Chemistry of PNP Bis(phosphide) Pincer Ligands and Palladium(II) Dimers as Robust, Versatile Precatalysts for Olefin Isomerization, Oligomerization, and Oxidation

Citation

Winston, Matthew Steven (2013) Chemistry of PNP Bis(phosphide) Pincer Ligands and Palladium(II) Dimers as Robust, Versatile Precatalysts for Olefin Isomerization, Oligomerization, and Oxidation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/GES9-N146. https://resolver.caltech.edu/CaltechTHESIS:10052012-153447918

Abstract

The first half of this thesis details the synthesis and coordination chemistry of a very unusual pyridine-linked bis(secondary phosphine) pincer ligand system. Despite the highly nucleophilic phosphide donors, this dianionic system is an unexpectedly poor pincer ligand. Crystallographic and DFT studies reveal that both phosphide-metal σ- and π-bonding is compromised by long metal-phosphorus bonds, which result in significant distortions to the chelate ring. The neutral ligand coordinates readily κ² (via phosphines) to late metals, such as palladium(II), affording P-chirogenic diastereomers. Crystallographic and spectroscopic analysis of a series of palladium(II) dihalides stabilized by this bis(phosphine) indicate that one diastereomer is enthalpically favored, while the other more structurally versatile diastereomer is favored entropically. There is also evidence of an interesting phosphine epimerization pathway assisted by the non-coordinated pyridine ring.

Ethylene polymerization and ethylene/1-hexene copolymerization activities of several zirconium(IV) and vanadium(III) polymerization precatalysts supported by heterocycle-linked bis(phenolate) ligands are also discussed. Activities as high as 106 g PE/(mol x h) were observed, but only the vanadium catalyst incorporates comonomer, albeit with low efficiency (<1 mol%).

Finally, catalytic applications of air- and water-tolerant bis(μ-hydroxy) palladium(II) dimers have been investigated. Mechanistic studies show that this precatalyst can oxygenate olefins via a Wacker-type mechanism upon dimer dissociation. In the absence of stoichiometric oxidant, the resulting palladium(II) hydride intermediate can then isomerize and oligomerize olefins with turnover numbers at room temperature as high as 2100/h and 600/h, respectively. We also show that the catalyst is insensitive to water and air, so that olefin isomerization and oligomerization can be carried out on the benchtop in the absence of activators. In the presence of excess tert-butylhydroperoxide, Wacker-type behavior is favored, and neither isomerization nor oligomerization is observed. These dimers can also catalyze the aerobic dehydrogenation of cyclohexene to benzene with relatively low turnover numbers (1/h). Nevertheless, mechanistic studies indicate a C-H activation/β-hydride elimination sequence that does not involve an allylic-activated species.

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