Synthesis,characterization, electrochemical and photophysical properties of bimetallic complexes of rhenium(I) and osmium(II)

Monometallic and bimetallic diimine complexes of rhenium(I) and osmium(II), [(CO)₃(bpy)Re(4,4′-bpy)](PF₆) I, [(CO)₃(bpy)Re(4,4′-bpy)Re(bpy)(CO)₃](PF₆)₂II, [Cl(bpy)₂Os(4,4′-bpy)](PF₆) III and [Cl(bpy)₂Os(4,4′-bpy)Os(bpy)₂Cl](PF₆)₂IV, and a new heterobimetallic complex of rhenium(I) and osmium(II) [(CO)₃(bpy)Re(4,4′-bpy)Os(bpy)Cl](PF₆)₂V (bpy = 2,2′-bipyridine; 4,4′-bpy = 4,4′-bipyridine) have been synthesized and characterized by various spectral techniques. The photophysical properties of all the complexes have been studied and a comparison is made between the heterobimetallic and corresponding monometallic and homobimetallic complexes. Emission and transient absorption spectral studies reveal that excited state energy transfer from the rhenium(I) chromophore (∗Re) to osmium(II) takes place. The energy transfer rate constant is found to be 8.7 × 10⁷ s⁻¹.     

N-heterocyclic carbene catalyzed reaction of enals and 1,2-dicarbonyl compounds: stereoselective synthesis of spiro gamma-butyrolactones

[reaction: see text]. Nucleophilic heterocyclic carbene (NHC) catalyzed annulation of enals and cyclic 1,2-dicarbonyl compounds, opening a route to gamma-spirolactones, has been observed for the first time. The strategy works well with isatins, leading to spiroannulated oxindole derivatives. It is conceivable that the spiroannulation protocol reported herein will be applicable to the synthesis of important natural products that are endowed with a spiro gamma-butyrolactone motif, especially oxindoles and norsesquiterpenoids.     

Water dissociation on Cu (111): Effects of molecular orientation, rotation, and vibration on reactivity

Three-dimensional time-dependent quantum mechanical method has been used to study the influence of orientation, rotation, and vibration on the dissociation of water molecule on Cu(111) surface, using London-Eyring-Polanyi-Sato potential energy surface. Our calculations show that dependency of dissociation probability on the initial orientation of the molecule changes with the vibrational state of the molecule. It has also been found that for v(0) = 0 and 1, where v(0) stands for the vibrational state of the pseudo diatomic HO-H, the rotational excitation of the molecule increases the reactivity, whereas for v(0) = 2, the rotational excitation of the molecule decreases the reactivity. Vibrational excitation of the molecule greatly enhances the dissociation probability.     

Recognition of aromatic amino acids and proteins with p‐sulfonatocalix[4]arene – A luminescence and theoretical approach

The host–guest interaction of p‐sulfonatocalix[4]arene (p‐SC4) with aromatic amino acids (AAs) and two proteins has been studied using UV–Vis absorption, fluorescence, and theoretical methods. Spectral studies supported by binding constant and calculated binding energy (BE) values show that p‐SC4 binds more strongly with tyrosine compared with other AAs. The application of Bader's theory of atoms in molecule shows the involvement of various types of noncovalent interactions in the formation of the host–guest complexes. Both tyrosine and histidine have strong electrostatic interaction with the sulfonato group and other two AAs have dominant π?π interaction with the aromatic rings of calixarene. In addition, the role of C?H···O, C?H···π and lone pair···π (lp···π) interactions in the stabilization of p‐SC4‐AA complexes has also been realized from the atoms in molecule analysis. The electron density at the bond critical points varies with the calculated BEs and trend in BEs is in good agreement with the experimental binding constant values. The work has been extended to the binding of p‐SC4 with proteins, bovine serum albumin and ovalbumin. Ovalbumin exhibits stronger binding with p‐SC4 than bovine serum albumin. Copyright © 2012 John Wiley & Sons, Ltd.     

Electron transfer reactions of iron(III)-polypyridyl complexes with organic sulfoxides

The redox reactions of four iron(III)-polypyridyl complexes with six aryl methyl sulfoxides have been investigated by spectrophotometric technique. The reaction follows clean second order kinetics and proceeds through rate determining electron transfer (ET) from organic sulfoxides to iron(III). The Marcus cross-reaction relation has been applied to obtain the self exchange rate constant for the ArSOR/ArS⁺(O)R couple as 1.3×10⁷ M⁻¹ s⁻¹. The application of Marcus theory to this ET reaction shows that the contribution of inner sphere reorganization energy is 0.4 eV. The rate constant and reaction constant values observed with organic sulfoxides are small compared with organic sulfides towards the same oxidant Fe(NN)₃³⁺.     

