A Rotaxane-like Cage-in-Ring Structural Motif for a Metallosupramolecular Pd₆ L₁₂ Aggregate

A BODIPY-based bis(3-pyridyl) ligand undergoes self-assembly upon coordination to tetravalent palladium(II) cations to form a Pd₆ L₁₂ metallosupramolecular assembly with an unprecedented structural motif that resembles a rotaxane-like cage-in-ring arrangement. In this assembly the ligand adopts two different conformations-a C-shaped one to form a Pd₂ L₄ cage which is located in the center of a Pd₄ L₈ ring consisting of ligands in a W-shaped conformation. This assembly is not mechanically interlocked in the sense of catenation but it is stabilized only by attractive π-stacking between the peripheral BODIPY chromophores and the ligands' skeleton as well as attractive van der Waals interactions between the long alkoxy chains. As a result, the co-arrangement of the two components leads to a very efficient space filling. The overall structure can be described as a rotaxane-like assembly with a metallosupramolecular cage forming the axle in a metallosupramolecular ring. This unique structural motif could be characterized via ESI mass spectrometry, NMR spectroscopy, and X-ray crystallography.