Host-Guest Complexes of C-Ethyl-2-methylresorcinarene and Aromatic N,N'-Dioxides.

The C-ethyl-2-methylresorcinarene (1) forms 1:1 in-cavity complexes with aromatic N,N'-dioxides, only if each of the aromatic rings has an N-O group. The structurally different C-shaped 2,2'-bipyridine N,N'-dioxide (2,2'-BiPyNO) and the linear rod-shaped 4,4'-bipyridine N,N'-dioxide (4,4'-BiPyNO) both form 1:1 in-cavity complexes with the host resorcinarene in C4v crown and C2v conformations, respectively. In the solid state, the host-guest interactions between the 1,3-bis(4-pyridyl)propane N,N'-dioxide (BiPyPNO) and the host 1 stabilize the unfavorable anti-gauche conformation. Contrary to the N,N'-dioxide guests, the mono-N-oxide guest, 4-phenylpyridine N-oxide (4PhPyNO), does not form an in-cavity complex in the solid state. The host-guest complexation and the relative guest affinities were studied through 1H NMR competition experiments in methanol. Single-crystal X-ray crystallography of the 1:1 complexes supports the proposed solution-state structures, also revealing strong hydrogen bonds between the host and the guests, not observed in solution owing to hydrogen/deuterium (H/D) exchange processes in methanol.