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Synthesis and Catalytic Activities of Nickel Complexes Bearing Flexible Tridentate Ligands

Nambukara Wellala, Nadeesha P
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1491561548324255

Abstract Details

2017, PhD, University of Cincinnati, Arts and Sciences: Chemistry.
This dissertation research focuses on the development of tridentate ligand supported nickel complexes for both catalytic and stoichiometric bond activation reactions. One of the main objectives of this research is to understand how the flexibility of ligands impacts the reactivity of transition-metal complexes. To this end, three different ligands with varying degree of flexibility have been studied. Chelating ligands have been used to enhance the stability of transition-metal complexes because compared to monodentate ligands these ligands are less likely to dissociate from the metal. Guided by this hypothesis, a chelating secondary phosphine oxide (SPO) ligand has been synthesized and shown to act as either a bidentate or a pseudo tridentate ligand depending on the electronic demand and geometry at the nickel center. Nickel(0) species supported by this SPO is a more effective catalyst than Ni(0)–Ph2P(O)H for cross-coupling of aryl iodides with aryl thiols. Both electron-donating and electron-withdrawing groups including OMe, CF3, CN, pyridyl or ester groups are tolerated in this catalytic system. However, low isolated yields are observed with sterically crowded aryl iodides and aryl thiols. Nickel complexes bearing a tridentate pyridine bis-amide ligand have also been studied for carbon-sulfur cross-coupling reaction of aryl iodide with aryl thiol. One of the nickel chloride complexes in which (o-Me)2C6H3 group is the substituent on the amide nitrogens has been used as the catalyst. Harsh reaction conditions including high catalyst loadings, high temperatures and long reaction times are required to obtain nearly quantitative yields. Two cationic nickel hydride complexes containing a PNHPiPr ligand have been synthesized. For the cationic hydride with Br- as the counter anion, NaBH4 is the main hydride source whereas alcohol and residual water are the proton sources for NH hydrogen. These cationic hydrides have been studied for the reduction of CO2. The rate of CO2 insertion is enhanced by using polar solvents or salt additives and LiBr in particular. Protonation of neutral nickel bromide, phenyl and methyl complexes bearing deprotonated PNHPiPr ligand have been investigated with different acids. These complexes are all protonated at the amido site. Nickel methyl complex can be fully converted to cationic nickel chloride complex in the presence of three equivalents of HCl•Et2O. Cationic nickel hydride complexes supported by PNHPiPr ligand undergo protonation at the hydride ligand. Neutral nickel bromide, phenyl and methyl complexes containing deprotonated PNHPiPr ligand and cationic nickel hydride complexes bearing PNHPiPr ligand are sensitive to molecular oxygen. The reaction of neutral nickel bromide complex with O2 forms the PNHPiPr ligated cationic nickel bromide complex as the major product. Similarly, cationic nickel phenyl complex supported by PNHPiPr ligand is the major product generated from the neutral nickel phenyl complex and molecular oxygen. Analysis of bond lengths and angles suggests that the aforementioned reactivity may be driven by ligand strain present in the neutral nickel complexes. Preliminary data reveal the presence of side-on O–O bond in the products generated from the reaction of cationic nickel hydrides with O2.
Hairong Guan, Ph.D. (Committee Chair)
William Connick, Ph.D. (Committee Member)
Allan Pinhas, Ph.D. (Committee Member)
208 p.
Chemistry; Inorganic Chemistry; Organic Chemistry
Nickel Complexes; Pincer Complexes; CO2 Reduction; C S Cross Coupling Reactions; Hydride Complexes; Protonation

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Citations

  • Nambukara Wellala, N. P. (2017). Synthesis and Catalytic Activities of Nickel Complexes Bearing Flexible Tridentate Ligands [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1491561548324255

    APA Style (7th edition)

  • Nambukara Wellala, Nadeesha. Synthesis and Catalytic Activities of Nickel Complexes Bearing Flexible Tridentate Ligands. 2017. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1491561548324255.

    MLA Style (8th edition)

  • Nambukara Wellala, Nadeesha. "Synthesis and Catalytic Activities of Nickel Complexes Bearing Flexible Tridentate Ligands." Doctoral dissertation, University of Cincinnati, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1491561548324255

    Chicago Manual of Style (17th edition)

ucin1491561548324255
263
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This open access ETD is published by University of Cincinnati and OhioLINK.