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Original article
10 (
2_suppl
); S2829-S2835
doi:
10.1016/j.arabjc.2013.11.005

Oxidative coupling of 2-aminophenol to 2-amino-phenoxazine-3-one catalyzed by organotin (IV)–copper (I) cyanide coordination polymers as heterogeneous catalysts

, , ,
 Department of Chemistry, Faculty of Science, Tanta University, Tanta 31527, Egypt

⁎Corresponding author. Tel.: +20 1007276665. saharelkhalfy@hotmail.com (Sahar H. El-Khalafy)

Received: , Accepted: ,
© The Author(s) 2013
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The activity of organotin (IV)–copper (I) cyanide coordination polymers [Ph3SnCu (CN)2 · L] where L = 1,2-bis(4-pyridyl)ethane (bpe) 1, 4,4′-bipyridine (bpy) 2, methylpyrazine (mepyz) 3 or trans-1,2-bis(4-pyridyl)ethene (tbpe) 4 has been investigated as heterogeneous catalysts for the biomimetic oxidative coupling of 2-aminophenol (OAP) to 2-aminophenoxazine-3-one (APX). The rate of OAP consumption was determined by measuring the amount of dioxygen consumed using a gas burette. All the coordination polymeric catalysts showed high activity for auto-oxidation of OAP. The coordination polymer 4 has been found to be the most reactive catalyst. The rate of auto-oxidation reaction of OAP catalyzed by the coordination polymer 4 has been found to increase with increasing the pH from 7 to 9 then decreased at higher pH. The rate of auto-oxidation reaction showed linear dependence on catalyst concentration and pressure of dioxygen. The rate of reaction has been found to fit a Michaelis–Menten kinetic model for saturation of catalyst sites with increasing OAP concentration. The catalyst 4 showed no deactivation after six runs.

Keywords

Coordination polymers
Organotin
Copper cyanide
Auto-oxidation
2-Aminophenol
2-Aminophenoxazine-3-one
1

1 Introduction

Coordination polymers are a new class of crystalline porous polymers that are formed by copolymerization of multidentate organic ligands with transition metal ions or metal ion clusters (Rowsell and Yaghi, 2004; Lin et al., 1999; Férey et al., 2005; Yaghi et al., 2003). Due to their zeolite-like properties, such as high surface areas, microporosity, well-defined structures and the ability to tune pore size on Å scale, recently they have been applied as effective catalysts for different reactions such as Knoevenagel condensation reaction (Hasegawa et al., 2007), Friedel–Crafts type reaction (Horcajada et al., 2007), aldol condensation reaction (Dewa et al., 2001), oxidation reactions (Llabrés i Xamena et al., 2008; Xiao et al., 2008; Qiu et al., 2009), asymmetric olefin epoxidation (Cho et al., 2006), asymmetric hydrogenation (Sabo et al., 2007) trans esterification (Seo et al., 2000; Zhou et al., 2009; Kwak et al., 2009), photochemical reactions (Pan et al., 2003) and as effective reagent for the oxidation of phenols (Hassanein and Etaiw, 1993).

The oxidative coupling of 2-aminophenol (OAP) to 2-amino-3H-phenoxazine-3-one (APX) (Scheme 1) through catalytic activation of dioxygen by transition metal complexes has been considered as one of the important reactions (McLain et al., 2000; Benedini et al., 1963; Simándi et al., 1987; Keum et al., 1990; Szeverényl et al., 1991; Simandi et al., 1993; Sakaue et al., 1993; Zaki et al., 2000; Kaizer et al., 2002; Horvath et al., 2004; Simándi et al., 2004; Maurya et al., 2005; Hassanein et al., 2008). 2-Amino-3H-phenoxazine-3-one also known as questiomycin A, is related to the naturally occurring antineoplastic agent actinomycin D, which acts by inhibiting DNA-directed RNA synthesis (Szigyártó et al., 2006; Homma and Graham, 1962) and is used clinically for the treatment of certain types of cancer (Homma and Graham, 1962; Nakazawa et al., 1981; Frei, 1974; Hollstein, 1974; Fukuda et al., 2005). APX has been used as a model for the behavior of actinomycin D. Phenoxazinone synthase catalyzes the oxidative coupling of two molecules of substituted 2-aminophenol to the phenoxazinone chromophore in the final step of the biosynthesis of actinomycin D.

Scheme 1

We report in this paper the catalytic activity of the organotin (IV)–copper (I) cyanide coordination polymers [Ph3SnCu(CN)2 · L] 1–4 (Scheme 2) for the oxidation of 2-aminophenol to 2-amino-phenoxazine-3-one.

Scheme 2

2

2 Experimental

2.1

2.1 Materials and reagents

All reagents were commercially available and used as received. OAP (Aldrich) was used as received. Water used in the reaction was double distilled deionized water.

