Advances in Chemistry

Volume 2014 (2014), Article ID 506851, 12 pages

http://dx.doi.org/10.1155/2014/506851

Research Article

Cobalt(II) and Manganese(II) Complexes of Novel Schiff Bases, Synthesis, Charcterization, and Thermal, Antimicrobial, Electronic, and Catalytic Features

Selma Bal and Sedat Salih Bal

Chemistry Department, Faculty of Arts and Science, Kahramanmaras Sutcu Imam University, Avsar Kampusu, 46100 Kahramanmaras, Turkey

Received 6 June 2014; Accepted 28 July 2014; Published 21 August 2014

Academic Editor: Alessandro D’Annibale

Copyright © 2014 Selma Bal and Sedat Salih Bal. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Carbazoles containing two new Schiff bases (Z,Z)-N,N′-bis[(9-ethyl-9H-carbazole-3-yl)methylene]propane-1,3 diamine (L¹) and (Z,Z)-N,N′-bis[(9-ethyl-9H-carbazole-3-yl)methylene]-2,2-dimethylpropane-1,3-diamine (L²) and their Co(II) and Mn(II) complexes were synthesized and characterized using various spectroscopic methods and thermal analysis, which gave high thermal stability results for the ligands and their cobalt complexes. The title compounds were examined for their antimicrobial and antifungal activities, which resulted in high activity values for the ligands and their manganese complexes. Oxidation reactions carried out on styrene and cyclohexene revealed that the complex compounds were the most effective catalysts for styrene oxidation, giving good selectivities than those of cyclohexene oxidation. Electronic features of the synthesized compounds were also reported within this work.

1. Introduction

Schiff bases and their complexes have been and are being employed to many reactions in synthetic chemistry. In particular, the oxidation of alkenes is important intermediate to get new, industrially important chemicals for both organic synthesis and pharmaceutical industry. Catalytic transformations of hydrocarbons into valuable oxygenated derivatives such as alcohols, aldehydes, and epoxides using peroxides as oxidants have been extensively studied over the last few decades [1–5]. In particular, the catalysis of alkene oxidation by soluble transition metal complexes is of great interest in both biomimetic chemistry and synthetic chemistry [6]. So far various Schiff base complexes have been employed to catalytic oxidation of olefins to epoxides and aldehydes, and it has been proved that many Schiff base complexes gave improved results as catalysts for these kinds of oxidation reactions [7–20]. In our research, the synthesized Schiff base complexes have been searched for their potential use as catalysts in oxidation reactions for both cyclohexene and styrene. Not only for these oxidation reactions but also for many kinds, it is important to use eco-friendly with easy recoverability oxidants such as H₂O₂ and air. It is also important that the catalysts are thermally stable enough to carry out these kinds of reactions which generally require elevated temperatures [21, 22].

In addition their catalytic activities, various Schiff bases have also been examined for their biological activities in many previous studies [23–25]. This interest comes from the fact that their metal complexes can be used as antimicrobial, antifungal, and antitumor agents [26–28].

In previous studies various carbazoles containing Schiff bases and their coordination compounds have been synthesized, characterized, and examined for their different features such as luminescence, thermal property [29–33], biological activity [34], and electrochemical and optical behaviour [35–37] and for their use as Langmuir-Blodgett film [38].

We report here total synthesis, spectral and thermal characterization of two carbazole derived novel Schiff bases and their copper(II) and manganese(II) complexes, their thermal, electrochemical, antimicrobial features, and their effect as catalysts in the oxidation reactions of cyclohexene and styrene.

2. Experiment

2.1. Materials and Instrumentation

9-Ethylcarbazole, phosphorus(V) oxychloride, 1,3-diaminopropane, and 2,2-dimethyl-1,3-diaminopropane, all the solvents used, and acetate salts of copper(II) and manganese(II) were purchased from Sigma Aldrich. Nuclear magnetic resonance spectra of the synthesized ligands were recorded on a Bruker AV 400 MHz spectrometer in the solvent CDCl₃. Infrared spectra were obtained using KBr discs on a Shimadzu 8300 FTIR spectrophotometer in the region of 400–4000 cm⁻¹. Ultraviolet spectra were run in ethanol on a Schimadzu UV-160A spectrophotometer. Magnetic measurements were carried out by the Gouy method using Hg[Co(SCN)₄] as a calibrant. Molar conductances of the ligands and their transition metal complexes were determined in MeOH (~10⁻³) at room temperature using a Jenway Model 4070 conductivity meter. Mass spectra of the ligand were recorded on a LC/MS APCI Agilent 1100 MSD spectrophotometer. The oxidation products were analyzed with a gas chromatograph (Shimadzu, GC-14B) equipped with a SAB-5 capillary column and a flame ionization detector. Elemental analyses were performed on a LECO CHNS 932 elemental analyzer and the metal analyses were carried out on an Ati Unicam 929 Model AA spectrometer in solutions prepared by decomposing the compounds in aqua regia and subsequently digesting them in concentrated HCl. Thermal analyses of synthesized ligands and their metal complexes were carried out on a Perkin-Elmer Thermogravimetric Analyzer TG/DTA 6300 instrument under nitrogen atmosphere between the temperature ranges 30°C and 988°C at a heating rate of 10°C/min. Cyclic voltammetry was performed using IviumStat electrochemical workstation equipped with a low current module (BAS PA-1) recorder.

