• Balachandran Unni Nair

Articles written in Journal of Chemical Sciences

• Approaches to the aqueous chemistry of chromium (IV) and chromium (V) in macrocyclic and Schiff base complexes

The stabilization of chromium(IV) and chromium(V) in aqueous systems has now been made through designed alterations in Franck-Condon barriers for the interconversion of various oxidation states of chromium. Equatorially coordinated macrocyclic and Schiff base ligands influence the reorganizational barriers for the conversion of Cr(III)-Cr(IV)-Cr(V)-Cr(VI) couples such that Cr(IV) and Cr(V) transients of sufficiently long life time can be generated. A series of macrocyclic and Schiff base complexes of Cr(III) has been synthesised and subjected to non-complementary redox reaction with Ce(IV) or electrochemical oxidation. A Cr(IV) macrocyclic complex, Cr(Me4[14]tetraene)(H2O)24+, 1 has been generated in aqueous acidic medium, through electrochemical oxidation. The Ce(IV) oxidation of Cr(III) Schiff base complexes, diaqua[N,N′-ethylenebis(salicylideneiminato) chromium(III)], and diaqua[N,N′-propylenebis(salicylideneiminato) chromium(III)] proceed in two stages with 1 Cr:l Ce(IV) redox stoichiometry. Suffīciently long-lived Cr(IV) intermediates and Cr(V) Schiff base products have been generated. ESR evidence for the formation of Cr(V) Schiff base products and the mechanistic outcome of the oxidation of Cr(III) Schiff bases are discussed.

• Effect of non-leaving ligand environment on aqua ligand substitution in chromium(III) schiff-base complexes

• Force field calculation on the structure of transition metal complexes: Application to schiff-base complexes of Ti(IV)

• Preparation, characterization and reactivities of chromium (III) complexes of a homologous series of Schiff-base ligands

Complexes of general formulation [Cr(Schiff-base)(H2O)2]X, where Schiff-base=1, 2-bis(salicylideneamino)ethane (salen), 1, 3-bis(salicylideneamino) propane (salprn) and 1, 4-bis(salicylideneamino)butane (salbuen) and X=ClO4 or Cl have been prepared and characterised. Single-crystal structure of the complexrans[Cr(salprn)(H2O)2]Cl has been resolved. Evidence for less distorted coordination geometry around chromium in the salprn analogue as compared to that intrans-[Cr(salen)(H2O)2]+ has been presented. The kinetics of the aqua ligand substitution in the complexes Cr(Schiff-base)(H2O)2+ by thiocyanate have been investigated under pseudo-first order conditions with [NCS]≫[Cr(III)],T=10–50°C, [H+]=10−8−10−1 M,I=1.0M (LiClO4). An attempt has been made to discuss the anomalous lability oftrans-[Cr(salen)(H2O)2]+ reported earlier in comparison to the reactivities of salprn and salbuen analogues. Bimolecular rate constants for the aqua ligand substitution by NCS in [Cr(Schiff-base)(H2O)2]+ at 25°C vary by more than two orders of magnitude depending on the nature of the Schiff-base ligand. The reactivity order is discussed in terms of the possible steric strain imposed by the equatorially coordinated quadridentate ligand.

• Chromium-induced molecular assemblies and long range ordering in collagenous tissues: A conceptual insight into chromium tanning

A molecular insight into the mechanism of stabilisation of collagen against the action of collagenase and wet heat leading to tanning has been gained. The variations in the extent of stabilisation of collagen with the nature of the molecular species of chromium(III) have been demonstrated experimentally. A case has been made to conceive tanning as emerging from an increase in long-range order and chromium(III)-induced molecular assembly of collagen.

• Synthesis of cadmium sulphide in pure and mixed Langmuir-Blodgett films ofn-octadecylsuccinic acid

Cadmium sulphide (CdS) nanoparticles were grown by the reaction of sodium sulphide (Na2S) with Langmuir-Blodgett (LB) films of cadmium salts ofn-octadecylsuccinic acid (ODSU) and with LB films of ODSU in mixtures of octadecylamine and octadecyl alcohol. The results indicate that heterogeneous nucleation and aggregation in the pure ODSU LB films due to processes like Ostwald ripening are destabilized by the presence of the long-chain amine and alcohol in mixed systems. CdS nanoparticles in the LB films were monitored by UV-visible absorption spectra, which allow an estimation of the size of the particles. The morphology, size and nature of the nanocrystallites formed depend on whether the sulphidation was done on the pure film or in the mixed films. It is seen that particles of size around 1.6 nm were formed in ODSU/octadecylalcohol and ODSU/ octadecylamine mixed LB films while in pure ODSU films the size was about 2.7 nm. These films showed typical needle-shaped structures, as observed by the optical microscopic technique. Mean size and morphology were confirmed by transmission and scanning electron microscopy, while selective area electron diffraction patterns showed six-fold symmetry and indicated that the CdS crystals grow epitaxially with respect to the monolayer. Further, the crystallisation enhanced in the mixed LB films showed a characteristic zinc oxide (Wurtzite) structure compared with the pure ODSU matrix.

