A Reduced-Symmetry Pd2L4 Cage from a Heterotopic Dipyridyl Ligand

Two dipyridyl ligands, L-3,L-3 and L-3,L-4, 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 (C-2h) lantern-type cage is formed in one instance, [Pd-2(L-3,L-3)(4)](4+), while the isomeric ligand yields a larger double-walled square complex, [Pd-4(L-3,L-4)(8)](8+). [Pd-2(L-3,L-3)(4)](NO3)(4) was isolated in crystalline form revealing two anions within the interior of the C-2h-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,L-4)(8)](NO3)(8 )being isolated as a crystalline solid. The double-walled square complex has a greater Pd<middle dot><middle dot><middle dot>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.