Beena Tyagi

Sulfated zirconia: an efficient solid acid catalyst for esterification of myristic acid with short chain alcohols

Sulfated zirconia (SZ) catalysts prepared by a two-step sol–gel method and calcined at 600–700 °C were evaluated for esterification of myristic acid with methanol using varied acid to alcohol ratio, reaction temperature and catalyst concentration. An exceptionally small concentration of SZ catalysts (0.125–0.5 wt% to acid) exhibited 98–100% conversion of myristic acid with methanol at 60 °C after 5 h. The conversion was decreased with an increase in the alkyl chain of alcohol from methanol to butanol, however, similar conversion was achieved by increasing the reaction temperature to 90 °C. The ester formation was selective, irrespective of alcohol and other reaction variables. The calcination temperature has strong influence on the structural, textural and acidic features and thus on the activity and re-usability of SZ catalysts. The reaction is sensitive to moisture present in methanol or reaction mixture. The studied reaction is Bronsted acid catalyzed and the SZ catalyst having higher number of Bronsted acid sites was re-used successfully without significant loss in activity; whereas the SZ catalyst having lower number of Bronsted acid sites showed a decrease in activity (∼28%) after five reaction cycles. The results clearly indicated the necessity of higher number of Bronsted acid sites for better performance and recycling of the SZ catalysts for esterification of myristic acid with methanol under the conditions studied.

Predominance of electron-withdrawing effect over angular strain in the metal-promoted hydrolysis of 2,4,6-tris(2-pyridyl)-1,3,5-triazine

The reaction of 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) and RhCl 3 ·3H 2 O in refluxing ethanol–water (1:1) resulted in the hydrolysis of tptz to bis(2-pyridylcarbonyl)amide anion (bpca) and afforded a complex of composition [Rh(bpca) 2 ][PF 6 ] 1. However, hydrolysis of tptz did not occur when it was treated with RuCl 3 ·3H 2 O under similar condition, yielding instead the complex [Ru(tptz) 2 ][PF 6 ] 2 ·H 2 O 2. The molecular structures of 1 and 2 have been established by single-crystal X-ray analysis. In complex 1 the bis(2-pyridylcarbonyl)amido moiety functions as a tridentate ligand with nitrogen donor atoms and is bound to Rh III in a mutually perpendicular fashion forming a distorted-octahedral geometry around the metal ion. In complex 2 two tridentate tptz are co-ordinated to Ru II in a similar manner as found for 1. From a comparison of bond lengths and angles in the co-ordination spheres of 1 and 2 it is suggested that the electron-withdrawing effect (L→M) of the metal ion is the predominant factor, rather than angular strain at the carbonyl carbon atoms, responsible for hydrolysis of tptz. Electrochemical studies of 1 revealed a metal-based two-electron couple (Rh III –Rh I ) at -1.15 V and two ligand-based redox couples at -1.44 and -1.84 V. In the case of 2 the Ru II –Ru III couple appears at +1.77 V and the ligand-based reduction at -0.62 and -0.80 V which are significantly positively shifted compared to those of free tptz.

Dinuclear ruthenium(II) and/or osmium(II) complexes of bipyridyl ligands bridged by rigid spacers: synthesis, characterization, electrochemical behavior and luminescence properties

Abstract New bridging ligands, 1,4-bis[4-(4′-methyl)-2,2′-bipyridyl)imine]benzene (BL1) and 4,4′-bis[4-(4′-methyl)-2,2′-bipyridyl)imine]benzene (BL2), and their dinuclear complexes of the type [(bpy)2Ru-BL-Ru(bpy)2]4+, [(bpy)2Os-BL-Os(bpy)2]4+ and [(bpy)2Ru-BL-Os(bpy)2]4+ (where BL=BL1 and BL2) have been synthesized and characterized. Mononuclear model complexes [(bpy)2RuL1]2+, [(bpy)2OsL1]2+ and [(bpy)2RuL2]2+, where L1=[4-(4′-methyl)-2,2′-bipyridyl)imine]-4-aminobenzene and L2=[4-(4′-methyl)-2,2′-bipyridyl)imine]-4-aminodiphenylene, have also been synthesized. Ligands and metal complexes were characterized on the basis of elemental analysis, FAB mass, IR and 1H NMR data. All metal complexes exhibit characteristic MLCT absorption and luminescence bands in the visible region. Cyclic and square wave voltammograms of all complexes exhibit metal-based oxidations (M(II)→M(III)) in the potential ranges +1.20 to +1.23 V for Ru(II) and +0.75 to +0.82 V for Os(II). Ligand-based reductions occurred in the potential range −1.28 to−1.90 V. Mixed-valence species of the dinuclear complexes were generated by the addition of a standard solution of ammonium cerium(IV) nitrate. The excited state life-time of all complexes were measured in acetonitrile and also in a methanol–ethanol (4:1) mixture at room temperature. The homodinuclear complexes exhibit significantly long life-times, which are attributed to delocalization of excited electron on the conjugated bridging unit. In heterodinuclear complexes the Ru-based emission life-time decreased significantly; this quenching process takes place via energy transfer from Ru(II) to Os(II) center through spacers. The rate constants for energy transfer are calculated.

