Articles written in Journal of Chemical Sciences

    • Iron(III) complexes of certain tetradentate phenolate ligands as functional models for catechol dioxygenases

      Mallayan Palaniandavar Marappan Velusamy Ramasamy Mayilmurugan

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      Catechol 1,2-dioxygenase (CTD) and protocatechuate 3,4-dioxygenase (PCD) are bacterial non-heme iron enzymes, which catalyse the oxidative cleavage of catechols tocis, cis-muconic acids with the incorporation of molecular oxygen via a mechanism involving a high-spin ferric centre. The iron(III) complexes of tripodal phenolate ligands containing N3O and N2O2 donor sets represent the metal binding region of the iron proteins. In our laboratory iron(III) complexes of mono- and bisphenolate ligands have been studied successfully as structural and functional models for the intradiol-cleaving catechol dioxygenase enzymes. The single crystal X-ray crystal structures of four of the complexes have been determined. One of thebis-phenolato complexes contains a FeN2O2Cl chromophore with a novel trigonal bipyramidal coordination geometry. The Fe-O-C bond angle of 136.1‡ observed for one of the iron(III) complex of a monophenolate ligand is very similar to that in the enzymes. The importance of the nearby sterically demanding coordinated -NMe2 group has been established and implies similar stereochemical constraints from the other ligated amino acid moieties in the 3,4-PCD enzymes, the enzyme activity of which is traced to the difference in the equatorial and axial Fe-O(tyrosinate) bonds (Fe-O-C, 133, 148‡). The nature of heterocyclic rings of the ligands and the methyl substituents on them regulate the electronic spectral features, FeIII/FeII redox potentials and catechol cleavage activity of the complexes. Upon interacting with catecholate anions, two catecholate to iron(III) charge transfer bands appear and the low energy band is similar to that of catechol dioxygenase-substrate complex. Four of the complexes catalyze the oxidative cleavage of H2DBC by molecular oxygen to yield intradiol cleavage products. Remarkably, the more basic N-methylimidazole ring in one of the complexes facilitates the rate-determining productreleasing phase of the catalytic reaction. The present study provides support to the novel substrate activation mechanism proposed for the intradiol-cleavage enzymes.

    • Fixation and sequestration of carbon dioxide by copper(II) complexes


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      The fixation of carbon dioxide (CO2) is an important global challenge. A significant increase of the atmospheric CO2 due to the industrial emissions and a steady increase in combustion of fossil fuels is widespread environmental concern. This article is a short literature review on the recent developments in the field of CO2 activation and fixation by bioinspired copper(II) catalysts. In our laboratory, copper(II) complexes of bidentate ligands have been reported as catalysts for the fixation of CO2. The molecular structure of one of the complexes has shown unusual trigonal bipyramid geometry (τ, 0.936) by the coordination of two ligand units and a water molecule. All the complexes exhibited a well-defined Cu(II)/Cu(I) redox potentials around 0.352 to 0.401 V in acetonitrile. The rhombic EPR spectra of the complexes indicate the existence of a geometrical equilibrium between trigonal bipyramidal and square pyramidal at 70 K. The d-d transitions around 750–800 and 930–955 nm further supports five coordination geometry in solution. These copper(II) complexes have successfully fixed atmospheric CO2 as CO322- by using Et3N as sacrificial reducing agent and afforded [Cu(L)CO3(H2O)]. The CO322-bound complex has shown a distorted square pyramidal geometry (τ, 0.369) around copper(II) center via the coordination of only one ligand unit, a carbonate, and water molecules. The catalysts are active enough to fix CO2 for eight repeating cycles without any change in the efficiency. The fixation of CO2possibly proceeds via the formation of Cu(I)-species. This is supported by X-ray structure, which reveals distorted tetrahedral geometry by the coordination of two units of ligand.

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