A Discrete Self-Assembled Pd₁₂ Triangular Orthobicupola Cage and its Use for Intramolecular Cycloaddition

Water-soluble Pd₁₂ L₆ coordination cage TC-1 was synthesized by coordination-driven self-assembly of symmetrical tetrapyridyl donor L with 90° ditopic acceptor cis-[Pd(NO₃ )₂ (tmeda)] [tmeda=N,N,N',N'-tetramethylethane-1,2-diamine]. The Pd₁₂ L₆ coordination assembly is an uncommon example of a coordination cage having triangular orthobicupola-like geometry. It was characterized by multinuclear NMR spectroscopy, ESI-MS, and single-crystal X-ray diffraction. Self-assembly of a tetratopic donor with a cis-blocked 90° ditopic acceptor generally yields tri-/tetra-/hexagonal barrels or closed cubic cages. However, in the present case the donor and acceptor are arranged in an unusual fashion to generate an orthobicupola geometry in which two triangular cupola share a common irregular hexagonal face. The cage was used to perform intramolecular cycloaddition reactions of O-propargylated benzylidinebarbituric acid derivatives in nitromethane. Several penta-/tetracyclouracil derivatives were synthesized through cage-catalyzed [4+2] cycloaddition reactions in a concerted manner with good to high conversion under mild reaction conditions, whereas in the absence of cage TC-1 similar reactions led to lower conversion to the cyclized products in organic solvent. This approach is of particular importance compared to the literature reports on the synthesis of similar compounds under high-temperature reflux conditions with high catalyst loading.