Interaction of phenylhydrazine with RuIII-EDTA complexes: reduction of phenylhydrazine to ammonia and aniline in aqueous acidic conditions

Abstract Reaction of the phenylhydrazinium (PhN2H4+)_ion with the labile aquo complexes of [RuIII(HED-TA)(H2O)] (EDTA = ethylenediaminetetraacetate) was studied electrochemically employing sampled DC, cyclic voltammetry and differential pulse techniques in 0.2 M CH3COONa and H2SO4 mixture between pH 1.0 and 4.5 at 25°C. It produced two phenylhydrazinium complexes, namely [RuIII(HEDTA)(N2H4Ph)]+ and [RuIII(EDTA)(N2H4Ph)] in the rapid aquo-substitution reactions. The second-order rate contants, k1, for the formation of the former, and k2 for the latter are 0.98 and 36.85 M−1 s−1, respectively, as determined spectrophotometrically by following the intensity of the LMCT band at 420 nm. The complex, [RuIII(HEDTA)(N2H4Ph)]Cl·2H2O was prepared and characterised by physicochemical methods. These phenlhydrazinium complexes were reduced by two electrons at E 1 2 = −0.175 V vs SCE , producing one mole of NH3 and the corresponding RuIII-NH2Ph complexes in subsequent rapid decomposition steps. The unstable RuIII-NH2Ph complexes were rapidly hydrolysed to one mole of NH2Ph and the respective aquo complex. One-electron reduction steps for the regenerated aquo complex at ( E 1 2 ) − 0.254 V and the unhydrolysed RuIII-NH4Ph complex at −0.417 V were also observed. Phenylhydrazine was reduced to ammonia and aniline by constant potential coulometry at −0.200 V (Hg) vs SCE, in the presence of [RuIII(HEDTA)H2O)] at 100: 1 molar ratio. The turnover number (per mole of catalyst per hour) with respect to ammonia is 5.98 and 2.82 at pH 2.8 and 1.9, respectively. The probable mechanisms for the electroreduction of the above phenylhydrazinium complexes and the catalytic reduction of phenylhydrazine to NH3 and NH2Ph, are proposed.

Synthesis and characterization of dinuclear platinum group metal complexes of a hexadentate ligandα,α′-bis(bis-(2-(diphenylphosphino)ethyl)amino)-m-xylene

Abstract A number of dinuclear complexes of platinum group metal ions with a hexadentate ligand, α , α ′-bis(bis-(2-(diphenylphosphino)ethyl)amino)- m -xylene (L), were synthesized. The reaction of [MCl n (COD)] x ( n = 1, x = 2 when M = Rh I and Ir I and n = 2, x = 1 when M = Pd II and Pt II ) with the ligand L resulted in the formation of square planar dinuclear complexes of the composition [M 2 LCl 2 ] n + ( n = 0 for Rh I and Ir I and n = 2 for Pd II and Pt II ). The reaction of [PtCl 2 (COD)] with L under different experimental conditions yielded a tetranuclear complex, [Pt 4 L 2 Cl 4 ]Cl 4 . The reaction of [M 2 LCl 2 ] (M = Rh I , Ir I ) with CO gas at ambient temperature resulted in the formation of the pentacoordinate CO adduct, [M 2 LCl 2 (CO) 2 ], with trigonal bipyramidal geometry. The reactionof [RhCl(CO)(AsPh 3 ) 2 ] and [IrCl(CO)(PPh 3 ) 2 ] with L gave complexes of the same composition as that of the CO adduct, [M 2 LCl 2 (CO) 2 ], with two isomeric forms in the case of iridium. The reaction of [Rh 2 LCl 2 ] with AgClO 4 in acetonitrile resulted in the complete displacement of chloride, with the formation of [Rh 2 L(NCCH 3 ) 2 ](ClO 4 ) 2 . A complex of the composition [Rh 2 L(PPh 3 ) 2 ](BPh 4 ) 2 was obtained when [RhCl(COD)] 2 was reacted with L in the presence of PPh 3 in equimolar ratio followed by addition of NaBPh 4 . The reaction of hydrated iridium trichloride gave a dinuclear octahedral complex of Ir III of composition [Ir 2 LCl 6 ]. The geometry of the complexes was established by 31 P{ 1 H} NMR spectral data.

SYNTHESIS, CHARACTERIZATION AND REACTIVITIES OF DINUCLEAR RHODIUM(I) COMPLEXES OF A HEXADENTATE LIGAND, N,N,N′,N′-TETRAKIS [2-(DIPHENYLPHOSPHINO) ETHYL] ETHANE-1,2-DIAMINE

Abstract A number of square planar dinuclear rhodium(I) complexes, [Rh2LX2] (X[dbnd]Cl, I, N3) and [Rh2L][ClO4]2 have been prepared with a hexadentate ligand having N2P4 donor sites, N,N,N′,N′-tetrakis[2-(diphenylphosphino) ethyl]ethane-1,2-diamine (L). The compound [Rh2LCl2] readily undergo oxidative addition reactions with Cl2, Br2, I2 and MeI which result in the formation of octahedral dinuclear complexes with additional ligands in axial positions. [Rh2LCl2] with NOBF4 forms a pentacoordinate complex [Rh2L(NO)2Cl2][BF4]2. Reaction of [Rh2LX2] (X[dbnd]Cl,N3) with CO gives pentacoordinate dinuclear complexes [Rh2L(CO)2X2] with trigonal bipyramidal geometry. However, a CO adduct [Rh2L(CO)4I]2 is obtained when [Rh(cod)I]2 reacts with L in CO-saturated dichloromethane solution. All CO adducts react with O2 to give octahedral carbonato complexes, [Rh2L(O)4(CO3)2X2] (X[dbnd]Cl, I, N3) in which CO3 2- acts as a bidentate ligand and PPh2 of L is oxidised to Ph2P[dbnd]O which coordinates to rhodium through oxyge...