2.1. Materials and methods

All commercial products were purchased from SigmaAldrich. Solvents were purified and dried by the standard methods. Melting points were determined in open capillary tubes on a Stuart melting point apparatus. The FTIR spectra were run in the single beam Nicolet IR 100 (Fourier-Transform); while UV of all the samples were taken in water using UV-Genesys spectrophotometer. Their mass spectral data were obtained from waters GCT premier spectrometer. The ¹H NMR and ¹³C NMR spectra were recorded in DMSO-d₆ using NMR Bruker DPX 400 spectrophotometer operating at 300 and 75MHz for ¹H and ¹³C NMR respectively. TMS was used as internal standard with the deuterium signal of the solvent as the lock and chemical shifts δ recorded in ppm. The elemental analysis (C, H, N, S) of the compounds was performed using Flash EA 1112 elemental analyzer while the pH was monitored using Portable pH Meter Model PHB4. Compounds were routinely checked by TLC on silica gel G plates using three different eluting solvents depending on the polarity disparity. The solvent systems are petroleum ether:chloroform (9:1, v/v), petroleum ether:chloroform (6:4, v/v) and chloroform:methanol (9:1, v/v). Also, the developed plates were visualized using a UV lamp for the presence of spots and R_f values were duly calculated. All of the crude products were isolated as solids and purified. Fastness to light was assessed in accordance with BS 1006-1978. Rubbing fastness was checked with an Atlas Crock meter in accordance with AATCC TM 8-1961 and the wash fastness was determined according to ISO: 765-1979 (^{Maradiya & Patel, 2002}; ^{Sakoma, Bello, & Yakubu, 2012}).

2.2. General procedure for the synthesis of 1-(p-sulphophenyl)-3-methyl-5-pyrazolone based acid dyes

Synthesis of acid dyes and their metal complexes involves three-step procedure which is as follows.

2.2.2. Diazotization and coupling with phenol derivatives

To the well stirred ice jacketed aqueous solution (2.69 g) of 1(p-sulphophenyl)-3-methyl-4-amino pyrazolone (at 0-5 °C) was added conc. HCl (3.5 ml) and sodium nitrite solution (0.7 g in 2 ml H₂O). The reaction mixture was vigorously stirred for 1 h at the above mentioned temperature to obtain the diazonium salt of 1-(p-sulphophenyl)-3-methyl-4-amino pyrazolone. The diazonium compound formed in this way was coupled to various coupler mentioned previously to synthesize our dyes. Thus 1.285 g (0.010 mol) 4-chlorophenol (5a) was dissolved in 200 ml water containing 0.45 g NaOH and coupled with prepared diazo. The coupling was facilitated using sodium carbonate as an acid binding agent. The reaction mixture was given 4-5 h to complete the coupling at 30-35 °C. The dye was cooled to room temperature. Its pH was reduced to 4.5 by HCl and filtered. The cake was dried in oven at 70-75 °C till constant weight. By adopting the same procedure other dyes 6b-f were prepared from couplers 5b-f as shown in Scheme 1.

Scheme 1 Synthesis of ligand acid dyes 6a-f and their Fe (II) and Cu (II) complexes (7a-l). 

2.3. Dyeing method

Dye solution (10 ml, 0.4%, w/v) was taken in a dye-bath. The pH of the dye-bath was adjusted to 6.5 by adding acetic acid solution (1.0 ml, 10%, w/v) solution. The total volume of the dye-bath was adjusted to 100 ml by adding required amount of water. The leather swatches were introduced into the dye-bath with stirring. The content of the dye-bath was stirred for 1 h at 25-30 °C. The temperature was then gradually raised to 55 °C over period of half hour and maintained for one hour. The dyebath was kept rotating during the process of dyeing. Then 2.0 ml formic acid was added to adjust pH to 2.0 for dye fixation. After this, the spent dye liquor was taken in 250 ml volumetric flask. The swatches were washed with cold water and the combined solution of dye liquor and washings was then further diluted to 250 ml with water. The absorbance of this solution was measured to find out the exhaustion of dye on leather. The dyed swatches were dried and mounted on shade card. A weighed amount of leather swatch was stirred in boiling acidified pyridine which dissolves the unfixed dye from swatch and from the absorbance of this solution percentage fixation was checked.

