Browsing by Author "Nurdin, Lucie"
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- ItemOpen AccessReactions of Neutral Co(II) Complexes of a Dianionic Tetrapodal Pentadentate Ligand: Co(III) Amides from Imido Radicals(2017-03-15) Piers, Warren E.; Nurdin, Lucie; Spasyuk, Denis M.; Maron, Laurent; Bi, David W.Neutral cobalt(II) complexes of the dianionic tetrapodal pentadentate ligand B2Pz4Py, in which borate linkers supply the anionic charges, are reported. Both the six-coordinate THF adduct 1-THF and the five-coordinate THF-free complex 1 are in a high-spin S = 3/2 configuration in the ground state and have been structurally characterized by X-ray crystallography. These two Co(II) starting materials react rapidly with aryl azides of moderate steric bulk. The thermodynamic products of these reactions are low-spin, diamagnetic, Co(III) amido complexes that are either monomeric, when an external hydrogen atom source such as 1,4-cyclohexadiene is present, or dimeric products formed via C–C coupling of the azide aryl group and internal transfer of H• to the nitrogen. These products are fully characterized and are rare examples of octahedral Co amido compounds; structural determinations reveal significant pyramidalization of the amido nitrogens due to π–π repulsion wherein the amido ligand is primarily a σ donor. The amido products arise from highly reactive Co(III) imido radical intermediates that are the kinetic products of the reactions of 1 or 1-THF with the azide reagents. The imido radicals can be detected by X-band EPR spectroscopy and have been probed by density functional theory computations, which indicate that this doublet species is characterized by a high degree of spin localization on the imido ligand, accounting for the reactivity with hydrogen atom sources and dimerization chemistry observed. The high coordination number and the electron-rich nature of the dianionic B2Pz4Py ligand framework render the imido ligand formed highly reactive.
- ItemOpen AccessUnraveling Reaction Mechanisms using Iron and Cobalt Complexes Supported by a Dianionic Pentadentate Ligand(2020-09-19) Nurdin, Lucie; Piers, Warren E.; Birss, Viola; Roesler, Roland; Jackson, Leland J.; Smith, JeremyThe search for carbon-neutral alternatives to fossil fuels has led to the investigation of fundamental reactions mediated by first-row transition metal complexes, such as the reduction of dioxygen to water or the oxidation of ammonia to dinitrogen. In living organisms, metalloenzymes can mediate these transformations under mild conditions, through complex mechanisms that are often difficult to study under biological conditions. Therefore, natural systems represent a constant source of inspiration for synthetic chemists who wish to develop artificial catalysts and understand their reaction mechanisms. In this context, strategic ligand design yielded a plethora of classes of ligands, which vary the reactivity and properties of transition metal complexes. In particular, pentadentate ligands have been widely employed across the periodic table to allow a single site for reactivity, thus providing a well-defined system for mechanistic studies. These systems represent a modular tool for chemists to study detailed mechanistic steps of chemical reactions and to understand the role of what are often fleeting intermediates in life-sustaining reactions. This thesis presents a concrete example of using a pentadentate ligand combined with iron and cobalt to access and stabilize high-valent metal-oxo and metal-imido complexes, which have been proposed as key intermediates in a variety of catalytic transformations. The syntheses of various iron and cobalt complexes supported by the tetrapodal pentadentate Pz4PyB2 ligand are discussed, and their reactivity is explored. As a result, these complexes have been investigated for nitrene transfers; dioxygen reduction to water, ammonia oxidation to dinitrogen, and as mimics to biological related systems. The mechanistic details of these processes were studied extensively, both experimentally and theoretically, and reveal a unique platform for reactivity.