.¹⁴

On the other hand, considerable attention has been paid to the functionalization of naphthoquinone derivatives and evaluation of their biological properties, mainly due to their association with multiple biological oxidative processes.¹⁵ Literature reports suggest that naphthoquinone derivatives possess a wide range of biological properties that include antibacterial and antifungal, antiviral, antimalarial and anticancer activity which have stimulated the study of these bioactive compounds in the field of medicinal chemistry.¹⁶ However, we could not find even a single report on the dithiocarbamate derivatives of naphthoquinone and their transition metal complexes. Although, an enormous number of dithiocarbamate complexes containing simple alkyl/aryl substituents have been synthesized and studied for their various applications, however, functionalization of the ligand backbone is still in its early stage.¹⁷

In the light of these observations, we have designed and synthesized a new secondary amine precursor 2-chloro-3-{2-(piperazinyl)ethyl}-amino-1,4-naphthoquinone (L) which is successfully used to deliver a series of 2-chloro-3-amino-1,4-naphthoquinone functionalized mononuclear transition metal dithiocarbamate complexes 1–5 (Scheme 1). All the compounds were thoroughly characterized by microanalysis and standard spectroscopic, optical, thermogravimetric and cyclic voltammetric methods. Further, L and 1–5 were screened against a panel of microbes such as S. aureus, B. subtilis, E. coli, P. aeruginosa, C. albicans and A. niger. Undoubtedly, the cumulative effect of various pharmacophores present in these compounds resulted into enhanced biological properties.

Scheme 1 Preparation of monometallic dithiocarbamate complexes.

2. Experimental section

2.1 Materials and measurement

All solvents were purchased from the commercial sources and were freshly distilled prior to use. All the reagents such as metal acetates Mn(OAc)₂·4H₂O, Co(OAc)₂·4H₂O, Ni(OAc)₂·4H₂O, Cu(OAc)₂·H₂O, Zn(OAc)₂·2H₂O and CS₂ were purchased from Merck and Sigma-Aldrich Chemicals Limited and these were used without further purification. Elemental analyses (C, H, N) were carried out on a Perkin-Elmer 2400 analyzers. Thin layer chromatography was performed on Merck 60 F254 aluminium coated plates. Magnetic moments were done by Faraday Valance-2002 (1.0 tesla), balance-Mettler UMx5, temperature controller OMEGA. FT-IR spectra were recorded in the 4000–400 cm⁻¹ range using a Perkin-Elmer FT-IR spectrometer as KBr pellets. The ¹H and ¹³C NMR spectra of relevant compounds were obtained on a Bruker AV-III 400 MHz spectrometer in spectrometer with CDCl₃ and DMSO-d6 as solvent and TMS as internal standard. Mass spectrum of L was recorded on Thermo-Fisher DSQ II GCMS instrument while ESI MS were obtained from AB SCIEX, 3200 Q TRAP LC/MS/MS system. UV-visible spectra were recorded on a Perkin-Elmer Lambda 35 UV-visible spectrophotometer and the optical characterization of solid samples was performed by using the UV-visible transmittance measurements. Fluorescence was recorded on JASCO make spectrofluorometer model FP-6300. TGA/DTA plots were obtained using SII TG/DTA 6300 in flowing N₂ with a heating rate of 10 °C min⁻¹. The antimicrobial activity was performed by Division of Central n kashiba, Advanced diagnostic laboratory, Surat, India.

2.2 Synthesis of 2-chloro-3-{2-(piperazinyl)ethyl}-amino-1,4-naphthoquinone (L)

2,3-Dichloro-1,4-naphthoquinone (DClNQ) (1.0 g, 4.404 mmol) and 2-(piperazin-1-yl)ethanamine (0.568 ml, 4.404 mmol) were dissolved in 30 ml of ethanol and the reaction mixture was allowed to stir for 6 hours at room temperature. A change in colour from yellow to red of reaction mixture was observed. The solvent was removed under rotatory evaporator and the red colour residue was washed several times with a saturated solution of Na₂CO₃, followed by distilled water. Finally the residue was dried under vacuum and kept in desiccators over calcium chloride for data collection. ¹H NMR (400 MHz, CDCl₃): δ 8.09–8.07 (dd, 1H, Ph), 7.97–7.95 (dd, 1H, Ph), 7.68–7.64 (td, 1H, Ph), 7.58–7.545 (td, 1H, Ph), 6.74 (s, 1H, N–H), 3.91–3.87 (m, 2H, CH₂), 3.06 (t, 2H, CH₂), 2.66–2.62 (m, 8H, CH₂), 2.51 (s, 1H, N–H). ¹³C NMR (400 MHz, CDCl₃): δ 180.66 (C=O), 175.71 (C=O), 148.05 (C–N), 134.82, 134.72, 132.76, 132.34, 131.97, 126.74, 126.69, 126.24, 126.18, 56.81, 55.66, 53.93, 53.68, 46.10, 45.99, 40.83, 38.51; ¹³C DEPT 135 NMR (400 MHz, CDCl₃): δ 134.91, 132.46, 126.73, 126.69, 56.37, 52.99, 52.29, 51.80, 45.67, 44.62, 40.85, 40.74, 39.99. Full MS (m/z): 319.32 [M]⁺, calcd mass: 319.1 [M]⁺.

