Phosphoranimide-supported Nickel Clusters for Hydrotreatment

• Author / Creator

  Zhao, Ting

• Fortuitous characterization of the cationic nickel cluster [Ni4(NPtBu3)4][Li3Br4(Et2O)3] led to the rational syntheses and characterization of the mixed-valence low-coordinate cationic nickel cluster: [Ni4(NPtBu3)4]BPh4 and [Ni4(NPtBu3)4]PF6, and subsequently the preparation of the anionic cluster Na[Ni4(NPtBu3)4]. The redox chemistry of the nickel series, [Ni4(NPtBu3)4]BPh4, [Ni(NPtBu3)]4, and Na[Ni4(NPtBu3)4] was investigated, with and without the presence of alkali metal cation. Oxidation of the cobalt analog [Co(NPtBu3)]4 results in the formation of dimetallic mixed-valence cluster [Co2(NPtBu3)4]PF6, which is likely due to the Na+ contamination in the starting material. Synthesis and characterization of the pure tetrametallic [Co(NPtBu3)]4 remains a challenge. A new preparation of the neutral cluster [Ni(NPtBu3)]4 was developed and turned out to be a key to address the reproducibility issue previously observed. The anionic cluster Na[Ni4(NPtBu3)4], an impurity present from the previous procedure, is the actual catalyst for the hydrogenation of diphenylacetylene. The hydrogenation investigation of the nickel series, [Ni4(NPtBu3)4]BPh4, [Ni(NPtBu3)]4, and Na[Ni4(NPtBu3)4] suggests that anionic cluster is a more active catalyst than the neutral cluster, which is more active than the cationic cluster. As the temperature increases, higher activities were observed for all three in the series, although much of that must be attributed to the catalysts transforming from homogeneous to heterogeneous. The hydrosilylation and hydrogenolysis of carbonyl compounds with the tetranickel series were investigated. A preliminary study of the hydrosilylation with impure precatalyst is mainly reproducible using the purified neutral catalyst, delivering good to excellent yields of hydrosilylation product using diphenylsilane as a selective reductant. Complete silylative deoxygenation of carbonyl compound was also investigated, using a more reactive silane and higher reaction temperatures to drive the conversion. In general, aldehydes show higher reactivity than ketones, and aromatic substrates show higher reactivity than aliphatic substrates, at elevated temperature (60–80 C). A high conversion (~ 90%) to deoxygenation products was achieved for cyclohexenone at 160 C, albeit at long reaction time and with the catalyst in uncharacterized heterogeneous form. In addition, several allyl-capped nickel clusters, [Ni(η3-allyl)(NPEt3)]2, [Ni(η3-allyl)(NPPh3)]2, and [Ni(η3-allyl)(NPtBu3)]2, were synthesized and characterized, as the second-generation of phosphoranimide-bridged first-row transition metal clusters. This new set of compounds is important for developing new hydrotreatment catalysts because they are diamagnetic, coordinatively saturated, thermally stable, and electronically tunable. The redox investigation of [Ni(η3-allyl)(NPEt3)]2 led to the discovery of the allyl-capped trimetallic nickel cluster, [Ni3(η3-allyl)3(µ3-NPEt3)2]PF6, and the heterotrimetallic cluster, [(η3-allyl)Ni]2CoCl2(μ3-NPEt3)2. These clusters provide a general method for preparing heteropolymetallic clusters in the future, which could potentially be powerful precatalysts for hydrotreatment. Finally, the hydrodesulfurization (HDS) activity of [Ni(NPtBu3)]4 and [Ni(η3-allyl)(NPEt3)]2 is discussed, together with the non-innocent role of potassium hydride and the specific promoting effect of potassium cation. The promising results obtained in these studies validate the proposal that structurally-engineered first-row transition metal compounds are capable of catalytic HDS under relatively mild conditions and warrant further investigation for the rational design of new phosphoranimide-bridged first-row transition metal clusters and the development of heterogeneous catalysts.

• Subjects / Keywords

  • Hydrodesulfurization
  • Low-coordinate nickel
  • Phosphorane imide

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3M32NR4W

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.