(482b) Modifying Palladium(II) Oxidation Catalyst Performance through the Introduction of a Novel Multifunctional Siloxane Framework

In the homogeneous Pd(OAc)2/pyridine-catalyzed aerobic oxidation of alcohols to carbonyl compounds, the Pd complex is known to be stabilized against agglomeration to Pd(0) particles by pyridine, which also attenuates activity through strong metal: ligand interactions. These competing interaction result in an inverse dependence of activity on concentration of pyridine. By incorporating pyridyl groups into a soluble, well-defined bidentate siloxane ligand, we seek to control the binding and catalytic properties through the sequence of organic functionality in the siloxane oligomer.

The bidentate bis(pyridyl)siloxane oligomer ligands were synthesized from a novel series of both meta-and para-pyridylsilanol, which were condensed with the appropriate organochlorosilanes to give stable products with controlled oligomer structure. Depending on system concentration and whether excess ligand was present, these flexible, bidentate ligands formed different distributions of monomeric or polymeric ring and chain Pd complexes species. The speciation was analyzed with 1H NMR, and it was found that more dilute conditions favored the formation of smaller ring species. For bis(meta-pyridyl)methylsiloxane oligomers greater than 5 Si-O units, monomeric bidentate complexes were predominant. The distribution of complex species could be explained with models based on statistics of ring-chain equilibrium.

The dependence of catalytic activity on ligand/Pd ratio depended on the siloxane and the chain length. Shorter bis(meta-pyridyl)siloxanes (less than 4 Si units) showed the typical inverse concentration dependence. The dependence almost disappeared for longer ligands. On the contrary, for bis(para-pyridyl)methylsiloxanes, the characteristic inverse dependence for activity on py:Pd remained even at oligomer lengths of 8 Si-O units. This difference in activity between meta and para isomers can be explained by their relative binding affinities to palladium with meta isomer ligands binding much stronger to Pd(OAc)2 than the corresponding para isomers, though both being greater than pyridine for longer oligomers. For both isomers, higher binding affinity for palladium centers than pyridine attenuated the aggregation of Pd(0) creating a more stable catalyst. The results from density functional theory can be used to explain the binding characteristics of this novel class of ligands. They showed that shorter chains could not form monomeric rings due to ring strain, as well as results of experiments of displacement of siloxane ligands by pyridine, which measured the relative binding constants of these ligands to Pd. Overall, we have shown that multiple functional moieties can interaction cooperatively to influence overall catalytic performance. The advantage of the described catalysts is that introduction of multiple organic functionalities is easily accomplished by manipulation of the siloxane backbone. Another potential application lies in the preparation of multifunctional silica surfaces capable of selectively binding and orienting transition metal centers, giving stable and well-defined catalytic surface sites.