2.3 Synthesis of triethylammonium dtc salt of 2-chloro-3-{2-(piperazinyl)ethyl}-amino-1,4-naphthoquinone

CS₂ (0.114 g, 1.5 mmol) was added in excess to a solution of ligand precursor 2-chloro-3-{2-(piperazinyl)ethyl}-amino-1,4-naphthoquinone (L) (0.319 g, 1.0 mmol) in 20 ml of triethyl amine. The reaction mixture was stirred at room temperature for 1 h and then solvent was evaporated by using rotatory evaporator. The residue was washed several times with petroleum ether and finally dried under high vacuum. The solid product was stored at 20 °C under nitrogen atmosphere and the data were collected. ¹H NMR (400 MHz, CDCl₃): δ 8.17–8.15 (dd, 1H, Ph), 8.05–8.03 (dd, 1H, Ph), 7.76–7.71 (td, 1H, Ph), 7.66–7.64 (td, 1H, Ph), 6.95 (s, 1H, N–H), 4.53 (m, 2H, CH₂), 4.01 (t, 2H, CH₂), 3.21–3.09 (m, 8H, CH₂), 2.75–2.58 (q, 6H, CH₂), 1.39 (t, 9H, CH₂). ¹³C NMR (400 MHz, CDCl₃): δ 193.69 (C=S), 180.58 (C=O), 176.74 (C=O), 148.65 (C–N), 134.96, 134.81, 132.73, 132.65, 132.52, 132.36, 129.87, 126.73, 56.77, 56.19, 56.05, 53.45, 52.37, 50.26, 45.90, 41.05, 40.88.

2.4 Synthesis of metal complexes 1–5

An excess amount of CS₂ (0.114 g, 1.5 mmol) was added in three portions over a period of 30 minutes to a solution of 2-chloro-3-{2-(piperazinyl)ethyl}-amino-1,4-naphthoquinone (L) (0.319 g, 1.0 mmol) in 20 ml of triethyl amine. The reaction mixture was stirred rigorously for 1 h at room temperature. During this time, a change in colour from red to pale yellow was observed, indicating the formation of 2-chloro-3-{2-(piperazinyl)ethyl}-amino-1,4-naphthoquinone dithiocarbamate ligand. This ligand was allowed to react in situ with Mn(OAc)₂·4H₂O (0.081 g, 0.33 mmol), Co(OAc)₂·4H₂O (0.084 g, 0.33 mmol), Ni(OAc)₂·4H₂O (0.124 g, 0.5 mmol), Cu(OAc)₂·H₂O (0.101 g, 0.5 mmol) or Zn(OAc)₂·2H₂O (0.110 g, 0.5 mmol) metal salts over a period of 7 h. Thereafter, the residue was filtered, washed several times with distilled water followed by petroleum ether. The residue was finally dried under high vacuum and the product was stored at 20 °C under nitrogen atmosphere for data collection.

1. Light pink solid (0.380 g, 92% yield); m.p. >325 °C (decomposes); IR (KBr): ν = 3265 (N–H), 2940 (C–H), 1676 (C=O), 1427 (C=N), 1010 (C–S) cm⁻¹; elemental analysis calcd (%) for C₅₁H₅₁Cl₃N₉O₆S₆Mn (M. wt. 1239.6): C 49.41, H 4.15, N 10.17, S 15.52; found: C 49.64, H 4.09, N 10.05, S 15.94. LC MS: 1240.4 (M + H).

