Prateeti Chakraborty

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.

Exploration of CH⋯π interactions involving the π-system of pseudohalide coligands in metal complexes of a Schiff-base ligand

A series of four mononuclear Schiff-base complexes, namely, [Zn(L)(NCS)2] (1), [Zn(L)(N3)2] (2), [Cu(L)(NCS)2] (3) and [Ni (L)(2bpy)(NCS)](ClO4) (4), [where L = N,N-dimethyl-N′-(phenyl-pyridin-2-yl-methylene)-ethane-1,2-diamine and 2bpy = 2-benzoylpyridine] were synthesized with the aim of investigating the role of different non-covalent weak interactions responsible for the crystal packing of the complexes. All of them were structurally characterised by X-ray diffraction analysis. In addition to conventional CH3⋯π and π⋯π interactions, the importance of unconventional C–H⋯π interactions in the crystal packing of compounds 1–4 was investigated by means of Hirshfeld surface analysis and DFT calculations. In these unconventional C–H⋯π interactions, the π-system (electron donor) is provided by the pseudohalide coligands. The interactions formed by the π-system depend on the nature of the pseudohalide (N3, NCO, NCS or NCSe) as demonstrated by molecular electrostatic potential calculations. Additionally, we have explored the photophysical properties of these complexes. Finally, we have combined a search in the Cambridge Structural Database and DFT energy calculations to analyse the rare ambidentate behaviour of SCN within the same complex.

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

An “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)(μ-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 cm−1. 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.

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.

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.

Role of solvent in the phosphatase activity of a dinuclear nickel(II) complex of a Schiff base ligand: mechanistic interpretation by DFT studies

A novel dinuclear μ-diphenoxo-μ-acetato metallohydrolase [Ni2L2(NCS)(Ac)(H2O)0.5(MeOH)0.5]·1.25H2O (HL = 2-((E)-(2-(pyridin-2-yl)ethylimino)methyl)-4-chlorophenol) with an intermetallic separation of 3.099 A has been synthesized and characterized both structurally and spectroscopically. Temperature dependent (2–300 K) magnetic susceptibility measurements show that the compound exhibits a global intramolecular ferromagnetic interaction through the diphenoxido and syn–syn acetate bridges (J = 7.21 cm−1). It is also noticed that a weak antiferromagnetic intermolecular interaction or ZFS effect of the Ni(II) ions is working for lowering of χMT at extreme low temperature. The compound’s phosphatase activity, investigated spectrophotometrically on 4-nitrophenylphosphate (4-NPP), demonstrates excellent catalytic efficiency. Additionally, using a DMF medium facilitated a better catalytic pathway (8.18 s−1) than when acetonitrile (MeCN) was used. A plausible 4-NPP hydrolytic reaction mechanism was proposed by utilizing DFT calculations and the results suggest a SN2-like addition–substitution pathway with two possible catalyst–substrate binding modes (RC1-1 and RC1-2). RC1-2 is regarded as the direct reactant since the water molecule is adjacent to the electron-deficient phosphorus center of the substrate. More importantly, the poor catalytic behavior in the MeCN medium could be ascribed to MeCN molecules having better coordination properties than DMF molecules, since the formation energies of [Ni–MeCN] and [Ni–DMF] are −2.5 and −1.2 kcal mol−1, respectively, according to PCM calculations.

A Deep Insight into the Photoluminescence Properties of Schiff Base CdII and ZnII Complexes

A tridentate N,N,O donor ligand 2,4-dichloro-2-[(2-piperazine-4-yl-ethylimino)-methyl]-phenol (HL) was designed, and eight new ZnII and CdII 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.

Role of para-substitution in controlling phosphatase activity of dinuclear NiII complexes of Mannich-base ligands: experimental and DFT studies

With an aim to study the electronic effect of the group lying in the para-position of phenol-based compartmental Mannich-base ligands, five dinuclear nickel(II) complexes [Ni2L1−5(μ-NO3)(NO3)2] have been synthesized [R = Me (1), CHMe2 (2), CMe3 (3), Cl (4), and OMe (5)] having octahedral structures (fac-manner) in each case as confirmed by single-crystal X-ray diffraction (1–4). The Ni⋯Ni distance (3.42–3.49 A) and Ni–O–Ni bridging bond angle (118.62–121.45°) of 1–4 is proportional to the electronic partial charge (I-effect) of the para-substituents as 3 2 > 5 > 1 > 4 in terms of kcat values (6–81 s−1), as per the Michaelis–Menten profile. DFT calculations establish that the electron-donating group decreases the reaction energy barrier via reducing the energy gap between the orbital of electron-sufficient para-substituted phenolate group and the electron-demanding leaving group. The exceptionally high activity of complex 3 also establishes the rationality of our catalyst design in our previous work (Inorg. Chem. 2015, 54, 2315–2324).

