Cesium Cation−π Interactions Stabilize Pyrogallol[4]arene Coordination Networks

Pyrogallol[4]arene macrocycles represent versatile precursors for the rational design of a rapidly expanding range of coordination networks, metal–organic nanocapsules, hierarchical nanocapsular assemblies, and elegant nanostructures. In this work, the solvothermal reaction of C-alkylpyrogallol[4]arene (alkyl = methyl, ethyl) in cesium-containing N,N-dimethylformamide (DMF) afforded coordination networks having a composition of [Cs2(C-methylpyrogallol[4]arene)(DMF)]·(DMF)2 and [Cs2(C-ethylpyrogallol[4]arene)(DMF)3], respectively. These extended coordination networks each display a duplicity of cesium environments featuring both (i) a "pseudocrown ether" environment involving O-coordination between Cs+ and the phenolic oxygens of four neighboring doubly deprotonated C-alkylpyrogallol[4]arenes, plus DMF ligation, and (ii) polyhapto η6-type coordination of a Cs+ center by the four phenyl rings of a C-alkylpyrogallol[4]arene receptor. The presence of both an exo coordination mode in which the cesium cation is bound to the "hard" phenolic/phenoxide upper rim pyrogallol[4]arene oxygens plus an endo one corresponding to cesium cation−π bonding by the "soft" aromatic rings defining the pyrogallol[4]arene cavity is unique among pyrogallol[4]arene chemistry. This duality is pivotal to the cross-linking arrangement which yields an extended network and suggests that a number of interesting architectures might be identified by further study of alkali metal-incorporated C-alkylpyrogallol[4]arene systems.