Designing the Metal- and Substrate-Binding Properties of Tripodal Organosiloxane Ligands: The Role of Intramolecularity

Recently we have investigated the synthesis, metal binding, and catalytic properties of a novel class of tripodal organosiloxane ligands, with the ultimate goal of designing the environment around the metal active site to attain substrate specificity.  The key feature of these systems is the mutual and coordinated interaction between the various organic ligands, the transition metal, the siloxane oligomer, and the substrate(s).  Quantifying the magnitude of these intramolecular interactions is of great importance in designing improved cooperative ligands for transition metal ion catalysis.  Here we present the results of ¹H NMR studies of the coordination oligomer distribution for complexes of palladium(II) acetate and heterotrifunctional carboxybutyl(meta-pyridyl)siloxanes.  The various coordination oligomers were unambiguously identified by the response of their NMR resonances to changes in system dilution, temperature, and the concentrations of monofunctional probe molecules.  These experimental data were then fitted to a modified Jacobson-Stockmayer model of ring-chain equilibrium, which provided accurate effective molarity parameters for the various ring- and cage-closing equilibria.  The hydrogen-bonding interaction between alcohols or diols and the active sites was also investigated by ¹H NMR at 298 K, and revealed an interesting perturbation of the coordination oligomer distribution by the substrate molecules.  Finally, the kinetics of aerobic oxidation catalysis using alcohol and diol substrates was studied for these complexes.  The results emphasize the interconnectedness of the ligand-metal, substrate-ligand, and substrate-metal interactions, and the potential for manipulating these interactions using robust synthesis techniques and modeling of intramolecularity.