2.2. Synthesis of 9-Ethyl-9H-carbazole-3-carbaldehyde

Formulation of 9-ethylcarbazole was done by using Vilsmeier formulating agents DMF and POCl₃. Inside a fume cupboard, DMF (32 mL, 0.4 mol) was put into a 250 mL round-bottom flask placed in an ice bath. Over DMF at 0°C 32 mL (0.32 mol) POCl₃ was added dropwise through a dropping funnel. Resulting solution was stirred at room temperature for 2 hours. To the stirring mixture, 9-ethyl-9H-carbazole (8 g, 0.04 mol) dissolved in 32 mL DMF was slowly added. The reaction mixture was heated at 80°C and left stirring for 24 hours. The resulting dark coloured mixture was poured into slurry of crushed ice and water (250 mL). The precipitate was washed with water and extracted by CHCl₃ and then washed with n-hexane. The dirty yellow precipitate was subjected to flash column chromatography with the eluent of ethyl acetate/hexane (1 : 10). Vilsmeier reaction always gives both mono- and dialdehyde of the formulated carbazole [39, 40]. The monoaldehyde was the first product eluated as yellowish-white crystals. The dialdehyde obtained was white solid. TLC chromatography,   elemental analysis, and spectral data confirm the purity and structure of synthesized mono- and dialdehyde products. Monoaldehyde 9-ethyl-9H-carbazole-3-carbaldehyde: yield: 40%. m.p.: 85–87°C. UV-Vis (ethanol) (, nm) (ε, M−1 cm⁻¹): 202(50000), 210(80000), 226(30000), 264(36000), 284(95000). FT-IR (KBr, cm⁻¹): 1591(w), 1622(w), 2850(w), 2922(w)(Ar-H and C-H); 1681(s)(CHO). ¹H NMR(400 MHz, CDCl₃): 10.1(CHO), 8.6 (H-4, d, ), 8.16 (H-5, brd, ), 8.02 (H-2, dd, ), 7.46 (H-1&H-8, d, ), 7.56 (H-7, dt, ), 7.35 (H-6, dt, ), 4.39 (2H-14, q, ), 1.47 (3H-15, t, ). ¹³C NMR(400 MHz, CDCl₃): 191.8(CHO), 127.2(C-1), 126.8(C-2), 128.5(C-3), 124.01(C-4), 120.8(C-5), 120.3(C-6), 108.7(C-7), 109.2(C-8), 143.5(C-10), 123.1(C-11), 123.0(C-12), 140.7(C-13), 37.9(C-14), 13.9(C-15). Mass spectrum (LC/MS APCI): m/z 223.7 [M]⁺.

2.3. Synthesis of the Ligands (Z,Z)-N,N′-Bis[(9-ethyl-9H-carbazol-3-yl)methylene]propane-1,3-diamine (L¹) and (Z,Z)-N,N′-Bis[(9-ethyl-9H-carbazol-3-yl)methylene]-2,2-dimethylpropane-1,3-diamine (L²)

Inside a 100 mL round-bottom flask, 0.45 g (2 mmol) 9-ethyl-9H-carbazole-3-carbaldehyde was put and dissolved in enough amount of ethanol. Over this, 0.1 g (1 mmol) diamine was added dropwise. The resulting solution was heated under reflux for four hours and left overnight. The white precipitate was recrystallized from ethanol (Figure 1).

Figure 1: Synthesis scheme of the synthesized compounds and the proposed structure for the metal complexes.

2.3.1. (Z,Z)-N,N′-Bis[(9-ethyl-9H-carbazole-3-yl)methylene]propane-1,3-diamine (L¹)

Yield: 80%, m.p.: 167°C, elemental analysis found % (calculated %): C 82.34(81.78) H 6.80(6.66) N 12.01(11.56). UV-Vis (ethanol) (, nm) (ε, M−1 cm⁻¹): 216(175000), 250(105000), 266(120000), 296(210000), 310(161000). FT-IR (KBr, cm⁻¹): 2843(w), 2920(w), 2971(w), 1598(w)(Ar-H and C-H); 806(s), 740(s)(Ar-H) 1637(s)(imine). Mass spectrum (LC/MS APCI): m/z 485.2 , 280.1 , which arises from the loss of ten hydrogen and one carbazole units leaving C₁₉H₁₀N₃. ¹H NMR(400 MHz, CDCl₃): 7.44 (H-1&H-8, d, ), 7.92 (H-2, dd, .5&1.5), 8.49 (H-4, d, ), 8.15 (H-5, d, ), 7.28 (H-6, dt, ), 7.51 (H-7, dt, .2&7.1), 4.41 (H-14, q, ), 1.47 (H-15, t, ), 3.84 (H-16&H-18, q, ), 2.26 (H-17, m, ), 8.5 (imine, s). ¹³C NMR(400 MHz, CDCl₃): 126.03(C-1), 125.9(C-2), 127.7(C-3), 123.1(C-4), 120.7(C-5), 119.4(C-6), 108.5(C-7), 108.7(C-8), 141.5(C-10), 127.2(C-11), 124.0(C-12), 140.5(C-13), 37.7(C-14), 13.8(C-15), 59.5(C-16&C-18), 32.3(C-17), 162.06(imine).

