44,637 research outputs found
Phenoxo bridged luminescent dinuclear zinc(II) and cadmium(II) complexes of 2-[[[2-(2-pyridyl)ethyl]imino]methyl]phenol: Crystal structure, photophysical and thermal studies
No full textReactions of zinc(II) and cadmium(II) halides with an N,N,O-donor Schiff-base ligand HL (obtained by the
1:1 condensation of salicylaldehyde and 2-(2-aminoethyl)pyridine) yield six new phenoxo bridged dinuclear
complexes of the general formula [Zn2L2X2] (1–3) and [Cd2L2X2] (4–6), where X = Cl, Br and I, respectively.
The complexes have been characterized by routine physicochemical techniques: elemental
analyses, IR, electronic spectral studies, conductivity and solid state thermal studies. Complexes 1, 2
and 6 have further been characterized by single crystal X-ray structural analyses. The ligands, as well as
all six complexes, are highly fluorescent. For the ligand, the emission band is attributed to a p–p⁄ transition,
whereas for the complexes the emissions may be assigned to ligand-to-metal charge transfers
(LMCT). Quantum yield calculations revealed that the metal complexes exhibit more intense fluorescence
compared to the ligand, which is supposed to be due to the enhancement of rigidity of the ligand on chelation,
which reduces the loss of energy through non-radiative channels of the intraligand emission
excited state. Thermogravimetric analyses of the complexes suggest that upon heating the thermally stable
final product in the case of complexes 1–3 is ZnO, 6 gives CdO, whereas for complexes 4 and 5 the final
product remains unidentified
Phosphatase models: Synthesis, structure and catalytic activity of zinc complexes derived from a phenolic Mannich-base ligand
No full textA series of dinuclear [Zn2(L1)2X2] (1–3) and mononuclear [Zn(HL2)X2] complexes (4–6), (X = Cl, Br, I) were synthesised from two Mannich-base compartmental ligands, namely [bis(2-methoxyethyl)aminomethyl]-4-chlorophenol (HL1) and 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-chlorophenol (HL2), respectively. They were characterised by routine physicochemical techniques (CHN, UV, IR, ESI-MS and NMR) and complex 2–5 was further structurally characterised by single crystal X-ray analysis where the Zn. . .Zn bond-distance is 3.10–3.12 Å. All the quintessential complexes exhibit excellent phosphatase activity and the experimental first order rate constant values (kcat) for the hydrolysis of 4-nitrophenyl phosphate ester (PNPP) reaction in methanol are in the range from 1.05 to 214 s1 at 25 C evaluated by monitoring spectrophotometrically the gradual release of p-n nitrophenolate (kmax = 427 nm, e = 18500 M1 cm1). The coordinated X halides affect the phosphatase activity in the order Br > Cl > I (in dinuclear complexes) and Cl > Br > I (in mononuclear) and the trend in the two cases has been well recognised to be due to a different rate determining step. Moreover the influence of chloro atom in para-position of the phenol ring and the role of solvent have been rationalised by comparing the kinetic
parameters with those obtained for the corresponding methyl analogues having reasonably close structural resemblance as reported by Sanyal et al. (2014)
Portraying the role of halo ligands and the auxiliary part of ligands of mononuclear manganese(iii)-Schiff base complexes in catalyzing phospho-ester bond hydrolysis
No full textFour mononucleating Schiff base ligands, namely HL1, HL2, HL3 and HL4, were prepared via condensation
between 2-hydroxybenzaldehyde and 2-morpholinoethanamine, 2-(piperazin-1-yl)ethanamine, 2-(piperidin-
1-yl)ethanamine and 2-(pyrrolidin-1-yl)ethanamine, respectively. Then, seven mononuclear manganese(III)
complexes were synthesized using the above-mentioned ligands. Complexes 1–3 were prepared with
ligand HL1 by using chloride, bromide and iodide salts of manganese(II), respectively. On the other hand,
complexes 4, 5, 6 and 7 were prepared by reaction of manganese chloride followed by sodium
thiocyanate with ligands HL2, HL3, HL4, and HL1, respectively. All the complexes were characterized
by using the usual physicochemical techniques and their solid state structures were obtained from
single crystal X-ray analysis. The phosphatase-like activity of these complexes was studied in a 97.5%
