Wide variability in the stability of Pd₆L_x-type coordination cages

The relative stability of hexanuclear Pd-based coordination cages has been investigated. For this purpose, we have synthesized eight cages of type [Pd₆L₁₂]¹²⁺ or [Pd₆L₈]¹²⁺. Six of these cages have been described before, and two are reported here for the first time, including crystallographic analyses. The stability of the cages was investigated by subjecting cage solutions in DMSO-d₆ or CD₃CN to different amounts of pyridine. The cage degradation was monitored by ¹H NMR. Our findings reveal significant variability in cage stability. The relative thermodynamic stability of the cages varies by nearly two orders of magnitude and the kinetic stability by more than four orders of magnitude. The stability of the cages is primarily governed by the nature of the donor group, with denticity playing a subordinate role. The high kinetic stability conferred by imidazole-based ligands provided access to a rare octanuclear cage of the type [Pd₈L₁₆]¹⁶⁺, featuring a square antiprismatic geometry.