(120c) 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.