Ayan Patra

Diversity of carboxylate binding in a new tetranuclear zinc cluster: correlation between spectroscopic investigations and carboxylate binding modes

A new tetranuclear ZnII4 cluster has been prepared and characterized by exploiting the flexibility, chelating ability and bridging potential of a carboxylate rich μ-bis(tetradentate) ligand. The acetate and benzoate functionalities of the ligand display both monodentate terminal and syn–anti bidentate bridging binding modes in the cluster. The binding modes of carboxylate have been investigated by a combined approach of X-ray crystallography, FTIR, solution and solid state 13C NMR spectroscopic techniques.

Exploring the catalytic activity of new water soluble dinuclear copper(II) complexes towards the glycoside hydrolysis

Two water soluble dinuclear copper(II) complexes of a new dinucleating ligand, H3phpda [H3phpda = N,N′-bis(2-pyridylmethyl)-2-hydroxy-1,3-propanediamine-N,N′-dipropionic acid] have been synthesized and characterized for the investigation of catalytic hydrolysis of glycosides. In methanol, the reaction of stoichiometric amounts of Cu(OAc)2·H2O and the ligand H3phpda in the presence of NaOH, produced a new water soluble dinuclear copper(II) complex, [Cu2(phpda)(μ-OAc)] (1). Similarly, the reaction of stoichiometric amounts of Cu(ClO4)2·6H2O and the ligand H3phpda in the presence of NaOH, in methanol, afforded a new water soluble dinuclear copper(II) complex, [Cu2(phpda) (H2O)2](ClO4) (2). Characterizations of the complexes have been performed using various analytical techniques including DFT calculation. The DFT optimized structure of complex 1 shows that two copper(II) centers are in a distorted square pyramidal geometry with Cu⋯Cu separation of 3.677 A. On the other hand, the DFT optimized structure of complex 2 reveals that one copper(II) center adopts a five-coordinate distorted square pyramidal geometry and the other copper(II) center is in a distorted square planar geometry with Cu⋯Cu separation of 3.553 A. Further, the mass spectroscopic analyses of complexes 1 and 2 reconfirm their dimeric nature, even in solution. Glycosidase-like activity of complexes 1 and 2 has been evaluated in aqueous solution at pH ∼ 10.5 by UV-vis spectrophotometric techniques using p-nitrophenyl-α-D-glucopyranoside and p-nitrophenyl-β-D-glucopyranoside as the model substrates. Both complexes are active in catalyzing the hydrolysis of glycosides. DFT calculation has been performed to find the Fukui functions at the metal centers in complexes 1 and 2 to predict the possible metal sites involved in the binding process with substrates during the catalytic hydrolysis reactions.

Hydrolytically active tetranuclear [NiII2]2 complexes: synthesis, structure, spectroscopy and phosphoester hydrolysis

Three tetranuclear nickel(II) complexes, [Ni4(H2chdp)2(H2O)4]Br2·4CH3OH·3H2O (1), [Ni4(H2chdp)2(H2O)4](PF6)2 (2) and [Ni4(H2chdp)2(H2O)4](ClO4)2·3.2CH3OH·0.8H2O (3) have been synthesized by exploiting the flexibility, chelating ability and bridging potential of a new symmetrical μ-bis(tetradentate) ligand, H5chdp (H5chdp = N,N′-bis[2-carboxybenzomethyl]-N,N′-bis[2-hydroxyethyl]-1,3-diaminopropan-2-ol). Complexes 1, 2 and 3 have been synthesized by carrying out reaction of the ligand H5chdp with stoichiometric amounts of NiCl2·6H2O/NaBr, NiCl2·6H2O/NH4PF6, and Ni(ClO4)2·6H2O, respectively, in methanol–water in the presence of NaOH at ambient temperature. Characterizations of the complexes have been done using various analytical techniques including single crystal X-ray structure determination of complexes 1 and 3. Molecular architecture of each complex is built from the self-assembly of two monocationic [Ni2(H2chdp)(H2O)2]+ units which are exclusively bridged by two benzoate functionalities of the ligands. Single crystal X-ray structure analyses reveal that the metallic cores of complexes 1 and 3 consist of four distorted octahedral nickel(II) ions with intra-ligand Ni⋯Ni separation of 3.527(7) A and 3.507(1) A, respectively. Complexes 1 and 3 display a rare μ3:η2:η1:η1 bridging mode of two benzoate groups of H2chdp3− ligand with each bridging among three nickel(II) ions. Mass spectrometric analyses suggest that all the tetranuclear complexes are stable in solution. Potentiometric titration results and the corresponding species distribution curves show that all the complexes exist predominantly in their tetrameric species in solution, in the pH range of 6–12. The catalytic activity of all the three complexes toward phosphoester hydrolysis has been investigated in methanol–water (1 : 1; v/v) solution by UV-vis spectrophotometric technique using bis(p-nitrophenyl)phosphate (BNPP) as a model substrate.