Journal of Chemical Sciences: Published by the Indian Academy of Sciences (v.116, #3)
Kinetics and mechanism of the reactions of hexaaqua rhodium (III) with sulphur (IV) in aqueous medium
by Suprava Nayak; Anadi C. Dash (pp. 133-142).
An O-bonded sulphito complex, Rh(OH2)5(OSO2H)2+, is reversibly formed in the stoppedflow time scale when Rh(OH2) 6 3+ and SO2/HSO 3 − buffer (1 2)5OH2++ SO2 □ Rh(OH2)5(OSO2H)2+ (k1/k-1), k1 = (2.2 ±0.2) × 103 dm3 mol−1 s−1, k−1 = 0.58 ±0.16 s−1 (25°C,I = 0.5 mol dm−3). The protonated O-sulphito complex is a moderate acid (K d = 3 × 10−4 mol dm−3, 25°C, I= 0.5 mol dm−3). This complex undergoes (O, O) chelation by the bound bisulphite withk= 1.4 × 10−3 s−1 (31°C) to Rh(OH2)4(O2SO)+ and the chelated sulphito complex takes up another HSO 3 − in a fast equilibrium step to yield Rh(OH2)3(O2SO)(OSO2H) which further undergoes intramolecular ligand isomerisation to the S-bonded sulphito complex: Rh(OH2)3(O2SO)(OSO2)- → Rh(OH2)3(O2SO)(SO3)− (k iso = 3 × 10−4 s−1, 31°C). A dinuclear (μ-O, O) sulphite-bridged complex, Na4[Rh2(μ-OH)2(OH)2(μ-OS(O)O)(O2SO)(SO3) (OH2)]5H2O with (O, O) chelated and S-bonded sulphites has been isolated and characterized. This complex is sparingly soluble in water and most organic solvents and very stable to acid-catalysed decomposition
Keywords: Rhodium-sulphur complexes; intramolecular ligand isomerisation; sulphite-bridged complexes
Equilibria and kinetics for pH-dependent axial ligation of ethylester and methylester(aquo)cobaloximes with aromatic and aliphatic N-donor ligands and a molecular mechanistic study of the Co-C bond
by J. V. Madhuri; V. Malathi; S. Satyanarayana (pp. 143-150).
Equilibrium constants are determined for the reaction of ethylester and methyl ester (aquo) cobaloximes with histamine, histidine, glycine and ethyl glycine ester as a function of pH at 25°C, using spectrophotometric technique. The functional dependence of pK a on the substitution rate of H2O varies with the pK a of the incoming ligand, establishing the existence of nucleophilic participation of the ligand in the transition state. This data is interpreted with the help of kinetic data where dissociation kinetic reactions were also studied as a function of pH. Binding and kinetic data were correlated based on the basicity, steric hindrance of the entering ligand and HSAB principle. To compare the rate constants of the entering ligands pH-independent second-order rate constants were calculated. The effect of incoming ligand on Co-C bond is studied using molecular mechanics
Keywords: Cobaloximes; equilibrium constants; ligand substitution reactions; molecular mechanics
Imidazolidine ring as a reduced heterocyclic spacer in a new all-N-donor μ-bis (bidentate) Schiff base ligand: Synthesis, characterization and electron transfer properties of imidazolidine-bridged dicopper complexes
by Manindranath Bera; Prasant Kumar Nanda; Uday Mukhopadhyay; Debashis Ray (pp. 151-158).
Low-temperature stoichiometric Schiff base reaction in air in 3:1 mole ratio between benzaldehyde and triethylenetetramine (trien) in methanol yields a novel tetraaza μ-bis (bidentate) acyclic ligand L. It was characterized by elemental analysis, IR, EI mass and NMR (1H and13C) spectra. The formation of a five-membered imidazolidine ring from the ethylenediamine backbone as a spacer-cumbridging unit gives rise to a new type of imidazolidine-bridged ligand. A geometric optimisation was made of the synthesized ligand and its complexes by the method of molecular mechanics (MM2) method in order to establish the stable conformations. This hitherto unknown tetraaza acyclic ligand affords new cationic dicopper(I/I) and dicopper(II/II) complexes in good yield. Dicopper(II/II) complex displays weakd-d transition bands in the visible region, while dicopper(I/I) complex displays strong MLCT band in the same region. Both the dinuclear complexes are of non-intimate nature and show interesting solution electrochemical behaviour. EPR spectral study of μ-bis (imidazolidino) bridged dicopper(II/II) complex also supports the non-communicative nature of the two copper centres within the same molecule
Keywords: Schiff base; dicopper (II/II) complexes; imidazolidine-bridged; molecular mechanics; cyclic voltammetry; EPR
Proline-catalysed asymmetric ketol cyclizations: The template mechanism revisited
by R. Malathi; D. Rajagopal; Zoltan G. Hajos; S. Swaminathan (pp. 159-162).
