Transformation of a Pd6 trifacial barrel to a Pd8 tetrafacial barrel by C70 as guest and oxidative photolysis of alkenes using the C70 encapsulated barrel under red light

Abstract

Fabricating discrete molecular cages as fullerene receptors has been a compelling task. The main objective of confining fullerene molecules is to utilize their physico-chemical properties in commonly used media in which they are insoluble. Herein, a fluorenone-based large trigonal Pd₆ molecular barrel (M1) was synthesized by coordination self-assembly to act as a potential fullerene trap. M1, in the presence of fullerene C₇₀, converted to a larger Pd₈ tetrafacial barrel (M2) forming stable host–guest adduct, (C₇₀)₃@M2. The PF₆⁻ analogues of both M1 and (C₇₀)₃@M2 were acetonitrile soluble, generated reactive oxygen species due to the presence of photosensitizing fluorenone moieties in their building blocks and catalyzed the oxidative transformation of alkenes into carbonyl compounds under 390 nm irradiation. The presence of encapsulated C₇₀ molecules in (C₇₀)₃@M2 enabled its photosensitizing wavelength to be tuned to 650 nm (red light). Subsequently, (C₇₀)₃@M2, at low catalyst loadings and under red light irradiation, catalyzed olefin oxidations in acetonitrile wherein free C₇₀ was completely inefficient due to insolubility. In summary, (C₇₀)₃@M2 was employed as a photocatalyst to mimic the ozonolysis of olefins without the use of ozone or other metal-oxide oxidants that produce over-oxidized products and generate toxic waste, under innocuous red light irradiation and environment-friendly reaction conditions.