Azo Reduction Research Articles

Chemistry of azopyrimidines. Part II. Synthesis, spectra, electrochemistry and X-ray crystal structures of isomeric dichloro bis[2-(arylazo)pyrimidine] complexes of ruthenium(II)

2-(Arylazo)pyrimidines (aapm, 3) are new N,N′- chelating ligands in the azoimine family and were reacted with RuCl 3 in ethanol under refluxing conditions. Three isomers of the composition Ru(aapm) 2Cl 2 have been chromatographically separated and are established as having trans–cis–cis (tcc), cis–trans–cis (ctc) and cis–cis–cis (ccc) configurations with reference to the order of coordination pairs as Cl; N(pyrimidine), N and N(azo), N′. Two of the three isomeric structures have been confirmed by X-ray crystallography. In both of these structures, the Ru–N(azo) distances are relatively shorter than those of Ru–N(pyrimidine), indicating stronger bonding in the former and the presence of a Ru-(aapm) π-interaction that is localised in the Ru-azo fragment. The isomer configuration is supported by IR and 1H NMR data. The complexes exhibit t 2(Ru)→π*(aapm) MLCT transitions in the visible region. Redox studies show the Ru(III)/Ru(II) couple in the green complexes [tcc-Ru(aapm) 2Cl 2] at 1.1–1.2 V and in the blue complexes [ctc- and ccc-Ru(aapm) 2Cl 2] at 1.2–1.4 V versus saturated calomel electrode (SCE) and two successive azo reductions. The difference in the first metal and ligand redox potentials is linearly correlated with ν CT [t 2(Ru)→π*(aapm)].

Polypyrrole Films Doped with an Electroactive Sulfonated Chelating Reagent: Electrochemical Characterization and the Detection of Metal Ions

Polypyrrole films doped with the electroactive chelating reagent 3-(2-pyridyl)-5,6-diphenyl-4,4′-disulfonate-1,2,4-triazine (PDTDS2–) were prepared and characterized. The electroactive azo bond in PDTDS2– was detected in the polymer film by cyclic voltammetry. The ratio of pyrrole to PDTDS2– in the films (as evaluated from azo reduction and polymer growth charges) was 11.7:1 (s =2.03, n=38), quite close to the expected ratio of 8:1 for a doubly charged anion-doped film. The azo bond electrochemistry in the polypyrrole exhibited Nernstian pH behavior (ΔEf/Δ pH=–54.7±1.3 mV dec–1). Copper detection by medium-exchange cyclic voltammetry at these films was examined and it was found that both reduction and stripping, oxidation responses were obtained but the latter was better as it was narrower and larger in magnitude. Thinner films gave better copper responses and detection of Cu2+ down to 1×10 –6 mol dm–3 was possible at films prepared with a growth charge of about 40 μC. Selectivity was evaluated in mixture experiments and a range of commonly occurring ions did not interfere in the Cu2+ electrochemical detection. The most severe interferents were mercury(II) and EDTA; new voltammetric peaks for Ag(I) and Hg(II) were observed.

Silicagel support mediated nonphotolytic cleavage of the rhenium–rhenium bond of [{NC 5H 4NNC 6H 4(R)}(CO) 3Re 0] 2. Synthesis of the monomeric species Re I{NC 5H 4NNC 6H 4(R)}(CO) 3Cl, crystal structure, spectroscopic and electron-transfer properties

