Chitta R. Saha

Dihydrogen reduction of nitroaromatics, alkenes, alkynes and aromatic carbonyls by ortho-metalated Pd(II) complexes in homogeneous phase

Abstract Dihydrogen reductions of nitroaromatics, alkenes, alkynes and aromatic groups have been investigated in DMF at 25 °C in the presence of Pd(II) and Pt(II) complexes with azo- and hydrazobenzenes. The optimum conditions have been determined in all cases and the final products were the corresponding anilines, alkanes and alcohols. Preferential and sequential reductions were observed in a few cases. The presence of a metalaryl σ-bond was essential for the catalytic activities of these complexes. During reduction, the azo complex was first converted to hydrazo complex, and the latter was reduced by the aromatic substrate, for which a rate equation of the type: rate = K 1 [Cat][H 2 ] 2 has been derived from kinetic data. For other substrates, the hydrazo complex behaves as the actual catalytic species, and a rate equation of the type: rate = K 2 [Cat][H 2 ] has been derived. Tentative mechanisms for these reduction processes have been proposed on the basis of experimental observations.

Dihydrogen reduction of organic substrates using orthometallated ruthenium (II) complex catalysts

Abstract Dihydrogen reduction of aliphatic and aromatic nitrocompounds and nitriles, aliphatic ketones and Schiff bases to their corresponding saturated products was efficiently carried out in presence of dinuclear orthometallated ruthenium(II) complexes of the type [RuL(CO)2Cl]2 (LH=2-phenylpyridine, benzo (h)-quinoline, 1-phenylpyrazole and azobenzene) as catalyst precursors under high pressure, high temperature conditions in either DMF or DMSO medium. The dinuclear complexes undergo dissociation to the corresponding monomer solvent adducts, [RuL(CO)2Cl·S] (S = DMF, DMSO) which are suggested to be the actual catalytic species. The probable mechanism for the reduction has been suggested on the basis of kinetic studies and physico-chemical properties of the catalyst solution during high pressure run.

Reductive carbonylation of nitroaromatics using RhA(CO)2)

Abstract The investigation of the catalytic activity of RhA(CO) 2 (HA=anthranilic acid) towards the reductive carbonylation of nitroaromatics in DMF medium under high P CO (8.0×10 3 kNm −2 ) at 140°C without cocatalyst showed moderate conversion of the substrates to the corresponding diphenylureas and anilines as major and minor products respectively. The presence of acidic cocatalysts increased the yield of diphenylurea while in the presence of basic cocatalysts and alcohols, corresponding carbamates were the main products. The maximum yield and selectivity of the carbamate were achieved using NaOMe and MeOH as cocatalyst and cosolvent respectively under optimum reaction conditions. A tentative reaction mechanism based on the identification of reactive intermediates was proposed for this carbonylation process.

Silver(I) and silver(III) complexes of biguanides and N′-amidinoisoureas

Abstract The preparation and characterization of some quadricovalent cationic complexes of silver(I) and silver(III) with biguanides and N′-amidinoisoureas have been described. Silver(I) complexes were characterized by elemental analysis, conductance measurements, IR and UV spectral studies and thermal analysis. These complexes are colourless and may be classified into 4 main types: [AgLA2], [AgL′A2|X, [AgL″2]X and [Ag′A2]X L= N′-amidinoisourea, L′ = biguanide, N-alkylbiguanide or O-alkyl N′-amidinoisourea, L″ = n-propylbiguanide, A = N2O or NH 3 , X = SO 4 2 , NO3, OH or HSO4. Silver(III) complexes are all brown-red to orange-red and may be represented by formulae [AgL2]X and [AgL′2]X3 = N′=amidinoisourea, L′ = biguadine or O-alkyl N′-amidinoisourea, X = SO 4 2 or OH. These complexes were characterized by elemental analysis, oxidimetric titrations, magnetic susceptibility measurements and IR spectral studies. Thermal analysis indicate the dimeric nature of some silver(I) complexes. Silver(I) and silver(III) complexes are suggested to have tetrahedral and square planar configuration, respectively. IR spectra indicate the presence of stronger metal-ligand bonding in silver(III)_complexes.

Use of polystyrene bound orthometalated Schiff base complexes of palladium (II) as catalysts for the dihydrogen reduction of nitroalkanes, nitriles and ketones

Abstract Dihydrogen reduction of nitroalkanes, aliphatic and aromatic nitriles and ketones to their corresponding saturated products was successively achieved in DMF medium using polystyrene based acetato-bridged orthometalated Schiff base complexes of palladium (II) as catalysts, at 80–130°C and 6.0–14.0 × 10 3 (kN m −2 ) of P H 2 . The acetato-bridged Schiff base complexes are the catalyst precursors and the actual catalysts are the corresponding hydrogen activated orthometalated complexes with the acetate bridge replaced by H and DMF. The immobilization of the palladium (II) complexes in the polymer matrix slightly decreased their catalytic activities on the basis of metal content but improved the chemical and thermal stabilities and product selectivities relative to those of the corresponding homogeneous ones. The same specimen of the catalyst can be used repeatedly for the reduction of different substrates and stored for a long time without suffering any appreciable loss of activity. XPS data suggest the presence of palladium (II) in the fresh and used catalyst and kinetic studies indicate 1st order rate dependence on palladium (II) content, second order on P H 2 and independent of substrate concentration. A plausible mechanistic route has been suggested on the basis of kinetic data and experimental observations.