2.3.2. (Z,Z)-N,N′-Bis[(9-ethyl-9H-carbazol-3-yl)methylene]-2,2-dimethylpropane-1,3-diamine (L²)

Yield: 75%, m.p.: 115°C, elemental analysis found % (calculated %): C 82.11 (81.99) H 7.530(7.08) N 11.01(10.93). UV-Vis (ethanol) (, nm) (ε, M−1 cm⁻¹): 268(115000), 300(78000), 306(61000), 334(99000). FT-IR (KBr, cm⁻¹): 2973(w), 2949(w), 2867(w), 2821(w), 1595(w) (Ar-H and C-H); 809(s), 747(s)(Ar-H) 1646(s)(imine). Mass spectrum (LC/MS APCI): m/z 514.3 , 308.1 , which arises from the loss of ten hydrogen and one carbazole units leaving C₂₁H₁₄N₃. ¹H NMR(400 MHz, CDCl₃): 7.44(H-1&H-8, d, ), 7.99 (H-2, dd, ), 8.49 (H-4, d, ), 8.16 (H-5, d, ), 7.29 (H-6, dt, ), 7.53 (H-7, dt, .1&7.0), 4.42 (H-14, q, ), 1.48 8H-15, t, ), 3.67 (H-16&H-18, s), 1.2 (H-19&H-20, s), 8.48 (imine, s). ¹³C NMR(400 MHz, CDCl₃): 125.9(C-1), 125.9(C-2), 128.0(C-3), 123.05(C-4), 120.8(C-5), 119.4(C-6), 108.5(C-7), 108.7(C-8), 141.4(C-10), 127.2(C-11), 123.1(C-12), 140.7(C-13), 37.7(C-14), 13.9(C-15), 58.2(C-16&C-18), 37.3(C-17), 24.8 (C-19&C-20).

2.4. Synthesis of Complex Compounds

The ratio of the metal salts and the ligands was taken as 1 : 1 (Figure 1). A solution of the metal salt (1 mmol) in 15 mL absolute ethanol was added into the solution of the ligand L¹/L² (1 mmol) in 15 mL ethanol. The mixtures were stirred under reflux overnight. The precipitates were filtered, washed with distilled water to get rid of the excess salt, and dried in vacuum.

2.4.1. Cobalt(II) Complex of L¹

[CoL¹(H₂O)₂]·2AcO complex: (C₃₃H₃₆CoN₄O₂), brown coloured, yield: 72%, m.p.: 205.5°C, Elemental Analysis found % (calculated %): C 63.65(63.38), H 6.71(6.26), N 9.66(9.67), Co 10.68(10.17). UV-Vis (ethanol) (, nm) (ε, M−1 cm⁻¹): 256(51000), 282(11600), 330(87000), 367(75000), 482(30000), 560(33000). FT-IR (KBr, cm⁻¹): 3350, 3220(O-H), 2972, 2822, 2745(Ar-H, C-H), 1575(imine), 439(M-N), 537(M-O). (Ω−1 cm2 mol⁻¹): 80.1, B.M.: 4.35.

2.4.2. Manganese(II) Complex of L¹

[MnL¹(H₂O)₂]·2AcO complex: (C₃₃H₃₆MnN₄O₂), brown coloured, yield: 64%, m.p.: 198°C, Elemental Analysis found % (calculated %): C 69.23(68.86), H 6.21(6.30), N 10.15(9.73), Mn 10.02(9.54). UV-Vis (ethanol) (, nm) (ε, M−1 cm⁻¹): 246(86000), 291(51000), 375(24000), 640(12000). FT-IR (KBr, cm⁻¹): 3342(O-H), 2847, 2730(Ar-H, C-H), 1589(imine), 481(M-N), 604(M-O). (Ω−1 cm2 mol⁻¹): 28.7, B.M.: 5.85.

2.4.3. Cobalt(II) Complex of L²

[CoL²(H₂O)₂]·2AcO complex: (C₃₅H₄₀CoN₄O₂), black coloured, yield: 60%, m.p.: 146.5°C, Elemental Analysis found % (calculated %): C 70.09(69.18), H 6.92(6.63), N 9.57(9.22), Co 10.01(9.70). UV-Vis (ethanol) (, nm) (ε, M−1 cm⁻¹): 230(120000), 284(61000), 384(43000), 620(10200). FT-IR (KBr, cm⁻¹): 3306(O-H), 2919, 2812(Ar-H, C-H), 1550(imine), 553(M-N), 654(M-O). (Ω−1 cm2 mol⁻¹): 67.3, B.M.: 4.89.

