Bipinbihari Ghosh

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.

Catecholase activity of Mannich-based dinuclear CuII complexes with theoretical modeling: new insight into the solvent role in the catalytic cycle

Four new dinuclear CuII complexes were synthesised from two Mannich-base ligands namely 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-chlorophenol (HL1) and 2-[bis(2-methoxyethyl)aminomethyl]-4-chlorophenol (HL2): [Cu2(L1)(OH)](ClO4)2·CH3OH (1), [Cu2(L2)2](ClO4)2·H2O (2), [Cu2(L2)2(NO3)2] (3) and [Cu2(L2)2(OAc)2]·H2O (4) and well characterised. X-ray diffraction analysis of the complexes reveals a Cu⋯Cu distance of 2.9183(13), 2.9604(6), 3.0278(4) and 3.0569(11) A, respectively. In 1 the metal coordination geometry is intermediate between trigonal bipyramidal (TBP) and square pyramidal (SP) (τ = 0.488), in 2 the geometry is TBP (0.828 and 0.639) and in 3 and 4 is SP (τ = 0.188 and 0.083, respectively). Spectrophotometric investigations to evaluate the catecholase activity of complexes against 3,5-di-tert-butylcatechol (3,5-DTBC) and tetrachlorocatechol (TCC) in three different solvents (acetonitrile, methanol and DMSO) under completely aerobic conditions reveal that complexes 1–4 are able to oxidise 3,5-DTBC in all the solvents, while TCC can be oxidised only in acetonitrile (kcat = 0.0002–0.02 s−1). Intensive DFT calculations prove an ionic pathway for 1–3 while a unique neutral catalytic cycle for 4.

Catecholase activity, DNA binding and cytotoxicity studies of a Cu(II) complex of a pyridoxal schiff base: synthesis, X-ray crystal structure, spectroscopic, electrochemical and theoretical studies

A binuclear Cu(II) complex of formula [Cu(L1Hpy)Cl]2(ClO4)2 (1), where L1H2 is a new tridentate ligand, formed by condensation of 2-aminomethyl pyridine and pyridoxal (one of the forms of vitamin B6), has been synthesized. X-ray crystal structure determination shows that in this complex two Cu(II) ions are interconnected by complementary hydroxymethyl bridges of the two pyridoxal moieties, which is a very rare example in the literature. However, with a Cu⋯Cu separation of 6.574(1) A and Cu–O(H)CH2– distance of 2.289 A, the bridge is very weak, and DFT calculations, as well as ESI-MS data and solution spectral studies indicate that in a MeOH solution the complex exists predominantly as a mixture of monomers [Cu(L1Hpy)Cl]+ and [Cu(L1Hpy)(MeOH)]2+ with the former being the predominant form. The DFT calculations as well as EPR spectra suggest that the SOMO is a metal dx2−y2 orbital. The complex shows highly efficient catecholase activity with kcat = 3·46 × 105 h−1 and kcat/KM = 1.00 × 108 M−1 h−1, which are the best values reported in the literature, so far, for catecholase mimicking model complexes. DFT calculations show that the reduction of the Cu(II)/Cu(I) by the coordinated catechol and the resultant structural changes is the rate determining step in the catalytic cycle. The complex also binds DNA quite strongly with a binding constant of ∼105 M−1. DFT calculations suggest that the most probable binding mode of the complex is intercalation of the pyridine ring of the complex between two adenine or adenine and cytosine base pairs. The complex shows low cytotoxicity towards HCT and HeLa cells, though cytotoxicity towards the latter cell line is much more than the former. It was also found that the complex can be used as a fluorescence probe for imaging HCT cells.