Structural and Co-conformational Effects of Alkyne-Derived Subunits in Charged Donor−Acceptor [2]Catenanes

Four donor−acceptor [2]catenanes with cyclobis(paraquat-p-phenylene) (CBPQT4+) as the
π-electron-accepting cyclophane and 1,5-dioxynaphthalene (DNP)-containing macrocyclic polyethers as
π-electron donor rings have been synthesized under mild conditions, employing Cu+-catalyzed Huisgen
1,3-dipolar cycloaddition and Cu2+-mediated Eglinton coupling in the final steps of their syntheses. Oligoether
chains carrying terminal alkynes or azides were used as the key structural features in template-directed
cyclizations of [2]pseudorotaxanes to give the [2]catenanes. Both reactions proceed well with precursors
of appropriate oligoether chain lengths but fail when there are only three oxygen atoms in the oligoether
chains between the DNP units and the reactive functional groups. The solid-state structures of the donor−acceptor [2]catenanes confirm their mechanically interlocked nature, stabilized by [π···π], [C−H···π], and
[C−H···Ο] interactions, and point to secondary noncovalent contacts between 1,3-butadiyne and 1,2,3-triazole subunits and one of the bipyridinum units of the CBPQT4+ ring. These contacts are characterized
by the roughly parallel orientation of the inner bipyridinium ring system and the 1,2,3-triazole and 1,3-butadiyne units, as well as by the short [π···π] distances of 3.50 and 3.60 Å, respectively. Variable-temperature 1H NMR spectroscopy has been used to identify and quantify the barriers to the conformationally
and co-conformationally dynamic processes. The former include the rotations of the phenylene and the
bipyridinium ring systems around their substituent axes, whereas the latter are confined to the circumrotation
of the CBPQT4+ ring around the DNP binding site. The barriers for the three processes were found to be
successively 14.4, 14.5−17.5, and 13.1−15.8 kcal mol-1. Within the limitations of the small dataset
investigated, emergent trends in the barrier heights can be recognized:  the values decrease with the
increasing size of the π-electron-donating macrocycle and tend to be lower in the sterically less encumbered
series of [2]catenanes containing the 1,3-butadiyne moiety.