Synthesis, characterization, thermal and catalytic properties of a novel carbazole derived Azo ligand and its metal complexes

2.1 Synthesis of the ligand

Formylation of 9-ethylcarbazole was done by using Vilsmeier formylating agent DMF and POCl₃ (Grigoras and Antonoaia, 2005; Yoon et al., 2007; Bal and Bal, 2014). The monoaldehyde was separated by flash chromatography with ethyl acetate/hexane (1:10) as eluent. N-Ethylcarbazole-3-carbaldehyde was then reacted with 1,3-diaminopropane to give the imine compound; (Z,Z)-N,N′-bis[(9-ethyl-9H-carbazol-3-yl)methylene]propane-1,3-diamine, which was previously synthesized and characterized by us (Bal and Bal, 2014). Finally, our ligand was synthesized through azo coupling reaction (Eren et al., 2014; Bal et al., 2014a,b) between diazonium salt of 2-amino-4-methylphenol and (Z,Z)-N,N′-bis[(9-ethyl-9H-carbazol-3-yl)methylene]propane-1,3-diamine. Synthesis scheme can be seen in Fig. 1.

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Figure 1

Reaction Scheme for the syntheses and proposed structures of the ligand (L) and its complexes.

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Chemical formula: C₄₇H₄₄N₈O₂.Yield: 70%, m.p.: 185–187 °C, Elemental Analysis found% (calculated%): C 75.21(74.98) H 6.1(5.89) N 14.91(14.88). UV–Vis (ethanol) (λ_max, nm) (ε, M⁻¹ cm⁻¹): 265, 358, 448. FT-IR (KBr, cm⁻¹): 3390 (brd) (Ar—OH); 2971 (w), 2847 (w), 2919 (w), 1598 (w) (Ar—H, C—H); 742(s) (Ar—H), 1615(s) (CH—N), 1471(s) (N—N). Mass spectrum (LC/MS APCI): m/z 737.4 [M-CH₃]⁺. ¹H NMR (300 MHz,CDCl₃): 8.1 (H-5, H-5′, s), 7.91 (H-2, H-2′, d, J = 8.4 Hz), 4.41 (H-14, H-14′, q, J = 7.2), 1.47 (H-15, H-15′, t, J = 7.2), 8.6 (H-16,H-16′, s), 3.84 (H-17&H-19, t, J = 6.8), 2.26 (H-18, pentet, J = 6.8), 7.2 (H-21, H-21′, s), 7.0 (H-23, H-23′, d, J = 7 Hz), 6.9 (H-24, H-24′, d, J = 8.1 Hz), 2.1 (3H-26, 3H-26′, s). ¹³C NMR (400 MHz,CDCl₃): 126.04 (C-1, C-1′), 125.9 (C-2, C-2′), 127.6 (C-3, C-3′), 123.1 (C-4, C-4′), 120.7 (C-5, C-5′), 119.4 (C-6, C-6′), 108.5 (C-7, C-7′), 108.7 (C-8, C-8′), 141.5 (C-10, C-10′), 127.2 (C-11, C-11′), 124.0 (C-12, C-12′), 140.5 (C-13, C-13′′), 37.7 (C-14, C-14′), 13.8 (C-15, C-15′), 162.06 (C-16, C-16′), 59.5 (C-17&C-19), 32.3 (C-18), 135.09 (C-20, C-20′), 120.34 (C-21, C-21′), 136.7 (C-22, C-22′), 120.84 (C-23, C-23′), 120.92 (C-24, C-24′), 150.0 (C-25, C-25′), 29.7 (C-26, C-26′). Ʌ_M (Ω⁻¹ cm² mol⁻¹): 7.

2.2 Synthesis of complex compounds

Acetate salts (1 mmol) of Cu(II), Co(II) and Ni(II) were mixed with ethanol solution of our ligand (1 mmol). The resulting mixtures were left refluxing overnight. Complex compounds were crystallized from ethanol.

2.3 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. Then 20 mmol hydrogen peroxide (30% in water) was added dropwise. The resulting mixture was vigorously stirred for 8 h at 60 °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.