2.4.4. Manganese(II) Complex of L²

[MnL²(H₂O)₂]·2AcO complex: (C₃₃H₄₀MnN₄O₂), dark green coloured, yield: 54%, m.p.: 119°C, elemental analysis found % (calculated %): C 70.03(69.64), H 7.14(6.68), N 9.67(9.28), Mn 9.25(9.10). UV-V is (ethanol) (, nm) (ε, M−1 cm⁻¹): 265(90000), 395(71000), 567(14000). FT-IR (KBr, cm⁻¹): 3252(O-H), 2920, 2835(Ar-H, C-H), 1540(imine), 490(M-N), 601(M-O). (Ω−1 cm2 mol⁻¹): 35.2, B.M.: 6.23.

2.5. Preparation of Microorganism Culture

The growth inhibitory activity of the synthesized compounds was tested against 4 gram negative, 4 gram positive bacteria (Klebsiella pneumoniae FMC 5, Escherichia coli DM, and Enterobacter faecium (clinic isolate) and Enterobacter aerogenes ATCC 13048, Bacillus subtilis IMG 22, Bacillus megaterium DSM 32, Staphylococcus aureus ATCC 25923, and Streptococcus faecalis) and 3 yeasts (Candida albicans ATCC 1023, Candida utilis, and Saccharomyces cerevisiae WET 136). These microorganisms were provided from Microbiology Laboratory Culture Collection, Department of Biology, Kahramanmaraş Sütçü İmam University, Turkey.

Antimicrobial activities of the compoundswere determined using the hollow agar. The bacteria were first incubated at °C for 24 h in nutrient broth (Difco), and the yeasts were incubated in Sabouraud dextrose broth (Difco) at °C for 24 h. The cultures of the bacteria and yeast were injected into the Petri dishes (9 cm) in the amount of 0.1 mL (McFarland OD: 0.5, 1.5 × 10⁸ bacteria/mL and 1.5 × 10⁶ yeast/mL) [38, 39, 41, 42]. Then, Mueller Hinton agar and Sabouraud dextrose agar (sterilized in a flask and cooled to 45–50°C) were homogeneously distributed onto the sterilized Petri dishes in the amount of 25 mL. Finally, 2 mg of each chemical compound dissolved in ethanol was placed inside the sterilised antibiotic discs. The prepared antibiotic discs were then placed in the bacterial medium.

Afterwards, the plates combined with the discs were left at 4°C for 2 h, the plates injected with yeast were incubated at °C for 24 h, and ones injected with bacteria were incubated at °C for 24 h. After 24 h, inhibition zones appearing around the disks were measured and recorded in mm [41–44].

2.6. Determination of Minimal Inhibitory Concentration (MIC)

A broth microdilution broth susceptibility assay was used, as recommended by NCCLS, for the determination of the MIC of the ligand and the complexes and some reference components [38, 41]. All tests were performed in Mueller Hinton broth (MHB) supplemented with Tween 80 detergent (final concentration of 0.5% (v/v)), with the exception of the yeasts (Sabouraud dextrose broth (SDB) + Tween 80). Bacterial strains were cultured overnight at 37°C in MHB, and the yeasts were cultured overnight at 30°C in SDB. Geometric dilutions ranging from 200 to 2500 μg/mL of the compounds were prepared including one growth control (MHB + Tween 80) and one sterility control (MHB + Tween 80 + test oil). Test tubes were incubated under normal atmospheric conditions at 37°C for 24 h for bacteria and at 30°C for 48 h for the yeasts. The microbial growth was determined by turbidimetric methods.

2.7. Oxidation Procedure

A mixture of 1.10−3 mol catalyst, 20 mL solvent (CH₃CN), and 10 mmol cyclohexene/styrene was stirred under nitrogen atmosphere in a 50 mL round-bottom flask equipped with a condenser and a dropping funnel at room temperature for 30 min. Then 20 mmol hydrogen peroxide (30% in water) was added (catalyst : substrate : oxidant ratio is 1 : 10 : 20). The resulting mixture was then refluxed for 8 h under nitrogen atmosphere at 90°C. After filtration and washing with solvent, the filtrate was concentrated and then subjected to GC analysis. The yields were recorded as the GC yield based on the starting styrene or cyclohexene. The identity of the oxidation products was confirmed by GC-MS. A blank reaction was also carried out without any catalyst for both oxidation reactions.

