Mechanism of Metal Ion Catalysis in the dissociation of cis-diaquobisoxalatochromate(III) ion

The metal ion (M²⁺) catalysed dissociation of cis-diaquobisoxalatochromate into the tetraaquomonooxalato complex in aqueous perchloric acid medium which follows the rate law — d(complex)/dt = {k_H[H⁺] + k_M[M²⁺]}[complex] has been studied. Based on k_M values the order of catalysing effect of the different metal ions studied is Cu²⁺ > Ni²⁺ > Co²⁺ > Mn²⁺, which is also the order of stabilities (K_MOx) of the monooxalato complexes of these metal ions; in fact the plot of log k_M vs. log K_MOx is linear. This together with the relative values of ΔH^≠ and ΔS^≠ for the H⁺ catalysed and M²⁺ catalysed paths is in agreement with a mechanism involving chelation of the catalysing cation through the free carbonyl oxygens of the oxalate ligand bound to Cr(III), followed by the dissociation of the Cr(III)? O bonds with simultaneous entry of two water molecules into the coordination sphere of Cr(III).     

Toluene oxidation on LaCoO3, LaFeO3 and LaCrO3 perovskite catalysts. A comparative study

Vapor phase catalytic oxidation of toluene over perovskites viz., LaCoO₃, LaCrO₃ and LaFeO₃ has been studied. The maximum activity was observed at 450°C. The oxidation is first order. The order of catalytic reactivity is LaCoO₃>LaFeO₃>LaCrO₃. The activation energy (E_a) values for LaCoO₃, LaFeO₃ and LaCrO₃ are 13.4, 23.93 and 25.31 kJ/mol, respectively.     

Chemoselective thioacetalisation and transthioacetalisation of carbonyl compounds catalysed by tetrabutylammonium tribromide (TBATB)

Thioacetals and thioketals of various aldehydes and ketones were obtained directly from carbonyl compounds or by a transthioacetalisation process from cyclic O,O-acetals in the presence of dithiols and a catalytic amount of tetrabutylammonium tribromide (TBATB). Chemoselective thioacetalisation of aromatic aldehydes containing an electron-donating group in the presence of an aldehyde containing an electron-withdrawing group, aldehydes in the presence of ketones, aliphatic cyclic ketones in the presence of aromatic ketones and less hindered ketones in the presence of more hindered ketones have been achieved. A cyclic acetal containing an electron-donating group has been chemoselectively transthioacetalised in the presence of an acetal having an electron-withdrawing substituent. These selectivities are due to the intrinsic reactivity of the substrate themselves and are independent of the catalyst and reaction conditions. Shorter reaction times, mild reaction conditions, stability of acid sensitive protecting groups, high efficiencies, facile isolation of the desired products and the catalytic nature of the reagent are the attractive features of the present method.     

Positron annihilation studies of poly(N‐isopropyl acrylamide) gel in mixed solvents

Positron annihilation lifetime spectroscopy was used to characterize the reentrant volume‐phase‐transition behavior of poly(N‐isopropyl acrylamide) hydrogel in an ethanol/water mixed solvent. The polymer gel was synthesized with γ irradiation. The ortho‐positronium lifetime (τ₃) in the gel slowly increased with an increase in the ethanol content in the mixed solvent. τ₃ was not influenced by the volume phase transition. The ortho‐positronium intensity decreased with the collapse of the gel in an approximately 10% ethanol/water mixture. When swelled in pure ethanol, τ₃ initially increased with the solvent amount in the gel, showing the destruction of intramolecular hydrogen bonding and the relaxation of polymer chains. The lower critical solution temperature of the gel in the 10% ethanol/water mixture was lower than that in pure water, and τ₃ for various solvent contents showed behavior similar to that seen in pure solvent. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1028–1036, 2002     

Sensitive LC MS quantitative analysis of carbohydrates by Cs<Superscript>+</Superscript> attachment

The development of a sensitive assay for the quantitative analysis of carbohydrates from human plasma using LC/MS/MS is described in this paper. After sample preparation, carbohydrates were cationized by Cs(+) after their separation by normal phase liquid chromatography on an amino based column. Cesium is capable of forming a quasi-molecular ion [M + Cs](+) with neutral carbohydrate molecules in the positive ion mode of electrospray ionization mass spectrometry. The mass spectrometer was operated in multiple reaction monitoring mode, and transitions [M + 133] --> 133 were monitored (M, carbohydrate molecular weight). The new method is robust, highly sensitive, rapid, and does not require postcolumn addition or derivatization. It is useful in clinical research for measurement of carbohydrate molecules by isotope dilution assay.     

Isomeric complexes of ruthenium(II) with neutral heterocyclic schiff base ligands. High resolution proton resonance spectra of trans-cis isomeric pairs of RuX2L2 (L = 2-arylpyridinecarboxaldimine, X = Cl, Br) and comparison of their physical properties

The reaction of 2-arylpyridinecarboxaldimine [RH₄C₆NC(H)Py, L (1)] with hydrated RuX₃ (X = Cl, Br) in boiling C₂H₅OH affords dark crystals of RuX₂L₂. Two geometrical isomers of the compound have been isolated and characterized by analytical and spectroscopic data. The trans isomer of RuCl₂L₂ shows a single sharp band for ν(Ru---Cl), whereas two bands are observed for the corresponding cis isomer. The highresolution ¹H NMR spectra of the isolated complexes are reported and completely assigned. All the complexes have multiple t₂→π^*(L) transitions in the visible region. Each of the complexes display a quasi-reversible oxidative response due to an Ru^III/Ru^II couple in the range 0.25–0.40 V vs S.C.E. at a platinum working electrode. The formal potentials of this couple obey the Hammett relationship. The ligand-based irreversible oxidations are also briefly noted.     