The reaction of Re 2(CO) 10 with the 2-arylazopyridine ligand, L [LNC 5H 4–NN–C 6H 4(R), RH, o-Me/Cl, m-Me/Cl] in dry THF under a dinitrogen atmosphere afforded a metal–metal bonded product of the type (L)(CO) 3Re 0–Re 0 (CO) 3(L) 1. On a silica gel column and in the presence of chlorinated solvents (CHCl 3 or CH 2Cl 2) the complex 1 transformed into a mononuclear complex of composition Re I(L)(CO) 3Cl 2, where the cleavage of Re–Re bond of 1 and the oxidative addition of chlorine to the rhenium center have taken place consecutively. The molecular structure of the complex 2a (RH) has been determined by single crystal X-ray diffraction. The crystal lattice consists of two crystallographically independent Re(L 1)(CO) 3Cl molecules which are non-superimposable mirror images; d and l enantiomers, exist in a 1:1 ratio. The complexes 2 display irreversible Re I→Re II oxidation processes near 1.5 V versus SCE and two quasi-reversible azo (NN) reductions in the ranges −0.28 to −0.48 V and −0.83 to −1.06 V versus SCE. The complexes of the type 1 exhibit only four successive azo reductions in the range 0 to −2 V versus SCE. Both the complexes ( 1 and 2) display dπ(Re)→π*L metal-to-ligand charge-transfer transitions near 500 nm and intraligand n–π* and π–π* transitions near 400 and 300 nm, respectively. The complexes 2 are susceptible to a spontaneous chloride exchange reaction.

Synthesis, spectral and electrochemical studies of isomeric dichloro-bis[N(1)-ethyl-2-(arylazo)imidazole]ruthenium(II) and the reaction of the green isomer with tertiary phosphines

N(1)-Ethyl-2-(arylazo)imidazoles (L) react with RuCl3 giving green (5) and blue RuL2Cl2 (6) isomers. The green isomer is quantitatively transformed into the blue isomer on boiling under reflux in 1,2-dichlorobenzene. The ligand is of the N,N′-chelating type and in principle, provides five geometrical isomers. Considering the coordination pairs in the order: Cl, N(imidazole), N and N(azo),N′ the green isomer is␣spectroscopically characterised as trans-cis-cis-tcc-RuL2Cl2 (5) and the blue isomer is cis-trans-cis-ctc-RuL2Cl2 (6). The green isomer reacts smoothly with tertiary phosphines giving species of type [RuCl(P)L2]+ (7,8) and [Ru(P-P)L2]+2 (9,10) in which Cl, P and P-P respectively occupy cis-positions [P = PPh3, PPh2Me, P-P = Ph2P(CH2)2PPh2 (dppe) and Ph2P(CH2)3PPh2 (dppp)]. All the complexes display allowed t2(Ru) → π* (L) transitions in the visible region. A systematic shift of this MLCT band to higher energy occurs in the order: tcc-RuL2Cl2 (5) 1.2V versus s.c.e.) in phosphine derivatives (7–10). The azo reduction (E0 L) appears at negative values with respect to s.c.e. and is found to be sensitive to substitution in the phenyl ring of the ligand frame. The potential difference (E0 M−E0 L) is linearly related to the MLCT band energies (νCT) of green and blue RuL2Cl2 complexes. The phosphine derivatives behave differently, a linear correlation being observed between νCT and E0 M. The isomer configuration of RuL2Cl2 and the stereochemistries of the phosphine derivatives are established by 1H n.m.r. spectral data. The green (5) and blue (6) isomers exhibit single -Me or -OMe signals indicatives of C2-symmetry. The phosphine complexes [Ru(P)Cl(L2)2]+/[Ru(P-P)(L2)2]+2 show two equally intense methyl signals suggesting C1-symmetry in the derivatives.

Reaction of trans-dichloro-bis[N(1)-methyl-2-(arylazo)imidazole] ruthenium(II) with tertiary phosphines

Trans-dichloro-bis[N(1)-methyl-2-(arylazo)imidazol e] ruthenium(II) (tcc-Ru(MeL)2Cl2) reacts with tertiary phosphines giving rise to species of type [RuCl(P)-(MeL)2]+ and [Ru(P-P)(MeL)2]2+ in which Cl, P and P-P respectively occupy cis-positions [P=PPh3, or PPh2Me; P-P=Ph2P(CH2)2PPh2 (dppe) or Ph2P (CH2)3PPh2 (dppp)]. The cations have been isolated as perchlorates. The complexes display allowed t2(Ru) → π*(MeL) transitions in the visible region and show the energy ordering [RuCl(P)(MeL)2]+ 1.1 V versus s.c.e. The azo reduction is sensitive to the nature of substituents in the ligand. The 1H n.m.r. spectra of the complexes are compatible with the isomer of C1-symmetry.