2. Brown solid (0.352 g, 85% yield); m.p. 270 °C (decomposes); ¹H NMR (400 MHz, CDCl₃): δ 8.13–8.03 (d, 6H, Ph), 7.70 (s, 6H, Ph), 6.80 (s, 3H, N–H), 3.95–3.71 (d, 20H, CH₂), 2.60 (d, 16H, CH₂). IR (KBr): ν = 3324 (N–H), 2933 (C–H), 1675 (C=O), 1490 (C=N), 1007 (C–S) cm⁻¹; elemental analysis calcd (%) for C₅₁H₅₁Cl₃N₉O₆S₆Co (M. wt. 1243.6): C 49.25, H 4.13, N 10.14, S 15.47; found: C 49.34, H 4.05, N 10.02, S 15.41. LC MS: 1244.8 (M + H). NMR could not be recorded due to poor solubility in DMSO even after acidification.

3. Dark brown solid (0.314 g, 74% yield); m.p. 315 °C (decomposes); ¹H NMR (400 MHz, CDCl₃): δ 8.18–8.16 (d, 2H, Ph), 8.05–8.03 (d, 2H, Ph), 7.75–7.73 (dd, 2H, Ph), 7.65–7.64 (dd, 2H, Ph), 6.79 (s, 2H, N–H), 4.00 (d, 2H, CH₂), 3.86 (d, 4H, CH₂), 3.62–3.60 (d, 6H, CH₂), 3.14–3.12 (d, 6H, CH₂), 2.75 (d, 2H, CH₂), 2.60 (d, 4H, CH₂). IR (KBr): ν = 3328 (N–H), 2911 (C–H), 1676 (C=O), 1440 (C=N), 1006 (C–S) cm⁻¹; elemental analysis calcd (%) for C₃₄H₃₄Cl₂N₆O₄S₄Ni (M. wt. 848.5): C 48.13, H 4.04, N 9.90, S 15.12; found: C 48.34, H 4.05, N 9.85, S 15.21. ESI-MS: 849.3 (M + H). ¹³C NMR could not be recorded due to poor solubility in DMSO even after acidification.

4. Dark brown solid (0.341 g, 80% yield); m.p. 245 °C (decomposes); IR (KBr): ν = 3331 (N–H), 2928 (C–H), 1676 (C=O), 1498 (C=N), 1007 (C–S) cm⁻¹; elemental analysis calcd (%) for C₃₄H₃₄Cl₂N₆O₄S₄Cu (M. wt. 853.4): C 47.85, H 4.02, N 9.85, S 15.03; found: C 47.98, H 4.08, N 10.05, S 15.18. LC MS: 854.0 (M + H).

5. Reddish yellow solid (0.380 g, 89% yield); m.p. 216 °C (decomposes); ¹H NMR (400 MHz, CDCl₃): δ 8.18–8.16 (dd, 2H, Ph), 8.06–8.04 (dd, 2H, Ph), 7.77–7.74 (td, 2H, Ph), 7.68–7.66 (td, 2H, Ph), 6.78 (s, 2H, N–H), 4.18 (m, 8H, CH₂), 4.02 (t, 4H, CH₂), 2.78–2.66 (m, 12H, CH₂). ¹³C NMR (400 MHz, CDCl₃): δ 203.08 (C=S), 180.85 (C=O), 176.77 (C=O), 155.19 (C–N) 147.84 (C–N), 144.50 (C–N), 134.97, 134.81, 132.66, 132.53, 129.84, 126.84, 126.7, 56.02, 54.73, 52.00, 51.81, 51.10, 40.89; ¹³C DEPT 135 NMR (400 MHz, CDCl₃): δ 134.97, 132.56, 126.84, 126.60, 56.02, 51.82, 51.10, 51.00, 40.90. IR (KBr): ν = 3313 (N–H), 2966 (C–H), 1677 (C=O), 1434 (C=N), 1008 (C–S) cm⁻¹; elemental analysis calcd (%) for C₃₄H₃₄Cl₂N₆O₄S₄Zn (M. wt. 855.2): C 47.75, H 4.01, N 9.83, S 15.00; found: C 47.95, H 4.03, N 10.01, S 15.01. ESI-MS: 856.1 (M + H).

