The Crystalline Sponge (CS) method is an approach to obtain single-crystal X-ray structures of compounds, that are difficult or impossible to crystallize, for example because they are of too small a quantity, are not solid at ambient conditions or form amorphous precipitates. In some cases spectroscopic or spectrometric techniques cannot give definitive answers to the analytical problems so a crystal structure is needed. The CS method utilizes a metal-organic-framework (MOF) as a host for analyte molecules (guests), see Figure 1. Analytes can diffuse into the pores of the MOF from solution or the gas-phase and align themselves in an ordered fashion, which is predetermined by binding sites of the MOF cavity. This induced-fit molecular recognition process can be understood as crystallization inside a crystal.
This method was already successfully applied in determination of bioactive compounds like terpenes or steroids, natural products, metabolites, reactive chemical species like ozonides, or volatile compounds.