Self-assembled palladium(ii) “click” cages: synthesis, structural modification and stability

Abstract

Readily synthesised and functionalised di-1,2,3-triazole “click” ligands are shown to self-assemble into coordinatively saturated, quadruply stranded helical [Pd₂L₄](BF₄)₄ cages with Pd(II) ions. The cages have been fully characterised by elemental analysis, HR-ESMS, IR, ¹H, ¹³C and DOSY NMR, DFT calculations, and in one case by X-ray crystallography. By exploiting the CuAAC “click” reaction we were able to rapidly generate a small family of di-1,2,3-triazole ligands with different core spacer units and peripheral substituents and examine how these structural modifications affected the formation of the [Pd₂L₄](BF₄)₄ cages. The use of both flexible (1,3-propyl) and rigid (1,3-phenyl) core spacer units led to the formation of discrete [Pd₂L₄](BF₄)₄ cage complexes. However, when the spacer unit of the di-1,2,3-triazole ligand was a 1,4-substituted-phenyl group steric interactions led to the formation of an oligomeric/polymeric species. By keeping the 1,3-phenyl core spacer constant the effect of altering the “click” ligands’ peripheral substituents was also examined. It was shown that ligands with alkyl, phenyl, electron-rich and electron-poor benzyl substituents all quantitatively formed [Pd₂L₄](BF₄)₄ cage complexes. The results suggest that a wide range of functionalised palladium(II) “click” cages could be rapidly generated. These novel molecules may potentially find uses in catalysis, molecular recognition and drug delivery.