Date of Award
Chemistry and Biochemistry
Bond Activation, Clusters, Cooperativity, Inorganic, Nickel, Organometallic
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This dissertation focuses on the activation and functionalization of inert-bonds by harnessing cooperativity between metals. Transition metal clusters can serve as molecular models for the poorly-understood solid-state catalysts used in most industrial scale processes. An understanding of how adjacent metals act together to coordinate substrates, cleave traditionally unreactive bonds and reform new skeletal bonds could inspire new classes of catalysts, both heterogeneous and molecular, that are able to utilize abundant and currently unusable species as chemical precursors. The pentanuclear Ni hydride cluster [(iPr3P)Ni]5H6 is an electron-deficient nido cluster that facilitates ultra-deep hydrodesulfurization by cleaving both C−S bonds of 4,6 dimethyldibenzothiophene and forming 3,3ʹ-dimethylbiphenyl and the tetranuclear Ni sulfide cluster [(iPr3P)NiH]4(μ4−S). The reaction of [(iPr3P)2Ni]2(μ-N2) with 4,6-dimethyldibenzothiophene gave no C−S activation product, which demonstrates the importance of intact cluster cooperativity in the activation this highly unreactive thiophene by [(iPr3P)Ni]5H6. The pentanuclear Ni cluster [(iPr3P)Ni]5H6 undergoes CC and CH bond cleavage along with CH bond formation upon reaction with ethylene at −30 °C to make the pentanuclear Ni-carbide cluster [(iPr3P)Ni]5H4(μ5−C), along with ethane and methane. The process is highly selective as the reaction proceeds in high yields in the presence of various functional group containing molecules. Isolated intermediates obtained from reacting [(iPr3P)Ni]5H6 with styrene and isobutylene feature a triple CH bond activated organic substrate bound to all 5 Ni centers. These intermediates give direct insight into the cooperative involvement of all five Ni centres in the binding and activation of these typically unreactive bonds. The reaction may give insight into the ability of Ni (1 1 1) surfaces to abstract C atoms from hydrocarbons selectivity at the step sites; this process is used to catalytically generate graphene on Ni metal surfaces. Main group elements other than C can also serve as central bridging ligands supporting Ni clusters. Reactions of [(iPr3P)Ni]5H6 resulted in the formation of square plane or tetrahedral clusters Ni clusters that incorporate μ4−O, μ4−NCH2Ph, μ2−Cl, and μ4−BH. A study of H/D exchange with [(iPr3P)Ni]5H6 and unactivated arenes such as deuterated benzene revealed a first order dependence of rate on [(iPr3P)Ni]5H6 and arene, and first-order inhibition by PiPr3. Structural analogues of likely intermediates were synthesized to give insight into this cooperative reactivity. The potential for [(iPr3P)Ni]5H6 to facilitate C−O bond cleavage was studied using vinyl ethers. These reactions demonstrate the ability of these clusters to engage in cooperative CC, C(sp3)O, C(sp2)O, and CH bond cleavages with minimal energy barriers. The selectivity in C(sp3)O versus C(sp2)O C−O bond cleavage is dependent on the group attached to the oxygen, with tBu causing C(sp3)O bond activation and SiMe3 and 1-Ad causing C(sp2)O bond cleavage. Functionalization of the carbide ligand in [(iPr3P)Ni]5H4(μ5−C) could allow for the use of ethylene as a catalytic C1 transfer reagent. However, reactivity studies with in [(iPr3P)Ni]5H4(μ5−C) demonstrate the utility of the carbide as an anchor-like ancillary ligand, which allows to access new cluster geometries and core oxidation states with the potential for improved stability under common catalytic conditions compared to [(iPr3P)Ni]5H6.
Shoshani, Manar Matti, "Cooperative Bond Activation by an Electron-Deficient Pentanuclear Nickel Hydride Cluster" (2018). Electronic Theses and Dissertations. 7481.