WATER-SOLUBLE, CATIONIC MANGANESE AND VANADIUM PORPHYRINS AS BIOMIMETIC MODELS AND PRACTICAL OXIDATION CATALYSTS

Abstract

Synthetic metalloporphyrins are studied as biomimetic models of heme- containing oxygenase and peroxidase enzymes, such as cytochrome P450. In many instances, model compounds have not only informed our understanding of enzymatic activity but also yielded practical catalysts and processes. The focus of the present work is the reactivity of synthetic manganese and vanadium porphyrin complexes bearing cationically-charged <italic>N</italic>-methyl pyridyl and <italic>N,N</italic>-dimethyl imidazolyl <italic>meso</italic>-substituents.

Cationic manganese porphyrins are shown herein to rapidly and efficiently catalyze the conversion of aqueous solutions of sodium chlorite to chlorine dioxide, an industrially useful bleach and disinfectant. The more electron-withdrawing porphyrin ligands were found to produce the fastest catalysts for this process (ca. 0.4 turnovers s<super>&minus;1</super>), and a cationic manganese porphyrazine was even more active (> 30 turnovers s<super>&minus;1</super>). This process was investigated by stopped-flow spectroscopy and computationally-assisted kinetic simulations, and the proposed mechanism involves rate-limiting oxidation of manganese(III) by chlorite ion to afford an oxomanganese(V) intermediate. High-valent oxomanganese(V) and -(IV) subsequently oxidizes other chlorite ions to produce chlorine dioxide in a peroxidase-like manner. Promising results for directly applying this new technology in water treatment are also presented.

High-valent oxomanganese porphyrin species, known intermediates in a number of manganese porphyrin-catalyzed oxidations, were characterized by XAS and EXAFS. Notably, the <italic>trans</italic>-dioxomanganese(V) porphyrin is shown to have two equivalent Mn-O bonds of 1.68 &Aring;, consistent with that predicted by vibrational spectroscopy. This investigation also revealed two intriguing species worthy of further study: an oxomanganese(IV) with an extremely short Mn-O bond length of 1.63 &Aring; and a manganese(V) with two anomalously long Mn-O/N bonds of 2.01 &Aring;.

Finally, the synthesis and spectroscopic characterization of a family of cationic vanadyl (oxovanadium(IV)) porphyrins is presented. These complexes resist facile chemical and electrochemical oxidation, although under extremely alkaline conditions an irreversible electrochemical oxidation event is detectable. Observed pKa transitions and electrochemistry allow the prediction that a putative <italic>trans</italic>-dioxovanadium(V) porphyrin could break a C-H bond with a bond dissociation energy > 99 kcal mol<super>&minus;1</super>. These results suggest the potential of these novel vanadium porphyrins to be further developed as catalysts for alkane functionalization.