Jnan Prakash Naskar

Occurrence of ribbons of cyclic water pentamers in a metallo-organic framework formed by spontaneous fixation of CO2

In the reaction of equimolar amounts of copper(II) acetate with 2,2′-dipyridylamine (DPA) in aqueous tetrahydrofuran, in presence of KOH, aerial CO2 is spontaneously fixed to the carbonate anion yielding [Cu(DPA)(CO3)]·3H2O (1). X-ray crystallography shows the presence of zigzag ribbons of cyclic water pentamers in the channels of a chain-like metallo-organic framework. The water ribbons are stabilised by hydrogen bonds to the metallo-organic backbone. Each (H2O)5 pentamer is approximately planar.

New bond-valence sum model

A new expression is devised empirically to accommodate zero and some negative oxidation states in the bond-valence sum approach. The method is worked out in detail for a number of homoleptic copper and nickel complexes of various coordinating atoms in several oxidation states of the metals. An implication of the expression is a linear variation between 1/req and 1/rax in octahedral MX6 moieties, where req and rax are, respectively, the average equatorial and axial bond lengths. This is verified in Cu2+X6 chromophores for X = F, O, N and S. The usefulness of the new expression in assessing the compatibility of a coordination sphere with an oxidation state of a metal ion is demonstrated by exemplary applications to some inorganic complexes, azurin and urease.

Chemistry of the copper( I )–water bond. Some new observations

One-dimensional polymeric copper(I) complexes of the type {[CuL(H2O)]BF4·H2O}n, where L = 2,3-diphenylquinoxaline, and {[CuL′(H2O)]X}n, where L′ = 2,3-dimethylquinoxaline and X− = ClO4− or BF4−, containing a rare copper(I)–water bond were synthesised. From the X-ray crystal structures of two of them, the copper(I) centres in these complexes are found to have a planar T-shaped N2O coordination sphere. It is concluded from the observed Cu(I)–O(water) bond lengths [2.167(7)–2.307(14) A] that the copper(I)–water bonds in these complexes are rather weak. With L, a monomeric complex of the type CuL2ClO4 has also been synthesised. But it has not been possible to obtain such a monomeric copper(I) complex with the BF4− anion or the ligand L′. In CuL2ClO4 the metal is also found to have, from the X-ray crystal structure, a planar T-shaped N2O coordination sphere with the perchlorate anion very weakly bound to the metal through an oxygen atom [Cu(I)–O(perchlorate) = 2.442(8) A]. While in the solid state electronic spectra, CuL2ClO4 displays a band at 346 nm, the aqua complexes show additional band(s) in the 400–480 nm range. CuL2ClO4 reacts with water in dichloromethane to yield an aqua copper(I) complex: CuL2ClO4 + H2O  → [CuL2(H2O)]ClO4 → 1/n{[CuL(H2O)]ClO4}n + L. In cyclic voltammetry at a glassy carbon electrode in anhydrous dichloromethane under N2 atmosphere, CuL2ClO4 shows a quasi-reversible CuII/I couple with a very high redox potential of 0.91 V vs. SCE, which is lowered to 0.79 V vs. SCE upon addition of water. This indicates that binding of water destabilises copper(I), a result expected on the basis of Pearsons HSAB Principle.

Synthesis and DFT calculations of oxido and phenylimido-rhenium(V) complexes incorporating the N, O donor ligand 2-[(2-hydroxyethylimino)methyl]phenol

Abstract The reactions of equimolar amounts of trans-[ReOC13(PPh3)2] or trans-[Re(NPh)(PPh3)2Cl3] with a Schiff base formed by condensation of 2-hydroxy-4-methoxybenzaldehyde and ethanolamine (H2L) result in the formation of cis-[ReO(HL)PPh3Cl2] (1a) and trans-[Re(NPh)(HL)(PPh3)Cl2] (2b), respectively, in good yields. 1a and 2b have been characterized by a range of spectroscopic and analytical techniques. The X-ray crystal structures of 1a and 2b reveal that 1a is an octahedral cis-Cl,Cl oxorhenium(V) complex, while 2b is a trans-Cl,Cl phenylimidorhenium(V) complex. The complexes are weakly emissive at room temperature with quantum yields of 10−4. Density functional theory calculations of the electronic properties of the complexes were performed and are in agreement with the experimental results. The complexes display quasi-reversible Re(V)/Re(VI) redox couples in acetonitrile. There is reasonable agreement between the experimental and calculated redox potentials of 1a and 2b.

