Use of an adjustable soft segment as an effective molecular design for crystal engineering of hydrogen-bonded tape motifs.

From all the 11 alkylsilylated guanosine and adenosine derivatives having different numbers and types of nonpolar and flexible alkylsilyl side chains, 13 crystals were obtained from appropriate solvents in spite of their low molecular symmetry, and their crystal structures were studied by X-ray crystallography and thermal analysis. In these crystals, a clear structural hierarchy was observed, and one-dimensional tape motifs were preferentially formed by multiple inter-base hydrogen bonds. The tape motifs were arranged in lamellar-like (L), herringbone (H) or widened lamella (WL) structures in the crystals. However, the alkylsilylated ribose unit adopted a variety of conformations with notable disorders at the alkylsilyl moiety. These results suggested that role of the adjustable and nonpolar alkylsilyl ribose unit was to provide cushioning or a filling effect as a molecular pad, which assisted the crystal packing of the robust tape motifs. The packing mode of the tape motifs could be understood from the size instead of the shape of the adjustable alkylsilyl ribose moiety, offering a novel approach to their crystal engineering.