Macrocyclic Dinuclear Palladium Complex as Novel Doubly Threaded [3]Rotaxane Scaffold and its Application to Rotaxane Cross-linker.

A dinuclear Pd(II) complex possessing a cyclic ligand was developed as a novel doubly threaded [3]rotaxane scaffold and applied to a rotaxane cross-linker reagent. The dinuclear complex (PdMC)2 was prepared by one-step macrocyclization followed by double palladation reactions. 1H NMR and UV-vis measurements revealed the formation of a doubly threaded pseudo[3]rotaxane by the complexation of (PdMC)2 with 2 eq. of 2,6-disubstituted pyridine 3 through double metal coordination. The treatment of (PdMC)2 with 2 eq. of 4-vinylpyridine (VP) afforded a doubly threaded [3]rotaxane cross-linker (PdMC-VP)2. Radical co-polymerization of VP and t-butylstyrene in the presence of (PdMC-VP)2 afforded a stable rotaxane cross-linked polymer (RCP). An elastic RCP was also prepared using n-butyl acrylate as a monomer. The obtained RCPs exhibited higher swellabilities and higher mechanical toughness compared with the corresponding covalently cross-linked polymers.