Use of steric crowding to synthesise ruthenium(II) N(1)-alkyl-2-(arylazo)imidazole isomers. Spectral characterization and redox studies

Complexes of N(1)-methyl- and N(1)-benzyl-2-(dimethylphenylazo)imidazoles with ruthenium(II) have been prepared and characterised by physico-chemical and spectroscopic means. The 7,8-dimethylphenylazo ligands gave four stereoisomers, whereas the 8,9-dimethylphenylazo ligands gave only two. Isomer assignments are made on the basis of i.r. and 1H-n.m.r. data. Redox studies show the RuIII/II couple at 0.4–0.5 V (versus s.c.e) for the trans,cis,cis-isomers, whereas the other isomers exhibit higher (0.6–0.7 V) potentials. Two successive azo reductions are observed at negative potentials. The difference between the first metal and ligand redox potentials is linearly correlated with νCT [t2(Ru) → π*(RL)].

Synthesis, structure and properties of [ReVL(O)Cl3], [ReVL(NR)Cl3], [ReIIIL(OPPh3)Cl3], and [ReIIIL(PPh3)Cl3] [L = 2-(arylazo)-1-methylimidazole, R = aryl] †

The reaction of KReO4 with L [2-(arylazo)-1-methylimidazole, with aryl = Ph (L1), C6H4Me-p(L2) or C6H4Cl-p(L3)] in concentrated HCl afforded [ReVL(O)Cl3] 1. Aromatic amines and PPh3 smoothly converted 1 into [ReVL(NR)Cl3] 2 and [ReIIIL(OPPh3)Cl3] 3 respectively. Treatment of 3 with PPh3 yielded [ReIIIL(PPh3)Cl3] 4. Complexes of type 3 and 4 display large paramagnetic shifts of 1H NMR lines which spread over ≈60 ppm. Structure determination of [ReL1(O)Cl3] 1a, [ReL2(NC6H4Me-p)Cl3] 2a, [ReL3(OPPh3)Cl3] 3c and [ReL3(PPh3)Cl3] 4c has revealed meridional geometry for all except 4c which is facial. In the latter Re–azo and Re–PPh3 back bonding is maximized. The metal atom is displaced away from the equatorial plane by ≈0.3 A towards the oxo ligand in 1a and the imido ligand in 2a. The imidazole nitrogen is co-ordinated trans to oxo, imido, Ph3PO and chloride ligands in 1a, 2a, 3c and 4c, respectively. The azo NN distance is lengthened by ≥0.05 A as a result of direct (3c, 4c) or indirect (1a, 2a) Re–azo back bonding. Azo reduction potential values are consistent with the low-lying nature of the azo(π*) orbital. The metal reduction potentials follow the trends: ReVI–ReV, 1 > 2 (imido better donor than oxo); ReIV–ReIII, 4 > 3 (stabilization of t2 by ReIII–PPh3 back bonding).

Studies on some reactions of cyclopalladated azobenzenes with N,O-chelators

The synthesis and characterization of new planar mixed chelates of the type [Pd(A)(N,O)] (A= ortho-metallated azobenzene or its substituted derivatives, A 1 to A 5, N,O=2-picolinato (pic), quinaldato (qnd), 8-quinolinolato (q) and 2-methyl-5-sulphonic-8-quinolinolato (msq)) are achieved by splitting the halogeno-bridge in [Pd 2(A) 2Cl 2] by aqueous AgNO 3 solution followed by the addition of N,O-chelator. [Pd(A 2)(N,O)] occurs as isomer [Pd(A 3)(N,O)] and the composition have been established using 1H NMR data. The complex composition is supported by elemental analyses and IR data. Cyclic voltammetric studies suggest two successive redox responses correspond to azo reduction.