3. Results and Discussion

3.1. IR and UV Spectra of the Ligands and the Complexes

IR spectrum of 9-ethyl-3-formylcarbazole revealed the characteristic weak C-H stretching bands belonging to aromatic rings at 1591 cm⁻¹, 1622 cm⁻¹, 2850 cm⁻¹, and 2922 cm⁻¹. The latter two bands might also be the C-H stretching frequencies generally observed for saturated methyl or methylene groups. The aromatic aldehyde was observed at 1681 cm⁻¹. Following the imine formation, this band was replaced for the imine stretching frequencies at 1638 cm⁻¹ for L¹ and at 1646 cm⁻¹ for L². In the IR spectra of L¹ and L², the weak bands between 2821 and 2973 cm⁻¹ belong to weak aromatic and saturated C-H stretchings. The medium strength bands at 1598 cm⁻¹ for L¹ and at 1595 cm⁻¹ for L² belong to aryl-H vibrations. If we examine the substitution patterns of the benzene rings, we can say that one metadisubstituted (strong aryl-H vibrations at 806 cm⁻¹ for L¹ and at 809 cm⁻¹ for L²) and one orthodisubstituted benzene ring (strong aryl-H vibrations at 740 cm⁻¹ for L¹ and at 747 cm⁻¹ for L²) are present in our ligands.

Following the coordination, the imine frequencies recorded for the ligands were red shifted and appeared at around 1575 cm⁻¹ for L¹-Co, 1589 cm⁻¹ for L¹-Mn, 1550 cm⁻¹ for L²-Co, and 1540 cm⁻¹ for L²-Mn complexes as weak signals. All the coordination compounds showed coordinated water frequencies between 3252 and 3306 cm⁻¹ [45]. The M-N stretching vibrations observed between 439 and 553 cm⁻¹ proved that the nitrogens of the imine groups coordinate with the central metal atoms.

The UV spectra of all ligands and the complexes were taken in ethanol in the range between 200 nm and 700 nm. For the complex compounds the d-d transitions were observed between 560 nm and 640 nm, and the bands observed in the 395–367 nm range for these complexes can be attributed to the charge transfer bands from ligand to metal or from metal to ligand centre [46, 47]. The and transitions of the synthesized ligands and complexes were observed in 202–334 nm region. The O-H stretching frequencies in water molecules assisting in coordination were seen as broad singlets between 3306 cm⁻¹ and 3350 cm⁻¹ [45]. All the infrared and u.v. results are given in the experimental Sections 2.2–2.4. The magnetic moment values for cobalt complexes were recorded as 4.35 B.M. and 4.89 B.M., which are characteristic values for tetrahedral cobalt complexes [46, 47]. The manganese(II) complexes gave 5.85 B.M. and 6.23 B.M. values, indicating a high spin complex and suggesting tetrahedral geometry [48, 49].

3.2. Electrochemical Properties of the Synthesized Compounds

Cyclic voltammogram studies were run in DMF (1 × 10−3 M)–0.1 M NBu₄BF₄ as supporting electrolyte at 293 K. Electronic spectra of all the compounds were taken for scan rates 100, 250, 500, 750, and 1000 mVs⁻¹ against an internal ferrocene-ferrocenium standard and we have obtained different oxidation and reduction processes for different scan rates. Examination of cv graphics of L¹ shows reversible oxidation-reduction processes for the scan rate 100 mVs⁻¹ (Figure 2(a)) at 0.87 V() and 0.88 V() ( :  = 1) and for the scan rate 500 mVs⁻¹ at 0.94 V() and 1.04 V() ( : ). The quasireversible processes were observed for 250 mVs⁻¹ scan rate at 0.87 V() and 1.0 V() and for 750 mVs⁻¹ at 0.9 V() and 1.12 V(). The rest of the peaks observed for different scan rates can be said to be irreversible. The cv graphics of cobalt(II) complex of this ligand shows reversible redox processes for the scan rates 100, 500, and 750 mVs⁻¹; for example, for 500 mVs⁻¹ (Figure 2(b)), the  :  ratio equals ( = 0.93 V and = 1.03 V). The other peak potentials recorded for this complex gave either quasireversible or irreversible redoxes. The manganese(II) complex of the same ligand gave reversible processes for the scan rates 100, 250, and 500 mVs⁻¹; for example, for 100 mVs⁻¹ at 0.98 V() and at 1.07 V(),  :  ratio equals around 1. The other ligand, L², revealed reversible redoxes for scan rates 250, 500, 750, and 1000 mVs⁻¹. For example, the scan rate 750 mVs⁻¹ (Figure 3(a)) gave cathodic peak potential at 0.98 V and anodic peak potential at 0.99 ( : =1). The cobalt complex of this ligand showed reversible redoxes for 100 and 250 mVs⁻¹ (Figure 3(b)) scan rates at 1.13 V() and 1.1 V() and at 0.99 V() and 1.1 V(), respectively. The manganese(II) complex of the same ligand gave reversible processes for 100 and 500 mVs⁻¹. The rest of the data can be seen in Table 1. The redox processes for all the complexes can be defined as the formation of M(II) or M(I) with a simple one-electron process [24, 25]. The possible redox process for the ligands can be shown as in Figure 4.

Table 1: Electrochemical data of the title compounds.