(v/v) N,N-dimethylformamide–water mixture using the disodium salt of 4-nitrophenylphosphate (4-NPP)
as a model substrate to evaluate the role of halo-anions and the auxiliary part of the ligand backbone in
the phosphatase like activity. Detailed experimental findings proved that complex 2 is the most active
catalyst among all seven complexes and the complex bearing a morpholine ring is the most active
catalyst among complexes 4–7
A series of mononuclear nickel(II) complexes of Schiff-base ligands having N,N,O- and N,N,N-donor sites: Syntheses, crystal structures, solid state thermal property and catecholase-like activity
No full textFour new mononuclear nickel(II) complexes, namely [NiL1(H2O)3](NO3)2 (1), [NiL2(H2O)3](NO3)2 (2),
[NiL3(H2O)3](NO3)2 (3) and [NiL4(ClBz)(H2O)] 1.25(H2O) (4) have been synthesized via Schiff-base formation
by condensation between 2-benzoylpyridine and N-(2-aminoethyl)pyrrolidine for L1, salicylaldehyde
and N-(2-aminoethyl)piperazine (L2), 5-chlorosalicylaldehyde and N-(2-aminoethyl)piperazine (L3), and
5-chlorosalicylaldehyde and N-(2-aminoethyl)morpholine (L4). These complexes are comprehensively
characterized via routine physicochemical techniques as well as by single crystal X-ray structural analyses.
Despite all the nickel complexes are mononuclear, the catecholase activity shows prominent variation
depending on the coordination environment around the metal center. Complexes 2 and 3 derived
from same amine bear an extra positive charge on the ligand system facilitating the substrate–catalyst
interaction to promote the oxidation of 3,5-DTBC to 3,5-DTBQ. On the contrary complexes 1 and 4 remain
inert in nature, although 1 shows structural similarities in terms of coordination environment with nickel
substituted catechol oxidase
Solvent dependent ligand transformation in a dinuclear copper(ii) complex of a compartmental Mannich-base ligand: Synthesis, characterization, bio-relevant catalytic promiscuity and magnetic study
No full textAn “end-off” pentadentate compartmental ligand HL has been synthesized by Mannich base condensation using p-cresol and 2-benzyl amino ethanol and structurally characterized. A dinuclear copper(II) complex, namely [Cu2(L)(m-OH)(H2O)(ClO4)2], has been prepared by treating HL with Cu(ClO4)2$6H2O in methanolic solution with the aim of investigating its catalytic promiscuity. Single crystal structural analysis reveals that the Cu–Cu separation is 2.9 °A. Catecholase activity of the complex has been investigated in anhydrous DMSO as well as in a DMSO–water mixture with progressively increasing the quantity of water up to a 1 : 1 volume ratio in order to assess the bio compatibility of the catalyst using 3,5-DTBC as a model substrate. In anhydrous DMSO the catalytic activity reaches its peak and decreases with increasing water concentration, a feature most likely due to insolubility of 3,5-DTBQ, the product formed in the catalysis, in water. The complex also shows excellent phosphatase-like activity by exploiting the Lewis acidity, the necessary requirement for that activity, under different pH. Thorough investigation reveals that no activity is observed at pH 6 but the activity increases with increasing pH and attains a maximum at pH 9. A variable temperature magnetic study shows that the two Cu centers are antiferromagnetically coupled at low temperature with a J value of 78.63 1.30 cm1. In acetonitrile medium the complex shows very exciting behavior. A new transformed ligand is generated that has been assigned as a Schiff-base ligand, 2,6-bis-[(2-hydroxy-ethylimino)-methyl]-4-methylphenol. The genesis of the new ligand is a consequence of dealkylation from HL followed by oxidation. This oxidation is counterbalanced by reduction of Cu(II) to Cu(I) as is evidenced from isolation of [Cu(MeCN)4](ClO4) from the mixture followed by X-ray structural characterization of the species
Analysis of a dual domain phosphoglycosyl transferase reveals a ping-pong mechanism with a covalent enzyme intermediate
