Chockalingam Srinivasan

Mechanism of oxidation of organic sulphides by oxo(salen)manganese(v) complexes

Abstract The kinetics of oxygen atom transfer from several cationic oxo(salen)manganese(V) [salen = N,N′-ethylenebis(salicylidineaminato)] complexes to organic sulphides have been studied spectrophotometrically in acetonitrile at 25 °C. The reaction follows an overall second-order kinetics, first-order each in sulphide and oxomanganese(V) complex. Electronic-substrate and electronicoxidant effect studies reveal that the single electron transfer from sulphide to the oxo complex is the rate-controlling step. The redox potentials of the couple MnV/MnIV have been estimated by applying Marcus theory to the experimentally observed rate constants.

Excited state electron transfer reactions of tris(4,4′-dialkyl-2,2′-bipyridine)ruthenium(II) complexes with phenolate ions: structural and solvent effects

Abstract The reductive quenching of three *RuL 3 2+ complexes (L≡4,4′-dialkyl-2,2′-bipyridine) by several substituted phenolate ions was investigated by the luminescence quenching technique. The decrease in the quenching constant k q with an increase in the size of the ligand is explained in terms of a change in Δ G and the electron transfer distance. The ϱ values obtained for the reactions of the three RuL 3 2+ complexes with phenolate ions are in accordance with the reactivity-selectivity principle. The influence of the solvent on the photoredox reaction of Ru(bpy) 3 2+ (bpy, 2,2′-bipyridine) reveals that, in addition to the bulk properties, the change in solvent reorganization energy (λ o ) and solvent longitudinal relaxation time (τ L ) influence the values of the quenching rate constants.

Steric effects in the photoinduced electron transfer reactions of ruthenium(II)-polypyridine complexes with 2,6-disubstituted phenolate ions

The rate constants (kq) for the photoinduced electron transfer reactions of Ru(II)-polypyridyl complexes (Ru(NN)32+) with 2,6-disubstituted phenolate ions in aqueous acetonitrile are highly sensitive to change in the bulkiness of the ligand in Ru(NN)32+ as well as the phenolate ion. The decrease in kq value with the increase in the size of the ligand in Ru(NN)32+ and the phenolate ion is ascribed to the decrease in the electronic coupling matrix element, ∣HDA ∣, between the donor and acceptor with the increase in the electron transfer distance. The hydrophobic interaction or possible π–π stacking between the pyridine rings of Ru(NN)32+ and the aryl moiety of ArO− leads to less steric effect.

EFFECT OF CYCLODEXTRIN ENCAPSULATION ON PHOTO-FRIES REARRANGEMENT OF BENZENSULPHONANILIDE

Abstract Photolysis of benzenesulphonanilide upon cyclodextrin encapsulation in solid phase yields exclusively 2-aminodiphenyl sulphone.

Oxidation of organic sulfides with clay-supported iodosylbenzene as oxygen donor

Abstract Alkyl, aryl and diaryl sulfides afford excellent yields of sulfoxides with PhIO supported on natural (montmorillonite, KSF and bentonite clay) as well as cation-exchanged clays of K10-montmorillonite in acetonitrile suspension and also in solid state. A ligand coupling mechanism for this facile oxidation is proposed.

Dual emission from 4-dimethylaminobenzonitrile in cyclodextrin derivatives

Abstract Dual fluorescence of 4-dimethylaminobenzonitrile (DMABN) in the presence of hydroxypropyl-β-cyclodextrin (HP-β-CD) and dimethyl-β-cyclodextrin (DM-β-CD) in aqueous media is reported. Equilibrium constants for the formation of inclusion complexes between DMABN and CD-derivatives are calculated for the ground as well as the excited states. Two maxima are observed in the excitation spectra, indicating the presence of two absorbing species. Fluorescence lifetimes of these complexes are also reported in addition to their ICD spectra. Based on the experimental observations, it is concluded that while the more polar TICT emission originates from the aqueous exterior, LE emission occurs predominantly from the complexed molecule. Higher order complexes are not observed. Absence of hydrogen bonding and a ‘tighter fit’ with the CD derivatives are the main factors responsible for this.

