Hetero-Diels–Alder Reaction between Singlet Oxygen and Anthracene Drives Integrative Cage Self-Sorting

Abstract

A Zn^II₈L₆ pseudocube containing anthracene-centered ligands, a Zn^II₄L'₄ tetrahedron with a similar side length as the cube, and a trigonal prism Zn^II₆L₃L'₂ were formed in equilibrium from a common set of subcomponents. Hetero-Diels-Alder reaction with photogenerated singlet oxygen transformed the anthracene-containing "L" ligands into endoperoxide "L^O" ones and ultimately drove the integrative self-sorting to form the trigonal prismatic cage Zn^II₆L^O₃L'₂ exclusively. This Zn^II₆L^O₃L'₂ structure lost dioxygen in a retro-Diels-Alder reaction after heating, which resulted in reversion to the initial Zn^II₈L₆ + Zn^II₄L'₄ ⇌ 2 × Zn^II₆L₃L'₂ equilibrating system. Whereas the Zn^II₈L₆ pseudocube had a cavity too small for guest encapsulation, the Zn^II₆L₃L'₂ and Zn^II₆L^O₃L'₂ trigonal prisms possessed peanut-shaped internal cavities with two isolated compartments divided by bulky anthracene panels. Guest binding was also observed to drive the equilibrating system toward exclusive formation of the Zn^II₆L₃L'₂ structure, even in the absence of reaction with singlet oxygen.