Cathodic stripping voltammetry of 2,3-dichloroquinoxaline and 1,4-dichlorophthalazine reactive dyes and their hydrolysis products: Reactive Red 41 and Reactive Red 96

Preliminary studies of the feasibility of monitoring by cathodic stripping voltammetry the hydrolysis of two further types of reactive dyes have been made. The azo reduction peak in differential pulse cathodic stripping voltammograms of the 2,3-dichloroquinoxaline reactive dye, Reactive Red 41, and in those of its hydrolysis product are sufficiently separated for the hydrolysis of Reactive Red 41 to be followed using the heights of these peaks. In the case of the 1,4-dichlorophthalazine reactive dye, Reactive Red 96, the azo peaks of the reactive and hydrolysed dyes are too close to be used to monitor the hydrolysis reaction, but peaks associated with reduction of the 1,4-dichlorophthalazine group are present which could be used to monitor the hydrolysis of Reactive Red 96.

Study of some important factors involved in azo derivative reduction by Clostridium perfringens

In order to design azo polymers having potential for use as colonic coating materials, the most important factors able to affect the bacterial degradation of azo derivatives were evaluated by a reliable method, using Clostridium perfringens ATCC 3626 as colonic bacteria. The azo degradation was followed by recording the decrease of the absorbance of azo dye solutions in phosphate buffer salt (PBS), pH 7.2, containing Clostridium perfringens, vs time. The results obtained show that the degradation of azo substrates is linear (zero order), faster in basic media and when a redox mediator, such as riboflavin or benzylviologen, is introduced in the incubation medium. Moreover, the substrate redox potential was shown not to affect significantly the degradation rates. However, when several azo dyes are introduced simultaneously into the incubation medium, the microbial azo reduction occurs sequentially as a function of the substrate redox potential, the substrate having the smallest negative redox potential being degraded first. Finally, when Eudragit® RL/RS films containing an azo dye (amaranth) are incubated with Clostridium perfringens or NADPH, the time required to bleach the films decreases dramatically when the percentage of Eudragit® RL in the films increases, as a result of the increase of the film permeability.

Cobalt-mediated selective C–H bond activation. Direct aromatic hydroxylation in the complexes [CoIII{o-OC6H3(R)NNC5H4N}2]ClO4· H2O (R = H, o-Me/Cl, m-Me/Cl or p-Me/Cl). Synthesis, spectroscopic and redox properties

The reactions of low-spin complexes [CoIIL3][ClO4]2·H2O 1 [L = 2-(arylazo)pyridine, (R)C6H4NNC5H4N (R = H, o-Me/Cl, m-Me/Cl or p-Me/Cl] with m-chloroperbenzoic acid (m-ClC6H4CO3H) in acetonitrile solvent at room temperature resulted in low-spin [CoIIIL′2]ClO4·H2O 2 [L′ = o-OC6H3(R)NNC5H4N]. In complexes 2 the o-carbon–hydrogen bond of the pendant phenyl ring of both parent ligands L has been selectively and spontaneously hydroxylated. During the transformation of 1 to 2 the metal ion is oxidised from the starting CoII to CoIII. The meridional configuration (cis-trans-cis with respect to the oxygen, azo and pyridine nitrogens respectively) of complexes 2 has been established by 1H and 13C NMR spectroscopy. When one methyl or chloro group was present at the meta position of the pendant phenyl ring of L the reaction resulted in two isomeric complexes due to free rotation of the singly bonded meta-substituted phenyl ring with respect to the azo group. In acetonitrile solvent, complexes 2 systematically display one d–d transition (1A1g → 1T1g) near 850 nm, two metal to ligand charge-transfer transitions in the visible region and intraligand transitions in the UV region. In acetonitrile solution all complexes 2 exhibit irreversible CoIII → CoIV oxidation near 2 V and reversible CoIII ⇌ CoII reduction near 0.0 V versus Ag–AgCl. The ligand-based expected four azo (NN) reductions are observed sequentially for all the complexes at the negative side of the reference Ag–AgCl. Complexes 2 can be quantitatively and stereoretentively reduced to the low-spin cobalt(II) congeners, [CoIIL′2] 2– electrochemically as well as chemically by using hydrazine hydrate. These complexes display eight-line EPR spectra in acetonitrile solution at 77 K. Complex 2a– exhibits a ligand to metal charge-transfer transition at 534 nm and intraligand transition at 345 nm. Two possible d–d transitions, 2E → 2T1 and 2E → 2T2 are observed at 700 and 800 nm respectively.

