Molecular catenation via metal-directed self-assembly and π-donor/π-acceptor interactions:: Efficient one-pot synthesis, characterization, and crystal structures of [3]catenanes based on Pd or Pt dinuclear metallocycles

Dinuclear square metallocycles 3a,b assemble spontaneously when M(en)(OTf)(2) (M = Pd, Pt) and a 4,4'-bipyridinium ligand are mixed in acetonitrile. Six new [3]catenanes were prepared in good yields by thermodynamically driven self-assembly reaction of molecular squares 3a,b and pi-complementary dioxoaryl cyclophanes. Single-crystal X-ray analyses of the [3]catenanes revealed the insertion of two aromatic units inside the metallocycle cavity. The structures are stabilized by means of a combination of pi-pi stacking, [C-H center dot center dot center dot pi] interactions, and [C-H center dot center dot center dot O] hydrogen bonds. [3]Catenane (DB24C8)(2)-(3a) showed in solid-state two external DB24C8 rings positioned over the Pd(en) corners, which are held in position by [N-H center dot center dot center dot O] hydrogen bonds. Furthermore, formation of catenane (DB24C8)(2)-(3a) can be switched off and on in a controllable manner by successive addition of KPF6 and 18-crown-6.