A convenient stereoselective synthesis of (R)-(−)-denopamine and (R)-(−)-salmeterol

β-Adrenoreceptor agonists (R)-(−)-denopamine (R)-1 and (R)-(−)-salmeterol (R)-2 have been prepared in good overall yield and high enantioselectivity through a biotransformative pathway.     

Platinum complexes of diazo ligands. Studies of regioselective aromatic ring amination, oxidative halogen addition and reductive halogen elimination reactions

2-(Arylazo)pyridine ligands, L1a-1c react with the salt K2[PtCl4] to give the mononuclear complexes [PtCl2(L1)](1), which readily react with ArNH2 to yield the monochloro complexes of type [PtCl(L2)](HL2= 2-[(2-(arylamino)phenyl)azo]pyridine)(2) via regioselective ortho-amine fusion at the pendent aryl ring of coordinated L1. Oxidative addition of the electrophiles Y2(Y = Cl, Br, I) to the square-planar platinum(II) complex, has led to syntheses of the corresponding octahedral platinum(IV) complexes, [PtY3(L2)](3) in high yields. Ascorbate ion reductions of the platinum(IV) complexes, , resulted in reductive halogen elimination to revert to the platinum(II) complexes almost quantitatively. Isolation of products and X-ray structure determination of the representative complexes followed all these chemical reactions. In crystal packing, the compound [PtCl2(L1c)](1c) forms dimeric units with a Pt...Pt distance of 3.699(1) A. In contrast, the crystal packing of 2b revealed that the molecules are arranged in an antiparallel fashion to form a noncovalent 1D chain to accommodate pi(aryl)-pi(pyridyl) and Pt-pi(aryl) interactions. Notably, the oxidation of [Pt(II)Cl(L2a)](2a) by I2 produced a mixed halide complex [Pt(IV)ClI2(L2a)](5), which, in turn, is reduced by ascorbate ion to produce [Pt(II)I(L2a)] with the elimination of ClI. All the platinum(II) complexes are brown, the platinum(IV) complexes, on the other hand, are green. Low-energy visible range transitions in the complexes of the extended ligand [L2]- are ascribed to ligand basedpi-pi* transitions. Cyclic voltammetric behaviour of the complexes is reported.     

Ruthenium complexes of 2-[(4-(arylamino)phenyl)azo]pyridine formed via regioselective phenyl ring amination of coordinated 2-(phenylazo)pyridine: isolation of products,X-ray structure,and redox and optical properties

Aromatic ring amination reactions in the ruthenium complex of 2-(phenylazo)pyridine is described. The substitutionally inert cationic brown complex [Ru(pap)(3)](ClO(4))(2) (1) (pap = 2-(phenylazo)pyridine) reacts smoothly with aromatic amines neat and in the presence of air to produce cationic and intense blue complexes [Ru(HL(2))(3)](ClO(4))(2) (2) (HL(2) = 2-[(4-(arylamino)phenyl)azo]pyridine). These were purified on a preparative TLC plate. The X-ray structure of the new and representative complex 2c has been solved to characterize them. The results are compared with those of the starting complex, [Ru(pap)(3)](ClO(4))(2) (1). The transformation 1 --> 2 involves aromatic ring amination at the para carbon (with respect to the diazo function) of the pendant phenyl rings of all three coordinated pap ligands in 1. The transformation is stereoretentive, and the amination reaction is regioselective. The extended ligand HL(2) coordinates as a bidentate ligand and chelates to ruthenium(II) through the pyridine and one of the azo nitrogens. The amine nitrogen of this bears a hydrogen atom and remains uncoordinated. Similarly, the amination reaction on the mixed-ligand complex [Ru(pap)(bpy)(2)](ClO(4))(2) produces the blue complex [Ru(HL(2))(bpy)(2)](ClO(4))(2) (3) as anticipated. The reactions of [RuCl(2)(dmso)(4)] and [Ru(S)(2)(L)(2)](2+) (dmso = dimethyl sulfoxide, S = labile coordinated solvent, L = 2,2'-bipyridine (bpy) and pap) with the preformed HL(2) ligand have been explored. The structure of the representative complex [RuCl(2)(HL(2a))(2)] (5a) is reported. It has the chlorides in trans configuration while the pyridine as well as azo nitrogens are in cis geometry. Optical spectra and redox properties of the newly synthesized complexes are reported. All the ruthenium complexes of HL(2) are characterized by their intense blue solution colors. The lowest energy transitions in these complexes appear near 600 nm, which have been attributed to intraligand charge-transfer transitions. For example, the lowest energy visible range transition in [Ru(HL(2b))(3)](2+) appears at 602 nm and its intensity is 65 510 M(-1) cm(-1). All the tris chelates show multiple-step electron-transfer processes. In [Ru(HL(2))(3)](2+), six reductions waves constitute the complete electron-transfer series. The electrons are believed to be added successively to the three azo functions. In the mixed-ligand chelates [Ru(HL(2))(pap)(2)](2+) and [Ru(HL(2))(bpy)(2)](2+) the reductions due to HL(2), pap, and bpy are observed.