Controlled Formation of Heteroleptic [Pd2(La)2(Lb)2]4+ Cages

Metallo­supra­molecular architectures are beginning to be exploited for a range of applications including drug delivery, catalysis, molecular recognition, and sensing. For the most part these achievements have been made with high-symmetry metallo­supra­molecular architectures composed of just one type of ligand and metal ion. Recently, considerable efforts have been made to generate metallo­supra­molecular architectures that are made up of multiple different ligands and/or metals ions in order to obtain more complex systems with new properties. Herein we show that the addition of an electron-rich 2-amino-substituted tripyridyl ligand, 2,6-bis­(pyridin-3-ylethynyl)­pyridine (2A-tripy), to a solution of the [Pd2(tripy)4]4+ cage resulted in the clean generation of a hetero­leptic [Pd2(tripy)2­(2A-tripy)2]4+ architecture. The formation of the mixed-ligand cage [Pd2(tripy)2­(2A-tripy)2]4+ was confirmed using 1H NMR spectroscopy, diffusion-ordered spectroscopy, and rotating-frame nuclear Overhauser effect spectroscopy and high-resolution electrospray ionization mass spectrometry. Density functional theory calculations suggested the cis isomer was more stable that the trans isomer. Additionally, the calculations indicated that the hetero­leptic palladium­(II) cages are kinetically metastable intermediates rather than the thermodynamic product of the reaction. Competition experiments supported that finding and showed the cages are long-lived in solution at room temperature. Finally, it was shown that the addition of 2A-tripy to a range of preformed [Pd2(Ltripy)4]4+ cages cleanly generated the mixed-ligand systems. Three other systems displaying different exo and endo functionalities within the cage assembly were generated, suggesting that this method could be applied to synthesize a range of highly functionalized hetero­leptic cis-[Pd2(La)2­(Lb)2]4+ cages.