An oxorhenium(V) Schiff-base complex: synthesis, structure, spectroscopic characterization, electrochemistry, and DFT calculations

Reaction of the Schiff base 2-[(2-hydroxyethylimino)methyl]phenol (H2L) with trans-ReOCl3(PPh3)2 in 1 : 1 M ratio in dichloromethane gives [ReOCl2(HL)(PPh3)] (1) in substantial yield. The compound has been characterized by C, H, and N microanalyses, IR, UV-Vis, 1H NMR, and ESI-MS spectra. The X-ray crystal structure of the title compound has been determined. The structure reveals that it is an octahedral cis-Cl, Cl oxorhenium(V) complex where the relative positions of the two chlorides are cis. 1 crystallizes in the monoclinic space group P21/n with a = 14.198(2), b = 10.970(1), c = 18.258(1) Å, β = 111.83(1)°, V = 2640.0(3) Å3, and Z = 4. Electrochemical studies in dichloromethane show Re(V) to Re(VI) oxidation at 1.51 V along with two successive reductions – Re(V) to Re(IV) and Re(IV) to Re(III) at −0.66 V and −1.03 V versus Ag/AgCl, respectively. Geometry optimization of 1 vis-à-vis its trans analog, trans-Cl, Cl complex 2, has been performed at the level of density functional theory; 2 is more stable than 1 by 2 kcal M−1.

A new phenylimidorhenium(V) compound containing the 2-[(2-hydroxyethylimino)methyl]phenol Schiff-base ligand: experimental and theoretical aspects

Reaction of equimolar trans-[Re(NPh)(PPh3)2Cl3] with H2L, a 1 : 1 Schiff-base condensate of salicylaldehyde and ethanolamine, in chloroform gives trans-[Re(NPh)(HL)(PPh3)Cl2] (1a) in good yield. 1a has been characterized by C, H, and N microanalyses, FTIR and UV–vis spectra. The X-ray crystal structure of 1a reveals that it is an octahedral trans-Cl,Cl phenylimidorhenium(V) complex. The rhenium center has an ‘N2OCl2P’ coordination sphere. 1a crystallizes in the monoclinic space group P21/c with a = 11.2391(5), b = 16.4848(7), c = 16.3761(8) Å, V = 3034.0(2) Å3 and Z = 4. The electrochemical aspects of 1a have been studied. Electrochemical studies of 1a in dichloromethane show a quasi-reversible Re(V) to Re(VI) oxidation at 1.128 V versus Ag/AgCl. This redox potential reasonably matches the calculated redox potential, 1.186 V versus Ag/AgCl. Geometry optimization of the trans-Cl,Cl 1a vis-à-vis its cis analog, cis-Cl,Cl 1b, have been performed at the level of density functional theory (DFT). It is revealed that 1a is more stable than 1b by 21.6 kcal per mole of energy in the gas phase.

A new trans-dioxorhenium(V) complex with 4-aminopyridine: synthesis, structure, electrochemical aspects, DFT, and TD-DFT calculations

The reaction of 1 : 4.4 M proportion of cis-[ReO2I(PPh3)2] and 4-aminopyridine (ampy) in acetone–water gives trans-[ReO2(ampy)4]I·2H2O (1a) in 85% yield. 1a has been characterized by C, H, and N microanalyses, FT-IR, UV–vis, 1H NMR spectroscopy, and molar conductivity. The X-ray crystal structure of 1a reveals an octahedral trans dioxorhenium(V) complex with a “N4O2” coordination for rhenium. 1a has an orthorhombic space group C2221 with a = 17.576(4), b = 19.370(4), c = 15.730(4) Å, V = 5355(2) Å3, and Z = 8. Geometry optimization of the trans-O,O complex, 1a and its cis-O,O analog, 1b performed at the level of density functional theory reveal that 1a is more stable than 1b by 25 kcal M–1 in the gas phase. The electronic spectrum of 1a was also analyzed at the level of time-dependent density functional theory. Excitation of 1a in methanol at 450 nm leads to a fluorescent emission at 505 nm with a quantum yield (Ф) of 0.04. Electrochemical studies of 1a in acetonitrile show a quasi-reversible Re(V) to Re(VI) oxidation at 0.618 V versus Ag/AgCl. This redox potential matches with the calculated redox potential of 0.621 V versus Ag/AgCl. Graphical Abstract