Compound
Scan rate (mV/s)
() (V)
(V)
(V)

L¹
100
0.87, −1.18 (−0.57, 0.88)
0.88
0.01

250
0.87, −0.06, −1.24 (−0.48, 1.00)
—
0.13

500
0.94, −0.08, −1.32 (−0.42, 1.04)
—
0.15

750
0.90, −0.12, −1.40 (−0.38, 1.12)
—
0.22

1000
0.90, −0.14, −1.42 (−0.35, 1.14)
—
0.24

[Co(L¹)(H₂O)₂]2AcO
100
0.72, −1.14 (−0.58, 0.80)
—
0.12

250
0.76, −1.28 (−0.50, 0.93)
—
0.17

500
0.93, −1.40 (−0.46, 1.03)
—
0.16

750
0.97, −1.47 (−0.46, 1.07)
—
0.21

1000
0.87, −1.54 (−0.42, 1.17)
—
0.30

[Mn(L¹)(H₂O)₂]2AcO
100
0.98, −1.11 (−0.52, 1.07)
—
0.09

250
0.98, −1.13 (−0.54, 1.02)
—
0.04

500
0.97, −1.28 (−0.40, 1.08)
—
0.13

750
0.96, −1.41 (−0.33, 1.26)
—
0.30

1000
0.97, −1.47 (−0.26, 1.22)
—
0.25

L²
100
0.09, −0.90 (−0.56, −0.01, 0.82)
—
−0.10

250
0.91, 0.03, −1.01 (−0.53, 0.93)
0.92
0.02

500
0.94, −0.02, −1.09 (−0.50, 0.95)
0.95
0.01

750
0.98, −1.12 (−0.44, 0.99)
0.99
0.01

1000
0.96, −1.31 (−0.35, 0.99)
0.98
0.03

[Co(L²)(H₂O)₂]2AcO
100
1.13, −0.03, −0.91 (−0.53, 0.14, 1.25)
—
0.17

250
0.99, −0.07, −0.96 (−0.47, 0.20, 1.10)
0.72
0.12

500
0.96, −0.08, −1.02 (−0.50, 1.13)
0.76
0.17

750
0.95, −1.19 (−0.41, 1.10)
—
0.15

1000
0.97, −1.42 (−0.27, 1.10)
—
0.13

[Mn(L²)(H₂O)₂]2AcO
100
1.10, −0.05, −0.92 (−0.54, 1.19)
—
0.20

250
0.96, 0.01, −1.00 (−0.49, 1.18)
0.75
0.22

500
0.97, 0.13, −1.12 (−0.42, 1.07)
—
0.12

750
0.96, −1.22 (−0.30, 1.11)
—
0.15

1000
0.93, −1.43 (−0.24, 1.15)
—
0.22

(a)

(b)

Figure 2: (a) Cyclic voltammogram of L¹ in the presence of 0.1 M NBu₄BF₄ in DMF solution at 1.10−3 M at 100 mVs⁻¹. (b) Cyclic voltammogram of L¹-Co in the presence of 0.1 M NBu₄BF₄ in DMF solution at 1.10-3 M at 500 mVs⁻¹.

(a)

(b)

Figure 3: (a) Cyclic voltammogram of L² in the presence of 0.1 M NBu₄BF₄ in DMF solution at 1.10−3 M at 750 mVs⁻¹. (b) Cyclic voltammogram of L²-Co in the presence of 0.1 M NBu₄BF₄ in DMF solution at 1.10−3 M at 250 mVs⁻¹.

Figure 4: Reversible reduction oxidation processes of the title compounds L¹ and L² in DMF (1.10−3 M) solution.

3.3. Thermal Analysis

Thermal gravimetric analysis was used to explore the thermal stability of these newly synthesized compounds and to verify the status of water or solvent molecules whether to be inside or outside of the coordination sphere of the complex compounds [26, 50]. The results of thermal analysis of the title compounds are in good agreement with the theoretical formulae as suggested from spectral analyses. The thermogram of L¹ (Figure 5(a)) shows decomposition starting around 220°C with the loss of two ethyl groups adjacent to nitrogen in the carbazole units with 12%(Calc.11.97%). The biggest loss was observed between the temperatures 340°C and 520°C with a percentage of 68.23(Calc.68%), which is assigned to the two carbazole units. The residual part was found as 19.87%(Calc.20%). One could also say that the 8.51%(Calc.8.67%) of the residual part may belong to the propyl unit which plays a bridge role between the two imine groups. TG plot of [CoL¹(H₂O)₂]2AcO complex gives somewhat distorted three decomposition steps (Figure 6(b)), the first of which is observed between the temperature ranges 150°C and 220°C with a 9.2%(Calc.10%) loss belonging to the two ethyl groups on the nitrogen atoms. The second decomposition belongs to two carbazole units with a loss of 58.22%(Calc.57.35%). The third decomposition step, however, was assigned as the loss of C₅H₈N₂. The residual part gave percentages for metallic Co and the two water molecules. Mn(II) complex of this ligand starts to decompose at around 90°C. Between the temperatures 200°C and 430°C, our complex shows the highest percentage loss with 57.11%(Calc.57.75%) belonging to the carbazole groups. Our second ligand L² decomposes between the temperatures 300°C and 500°C with a total loss of 76.32%(Calc.76.17%) corresponding to the two carbazoles in our structures (Figure 6(a)). TG graphics of Mn(II) complex of L² (Figure 5(b)) shows three decomposition steps. The first one at 110°C corresponds to the loss of ethyl groups on the carbazoles. The second stage at 150°C corresponds to the loss of carbazole groups without the ethyls. The residual part gives a total mass loss of 13.71%(Calc.14.53%). The cobalt complex of the same ligand, however, gives the biggest mass loss of 54.91%(Calc.54.7%) between 200°C and 500°C belonging to the carbazole units. The residual part for this complex gives a total mass loss of 24.61%(Calc.24.57%), 9.24%(Calc.8.96%) of which can be assigned as the ethyl groups adjacent nitrogen on carbazole rings. The thermal data can be examined in Table 2.

