Stability of [2]Pseudorotaxanes Templated Through Second-Sphere Coordination

A series of nine trans-dichlorobis(pyridine)palladium(II) complexes were prepared (2a-i), containing different 4-substituted pyridine co-ligands. The association constants between these Pd-complexes and 1 were measured, and their values plotted against corresponding σp° values. Measurement of the hydrogen bond acceptor capability of the chloride co-ligands revealed the presence of a linear free energy relationship between the electronic induction of a given 4-substituted pyridine co-ligand and the subsequent complexation strength in [2]pseudorotaxane formation. These trends also extended to trans-dibromobis(pyridine)palladium(II) (3a-e) and trans- dichlorobis(pyridine)platinum(II) complexes (5a-e) when plotted against σp° values. In addition, solid-state structures of three [2]pseudorotaxanes (1·2h, 1·2i, and 1·5e) were determined by single crystal X-ray diffraction further confirming the viability of this template in forming interpenetrated molecular architectures.