3.1 Spectral analysis

¹H NMR spectrum of our ligand (Fig. 2) showed the ethyl group protons on the carbazole units at 4.41 ppm (2H-14) as quartet (J = 7.2 Hz) and at 1.47 ppm (3H-15) as triplet (J = 7.2 Hz). The protons on the propane moiety were appeared at δ 3.84 (2H-17 & 2H-19) as triplet (J = 6.8 Hz) and at δ 2.26 (2H-18) as pentet (J = 6.8 Hz). The methyl protons on the two phenolic rings appeared at δ 2.1 and the hydroxyl protons were at around δ 10 ppm (Kose et al., 2013). We had identified the proton signals belonging to the carbazole unit previously (Bal and Bal, 2014). In the proton NMR of our newly synthesized ligand, we were able to see the hydrogens of 4 and 4’ at 8.6 ppm as singlet overlapping with the imine protons. The signal seen as a singlet at 8.1 ppm belongs to H-5 and H-5′ on carbazoles and the doublet observed at δ 7.91 (J = 8.4 Hz) was attributed to H-2 and H-2′. The other carbazole ring protons were seen between 7.3 ppm and 7.7 ppm. Considering the phenolic rings in our compound, we have seen the two isolated hydrogens of 21 at 7.2 as a singlet and the hydrogens of 23 and 24 were seen as doublets at 7 ppm and 6.9 ppm respectively. ¹³C NMR spectrum (Fig. 3) showed aromatic carbons between 100 ppm and 150 ppm. Among them, the peaks at 108.7 ppm and at 108.4 ppm belong to C-8, C-8′ and C-7, C-7′ on the carbazole units. The quaternary carbons C-10, C-10′ and C-13, C-13′ gave peaks at δ 141.5 and δ 140.4. The imine carbons bridging the two carbazole units appeared at 162.1 ppm. The three quaternary carbons on the phenolic aromatic rings, attached to the carbazole units through azo linkage, appeared at 150 ppm (C-25, C-25′), 135.09, (C-20, C20′) and 136.7 ppm (C-22, C-22′) (Gozel et al., 2014; Kaya et al., 2012). The methyl carbons para to the hydroxyls were seen at 29.7 (Gozel et al., 2014). The ethyl groups attached to Nitrogen on the carbazole units were at 37.7 ppm (C-14, C-14′), and at 13.9 (C-15, C-15′). The carbons belonging to 1,3-diaminopropane between the two carbazoles were observed at δ 59.5 (C-17, C-19); δ 32.3 (C-18). The IR spectra of all the complexes showed a considerable red shift in their imine stretching frequencies compared to that of the ligand itself, which proves that the complexation took place on the imine Nitrogens. Also, it is a common knowledge that the imine Nitrogens are always more likely to undergo complexation in the presence with transition metals. Although it is changeable, depending on the metal (Shindo and Brown, 1965) and the structures of the ligands (e.g. containing electronegative atoms) (Cabaniss and McVey, 1995), it is generally known that the bands between 1380 and 1600 cm⁻¹ are attributed to the symmetric and anti-symmetric stretching modes of carboxylate groups respectively (Finnie et al., 1998). The IR spectra of all complexes showed symmetric and asymmetric carboxylate stretchings, proving the acetate coordinations to the metals, and the difference between the splitting of carboxylate stretching bands (Δ = Ѵ_as–Ѵ_s) in our complexes were found to be between 173 and 181 as shown below. These results prove the unidentate pattern of carboxylate coordinations (Finnie et al., 1998).

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Figure 2

¹H NMR spectrum of the ligand (L).

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Figure 3

¹³C NMR spectrum of the ligand (L).

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3.2 Thermal analysis

TG traces of the synthesized compounds provided additional information regarding their thermal stabilities and thermal degradation behaviors (Fig. 4). Examination of the thermogram of our ligand shows continuous weight losses, which can be divided somewhat distorted intervals as shown in Table 1. The biggest weight loss was observed for the loss of two carbazole units with a percentage of 47.9 (calc. 47.3). The residual part, however, was around 12.3% (calc. 12.7%) and this percentage can be attributed for the loss of C₃H₆N₂ unit that is the 1,3-diaminopropane unit in the middle of the two carbazoles. TG graphic of the Copper complex of the ligand reveals the loss of two ethyl groups on carbazoles between 100 and 180 °C. The second loss between 220 and 300 °C was assigned to two molecule azo containing phenolic groups with loss of 28.6%. Above 300 °C, the biggest weight loss was recorded as 47.1% (Calc.46.9%), corresponding to two carbazole units, diaminopropane bridge (C₃H₆N₂) and the water molecules in the lattice. Cobalt coordination compound shows two degradation steps, the biggest mass loss happens between 110 °C and 700 °C with a percentage of 75 (Calc. 75.6%) which corresponds to the distinct parts of the whole molecule. The residual parts of all complexes of our ligand gave the masses of the two acetate units and the Copper and Cobalt metallic losses were seen in the residual parts of CuL(H₂O)₂(AcO)₂ and CoL(H₂O)₂(AcO)₂; however, the Nickel metallic loss was found with the losses of two ethyl groups and two water molecules in the first half of the thermal graphic of NiL(H₂O)₂(AcO)₂, the second half of which gave the biggest loss with a mass loss of 70.8% (Calc. 71.1).

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Figure 4

TG plots of L and its complexes recorded under nitrogen atmosphere between the temperature range 30 °C and 988 °C at a heating rate of 10 °C/min.

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3.3 Catalytic activity

Among the complex compounds, Copper (II) coordination compound showed better catalytic activity compared to the other coordination compounds. Both styrene and cyclohexene conversion percentages were 75.3% and 70.2% respectively and the styrene oxide selectivity was recorded as 22.2% while the cyclohexene oxide selectivity was 25.3%. Cobalt complex can be recorded as the second best catalyst in these oxidation reactions with 61.3% styrene and 59.7% cyclohexene conversion. The Nickel (II) complex, however, gave poorer results in both styrene oxidation and cyclohexene oxidation reactions. Catalytic activity results can be seen in Table 2 and Fig. 5 shows the possible oxidation reaction schemes.

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Figure 5

Various oxidation products of cyclohexene (a) and styrene (b).

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