3. Result and discussion

3.1 Synthesis

2-Chloro-3-{2-(piperazinyl)ethyl}-amino-1,4-naphthoquinone (L) was synthesized by nucleophilic substitution reaction of 2,3-dichloro-1,4-naphthoquinone with 2-(piperazin-1-yl)ethanamine and characterized. One pot three-component reaction of L, CS₂ and M(OAc)₂ in Et₃N solvent at room temperature affords access to a new series of mononuclear transition metal dithiocarbamate complexes [M{κ²S,S-S₂C–piperazine–C₂H₄N(H)ClNQ}_n] M = Mn(III) 1, Co(III) 2, Ni(II) 3, Cu(II) 4, Zn(II) 5; ClNQ = 2-chloro-1,4-naphthoquinone; n = 2 for 3–5 and n = 3 for 1–2} bearing pendant 2-chloro-3-amino-1,4-naphthoquinone groups (Scheme 1). The required dithiocarbamate derivative of L was synthesized in situ. Complexes 1–5 display moderate to poor solubility in common organic solvents however, there are quite soluble in DMSO/DMF, solvents of high nucleophilicity. These are found to be air-stable in solid state and in the solution over a period of days. Our attempts to grow single crystals suitable for single crystal X-ray diffraction study were unsuccessful.

It has been experienced by us¹⁸ and Professor Hogarth's coworkers¹⁹ that transition metal dithiocarbamate complexes with functionalised backbones are easy to synthesize but their crystallization can be more problematic. L and 1–5 were characterized by microanalysis, standard spectroscopy viz. ESI MS, FT-IR, ¹H and ¹³C NMR, UV-visible absorption, emission and by thermogravimetric methods. The ESI-MS/LC-MS of the complexes 1–5 (ESI†) gave [M + H]⁺ molecular ion peak at m/z 1240.4, 1244.8, 849.3, 854.0 and 856.1, respectively.

3.2 IR spectral study

The IR spectrum of L displays characteristic ν(N–H) bands at 3320 and 3252 cm⁻¹due to presence of naphthoquinone-conjugated amine and piperazinyl amine groups, respectively. While a strong band at 1677 cm⁻¹ arises due to ν(C=O) stretch of naphthoquinone moiety. The IR spectra of 1–5 clearly showed the disappearance of one of the ν(N–H) band associated with piperazinyl amine and the appearance of two new single sharp medium intensity bands in 1427–1498 cm⁻¹ and 1006–1010 cm⁻¹ regions due to ν(N–CS₂) and ν(C–S) stretching vibrations. These are two regions of particular interest in the IR spectral study of dithiocarbamates complexes and confirm the bidentate coordination of dithiocarbamate ligands.²⁰ The observed ν(N–CS₂) stretchings for 1–5 are indeed intermediates of C=N double bond and C–N single bond stretchings, arises due to the mesomeric drift of the electron cloud of the dithiocarbamate (–NCS₂) moiety towards the metal ion and confirmed an increase in the C–N double bond character. Notably, the naphthoquinone-conjugated amine band ν(N–H) along with ν(C=O) and ν(C–Cl) bands for complexes 1–5 remain present almost at the same positions viz. 3320, 1677 and 722 cm⁻¹ respectively, compared to their positions in L (Fig. 1). This gives a strong evidence for the presence of pendent 2-chloro-3-amino-1,4-naphthoquinone groups in 1–5 which is further confirmed by NMR study.

Fig. 1 IR spectrum of 2-chloro-3-{2-(piperazinyl)ethyl}-amino-1,4-naphthoquinone (L) and its dithiocarbamate complexes 1–5.

3.3 NMR spectral study

The ¹H NMR spectrum of precursor L recorded in CDCl₃ display most characteristic signals at δ 2.51 ppm and 6.74 ppm due to piperazinyl and naphthoquinone-conjugated –NH protons, whereas signals appeared in 3.91–2.62 ppm and 8.09–7.54 ppm regions are attributed to the aliphatic methylene and aromatic protons, respectively. The ¹³C NMR spectrum of L in CDCl₃ displays characteristic signals due to ketonic groups (>C=O) of naphthoquinone at 180.66, 176.71 ppm along with expected signals in aromatic/aliphatic regions. For comparison purpose, in situ prepared triethylammonium salt of a dithiocarbamate derivative of L was also characterized by ¹H and ¹³C NMR spectra (ESI†). The ¹³C NMR spectrum displays most characteristic signal at 193.69 ppm due to –NCS₂ moiety and confirms the formation of dithiocarbamate derivative of L in situ. In the ¹H NMR spectra of complexes 3 and 5, characteristic piperazinyl –NH signal is disappeared and a significant downfield shifting of aromatic/aliphatic signals was observed when compared to their respective positions in L (ESI†). The ¹³C NMR spectrum of 5 displays a very downfield signal at ∼203 ppm, a characteristic feature of coordinated dithiocarbamate moiety derived from secondary amines.²¹ Notably, the signals due to ketonic groups (>C=O) of naphthoquinone appear almost at same positions viz. δ = 180.85 and δ = 176.77 ppm when compared to their positions in L. This confirms the non-involvement of ketonic groups in complexation. The assignments of ¹H and ¹³C signals are supported by DEPT 135 NMR spectral study, which clearly distinguished the aromatic carbons (appear in the positive region) from aliphatic methylene carbons (appear in negative region). The position and features of the NMR signals confirm the purity and composition of the complexes and also reveal that no ligand-exchange reaction occurs in solution.

