Jaydeep Adhikary

A Combined Experimental and Theoretical Investigation on the Role of Halide Ligands on the Catecholase-like Activity of Mononuclear Nickel(II) Complexes with a Phenol-Based Tridentate Ligand

Three new mononuclear nickel(II) complexes, namely, [NiL(1)(H2O)3]I2·H2O (1), [NiL(1)(H2O)3]Br2·H2O (2), and [NiL(1)(H2O)3]Cl2·2H2O (3) [HL(1) = 2-[(2-piperazin-1-ylethylimino)methyl]phenol], have been synthesized and structurally characterized. Structural characterization reveals that they possess similar structure: [NiL(1)(H2O)3](2+) complex cations, two halide counteranions, and lattice water molecules. One of the nitrogen atoms of the piperazine moiety is protonated to provide electrical neutrality to the system, a consequence observed in earlier studies (Inorg. Chem. 2010, 49, 3121; Polyhedron 2013, 52, 669). Catecholase-like activity has been investigated in methanol by a UV-vis spectrophotometric study using 3,5-di-tert-butylcatechol (3,5-DTBC) as the model substrate. Complexes 1 and 2 are highly active, but surprisingly 3 is totally inactive. The coordination chemistries of 1 and 2 remain unchanged in solution, whereas 3 behaves as a 1:1 electrolyte, as is evident from the conductivity study. Because of coordination of the chloride ligand to the metal in solution, it is proposed that 3,5-DTBC is not able to effectively approach an electrically neutral metal, and consequently complex 3 in solution does not show catecholase-like activity. Density functional theory (DFT) calculations corroborate well with the experimental observations and thus, in turn, support the proposed hypothesis of inactivity of 3. The cyclic voltametric study as well as DFT calculations suggests the possibility of a ligand-centered reduction at -1.1 V vs Ag/AgCl electrode. An electron paramagnetic resonance (EPR) experiment unambiguously hints at the generation of a radical from EPR-inactive 1 and 2 in the presence of 3,5-DTBC. Generation of H2O2 during catalysis has also been confirmed. DFT calculations support the ligand-centered radical generation, and thus a radical mechanism has been proposed for the catecholase-like activity exhibited by 1 and 2. Upon heating, 2 and 3 lose water molecules in two steps (first lattice waters, followed by coordinating water molecules), whereas 3 loses four water molecules in a single step, as revealed from thermogravimetric analysis. The totally dehydrated species are red, in all cases having square-planar geometry, and have amorphous nature, as is evident from a variable-temperature powder X-ray diffraction study.

Preparation and characterization of ferromagnetic nickel oxide nanoparticles from three different precursors: application in drug delivery

Three varieties of nickel oxide nanoparticles [NiO(I), NiO(Br) and NiO(Cl)] have been prepared from three simple mononuclear nickel(II) Schiff-base complexes using a pyrolytic technique. The synthesized nanoparticles are characterized by FT-IR, UV-Vis, XRPD, DLS, SEM, TEM and EDX methods. All the techniques suggest the production of highly pure nickel oxides. The magnetic measurements reveal a small hysteresis loop at room temperature, confirming the super-paramagnetic (weak ferromagnetic) nature of the synthesized NiO nanoparticles. We have applied these nanoparticles for drug delivery. For this purpose, erythromycin, the well known broad spectrum antibiotic is conjugated with the NiO nanoparticles to develop NiO(I)-Ery, NiO(Br)-Ery and NiO(Cl)-Ery. These conjugated nanoparticles successfully deliver erythromycin towards both Gram positive and Gram negative bacteria and show effective antimicrobial activity against erythromycin resistant Staphylococcus aureus and Escherichia coli as model microbial species, evidenced from the Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) values. The order of efficiency toward drug delivery is NiO(I)-Ery > NiO(Br)-Ery > NiO(Cl)-Ery. Thus these conjugates can be applied to overcome the drug resistant properties of bacteria which will be a beneficial strategy in anti-bacterial therapy.

Development of an efficient magnetically separable nanocatalyst: theoretical approach on the role of the ligand backbone on epoxidation capability

Three chiral Schiff base ligands H2L1, H2L2, H2L3 have been synthesized by treating (R)-1,2-diaminopropane separately with 3,5-dichlorosalicylaldehyde, 3,5-dibromosalicylaldehyde and 3,5-diiodosalicylaldehyde, respectively. Three new asymmetric FeIII complexes, namely, FeL1Cl (1), FeL2Cl (2), FeL3Cl (3) have been prepared from their corresponding ligands. The crystal structure of 2 reveals that the complexes are mononuclear in nature. Circular dichroism (CD) studies suggest that the ligands and their corresponding complexes contain an asymmetric center. The catalytic activity of these complexes toward the epoxidation of alkenes has been investigated in the presence of iodosylbenzene (PhIO), in two solvents CH3CN and CH2Cl2. The epoxide yield suggests that the order of their catalytic efficiency is 3 > 2 > 1. This trend as well as the role of substitution on the ligand backbone on alkene epoxidation has also been confirmed by density functional theory (DFT) calculations. For further adaptation, we attached our most efficient homogeneous catalyst, 3, with surface modified magnetic nanoparticles (Fe3O4@dopa) and thereby obtained the new magnetically separable nanocatalyst Fe3O4@dopa@FeL3Cl. This catalyst has been characterized and its olefin epoxidation ability investigated in similar conditions to those used for homogeneous catalysts. The enantiomeric excess of the epoxide yield reveals the retention of chirality of the active site of Fe3O4@dopa@FeL3Cl. The catalyst can be easily recovered by magnetic separation and recycled several times without significant loss of its catalytic activity.

