Ramgopal Bhattacharyya

Oxoperoxo molybdenum(VI) and tungsten(VI) and oxodiperoxo molybdate(VI) and tungstate(VI) complexes with 8-quinolinol: synthesis, structure and catalytic activity

A solution obtained by dissolving MoO3 in a moderate excess of H2O2 reacts with 8-quinolinol (QOH) to give [MoO(O2)(QO)2] (1), but, when the same reaction is conducted with a large excess of H2O2, an anionic complex is formed, which reacts with PPh4Cl to give the corresponding salt [MoO(O2)2(QO)][PPh4] (2·PPh4). Freshly prepared WO3 behaves the same way and, depending on the amount of H2O2 used, as above, produces either [WO(O2)(QO)2] (3) or [WO(O2)2(QO)][PPh4] (4·PPh4), respectively. Crystallographic analyses reveal the coordination geometries around the metal center in these complexes to be distorted pentagonal bipyramids. These compounds show interesting catalytic properties in the oxidation of alcohols using H2O2 as the terminal oxidant. In the case of aromatics, including benzylic and cinnamylic alcohols, the oxidation occurs selectively, affording aldehydes or ketones with reasonably high turnover numbers. Taking benzyl alcohol as a representative case, a probable mechanism of the alcohol-to-aldehyde conversion mediated by the prepared catalysts is suggested. The oxidation of aliphatic primary alcohols, under the same conditions, does not show the above selectivity: the reaction yields the corresponding aldehydes as well as carboxylic acids. The work was also extended to study the catalytic activity towards the oxidation of phenol and various sulfides and amines using the same oxidants.

Highly facile homogeneous epoxidation of olefins using oxo-diperoxo tungstate(VI) complex as catalyst, bicarbonate as co-catalyst and hydrogen peroxide as a terminal oxidant

Addition of a dilute acetic acid solution of 8-quinolinol to an H2O2 solution of freshly precipitated H2WO4·2H2O furnishes a yellow adduct [WO(O2)2·2QOH] 1 which, on crystallization from a suitable solvent, affords orange-yellow complex [WO(O2)(QO)2] 2. When 2 reacts stoichiometrically with olefinic compounds in a 1∶1 molar ratio, the respective olefins are epoxidized and 2 is converted to the orange-red [WO2(QO)2] 3. When 1 is treated with an excess of H2O2 (greater than 6 equiv.) and PPh4Cl, an anionic light yellow complex PPh4[WO(O2)2(QO)] 4 is obtained. 4 reacts with cyclopentene (a representative olefin) in a 1∶1 molar ratio producing cyclopentene oxide and itself is converted to PPh4[WO2(O2)(QO)] 5. If the above reaction is conducted at a 1∶2 molar ratio (instead of 1∶1) then 2 moles of the corresponding epoxide is formed and 4 is converted to PPh4[WO3(QO)] 6. All these peroxo complexes have remarkable catalytic efficiencies in the epoxidation of olefinic compounds when used in tandem with NaHCO3 as co-catalyst and H2O2 as oxidant in a CH3CN medium at room temperature, the method being green and economical. The catalyst 4under the above experimental conditions shows so far unmatched efficiency in epoxidizing a wide variety of olefinic substrates.

Interaction of MgATP2− with DNA: assessment of metal binding sites and DNA conformations by spectroscopic and thermal denaturation studies

Abstract Spectroscopic (IR, UV, CD and fluorescence) and thermal denaturation studies of native calf thymus DNA, DNAMgATP2− and DNAMg2+ have been carried out in aqueous KBr medium (introduced by the present authors as a very effective solvent for DNA). The IR data recorded for the systems indicate that MgATP2− binds to the N7 and C6O of the guanine residue of DNA forming a five-membered chelate ring. The data also suggest that despite binding to the guanine bases, Mg2+ binds more strongly to the phosphate moiety of DNA. Solution CD spectra of DNA, DNAMgATP2− and DNAMg2+ indicate that in each case DNA exists in the B conformation. Thin-film CD studies reveal that irrespective of the relative humidity conditions, pure DNA as well as that after interaction with Mg2+ show a structural transition B → C, conformationally, although belonging to the B family. A similar study shows that DNA on interaction with MgATP2− assumes a more packed conformation (B)n giving rise to a ψ− spectrum. Steady-state as well as dynamic fluorimetric studies clearly indicate that MgATP2− does not intercalate between CGGC base pairs. The thermal denaturation studies support the IR data with respect to the metal binding sites and the mode of binding in both cases.

Formation of magnetic discontinuities through viscous relaxation

According to Parkers magnetostatic theorem, tangential discontinuities in magnetic field, or current sheets (CSs), are generally unavoidable in an equilibrium magnetofluid with infinite electrical conductivity and complex magnetic topology. These CSs are due to a failure of a magnetic field in achieving force-balance everywhere and preserving its topology while remaining in a spatially continuous state. A recent work [Kumar, Bhattacharyya, and Smolarkiewicz, Phys. Plasmas 20, 112903 (2013)] demonstrated this CS formation utilizing numerical simulations in terms of the vector magnetic field. The magnetohydrodynamic simulations presented here complement the above work by demonstrating CS formation by employing a novel approach of describing the magnetofluid evolution in terms of magnetic flux surfaces instead of the vector magnetic field. The magnetic flux surfaces being the possible sites on which CSs develop, this approach provides a direct visualization of the CS formation, helpful in understanding the governing dynamics. The simulations confirm development of tangential discontinuities through a favorable contortion of magnetic flux surfaces, as the magnetofluid undergoes a topology-preserving viscous relaxation from an initial non-equilibrium state with twisted magnetic field. A crucial finding of this work is in its demonstration of CS formation at spatial locations away from the magnetic nulls.