2.2

2.2 Preparation of the coordination polymers 1–4

The coordination polymers 1–4 were prepared by the reported method (Etaiw et al., 2011).

Typical procedure: 90 mg (1 mmol) of CuCN was dissolved in a solution of KCN (90 mg, 1.5 mmol) in 10 mL H2O. Dissolution was assisted by heating and magnetic stirring. The resulting mixture was added to the solution of Ph3SnCl (384 mg, 1 mmol) and 1 mmol of the bipodal ligand; L (bpe, bpy, mepyz and tbpe) in 20 mL of acetonitrile. At once, precipitates of 1–4 were formed. After filtration, washing with small portions of water and acetonitrile, the products were dried overnight. Elemental analyses supported the chemical formulae of 1–4. Anal. Calc. for 1: C, 59.14; H, 4.19; N, 8.62%. Found: C, 58.78; H, 4.23; N, 8.08%. For 2: C, 57.95; H, 3.73; N, 9.01%. Found: C, 58.03; H, 4.82; N, 8.52%. For 3: C, 53.65; H, 3.78; N, 10.01%. Found: C, 53.27; H, 3.91; N, 9.82%. For 4: C, 59.33; H, 3.89; N, 8.65%. Found: C, 59.10; H, 4.01; N, 8.34%.

2.3

2.3 Auto-oxidation reactions

Oxidations of 2-aminophenol were performed as previously by stirring of 100 ml of an aqueous mixture containing 5 volume% of methanol in a 250-ml round bottomed flask attached to a gas burette (Hassanein et al., 2008). The coordination polymers 1–4 were insoluble in the reaction medium. The pH was adjusted to 9.0 using borate buffer. All reactions were carried out at 38 °C and at constant oxygen pressure of 730 mmHg. Lower partial pressures of oxygen were obtained by use of oxygen/nitrogen mixture at 1 atm total pressure. After completion of the reaction, the mixture was extracted by diethyl ether. The extract was purified by means of silica gel column chromatography using chloroform/methanol (volume ratio 20:1) to give APX as red crystals, m.p. 254–256 °C and shows absorbance at 435 nm, which is identical with the data reported in the literature (Simándi et al., 1987). All kinetic experiments were carried out in duplicate and reproducible results were obtained.

3

3 Results and discussion

3.1

3.1 Spectral characterization of the coordination polymers 1–4

The IR spectra of the coordination polymers 1, 2 and 4 display one strong band for νCN in the range of 2086–2094 cm−1. However, the IR spectrum of 3 displays two bands at 2081 and 2112 cm−1. The position of these bands reflects the covalent nature of the Cu—C≡N → Sn bridge unit since they usually occur at higher wavenumbers than νCN bands of the corresponding salt, K3[Cu(CN)4] (Eller et al., 1993). The IR spectra of 1–4 show a band due to stretching vibrations of the Sn—C bond at 540–570 cm−1. The presence of the Sn—C stretching bands is a good evidence of the presence of the Ph3Sn connecting units, which play the role of linking [Cu-(CN)2]− building blocks forming polymeric network. The presence of the aromatic ligands was confirmed by the bands characteristic of these ligands, Table 1. 1H and 13C NMR spectra of 1–4 display the same bands characteristic of the phenyl, the ligand (L) and the cyanide groups, Tables 2 and 3. 1H NMR spectra of 1–4 display broad-multiplet bands at δ 7.47–7.92 ppm that are due to the protons of the phenyl groups (Ronconi et al., 2003) while 13C NMR spectra exhibit a singlet band at δ 137 ppm due to the cyanide group and multiplet bands between δ 129.00 and 131.50 ppm due to the phenyl groups (Brimah et al., 1994), Tables 2 and 3. Thus, the IR and NMR spectra confirm the presence of the cyanide ligand, the ligands (L) as well as the Ph3Sn units as bridging groups.

Table 1 Wavenumbers (cm−1) of different vibrational modes of 1–4.
Com-pound νCH (arom.) νCH (aliph.) νCN ν C⚌N νC⚌C νSn-C
1 3067,3018 2930 2091 1604 1558,1520,1479 540
2 3045,2988 – 2094 1596 1565,1530,1479 570
3 3067,3020 2991 2081,2112 1595 1575,1515,1478 570
4 3062, 3045 2989 2086 1600 1552,1518,1480 550
Table 2 1H NMR data for 1–4.
Compound δ(Ph-Sn) δ(Ligand)
1 7.47–7.92 (m) 8.41 (d), 7.22 (d), 2.91 (s)
2 7.49–7.82 (m) 8.71 (d), 7.82 (d)
3 7.50–7.79 (m) 8.41–8.51 (m), 1.05 (s)
4 7.52–7.78 (m) 8.58 (d), 7.60 (d), 6.93 (s)
Table 3 13C NMR data for 1–4.
Compound δ(Ph-Sn) δ(CN) δ(Ligand)
1 130.72–129.53 (m) 137 (s) 36 (s), 124 (s), 150 (s), 151(s)
2 130.04–129.20 (m) 137 (s) 122 (s), 142 (s), 151 (s)
3 130.66–129.00 (m) 137 (s) 22 (s), 141 (s), 142 (s), 144 (s), 145 (s)
4 131.50–129.84 (m) 137 (s) 122 (s), 132 (s), 144 (s), 151(s)