Thiocyanate mediated structural diversity in phenol based “end-off” compartmental ligand complexes of group 12 metal ions: Studies on their photophysical properties and phosphatase like activity

The reaction of a pentadentate compartmental ligand LH, namely 4-tert-Butyl-2,6-bis-[(2-pyridin-2-yl-ethylimino)-methyl]-phenol, with group 12 metal ions (ZnII, CdII, HgII) followed by addition of NaSCN afforded one discrete dinuclear complex [Zn2(L)(SCN)3](1), and two polymeric 1D species [Cd2.5(L)(SCN)3(AcO)]n (2) and [Hg2(L)(SCN)3]n (3). All the complexes have been structurally characterized by single crystal X-ray diffraction. The crystal structure of the complexes reveals different coordination modes of thiocyanate anion that affect the different topology detected in the compounds: the anions are μ1-NCS and μ1,1-NCS connected in complex 1, while μ1,3-NCS bridging mode is observed in 2, and μ1-SCN and μ1,3-NCS in 3. The polymeric Hg complex of the bicompartmental ligand system here reported is unprecedented. Detail study of their photophysical properties including the phosphorescence spectra at 77K has been done. Phosphatase like activity of all the three complexes has been performed in DMSO-H2O medium and their activity follows the order of 1>2>>3.

Surface modification minimizes the toxicity of silver nanoparticles: an in vitro and in vivo study

Currently toxicological research in Silver nanoparticle is a leading issue in medical science. The surface chemistry and physical dimensions of silver nanoparticles (Ag-NPs) play an important role in toxicity. The aim of this present study was to evaluate the in vitro and in vivo toxicity of Ag-NPs as well as the alteration of toxicity profile due to surface functionalization (PEG and BSA) and the intracellular signaling pathways involved in nanoparticles mediated oxidative stress and apoptosis in vitro and in vivo system. Ag-NPs released excess Ag+ ions leads to activation of NADPH oxidase and helps in generating the reactive oxygen species (ROS). Silver nanoparticles elicit the production of excess amount of ROS results activation of TNF-α. Ag-NPs activates caspase-3 and 9 which are the signature of mitochondrial pathway. Ag-NPs are responsible to decrease the antioxidant enzymes and imbalance the oxidative status into the cells but functionalization with BSA and PEG helps to protect the adverse effect of Ag-NPs on the cells. This study suggested that Ag-NPs are toxic to normal cells which directly lead with human health. Surface functionalization may open the gateway for further use of Ag-NPs in different area such as antimicrobial and anticancer therapy, industrial use or in biomedical sciences.

Macrocyclization of N,N′-propylenebis(3-formyl-5-tert-butylsalicylaldimine): a ratiometric fluorescence chemodosimeter for ZnII

Addition of 1,3-propane diamine to 2,6-diformyl-4-tert-butyl phenol in ethanol produces a site-selective imination product N,N′-propylenebis(3-formyl-5-tert-butylsalicylaldimine), an acyclic side-off compartmental ligand (H2L). In the presence of zinc nitrate the ligand goes on hydrolysis in 50 : 50 water–acetonitrile medium and forms a partially hydrolyzed ligand (H2L′) which slowly metallates to generate a macrocyclic dinuclear zinc(II) complex (1), as characterized by single crystal X-ray analyses. The formation of H2L′ is believed to occur through the cleavage of an imine bond of the acyclic compartmental ligand (H2L) in the presence of zinc nitrate which acts as a Lewis acid. The formation of H2L′ has been monitored by means of 1H NMR and further confirmed by HRMS spectroscopic studies. The interactions of H2L with nickel(II) and copper(II) nitrate produce dinuclear complexes 2 and 3 (reported in Inorg. Chem. Commun. 2012, 15, 266–268) respectively, which are formed with unchanged ligand. Various spectroscopic techniques have been used to further characterize the complexes. H2L hardly exhibits yellowish green fluorescence emission at 523 nm when excited at 437 nm in 1 : 1 water–acetonitrile. Upon addition of Zn2+, a new fluorescence emission band at 481 nm appears, the intensity of which slowly enhances. Thus, the ligand H2L is a ratiometric fluorescence chemodosimeter for the selective detection of Zn2+ ions. On addition of CaII, MgII, NaI and KI in the same concentrations as that of Zn2+, the emission band at 523 nm is slightly enhanced, whereas the addition of paramagnetic metal cations like CuII, FeII, NiII, CoII, and MnII resulted in quenching of fluorescence. The quenching effect is also observed in the presence of CdII, a d10 metal cation exhibiting similar coordination properties to ZnII. The ZnII ion selectivity has also been studied in the presence of other biologically relevant metal ions in 50 : 50 water–acetonitrile.