3.4 Electronic absorption spectral and magnetic moment studies

The electronic emission spectral data of L and its mononuclear dithiocarbamate complexes 1–5 were investigated at room temperature from 10⁻⁵ M DMSO solution samples and the pertinent results are summarized in Table 1. The assignment of UV-visible absorption bands are based on the literature reports on closely related compounds^22–24 The UV-visible absorption spectra of L and 1–5 displayed expected benzene and naphthoquinone bands in 275–277 nm and 324–340 nm regions mainly arises due to π → π* electronic transitions.²² In addition, a low energy broad band is observed in the visible regions between 443–468 nm, assignable to CT transitions and weak n → π* transitions of the carbonyl group of the quinone.²⁵ Notably, this broad band in the visible region for all the complexes 1–5 showed a significant hypsochromic shift (blue shift) relative to its position in L (Fig. 2). A new band clearly seen at 389 nm in the electronic absorption spectrum of nickel complex 3, probably arises due to intra ligand charge transfer transitions associated with N–C=S group of coordinated dithiocarbamate ligands. Similar band for other complexes is expected, however, it could not be seen due to broad nature of the band arises from the carbonyl group of the quinone (vide supra). Further, cobalt(III) complex 2 exhibits a weak absorption band at ∼649 nm, which is consistent with the absorption behavior of [Co{κ²S,S-S₂C–piperazine–C₂H₄N=C(Ph)}₃]¹⁸ and other cobalt(III) complexes.²⁶ The UV-visible absorption behavior of copper(II) 3 and nickel(II) 4 complexes are quite similar to those of closely related compounds.^18,27 The Zn(II) complex 5 is of white colour and presents a featureless electronic spectrum. Unlike manganese(II)–bis-dithiocarbamate complexes,²⁸ the manganese(III) complex 1 is of light pink in colour and stable in the solution and in solid state. Literature report²⁹ suggests the existence of divalent Mn(R₂dtc)₂ species as yellow solids which are highly unstable and readily oxidize to the dark-violet trivalent complexes Mn(R₂dtc)₃. Moreover, the d–d bands of high-spin Mn(III) complexes possesses extremely low intensity and in the presence of organic ligands and they could rarely be detected.³⁰ The magnetic moment values (Table 1) along with UV-visible absorption bands suggest an octahedral environment around manganese(III)/cobalt(III), the square planar environment around nickel(II)/copper(II) and tetrahedral environment around zinc(II) in their respective mononuclear dithiocarbamate complexes 1–5.

Table 1 UV-visible, magnetic and fluorescence spectral data of compounds 10⁻⁵ M in DMF solution

Entry	λmax nm (ε, L mol^−1 cm^−1)	Wave number cm^−1	Magnetic moment μeff (BM)	Fluorescence spectral data
				λex (nm)	λem nm (intensity)
L	277 (65 608) π → π*	36 101	—	277	557 (280) π* → π
	335 (10 998) sh, π → π*	29 850
	487 (14 902) n → π*	20 533
1	276 (73 761) π → π*	36 231	4.95	276	557 (129) π* → π
	340 (13 254) sh, π → π*	29 411
	465 (10 178) n → π*	21 505
2	277 (75 010) π → π*	36 101	Dia	277	556 (172) π* → π
	324 (32 041) sh, π → π*	30 864
	468 (8536) n → π*	21 367
	649 (1365) d–d	15 408
3	276 (50 550) π → π*	36 231	Dia	276	555 (621) π* → π
	324 (49 915) π → π*	30 864
	389 (14 902) n → π*	25 706
	460 (8536) n → π*	21 739
4	275 (65 608) π → π*	36 363	1.91	275	552 (481) π* → π
	333 (8808) sh, π → π*	30 030
	443 (13 629) n → π*	22 573
5	275 (80 029) π → π*	36 363	Dia	272	547 (53) π* → π
	339 (7031) sh, π → π*	29 498			576 (76) π* → π
	465 (8536) n → π*	21 505

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Fig. 2 UV-visible absorption spectra of the ligand and dithiocarbamate metal complexes at room temperature in 10⁻⁵ M DMF solution.