• DNA binding and cleavage activity by a mononuclear iron(II)Schiff base complex: Synthesis and structural characterization

Synthesis and characterization of a mononuclear Fe(II) compound [Fe(L)](ClO4)2 (1) [L = N-(1-pyridin-2-yl-phenylidene)-N'-[2-({2-[(1-pyridin-2-ylphenylidene)amino]ethyl}amino)ethyl] ethane-1,2-diamine] (1) is reported. 1 crystallizes in P-1 space group with a = 11.9241(3) Å, b = 12.1994(3) Å and c = 13.0622(4) Å. The binding property of the complex with DNA has been investigated using absorption and emission studies, thermal melting, viscosity experiments and circular dichroism studies. The binding constant ($K_b$) and the linear Stern-Volmer quenching constant ($K_{\text{sv}}$) of the complex have been determined as $3.5 \times 10^3$M-1 and $2.73 \times 10^4$M-1, respectively. Spectroscopic and hydrodynamic investigations revealed intercalative mode of binding of 1 with DNA. 1 is also found to induce oxidative cleavage of the supercoiled pUC 18 DNA to its nicked circular form in a concentration dependent manner.

• Effective synthesis route for red-brown pigments based on Ce - Pr - Fe - O and their potential application for near infrared reflective surface coating

New cerium-based ceramic pigments, displaying Ce2Pr0.2Fe$_x$O$_{4.3+y}$ stoichiometry, were obtained at low temperature using a sol-gel method. The powder precursor dissolved in 80% ethylene glycol was precipitated using ammonia and the obtained gel calcined at 800°C for 2 h to yield homogeneous and crystalline particles with a diameter of around 150 nm. The oxide was composed of cerium in its +4 oxidation state and Pr in its +3 oxidation state. The oxides with varying Fe content had an intense red-brown colour, with bandgap energy of around 2.2 eV at 0.1 mol% Fe doping. The near infra red reflectance from these pigments, a measure of their ability to reflect rather than absorb heat waves from sunlight was found to be 82.7%, even in the absence of a white reflective base. Such high near infra red reflectance from these pigments qualify them for being ideal cool pigments for surface coating applications.

• DNA binding and cleavage activity of a structurally characterized Ni(II) Schiff base complex

Synthesis and characterization of a mononuclear Ni(II) compound [Ni(L)(H2O)2](NO3)2 [L = N,N'-bis((pyridine-2-yl)phenylidene)-1,3-diaminopropan-2-ol] (1) is reported. 1 crystallizes in triclinic P-1 space group with a = 8.1911(2) Å, b = 11.6624(3) Å, c = 16.5356(4) Å and𝛼 = 108.8120(10)° , 𝛽 = 91.2010(10)° , 𝛾 = 91.1500(10)° . The binding property of the complex with DNA has been investigated using absorption and emission studies, and viscosity experiment. The binding constant (Kb) and the linear Stern-Volmer quenching constant (Ksv) of the complex have been determined as 9.23 × 10 4 M−1 and 2.0 × 10 4 M−1, respectively. Spectroscopic and hydrodynamic investigations revealed groove or electrostatic nature of binding of 1 with DNA. 1 is also found to induce oxidative cleavage of the supercoiled pUC 18 DNA to its nicked circular form in a concentration dependent manner.

• Frequency upconversion in catechin assisted LaF₃: Yb³⁺-Er³⁺ square nanoplates

Yb ³⁺ -Er³⁺ codoped lanthanum upconverting nanostructures with varying concentration of dopants have been synthesized by catechin method. The prepared upconverting nanostructures were characterized through XRD,TEM,XPS, NIR absorption,TGAandDSCanalysis.TEMstudy confirms the formation of squareshaped nanoplates with size less than 100 nm. XPS peaks around 830–854eV further reveal the formation of LaF ₃ in the prepared samples. Frequency upconversion spectra from the developed nanostructures corresponding to the ²H₁₁/₂, ⁴S₃/₂ → ⁴ I₁₅/₂ and ⁴F₉/₂ → ⁴I₁₅/₂ transitions upon 980nm excitation are reported. Enhancement in the upconversion emission of catechin-based lanthanum nanostructures has been observed when compared with that prepared by ammonium di-n-octadecyldithiophosphate (AODTP) method.

• # Journal of Chemical Sciences

Volume 132, 2019
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019