Redox‐Induced Spin‐State Switching and Mixed Valency in Quinonoid‐Bridged Dicobalt Complexes

The complexes [{(tmpa)Co^II}₂(μ‐L¹)^2?]²⁺ ( 1²⁺ ) and [{(tmpa)Co^II}₂(μ‐L²)^2?]²⁺ ( 2²⁺ ), with tmpa=tris(2‐pyridylmethyl)amine, H₂L¹=2,5‐di‐[2‐(methoxy)‐anilino]‐1,4‐benzoquinone, and H₂L²=2,5‐di‐[2‐(trifluoromethyl)‐anilino]‐1,4‐benzoquinone, were synthesized and characterized. Structural analysis of 2²⁺ revealed a distorted octahedral coordination around the cobalt centers, and cobalt–ligand bond lengths that match with high‐spin Co^II centers. Superconducting quantum interference device (SQUID) magnetometric studies on 1²⁺ and 2²⁺ are consistent with the presence of two weakly exchange‐coupled high‐spin cobalt(II) ions, for which the nature of the coupling appears to depend on the substituents on the bridging ligand, being antiferromagnetic for 1²⁺ and ferromagnetic for 2²⁺ . Both complexes exhibit several one‐electron redox steps, and these were investigated with cyclic voltammetry and UV/Vis/near‐IR spectroelectrochemistry. For 1²⁺ , it was possible to chemically isolate the pure forms of both the one‐electron oxidized mixed‐valent 1³⁺ and the two‐electron oxidized isovalent 1⁴⁺ forms, and characterize them structurally as well as magnetically. This series thus provided an opportunity to investigate the effect of reversible electron transfers on the total spin‐state of the molecule. In contrast to 2²⁺ , for 1⁴⁺ the metal–ligand distances and the distances within the quinonoid ligand point to the existence of two low‐spin Co^III centers, thus showing the innocence of the quintessential non‐innocent ligands L. Magnetic data corroborate these observations by showing the decrease of the magnetic moment by roughly half (neglecting spin exchange effects) on oxidizing the molecules with one electron, and the disappearance of a paramagnetic response upon two‐electron oxidation, which confirms the change in spin state associated with the electron‐transfer steps.     

Mechanism of Evolution of Koneramine Complexes from One‐Pot Reactions: Snapshots of Intermediates Offer Facile Routes to New Dipicolylamines

Koneramines (L^ROR′, R=Ph or Ts; R′=Me, iPr) and their complexes were found to emerge from the system of pyridine‐2‐carboxaldehyde and N‐phenyl/tosylethylenediamine when a primary or secondary alcohol was used as solvent. Imidazolidinylpyridines (L^R, R=Ph or Ts) became major emergents whereas hemi‐aminals (L^ROH, R=Ph or Ts) are minor emergents of the system when tertiary butanol was used as the solvent; the bulky tertiary butyl group prevented the addition of alcohol to the iminium ion that diverted the equilibrium towards imidazolidinylpyridines. By playing with the components of the reaction mixture, crystals of the metastable intermediates bound to copper(II) and/or zinc(II) were obtained and the structures were determined by X‐ray diffraction analysis. The reported results shed light on how to control the emergents of the multicomponent reaction mixture that forms koneramines. Reactivity studies of the intermediates pave the way for a new type of koneramine complexes that are new dipicolylamines where the two pyridine moieties of the resulting koneramine are not the same.     

Investigations on temperature dependent structural evolution of NaPO3 glass

The understanding of molecular level structural information of phosphate glasses is very much essential. The unique microwave-absorbing ability of NaH₂PO₄·2H₂O was found to be very useful for preparing crystal and glassy sodium super ionic conductors (Nasicon's) as a component of batch mixtures. In this work NaPO₃ glass was prepared by both conventional melt quench and microwave heating from NaH₂PO₄·2H₂O as a starting material. The structure of NaPO₃ glass and their structural evolution upon heating through glass transition were probed by combination of complementary techniques like differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD), Fourier transform infrared (FT-IR) and thermo-Raman spectroscopy.This revised version was published online in November 2005 with corrections to the Cover Date.     

