| Multifunctional oligomeric siloxanes are effective platforms for modification of the active site environment in transition metal complex catalysis. They accomplish this through binding the metal center with variable strength, projecting steric bulk around the active site in a controlled manner, and allowing for pre-association of the substrate through hydrogen-bond donor or acceptor sites. En route to preparation of these ligands, the basic chemistry of pyridylsilanols and pyridylsiloxanols was developed and extended, as was the convenient preparation, handling, and use of methylchlorosilane in the synthesis of organosilicon hydride halides. The selective oligomerization of these monomers into the desired structures was accomplished using a step growth process. The palladium-binding properties of the oligomeric bis(3-pyridyl)siloxanes were studied in detail by proton NMR spectroscopy, using pyridine as a monofunctional probe; experimental observations of the coordination oligomer distribution were modeled using a modified Jacobson-Stockmayer theory. The observed effective molarities of the bidentate coordination complexes were greatest for bis(3-pyridyl)siloxane ligands with six or seven siloxane spacer units between the ligand groups. The oligomer distribution was also found to be nearly independent of temperature, suggesting that entropic effects are dominant in these systems. The bifunctional ligands significantly affected the kinetics of aerobic oxidation of benzyl alcohol to benzaldehyde, with stronger intramolecular interaction leading to a weaker dependence of the initial rate on the ligand-to-metal ratio, as well as improvement in the stability of the catalyst under demanding reaction conditions. When a carboxylic acid group was introduced into the bis(pyridyl)siloxane structure, an equivalent of acetate was quantitatively displaced to form a distribution of cage-like structures, including a bicyclic coordination cage. These complexes exhibited an unusual hydrogen-bonding interaction with benzyl alcohol and 1,3-benzenedimethanol, evidence for a supramolecular substrate-catalyst interaction which may provide a basis for the attainment of substrate specificity using artificial active site environments. |
