Densely Charged Dodecacationic [3]- and Tetracosacationic Radial [5]Catenanes

Abstract

Here, we report the efficient syntheses of a dodecacationic [3]catenane, which consists of three mechanically interlocked 4+ charged rings despite their mutual Coulombic repulsion, and a tetracosacationic radial [5]catenane, which consists of four 4+ charged rings mechanically interlocked around an 8+ charged ring and bears up to 24 positive charges in its co-constitution. The solid-state structures, determined by single-crystal X-ray crystallography, show that the two mechanical bonds in the [3]catenane result in the nano-confinement of 12 positive charges within a volume of less than 1.65 nm3, corresponding to 7.3 positive charges per nm3, whereas the radial [5]catenane has a charge density of 6.0 positive charges per nm3 and a molecular length of up to 3.7 nm. The template-directed strategy for constructing mechanically interlocked molecules, consisting of multiply charged rings, represents a departure in chemistry tantamount to producing a blueprint for exploring a novel class of organic molecules that boast large numbers and high densities of like charges. It has been known from time immemorial that like charges repel each other according to Coulomb's law. At the molecular level, this law holds good for ions of the same charge, which are unlikely to become tightly packed without the existence of solvents or counterions to temper repulsive interactions. In this research, we demonstrate that stable macrocyclic polycations can be confined in nanospace by introducing mechanical bonds. Generally, the formation of mechanical bonds requires template-directed protocols that balance the entropy loss from mechanical interlocking with the enthalpic gain resulting from molecular templation. We appeal, under reducing conditions, to radically based templation that is negated upon oxidation, leading to the production of enthalpically and entropically demanding [n]catenanes. The possibility of switching off templating interactions post-synthetically paves the way for the isolation of organic compounds with unprecedented high densities of like charges. Notwithstanding the many synthetic protocols for producing mechanically interlocked molecules (MIMs)—whose unique structural features raise the prospect of their potential use in areas as diverse as molecular electronics and artificial molecular machines—mechanically interlocked structures containing multiple components, which repel each other, remain undocumented for the most part. Herein, we report the efficient syntheses of a 12+ charged [3]catenane and a 24+ charged radial [5]catenane, which consist of three and five mechanically interlocked polycationic rings, respectively, despite their mutual Coulombic repulsion.