• M Padmanabhan

      Articles written in Journal of Chemical Sciences

    • Molecular structure of electron donor-acceptor complexes of metallotetraphenylporphyrins with trinitrobenzene

      M Padmanabhan M Damodara Poojari V Krishnan

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      The metallotetraphenylporphyrins, MTPPs, where M=Co(II), Cu(II) and Ag(II) form one to one molecular complexes in solution with 1,3,5-trinitrobenzene (TNB). The crystal structure of CoTPP.TNB.2CH3OH revealed that the mean separation between the porphyrin and TNB planes is 3·27 Å and the centre of the aryl ring of TNB is displaced by 0·90 Å from the centre of the porphyrin plane. Extended Huckel molecular orbital calculations suggest that acceptor orbitals are predominantly nitro-group based rather than an aryl π framework. The contribution of the metal orbitals in the donation is also seen as predicted by the structural data. Electrochemical redox measurements in solution provide evidence for charge-transfer stabilisation in these complexes.

    • Studies on polymer supported metalloporphyrins with modified electronic properties

      Tessymol Mathew M Padmanabhan

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    • Studies on a catalase model system based on immobilised metalloporphyrins on modified solid polystyrene support

      Mikki V Vinodu M Padmanabhan

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      Metalloporphyrins immobilised on solid polymer supports are developed as efficient model systems for catalyse enzyme, which dismutate hydrogen peroxide in many living organisms, saving them from several harmful oxidative processes. The metalloporphyrins employed are Mn(III), Fe(III) and Co(III) derivatives of anionic tetrasulphonated tetraphenylporphyrins (H2TPPS). The polymer support used was divinylbenzene crosslinked polystyrene (in bead form) which was first surface functionalised to chloromethyl derivative and then to N-alkyl pyridinium derivatives (PS). The negatively charged MTPPS were then appended on to the polymer support by ion exchange method. The catalytic efficiency of these immobilised MTPPS was found to be far superior to other supported systems reported earlier. The relative efficiency of the three supported metalloporphyrin systems was studied and compared in varyingpH and temperature conditions. In acidicpH, the order of efficiency of the metalloderivatives was Fe(III)>Mn(III)>Co(III) while in basicpH, the order changed to Mn(III)≈Co(III)>Fe(III). The maximum efficiency was found to be aroundpH 10.0 in all the cases. While the Fe(III) derivatives showed greater sensitivity to temperature, the Mn(III), on the contrary, showed the least sensitivity. The catalytic efficiency of the immobilised systems was found to be retained even after repeated use of the catalysts indicating a high degree of recyclability.

    • Peroxidase-like catalytic activities of ionic metalloporphyrins supported on functionalised polystyrene surface

      Mikkiv Vinodu M Padmanabhan

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      Metalloderivatives of anionic tetrasulphonated tetraphenylporphyrin (MTPPS, M = Mn(III), Fe(III) and Co(III)) were synthesized and immobilized on cationically functionalised divinylbenzene(DVB)-crosslinked polystyrene(PS). These supported catalysts (PS-MTPPS) were found to exhibit peroxidase-like activity. The co-oxidation of 4-aminoantipyrine and phenol by H2O2 was attempted with these catalysts to mimic this enzyme function. The catalytic efficiency of all these immobilized MTPPS was found to be superior to the corresponding unsupported MTPPS in solution. The effect of the central metal ion of the porphyrin, pH of the reaction medium and also the temperature effect are investigated. The ideal pH was seen to be in the 8.0–8.5 range, with maximum effect at 8.2. The efficiency order for the various PS-MTPPS was seen to be Co>Mn>Fe, with CoTPPS showing efficiency comparable to that of horseradish peroxidase. The catalytic efficiency was found to be increasing with temperature for all the catalysts. The re-usability of these PS-MTPPS systems for peroxidase-like activity was also studied and it was found that they exhibited a very high degree of recyclability without much poisoning.

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