Structural deformation and host-guest properties of doubly-reduced cycloparaphenylenes, []CPPs ( = 6, 8, 10, and 12).

Chemical reduction of several cycloparaphenylenes (CPPs) ranging in size from [8]CPP to [12]CPP has been investigated with potassium metal in THF. The X-ray diffraction characterization of the resulting doubly-reduced []CPPs provided a unique series of carbon nanohoops with increasing dimensions and core flexibility for the first comprehensive structural analysis. The consequences of electron acquisition by a []CPP core have been analyzed in comparison with the neutral parents. The addition of two electrons to the cyclic carbon framework of []CPPs leads to the characteristic elliptic core distortion and facilitates the internal encapsulation of sizable cationic guests. Molecular and solid-state structure changes, alkali metal binding and unique size-dependent host abilities of the []CPP series with = 6-12 are discussed. This in-depth analysis opens new perspectives in supramolecular chemistry of []CPPs and promotes their applications in size-selective guest encapsulation and chemical separation.