Highly reactive porphyrin-iron-oxo derivatives produced by photolyses of metastable porphyrin-iron(IV) diperchlorates.
Photolyses of metastable porphyrin-iron(IV) diperchlorates in laser flash photolysis reactions gave highly reactive transients. The systems studied were 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetramesitylporphyrin (TMP), and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP). The new species, which decayed within milliseconds in acetonitrile solutions, were shown to react with organic substrates by oxo-transfer reactions involving insertions into carbon-carbon double bonds of alkenes and styrenes or benzylic carbon-hydrogen bonds of arenes. The order of reactivity was OEP > TPP > TMP. Second-order rate constants for reactions with several substrates at 22 degrees C were determined; representative values of rate constants for the TPP derivative were k = 8.6 x 10(5) M(-1) s(-1) for styrene, k = 2.5 x 10(6) M(-1) s(-1) for cyclohexene, and k = 7.7 x 10(4) M(-1) s(-1) for ethylbenzene. These porphyrin-iron-oxo transients reacted 4-5 orders of magnitude faster than the corresponding iron(IV)-oxo porphyrin radical cations with rate constants similar to those of porphyrin-manganese(V)-oxo derivatives. Rate constants for oxidations of benzylic C-H positions of arenes correlated with the C-H bond dissociation energies, and Hammett correlations for reactions with substituted styrenes had rho(+) values ranging from -0.5 to -0.7, reflecting electrophilic character of the oxidants and their high reactivity. On the basis of their unique UV-visible spectra, high reactivities, and oxo-transfer properties, the new transients are tentatively identified as porphyrin-iron(V)-oxo perchlorates, electronic isomers (or valence tautomers) of well-known iron(IV)-oxo porphyrin radical cations.
Pan
Zhengzheng
Z
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, USA.
Wang
Qin
Q
Sheng
Xin
X
Horner
John H
JH
Newcomb
Martin
M
D005290:Ferric Compounds / Q000737:chemistry; D006418:Heme / Q000737:chemistry; D058085:Iron Compounds / Q000737:chemistry*; D007700:Kinetics; D008665:Metalloporphyrins / Q000737:chemistry*; D010084:Oxidation-Reduction; D010782:Photolysis; D011166:Porphyrins / Q000737:chemistry; D019163:Reducing Agents / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet
C509964:5,10,15,20-tetraphenylporphyrin; D005290:Ferric Compounds; D058085:Iron Compounds; D008665:Metalloporphyrins; D011166:Porphyrins; D019163:Reducing Agents; C488938:octaethylporphyrin; D006418:Heme