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

Abstract

Self-assembly of naked PdII 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 Pd4L8 (L= L1 or L2) tetrahedral cages and their homologous Pd3L6 (L= L1 or L2) double-walled triangular macrocycles. The resulting assemblies exhibited solvent, temperature, and counteranion induced dynamic equilibrium. Treatment of L1 with Pd(BF4)2 in acetonitrile (ACN) resulted in selective formation of a tetrahedral cage [Pd4(L1)8](BF4)8 (1a), which is in dynamic equilibrium with its homologue triangle [Pd3(L1)6](BF4)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 [Pd4(L2)8](BF4)8 (3a) and triangle [Pd3(L2)6](BF4)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 1H NMR. The equilibrium constant K = ([Pd4L8]3/[Pd3L6]4) was calculated at each experimental temperature and fitted with the Van't Hoff equation to determine the standard enthalpy (ΔH°) and entropy (ΔS°) associated with the interconversion of the double-walled triangle to tetrahedral cage. The thermodynamic feasibility of structural interconversion was analyzed from the change in ΔG°, which suggests favorable conversion of Pd3L6 triangle to Pd4L8 cage at elevated temperature for L1 in DMSO and L2 in ACN. Interestingly, similar self-assembly reactions of L1 and L2 with Pd(NO3)2 instead of Pd(BF4)2 resulted in selective formation of a tetrahedral cage [Pd4(L1)8](NO3)8 (1b) and double-walled triangle [Pd3(L2)6](NO3)6 (4b), respectively.