Kinetic control through oxidative locking in metallosupramolecular self-assembly

Abstract

Metallosupramolecular self-assembly has fast expanded as a field due to the possibility for relatively facile construction of large assemblies through reversible non-covalent interactions, compared to their more synthetically challenging covalent counterparts. Not least, it provides a fast and often quantitative route to the construction of three-dimensional structures with a cavity. These internal spaces have been shown to be effective for a variety of applications, including but not limited to catalysis, drug delivery, use as a noncovalent protecting group, a separations material etc. Thermodynamic processes, with the inherent advantages of atom efficient, high-yielding reactions, usually control these systems. However this can also be a double-edged sword, with these systems susceptible to changes to specific ambient conditions, and are thus often not kinetically stable. Herein, we report the expansion of a method utilising the one electron oxidation of high spin d7 cobalt(II) to low spin d6 cobalt(III) as a molecular locking mechanism as part of the assembly process. This allows for the formation of species under thermodynamic control in the CoII manifold, with the kinetic stability of these assemblies in the oxidised CoIII and has been used to synthesise a variety of tetrahedra and helicates with a series of bis-bidentate N,N’-chelate ligands, which have shown to be stable away from their thermodynamically preferred conditions for long periods of time. These containers can be made both water and organic soluble via counteranion exchange, and a series of guests have been shown to bind in the tetrahedral species. Alongside on going biological viability tests, these guests show promise for a variety of applications including fluorescent tagging and radio-diagnostic agents. Novel switching methods have also been demonstrated for transformations between these species going both energetically down and up hill.