1,721,108 research outputs found
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)
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
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
Azido bridge mediated catecholase activity, electrochemistry and magnetic behavior of a dinuclear copper(II) complex of a phenol based "end-off" compartmental ligand
No full textA dinuclear Cu(II) species [Cu2L2(H2O)2(N3)](NO3)2 (L = 2,6-bis(N-ethylpyrrolidine-iminomethyl)-4-methyl-phenolato) where two Cu centers are bridged by phenoxido and l1,1-azido bridges with Cu–Cu separation of 3 Å have been synthesized with the view to explore the role of azido bridge on catecholase activity and electrochemical property and the roles of both the bridging groups on magnetic coupling of two copper centers. The complex exhibits excellent catecholase activity in acetonitrile as well as in DMSO medium not only by oxidizing 3,5-di-tert-butylcatechol (3,5-DTBC) but also tetrachlorocatechol (TCC), a catechol which is very thorny to oxidize, under aerobic conditions and becomes the first example of its own kind. CV study reveals three quasi-reversible reductive couples which are tentatively assigned as Cu2 II to CuIICuI and CuICuI reduction followed by reduction of CuICuI complex to Cu0Cu0 species. Variable temperature magnetic study suggests the presence of an antiferromagnetic spin–exchange interaction
between Cu(II) ions in the dimer via double bridge where the antiferromagnetic contribution of phenoxido bridge predominates over the ferromagnetic interaction of azido bridge
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
Dinuclear cobalt(II) complexes of Schiff-base compartmental ligands: Syntheses, crystal structure and bio-relevant catalytic activities
No full textThree dicobalt(II) complexes, namely [Co2(L1H)(H2O)2(OAc)2](OAc)2 (1), [Co2(L2)(H2O)2(OAc)2](OAc) (2)
and [Co2(L3)(H2O)2(OAc)2](OAc) (3) of the p-cresol based ‘‘end-off’’ compartmental ligands 2,6-bis(R-iminomethyl)-
4-methyl-phenolato, where R = N-ethylpiperazine for L1, 2-ethylpyridine for L2 and N-ethylpiperidine
for L3, have been synthesized and characterized by common physicochemical techniques, and in
the case of complex 1 also by single crystal X-ray diffraction analysis. All the complexes show excellent
catecholase-like activity, monitored not only with 3,5-di-tert-butylcatechol but also with tetrachlorocatechol,
a substrate reluctant to be oxidized. To the best of our knowledge, to date no cobalt complex has
been found in the literature to manifest such activity. The complexes are observed to interact efficiently
with CT-DNA and on incubation (employing plasmid pTZ57/R/T DNA) they exhibit concentration dependent
DNA cleavage activity. The mechanisms related to the DNA cleavage and catecholase-like activities
have been investigated. The cytotoxicity of the complexes has also been examined through an MTT assay
Influence of the coordination environment of zinc(II) complexes of designed mannich ligands on phosphatase activity: A combined experimental and theoretical study
No full textA mononucleating (HL1) and a dinucleating (HL2) “endoff”
compartmental ligand have been designed and synthesized by
controlled Mannich reaction using p-cresol and bis(2-methoxyethyl)amine,
and their formation has been rationalized. Six complexes have been
prepared on treating HL1 and HL2 with ZnIIX2 (X = Cl−, Br−, I−) with the
aim to investigate their hydrolytic activity on phosphoester bond cleavage.
Interestingly, the mononucleating ligand was observed to yield dinuclear
complexes, [Zn2(L1)2X2] (1−3), while the potential dinucleating ligand
generated mononuclear complexes, [Zn(HL2)X2] (4−6). Four (1−4), out
of six complexes studied, were characterized by single-crystal X-ray
diffraction (XRD): the Zn ion exhibits trigonal bipyramidal and tetrahedral
coordination spheres in the di- and mononuclear complex, respectively.
The hydrolytic kinetics, followed spectrophotometrically with 4-nitrophenylphosphate
(4-NPP) in buffered dimethylformamide (DMF) (97.5% DMF, v/v) because of solubility reasons, under excess
substrate conditions (substrate:complex = 20:1), indicated that the complexes enormously accelerate the rate of
phosphomonoester hydrolysis with first order rate constants (kcat) in the range 2−10 s−1 at 25 °C. In each case kinetic data
analyses have been run by Michaelis−Menten treatment. The efficacy in the order of conversion of substrate to product (pnitrophenolate
ion) follows the trend 1 > 2 > 3 > 4 > 5 > 6, and the ratio of kcat of an analogous dinuclear to mononuclear
complex is ≃2. An electrospray ionization-mass spectrometry (ESI-MS) study has revealed the dissociation of the
centrosymmetric dinuclear complex to two mononuclear species instead of a syn-cooperative catalysis. Density functional
theory (DFT) calculations have been performed to rationalize our proposed mechanistic pathway for phosphatase activity. The
comparative analysis concludes the following facts under experimental conditions: (1) the halide bound to the active site affects
the overall rate in the order: Cl− > Br− > I− regardless of nuclearity; (2) dinuclear complexes prevail over the mononuclear ones
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
Dinuclear copper(II) complexes: Solvent dependent catecholase activity
No full textFour new dicopper(II) complexes of phenol based compartmental ligands, namely [Cu2(L1H)2(H2O)2(NO3)2]
(NO3)2 (1), [Cu2(L2)(OH)(H2O)(NO3)](NO3) (2), [Cu2(L3)2(H2O)(NO3)](NO3) (3) and [Cu2(L4)(H2O)2(NO3)]
(NO3)2 (4) [where L1 = 2-formyl-4-methyl-6-(4-(aminomethyl)-piperidine)iminomethyl-phenolato, L2 =
2,6-bis(2-amino-2-methyl-1-propanol)iminomethyl-4-methyl-phenolato, L3 = 2-formyl-4-methyl- 6-(benzylamine)
iminomethyl-phenolato and L4 = 2,6-bis(2-aminoethylpyridine)iminomethyl-4-methyl-phenolato]
have been synthesized and structurally characterized. The single crystal X-ray analyses reveal that
all four complexes are dinuclear in nature; complexes 2 and 4 comprise of one respective ligand, whereas
1 and 3 are contain two respective ligands, and the Cu–Cu separation in each case is ca. 3.0 Å. All four
complexes are soluble in dichloromethane (DCM), methanol, acetonitrile (ACN), dimethylsulfoxide
(DMSO), water–methanol (50:50, v/v), and this property has been exploited to access the solvent effect
on the catecholase activity of the complexes towards the aerobic oxidation of 3,5-DTBC to 3,5-DTBQ. A
UV–Vis spectral study in the different solvents, followed by a kinetic investigation, suggests that the
change in spectral behavior follows a similar trend, being dependent on the coordinating ability of the
solvent, irrespective of the complex used. The commonly known physical parameters of the solvents, like
the dielectric constant, dipole moment, polarity, etc., do not seem to be a key factor in controlling the catecholase
activity. However, protic solvents are observed to be a better choice than aprotic solvents for the
oxidation of 3,5-DTBC
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