Zinc(II) complexes with uncommon aminal and hemiaminal ether derivatives: synthesis, structure, phosphatase activity and theoretical rationalization of ligand and complex formation

Condensation of N-(2-hydroxyethyl)ethylenediamine and pyridine-2-carbaldehyde in the presence of ZnX2 salts (X = ClO4−, Cl−, Br−, I−) generates four complexes, namely, [ZnL1Cl](ClO4) (1), [ZnL1Cl][ZnCl3(H2O)]·2H2O (2), [ZnL1Br][ZnBr3(H2O)]·2H2O (3), and [Zn(L2)3]I2 (4), where L1 and L2 are aminal and hemiaminal ether derivatives, i.e., (E)-N-((pyridine-2-yl)methylene)-2-((2-pyridine-2-yl)oxazolidin-3-yl)ethanamine and 2-(2-pyridin-2-yl-imidazolidin-1-yl)-ethanol, respectively. No complex with the expected Schiff-base ligand (E)-2-(2-(pyridin-2-ylmethyleneamino)ethylamino)ethanol (L) or with 2-(2-pyridyl-3-(2-hydroxyethyl))oxazolidine (L3) was obtained. The structures of complexes 1, 3, and 4 have been elucidated by single crystal X-ray diffraction. All the complexes have been characterized by detailed NMR (1H, 13C and DEPT-135) and ESI-MS analyses, indicating retention of solid state structures in the solution phase. Complex 1 deserves special mention as during its formation, the ClO4− ion undergoes reduction to Cl− and thus, it forms a mixed anionic ligand complex. Thorough DFT calculations have been performed at the BP86-D3/def2-TZVP level of theory to rationalize the ligand and complex formation. The calculations suggest that the aminal form (L1) is the most favored species followed by the Schiff-base, whereas the hemiaminal ether form (L3) is the least preferred one. The role of halides during the formation of the monoanionic [ZnX3(H2O)]− species is crucial, and the achieved complex is highly favored when X = Cl and disfavored for X = I; this trend has been rationalized by the DFT calculations. The phosphatase activities of the complexes have been investigated, and their efficiencies follow the order 2 > 1 > 3 > 4.

Bioactive Heterometallic CuII–ZnII Complexes with Potential Biomedical Applications

A series of multinuclear heterometallic Cu–Zn complexes of molecular formula [(CuL)2Zn(dca)2] (1), [(CuL)2Zn(NO3)2] (2), [(CuL)2Zn2(Cl)4] (3), and [(CuL)2Zn2(NO2)4] (4) have been synthesized by reacting [CuL] as a “metalloligand (ML)” (where HL = N,N′-bis(5-chloro-2-hydroxybenzylidene)-2,2-dimethylpropane-1,3-diamine) and by varying the anions or coligands using the same molar ratios of the reactants. All of the four products including the ML have been characterized by infrared and UV–vis spectroscopies and elemental and single-crystal X-ray diffraction analyses. By varying the anions, different structures and topologies are obtained which we have tried to rationalize by means of thorough density functional theory calculations. All of the complexes (1–4) have now been applied for several biological investigations to verify their therapeutic worth. First, their cytotoxicity properties were assessed against HeLa human cervical carcinoma along with the determination of IC50 values. The study was extended with extensive DNA and protein binding experiments followed by detailed fluorescence quenching study with suitable reagents to comprehend the mechanistic pathway. From all of these biological studies, it has been found that all of these heterometallic complexes show more than a few fold improvement of their therapeutic values as compared to the similar homometallic ones probably because of the simultaneous synergic effect of copper and zinc. Among all of the four heterometallic complexes, complex 3 exhibits highest binding constants and IC50 values suggest for their better interaction toward the biological targets and hence have better clinical importance.