No full textPhosphoglycosyl transferases (PGTs) are integralmembrane proteins with diverse architectures that catalyze the formation of polyprenol diphosphate-linked glycans via phosphosugar transfer from a nucleotide diphosphate-sugar to a polyprenol phosphate. There are two PGT superfamilies that differ significantly in overall structure and topology. The polytopic PGT superfamily, represented by MraY and WecA, has been the subject of many studies because of its roles in peptidoglycan and O-antigen biosynthesis. In contrast, less is known about a second, extensive superfamily of PGTs that reveals a core structure with dual domain architecture featuring a C-terminal soluble globular domain and a predicted N-terminal membraneassociated domain. Representative members of this superfamily are the Campylobacter PglCs, which initiate N-linked glycoprotein biosynthesis and are implicated in virulence and pathogenicity. Despite the prevalence of dual domain PGTs, their mechanism of action is unknown. Here, we present the mechanistic analysis of PglC, a prototypic dual domain PGT from Campylobacter concisus. Using a luminescence-based assay, together with substrate labeling and kinetics-based approaches, complementary experiments were carried out that support a ping-pong mechanism involving a covalent phosphosugar intermediate for PglC. Significantly, mass spectrometrybased approaches identified Asp93, which is part of a highly conserved AspGlu dyad found in all dual domain PGTs, as the active-site nucleophile of the enzyme involved in the formation of the covalent adduct. The existence of a covalent phosphosugar intermediate provides strong support for a ping-pong mechanism of PglC, differing fundamentally from the ternary complex mechanisms of representative polytopic PGTs. Keywords: phosphoglycosyl transferase; membrane protein; dual domain PGT; covalent intermediate; glycoconjugate biosynthesisNational Institutes of Health (U.S.) (Grant GM-039334
Syntheses, characterization, and magneto-structural analyses in μ1,3-acetato-bridged tetracopper(II) and μ1,3- and μ1,1,3-acetato-bridged pentanickel(II) clusters
No full textTwo pentanuclear NiII complexes, [Ni5(L1)2(CH3COO)6(OH)2-
(MeOH)2] (1) and [Ni5(L2)2(CH3COO)6(OH)2(H2O)2] (2), and
one tetranuclear CuII complex, [Cu4(L3)2(CH3COO)4(O)] (3),
have been synthesized from phenol-based “end-off” compartmental
ligands HL1 to HL3 {HL1 = 2,6-bis[ethyl(2-
thienyl)iminomethyl]-4-tert-butylphenol; HL2 = 2,6-bis-
[ethyl(2-thienyl)iminomethyl]-4-chlorophenol and HL3 = 2,6-
bis[ethyl(2-thienyl)iminomethyl]-4-methylphenol, respectively}.
The complexes have been structurally characterized
and their magnetic properties have been investigated within
the temperature range 2.2–300 K. Complexes 1 and 2 comprise
two dinuclear [Ni2L2] units linked to a central Ni ion by bridging μ3-hydroxo groups
Anion mediated formation of different Schiff bases from the same precursors and their nickel(II) complexes with different nuclearity
No full textA Deep Insight into the Photoluminescence Properties of Schiff Base Cd(II) and Zn(II) Complexes
No full textA tridentate N,N,O donor ligand 2,4-dichloro-2-[(2-piperazine-4-yl-ethylimino)-methyl]-phenol (HL) was designed, and eight new Zn(II) and Cd(II) complexes, namely, [Zn(LH)(SCN)2] (1), [Zn(LH)(N3)2] (2), [Zn(LH)(NO2)2] (3), [Zn(LH)(dca)(OAc)] (4), [Cd2(LH)2(SCN)4] (5), [Cd(LH)(N3)2] (6), [Cd(LH)(NO2)2] (7), and [Cd(LH)(dca)(OAc)] (8) [where dca = dicyanamide anion] were synthesized. Five of them (1, 2, 4, 5, 7) were structurally characterized through single-crystal X-ray diffraction analysis. H-Bonding interactions are found to be the major stabilizing factor for crystallization in the solid state. Experimental and computational studies were performed in cooperation to provide a rationalization of the photoluminescence properties of those complexes. The quantum yields are anion-dependent, with enhanced efficiencies in the following order: LH < Cd-SCN(5) < Cd-dca(8) < Cd-N3(6) < Cd-NO2(7) < Zn-dca(4) < Zn-N3(2) < ZnNO2(3) < ZnSCN(1). By using quantum chemical calculations we rationalized the above trends. Moreover, the diverse lifetimes observed for those eight complexes were also quantitatively explained by considering the subtle competition between different photo-deactivation pathways
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