3.5 Fluorescence emission spectral study

The fluorescence properties of L and its dithiocarbamate complexes 1–5 were studied at room temperature (298 K) from 10⁻⁵ M DMF solution samples and the spectral data is summarized in Table 1. The fluorescence spectra of these are shown in (Fig. 3). Apparently all compounds exhibit medium to very strong fluorescence emission bands in the visible region upon excitation at their respective λ_max values. For instance, L and 1–5 fluoresces strongly at ∼550 nm with concomitant Stokes shifts of ≈280 nm, except zinc complex 5 which shows a weak broad fluorescence emission band at 547 upon excitation at 272 nm. The weak fluorescence behaviour of zinc complex 5 is somewhat in contrast to many other zinc complexes that are shown to exhibit maximum fluorescence emissions upon excitation at lower wavelengths.^18,31 Contrarily,³² complexes 3 and 4 showed excellent fluorescence emission behaviour upon excitation of intra ligand charge transfer bands. Such a trend of fluorescence spectra and concomitant bathochromic shifts of intramolecular charge-transfer emissions by coordination compounds were previously observed in dithiocarbamate³³ complexes. The fluorescence bands appeared in the spectra of all the complex 1–5 are attributed to π* → π transitions. None of the compounds display any noticeable fluorescence upon excitation at their low energy bands such as 487, 465, 468, 460, 443 and 465 nm, respectively.

Fig. 3 Fluorescence spectra of the ligand and dithiocarbamate metal complexes at room temperature in 10⁻⁵ M DMF solution.

3.6 Cyclic voltammetric study

Since a large part of the biological activity of naphthoquinone derivatives is related to their electron-transfer ability,¹⁵ it becomes pertinent to investigate the electrochemical behaviour of these newly synthesized compounds. Electrochemical investigations of L and 1–5 were performed using cyclic voltammetry in the potential range of +2.1 to −2.1 V from 1.0 mM CH₂Cl₂ solutions containing n-Bu₄NPF₆ (0.1 M) as supporting electrolyte at a scan rate of 100 mVs⁻¹. The voltammograms for these are shown in Fig. 4 and electrochemical data is represented in Table 2. The electrochemical examination of 1–5 clearly demonstrates the quasi-reversible redox behavior of L, 1 and 5.

Fig. 4 Cyclic voltammograms of a 1 mM solution of ligand precursor L and compounds 1–5 in CH₂Cl₂ containing 0.1 M tetra-n-butylammonium hexafluorophosphate as the supporting electrolyte.

Table 2 Electrochemical data for the compounds L and 1–5

Entry	EpIc (V)	EpIIc (V)	EpIa (V)	EpIIa (V)	ΔEpI (V) (EpI,a − EpI,c)	ΔEpII (V) (EpII,a − EpII,c)
L	−1.350	−1.923	−1.013	−1.468	0.337	0.455
1	−1.210	−1.730	−1.008	−1.456	0.202	0.274
2	−1.469	−2.046	−0.995	−1.550	0.474	0.496
3	−1.491	−1.969	−1.008	−1.639	0.483	0.330
4	−1.462	−1.887	−1.037	−1.627	0.425	0.260
5	−1.291	−1.834	−0.995	−1.515	0.296	0.319

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First, electrochemical examination of L clearly demonstrates its quasi-reversible redox behavior. It primarily exhibits two single-electronic waves due to electroreduction of naphthoquinone moiety to semiquinone radical anion and then to dianion, respectively. The separation between each cathodic and corresponding anodic peak, ΔE_p at 100 mV s⁻¹ scan rate is larger than 59 mV (Table 2) and the ratio of the current intensity of the cathodic and anodic peaks are different from unity, suggesting a quasi-reversible nature of both the redox couples of L. Compared to the cyclic voltammogram of L, its complexes 1–5 did not display any additional peak, in the cathodic or anodic scan under the similar experimental conditions, i.e. voltammograms of 1–5 exhibit similar feature, except the differences in the cathodic/anodic current peak heights (Fig. 4). This implies that the complexes 1–5 are primarily electro active with respect to the dominant redox-active naphthoquinone moieties. The redox active transition metal cations in the dithiocarbamate complexes 1–5 are apparently present in silent mode. The silent nature of transition metal ions in the presence of dominant electro-active ligands has previously been observed in a number of cases.^10b,18,34 Further, electrochemically equivalence of multiple naphthoquinone units of 1–5 is confirmed by the emergence of a single quasi-reversible wave. When compared the electrochemical responses of these complexes with corresponding ligand precursor L, complexes 1 and 5 exhibit redox potentials that are significantly shifted anodically whereas the redox potentials of the rest of the complexes are shifted cathodically.