A modified template mechanism based on modelling studies of energy minimised complexes is presented for the asymmetric proline-catalysed cyclization of triketones1,2 and3 to the 2S,3S-ketols1a,2a and3a respectively. The template model involves a three-point contact as favoured in enzyme-substrate interactions. Our minimisation studies are in agreement with the divergent behaviour of the 6,5-, 6,6-and 6,7-bicyclic systems. They support the high 93.4%ee observed with the 6,5-bicyclic ketol and the lower 73%ee found with the 6,6-bicyclic ketol. The calculations also explain the lack of asymmetric induction with the 6,7-bicyclic system
Keywords: Proline-catalysed ketol cyclization; template mechanism
Microwave-mediated reductive amination-cyclization of 4-aryl-4-oxobutanoates: Facile synthesis of 3-methylidene-5-phenyl-2,3-dihydropyrrolidones
by H. Surya Prakash Rao; S. P. Senthilkumar (pp. 163-168).
Microwave-mediated three-component condensation of 4-aryl-4-oxobutanoates with ammonium formate furnishes 3-methylidene-5-phenyl-2,3-dihydropyrrolidones in good yield within 2 min. The pyrrolidone products were characterized on the basis of spectral data and X-ray crystal structure analysis. The reaction is found to be general and a variation in the ester and aryl moieties is possible. However, when alkylammonium formate is used only amide products are formed
Keywords: Microwaves; three-component coupling; pyrrolidones; ammonium formate
Solubilization of silica: Synthesis, characterization and study of penta-coordinated pyridine N-oxide silicon complexes
by Subramania Ranganathan; Ch Chandrashekhar Rao; Suvarchala Devi Vudayagiri; Y. B. R. D. Rajesh; B. Jagadeesh (pp. 169-174).
In an effort to design agents that could solubilize silica in water, under ambient conditions and pH, as takes place in nature, novel zwitterionic, penta-oxo-coordinated silicon compounds with siliconate cores have been prepared from 4-substituted pyridine N-oxides (H, OMe, morpholino, NO2) as donor ligands, their structures established by1H,13C and MS, and the coordination number of silicon, by29Si NMR. The formation of complexes from pyridine N-oxides is noteworthy since they arise from interaction with a weakly nucleophilic oxygen centre. The ability of the pyridine N-oxides to enhance the solubilization of silica in water has been experimentally demonstrated. Possible rationalization of this observation on the basis of O → Si coordination via the oxygen atom of pyridine N-oxide is suggested
Keywords: Solubilization of silica in water; step-wise design; penta-oxo coordinated silicon compounds
Acylation of aromatic alcohols and phenols over InCl3/montmorillonite K-10 catalysts
by Vasant R. Choudhary; Kailash Y. Patil; Suman K. Jana (pp. 175-177).
Montmorillonite K-10 clay supported InCl3 is a highly active catalyst for the acylation of aromatic alcohols and phenols with different acyl chlorides. This catalyst can be reused in reactions a number of times without very significant loss of catalytic activity
Keywords: Acylation of aromatic alcohols; acylation of phenols; InCl3/montmorillonite K-10 catalyst; acyl chloride
Studies of surfactants in photogalvanic cells — NaLS-EDTA and azur-B system
by R. C. Meena; Gautam Singh; Nidhi Tyagi; Manju Kumari (pp. 179-184).
Photogalvanic effects were studied in photogalvanic cells containing sodium lauryl sulphate as surfactant, EDTA as reductant and azur-B as photosensitizer. The photopotential and photocurrent generated were 814#x00B7;0 mV and 255·0 μA respectively. The observed conversion efficiency was 1·2% and the maximum power of cell was 207·57 μW. The storage capacity of the cell was 26#x00B7;0 min in dark. The effect of different parameters on electrical output of the cell was observed. A mechanism has been proposed here for the generation of photocurrent in photogalvanic cells
Keywords: Sodium lauryl sulphate; fill factor; conversion efficiency; power point