Solvates of Sildenafil Saccharinate. A New Host Material

Abstract

Sildenafil saccharinate (SS) forms solvates with a large number of solvents, which are included in distorted hexagonal cavities constituted with N(+)−H···N(-) and C−H···O interactions. All these solvates, except the methanolate, are isostructural and contain various host−guest O−H···O and C−H···O interactions and also an intramolecular N−H···O hydrogen bond in the sildenafil entity. The C−H···O bonds, in particular, are strong and numerous and play an important role in stabilizing the crystal packing at several levels. The solute−solvent ratio varies among the isostructural solvates because of differences in guest size. Solvents with smaller volume occupy the peripheries of the cavity, while larger solvents are located within the cavity. All the solvates are converted easily to the apohost upon heating, and this transformation is associated with a rearrangement of several C−H···O interactions in the structure. The apohost could also be prepared by grinding the two components, and this procedure led to mixtures of crystalline and amorphous forms. When this ground sample was heated at 120 °C, it was converted into the crystalline SS apohost. In this family of host−guest compounds the host framework is largely invariant and robust, and the guest molecules only play a bystander role in the packing. The totally different crystal structure of the methanol solvate argues in favor of using the term pseudopolymorph to describe modulations in the structural landscape of a host−guest system.