Shyamal Kumar Chattopadhyay

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.

Ruthenium(II) complexes containing the pentadentate SNNNS chelating ligand 2,6–diacetylpyridine bis(4–(p-tolyl)thiosemicarbazone). Synthesis, reactivity and electrochemistry

Ruthenium(II) heptacoordinate complexes containing the pentadentate SNNNS chelating ligand 2,6–diacetylpyridine bis(4–(p-tolyl)thiosemicarbazone) (L1H2) have been prepared. The compounds were of the type Ru(L1H2)X2 [X=Cl (1);Br (2); SCN (3)],[Ru(L1H2)- (Y)Cl]Cl [Y=imidazole (4); pyridine-N-oxide (5)] and [Ru(L1H2)(PPh3)X]Y, [X=Cl (6), (7);Br (8); Y=ClO4/ PF6]. The complexes were characterised by i.r., u.v.–vis. and n.m.r. spectroscopy and their electrochemical behaviour was examined by cyclic voltammetry. They exhibit a reversible to quasi-reversible RuII/RuIII couple in MeCN solution at a glassy carbon working electrode using an Ag/AgCl electrode as the reference.

Studies of nickel(II) complexes of 3-hydroxyiminobutanone thiosemicarbazone and 3-hydroxyiminobutanone (4-phenyl thiosemicarbazone). Crystal structure of bis(3-hydroxyiminobutanone (4-phenyl thiosemicarbazone) nickel(II) nitrate, monohydrate, [Ni(C11H14N4OS)2](NO3)2·H2O

Abstract Nickel(II) complexes of two tridentate N.N.S-chelating ligands, 3-hydroxyiminobutanonethiosemicarbazone (L1H2) and 3-hydroxyiminobutanone (4-phenylthiosemicarbazone) (LH2), have been synthesized and characterized by elemental analysis, IR, UV-vis spectroscopy and cyclic voltammetric studies. The structure of Ni(LH2)2(NO3)2·H2O has been determined by single-crystal X-ray diffraction. Nickel(II) is in a distorted octahedral N4S2 donor environment; one of the nitrate groups exhibits partial disorder.

Synthesis, structural and magnetic studies of imidazolium bis(oxalato)cuprate(II)

Abstract Imidazoliumm bis(oxalato)cuprate(II) has been synthesized and its structure determined by X-ray crystallography. Variable temperature magnetic susceptibility measurements, as well as EPR and UV-vis spectroscopic studies, have been carried out. The results show that in the solid state the compound exists in a chain-like structure, with an asymmetric one-atom weak oxalate bridge joining adjacent CuII centres. Of the two oxygen atoms of each coordinated oxalate only one participates in bridging; thus, each oxalate ultimately achieves three-point coordination. Each of the imidazolium ions participates in two hydrogen through the N—H moieties. The compound exhibits a weak antiferromagnetic interaction (J = −0.40 cm−1).EPR spectra reveal that the triplet state is appreciably populated at both room and liquid nitrogen temperatures.

Studies of Copper(II) Thiosemicarbazide Complexes and their Reactivities. X-Ray Structure of Two Unusual Reaction Products, [Cu(bpy)(H2O)2SO4] and [(bpy)2Cu2(C2O4)Cl2]·H2O

A number of ternary complexes of Cu2+ with thiosemicarbazide (TSC) including [Cu(bpy)(TSC)2]Cl2, [Cu(o-phen)(TSC)2]Cl2 and [Cu(bpy)(TSC)2][CuCl4] [Cu(o-phen)(TSC)2] [CuCl4] have been synthesised and characterized. Their reactivities in aqueous and DMF solutions were studied. It was observed that while on coordination to Cu2+ thiosemicarbazide is converted to thiocyanate in aqueous solution, in DMF solution the thiocyanate is further aerially oxidised to sulfate. In the presence of hydrogen peroxide, oxidation to sulfate is rapid and from the mother liquor we could isolate [(bpy)2Cu2(C2O4)Cl2]·H2O whose structure was determined by X-ray crystallography. X-ray crystal structure of [Cu(bpy)(H2O)2SO4] is also discussed.*

Synthesis, characterization, X-ray crystal structure, DFT calculations, and catalytic properties of a dioxidovanadium(V) complex derived from oxamohydrazide and pyridoxal: a model complex of vanadate-dependent bromoperoxidase.

