Susmita Bhunia

Layered transition metal carboxylates: efficient reusable heterogeneous catalyst for epoxidation of olefins.

Layered metal carboxylates [M(malonato)(H(2)O)(2)](n) (M = Ni(II) and Mn(II)) that have a claylike structure have been synthesized hydrothermally and characterized. The interlayer separation in these layered carboxylates is comparable to that of the intercalation distance of the naturally occurring clay materials or layered double hydroxides (LDHs). In this study, we have demonstrated that, instead of intercalating the metal complex into layers of the clay or LDH, layered transition metal carboxylates, [M(malonato)(H(2)O)(2)](n), as such can be used as a recyclable heterogeneous catalyst in olefin epoxidation reaction. Metal carboxylates [M(malonato)(H(2)O)(2)](n) exhibit excellent catalytic performance in olefin epoxidation reaction.

MCM-41-supported oxo-vanadium(IV) complex: a highly selective heterogeneous catalyst for the bromination of hydroxy aromatic compounds in water.

An ecofriendly solid catalyst has been synthesized by anchoring vanadium(IV) into organically modified MCM-41. First, the surface of Si-MCM-41 was modified with 3-aminopropyl-triethoxysilane (3-APTES), the amine group of which upon condensation with ortho-hydroxy-acetophenone affords a N(2)O(2)-type Schiff base moiety in the mesoporous matrix. The Schiff base moieties were used to anchor oxo-vanadium(IV) ions. The prepared catalyst has been characterized by UV-vis, IR spectroscopy, small-angle X-ray diffraction (SAX), nitrogen sorption, and transmission electron microscopy (TEM) studies. It is observed that the mesostructure has not been destroyed in the multistep synthesis procedure, as evidenced by SAX and TEM measurements. The catalyst has shown unprecedented high conversion as well as para selectivity toward the bromination of hydroxy aromatic compounds using aqueous 30% H(2)O(2)/KBr in water. The reaction proceeds according to the stoichiometric ratio, and the monobrominated product was obtained as the major product using a stoichiometric amount of the bromine source. The immobilized complex does not leach or decompose during the catalytic reactions, showing practical advantages over the free metal complex.

Catalytic olefin epoxidation over cobalt(II)-containing mesoporous silica by molecular oxygen in dimethylformamide medium

A cobalt(II) Schiff base complex has been immobilized onto the surface of Si–MCM-41 to prepare a new catalyst. The amine group-containing organic moiety 3-aminopropyl-triethoxysilane had first been anchored on the surface of Si–MCM-41 via a silicon alkoxide route. Upon condensation with salicylaldehyde, the amine group affords a bidentate Schiff-base moiety in the mesoporous matrix, which is subsequently used for anchoring of cobalt(II) centers. The prepared catalyst has been characterized by UV-vis, infrared (IR), EPR spectroscopic and small angle X-ray diffraction (XRD) analyses, and N2 sorption studies. The catalytic activity was tested in epoxidation reactions of olefinic compounds, including styrene and allyl alcohol, with molecular oxygen at atmospheric pressure in dimethylformamide medium in the absence of additional sacrificial reductant. The reactions seemed to proceed through a radical formation mechanism. The immobilized catalyst showed good activity and epoxide selectivity in the alkene epoxidation. Notably, the catalyst can be recovered and reused without any loss of activity.

A family of ligand and anion dependent structurally diverse Cu(II) Schiff-base complexes and their catalytic efficacy in an O-arylation reaction in ethanolic media

Two nitrato bridged dinuclear systems [Cu2(L1)2(NO3)3]NO3·H2O (1) and [Cu2(L2)2(NO3)3]NO3·MeOH (2), five monomeric complexes viz. [Cu(L3)(NO3)]NO3 (3), [Cu(L4)(NO3)]NO3 (4), [Cu(L5)(NO3)]NO3 (5), [Cu(L6)(NO3)NO3] (7), [Cu(L7)(NO3)]NO3 (8) and one hetero bi-bridged (phenoxido and water) dinuclear complex [Cu2(L2)2(H2O)2](ClO4)4·4H2O (6) have been synthesized and characterized using several physicochemical methods (L1 = 1-(N-3-methoxysalicylideneimino)-ethane-2-piperazine, L2 = 1-(N-3-ethoxysalicylideneimino)-ethane-2-piperazine, L3 = 1-(N-4′-ethoxy-α-methylasalicylideneimino)-ethane-2-piperazine, L4 = 1-(N-5′-chloro-α-methylasalicylideneimino)-ethane-2-piperazine, L5 = 1-(N-5-chlorosalicylideneimino)-ethane-2-piperazine, L6 = 1-(N-4-methoxysalicylideneimino)-ethane-2-piperazine and L7 = 1-(N-4′-methoxy-α-methylasalicylideneimino)-ethane-2-piperazine). X-ray structural analysis showed that complexes 1 and 2 are discrete dinuclear species where the pentacoordinated metal centers are bridged through a nitrate ion. In 3, 4, 5 and 8 the monomeric copper center displays a square pyramidal geometry with a weak axial Cu–O bond. In 7, the monomeric copper center shows a distorted octahedral geometry with two coordinated nitrate anions. However, in 6 the two copper centers coordinate in different manners (one is square-pyramidal and the other is distorted octahedral) and are bridged through a phenoxido group and a water molecule. All complexes efficiently catalyze the C–O coupling reaction under homogeneous conditions at 80 °C to afford unsymmetrical diaryl ethers using nitroarenes to act as an excellent electrophile. Notably, the reaction is carried out in ethanol media which facilitates the avoidance of toxic wastes. Structurally diverse copper(II) Schiff-base complexes have rarely been used systematically in catalytic C–O coupling reactions.

Heterogeneous O-arylation of nitroarenes with substituted phenols over a copper immobilized mesoporous silica catalyst

Highly porous and robust mesoporous silica, SBA-15 has been subjected to post-synthesis modification for the anchoring of copper through Schiff base moiety formation using the silicon alkoxide route. The hybrid porous material has been fully characterized by powder-XRD, electronic spectra, EPR, thermogravimetric analysis, N2 sorption measurements, and TEM and SEM/EDS studies. The efficiency of the catalyst has been assessed in the O-arylation reaction using various substituted phenols and nitroarenes in heterogeneous conditions. The catalytic coupling reaction efficiently produces unsymmetrical diaryl ethers. The impressive capability to activate substrates having electron-donating or electron-withdrawing substituents and to have a high turnover frequency in the catalytic reactions made the catalyst highly desirable.