Table 2: Thermal analysis of the ligands and their complexes.

Compound
M.W.
/°C
Mass loss/%

found (calculated)
Assignment
Residual/%

found (calculated)

L¹
484.26
220–330

340–520
12.00 (11.97)

68.23 (68)
C₂H₅ 

C₁₂H₈N
19.87 (20)

[Co₂L¹(H₂O)₂]2AcO
579.60
150–220

220–500

500–700
9.2 (10)

58.22 (57.35)

16.17 (16.5)
C₂H₅ 

C₁₂H₈N

C₅H₈N₂
10.80 (10) (Co)

6.54 (6.15) (H₂O)

[Mn₂L¹(H₂O)₂]2AcO
575.60
90–200

200–430

430–590
10.3 (10)

57.11 (57.75)

16.8 (16.6)
C₂H₅ 

C₁₂H₈N

C₅H₈N₂
9.61 (9.5) (Mn)

5.94 (6.2) (H₂O)

L²
512.69
300–500
76.32 (76.17)
C₁₄H₁₃N
24.04 (23.83)

[Co₂L²(H₂O)₂]2AcO
607.65
200–500

830–900
54.91 (54.7)

20.48 (20.73)
C₁₂H₈N

C₇H₁₄N₂
9.48 (9.69) (Co)

5.89 (5.92) (H₂O)

9.24 (8.96) (undetermined)

[Mn₂L²(H₂O)₂]2AcO
603.66
90–150

150–320

400–490
10.16 (9.6)

55.23 (55)

20.9 (20.87)
C₂H₅ 

C₁₂H₈N

C₇H₁₄N₂
8.48 (9.1) (Mn)

5.23 (5.43) (H₂O)

All thermal analyses were done under nitrogen atmosphere between the temperature ranges 30°C and 988°C at a heating rate of 10°C/min.

(a)

(b)

Figure 5: (a) TG plot of L¹ recorded under nitrogen atmosphere between the temperature ranges 30°C and 988°C at a heating rate of 10°C/min. (b) TG plot of L²-Mn recorded under nitrogen atmosphere between the temperature ranges 30°C and 988°C at a heating rate of 10°C/min.

(a)

(b)

Figure 6: (a) TG plot of L² recorded under nitrogen atmosphere between the temperature ranges 30°C and 988°C at a heating rate of 10°C/min. (b) TG plot of L¹-Co recorded under nitrogen atmosphere between the temperature ranges 30°C and 988°C at a heating rate of 10°C/min.

3.4. Catalytic Activity

Results of the oxidation reactions (Table 3) show the catalytic activity of synthesized complexes. Comparison between neat and complex catalysed reactions proves that the oxidation reactions using catalysts give higher conversions than their corresponding neat reactions. Examination of Table 3 reveals that the coordination compounds were most effective towards the oxidation of styrene; in particular, the manganese(II) complex gave 28.2% and 31.8% selectivities for styreneoxide. Oxidation of cyclohexene, on the other hand, gave poorer results compared to those of styrene oxidation. Higher selectivities (14.2% and 18.7%) were obtained for cyclohexeneoxide by using cobalt(II) complexes as catalyst. Comparing our results to those reported previously, we can say that our compounds and their catalytic activity results, although not whole, have similarities with salen and salophane type Schiff bases reported previously [8, 12, 18, 51]. The oxidation schemes for both styrene and cyclohexene can be seen in Figure 7.

Table 3: Oxidation of styrene and cyclohexene.

Entry
Catalyst
Styrene conversion (%)
Selectivity (%)

Styreneoxide
Benzaldehyde
Others

1
Blank
11.5
4.5
10.0
85.5

2
[CoL¹(H₂O)₂]2AcO
76.7
21.8
44.5
33.7

3
[CoL²(H₂O)₂]2AcO
81.3
17.4
56.1
26.5

4
[MnL¹(H₂O)₂]2AcO
85.3
28.2
38.9
32.9

5
[MnL²(H₂O)₂]2AcO
77.8
31.8
40.5
27.7

Entry
Catalyst
Cyclohexene conversion (%)
Selectivity (%)

Cyclohexeneoxide
2-Cyclohexen-1-one
Others

1
Blank
17.7
7
14.3
78.7

2
[CoL¹(H₂O)₂]2AcO
65
14.2
61.2
24.6

3
[CoL²(H₂O)₂]2AcO
69.7
18.7
58.3
23

4
[MnL¹(H₂O)₂]2AcO
71.2
9.3
52.4
38.3

5
[MnL²(H₂O)₂]2AcO
80.9
11.2
64
24.8

Reaction conditions: alkene (10 mmol), H₂O₂ (20 mmol), catalyst (1 mmol), CH₃CN (20 mL), and nitrogen atmosphere at 90°C.

