Influence of cis-protecting groups toward ligand exchange reactions in polynuclear Pd(II)-based coordination cages

Polynuclear self-assembly molecules of general formula [{Pd(en)}x(ligand)y](NO3)2x (A) undergo ligand exchange reaction when heated in DMSO. A mixture of [Pdm(ligand)n](NO3)2m (B) and [Pd(en)2](NO3)2 (C) is generated in this process. The binuclear compound (A) containing a bidentate, non-chelating ligand 1,4-bis(4′-pyridylmethyl)-2,3,5,6-tetrafluorobenzene, is subjected to ligand exchange where upon the compound (A) remains in a dynamic equilibrium with the mixture of ensuing (B) and (C). Combination of separately prepared (B) and (C) also generates (A), thus equilibrium of (A) with (B) and (C) is again observed. We predict [{Pd(bpy)}x(ligand)y](NO3)2x (A′) where 2,2′-bipyridyl (bpy) is the cis-protecting group would not probably undergo ligand exchange. The idea was conceived from the fact that (bpy) is more rigid compared to (en) moreover one of the expected products in the event of ligand exchange [Pd(bpy)2](NO3)2 (C′) is not really very stable unlike (C). In fact, when (A′) is heated in DMSO no ligand exchange is observed at all. More interestingly combination of (B) and (C′) generated (A′) smoothly. Mononuclear complexes obtained from the ligand 4-phenylpyridine are also used for similar study for comparison. It is suggested that (bpy) could serve as a better cis-protecting group for Pd(II)-based self-assembly coordination cage compounds particularly when dissolution of the assemblies in polar solvents and heating of the resultant solution is required.

Graphical abstractSelf-assembly [{Pd(en)}x(ligand)y](NO3)2x (A) used in this work undergoes ligand exchange to give a dynamic equilibrium of (A) with a mixture of [Pdm(ligand)n](NO3)2m (B) and [Pd(en)2](NO3)2 (C) when heated in DMSO. However, when [{Pd(bpy)}x(ligand)y](NO3)2x (A′) is heated in DMSO the otherwise expected mixture of [Pdm(ligand)n](NO3)2m (B) and [Pd(bpy)2](NO3)2 (C′) is not detected at all but (A′) remains intact. Moreover, combination of (B) and (C′) generates (A′) very smoothly. The complex [Pd(en)(bpy)](NO3)2 also remains intact in the event of heating in DMSO.Figure optionsDownload full-size imageDownload as PowerPoint slide