3.1. Synthesis and spectral characterization of ligand acid dyes and their metal complexes

Synthesis of acid dyes and their iron (Fe, II) and copper (Cu, II) complexes were prepared in five step procedure as shown in Scheme 1. SPMP [1-(4-sulphophenyl)3-methyl-2-pyrazolin-5one] was nitrosated at 0-5 °C using NaNO₂ and HCl as described by Knorr. The nitroso compound was filtered to remove some terry material. The clarified nitroso derivative [that usually exists in an oxime form (as indicated by its FTIR)] was salted out by common salt. It was dried after filtration. Reduction of oxime was carried at 100-105 °C using zinc and HCl. The oxime and zinc were alternatively added in small portions in boiling HCl solution. The reduction was completed as the solution became colorless. A small amount of additional zinc dust was also added to prevent aerial oxidation on cooling. The resultant amine hydrochloride was quenched to −7 °C. The excessive un-reacted zinc was removed by filtration. This amine hydrochloride was diazotized using an aqueous solution of NaNO₂ (6.9 g dissolved in 250 ml of solution) and HCl at −5 to −2 °C to avoid the formation of Rubazoic acid, which is automatically formed during this reaction with increasing temperature due to oxidizing action of nitrous acid, formed in situ. The diazonium salt prepared in this way was coupled with different para substituted phenols, like, p-chlorophenol, p-nitrophenol, phenol-4-suphonic acid, 2nitrophenol-4-suphonic acid and bisphenols (bisphenol S and bisphenol A). The coupling was carried out in alkaline medium at pH 8-9. The synthesis of this diazo has been confirmed from X-ray structure of its crystal. Coupling in alkaline medium occurred at ortho position to the hydroxyl groups of phenol and bisphenol derivatives, as para position was blocked. The synthesized dyes 6a-f were precipitated on completion of reaction by reducing the pH of solution to 1.0 with HCl. The filtered dyes were dried and purified in ethanol. Metallization of these dyes andwas done by treating their alkaline °C. It took solutionabout with4-5 hFeSOto complete₄·7H₂O and CuSO₄·5H₂O at 65-70 °C. It took about 4-5 h to complete the metallization as was observed by taking the TLC of reaction mixture in 9:1 chloroform and methanol. Dyes (7a-l) were precipitated with addition of HCl, filtered and dried in oven at 80 °C. These dyes were again purified from ethanol, dried, weighed and determined the percentage yield. These unmetallized dyes 6a-f are tridentate ligands which form complexes with iron (Fe, II) and copper (Cu, II) through 1:1 metal and ligand stoichiometric ratio (Fig. 2). In case of Fe²⁺ and Cu²⁺ complexes lone pairs of electrons are donated by two oxygen atoms and one nitrogen atom of the diazo linkage, while the other three coordination numbers of these metals have been satisfied by three water molecules. The complex formation pattern has been verified by the UV-vis spectrophotometric studies of these dyes 7a-l. In case of IR spectra of compounds different moieties showed stretching and bending bands characteristic of the synthesized compounds. The infrared spectra of the synthesized acid dyes and their metal complexes exhibited absorption peaks due to O H, Ar H, C H, C O, C C, N N, SO₃H, C O and O M stretching and bending vibrations at 3399, 3050, 2926, 1550, 1472, 1272, 1164, 1004, 834,610cm⁻¹ as depicted from their FTIR spectra (Fig. 1). Similarly other metal complexes have been confirmed from their respective IR spectra.

Fig. 1 FTIR spectrum of Iron complex of dye 6b. 

