Highly Reactive Porphyrin−Iron−Oxo Derivatives Produced by Photolyses of Metastable Porphyrin−Iron(IV) Diperchlorates

Authors:
Publisher website
Google Scholar
Abstract
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 °C were determined; representative values of rate constants for the TPP derivative were k = 8.6 × 105 M−1 s−1 for styrene, k = 2.5 × 106 M−1 s−1 for cyclohexene, and k = 7.7 × 104 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 ρ+ 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.

This publication has 43 references indexed in Scilit: