Solvent and Counteranion Assisted Dynamic Self-Assembly of Molecular Triangles and Tetrahedral Cages.

Self-assembly of naked Pd II ions separately with newly designed bis(3-pyridyl)benzothiadiazole ( L1 ) and bis(3-pyridyl)thiazolo[5,4- d ]thiazole ( L2 ) donors separately, under varying experimental conditions, yielded Pd 4 L 8 (L= L1 or L2 ) tetrahedral cages and their homologous Pd 3 L 6 (L= L1 or L2 ) double-walled triangular macrocycles. The resulting assemblies exhibited solvent, temperature, and counteranion induced dynamic equilibrium. Treatment of L1 with Pd(BF 4 ) 2 in acetonitrile (ACN) resulted in selective formation of a tetrahedral cage [Pd 4 ( L1 ) 8 ](BF 4 ) 8 ( 1a ), which is in dynamic equilibrium with its homologue triangle [Pd 3 ( L1 ) 6 ](BF 4 ) 6 ( 2a ) in dimethyl sulfoxide (DMSO). On the other hand, similar self-assembly using L2 instead of L1 yielded an equilibrium mixture of tetrahedral cage [Pd 4 ( L2 ) 8 ](BF 4 ) 8 ( 3a ) and triangle [Pd 3 ( L2 ) 6 ](BF 4 ) 6 ( 4a ) forms in both ACN and DMSO. The assembles were characterized by multinuclear NMR and ESI-MS while the structure of the tetrahedral cage ( 1a ) was determined by single crystal X-ray diffraction. Existence of a dynamic equilibrium between the assemblies in solution has been investigated via variable temperature 1 H NMR. The equilibrium constant K = ([Pd 4 L 8 ] 3 /[Pd 3 L 6 ] 4 ) was calculated at each experimental temperature and fitted with the Van't Hoff equation to determine the standard enthalpy (Δ H °) and entropy (Δ S °) associated with the interconversion of the double-walled triangle to tetrahedral cage. The thermodynamic feasibility of structural interconversion was analyzed from the change in Δ G °, which suggests favorable conversion of Pd 3 L 6 triangle to Pd 4 L 8 cage at elevated temperature for L1 in DMSO and L2 in ACN. Interestingly, similar self-assembly reactions of L1 and L2 with Pd(NO 3 ) 2 instead of Pd(BF 4 ) 2 resulted in selective formation of a tetrahedral cage [Pd 4 ( L1 ) 8 ](NO 3 ) 8 ( 1b ) and double-walled triangle [Pd 3 ( L2 ) 6 ](NO 3 ) 6 ( 4b ), respectively.