¹H NMR and ¹³C NMR spectra were taken for ligand dyes which showed characteristic signals for different protons and carbons at different positions which evidenced the synthesis of dyes. In case of compounds 6b signal for hydroxyl group was absent due to exchangeable proton with DMSO. The doublet signal of mutually coupled set of four protons of phenyl group substituted with sulfonic group at present 7.95 and 7.68 ppm having coupling constant respectively signals 8.6 Hz. The multiplet signal for one proton of aromatic ring of coupling moiety is present at 7.83-7.90 ppm while the same ring bearing another single proton shows signal at 8.07 ppm with coupling constant 2.35 Hz which evidenced the meta relationship with another proton at the same ring. The methyl group present at pyrazolone ring exhibited singlet signal at 2.20ppm (Fig. 2). In ¹³C NMR spectrum the signal for methyl group is present at 12.14 ppm. There are ten signals in the range 117.06-158.00 ppm for different carbon nuclei in the compound (Fig. 3). In this way other ligand acid dyes were confirmed from their respective ¹H NMR and ¹³C NMR spectra. When the ¹H NMR spectra of metal complex dyes were conducted they showed very broad and distorted signals due to paramagnetic nature of metals used for complexation and the so the NMR study of complexes was not fruitful for structure elucidation but in other words the distorted broad signals proved the complexation when the ¹H NMR of ligands acid dyes and complexes were compared.

Fig. 2 ¹H NMR spectrum of ligand acid dye 6b. 

Fig. 3 ¹³C NMR spectrum of ligand acid dye 6b. 

3.2. Dyeing properties of substituted ligand acid dyes and their metal complexes (6a-f and 7a-l)

The dyeing properties of p-substituted phenols and bis phenol dyes have been found to be very attractive. Almost all properties have been found to be of very high value (4-5). Ligand dyes had low values as per our expectation due to the presence of free hydroxyl groups. The dyeing properties of these dyes are given in Table 1.

Almost all un-metalized p-substituted phenol dyes have been found to be different; both in 2% and 5% dyed leathers, along with a variation of the depth of shades. This can be attributed to the difference of chromophoric systems in all dyes. However, it is clear from shades that this difference is due to the difference of p-substituents in different phenols. All of the phenol homologues had dark and redder shades except p-sulphophenol substituted dyes. The variation of the depth can be attributed to the participation of peripheral group’s variation in different phenols.

Almost all iron-metalized phenol dyes had been found to be similar (olive); both in 2% and 5% dyed leathers, along with a variation of depth of shades. However, it is clear from shades that the depth difference is due to the difference of p-substituents in different phenols. All the phenol homologues had dark and redder shades except p-sulphophenol. Both nitro phenols had similar shades with yellowish tone. Almost all copper-metalized phenol had been found to be different; both in 2% and 5% dyed leathers, along with a variation of depth of shades. This can be attributed to the difference of chromophoric systems in all dyes. However, it is clear from shades that this difference is due to the difference of p-substituents in different phenols. All of the phenol homologues had dark and redder shades except p-sulphophenol. Both nitro phenols had similar shades with yellowish red tone.

The dyeing properties of bis-phenols dyes have been found to be very good and attractive. Almost all properties have been found to be of very high value (4-5). The dyes with bisphenol-A have been found to be much darker as compared to the dyes of bisphenol-S. This can be attributed to the n → п* electronic transitions occurring in sulphone group of bisphenol-S.

As it is clear from dye shades among the un-metalized bisphenol dyes, the dye with bisphenol-A had a high color value while bisphenol-S dye gave a low color yield on leather. This difference can be ascribed to the participation of sulphone group present in bisphenol-S.

It is clear from the depth of shades that iron metalized bisphenol dyes had greater color value than their parent un-metalized bisphenol dyes. These dyes are much greener as they were expected. The iron complex with ligand acid dye based on bisphenol-S had a low color value while bisphenol-A dye provided a darker shade and high color yield on leather along with a much redder effect. This difference can be attributed to the participation of sulphone group in bisphenol-S.