(a)

(b)

Figure 7: (a) Oxidation reaction of styrene. Catalyst : substrate : oxidant ratio: 1 : 10 : 20, under nitrogen atmosphere at 90°C. (b) Oxidation reaction of styrene. Catalyst : substrate : oxidant ratio: 1 : 10 : 20, under nitrogen atmosphere at 90°C.

3.5. Antimicrobial Activity and Minimal Inhibitory Concentration (MIC) Analysis

The title compounds were evaluated for antimicrobial activity against four gram negative (K. pneumoniae, E. aerogenes, E. faecium, and E. coli) and four gram positive bacteria (B. subtilis, S. aureus, S. S. feacalis, and B. megaterium) and fungi (C. albicans, C. utilis, and S. cerevisiae). All antimicrobial and antifungal tests were performed in Mueller Hinton broth. Test tubes were incubated under normal atmospheric conditions at 37°C for 24 h for bacteria and at 30°C for 48 h for the yeasts and the microbial growth was determined by turbidimetric methods. All the synthesized compounds were effective for almost all of the microorganisms; in particular, for the microorganisms K. pneumoniae, E. aerogenes, S. faecalis, and S. aureus the title ligands brought about bigger inhibition zones. Generally, they were most effective, both antimicrobial and antifungal. Among the fungi, they gave the biggest inhibition zones for C. utilis with 27 mm and 25 mm zones, respectively. Among the coordination compounds, the higher values, that are both antimicrobial and antifungal, were recorded for the manganese(II) complex compounds (Table 4). Minimal inhibition zone experiments revealed that the microorganisms K. pneumoniae, E. aerogenes, S. faecalis, S. aureus, and C. utilis were the most sensitive microorganisms with their MIC values of 500 μg/mL to our ligands and their manganese complexes. The other microorganisms were moderately resistant to the synthesized compounds (Table 5).

Table 4: Antimicrobial data of synthesized compounds.

Microorganisms
Compounds

L¹
L²
L¹–Co
L²–Co
L¹–Mn
L²–Mn

Gram (−)

    K. pneumoniae
18a
15
7
11
15
17

    E. aerogenes
22
17
8
6
12
11

    E. faecium
10
7
12
15
19
21

    E. coli
12
11
10
9
14
16

Gram (+)

    B. subtilis
9
7
—b
—
8
10

    S. aureus
24
21
11
9
13
12

    S. faecalis
22
19
9
7
11
12

    B. megaterium
15
12
—
7
11
9

Fungi

    C. albicans
11
12
—
7
9
8

    C. utilis
27
25
9
10
16
17

    S. cerevisiae
16
14
11
9
12
15

aInhibition zone, mm.

bUndetermined inhibition zone.

Table 5: MIC values of the synthesized compounds.

Microorganismsa
Compounds

L¹
L²
L¹–Co
L²–Co
L¹–Mn
L²–Mn

Gram (−)

    K. pneumoniae
500
500
1000
1000
500
500

    E. aerogenes
500
500
1000
1000
500
500

    E. faecium
750
1000
750
750
500
500

    E. coli
1000
1000
1000
1000
1000
750

Gram (+)

    B. subtilis
1000
1000
>2500
>2500
1000
1000

    S. aureus
500
500
750
1000
500
500

    S. faecalis
500
500
1000
1000
500
500

    B. megaterium
750
1000
>2500
1000
750
1000

Fungib

    C. albicans
1000
1000
>2500
1000
1000
1000

    C. utilis
500
500
1000
1000
500
500

    S. cerevisiae
750
750
1000
1000
750
750

aAll microorganisms tests were performed in Mueller Hinton broth (MHB).

bAll fungi tests were performed in Sabouraud dextrose broth (SDB).

4. Conclusion

With this work, carbazoles containing efficient ligands and catalysts have been synthesized, characterized, and used for the oxidation reactions of styrene and cyclohexene. Catalytic activity results show the highest selectivities for styrene oxide formation and moderate results have been obtained for the oxidation of cyclohexene. Thermal analysis results are in agreement with the proposed structures of the ligands and their coordination compounds. Thermally most stable compound is the ligand L² with the decomposition temperature starting at 300°C. Following this is the ligand L¹. Among the coordination compounds, cobalt complexes seem to be more resistant to temperature than manganese ones. The biological activity results reveal that both ligands and their Mn(II) complexes are effective as being antimicrobial and antifungal. The cobalt(II) complexes, however, show moderate activities. Finally, the electronic features of these compounds have also been reported.

Conflict of Interests

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

Acknowledgment

The authors would like to thank Kahramanmaras Sutcu Imam University Research Projects Coordination Unit for the financial support.

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