Subtle Ligand Modification Inverts Guest Binding Hierarchy in M$^{II}$$_{8}$L$_{6}$ Supramolecular Cubes

Abstract

Zinc(II), a dimolybdenum(II) paddlewheel tetramine A, and 2-formylpyridine self-assembled to generate a cubic Zn$^{II}$$_{8}$(L$^{A}$)$_{6}$ assembly. The paddlewheel faces of this assembly exhibited two distinct conformations, whereas the analogous Fe$^{II}$$_{8}$(L$^{A}$)$_{6}$ framework displayed no such perturbation to its structure. This variation in behavior is attributed to the subtle difference in ligand rotational freedom between the Zn$^{II}$- and Fe$^{II}$-cornered cubes. The incorporation of a fluorinated Mo$^{II}$$_{2}$ paddlewheel, B, into analogous Zn$^{II}$$_{8}$(L$^{B}$)$_{6}$ and Fe$^{II}$$_{8}$(L$^{B}$)$_{6}$ structures resulted in changes to the rotational dynamics of the ligands. These differing dynamics perturbed the energies of the frontier orbitals of these structures, as determined through spectroscopic and electrochemical methods. The result of these perturbations was an inversion of the halide binding preference of the Zn$^{II}$$_{8}$(L$^{B}$)$_{6}$ host as compared to its Zn$^{II}$$_{8}$(L$^{A}$)$_{6}$ congener, whereas the Fe$^{II}$$_{8}$(L$^{B}$)$_{6}$ host maintained a similar binding hierarchy to Fe$^{II}$$_{8}$(L$^{A}$)$_{6}$.