The structure of iron(III) tetra(4-N-methylpyridyl)porphyrin species bound to synthetic water–soluble anionic polymers and their reactions with tert-butyl hydroperoxide in aqueous solution

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Abstract
This paper describes studies carried out to investigate the nature of the species that are formed when the cationic iron(III) tetra(4-N-methylpyridyl)porphyrin (FeIIIT4MPyP) is bound to three water-soluble anionic polymers, namely, poly(styrene-4-sulfonate), poly(vinylsulfonate) and poly(acrylate), in water at pH 9.2. GPC studies with FeIIIT4MPyP and its free base, H2T4MPyP, show that the iron porphyrin is bound by electrostatic interactions and by ligation to the metal. UV–VIS spectroscopy reveals that the polymers induce a broadening and a blue-shift of the metalloporphyrin Soret band and for poly(styrene-4-sulfonate) the extent to which this occurs depends on the method of preparation of the polymer-bound porphyrin. Resonance Raman spectroscopy shows that with poly(styrene-4-sulfonate) the iron porphyrin is bound as a low spin iron species whilst with poly(acrylate) it is a mixture of high and low spin species. The rates of reactions of polymer-bound FeIIIT4MPyP with imidazole and with ButO2H in the presence of 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate)(ABTS) are found to be dependent on the nature of the polymer and, for poly(styrene-4-sulfonate), on the method of preparation of the bound catalyst. Products from the polymer–FeIIIT4MPyP-catalysed decomposition of tert-butyl hydroperoxide in the presence and absence of ABTS have been determined and the oxidant account-ability is excellent. Evidence for the formation of polymer-bound oxoiron(IV) tetra(4-N-methylpyridyl)porphyrin in the reactions with ButO2H is presented. Possible structures for the polymer-bound FeIIIT4MPyP are discussed and it is concluded that the metalloporphyrin is extensively aggregated on the polymer chains as µ-oxo x-mers.

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