Conformational Flexibility of Tetralactam Macrocycles and their Intermolecular Hydrogen-Bonding Patterns in the Solid State.
S.S. Zhu, M. Nieger, J. Daniels, T. Felder, I. Kossev, T. Schmidt, M. Sokolowski, F. Vögtle, C.A. Schalley – 2009
Flexible rigidity: Tetralactam macrocycles of the Hunter type bear a rigid scaffold (see space-filling representation), but can still widely adapt to the properties of a guest molecule inside their cavities. X-ray crystal structures of a series of differently substituted macrocycles reveal a remarkably broad variety of intermolecular hydrogen-bonding patterns organizing the macrocycles in the crystals in intriguingly different ways. Despite their rigid scaffold, tetralactam macrocycles (TLMs) display a remarkable degree of conformational flexibility, as revealed by analysis of the corresponding X-ray crystal structures. This flexibility is not limited to the rotatability of the TLM amide groups but also applies to the m-xylene rings, and it thus has a great impact on the overall shape of the macrocycle cavity. The conformational properties of the TLMs give rise to a broad variety of intermolecular hydrogen-bonding patterns, including infinite ladders, an interesting catemer motif, and short C[BOND]H⋅⋅⋅O[DOUBLE BOND]C hydrogen bonds. These results are in accord with previous theoretical calculations, support a structural model proposed earlier for an interpretation of scanning tunneling microscopy images, and substantially contribute to the understanding of the adaptability of macrocyclic scaffolds, which is crucial for guest binding or templated syntheses with TLMs.