Alkane Metathesis via Tandem Catalysis

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  • March 19, 2019
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  • Huang, Zheng
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • Alkane metathesis (AM) has potentially tremendous applicability via converting low-value alkanes (C3-C9) from the Fisher-Tropsch process into linear alkanes in the diesel fuel range (C10-C19). A well-defined and highly efficient tandem catalytic system for the metathesis of n-alkanes has been developed. The system is comprised of one pincer Ir catalyst that effects alkane dehydrogenation and olefin hydrogenation, and a second catalyst for olefin metathesis. The catalytic system shows complete selectivity for linear alkane products. Chapter 2 presents the mechanistic studies of AM. The (tBu-PCP)Ir [tBu-PCP = C6H3(CH2PtBu2)2-1,3] system shows higher product selectivity than the (tBu-POCOP)Ir [tBu-POCOP = C6H3(OPtBu2)2-1,3] system because of the different resting states under AM. Both of steric and electronic factors favor the formation of (tBu-PCP)IrH2 and (tBu-POCOP)Ir-olefin as the catalytic resting states. Experimental evidence and DFT calculations suggest that olefin isomerization by the Ir complex occurs from a (pincer)Ir(I)-olefin complex via formation of a (pincer)Ir(III)(allyl)(H) intermediate, not via a (pincer)Ir(H)2(olefin) intermediate. Syntheses of eight new Ir pincer complexes for transfer dehydrogenation and alkane metathesis are outlined in chapter 3. Among these iridium complexes, the least bulky, (iPr-POCOP)Ir-C2H4, exhibits the highest activity in both transfer dehydrogenation and alkane metathesis. Compared to the parent (tBu-POCOP)Ir-C2H4, the sterically more hindered complex (tBu-PSCOP)Ir-C2H4 shows lower transfer dehydrogenation and alkane metathesis activity, but the product selectivity in alkane metathesis was improved as a result of a dihydride resting state ((tBu-PSCOP)Ir-H2). Chapters 4 and 5 describe the synthesis of alumina-supported Ir pincer complexes which are recyclable and highly active in transfer dehydrogenation reactions. In addition, use of this supported catalyst in combination with Re2O7/Al2O3 or MoO3/CoO/Al2O3 catalyst provides an efficient alkane metathesis catalytic system in which both catalytic components are heterogeneous. Chapter 6 presents a series of unprecedented single-crystal-to-single-crystal transformations involving interchange of multiple small gaseous ligands (N2, CO, NH3, C2H4, H2, O2) at an iridium center of a pincer iridium(I) complex. The single crystal remains intact during these ligand exchange reactions which occur within the crystal and do not require prior ligand extrusion. Single crystals bearing nitrogen, ethylene or hydrogen exhibit highly selective hydrogenation of ethylene relative to propylene (25:1) when surface sites are passified by CO.
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  • Brookhart, Maurice
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