Complexation of C-methyl pyrogallarene with small quaternary and tertiary alkyl ammonium cations

Abstract

Complexation properties of pyrogallarene 1 towards small quaternary and tertiary alkyl ammonium cations were studied in gas phase, solution and in solid state. In gas phase both dimeric capsules and monomeric 1 : 1 complexes of all cations 2a⁺–d⁺ are detected but only in the case of 2a⁺ is the abundance of the capsule form higher than the monomeric 1 : 1 complex. A similar trend is observed in NMR experiments, which reveal a favourable dimeric complex for 2a⁺ and a weaker dimeric complex for 2b⁺ but only monomeric complexes for 2c⁺ and 2d⁺. Also in solid state, 2a⁺ and 2b⁺ form capsules when crystallized from MeOH while 2c⁺ and 2d⁺ form dimeric 1 : 1 complexes. As a reference, hetero-conformational dimeric capsule EtOH@1₂ obtained from the crystallization without any guest, is reported. The capsule consists of two directly hydrogen-bonded pyrogallarenes 1, one in boat conformation and the other in crown conformation, and encloses a molecule of ethanol.