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

Carbazoles containing two new Schiff bases (Z,Z)-N,N-bis[(9-ethyl-9H-carbazole-3-yl)methylene]propane-1,3 diamine (L) and (Z,Z)-N,N-bis[(9-ethyl-9H-carbazole-3-yl)methylene]-2,2-dimethylpropane-1,3-diamine (L) 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 compoundswere 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.


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][2][3][4][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][8][9][10][11][12][13][14][15][16][17][18][19][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 H 2 O 2 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][24][25].This interest comes from the fact that their metal complexes can be used as antimicrobial, antifungal, and antitumor agents [26][27][28].
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

Experiment
2.1.Materials and Instrumentation.9-Ethylcarbazole, phosphorus(V) oxychloride, 1,3-diaminopropane, and 2,2dimethyl-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 CDCl 3 .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.

Synthesis of the Ligands (Z,Z)-N,N
). 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).

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 L 1 /L 2 (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.

Preparation of Microorganism
Afterwards, the plates combined with the discs were left at 4 ∘ C for 2 h, the plates injected with yeast were incubated at 25 ± 0.1 ∘ C for 24 h, and ones injected with bacteria were incubated at 37 ± 0.1 ∘ C for 24 h.After 24 h, inhibition zones appearing around the disks were measured and recorded in mm [41][42][43][44].

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 g/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.

Oxidation Procedure.
A mixture of 1.10 −3 mol catalyst, 20 mL solvent (CH 3 CN), 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.

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 L 1 and at 1646 cm −1 for L 2 .In the IR spectra of L 1 and L 2 , 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 L 1 and at 1595 cm −1 for L 2 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 L 1 and at 809 cm −1 for L 2 ) and one orthodisubstituted benzene ring (strong aryl-H vibrations at 740 cm −1 for L 1 and at 747 cm −1 for L 2 ) 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 L 1 -Co, 1589 cm −1 for L 1 -Mn, 1550 cm −1 for L 2 -Co, and 1540 cm −1 for L 2 -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].

Electrochemical Properties of the Synthesized Compounds.
Cyclic voltammogram studies were run in DMF (1 × 10 −3 M)-0.1 M NBu 4 BF 4 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 ferroceneferrocenium standard and we have obtained different oxidation and reduction processes for different scan rates.Examination of cv graphics of L 1 shows reversible oxidationreduction processes for the scan rate 100 mVs −1 (Figure 2(a)) at 0.87 V( pc ) and 0.88 V( pa ) ( pc :  pa = 1) and for the scan rate 500 mVs −1 at 0.94 V( pc ) and 1.04 V( pa ) ( pc :  pa ≅ 1).The quasireversible processes were observed for 250 mVs −1 scan rate at 0.87 V( pc ) and 1.0 V( pa ) and for 750 mVs −1 at 0.9 V( pc ) and 1.12 V( pa ).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  pc :  pa ratio equals 0.9 ≅ 1 ( pc = 0.93 V and  pa = 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( pc ) and at 1.07 V( pa ),  pc :  pa ratio equals around 1. The other ligand, L 2 , 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 ( pc :  pa =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( pc ) and 1.1 V( pa ) and at 0.99 V( pc ) and 1.1 V( pa ), 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.

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 L 1 (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 [CoL 1 (H 2 O) 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 C 5 H 8 N 2 .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 L 2 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 L 2 (Figure 5   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.

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.   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

Figure 4 :
Figure 4: Reversible reduction oxidation processes of the title compounds L 1 and L 2 in DMF (1.10 −3 M) solution.

Figure 5 :
Figure 5: (a) TG plot of L 1 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 L 2 -Mn recorded under nitrogen atmosphere between the temperature ranges 30 ∘ C and 988 ∘ C at a heating rate of 10 ∘ C/min.

Figure 6 :
Figure 6: (a) TG plot of L 2 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 L 1 -Co recorded under nitrogen atmosphere between the temperature ranges 30 ∘ C and 988 ∘ C at a heating rate of 10 ∘ C/min.
(b)) shows three decomposition steps.The first one at 110 ∘ C corresponds to the loss of ethyl groups on the

which arises from the loss of ten hydrogen and one carbazole units leaving C 19 H 10 N 3 .
1 [ − 204], which arises from the loss of ten hydrogen and one carbazole units leaving C 21 H 14 N 3 .

Table 1 :
Electrochemical data of the title compounds.

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

Table 3 :
Oxidation of styrene and cyclohexene.

Table 4 :
Antimicrobial data of synthesized compounds.
a Inhibition zone, mm.b Undetermined inhibition zone.