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Title:Ligand design in nickel and cobalt complexes for homogeneous catalysis
Author(s):Nugent, Joseph William
Director of Research:Fout, Alison R
Doctoral Committee Chair(s):Fout, Alison R
Doctoral Committee Member(s):Girolami, Gregory S; Murphy, Catherine J; Vura-Weis, Joshua
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:The environmental impact and increased cost incurred utilizing second- and third-row transition metals in catalysis has pushed scientists to develop more earth-abundant first-row transition metal catalysts which can match or surpass the activity and selectivity of their less earth-abundant counterparts. Moving towards these first-row transition metals has presented challenges in controlling the inherent one-electron modes of reactivity. The work presented herein presents strategies to allow cobalt and nickel to participate in productive catalytic reactions. In early work in the Fout lab, the design and metalation strategies for a monoanionic, bidentate NHC-CAryl ligand framework ([H(BrCC)]Br) were developed. The metalation of this ligand precursor with nickel was achieved by both initial metalation with Ni(II) followed by reduction and direct metalation with Ni(0). The presence of a Ni-CAryl bond in the resulting complexes allowed for the modulation of the ligand backbone via insertion of isocyanides, forming the corresponding NHC-η2-iminoacyl nickel complexes. Oxidation of the η2-iminoacyl-containing complex resulted in this moiety releasing from the nickel center to form a nitrilium species which upon reduction reformed the η2-iminoacyl fragment. Work with nickel continued with the development of an amination procedure for (hetero)aryl bromides and iodides catalyzed by simple nickel salts. This protocol allowed for the functionalization of a variety of (hetero)aryl bromides and iodides with a variety of nickel sources. This protocol exhibited a good functional group tolerance and was amenable to low catalyst loadings (0.1 – 5%). Experiments with (PPh3)2Ni(N(SiMe3)2 as the catalyst showed increased rate and reduced induction periods which suggest that this species is achieved in solution by reduction of (PPh3)2NiCl2 to the Ni(I) species. This nickel-catalyzed protocol offers a simple procedure for the formation of valuable aniline products using commercially available nickel sources while avoiding the use of palladium and tailored ligand which may not be readily available in some research settings. Work with the electron-rich monoanionic bis(carbene) aryl pincer ligand (MesCCC) was also carried out to explore bond activation on low valent cobalt complexes. The catalyst (MesCCC)Co-py was found to catalytically dehydrogenate ammonia borane (AB) and other amine boranes, releasing 1.7 eq H2 for AB. Stoichiometric addition of AB to (MesCCC)Co-py yielded the hydride-amidoborane complex (MesCCC)CoH(NH2BH3). This species is formed almost immediately in THF and is the catalytic resting state of the catalyst during catalysis. Formation of this species via N-H activation of AB onto the Co(I) center was confirmed by reaction of (MesCCC)Co-py with the deuterated isotopologues and DFT calculations. Isolated (MesCCC)CoH(NH2BH3) was shown to catalyze this reaction, forming similar BN-containing products to (MesCCC)Co-py, albeit at a slower rate. Finally, a variety of (MesCCCCF3)Co and (DippCCCCF3)Co complexes were synthesized to serve as analogues to determine the effect of ligand electronic modification on the chemistry of the resulting cobalt complexes. The electron withdrawing CF3 group lowered the electron density of the cobalt center in the (CF3CCC)Co complexes, but had a relatively small effect on the corresponding hydrogenation chemistry. Instillation of a Lewis acid secondary coordination sphere on a CCC framework was achieved via sequential metalation and hydroboration. Finally, a rhodium metalation procedure for the DippCCC ligand was achieved resulting in the isolation of (DippCCC)RhN2. This complex was shown to participate in hydrofunctionalization reactions in a similar fashion to (DippCCC)CoN2, catalyzing the hydroboration/silylation of olefins.
Issue Date:2020-07-02
Rights Information:Copyright 2020 Joseph Nugent
Date Available in IDEALS:2020-10-07
Date Deposited:2020-08

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