Synthesis, spectral and electrochemical characterization of (dithiocarbamato)(arylazoimidazole)Palladium(II) perchlorates

The ternary complexes [Pd(RaaiX)(SS)ClO4) where RaaiX is a N(1)-alkyl-2-(arylazo)imidazole (p-RC6H4N =NC3H2NN(1) X; X = Me, or Et, and R = H, Me or Cl) and SS = N,N-diethyldithiocarbamate or morpholinedithiocarbamate have been prepared and characterized by elemental analysis, i.r., u.v.-vis. and 1H-n.m.r. data. Electrochemical studies show azo reduction. The complexes are thermally unstable and decompose to bis(dithiocarbamato)palladium(II) in solution.

Investigations into the azo reducing activity of a common colonic microorganism

This work was undertaken to study some factors affecting the bacterial reduction (cleavage) of azo compounds, knowledge of which will be of use in the development of azo cross-linked polymers for colon-specific drug delivery. A common colonic bacterium, Bacteroides fragilis was used as test organism and the reduction of azo dyes amaranth, Orange II and tartrazine were studied; also a model azo compound, 4,4′-dihydroxyazobenzene. It was found that the azo compounds were reduced at different rates and the rate of reduction could be correlated with the half-wave (redox) potential of the azo compounds. 4,4′-Dihydroxyazobenzene (E 1/2 −470 mV) was reduced at the fastest rate of 0.75 mol l −1 h −1, amaranth (E 1/2 −568 mV) at 0.30 mol l −1 h −1, Orange II (E 1/2 −648 mV) at 0.2 mol l −1 h −1 and tartrazine (E 1/2 −700 mV) at 0.08 mol l −1 h −1. Similar observations were made with another colonic bacterium Eubacterium limosum. Reduction of 4,4′-dihydroxyazobenzene did not occur under conditions of aeration, but was enhanced by the low molecular weight electron carrier benzyl viologen, with time for 50% azo reduction being decreased from 120 min to 30 min. These studies with a common, numerically important, colonic bacterium indicate that the reduction of an azopolymer may be influenced by the chemical nature of the azo compound used as cross-linker.

Biodegradation of selected azo dyes under methanogenic conditions

Biological treatment of wastewaters discharged by the textile industry could potentially be problematic due to the high toxicity and recalcitrance of the commonly-used azo dye compounds. In the present report, the fate of two azo dyes under methanogenic conditions was studied. Mordant Orange 1 (MO1) and Azodisalicylate (ADS) were completely reduced and decolorised in continuous UASB reactors in the presence of cosubstrates. In the MO1 reactor, both 5-aminosalicylic acid (5-ASA) and 1,4-phenylenediamine were identified as products of azo cleavage. After long adaptation periods, 5-ASA was detected at trace levels, indicating further mineralization. ADS, a pharmaceutical azo dye constructed from two 5-ASA units, was completely mineralized even in the absence of cosubstrate, indicating that the metabolism of 5-ASA could provide the reducing equivalents needed for the azo reduction. Batch experiments confirmed the ADS mineralization. These results demonstrate that some azo dyes could serve as a carbon, energy, and nitrogen source for anaerobic bacteria.

Azo reduction catalyzed by cytochrome P450 1A2 and NADPH-cytochrome P450 reductase

Azo reduction catalyzed by cytochrome P450 1A2 and NADPH-cytochrome P450 reductase

Chemistry of azoimidazoles. Synthesis, spectral characterization and redox studies of N(1)-benzyl-2-(arylazo)imidazolepalladium(II)chloride

Arylazoimidazoles (2) are N,N-chelating ligands. The polymerization trend of the azolate system is restricted via N(1)-benzylation. The parent molecules (2), N(1)-benzylated products (3) and palladium complexes (4) were made by standard methods. The ligands (3) and complexes (4) are new. They have been characterized by elemental analysis, i.r., u.v.-vis. and high resolution 1H-n.m.r. spectral data. Redox studies were carried out by cyclic voltammetry. On complexation, azo reduction is shifted anodically.