INFLUENCE OF HYDRATION ON THE INTERNAL DYNAMICS OF HEN EGG WHITE LYSOZYME IN THE DRY STATE

Abstract— Proteins exist in a predominately aqueous solvent environment. Hydration of the protein surface significantly affects many aspects of the protein's structure and function; these effects may be related to the molecular dynamics of the protein. We have examined the influence of hydration on the internal dynamics of hen egg white lysozyme using room-temperature phosphorescence from the intrinsic tryptophan residues. Powders of lyophilized lysozyme were hydrated in a phosphorimeter using a flow system that allowed for continuous manipulation of relative humidity over the range 0–92%; this system allowed us to directly compare intensity differences that result from changes in hydration. Lysozyme phosphorescence intensity decreased as a function of hydration over the entire relative humidity range; the decrease was not linear but appeared to occur in distinct phases. The phosphorescence intensity decays were multiexponential over the hydration range studied, and hydration had the largest influence on the long lifetime component. These data suggest that the protein exists in multiple, static conformations in the dry state and that water binding to polar (as opposed to charged) sites on the protein surface induces local and/or global softening of the protein structure.     

Ultrasound‐Assisted Pechmann Condensation of Phenols With β‐Ketoesters to Form Coumarins,in the Presence of Bismuth(III) Chloride Catalyst

Ultrasound was found to synergistically accelerate the condensation of phenol with β‐ketoesters in the presence of BiCl₃. In the absence of ultrasound, under the same conditions, the reaction was found to be slow. Thus, the reaction can be carried out in the presence of ultrasound at room temperature (28–30°C), with a considerable reduction of reaction time, with high yield and high purity of coumarins.     

Thermal decomposition mechanism of synthesized copper octoate

Present study describes the synthesis and characterization of copper octoate. Attenuated total reflectance–Fourier transformation infra red (ATR–FTIR) and energy dispersive X-ray (EDX) spectrometric techniques have been used for the characterization of the synthesized compound. The surface morphology of the compound has been studied using scanning electron microscopy (SEM). Thermal behavior and decomposition mechanism of copper octoate has been explained on the basis of simultaneous thermo-gravimetry–differential thermal analysis–evolved gas analysis (TG–DTA–EGA) and high temperature X-ray diffraction (HTXRD) measurements. Copper octoate is stable up to 250 °C. The decomposition process consists of two overlapping steps. A plausible decomposition mechanism is proposed and details of the studies carried out are being discussed here.     

Silver Corrole Complexes: Unusual Oxidation States and Near‐IR‐Absorbing Dyes

Macrocycles such as porphyrins and corroles have important functions in chemistry and biology, including light absorption for photosynthesis. Generation of near‐IR (NIR)‐absorbing dyes based on metal complexes of these macrocycles for mimicking natural photosynthesis still remains a challenging task. Herein, the syntheses of four new Ag^III corrolato complexes with differently substituted corrolato ligands are presented. A combination of structural, electrochemical, UV/Vis/NIR‐EPR spectroelectrochemical, and DFT studies was used to decipher the geometric and electronic properties of these complexes in their various redox states. This combined approach established the neutral compounds as stable Ag^III complexes, and the one‐electron reduced species of all the compounds as unusual, stable Ag^II complexes. The one‐electron oxidized forms of two of the complexes display absorptions in the NIR region, and thus they are rare examples of mononuclear complexes of corroles that absorb in the NIR region. The appearance of this NIR band, which has mixed intraligand charge transfer/intraligand character, is strongly dependent on the substituents of the corrole rings. Hence, the present work revolves round the design principles for the generation of corrole‐based NIR‐absorbing dyes and shows the potential of corroles for stabilizing unusual metal oxidation states. These findings thus further contribute to the generation of functional metal complexes based on such macrocyclic ligands.