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Hydrogen Transfer Reaction Involving Nickel POCOP-Pincer Hydride Complexes

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2015, PhD, University of Cincinnati, Arts and Sciences: Chemistry.
The development of efficient and low-cost catalysts from Earth abundant metals with an objective to replace expensive and precious metal-based catalysts is important for the affordability of commodity chemicals in the future. Therefore, study of nickel based POCOP-pincer hydride complexes as catalysts is of interest due to the use of an abundant metal and the known benefits of the POCOP-pincer ligand such as rigid backbone, high thermal stability and easy to tune the electronic and steric properties. Extensive research of the structure and reactivity of these nickel POCOP-pincer hydride complexes is of great importance to the understanding and development of relatively inexpensive nickel-based catalysts. Progress has been made in three research projects with a unifying theme of hydrogen transfer either from or to nickel POCOP-pincer hydride complexes. Nickel POCOP-pincer hydride complexes react with phenylacetylene to generate two insertion products. The 2,1-insertion is more favorable than the 1,2-insertion and both pathways involve cis addition of Ni–H across the C=C bond. Unlike the palladium case, alkynyl complexes and H2 are not produced in the nickel system. The more bulky hydride complex [2,6-(tBu2PO)2C6H3]NiH shows no reactivity towards phenylacetylene. Catalytic hydrogenation of phenylacetylene with [2,6-(iPr2PO)2C6H3]NiH and [2,6-(cPe2PO)2C6H3]NiH takes place at an elevated temperature (70-100 °C) and proves to be heterogeneous. The structures of the insertion products have been studied by X-ray crystallography. Nickel POCOP-pincer hydride complexes have been found to be active catalysts for dehydrogenation of dimethylamine borane, a viable solid chemically stored hydrogen source. The intermediates and products have been determined by spectroscopic techniques. The homogeneous nickel POCOP-pincer hydride complexes catalyze the dehydrogenation of dimethylamine borane, and heterogeneous nickel particles leached from the POCOP-pincer complexes are believed to also act as a catalyst. Hydride transfer is an essential step in a vast array of stoichiometric and catalytic transformations. Although metal-hydride complexes formally deliver a hydride in those reactions, it may proceed via direct hydride transfer or single electron transfer (SET) followed by hydrogen atom transfer. Reactions between trityl cation and nickel POCOP-pincer hydrides support that hydride transfer proceeds solely via direct hydride transfer for [2,6-(tBu2PO)2C6H3]NiH. The complexes [2,6-(iPr2PO)2C6H3]NiH and [2,6-(cPe2PO)2C6H3]NiH transfer hydride by direct hydride transfer (major pathway) and SET followed by hydrogen transfer (minor pathway), supported by 1H NMR spectroscopy. Upon these findings, the reaction of N-(1-phenylethylidene) pyrrolidinium tetrafluoroborate with nickel POCOP-pincer hydrides was also studied in efforts to broaden the scope of hydride transfer reactions of the nickel POCOP-pincer hydrides. Stoichiometric reactions confirm that the hydride complex [2,6-(iPr2PO)2C6H3]NiH reduces the iminium cation to a tertiary amine completely while the complexes [2,6-(tBu2PO)2C6H3]NiH and [2,6-(cPe2PO)2C6H3]NiH show evidence of partial iminium cation reduction.
Hairong Guan, Ph.D. (Committee Chair)
William Connick, Ph.D. (Committee Member)
James Mack, Ph.D. (Committee Member)
105 p.

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Citations

  • Wilson, G. (2015). Hydrogen Transfer Reaction Involving Nickel POCOP-Pincer Hydride Complexes [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1447688935

    APA Style (7th edition)

  • Wilson, Gleason. Hydrogen Transfer Reaction Involving Nickel POCOP-Pincer Hydride Complexes. 2015. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1447688935.

    MLA Style (8th edition)

  • Wilson, Gleason. "Hydrogen Transfer Reaction Involving Nickel POCOP-Pincer Hydride Complexes." Doctoral dissertation, University of Cincinnati, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1447688935

    Chicago Manual of Style (17th edition)