Kusum K. Bania

Enhanced Catalytic Activity of Zeolite Encapsulated Fe(III)-Schiff-Base Complexes for Oxidative Coupling of 2-Napthol

Iron(III) Schiff-base complexes of general formula [Fe(L)(2)Cl]·2H(2)O, where L = N,Ń-bis(salicylidene)ethylenediamine and N,Ń-disalicylidene-1,2-phenylenediamine have been encapsulated within various alkali exchanged zeolites viz. LiY, NaY, and KY by flexible ligand method. The encapsulated complexes are characterized by EDX, scanning electron microscopy (SEM), powder X-ray diffraction (XRD), FT-IR, UV-vis, diffuse reflectance spectroscopy (DRS), electron spin resonance spectroscopy (ESR) and cyclic voltammetry studies. The diffuse reflectance UV-vis spectra of encapsulated complexes show a dramatic red shift of the charge transfer band with increasing electropositivity of the exchangeable cations. The electrochemical analysis predicts the shifting of the reduction potential toward negative values with increasing size of the alkali exchanged cations. The zeolite encapsulated Schiff-base complexes of iron are found to be catalytically active toward the oxidative coupling of 2-napthol. Metal complexes incorporated in potassium exchanged zeolite-Y are found to be more effective for catalytic conversion of 2-naphthol to binaphthol and induces higher selectivity toward the R-conformation. The catalytic conversion of 2-napthol to BINOL is found to depend on the reduction potential of the catalyst, with a more negative reduction potential being better for the catalytic conversion. Density functional calculation is being carried out on both the neat Fe-Salen and Fe-Salophen complexes and those encapsulated in NaY zeolite to investigate change in structural parameters, energies of the HOMO and LUMO, and global hardness and softness. Fukui functions, as local descriptors, are used to analyze the hard-hard interaction at a particular site of the complexes.

Peroxoniobium(V)-catalyzed selective oxidation of sulfides with hydrogen peroxide in water: a sustainable approach

An efficient and eco-compatible route for the selective oxidation of a variety of thioethers to the corresponding sulfoxide or sulfone with 30% aqueous H2O2 in water, using newly synthesized peroxoniobium (pNb) complexes as catalysts, is described. The catalysts with formulas Na2[Nb(O2)3(arg)]·2H2O (arg = arginate) (NbA) and Na2[Nb(O2)3(nic)(H2O)]·H2O (nic = nicotinate) (NbN) have been synthesized from the reaction of sodium tetraperoxoniobate with 30% H2O2 and the respective organic ligand in an aqueous medium, and these have been comprehensively characterized by elemental analysis, spectral studies (FTIR, Raman, 1H NMR, 13C NMR and UV-vis), EDX analysis and TGA-DTG analysis. The density functional theory (DFT) method has been used to investigate the structure of the synthesized pNb complexes. The catalysts are physiologically safe and can be reused for at least six reaction cycles without losing their activity or selectivity. The oxidation is chemoselective for sulfides or sulfoxides leaving the CC or alcoholic moiety unaffected. The developed methodologies, apart from being high yielding and straightforward, are completely free from halogen, organic co-solvent, or co-catalysts.

Oxidative coupling of 2-naphthol by zeolite-Y supported homo and heterometallic trinuclear acetate clusters

Two trinuclear acetate clusters of iron and cobalt of general formula [Fe3O(O2CCH3)6(H2O)3]NO3·2H2O and Fe2Co(O)[(OOCC6H4NO2)6]NO3·2H2O are synthesized and characterized. The synthesized trinuclear clusters are supported on zeolite-Y via an ion exchanged method. FTIR study reveals that the two complexes are tethered via formation of Si–O–H⋯O–H hydrogen bond linkages with a zeolite-Y matrix. Homogeneous and heterogeneous trinuclear catalysts are found to be efficient catalysts for oxidative coupling of 2-naphthol. Compared to homometallic oxo-clusters, bimetallic complexes are found to show better catalytic activity. Besides obtaining BINOL as a major product, these metal clusters also lead to formation of the tautomeric form of BINOL. The crystal structure of the by-product indicates the formation of a tetrahedral chiral centre in the molecule via the attachment of the solvent. Density Functional Theory (DFT) calculations have been performed to elucidate the structural and electronic properties of both homogeneous and heterogeneous complexes.

