A Reduced-Symmetry Pd₂L₄ Cage from a Heterotopic Dipyridyl Ligand

Two dipyridyl ligands, L and L, have been used in combination with palladium(II) in the construction of metallosupramolecular species that show anion-dependent behavior in solution. A rare example of a low-symmetry (C₂) lantern-type cage is formed in one instance, [Pd₂(L_3,3)₄]⁴⁺, while the isomeric ligand yields a larger double-walled square complex, [Pd₄(L_3,4)₈]⁸⁺. [Pd₂(L_3,3)₄](NO₃)₄ was isolated in crystalline form revealing two anions within the interior of the C₂-symmetry cage. The cage itself is held together by hydrogen bonding between "head-to-tail" pairs of ligands that reinforces the symmetry generated by the ditopic ligands. In solution, the cage with NO₃^- has sharp ¹H nuclear magnetic resonance (NMR) signals at room temperature, while the BF₄^- analogue has broad signals that sharpen at higher temperatures or upon addition of (Bu₄N)(NO₃), highlighting the importance of the anion in templating or otherwise influencing self-assembly in solution. Altering the substitution position of one of the pyridyl rings yields a more "open" complex, with [Pd₄(L_3,4)₈](NO₃)₈ being isolated as a crystalline solid. The double-walled square complex has a greater Pd···Pd separation due to the increased angle that the pyridyl groups subtend at the core of the ligand. NMR spectroscopy and mass spectrometry studies suggest a single species in the presence of nitrate but multiple species with tetrafluoroborate.