Highly efficient epoxidation method of olefins with hydrogen peroxide as terminal oxidant, bicarbonate as a co-catalyst and oxodiperoxo molybdenum(VI) complex as catalyst

A combination of the newly synthesized and structurally characterized compound, [MoO(O2)2(saloxH)](saloxH2= salicylaldoxime) as catalyst, H2O2 as terminal oxidant and NaHCO3 as co-catalyst when stirred in CH3CN (10 cm3) at room temperature (rt) shows a very pronounced efficiency epoxidation of olefinic compounds, the method being green and economical.

Reductive nitrosylation of tetraoxometallates. Part 3. Generation of {Mo(NO)}4, {Mo(NO)2}4, and {Mo(NO)2}6 moieties: synthesis of 2,2′- bipyridine, 1,10-phenanthroline, thiocyanato-, and diethyldithiocarbamato-complexes of mono- and di-nitrosylmolybdenum directly from MoO42– in aqueous and aerobic media

In an aqueous aerobic medium MoO42– can be reductively nitrosylated using excess of NH2OH·HCl and SCN– in the range pH 4–4.5, generating selectively the {Mo(NO)}4 moiety. This has been confirmed by synthesising complexes of the types [Mo(NO)(NH2O)(NCS)4]2–, [Mo(NO)(NH2O)(S2CNEt2)2], and the isomeric [Mo(NO)(NH2O)(NCS)2(L–L)][L–L = 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen)] almost in quantitative yield. However, if the same reaction is conducted at pH 5.2–5.4 a selective generation of the {Mo(NO)2}4 moiety occurs leading to the stereoselective synthesis of [Mo(NO)2(NHO)(NCS)4]2– and [Mo(NO)2(NHO)(NCS)2(L–L)]. In the range pH 5.7–6 the initially formed Mo(NO)3+ species undergoes disproportionation to a formally molybdenum(0) species, {Mo(NO)2}6, viz.[Mo(NO)2(NCS)4]2– or [Mo(NO)2(NCS)2(L–L)] along with an oxomolybdenum(V) species, viz.[Mo2O4(NCS)6]4– or [Mo2O4(NCS)2(L–L)2]. Here also, the dinitrosylmolybdenum moiety is formed stereoselectively.

Syntheses, spectroscopic and X-ray structure analyses of two dioxouranium (VI) complexes : supramolecular framework built from O-H...O, C-H...O and stacking interactions

Abstract Two dioxouranium(VI) complexes, [UO2(PBHA)2(DMSO)] (I) and [UO2(SALOP)(H2O)] (II), PBHA = N-phenyl benzohydroxamate, DMSO = dimethyl sulfoxide, SALOP = N,N′-bis (salicylidene)-o-phenylene diaminate have been synthesized, and characterized by spectroscopic and X-ray studies. Both compounds (I) and (II) crystallize in orthorhombic system with space group Pbca. The molecules in (I) are linked by intermolecular C–H…O hydrogen bonds into 32-membered tetrameric clusters (U4C12O8N4H4), which are further connected via uranyl oxygen atoms forming a three-dimensional supramolecular framework. In (II), intermolecular O—H…O hydrogen bonds between molecules related by inversion and translation generate infinite zigzag chains of R22(8) rings running along the [010] direction. The parallel chains are interlinked through C—H…O hydrogen bonds producing a complex supramolecular architecture in (II).

Novel oxo-peroxo molybdenum(VI) complexes incorporating 8-quinolinol: synthesis, structure and catalytic uses in the environmentally benign and cost-effective oxidation method of methyl benzenes: Ar(CH3)n (n = 1, 2)

A hitherto unknown distorted pentagonal bipyramidal complex, [MoO(O2)(QO)2], very efficiently catalyses homogeneous liquid phase oxidation of methylbenzenes, viz. toluene and o- and p-xylenes to benzoic acid, phthalic acid and p-toluic acid respectively, using H2O2 and O2 as oxidants.

Reductive Nitrosylation of ReO− 4 and Synthesis of an Azido Nitrosyl Complex of Rhenium and its 1,10-Phenanthroline and 2,2 ' -Bipyridine Derivatives Directly from ReO− 4

Abstract Reductive nitrosylation of ReO- 4 by any of the known nitrosylating agents1 and subsequent synthesis of rhenium nitrosyl derivatives have not yet been reported. The known nitrosyl compounds of this metal are very few and were prepared from low valent rhenium compounds (cyano, chloro or carbonyl complexes) by the treatment with HNO3 2.3 or NO4,5 or NOX6. We have recently reported7 the reduction and subsequent nitrosylation of ReOi using NH2OHHCI and NCS- in alkaline medium. Herein is described the direct reductive nitrosylation of ReO- 4 using NH2OHHCl and N- 3 as evidenced by synthesing the complex ion [Re(NO)(N3)3H2O]- and its 1,10-phenanthroline and 2,2′-bipyridine derivatives from an aqueous-aerobic medium. It may be mentioned that azido nitrosyl complexes of any metal are extremely rare (only one is known).6

Reductive nitrosylation of tetraoxometallates—XI. Molybdate hydroxylamine reaction in the presence of cyanide: Synthesis of mono- and dinitrosyl cyan

Abstract The new molybdenum cyanonitrosyl complexes, R2[Mo(NO)(CN)5]·2H2O (R = Ph4P and Bu4N) and [Mo(NO)(CN)3(L-L)]·H2O [L-L =