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Title:High and low valent nickel complexes supported by tridentate and tetradentate ligands relevant for C–C bond formation reactions
Author(s):Griego, Leonel
Director of Research:Mirica, Liviu M
Doctoral Committee Chair(s):Mirica, Liviu M
Doctoral Committee Member(s):Rauchfuss, Thomas B; Fout, Allison R; Guironnet, Damien
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Nickel (Ni) complexes stabilized by multidentate ligands are known to be efficient catalysts for a wide range of cross-coupling reactions, including those involving Csp3–Csp3 coupling. Although many Ni0/NiII systems have been reported as potential species in catalysis, newer studies suggest that their odd-electron NiI/NiIII counterparts are also viable catalytic intermediates. Our goal is to isolate these rare oxidation states especially those containing organometallic intermediates. Herein, we discuss the isolation of key Ni intermediates in the +1, +2, and +3 oxidation states supported by various N-based macrocycles. In this work, we provide direct evidence for a NiI/NiII/NiIII catalytic cycle by using a bulky 1,4,7-triisopropyl-1,4,7-triazacyclononane ligand (iPr3TACN). The oxidation of the corresponding NiII-dimethyl complex with weak oxidants yields a paramagnetic NiIII-dimethyl complex that was characterized by EPR. In addition, reductive elimination of ethane in quantitative yields was detected by NMR, while a direct formation of NiI species was observed by carefully monitoring the 2e- transformation by EPR. In addition, various NiI species were characterized via EPR and reactivity tested. The combined results further confirmed that the bulky iPr3TACN ligand framework can stabilize Ni in several oxidation states that are relevant for cross-coupling catalysis. Furthermore, two new classes of macrocyclic ligands and their ability to stabilized high- valent Ni centers have been studied. The first class includes chiral macrocyclic ligands that have been successfully synthesized and characterized. The R*N3CBr and R*N4 (R* = phenethyl, PE or methylneopentyl, MeNp) pyridinophanes allowed for the isolation of various Ni complexes including the rare high-valent [(PEN3C)NiIII(MeCN)2](PF6)2 complex. The reactivity of the chiral Ni complexes has been tested demonstrating slight enantioselectivity induced by the chiral ligands. In addition, a second class of cross-bridged cyclam/cyclen macrocyclic ligands was studied as their Ni(II)Cl2 complexes demonstrated accessible high-valent Ni(III) species. To conclude, preliminary results have shown the (iPr3TACN)Ni complexes are efficient catalysts in the Kumada coupling reactions of alkyl electrophiles with alkyl Grignards, raising the possibility of exploring other types of cross-coupling reactions. In addition, the iPr3TACN ligand framework has allowed for the isolation of a novel [(iPr3TACN)NiIIMe]PF6, a Ni-monomethyl complex that has been elusive with RN4 (R = Me, iPr, tBu) and MeTACN ligand systems. The isolation of this complex opens the possibility of exploring a more bioinorganic side of the iPr3TACN, as Ni–Me complexes are proposed as key intermediates CODH/acetyl coenzyme A synthase (ACS) and methyl coenzyme M reductase (MCR) systems.
Issue Date:2021-12-03
Rights Information:Copyright 2021 Leonel Griego
Date Available in IDEALS:2022-04-29
Date Deposited:2021-12

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