Copper(II) thiocyanate complexes of 2-(2-pyridinyl)-benzthiazole: synthesis, structure, redox behavior, thermal aspects, and DFT calculations

Reaction of 2-(2-pyridinyl)-benzthiazole (Pbt), KSCN, and copper(II)perchlorate hexahydrate in 1 : 2 : 1 molar proportion in dimethyl sulfoxide gives monomeric [Cu(Pbt)(SCN)2DMSO] (1a). In another run, identical proportions of the same reactants in methanol generates a symmetric thiocyanate bridged dimeric copper(II) compound, [Cu2(Pbt)2(SCN)2] (2a), in 72% yield. Both 1a and 2a have been characterized by C, H, and N microanalyses, copper estimation, FTIR, UV–vis spectra, and room temperature magnetic susceptibility measurements. The X-ray crystal structures of 1a and 2a have been determined. Electrochemical studies of 1a and 2a show Cu(II) to Cu(III) oxidation in solution. The thermal behaviors of 1a and 2a have been studied. Bond valence sum method of analysis was performed to assign the oxidation state for each copper center in 1a and 2a. Geometry optimization of 1a, 2a, and their linkage isomers has also been performed at the level of the density functional theory to show that 1a and 2a are the most stable congeners. These results corroborate our experimental findings.

Plasma-aminothiols status and inverse correlation of total homocysteine with B-vitamins in arsenic exposed population of West Bengal, India

ABSTRACT Chronic arsenic toxicity is a serious environmental health problem across the world. Bangladesh and India (particularly the state of West Bengal) are the worst affected countries with such problem. The present study reports plasma-aminothiols (p-aminothiols) like L-cysteine (L-Cys), cysteinyl glycine (Cys-gly), total homocysteine (t-Hcy) and glutathione (GSH) status, and the inverse relationship of t-Hcy with B-vitamins (B1, B6, B9 and B12) in arsenic exposed population of West Bengal, India. Reverse phase HPLC was used to measure p-aminothiols and serum B-vitamins in different arsenic exposed population. Arsenic in drinking water and urine were measured by flow injection analysis system – Atomic Absorption Spectrometry (FIAS-AAS) and Transversely heated graphite atomizer (THGA-AAS) techniques, respectively. Water arsenic exposure was >50 µg/L in 50% population, of which majority (33.58%) belong to the range of >50–500 µg/L and more than 8% were even >1000 µg/L. Urine arsenic (µg/g creatinine) levels increased with arsenic exposure. The variability among p-aminothiols was also observed with higher exposure to arsenic in drinking water. A significant difference between exposed and control population was noticed for plasma L-Cys. The difference of B-vitamins between the population exposed to <50 and >50 µg/L arsenic in drinking water was also found to be significant. B9 and B12 deficiency with increased consumption of arsenic in water corroborates the anemic conditions commonly observed among arsenic exposed population. The aminothiol status indicated oxidative stress in exposed population. This study demonstrated progressive increase in plasma t-Hcy as well as inverse relationships of serum B-vitamins with increased water arsenic concentration.

A copper(II) complex of benzimidazole-based ligand: synthesis, structure, redox aspects and fluorescence properties

Abstract Employing 1-(2-methoxybenzyl)-2-(2-methoxyphenyl)-1H-benzimidazole (bpb) as a monodentate ligand, a new greenish-blue copper(II) complex, [Cu(bpb)2(NO3)2] (1a), has been synthesized. 1a has been characterized analytically and spectroscopically. The X-ray crystal structure of 1a reveals that it adopts a cis disposition with respect to the ligands. The solid state structure of 1a is stabilized by intramolecular offset face-to-face π–π stacking. Non-covalent supramolecular edge-to-face C–H⋯π interactions with neighboring molecules give 1-D supramolecular chains that further lead to the formation of an assembled 3-D supramolecular metal-organic framework via hydrogen bonding interactions. 1a shows blue fluorescence most likely due to intramolecular offset face-to-face π–π stacking. At room temperature, 1a is one-electron paramagnetic. It shows a rhombic EPR spectrum with g1 = 2.12, g2 = 2.42, and g3 = 2.52 in the solid state at liquid nitrogen temperature. In cyclic voltammetry, 1a displays a one-electron oxidative Cu(II)/Cu(III) couple. Our DFT calculations, corroborate the observed experimental results of 1a.