Bis(pyridyl)siloxane Oligomeric Ligands for Palladium(II) Acetate: Synthesis and Binding Properties

Abstract

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. Theref...