Toward the Upgrading of Hydrocarbons: Synthesis, Characterization, and Reactivity of Platinum and Palladium Complexes

Abstract

Herein, hydroxyl-dimers of platinum and palladium supported by diimine ligands have been prepared and studied. It was found that they are capable of activating allylic, benzylic, and aromatic carbon-hydrogen bonds. The kinetics of the C-H activation of cyclohexene with the platinum system and of indene with the palladium system were studied. In each case the rate-limiting step was found to be associative substitution. The catalytic dehydrogenation of cyclohexene to benzene with the palladium hydroxyl-dimer was investigated. The chemical oxidation of the η3-organometallic products synthesized via C-H activation of cyclohexene and indene with platinum and palladium, respectively, was explored. Treatment of these products with strong halogenating oxidants resulted in the liberation of halogenated substrates and bridging halide metal complexes. Uncharged platinum and palladium polypyrazoleborate complexes were synthesized and characterized. The kinetics of the degenerative ligand exchange of DMSO with palladium methyl DMSO complexes ligated with a polypyrazoleborate or a diimine ligand were studied and it was found that the associative substitution mechanism through which the diimine ligated complexes proceeds was unavailable to the polypyrazoleborate palladium complex. In the polypyrazoleborate system ligand exchange proceeded through a dissociative mechanism. The C-H activation substrate scope of the platinum and palladium hydroxyl-dimers was further explored. The diimine platinum hydroxyl-dimer was found to activate an allylic C-H bond in cyclopentene and form an η3-cyclopentenyl complex when heated with acid and dilute stoichiometric amounts of cyclopentene. Conversely, dehydrogenation of cyclopentene to a platinum-bound η5-cyclopentadiene resulted from the treatment of the platinum hydroxyl-dimer with super-stoichiometric amounts of cyclopentene at room temperature. This difference in reactivity with the same substrate may be due to the relative reactivities of allylic versus homoallylic C-H bonds. Further evidence of a homoallylic C-H activation mechanism was demonstrated by treating the diimine platinum hydroxyl-dimer with neo-hexene to form a cyclometalated neo-hexenyl platinum complex. The diimine palladium hydroxyl-dimer was also found to catalytically oligomerize and isomerize olefins. Furthermore, the catalytic activity was supported by the presence of oxygen and stable in the presence of water. Most likely the oligomerization proceeded through a palladium hydride that can decompose to palladium(0) complex, which can be reoxidized by oxygen. The photochemical oxidation of platinum complexes with Ru(bipy)_3^(2+) was also investigated. Finally, the coordination chemistry of the tris(triphenylphosphino)silyl ligand was explored with cobalt, ruthenium, nickel, and platinum.