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

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 (Fe^IIIT4MPyP) 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 Fe^IIIT4MPyP 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 Fe^IIIT4MPyP with imidazole and with Bu^tO₂H 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–Fe^IIIT4MPyP-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 Bu^tO₂H is presented.

Possible structures for the polymer-bound Fe^IIIT4MPyP are discussed and it is concluded that the metalloporphyrin is extensively aggregated on the polymer chains as µ-oxo x-mers.