Synthesis, structural characterization and anion binding studies of palladium macrocycles with hydrogen-bonding ligands

The reactions between [Pd(P-P)(OTf)₂] (where P-P = dppp or dppf) and two different bipyridyl ligands (L₁ = 1,3-bis(4-pyridylmethyl)urea and L₂ = 1,3- bis(pyridinylmethyl)benzenedicarboxamide) containing hydrogen-bonding units have been studied. The X-ray crystal structures of three of these assemblies have been solved showing them to be the [2 + 2] metallo-macrocycles [Pd(P-P)(L_n)]₂(OTf)₄ [P-P = dppp, n = 1, (1); P-P = dppp, n = 2, (2); P-P = dppf, n = 1, (3)]. To confirm whether the dimeric assembly of one of these species (1) is retained in solution, several investigations have been carried out. ¹H NMR studies in DMSO and high resolution ESI mass spectrometry have shown that 1 is in equilibrium with a larger [3 + 3] metallo-macrocycle. The equilibrium between these two species can be modified by changing the temperature, concentration or solvent. Also, addition of certain anions (e.g. [H₂PO₄]^–) to the mixture shifts the equilibrium favoring the formation of the [2 + 2] metallo-macrocycle over the [3 + 3] (initially present in a larger proportion).