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Title:Synthesis, characterization, and catalytic reactivity of first-row transition metal CCC pincer complexes
Author(s):Jackson, Bailey Jeanne
Director of Research:Fout, Alison
Doctoral Committee Chair(s):Fout, Alison
Doctoral Committee Member(s):Girolami, Gregory; Murphy, Catherine; van der Veen, Renske
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
transition metal
Abstract:Catalysis is such a ubiquitous process that over 75% of existing chemical processes and 90% of newly developed processes utilize catalysts. Understanding how to improve and create new catalysts is therefore essential to developing new products as well as greener and more efficient processes. One way to improve the environmental impact of these reactions is to move away from using less biocompatible second- and third-row transition metal catalysts and replace them with first-row transition metal catalysts. However, first-row metals tend to engage in one-electron radical chemistry much more readily than the typically desired two-electron reactions. One way to induce first-row transition metals to engage in the desired reactions is to use strong-field ligand frameworks. Strong-field ligands cause the d-orbital splitting of the metal center to increase in energy, often leading to low-spin electronic configurations which favor two-electron reactivity. N-heterocyclic carbenes (NHCs) are popular strong-field ligands due to their inherent stability and oxidative robustness, especially compared to phosphines. Combining NHCs with pincer ligand frameworks provides even more stable complexes. Therefore, we investigated a variety of monoanionic, bis(carbene), strong-field pincers with first-row transition metals. A variety of ArCCC (Ar = Mes, DIPP) and RCcCcC (R = benzyl, t-butyl) pincer ligand frameworks were developed and their metalation investigated. The RCcCcC pincers were successfully metalated with nickel generating (RCcCcC)Ni(II)Br complexes (R = benzy, t-butyl). DIPPCCC pincers, previously metalated with nickel and cobalt in our group, have been extended to iron as well. The metalation method first goes through a zwitterionic intermediate which was isolated and characterized before being reduced in situ to yield the (DIPPCCC)Fe(II)H(L)(L’) complexes. A distinct electronic effect was observed on the Fe−H when varying the L-type ligands coordinated to the iron center. Reactivity of these iron hydrides with CO2 showed insertion into the Fe−H bond to form a κ2-OOCH complex (L = PMe3, L’ = N2). This complex was also independently synthesized starting from (DIPPCCC)Fe(II)Cl(PMe3)2. The zwitterionic metalation method was also extended to other first-row transition metals. The synthesis and characterization of zwitterionic complexes, H2(DIPPCCC)M(II)Cl3 (M = Mn, Co, Ni), was accomplished and the in situ reduction lead to metalation using cobalt and nickel. An alternate metalation method, transmetalation from zirconium, into similar CCC pincer ligand frameworks has been previously established by the Hollis group. Utilizing a similar procedure zirconium complexes were synthesized and characterized (ArCCC)Zr(IV)X3 (Ar = Mes, DIPP). The transmetalation to iron, cobalt, and nickel were all successful with both ligand derivatives. Our group has studied cobalt catalysts, (MesCCC)Co(I)L, for the hydrogenation of alkenes and semi-hydrogenation of alkynes and were interested in extending the hydrogenation further to more polar functional groups. Nitrile hydrogenation is a difficult reaction both in terms of activating the C−N triple bond and selectivity among products. We discovered a mild set of reaction conditions that selectively formed the primary amine for a variety of different substrates. During our mechanistic work we determined that the process is actually Lewis acid-assisted and is in fact undergoing a two-electron catalytic process. During the investigation and subsequent publication of this work another product, the secondary aldimine, was seen under some reaction conditions. Modifying our reaction parameters, we discovered we could also selectively form the secondary aldimine product starting from the nitrile. This represents only the second example of a cobalt catalyst that can perform the hydrogenation of nitriles with selectivity to the secondary aldimine. Our condition switchable system also provided exceptionally mild reaction conditions compared to other first-row transition metal catalysts for nitrile hydrogenation.
Issue Date:2018-11-12
Rights Information:Copyright 2018 Bailey Jackson
Date Available in IDEALS:2019-02-07
Date Deposited:2018-12

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