Development of ligands intended to support first-row transition imidometal complexes

• Main Author: Ang, Zhi Zhong
• Other Authors: Richard D. Webster
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Chemistry; Transition metal; High oxidation state; Redox active ligand; Mesoionic carbene
• Online Access: https://hdl.handle.net/10356/175898
• Institution: Nanyang Technological University

First-row transition metal complexes bearing high oxidation state containing metal―E (where E is C, N, or O) multiple bonds are the active oxidants in the catalysis of a series of biological and chemical reactions. Due to its reduced stability, late transition metals of this nature are rarely isolated and characterized. However, the lower stability also makes them highly reactive intermediates serving as catalysts for C-H bond activation and group transfer reaction. We have designed 2 series of ligands intended to support complexes containing iron, cobalt or nickel core which would enable us to achieve high oxidation chemistry. The first ligand system, H4L, is a redox-active ligand that serves as an ideal electron reservoir with a highly conjugated system to delocalize the electron radical during oxidation. This is helpful as they can allow complexes to behave like higher oxidation state species, which are difficult to access in late-transition metals The coodination of (L)4- to cobalt(II) and nickel(II) produce square planar complexes of [CoII(L4-)]2- and [NiII(L4-)]2-, respectively. Both these complexes can undergo successive one electron oxidation producing a five-membered redox series where all these species were successfully characterized by X-ray crystallography except for [CoIII(L2-)]1+ and [CoIII(L2-)(X)]0. The five-electron oxidized species [CoII(L0)(X)2]0 and [NiII(L0)(X)2]0 are isolated as a six-coordinated complex and are the first instance of this redox state isolated for aminophenol cobalt complexes. Furthermore, charge assignment between the metal centre and ligand can easily be done using an established metrical oxidation state calculation, which uses the C—C bond distance of the aryl ring to determine the ligand oxidation state. The reaction between [M(L)]n and various nitrene transfer agents shows that the formation of an imidometal complex is achievable under the use of strong nitrene transfer agents such as PhINNs or PhINTs. However, preliminary characterization of these species was performed using UV-Vis spectroscopy and mass spectrometry. Ligand 8 utilized a class of carbene known as mesoionic carbenes (MICs) that have all their resonance forms assigned charges in two of the atoms. They are stronger σ-donors but poorer π-acceptors than traditional NHCs, thus strengthening the M-C bond and preventing insertion reaction to the carbenic donor from occurring. Electron-withdrawing trifluoromethyl group incorporated into aryl structure to make them more robust toward ligand oxidation. The ligand precursor [6a - d]3+ can be synthesized through the alkylation of a triazole which is a product of a copper-catalyzed “click” reaction between an alkyne and an organic azide. Through the use of a solid-state mechanochemical process, the synthesis of [6a - d]3+ shows significant improvement in the chemo- and regioselectivity of the reaction relative to the conventional solution-state methods. The reaction between [6a - d]3+ and Ag2O produces trimetallic sandwich complex [Ag3(8a-d)2]3+ which is a common precursor to transmetallate the ligand to other metal centres (Chapter 4). The silver complexes are structurally and spectroscopically characterized, where we observed the steric bulkiness of the aryl group can enforce or prevent argentophilic bonding. Similarly, after transferring the ligand to cobalt ion, [Co(8a)(MeCN)]2+ adopts a four-coordinate geometry while that of cobalt complex supported by ligand 8b, which contains smaller aryl substituents, adopts a six-coordinate geometry. Subsequently, we have successfully synthesised the halide, azido and hydroxometal complexes supported by 8a and 8c, which are potential precursors to the nitrido and oxometal complexes, respectively.