Micellar effect on the photoinducedelectron-transfer reactions ofruthenium(II)–polypyridyl complexes with phenolateions. Effect of cetyltrimethylammoniumchloride

Although the emission spectra and excited-state lifetime of Ru(bpy) 3 2+ (bpy=2,2′-bipyridine), Ru(dmbpy) 3 2+ (dmbpy=4,4′- dimethyl-2,2′-bipyridine) and Ru(phen) 3 2+ (phen=1,10-phenanthroline) are little affected by the addition of the cationic surfactant cetyltrimethylammonium chloride (CTAC), an appreciable shift in λ max em , emission intensity and emission lifetime are observed for Ru(dtbpy) 3 2+ (dtbpy=4,4′-di-tert-butyl-2,2′-bipyri dine) and Ru(dpphen) 3 2+ (dpphen=4,7-diphenyl-1,10-phenanthroline). These results point out the importance of hydrophobic interactions over the coulombic repulsion thereby bringing the ruthenium(II) complexes close to the cationic micelles. The rate of luminescence quenching of five *Ru(NN) 3 2+ complexes, with alkyl-substituted phenolate ions in the presence of CTAC, follows different trends depending on the nature of the ligand in Ru(NN) 3 2+ as well as the quencher. The micellar inhibition is explained by considering ArO - being strongly associated with CTAC leaving Ru(NN) 3 2+ in the aqueous phase. The micellar catalysis at high [CTAC] with Ru(dtbpy) 3 2+ and Ru(dpphen) 3 2+ may be due to the operation of predominant hydrophobic interaction over the electrostatic repulsion.

Photoinduced oxidation of benzhydrol and reduction of benzil on titanium dioxide

Abstract In the TiO2-photocatalyzed oxidation of benzhydrol in oxygen-purged acetonitrile, benzophenone was observed to be the sole product. The influence of various parameters, like irradiation time, amount of catalyst, concentration of catalyst and other factors on the photocatalytic oxidation, has been investigated. The proposed mechanism envisages the oxidation of benzhydrol by a hole to give the radical cation of benzhydrol and its subsequent reaction with a superoxide radical anion produced by the transfer of a conduction band electron to oxygen. The involvement of superoxide is revealed by (i) the 9,10-dicyanoanthracene-photosensitized oxidation of benzhydrol to benzophenone and also (ii) the absence of photocatalytic oxidation in the presence of nitrogen. Benzil, an example of 1,2-diketone, was converted to benzoin in the TiO2-mediated reduction in nitrogen atmosphere.

Reductive quenching of excited states of chromium(III)-polypyridyl complexes with alkyl aryl sulphides

Abstract The variation in the quenching rate constants,kq, of *Cr(NN)33+ with the change in the structure of organic sulphides in deaerated acetonitrile solutions is explained with Marcus model. The retardation in kq values for sulphides with bulky alkyl groups in RSPh is accounted for in terms of change of electron transfer distance. The formation of oxidised products in aerated solutions is explained by the formation of singlet oxygen in the system.

Kinetics and mechanism of oxygenation of aromatic sulfides and arylmercaptoacetic acids by peroxomonophosphoric acid

Abstract The oxygenation of aryl methyl sulfides and diaryl sulfides by peroxomonophosphoric acid in acetonitrile–water mixture follows an overall second-order kinetics, first-order in each reactant. The study of the influence of [H + ] on the oxidation of phenyl methyl and diphenyl sulfides reveals that H 2 PO 5 − and HPO 5 2− are the oxidising species in the oxidation process. In substituted phenyl methyl sulfides and diphenyl sulfides, the rate of oxygenation is accelerated by electron-donating substituents and retarded by electron-withdrawing groups, indicating the nucleophilic displacement by the sulfide sulfur on the peroxide oxygen. The negative ρ value obtained in the correlation analysis of rate constants with σ constants also shows the formation of a more positively charged sulfur species in the rate-limiting step. Studies with different alkyl phenyl sulfides (C 6 H 5 SR; R=Me, Et, Pr i or Bu t ) indicate that the rate is retarded by bulky R groups. Similar kinetic results are also obtained in the peroxomonophosphoric acid oxidation of sodium phenylmercaptoacetate. On the basis of the kinetic studies, a common mechanism has been proposed.