Averi Guha

Radical Pathway in Catecholase Activity with Zinc-Based Model Complexes of Compartmental Ligands

Four dinuclear and three mononuclear Zn(II) complexes of phenol-based compartmental ligands (HL(1)-HL(7)) have been synthesized with the aim to investigate the viability of a radical pathway in catecholase activity. The complexes have been characterized by routine physicochemical studies as well as X-ray single-crystal structure analysis: [Zn(2)(H(2)L(1))(OH)(H(2)O)(NO(3))](NO(3))(3) (1), [Zn(2)L(2)Cl(3)] (2), [Zn(2)L(3)Cl(3)] (3), [Zn(2)(L(4))(2)(CH(3)COO)(2)] (4), [Zn(HL(5))Cl(2)] (5), [Zn(HL(6))Cl(2)] (6), and [Zn(HL(7))Cl(2)] (7) [L(1)-L(3) and L(5)-L(7) = 2,6-bis(R-iminomethyl)-4-methylphenolato, where R= N-ethylpiperazine for L(1), R = 2-(N-ethyl)pyridine for L(2), R = N-ethylpyrrolidine for L(3), R = N-methylbenzene for L(5), R = 2-(N-methyl)thiophene for L(6), R = 2-(N-ethyl)thiophene for L(7), and L(4) = 2-formyl-4-methyl-6-N-methylbenzene-iminomethyl-phenolato]. Catecholase-like activity of the complexes has been investigated in methanol medium by UV-vis spectrophotometric study using 3,5-di-tert-butylcatechol as model substrate. All complexes are highly active in catalyzing the aerobic oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ). Conversion of 3,5-DTBC to 3,5-DTBQ catalyzed by mononuclear complexes (5-7) is observed to proceed via formation of two enzyme-substrate adducts, ES1 and ES2, detected spectroscopically, a finding reported for the first time in any Zn(II) complex catalyzed oxidation of catechol. On the other hand, no such enzyme-substrate adduct has been identified, and 3,5-DTBC to 3,5-DTBQ conversion is observed to be catalyzed by the dinuclear complexes (1-4) very smoothly. EPR experiment suggests generation of radicals in the presence of 3,5-DTBC, and that finding has been strengthened by cyclic voltammetric study. Thus, it may be proposed that the radical pathway is probably responsible for conversion of 3,5-DTBC to 3,5-DTBQ promoted by complexes of redox-innocent Zn(II) ion. The ligand-centered radical generation has further been verified by density functional theory calculation.

Preparation of antiferromagnetic Co3O4 nanoparticles from two different precursors by pyrolytic method: in vitro antimicrobial activity

Two varieties of Co3O4 nano particles (Co3O4-I and Co3O4-II) have been synthesized from two different precursors using a pyrolytic technique. Co3O4-I was prepared by using a coordination polymer [Co(dca)2(2-benzoylpyridine)]n (dca = dicyanamide) as sole precursor, whereas Co3O4-II was obtained from a dinuclear complex [Co2(HL)(OAc)2](OAc)2·4H2O [HL = 2,6-bis(N-ethylpiperazine-iminomethyl)-4-methyl phenol]. The synthesized nanoparticles were characterized by FTIR spectroscopy, magnetic measurements and X-ray diffraction studies. Both Co3O4-I and Co3O4-II are high-quality mono-dispersed, stable and defect-free nanoparticles. The surface morphology of these nanoparticles was revealed by scanning electron microscopy. Co3O4-I nanoparticles have square shape and size ranging from 10 to 25 nm, whereas Co3O4-II nanoparticles have hexagonal shape with larger particle size (100–150 nm). The size distribution of the nanoparticles was determined by dynamic light scattering. The particle size and microstructure were studied by transmission electron microscopy (TEM) images. These nanoparticles show an effective anti-microbial activity, employing Staphylococcus aureus and Escherichia coli as model microbial species, evidenced from the Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) values.

Anion mediated formation of different Schiff bases from the same precursors and their nickel(II) complexes with different nuclearity

Two nickel(II) complexes, [Ni2 (H2O)4](NO3)2 (1) and [Ni5 (OAc)6(OH)2] · 5.5H2O (2), have been synthesized and structurally characterized by X-ray crystallographic study, where L1 and L2 represent 2-formyl-4-methyl-6-(1-(2-aminomethyl)piperidine)-iminomethylphenolate and 4-methyl-2,6-bis(1-(2-aminomethyl)piperidine)-iminomethylphenolate, respectively. One-pot reactions between 2,6-diformyl-4-methylphenol with 1-(2-aminoethyl)piperidine, in the presence of nickel(II) nitrate and nickel(II) acetate, afforded in situ [1 + 1] and [1 + 2] condensation leading to Schiff-bases HL1 and HL2, and to 1 and 2 of different nuclearity. Both processes were conducted under the same reaction conditions in anhydrous EtOH. Preferential attainment of bi- or pentanuclear compounds is mediated by the anion of the Ni(II) salt. Complex 1 is formed by two unsymmetrical tridentate L1 chelating in a head-tail fashion to make a centrosymmetric phenoxido-bridged Ni(II) dimer. Two waters complete octahedral coordination. Conversely in 2, two [Ni2(OAc)L2]+ units, having a syn-syn bidentate bridging acetate, embrace a third Ni through bridging acetate and μ3-hydroxide, giving rise to an adduct of C2 symmetry where the compartmental ligand planes form a dihedral angle of ca. 82°. The catecholase activity of these complexes were explored and both complexes effectively catalyze the conversion of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ).

Structure and luminescence of a nitrate-bridged heterotrinuclear Cu2-Pr complex with compartmental Schiff base ligand

A unique heterotrinuclear nitrate-bridged complex of hexanitrate praseodymium(III) and dicopper(II) compartmental species has been synthesized and characterized by X-ray single crystal structure analysis. The structure determination indicates that the dinuclear copper moiety undergoes a tilted deformation (with respect to the dicopper complex) upon connection to the lanthanide species via a rare nitrate bridge. The trinuclear species is highly fluorescent owing to the presence of praseodymium.