3.2

3.2 Auto-oxidation of 2-aminophenol

The activity of coordinate polymers [Ph3SnCu(CN)2 · L] 1–4 was investigated as heterogeneous catalysts in the auto-oxidation of 2-aminophenol to 2-amino-3H-phenoxazine-3-one (APX). The rate of 2-aminophenol consumption was determined by measuring the amount of dioxygen consumed using a gas burette. After a short induction period, the volume of dioxygen consumed was linear with time, indicating a zero-order dependence on substrate concentration.

The data summarized in Table 4 show the activity of the coordination polymers 1–4 in the auto-oxidation of 2-aminophenol in aqueous mixture at pH 9.0 and slightly less than 1 atm of dioxygen at 38 °C. All the coordination polymers showed high activity for auto-oxidation of 2-aminophenol. The relative zero-order rate constants kobs show that the coordination polymer 4 is the most reactive catalyst. Auto-oxidation of o-aminophenol catalyzed by the coordination polymer 4 gave within 2.5 h APX in 65% yield and 20% unreacted 2-aminophenol in addition to the unidentified material. Detailed study of the auto-oxidation of 2-aminophenol was carried out using the coordination polymer 4 as catalyst.

Table 4 Oxidation of 2-aminophenol by copper (I)–tin coordination polymers a 1–4.
Catalyst kbobs (mol L−1 min−1) × 105
– 0.4538
1 0.8403
2 0.8529
3 0.9887
4 1.2286
a All reactions were carried out at 38 °C and dioxygen pressure of 730 mmHg with magnetic stirring of 1.5 mmol of OAP dissolved in 5.0 ml of methanol and 0.015 mmol of catalyst. The total volume of reaction mixture was maintained at 100 ml. The pH was adjusted to 9.0 using borate buffer.
b Initial rate constant calculated from the plot of oxygen consumption vs. time.

3.3

3.3 Effect of pH on the auto-oxidation of 2-aminophenol catalyzed by coordination polymer 4

The auto-oxidation of 2-aminophenol was studied in the pH range 7.0–11.0, using sodium borate and sodium phosphate buffers. The zero-order rate constant kobs of auto-oxidation reactions reached an optimum at pH 9.0 and then decreased at higher pH values (Fig. 1). The decrease of reaction rate at pH values higher than 9 indicates that the o-aminophenoxide anion is not the active species. In acidic media; pH = 3 and 4, no oxidation takes place.

Effect of pH on the rate of auto-oxidation of 2-aminophenol. All reactions were carried under conditions reported in Table 1 using 2 mmol of 2-aminophenol and 0.02 mmol of catalyst 4.The pH 7.0 was adjusted using a mixture of Na2HPO4 and HCl. The pH was adjusted to 8.0 and 9.0 by using sodium borate and HCl mixture and the pH was adjusted to 10.0 using NaHCO3 and NaOH mixture.
Figure 1
Effect of pH on the rate of auto-oxidation of 2-aminophenol. All reactions were carried under conditions reported in Table 1 using 2 mmol of 2-aminophenol and 0.02 mmol of catalyst 4.The pH 7.0 was adjusted using a mixture of Na2HPO4 and HCl. The pH was adjusted to 8.0 and 9.0 by using sodium borate and HCl mixture and the pH was adjusted to 10.0 using NaHCO3 and NaOH mixture.

3.4

3.4 Effect of concentration of the catalyst 4 on the auto-oxidation of 2-aminophenol

Data in Fig. 2 illustrate the effect of concentration of the catalyst 4 on the observed rate constant kobs of the auto-oxidation of 2-aminophenol. The observed rate constant kobs increased linearly with increasing the concentration of the catalyst 4 from 8 × 10−5 M to 3 × 10−4 M, indicating a first order dependence of rate of oxidation reaction on catalyst concentration studied in this range.

The dependence of rate constant on coordination polymer 4 concentration at pH 9. For reaction conditions, see Fig. 1.
Figure 2
The dependence of rate constant on coordination polymer 4 concentration at pH 9. For reaction conditions, see Fig. 1.

3.5

3.5 Effect of temperature on the auto-oxidation of 2-aminophenol catalyzed by the coordination polymer 4

The temperature dependence of the rate constant kobs from 28 °C to 55 °C gave Arrhenius activation energy of 23.0 kJ/mol (Fig. 3).

