Investigations of C-H and N-H activation with electron deficient iridium pincer complexes

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  • March 20, 2019
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  • Sykes, Alison Cartwright
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • Transition metal reactions that involve C-H and N-H bond activation provide a potential method for functionalizing hydrocarbons and amines. For the past three decades there have been numerous advances in this field; however, developing new approaches to activation of these bonds continues to be a significant endeavor. Recently pincer ligands have been successfully used to stabilize metal complexes at elevated temperatures. When chelated to iridium, these pincer complexes have been shown to be very active as transfer dehydrogenation catalysts. This has been exemplified by Kaska, Goldman, and Jensen using Ir complexes containing the 2,6-(CH2PtBu2)2C6H3 (PCP) ligand and by Brookhart using the Ir complexes of the more electron-deficient bis(phosphinite) pincer ligand (POCOP). This dissertation explores alternative reactivity of these bis(phosphinite)iridium complexes. Chapter 2 focuses on the reactions of [(POCOP)Ir] with N-H bonds, in particular those of both anilines and benzamides. The electronic character of the anilines plays an important role in the reactivity; more electron-withdrawing anilines favor oxidative addition while the more electron-donating anilines favor ?-bond formation. An explanation for this behavior is provided along with a kinetic analysis of the oxidative addition and reductive elimination of anilines. A comparison of the reactivity of anilines to that of benzamides is also discussed. In Chapter 3, cationic [(POCOP)Ir(H)(L)][BArF] complexes are examined that contain tetrakis(trifluoromethylphenyl)borate (BArF) as a non-coordinating counterion. Since these complexes are cationic, the metal center is even more electron deficient than those mentioned above. The structure of [(POCOP)Ir(H)(H2)][BArF] was determined based on an H-D coupling constant of 33 Hz. In addition to the structural analysis, the rate of exchange between the hydride and the dihydrogen has also been determined. Cationic olefin complexes, [(POCOP)Ir(H)(L)][BArF] (L = C2H4, C3H6, norbornene, methyl acrylate) have also been synthesized. Their dynamic behavior has been assessed using both line-broadening and spin-saturation transfer NMR techniques. The rates of olefin rotation and insertion into the Ir-H bond were analyzed and energy barriers to these processes were calculated. A brief examination of the reactivity of the cationic norbornene complex with nucleophiles has also been explored; however the results showed no C-Nuc bond formation.
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  • Brookhart, Maurice
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  • October 19, 2010

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