Substituent and Solvent Effects on the Absorption Spectra of Cation-π Complexes of Benzene and Borazine: A Theoretical Study.

Time-dependent density functional theory (TDDFT) has been used to predict the absorption spectra of cation-π complexes of benzene and borazine. Both polarized continuum model (PCM) and discrete solvation model (DSM) and a combined effect of PCM and DSM on the absorption spectra have been elucidated. With decrease in size of the cation, the π → π* transitions of benzene and borazine are found to undergo blue and red shift, respectively. A number of different substituents (both electron-withdrawing and electron-donating) and a range of solvents (nonpolar to polar) have been considered to understand the effect of substituent and solvents on the absorption spectra of the cation-π complexes of benzene and borazine. Red shift in the absorption spectra of benzene cation-π complexes are observed with both electron-donating groups (EDGs) and electron-withdrawing groups (EWGs). The same trend has not been observed in the case of substituted borazine cation-π complexes. The wavelength of the electronic transitions corresponding to cation-π complexes correlates well with the Hammet constants (σp and σm). This correlation indicates that the shifting of spectral lines of the cation-π complexes on substitution is due to both resonance and inductive effect. On incorporation of solvent phases, significant red or blue shifting in the absorption spectra of the complexes has been observed. Kamlet-Taft multiparametric equation has been used to explain the effect of solvent on the absorption spectra of complexes. Polarity and polarizability are observed to play an important role in the solvatochromism of the cation-π complexes.

Cation–π interaction in cofacial molecular dyads: a DFT and TDDFT study

In this study we proposed a cofacial molecular dyad or a tweezer (4,5-biphenyl acridine, BPA) with the acridine moiety serving as the spacer and two phenyl rings as the wings. Density functional theory (DFT) calculation was implemented to illustrate the pivotal role of the phenyl rings in stabilizing the cation (Li+, Na+ or K+) at the tweezers via cation–π interaction. Our calculations reveal that apart from the covalent interaction, the cation–π interaction energy contributes to the stabilization of cations inside the molecular pocket. The effect of substituents on the structure and interaction energy of the cation–π complexes was also exemplified. An electron donating group favored the cation–π interaction while an electron withdrawing group lowered the strength of cation–π interaction. Solvent polarity was found to play a critical role in stabilizing the cation–π interaction. Solvents with a low dielectric constant mostly favored such interaction while highly polar solvents resulted in repulsive interaction energy. TDDFT (time dependent density functional theory) calculations were performed to understand the effect of the cation–π interaction on the absorption spectra of such molecular dyads.

Metallogel templated synthesis and stabilization of silver-particles and its application in catalytic reduction of nitro-arene

Metallogel of iron-carboxylates was obtained from trans-1,2-cyclohexanedicarboxylic acid in dimethylformamide (DMF) at basic condition. Spectroscopic and SEM morphology study of the iron-metallogel revealed that the iron complex with dicarboxylic acid was linked together via carboxylates and led to a supramolecular helical like architecture. The synthesized metallogel served as an excellent template for in-situ reduction of silver ion to silver particles micro to nano scale range. Variation of AgNO3 concentration shepherd to change the morphology of the Ag-particles. AgNO3 concentration was found to affect the shape and size of silver particles. On going from lower to higher concentration shape of silver particles changed from spherical to large agglomerated particles. Cubic shape Ag-particles were found on treatment of 0.05M AgNO3 solution with metallogel. Cubical shape silver particles were found to be effective catalyst for nitro-arene reduction in presence of NaBH4. Density functional theory (DFT) calculations were performed to rationalize the role of Ag-particles in catalytic reduction of 4-nitrophenol to 4-aminophenol. Based on DFT study, we proposed that catalytic reduction occurred via Ag-hydride complex formation. Since metallogels as well as the 4-aminophenol are finding large application in pharmaceuticals industries therefore the current work can provide an alternatives path in production of 4-aminophenols. In addition to this, the synthesis of Ag-nanomaterials using metallogel as template can pave a new direction in the development of nanotechnology and might find wide applications in catalytic industrial processes.

