The Synthesis, Characterization and Study of Transition Metal Complexes for the Oxidation and Activation of Hydrocarbons

Citation

Blake, Robert Edward, Jr. (1996) The Synthesis, Characterization and Study of Transition Metal Complexes for the Oxidation and Activation of Hydrocarbons. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/s731-np77. https://resolver.caltech.edu/CaltechTHESIS:11052019-113518368

Abstract

The reaction of previously characterized ruthenium oxo complex, [L_OEtRu^V(O)(µ-O)]₂ with alcohol substrates was undertaken to elucidate the mechanism of the oxidation reaction. Unfortunately, an autocatalytic reaction between the organometallic product, [L_OEtRu^IV(OH)(µ-O)]₂ and the reactant alcohol, as well as catalyst decomposition made exact determinations of rate constants impossible. During the course of this investigation, the free acid form of the ligand, L_OEtH, was isolated as a viscous oil. Subsequent investigation of the reactions of [L_OEtRu^V(O)(µ-O)]₂ with other species such as acids, salts and bases demonstrated the inherent instability of the complex. In several cases, initial products were spectroscopically characterized, but isolation of pure compounds was not achieved.

Given the problems with the decomposition of ruthenium complexes which utilized the Klaui ligand, trimetaphosphate was studied as a potentially oxidation-resistant alternative for the development of oxidation catalysts. Attempts were made to prepare salts of the trimetaphosphate ligand which are soluble in nonpolar media and free of water of hydration. Coordination complexes of the trimetaphosphate anion and transition elements were synthesized. The mode of coordination and stability of the ligand was examined by infrared and visible spectroscopy. In all cases, the very weakly coordinating trimetaphosphate anion failed to displace other weakly associated ligands from the metal, failed to adopt the proper coordination geometry or was easily removed from the metal center by water.

Hexabenzyloxycyclotriphosphatriene was synthesized and thermally rearranged to 1,3,5 - tribenzyl - 2,4,6 tribenzyloxy - 2,4,6 trioxocyclotriphosphazane as per published procedures. Characterization of the rearranged product by NMR techniques revealed the previously undetermined stereochemistry of the product. This ligand precursor has been shown to react with trialkyl silyl chlorides, although the products have not been characterized.

Cp*₂Ta(=N^tBu)(THF)[B(C₆F₅)₄] (1) was synthesized according to the method developed previously in our group. A cationic analog to Bergman's Cp₂Zr(=N^tBu), the reactivity of this complex to hydrocarbon substrates was studied. Contrary to previous reports, this complex does not react with methane, but C-H activation reactions were observed for propyne and phenyl acetylene. In the propyne case, an initial mixture of the [2+2] and C-H activation products was driven to exclusively the C-H activation product thermally. An interesting intramolecular activation of a Cp* methyl group precludes much of the desired C-H activation chemistry. The steric demands of the active site was demonstrated by the observed reaction with ethylene, but the lack of reactivity towards propene. A very interesting dealkylation of the imido group was observed upon reaction with carbon dioxide, which is proposed to involve the intermediacy of a coordinated isocyanate. Cp*₂Ta(=N^tBu)(THF)[B(C₆F₅)₄] reacts as expected with water, HCl and dihydrogen, and reacts cleanly with methylene chloride to give Cp*₂Ta(NH^tBu)Cl[B(C₆F₅)₄]. Many of the new compounds have been crystallographically and spectroscopically characterized. The reactivity of this complex can be rationalized in terms of the presence of both electrophilic and nucleophilic sites in the same molecule.

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