Electron transfer reactions of organic sulfoxides with photochemically generated ruthenium(III)-polypyridyl complexes

The electron transfer (ET) reaction of aryl methyl sulfoxides with ruthenium(III)-polypyridine complexes is sensitive to the change of substituent in the aryl moiety of ArS(O)CH₃ and ligand of Ru(III) complex. The detection of sulfoxide radical cation as the transient by conventional flash photolysis confirms ET in the rate-controlling step. The successful application of Marcus cross relation of ET leads to the evaluation of self-exchange rate constant of ArS⁺(O)CH₃/ArS(O)CH₃ as 4.0×10⁵ M⁻¹ s⁻¹ similar to organic sulfides. Comparison with the reactivity of iron(III)-polypyridyl complexes points out that both reactivity and ρ values are higher with Ru(III) complexes.     

Theoretical studies on hydroquinone-benzene clusters

High-level ab initio calculations were carried out to evaluate the interaction between the hydroquinone and benzene molecules. The intermolecular interaction energy was calculated using the M?ller-Plesset second-order perturbation theory at the complete basis set limit and also at the coupled cluster theory with single, double, and perturbatively triple excitations. The calculated binding energy is larger than the benzene dimer interaction energy. The T-shaped cluster (T-a) and the parallel conformation (P-a) are calculated to be nearly isoenergetic. Owing to the large energy gain in the attraction by electron correlation, the dispersion interaction is important for the attraction.     

Iron(III) [bond] Salen complexes as enzyme models: mechanistic study of oxo(salen)iron complexes oxygenation of organic sulfides

The oxidation of a series of para-substituted phenyl methyl sulfides was carried out with several oxo(salen)iron (salen = N,N'-bis(salicylidine)ethylenediaminato) complexes in acetonitrile. The oxo complex [O=Fe(IV)(salen)](*+), generated from an iron(III) [bond] salen complex and iodosylbenzene, effectively oxidizes the organic sulfides to the corresponding sulfoxides. The formation of [O [double bond] Fe(IV)(salen)](*+) as the active oxidant is supported by resonance Raman studies. The kinetic data indicate that the reaction is first-order in the oxidant and fractional-order with respect to sulfide. The observed saturation kinetics of the reaction and spectral data indicate that the substrate binds to the oxidant before the rate-controlling step. The rate constant (k) values for the product formation step determined using Michaelis-Menten kinetics correlate well with Hammett sigma constants, giving reaction constant (rho) values in the range of -0.65 to -1.54 for different oxo(salen)iron complexes. The log k values observed in the oxidation of each aryl methyl sulfide by substituted oxo(salen)iron complexes also correlate with Hammett sigma constants, giving positive rho values. The substituent effect, UV-vis absorption, and EPR spectral studies indicate oxygen atom transfer from the oxidant to the substrate in the rate-determining step.     

Kinetics and mechanism of (salen)MnIII–catalysed hydrogen peroxide oxidation of alkyl aryl sulphides

The kinetics of (salen)Mn^III complexes catalysed oxidation of aryl methyl and alkyl phenyl sulphides with hydrogen peroxide have been investigated at 25°C in 80% acetonitrile – 20% water spectrophotometrically. The reaction follows first‐order kinetics in (salen)Mn^III complex and zero‐order kinetics in hydrogen peroxide. The order of the reaction with respect to sulphide is fractional and saturation in reaction rate occurs at higher sulphide concentrations. The pseudo first‐order rate constants have been analysed as per Michaelis–Menten kinetics to obtain the values of k₂, the oxidant‐substrate complex decomposition rate constant, and K, the oxidant‐substrate complex formation constant. The effects of nitrogenous bases, free radical inhibitor and changes in solvent composition have also been studied. A suitable mechanism, supported by electronic‐oxidant and electronic‐substrate effect studies, involving a manganese(III)‐hydroperoxide complex as reactive species has been proposed. Copyright © 2007 John Wiley & Sons, Ltd.     

Methyl-2-arylidene hydrazinecarbodithioates: synthesis and biological activity

Methyl-2-arylidene hydrazine-carbodithioate has not been of particular interest to researchers even though its metal complexes are extensively reported on due to their biological activity. This study examined the cytostatic and antiviral activity of twelve methyl-2-arylidene hydrazinecarbodithioates reported by many researchers as intermediates for the synthesis of thiosemicarbazides and the preparation of their metal complexes. Compounds IIc, IIi, and IIl with tridentate ligand features were found to have the lowest IC₅₀ value (6.5 μM, ≈ 1 μM, and 0.8 μM, respectively) against HL60 human promyelocytic leukemia cells. They were also most inhibitory to human embryonic lung (HEL) fibroblast proliferation (5.3 μM, 17 μM, and 2.6 μM). Compound IIc and IIl show antiviral activity against wild-type herpes simplex virus (HSV), varicella zoster virus (VZV), and acyclovirresistant HSV; however, these activities were observed at concentrations at which the compounds also markedly inhibit HL60 and HEL cell proliferation.