Homo- and Heteromultinuclear Pseudocapsules Assembled from Calix[6]-mono-crown-5 and Alkali Metal Ions

Abstract

Theformation of pseudocapsule type homo- and heteromultinuclearcomplexes of calix[6]-mono-crown-5 (H-4 L) encapsulating from 4 to 6 alkali metal ions is reported.H-4 L reacts with KOH to yield a hexanuclearpotassium-(I) complex [K-6(HL)(2)(CH3OH)(2)]center dot CHCl3 (1) inwhich two bowl-shaped tripotassium-(I) complex units are linked ina rim-to-rim fashion via the interligand C-H center dot center dot center dot pi interactions. In the same reaction condition, RbOH afforded a tetranuclearrubidium-(I) complex [Rb-4(H-2 L)(2)(CH3OH)(2)(mu-H2O)(2)]center dot 6CHCl(3) (2). In 2, again two bowl-shaped dirubidium-(I) complex units are held togetherby two bridging water molecules and C-H center dot center dot center dot pi interactions that act as a glue to generate such an elegant pseudocapsule.Interestingly, a mixture of KOH and RbOH yielded a heterotetranuclearcomplex [K2Rb2(H-2 L)(2)(CH3OH)(2)(mu-H2O)(2)]center dot 6CHCl(3) (3). Similarly, twoheterodinuclear bowl units [KRb-(H-2 L)] in 3 are held together by two bridging water molecules and C-H center dot center dot center dot pi interactions to form a heteromultinuclear pseudocapsule. In each heterodinuclearK(+)/Rb+ bowl unit of 3, Rb+ occupies the center of the crown loop while K+ locatesinside the calix rim. Consequently, the proposed host discriminatesnot only on the types and numbers of the metal ions but also on theirpositional preferences in forming pseudocapsules. Solution studiesby nuclear magnetic resonance and electrospray ionization-mass supportthe heterometallic (K+/Rb+) complexation byshowing the superior binding affinity of Rb+ over K+ toward the crown loop. These results demonstrate how themetal-driven pseudocapsules are formed and present a new perspectiveon the metallosupramolecules of the calixcrown scaffold. The isolation of the pseudocapsule typehomo- and heteromultinuclearcomplexes of A,D-bridged calix[6]-mono-crown-5 (H-4 L) encapsulating from 4 to 6 alkali metal ionsis reported.