How guests settle and plan their escape route in a crystal - Structural metrics of solvation and desolvation for inorganic diols

Wheel-and-axle organic diols are host systems well known in the solid state. The introduction of a metal into the wheel-and-axle molecular frame opens the way for the design of materials with multiple functions, joining chemical and structural properties of the metal centre, with the steric and supramolecular attitudes of the organic matrix. This chapter treats the rationalization of host-guest properties of wheel-and-axle inorganic diols. A reversible dynamic reorganization between the pure host (apohost) and the host-guest phases requires two conditions: a low-cost structural rearrangement between two states represented by the close apohost and the final open host framework, and an easily accessible path of migration for the outcoming and incoming guest molecules. The first requirement is tackled by realizing a bistable system, capable of reversibly switching between networks based on host-host (self-mediated) and host-guest (guest-mediated) interactions. The use of a simple geometric model for the analysis of the basic interactions responsible of the cohesion in the structures of the apohosts and in those of the host-guest complexes shows that the 'Venetian-blind' mechanism allows switching between two networks with modest structural changes. The comparison of the crystal structures of the initial and final states of the guest removal for inorganic wheel-and-axle diols allows an understanding of the effects of guest inclusion on the host framework, and provides clues to postulate a mechanism for the structural rearrangements accompanying the process. The preferential mechanism of escape of guests from the crystal bulk of inorganic wheel-and-axle diols is studied by using the knowledge base Isostar for intermolecular interactions. Two preferential directions of coordination to guest molecules are discovered, related to two possible solvation sites within the guest-mediated networks. Application of the principles of structure correlation to supramolecular host-guest interactions indicates that, for inorganic wheel-and-axle diols, the preferential way for guests to escape from the crystal bulk corresponds to a migration of a guest along an array of alternate guest sites.