Photoinduced Homolysis of Alkyl–Cobalt(III) Bonds in a Cyclodextrin Cage

Abstract

Photodecomposition of methyl– and ethyl–Co^III complexes of meso-tetrakis(4-sulfonatophenyl)porphyrin (CH₃– and C₂H₅–Co^IIITPPSs) was used as a reaction probe to study the cage effect of cyclodextrin capsules formed by two per-O-methylated β-cyclodextrin (TMe-β-CD) molecules and their covalently linked dimer, Ph2CD. The photodecomposition of CH₃–Co^IIITPPS under aerobic conditions was markedly suppressed in the presence of TMe-β-CD and Ph2CD, while C₂H₅–Co^IIITPPS was less affected. Alkyl–Co^IIITPPS formed two types of inclusion complex with Ph2CD, the alkyl groups in Type 1 being located at the opposite side of the phenyl linker of Ph2CD and those in Type 2 being located at the same side. The photodecomposition of C₂H₅–Co^IIITPPS in Type 1 proceeded via an ethylperoxo complex, while that in Type 2 occurred via a radical pair generated in a narrow, rigid cage to form ethylene and Co^IITPPS.

Photoinduced homolysis of the methyl–Co^IIIL [L: meso-tetrakis(4-sulfonatophenyl)porphinato] bond was remarkably inhibited by including the porphyrin in per-O-methylated β-cyclodextrin capsules, which acted as a molecular cage that interfered with the contact of O₂ with the methyl radical, leading to recombination of a photochemically generated radical pair.

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