Metal complexes of azo compounds derived from 4-acetamidophenol and substituted aniline

2.1 Materials and methods

All chemicals used were of the analytical reagent grade. They included 4-acetamidophenol (Sigma), aniline, 4-methoxyaniline, 4-nitroaniline and 4-bromoaniline (Aldrich), sodium nitrite and hydroxide (BDH), ethyl alcohol (Adwic), and the disodium salt of ethylenediaminetetraacetic acid (Adwic). CuCl₂·2H₂O (Prolabo) and NiCl₂·6H₂O (BDH) were used as received. The elemental analyses (C, H, and N) were made at the Microanalytical Center at Cairo University. IR spectra were recorded on a Perkin-Elmer FT-IR type 1650 spectrophotometer. The solid reflectance spectra were measured on a Shimadzu 3101 PC spectrophotometer. The molar magnetic susceptibilities were measured on powdered samples using the Faraday method. The conductance measurements were carried out using a Sybron–Barnstead conductometer. Shimadzu TG-50H and DTA-50H thermal analyzers were used to record simultaneously TG, DTG, and DTA curves. The experiments were carried out in dynamic nitrogen atmosphere (20 ml min⁻¹) with a heating rate of 10 °C min⁻¹ in the temperature range of 20–1000 °C using platinum crucibles. Metal contents were determined by titration against standard EDTA after complete decomposition of the complexes with aqua regia in a Kjeldahl flask several times.

2.2 Synthesis of azo ligands (HL^1–4) (Mohamed, 1996)

p-X-Aminobenzene (0.93, 1.72, 1.23, and 1.38 g for X = p-H, -Br, -OMe, and -NO₂, respectively, 10 mmol) was mixed with HCl (11.5 M, 5 ml) and diazotized below 5 °C with NaNO₂ (2.07 g, 10 mmol). The resulting diazonium chloride was coupled with an alcoholic NaOH (3 g, 25 ml) solution of 4-acetamidophenol (4.53 g, 10 mmol) below 5 °C. The product was separated by filtration, purified by crystallization from diethyl ether and dried in vacuo over anhydrous calcium chloride. The red azo products are produced in 69–78% yield.

2.3 Synthesis of the complexes

The appropriate metal chloride (Cu(II) and Ni(II) ions) (10 mmol) in ethanol–water (1:1) (25 mL) mixture was added to the solution of the azo compound (0.40 g, 10 mmol) in the same solvent mixture (50 mL). The resulting solution was stirred under reflux for 0.5 h whereupon the complexes were precipitated. They were removed by filtration, washed with 1:1 ethanol/water and with Et₂O.

3.1 IR spectra and mode of bonding

As mentioned, the ligands have different coordinating sites. The IR has proven to be, in this particular case, a suitable technique to give enough information to elucidate the way of bonding of the ligands. Thus a detailed interpretation of IR spectra of these and the effect of binding of Ni(II) and Cu(II) ions on the vibration frequencies of the free ligands are discussed in this paper. The IR spectra of the free ligands and their metal complexes are carried out in the 4000–200 cm⁻¹ range (Table 2).

The IR spectra show broad-stretching vibration bands at 3284, 3321, 3285, and 3284 cm⁻¹ for H₂L¹, H₂L², H₂L³, and H₂L⁴, respectively, which could be attributed to OH of the phenolic group. The presence of coordinated water molecules renders it difficult to confirm the participation of the phenolic OH group in chelate formation. The bands observed at 1247, 1278, 1271, and 1270 cm⁻¹ are attributed to the ν(C–O) stretching vibration of the phenolic group for H₂L¹, H₂L², H₂L³, and H₂L⁴, respectively. These bands are shifted to higher or lower frequencies in all the complexes indicating coordination through the deprotonated phenolic OH group (Mohamed et al., 2001a,b, 2002). The strong bands observed at 1509, 1519, 1483, and 1492 cm⁻¹ in the spectra of the H₂L¹, H₂L², H₂L³, and H₂L⁴, respectively, may be attributed to ν(N⚌N) which shifted to higher or lower frequencies in all the complexes, indicating its involvement in the coordination of the ligands to the metal ions (Mohamed et al., 2001a,b, 2002). In the far IR spectra of all the complexes, there are new bands observed in the region of 580–400 cm⁻¹ which are absent in the spectrum of the free ligand. The bands observed at 500–529 cm⁻¹ (M–N) and 530–572 cm⁻¹ (M–O) (Table 2) provide conclusive evidence concerning the bonding of nitrogen and oxygen to the metal ions (Zaki et al., 1998). Therefore, the IR spectra indicate that the ligands coordinate through the deprotonated phenolic O and azo N.

3.2 Electronic spectra measurements

As further structural tools, solid reflectance spectral studies have been used to confirm the geometry of the complexes.

The diffused reflectance spectra of the Ni(II) complexes display three bands at ν₁: 14,771–15,625 cm⁻¹: ³A_2g → ³T_2g; ν₂: 17,361–18,484 cm⁻¹: ³A_2g → ³T_1g(F), and ν₃: 20,704–21,978 cm⁻¹: ³A_2g → ³T_1g(P) indicating octahedral configuration of the chelates (Mohamed et al., 2001a,b; Mohamed, 1996; Cotton et al., 1999). The spectra show also bands at 25,227–26,465 cm⁻¹ which may be attributed to ligand and to metal charge transfer.

The reflectance spectra of the Cu(II) complexes consist of low intensity shoulder bands centered at 14,286–14,970 and 17,241–17,391 cm⁻¹ (Mohamed et al., 2001a,b; Mohamed, 1996; Cotton et al., 1999), indicating the existence of the Cu(II) complexes in octahedral geometry.

The observed magnetic moment value of the Cu(II) and Ni(II) complexes are listed in Table 1 and the obtained values confirm the octahedral geometry.

3.3 Thermal analyses (TG and DTG) studies

Table 3 shows the TG and DTG results of thermal decomposition of the metal chelates. The thermograms of the [Ni(HL¹)Cl(H₂O)₃], [Cu(HL⁴)Cl(H₂O)₄], and [Ni(HL⁴)Cl(H₂O)₃] chelates show four decomposition steps within the temperature range of 50 to 1000, 30 to 900, and 30 to 900 °C, respectively. The first step of decomposition within the temperature range of 50–230, 30–120, and 30–900 °C occurred with the loss of two water molecules and HCl, HCl, and HCl gases with an estimated mass loss of 16.29% (16.76), 7.26% (8.99%), and 10.68% (calcd. 9.09%) for [Ni(HL¹)Cl(H₂O)₃], [Cu(HL⁴)Cl(H₂O)₄], and [Ni(HL⁴)Cl(H₂O)₃] chelates, respectively. The remaining three steps of decomposition take place in the temperature range of 230 to 1000, 120 to 900, and 100 to 900 °C which are attributed to the loss of HL¹, two water molecules and HL⁴ and two water molecules and HL⁴ for [Ni(HL¹)Cl(H₂O)₃], [Cu(HL⁴)Cl(H₂O)₄], and [Ni(HL⁴)Cl(H₂O)₃] chelates, respectively.

It is obvious that, [Cu(HL¹)Cl(H₂O)₃]·1.5H₂O and [Ni(HL²)Cl(H₂O)₃]·2H₂O chelates show five steps of decomposition. The first step of decomposition within the temperature range of 25–130 and 30–100 °C corresponds to the loss of one and half and two hydrated water molecules with an estimated mass loss of 6.22% (5.82%) and 7.24% (calcd. 7.45%), for [Cu(HL¹)Cl(H₂O)₃]·1.5H₂O and [Ni(HL²)Cl(H₂O)₃]·2H₂O chelates, respectively. While the second and third steps of decomposition correspond to the loss of coordinated water molecules and HCl gas within the temperature range of 130–330 and 100–300 °C with an estimated mass loss of 16.35% (15.63) and 18.49% (calcd. 18.72%) for [Cu(HL¹)Cl(H₂O)₃]·1.5H₂O and [Ni(HL²)Cl(H₂O)₃]·2H₂O chelates, respectively. In the final steps of decomposition within the temperature range of 330–900 and 300–900 °C, estimated mass losses of 61.16% (calcd. 62.21%) and 57.17% (calcd. 58.74%) are observed and can be attributed to the loss L¹ and L² ligand molecules leaving CuO and NiO as a residue for [Cu(HL¹)Cl(H₂O)₃]·1.5H₂O and [Ni(HL²)Cl(H₂O)₃]·2H₂O chelates, respectively.

In addition, [Cu(HL²)Cl(H₂O)³] and [Ni(HL³)Cl(H₂O)₃] chelates show five steps of decomposition. The first two steps of decomposition within the temperature range of 30–190 and 30–100 °C correspond to the loss of HCl and two coordinated water molecules and HCl and HBr and two water molecules of coordination with an estimated mass loss of 15.48% (16.04%) and 31.82% (calcd. 31.88%), for [Cu(HL²)Cl(H₂O)³] and [Ni(HL³)Cl(H₂O)₃] chelates, respectively. While the remaining steps of decomposition correspond to the loss of HL² and HL³ ligands within the temperature range of 190–800 and 350–850 °C with an estimated mass loss of 67.81% (calcd. 66.59) and 57.34% (calcd. 57.11%) for [Cu(HL²)Cl(H₂O)³] and [Ni(HL³)Cl(H₂O)₃] chelates, respectively.

On the other hand, in the [Cu(HL³)Cl(H₂O)₃] complex, the first two steps in the temperature range of 30–250 °C, with an estimated mass loss of 20.98% (calcd. 19.06%) can be attributed to the loss of HCl gas and three water molecules. The mass losses of the remaining decomposition steps within the temperature range of 250–950 °C correspond to the decomposition of HL³ ligand leaving CuO as a residue (Table 3).

3.4 Structural interpretation

From all of the above observations, the structure of all complexes for four ligands may be interpreted with a similar distribution of like coordinating sites, which coordinated to Cu(II) and Ni(II) ions and takes place via the phenolic O and azo N atom. Although ligands do not have the same fragments, they all contain the same coordinating sites, arranged in the same way. Thus, their coordinating behaviour described in the crystal structure of their complexes is relevant for the interpretation of the ligands’ coordinating capacity. The structure information from these complexes is in agreement with the data reported in this paper based on the elemental analysis, IR, and electronic spectra measurements. Consequently, the structures proposed are based on octahedral geometry. The proposed general structures are shown in Fig. 2.

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Figure 2

Proposed structural formulae of metal complexes.

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