Jean de Montmollin

Burnside's lemma was employed to enumerate the potential isomers of cages based on square planar metal complexes and low-symmetry ligands.

Coordination cages based on heteroditopic N-donor ligands can potentially form isomers, which differ in the relative orientation of the ligands. We have explored the construction of octahedral cages using small low-symmetry N-donor ligands (L). When the ligands were combined with [Pd(CH3CN)4](BF4)2, hexanuclear Pd6L12-type cages were obtained as a mixture of isomers, as evidenced by NMR spectroscopy and single-crystal X-ray crystallography. Attempts to resolve the isomers by cyclic ion mobility-mass spectrometry (cIM-MS) are described. Structurally defined cages of the general formula [Pt2Pd4Cl8L8](BF4)4 were formed in kinetically controlled reactions of homoleptic [PtL4](BF4)2 complexes with PdCl2(CH3CN)2. Crystallographic analyses of the products have revealed antipodal Pt2+ centers, the pyridine or pyrimidine donor groups of the bridging ligands being exclusively bound to the four equatorial PdCl2 complexes.

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.

The combination of three structurally distinct dipyridyl ligands with Pd ₂₊ results in the formation of a heteroleptic [Pd ₄ L ₂ L′ ₂ L′′ ₄ ] ₈₊ cage, which could be characterized by single crystal X-ray crystallography.