Orthometalated ruthenium-complex-catalysed reductive carbonylation of nitroaromatics

Abstract Orthometalated ruthenium(II) complexes of the type [RuL(CO)2Cl]2 (LH=2-phenylpyridine, 1-phenylpyrazole, azobenzene and benzo(h)quinoline) behaved as efficient catalysts for the reductive N-carbonylation of nitroaromatics in mild coordinating solvents containing alcohol and a basic cocatalyst under high pressure, high temperature conditions. The products were mainly the corresponding N-phenylcarbamates and aniline with a small amount of azobenzene detected in some isolated cases. The effects of various reaction parameters such as PCO, temperature, concentration of alcohol and cocatalysts on the nature and yields of products have been studied. A tentative reaction mechanism has been proposed on the basis of experimental findings.

Reusuable polymer-anchored catalyst for carbonylation reaction: kinetics and mechanism for diphenylurea formation

Abstract The investigation of the catalytic activity of [RhA(CO) 2 ] (HA; anthranilic acid) and its polymer analogue [Rh(OOCC 6 H 4 NHCH 2 -P)(CO) 2 ]; P=polystyrene moiety, towards the reductive carbonylation of nitrobenzene in DMF medium has been reported. The maximum yield and selectivity of diphenylurea was achieved using methanol as cosolvent under optimum reaction conditions. Spectroscopic and kinetic studies showed that the reaction proceeds through a species [RhA(C(O)OCH 3 ) 2 ] and the isocyanate formed at an intermediate stage is immediately scavenged by excess amine to form N , N ′-diphenylurea. A tentative reaction mechanism based on the identification of reactive intermediates has been proposed for the carbonylation process.

Chemical synthesis of nanocrystalline tin-doped cubic ZrO2 powders

Abstract The nanocrystalline powders of the system SnO 2 –ZrO 2 has been prepared using zirconium oxalate and tin tartarate, which are also synthesised from their organic and inorganic precursors. The aqueous solutions of oxalate and tartarate are mixed with proper proportions and with polyvinyl alcohol to form the polymer precursor solution. This is evaporated, pyrolysed and calcined to nanocrystalline powders. The phase of the powders is cubic at the temperature of calcination of 700 °C with the crystallite size ranges from 15 to 25 nm, whereas the particles size ranges from 30 to 50 nm for the sample containing 5 mol% SnO 2 . The shapes of the particles are oval and spherical. The alloying can be done up to 20 mol% with SnO 2 . The material is promising for chemical sensors.

SYNTHESIS AND STRUCTURAL INVESTIGATION OF METAL COMPLEXES OF 1-AMIDINO-2-THIOUREAS

Abstract Some new metal complexes of 1-amidino-2-thioureas† and its S- and N-alkyl derivatives have been isolated and characterized by electronic, vibrational and pmr spectral studies, magnetic and conductance measurements and thermal analysis. The ligands behave as SN or NN donors depending mainly on the pH of the medium and the nature of the metal atom. Protonation at the central nitrogen atom of the chelate ring converts the inner complexes to cationic ones and vice-versa. Electronic and pmr spectral data confirm the presence of strong ring currents in these chelates. Probable structures of the complexes have been suggested on the basis of their physicochemical properties.

SYNTHESES AND PROPERTIES OF SOME FIVE-AND SIX-COORDINATE COPPER(II) COMPLEXES WITH ACETYLACETONATE AND THE STRUCTURAL CHARACTERISATION OF A BIGUANIDE-ACETYLACETONATE DOUBLE SALT

Abstract Compounds of formulae [M(Hbg)2] [Cu(acac)2 X]2 (1) and [Ni(Rbg)2] [Cu(acac)2 Cl2] (2) (acac = acetylacetonate, X = Cl, Br, I; M = Cu, Ni; Hbg = biguanide, Rbg = N-alkylbiguanide, R = CH3 C2 H5) have been prepared by the interaction of the respective chelates in methanol. The presence of five and six coordinate copper(II) in the anions of (1) and (2) respectively is suggested from their conductance data, electronic and vibrational spectra, magnetic moments and thermal analyses. Compound (1), where M = Cu and X = Cl has been characterized by single crystal X-ray crystallography. Crystals are orthorhombic, space group Pbca with Z = 4 with a = 15.218(2), b = 15.986(3), c = 14.565(3)A. The structure was solved by Patterson and Fourier methods and refined by least-squares methods to R = 0.052 and RW = 0.052 for 4267 reflexions and R = 0.065 for all 5206 reflexions. The structure consists of square-planar [Cu(Hbg)2]2+ cations and square-pyramidal [Cu(acac)2 Cl]− anions. The nonhydrogen atoms of the cati...