Bis(pyridyl)siloxane oligomeric ligands for palladium(II) acetate: Synthesis and binding properties

The complexation of a series of new bis(meta-pyridyl)methylsiloxane ligands with palladium(II) acetate in dilute toluene-d₈ solution was studied by ¹H NMR at 233-363 K, measuring the binding affinity and ring-chain distribution as a function of ligand structure, temperature, and concentration. Significant differences in the ring distribution and Pd binding affinity were observed as a function of siloxane chain length, with hexa- and heptasiloxane spacers binding Pd most effectively. The ¹H NMR resonance of the singlet ortho aromatic proton was shifted unusually far downfield for the bidentate complex; the effect was strongest for the shortest chains and disappeared for chains with 10 or more siloxane units. Fitting the experimental data to a modified Jacobson-Stockmayer model demonstrated that the bidentate bis(pyridyl)siloxane complexes function as chelates in the concentration range 1-10 mM. Variable-temperature studies showed that all of the observed cyclic coordination oligomers are strainless. Therefore, entropic effects are implicated as the main cause of the observed trends in binding affinity, particularly the loss of conformational freedom of the siloxane chain upon binding.