Similarly, copper metalized bisphenol dyes had also greater color value than their parent un-metalized bisphenol dyes. These dyes are much redder as per our expectation. The copper complex dye with bisphenol-A had a high color value while bisphenol-S dye gave a lighter and low color yield on leather along with a much redder effect. This difference can be attributed to the participation of sulphone group in bisphenol-S (Figs. 4 and 5).

Fig. 4 2% shade of dyes 6a-f and 7a-l on leather. 

Fig. 5 5% shade of dyes 6a-f and 7a-l on leathe 

3.3. Optical studies of dyes

In case of optical studies, dyes were conducted their UV-vis studies to observe the changes rendered by metal complex formation. The UV-vis data presented in Table 2 is supported by combined UV-vis spectra of ligand acid dyes 6a-f. The un-metalized dye-6a was an orange dye with λ_max 460 nm while on metallization of this with iron, Olive Brown dye (Dye-7a) was produced with λ_max 490 nm undergoing a bathochromic shift of 30 nm along with hypochromic effect. Metallization of dye 6a with copper resulted in the formation of a Reddish Violet dye (Dye-7b) with λ_max 500 nm and showed a bathochromic shift of 40 nm with hypochromic shift in intensity.

Three dyes were prepared with p-nitrophenol. The first one being an un-metalized dye and the other two were iron and copper complexes, respectively. The detailed properties of pnitrophenol dyes are given in Table 1. The unmetallized dye 6b was an orange dye with λ_max 480 nm which developed yellowish brown dye on complexation with iron (Dye 7c) with λ_max 440 nm. It showed hypsochromic shift of 40 nm along with hyperchromic effect. Similar behavior was observed for copper complexes of this dye. The dye 6c was prepared from phenol-4-sulphonic acid which was Yellowish Orange colored dye having λ_max 460 nm. Metallization of the dye 6c with iron and copper produced yellowish brown (Dye 7e) and Reddish Orange dye (Dye 7f) dyes with λ_max 430 and 460 nm respectively which exhibited pattern of hypso and hypochromic effects. Dyes 7g and 7h were iron and Cu complexes of 1:1 type of dye 6d. The un-metalized dye 6d was a Reddish Orange dye with λ_max 520 nm. Its metallization with Iron produced a yellowish brown dye (Dye 7g) with λ_max 470 nm and enhanced the absorbance intensity than parent dye and showed a hypsochromic shift of 50 nm. While metallization of the same with copper resulted in the formation of a Reddish Brown dye (Dye 7h) with λ_max 500 nm and a hypsochromic shift of 20 nm was observed.

In the bisphenol series six dyes were prepared. For this purpose two different bisphenols, namely 4,4′-dihydroxy biphenyl sulphone (BPS), 4,4′-dihydroxy biphenyl propane (BPA), were used as couplers. Three disazo dyes were accomplished with bisphenol-S. The first dye being an un-metalized one and another two were iron and copper bis-metal complexes, respectively. The un-metalized dye 6e was a Reddish Orange dye with λ_max 420 nm. Its metallization with iron produced a Reddish Brown dye (Dye 7i) with λ_max 410 nm and a hypsochromic shift of 10 nm was seen. While the metallization of same dye 6e with copper resulted in the formation of a Yellow dye (Dye 7j). It had λ_max 370 nm and absorbance 1.6, showed a hypsochromic shift of 50 nm. The metal complexes of dye 6f with iron and copper exhibited the similar behavior like bisphenol S dye metal complexes in absorption intensity while no conceivable change was found in λ_max position. From the detailed UV-vis study of dyes it was gathered that metal complexes of ligand dyes bearing electron with drawing groups showed hypsochromic shit and those having electron donating groups showed bathochromic shift in absorbance which is in accordance with well established UV-vis absorption pattern of compounds (Figs. 6 and 7).

Fig. 6 UV-vis spectrum of ligand acid dyes 6a-f. 

Fig. 7 Effect of metal ions on λ_max of ligand acid dyes 6a-f.4.