3.7 Biological evaluation

The development of new antimicrobial agents is of considerable interest because of unprecedented increase of multidrug resistance in common pathogens and the hasty appearance of new infections. Reports suggest that a major part of fatal nosocomial infection as well as community-acquired infections are caused by S. aureus and P. aeruginosa.³⁵ While the most prominent fungal pathogens affecting human being through invasive candidiasis³⁶ is C. albicans. The spread of these organisms in healthcare settings is often difficult to control due to the presence of multiple intrinsic and acquired mechanisms of antimicrobial resistance. Thus, it becomes necessary to investigate the biological significance of newly synthesized compounds.

The ligand precursor L and its dithiocarbamate transition metal complexes 1–5 were screened by Broth dilution method³⁷ for their in vitro antibacterial activity against two Gram positive bacteria S. aureus, B. subtilis, and two Gram negative bacteria E. coli, P. aeruginosa. They were also evaluated for their in vitro antifungal activity against C. albicans and A. niger. Concentration of compounds was ranging from 10 μg to 600 μg (Fig. 5). The lowest concentrations of the compounds that prevented visible growth are provided in Table 3. It was established that the solvent had no antibacterial or antifungal activities against any of the test microorganisms. Ciprofloxacin and Fluconazole were used as standard reference drugs and these were tested under the similar conditions for comparison.

Fig. 5 Antimicrobial MIC (μg ml⁻¹) values for L and its dithiocarbamate complexes 1–5.

Table 3 MIC determination of antibacterial and antifungal agent (μg)

Entry	MIC (μg ml^−1)
	Bacteria (Gram +ve)		Bacteria (Gram −ve)		Fungi
	S. aureus	B. subtilis	E. coli	P. aeruginosa	C. albicans	A. niger
L	<10	<10	600	200	100	200
1	200	50	200	200	200	>600
2	600	600	300	200	100	200
3	200	20	200	200	200	200
4	10	50	200	200	100	50
5	50	20	300	50	200	300
Ciprofloxacin	15	5	15	10	—	—
Fluconazole	—	—	—	—	10	40

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L and its dithiocarbamate complexes 1–5 exhibit moderate to excellent activity against all the tested bacteria (Table 3), except the activity of L (MIC = 600 μg ml⁻¹) against E. coli, a Gram −ve bacteria and activity of cobalt(III) complex 2 (MIC = 600 μg ml⁻¹) against both of the Gram +ve bacteria. In particular, L (MIC < 10 μg ml⁻¹ against S. aureus and B. subtilis) and copper(II) complex 4 (MIC 10 μg ml⁻¹ against S. aureus) exhibit a remarkable activity (Fig. 5 and 6). In fact, a very good antibacterial activity of copper(II) 4 and zinc(II) 5 complexes were found against both of the Gram positive bacteria S. aureus and B. subtilis whereas Mn(III) 1, Ni(II) 3 complexes displayed good activity only against B. subtilis. All the compounds displayed moderate activity against both of the Gram −ve bacteria, except 5 which showed very good activity against P. aeruginosa. Apart from this, compounds L and 1–5 showed moderate activity against both of the fungi (Fig. 5), except manganese(III) complex 1 (poor; MIC = 600 μg ml⁻¹) and copper(II) complex 4 (very good; MIC = 50 μg ml⁻¹) against A. niger. Moreover, the activity of these dithiocarbamate complexes 1–5 are found superior against a number of microbes than many naphthoquinone derivatives, recently reported by us.³⁸ Although, it is difficult to rationalize the biological results, why certain compounds are active and other compounds are less active. However, the diversity in the anti-microbial activity of the synthesized compounds may be attributed to the differences in their electronic, redox and structural parameters associated with various transition metal ions. The enhanced antibacterial activity of L and copper(II) complex 4 against S. aureus (Table 3) apparently bring them about superior antibacterial agent than the reference drug ciprofloxacin. The activity of these compounds may either be associated with cell wall destruction, DNA damage,³⁹ protein synthesis inhibition or chelation with metal ions present in fungal cells, thus grudging them of the needed ions which would lead to cell mortality.⁴⁰

Fig. 6 Zone of inhibition of ligand precursor L against (a) S. aureus, (b) B. subtilis; 3 against (c) B. subtilis; 4 against (d) S. aureus, (e) B. subtilis and (f) A. niger in different concentration ranging from 10 μg to 600 μg ml⁻¹ in DMSO solution.