Cobalt-mediated direct and selective aromatic thiolation in the complex [CoIII(o-SC6H4NNC 5H4N)2]ClO4. Synthesis, spectroscopic characterisation and electron-transfer properties

The reaction of K[SC(S)OR] (R = Me, Et, Pr n , Pr i , Bu n , Bu i or CH 2 Ph) with the complex [Co II L 3 ][ClO 4 ] 2 ·H 2 O 1 [L = phenyl(2-pyridyl)diazene, C 6 H 5 NNC 5 H 4 N] in boiling dimethylformamide resulted in [Co III L′ 2 ]ClO 4 2 (L′ = o-SC 6 H 4 N NC 5 H 4 N). In complex 2 the o-carbon–hydrogen bond of the pendant phenyl ring of both the parent ligands L has been selectively and directly thiolated via carbon–sulfur bond cleavage of the dithiocarbonate. During the thiolation the metal ion is oxidised from the starting Co II in 1 to Co III in the final product 2. The reaction is highly sensitive to the nature of the solvent used, taking place only in those having high boiling points and relative permittivities. Its rate is dependent on the nature of the R group present in the thiolating agent, following the order Me ≈ Et > Pr n > Bu n > Pr i > Bu i benzyl. A meridional configuration (cis-trans-cis with respect to the sulfur, azo and pyridine nitrogens respectively) has been established by 1 H and 13 C NMR spectroscopy. The complex exhibits reversible Co III ⇌ Co II reduction at -0.135 V and four ligand-based azo (NN) reductions at -0.51 (one electron) and at -1.175 V (simultaneous three-electron reduction) respectively versus saturated calomel electrode. The oxidation of the co-ordinated thiol group occurs at 0.90 V.

Cathodic stripping voltammetry of copper-complexed reactive dyes at a hanging mercury drop electrode: Reactive violet 5

Reactive Violet 5 and its hydrolysis product, which is produced as a side product in the dyeing process, can be determined in an admixture at sub-ppb levels by cathodic stripping voltammetry because the potentials of their azo reduction peaks are separated sufficiently. For both dyes, at intermediate pH values the azo peak is preceded by a complexed -copper reduction peak at a less negative potential, which aids the identification of the dyes. The use of pH 6 EDTA buffer removes the complexed-copper peak, as does the use of an acidic buffer (pH < 3). This unusual use of EDTA as a pH buffer facilitates the determination of mixtures of the dye and its hydrolysis product.

Biodegradation of selected azo dyes under methanogenic conditions

Biological treatment of wastewaters discharged by the textile industry could potentially be problematic due to the high toxicity and recalcitrance of the commonly-used azo dye compounds. In the present report, the fate of two azo dyes under methanogenic conditions was studied. Mordant Orange 1 (MO1) and Azodisalicylate (ADS) were completely reduced and decolorised in continuous UASB reactors in the presence of cosubstrates. In the MO1 reactor, both 5-aminosalicylic acid (5-ASA) and 1,4-phenylenediamine were identified as products of azo cleavage. After long adaptation periods, 5-ASA was detected at trace levels, indicating further mineralization. ADS, a pharmaceutical azo dye constructed from two 5-ASA units, was completely mineralized even in the absence of cosubstrate, indicating that the metabolism of 5-ASA could provide the reducing equivalents needed for the azo reduction. Batch experiments confirmed the ADS mineralization. These results demonstrate that some azo dyes could serve as a carbon, energy, and nitrogen source for anaerobic bacteria.

Reduction of nitro and azo compounds by NADPH-cytochrome P-450 reductase-cytochrome c heme peptide system.

Octa heme peptide, an enzymic digestion product of Candida Krusei cytochrome c, was found to catalyze nitro and azo reduction in the presence of NADPH and NADPH-cytochrome P-450 reductase under anerobic conditions. The reduction was dependent on the concentrations of heme peptide and reductase, and inhibited by carbon monoxide and oxygen. Comparison of the activities of the reductase-heme peptide system with liver microsomes revealed that heme peptide was as effective as cytochrome P-450 in nitro reduction, although less effective in azo reduction. Thus, cytochrome c heme peptide may serve as an artificial substitute for cytochrome P-450 at least in the reductive metabolism of xenobiotics.