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Photochemical oxygen atom transfers with metalloporphyrins
342
Abstracts
DO75
PHOTOCHEMICAL OXYGEN ATOM TRANSFERS WITH METALLOPORPHYRINS
Kenneth S. Suslick School of Chemical Sciences, University of Illinois at Urbana-Champaign, 505 S. Mathews Ave., Urbana, IL 61801, USA There now exists a rich photochemistry of porphyrin complexes of the first row transition metals, particularly those of chromium, manganese, and iron [l]. In addition to the well-explored photodissociation of small ligands (e.g., CO), recent photochemical studies have revealed a diverse set of reactions, including oxygen and nitrogen atom transfers, photoreductions, photooxidations, photocatalysis, and radical chain initiations. Much of this diversity actually represents secondary thermal reactions. In many cases, the primary photoprocess is homolytic loss of an axial ligand, resulting in photoreduction of the metal and production of a reactive radical from the lost ligand. Subsequent fast thermal reactions can then lead to the formation of the wide range of reactivity observed. Consistent with this, irradiation of the low energy 1r+7r transitions does not produce photochemical reactions. Instead, the metalloporphyrin excited states that show photochemistry are those involved in charge transfer transitions, either from the axial ligand to the metal or from the porphyrin itself to the metal. Thus, metalloporphyrin photochemistry is observed primarily from complexes with hyper spectra. Recent work on photochemical oxygen atom transfers include examples of chain autoxidations, stoichiometric and catalytic oxidation of hydrocarbons, and cases of both homolytic and heterolytic 0 transfer. Unusual photochemistry is observed with transition metalloporphyrin perchlorates, periodates, nitrates, nitrites, and sulfates [2-41. Mn(TPP)(XOJ (X = Cl, I) can be photochemically converted cleanly to Mn(TPP)(X), with concomitant quantitative alkane hydroxylation or alkene epoxidation using all four oxygen atoms; this can be made photocatalytic. With Cr(TPP)(ClOJ, the photochemical product is O=CrrV(TPP). The photochemistry of Mn(TPP)(N03) and Mn(TPP)(N02) have also been examined. Both undergo homolytic 0 atom transfer, upon irradiation, cleanly yielding 0 = Mn”(TPP) initially. Subsequent substrate oxidation can be nearly quantitative. Tn contrast, photochemical 0 transfer from Fe(TPP)(N02) is heterolytic and leads to even more reactive metal 0x0 complexes. Extensions to the photoactivation of sulfate complexes will also be discussed. Applications of pseudo-matrix isolation photochemistry to these systems [4] will also be discussed. 1. 2. 3. 4.
Suslick, Suslick, Suslick, Suslick,
K. K. K. K.
S.; S.; S.; S.;
Watson, Watson, Watson, Bautista,
R. R. R. J.
A. A. A.; F.;
New J. Chem. 16, 633 (1992). Znorg. Chem. 30, 912 (1991). Wilson, S. R. Znorg. Chem. 30, 2311 (1991). Watson, R. A. J. Am. Chem. Sot. 113, 6111 (1991).
Abstracts
DO75
PHOTOCHEMICAL OXYGEN ATOM TRANSFERS WITH METALLOPORPHYRINS
Kenneth S. Suslick School of Chemical Sciences, University of Illinois at Urbana-Champaign, 505 S. Mathews Ave., Urbana, IL 61801, USA There now exists a rich photochemistry of porphyrin complexes of the first row transition metals, particularly those of chromium, manganese, and iron [l]. In addition to the well-explored photodissociation of small ligands (e.g., CO), recent photochemical studies have revealed a diverse set of reactions, including oxygen and nitrogen atom transfers, photoreductions, photooxidations, photocatalysis, and radical chain initiations. Much of this diversity actually represents secondary thermal reactions. In many cases, the primary photoprocess is homolytic loss of an axial ligand, resulting in photoreduction of the metal and production of a reactive radical from the lost ligand. Subsequent fast thermal reactions can then lead to the formation of the wide range of reactivity observed. Consistent with this, irradiation of the low energy 1r+7r transitions does not produce photochemical reactions. Instead, the metalloporphyrin excited states that show photochemistry are those involved in charge transfer transitions, either from the axial ligand to the metal or from the porphyrin itself to the metal. Thus, metalloporphyrin photochemistry is observed primarily from complexes with hyper spectra. Recent work on photochemical oxygen atom transfers include examples of chain autoxidations, stoichiometric and catalytic oxidation of hydrocarbons, and cases of both homolytic and heterolytic 0 transfer. Unusual photochemistry is observed with transition metalloporphyrin perchlorates, periodates, nitrates, nitrites, and sulfates [2-41. Mn(TPP)(XOJ (X = Cl, I) can be photochemically converted cleanly to Mn(TPP)(X), with concomitant quantitative alkane hydroxylation or alkene epoxidation using all four oxygen atoms; this can be made photocatalytic. With Cr(TPP)(ClOJ, the photochemical product is O=CrrV(TPP). The photochemistry of Mn(TPP)(N03) and Mn(TPP)(N02) have also been examined. Both undergo homolytic 0 atom transfer, upon irradiation, cleanly yielding 0 = Mn”(TPP) initially. Subsequent substrate oxidation can be nearly quantitative. Tn contrast, photochemical 0 transfer from Fe(TPP)(N02) is heterolytic and leads to even more reactive metal 0x0 complexes. Extensions to the photoactivation of sulfate complexes will also be discussed. Applications of pseudo-matrix isolation photochemistry to these systems [4] will also be discussed. 1. 2. 3. 4.
Suslick, Suslick, Suslick, Suslick,
K. K. K. K.
S.; S.; S.; S.;
Watson, Watson, Watson, Bautista,
R. R. R. J.
A. A. A.; F.;
New J. Chem. 16, 633 (1992). Znorg. Chem. 30, 912 (1991). Wilson, S. R. Znorg. Chem. 30, 2311 (1991). Watson, R. A. J. Am. Chem. Sot. 113, 6111 (1991).