The Arrhenius plot of rate data at 28–55 °C under conditions given in Fig. 1 at pH 9.
Figure 3
The Arrhenius plot of rate data at 28–55 °C under conditions given in Fig. 1 at pH 9.

3.6

3.6 Effect of concentration of 2-aminophenol on the rate of auto-oxidation

The dependence of the rate constant kobs of oxidation reaction on the concentration of 2-aminophenol was investigated in the range 7.5 × 10−3 M to 3 × 10−2 M, Fig. 4. The reaction rate constants kobs increased with increasing the concentration of 2-aminophenol to 2 × 10−2 M and then leveled off. A double reciprocal Line weaver–Burk plot, Fig. 5 shows that the rate fits a Michealis–Menten kinetic model for saturation of catalyst site with increasing concentration of 2-aminophenol.

The dependence of rate constant on 2-aminophenol concentration under conditions given in Fig. 1 at pH 9.
Figure 4
The dependence of rate constant on 2-aminophenol concentration under conditions given in Fig. 1 at pH 9.
Lineweaver–Burk plot of the data in Fig. 4.
Figure 5
Lineweaver–Burk plot of the data in Fig. 4.

3.7

3.7 Effect of partial pressure of dioxygen on the auto-oxidation of 2-aminophenol catalyzed by catalyzed by coordination polymer 4

The effect of partial pressure of dioxygen on the auto-oxidation of 2-aminophenol was investigated by using oxygen/nitrogen mixture to obtain reduced partial pressure of 1 atm total pressure on the reaction mixture. The data illustrated in Fig. 6 show a linear dependence of the rate constant kobs on the partial pressure of dioxygen.

The dependence of rate constant in the oxidation of 2-aminophenol on dioxygen pressure under conditions given in Fig. 1 at pH 9.
Figure 6
The dependence of rate constant in the oxidation of 2-aminophenol on dioxygen pressure under conditions given in Fig. 1 at pH 9.

The linear dependence of zero-order rate constant kobs on the concentration of the catalyst 4 and on the partial pressure of dioxygen indicates that the rate of auto-oxidation of 2-aminophenol is not limited by mass transfer or intraparticle diffusion of dioxygen or 2-aminophenol to the active site in the coordination polymer particles.

A suggested mechanism for the auto-oxidation of 2-aminophenol catalyzed by the catalyst 4 is shown in Scheme 3. The observed rate dependence on 2-aminophenol concentration indicates coordination of 2-aminophenol with the catalyst 4 through the lone pair of electrons on the nitrogen (Szeverényl et al., 1991). The mechanism of auto-oxidation of 2-aminophenol catalyzed by the coordination polymer 4 (Scheme 3) probably involves a superoxo derivative containing one 2-aminophenol molecule in the axial ligand (Szeverényl et al., 1991; Simandi et al., 1993; Horvath et al., 2004). The copper superoxo adduct reacts with 2-aminophenol by abstracting H-atom forming the reactive 2-aminophenoxyl radical which disproportionates leading to the key intermediate o-benzoquinone monoimine. Its further conversion into the product APX is shown in Scheme 3. The overall reaction requires several oxidative dehydrogenation steps involving OAP, dioxygen and o-benzoquinone monoimine as reactants on the way to APX (Simándi et al., 1987; Zaki et al., 2000; Kaizer et al., 2002; Horvath et al., 2004; Hassanein et al., 2008).

Scheme 3

3.8

3.8 Catalyst reuse

Reuse of the catalyst 4 in the auto-oxidation of 2-aminophenol was tested as follows, auto-oxidation of 2-aminophenol was carried out under reaction conditions of Table 4 using 0.03 mmol of catalyst 4. The reaction was followed by dioxygen uptake. After the completion of the reaction, the products were extracted with diethyl ether and the catalyst was separated by filtration form the aqueous layer. The reused catalyst 4 showed no change in reactivity after six cycles.

Comparison of the results in the present study with our previously reported data on the auto-oxidation of OAP catalyzed by cobalt (II) phthalocyaninetetrasodiumsulfonate (Hassanein et al., 2008) and 5,10,15,20-Tetrakis-(p-sulfonatophenyl) porphinatocobalt (II) (El-Khalafy and Hassanein, 2012) indicated that the heterogeneous catalyst 4 has the same activity as soluble cobalt (II) phthalocyaninetetrasodiumsulfonate and soluble 5,10,15,20-Tetrakis-(p-sulfonatophenyl) porphinatocobalt (II) under the same conditions. However, the catalyst 4 is reusable and can be easily separated from reaction mixture by simple filtration. It is difficult to compare the catalytic activity of the other catalysts for the oxidation of OAP from the literature data due to the difference of experimental conditions.

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