3.8 TG/DTA studies

Mononuclear transition metal dithiocarbamate complexes 1–5 were further studied by thermogravimetric method to investigate their thermal decomposition patterns in the temperature ranges from room temperature to 750 °C. Rightfully, the heating rate was controlled at 10 °C min⁻¹ under nitrogen atmosphere. The temperature range and percentage weight loss during the decomposition as well as the temperature corresponding to the maximum rate of decomposition are summarized in Table 4. Thermogravimetric plot for these compounds (Fig. 7) clearly reveals a single or multi stage mass loss on DTG and corresponding DTA curves, attributed to endothermic and/or exothermic elimination of molecular fragments due to the thermal degradation. It may be noted that the thermal decompositions of all the complexes 1–5 start before their melting point and accompanied by the appearance of one or more endothermic peaks on corresponding DTA curves. Further, it appears that manganese(III) complex 1 and zinc complex 5 are thermally unstable and their degradations start at a lower temperature (120–140 °C), compared to other complexes which are indeed stable up to ∼200 °C. All the complexes exhibit only ≈50% of degradations on TG curves and a stable residual mass could not be obtained up to 750 °C. Remarkably, thermal decompositions of zinc(II) complex 5 is essentially taking place in two stages. The first stage starts at 120 °C with a quick mass loss of 8.9% on TG curve which corresponds to the loss of CS₂ molecule (calc. 8.8%). The second stage starts at 151 °C and only 48.1% mass loss was observed on TG curve up to 750 °C. It gives a maximum rate of decomposition of 0.166 mg min⁻¹ on DTG curve at 212.4 °C.

Table 4 Thermogravimetric data of the dithiocarbamate metal complexes 1–5

Entry	Steps	Temp. range (°C)	Weight loss (%)	DTA peak (°C)	DTG peak (°C)	Rate of decomposition (mg min^−1)	Residual mass (%) found
1	I	140–750	35.4	190.2	177.1	0.078	64.6
2	I	230–750	46.3	222.5	240.4	0.239	53.7
				270.2	264.6	0.237
3	I	240–750	53.4	234.9	244.4	0.306	46.6
				309.5	309.5	0.505
4	I	190–750	47.5	234.2	196.5	0.165	52.5
					225.2	0.242
5	I	120–130	8.9	210.0	125.3	0.120	51.9
	II	151–750	39.2		212.4	0.166

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Fig. 7 TG/DTA curves of the dithiocarbamate metal complexes 2–5.

4. Conclusion

In conclusion, a novel multifunctional ligand 2-chloro-3-{2-(piperazinyl)ethyl}-amino-1,4-naphthoquinone (L) was synthesized and successfully used to obtain [M{κ²S,S-S₂C–piperazine–C₂H₄N(H)ClNQ}_n] {M = Mn(III) 1, Co(III) 2, Ni(II) 3, Cu(II) 4, Zn(II) 5; ClNQ = 2-chloro-1,4-naphthoquinone; n = 2 for 3–5 and n = 3 for 1–2} in a single step involving self-assembly of L, CS₂ and corresponding metal acetates. L and complexes 1–5 bearing functionalized ligand backbone were characterized satisfactorily by microanalysis and standard spectroscopy. Significantly, complexes 3 and 4 showed excellent fluorescence emission behaviour upon excitation of intra ligand charge transfer bands with concomitant Stokes shifts of ≈280 nm. L and 1–5 are electro active with respect to the dominant redox-active naphthoquinone moiety and redox active transition metal cations in 1–5 are apparently present in silent mode. A broad spectrum of the strongest antimicrobial activity was determined for the copper(II) complex 4 and L. Interestingly, the enhanced antibacterial activity of L (MIC < 10 μg ml⁻¹) as well as copper(II) complex 4 (MIC 10 μg ml⁻¹) against S. aureus apparently brings them about potent antibacterial agent than the reference drug ciprofloxacin (MIC 15 μg ml⁻¹).

Acknowledgements

VKS acknowledges CSIR, New Delhi for financial support (Project no. 01/(2733)/13/EMR-II) and SKV acknowledges the UGC for RFSMS (BSR) fellowship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08065g

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