Constructing two dimensional amide porous polymer to promote selective oxidation reactions

The covalent organic polymer synthesis is an elegant way to design materials with various topologies and explore new structures and properties. Herein, we report the synthesis of a novel porous polymer framework with the [3 + 2] structure motif via the condensation of 1,3,5-benzenetricarbonyl trichloride and p-phenylenediamine. The resulting highly stable mesoporous 2D framework material exhibits continuous conjugation of π-electronic system. Apart from gas adsorption properties, this material shows remarkable catalytic activity manifested by the conjugated π-cloud and amide functionality in selective oxidation reactions. Utmost selectivity of the reaction that exclusively yields aldehydes from benzyl alcohols via a free radical pathway is proposed and demonstrated. This is one of the rare cases of any organic polymer material acting as an outstanding organic catalyst with no metal add-ons.

Oxidative coupling of 2-naphthol to (R)/(S)-BINOL by MCM-41 supported Mn-chiral Schiff base complexes

Three Mn(III)-chiral Schiff base complexes supported on MCM-41 are found to be effective reusable catalysts for enantioselective oxidation of 2-naphthol to (R)- and (S)-BINOL (1,1′ bi-2-naphthol) in the presence of oxygen. The supported Mn(III)-complexes are characterized by PXRD, FTIR, solid state-NMR, BET, and cyclic voltammetry study. The homo-coupling reaction with oxygen as the oxidant is promoted by 20 mg of Mn(III) Schiff base complexes to afford binaphthols in nearly quantitative yields with high enantioselectivity of up to 91% ee. The catalytic activities of the homogeneous and heterogeneous chiral catalyst are found to be almost similar. However, the heterogeneous counterparts are found to be advantageous in terms of recyclability and storability. Oxygen partial pressure, the nature of the solvent, temperature and the amount of catalyst affect the catalytic oxidation process. High temperature and highly polar solvent are found to have adverse effects on the catalytic oxidation process.

Fractal to monolayer growth of AgCl and Ag/AgCl nanoparticles on vanadium oxides (VOx) for visible-light photocatalysis

A facile and simple methodology was adopted for the trapping of highly crystalline AgCl and Ag/AgCl nanoparticles (NPs) into the interlayer spacings of vanadium oxides (VOx). Self-organization of AgCl and Ag/AgCl-NPs on VOx was found to be governed by the nature of the dicarboxylic acids used during the synthesis of the nanocomposites. A “fractal-like” morphology of the AgCl@VOx nanocomposite was achieved in the presence of cis-1,2 cyclohexanedicarboxylic acid. Heating of the AgCl@VOx nanocomposite above 68 °C resulted in the growth of polydispersed and ultrafine (3–4 nm) Ag/AgCl-NPs and its self-organization into monolayer formation on a partly crystalline VOx matrix. Change in the conformation of the dicarboxylic acid to the trans-isomer resulted in the formation of a ‘rod-like’ structure of Ag/AgCl-NPs on a highly crystalline VOx matrix. The band gaps of the nanocomposites were within the range of 1.8 to 2.9 eV. Because of such a low band gap, the synthesized nanocomposites were found to be highly active toward the photooxidation of methylene (MB) and methyl orange (MO) under sunlight.

DFT and TDDFT study on cation-π complexes of diboryne (NHC → B ≡ B←NHC)

In this study, density functional theory calculation on mono-cationic cation-π complexes of diborynes has been made to understand the interaction in cation-π complexes of diboryne. Results suggest that apart from the smaller cations Li+ and Na+, larger cation like K+ ion could also form complexes with diboryne compounds via cation-π interaction. From the calculated structural and spectroscopic analysis 11B, 13C NMR (Nuclear Magnetic Resonance), FTIR (Fourier Transform Infra red) (force constant, value), and UV-vis spectra, it is found that the interaction between the cations and π-electron cloud of the diboryne is purely electrostatic. It is also observed that smaller cation (Li+) with high electronegativity interacts more strongly compared to larger cation (K+). Calculated interaction energy advocates that the π-electron cloud of the B2 unit contributes more to the cation-π interaction than the two aromatic phenyl rings of the NHC (N-heterocyclic carbene) substituted with 2,6-diisopropylphenyl group. The aryl substituent at the NHC-ligands undergoes a change in spatial orientation with respect to the size of cations in order to provide suitable space to the cations for effective cation-π interaction. Quantum theory of atoms in molecules study clarifies further the nature and extent of B-B and B2-cation interactions.11B-NMR, 13C-NMR, and time dependent density functional theory analysis indicate that cation-π interaction annihilates the B → C (NHC) π-back donation and favours the B≡B bond formation.