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(1) First 4-Pyridylidene Carboxylate Remote N-Heterocyclic Carbene Complexes of Ruthenium and their Water Oxidation Catalysis (2) Origin of High (E)-selectivity in Alkene Isomerization by a CpRu(PN) Catalyst

  • Author(s): Cao, Thomas
  • Advisor(s): Grotjahn, Douglas B
  • et al.
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Abstract

Part 1 of this dissertation entails the synthesis and characterization of a new family of ruthenium complexes bearing remote NHC, 4-pyridylidene ligands for water oxidation. Catalytic water oxidation is important for the development of cost-effective and carbon-neutral methods to produce hydrogen fuel from water. We describe the Zincke pyridine synthesis of ligand precursors OCR, where OC = 3-carboxypyridinium and the nitrogen atom of the pyridinium has been quaternized has been functionalized such that R = methyl (Me), adamantyl (Ad), and 2,6-dimethylphenyl (Ar). All three ligand precursors were successfully cyclometallated to yield coordination complexes of the formula [RuII(OCR)(tpy)(L)]+[PF6]- (tpy = 2,2';6',2"-terpyridine, L = CH3CN or H2O). The 4-pyridylidene coordination of the OCR ligand was confirmed by NMR and x-ray diffraction studies. Ru(OCAd) exhibited a reversible RuII/RuIII with an E1/2 of 0.441 V at pH 1. Combined UV-vis, QTOF-MS, and TDDFT were used to study the stability of the complexes in solution. During catalytic screening for water oxidation activity with ceric ammonium nitrate as the sacrificial oxidant, Ru(OCR) appeared to exhibit an initial burst of O2 production followed by a plateau after 40-100 turnovers.Part 2 of this dissertation examines an alkene isomerization catalyst Ru(acn), that was previously reported by the Grotjahn lab to exhibit high activity for isomerizing terminal alkenes to their internal isomers. The catalyst was noteworthy for its fast reaction times, mild reaction conditions, and significantly, its unprecedented kinetic (E)-selectivity. We conduct the first combined experimental and theoretical study to unveil the mechanism of isomerization and the origin of the high (E)-selectivity exhibited by the catalyst. In-situ NMR reaction monitoring were used to acquire time-dependent kinetics data for the Ru(acn)-mediated isomerization of but-1-ene into its (E)-and (Z)-but-2-ene isomers, the latter of which required special reaction conditions. An extensive computational model was developed, and lowest energy pathways validated by experimental Arrhenius activation energies derived from the experimental data. The experimental activation energies compared well to the DFT energies for the isomerization pathway featuring an exo-coordinated η3-allyl, and the structures of key intermediates reveal inner-sphere steric interactions that favor the (E)-allyl intermediate over the (Z)-isomer.

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This item is under embargo until June 3, 2022.