A vanadium(V) complex with the formula [Et3NH][V(V)O2(sox-pydx)] with a new tridentate ligand 2-[2-[[3-hydroxy-5-(hydroxymethyl)-2-methylpyridin-4-yl]methylene]hydrazinyl]-2-oxoacetamide (soxH-pydxH), obtained by condensation of oxamohydrazide and pyridoxal (one of the forms of vitamin B6), has been synthesized. The compound was characterized by various analytical and spectroscopic methods, and its structure was determined by single-crystal X-ray diffraction technique. Density functional theory (DFT) and time-dependent DFT calculations were used to understand the electronic structure of the complex and nature of the electronic transitions observed in UV-vis spectra. In the complex, vanadium(V) is found to be pentacoordinated with two oxido ligands and a bianionic tridentate ONO-donor ligand. The vanadium center has square-pyramidal geometry with an axial oxido ligand, and the equatorial positions are occupied by another oxido ligand and a phenolato oxygen, an imine nitrogen, and a deprotonated amide oxygen of the hydrazone ligand. A DFT-optimized structure of the complex shows very similar metrical parameters as determined by X-ray crystallography. The O4N coordination environment of vanadium and the hydrogen-bonding abilities of the pendant amide moiety have a strong resemblance with the vanadium center in bromoperoxidase enzyme. Bromination experiments using H2O2 as the oxidizing agent, with model substrate phenol red, and the vanadium complex as a catalyst show a remarkably high value of kcat equal to 26,340 h(-1). The vanadium compound also efficiently catalyzes bromination of phenol and salicylaldehyde as well as oxidation of benzene to phenol by H2O2.

Study of Cu2+ mediated oxidation of thiosemicarbazide, thiocarbohydrazide and thiourea

Abstract Thiosemicarbazide, thiocarbohydrazide and thiourea undergo Cu2+ mediated oxidation in DMF in the presence of 2,2′-bipyridine (bpy) to produce cis-[Cu(bpy)(H2O)2SO4].

Synthesis and characterization of bis(2,2'- bipyridine) ruthenium complexes containing thiosemicarbazide ligands: unique redox series

Abstract Two series of heterochelates of ruthenium(II) containing two bipyridyl molecules and a bidentate chelating sulfurnitrogen donor ligand in the form of 4-aryl substituted thiosemicarbazides have been synthesized and characterized. The first series of complexes are dicationic in which the ring substituted 4-aryl thiosemicarbazides (NS) are chelated in the keto form through the hydrazinic nitrogen and the thione sulfur atom. They are of the [Ru(bpy)2NS]+2 type. The second series have the general formula [Ru(bpy)2NS]+1 in which the thiosemicarbazide moiety remains chelated to the RuII centre through the hydrazinic nitrogen and the deprotonated thiolato S-atom. All the complexes have been characterized by elemental analysis, UV-vis, IR and EPR spectroscopy. The complexes were found to constitute a three membered redox series which were investigated by cyclic voltammetry.

Synthesis, characterisation and crystal structure analysis of bis (pyridine-2-carbaldehyde thiosemicarbazonato) cobalt(III) thio-cyanate monohydrate

The crystal structure of [CoL2](SCN)·H2O (L = pyridine-2-carbaldehyde thiosemicarbazonato) is reported. The structure, along with other physico-chemical studies, establishes that the compound is a low spin CoIII complex. This resolves confusion arising out of an e

SYNTHESIS, CHARACTERIZATION AND REACTIVITY OF CIS-RU(L)(PPH3)CL2; L = 2-PYRIDYL-N-(2'-METHYLTHIOPHENYLMETHYLENEIMINE) : CRYSTAL STRUCTURE OF CIS-RU(L) (PPH3)CL2 AND RU(L)(PPH3)(BPY)(CLO4)2

A number of ruthenium(II) complexes containing an NNS chelating thioether ligand (L) have been synthesized and characterized by elemental analysis, UV-visible, IR and 1 H NMR spectroscopy and cyclic voltammetry. Ru(L)(PPh 3 )Cl 2 ( 1 ) has been characterized by X-ray diffraction. Structural features of ( 1 ) indicate a distorted octahedral coordination environment around the ruthenium(II) centre. The two chlorides are cis to each other with the RuCl(1) bond trans to PPh 3 longer than the RuCl(2) bond. Several mixed-ligand complexes containing bipyridine, phenanthroline, 2-picolinate and oxalate were also isolated and the structure of one of the heterochelates has been solved. [Ru(L)(PPh 3 )(bpy)](ClO 4 ) 2 ( 2 ) has also been structurally characterized. The structure and properties of ( 2 ) correspond to the predictions based on the structure of ( 1 ).