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Tethered mixed anion aggregates, a structural basis for superbasic activity

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Harris, SR 2021 , 'Tethered mixed anion aggregates, a structural basis for superbasic activity', PhD thesis, University of Tasmania.

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Abstract

This thesis discusses the synthesis, characterisation and structural features of a number of mixed anion lithium complexes. These complexes were developed with the intention of improving our understanding of organolithium aggregation tendencies for compounds containing mixed anionic ligands, with a secondary goal of exploring for unexpected reaction outcomes as a result of superbasic activity.
Chapter 2 details the design and synthesis of a series of aniline coupled methoxybenzene derivatives for their use in the investigation into mixed anion lithium complexes. These ligand precursors possessed amine and alcohol groups tethered together through an aryl backbone. A series of 2-aminophenyl methanol derivatives (O’NRH\(_2\)) were prepared by the coupling of an aniline molecule onto 2-bromobenzoic acid and subsequently reducing the product to give ligand precursors (2-(phenylamino)phenyl)methanol (O’NPhH\(_2\)), (2-((2,4-dimethylphenyl)amino)phenyl)methanol (O’NPhMe\(_2\)H\(_2\)), (2-((4-isopropylphenyl)amino)phenyl)methanol (O’NPhiPrH\(_2\)), (2-(mesitylamino)phenyl)methanol (O’NPhMesH\(_2\)). A fifth O’NRH\(_2\) ligand precursor, (2-aminophenyl)methanol (O’NH\(_3\)), was prepared directly by the reduction of 2-aminobenzoic acid.
A second set of ligands was based off the same ligand scaffold with a methylene linker incorporated in between the amine and aryl backbone. The initial planned synthetic path mimicking that used for the synthesis of O’NRH\(_2\) was found to result in undesired ring closure, preventing the desired ligand precursor from being obtained. A second synthetic pathway was developed which used the reduction of phthalic acid and a subsequent desymmetrising halide substitution to produce the chloroalcohol (2-(chloromethyl)phenyl)methanol. This molecule was then able to be coupled with three anilines to give the ligand precursors (2-((phenylamino)methyl)phenyl)methanol (O’N’PhH\(_2\)), (2-((mesitylamino)methyl)phenyl) methanol (O’N’MesH\(_2\)) and (2-(((2,6-diisopropylphenyl)amino)methyl)phenyl)methanol (O’N’DIPPH\(_2\)) as well as isopropylamine to give (2-((isopropylamino)methyl)phenyl)methanol (O’N’iPrH\(_2\)).
Chapter 3 discusses organolithium aggregate structures prepared through the lithiation of O’NRH\(_2\) ligand precursors with nBuLi in various Lewis basic and non-Lewis basic solvents. Two structures showed existing aggregate motifs, with a monolithiated tetrameric cube [Li\(_4\)(O’NMesH)\(_4\)] and a stepped ladder [Li\(_4\)(O’NPh)\(_2\)(THF)\(_5\)] discovered. However the remaining structures isolated were found to take novel aggregate arrangements. The compounds [Li(DME)\(_3\)][Li\(_{11}\)(O’NPh)\(_6\)(DME)\(_3\)], [Li(DME)\(_3\)][Li\(_{11}\)(O’NPh)\(_6\)(DME)\(_3\)], and [Li\(_{11}\)(O’NMes)\(_5\)(O’NMesH)] showed lithium atoms loaded around a lithium alkoxide hexamer to form what was described as a triple winged capped hexamer. A partially complete winged hexamers, [Li\(_7\)(O’NPhMe\(_2\))(O’NPhMe\(_2\)H)\(_5\)(OEt\(_2\))], was found containing a mixture of monolithiated and dilithiated ligands. The compound [Li(THF)\(_4\)]\(_2\)[Li\(_10\)(O’NPhMe\(_2\))\(_6\)) showed an alternate winged hexamer structure with one “capping” lithium absent and altered connectivity of lithium atoms in the main cluster. Comparisons of these structures led to the proposal that the connectivity of the lithium atoms around the hexameric core could be altered through the availability of Lewis basic solvent molecules. This hypothesis was supported by \(^1\)H NMR spectroscopic studies as well as the isolation of a second partially complete capped hexamer [Li\(_{10}\)(O’NPhMe\(_2\)H)\(_2\)(O’NPhMe\(_2\))\(_4\)]. In addition to the triple winged capped hexameric structures, [Li\(_{10}\)(O’NH)\(_4\)(O’NH\(_2\))\(_2\)(OEt\(_2\))\(_4\)] was found containing two lithium amide wings around a hexameric core. Furthermore, a unique winged tetramer arrangement [Li(DME)\(_3\)]\(_2\)[Li\(_6\)(O’NMes)\(_4\)] showed lithium loading around a lithium alkoxide cube.
Chapter 4 discusses the synthesis and characterisation of a set of five aggregates containing anionic fragments from cleaved ether solvent molecules. Four structures, [Li\(_{10}\)(O’NPh)\(_4\)(OEt)\(_2\)(Et\(_2\)O)\(_4\)], [Li\(_{10}\)(O’NPhMe)\(_4\)(OEt)\(_2\)(OEt\(_2\))\(_4\)], [Li\(_{10}\)(O’NMes)\(_4\)(OEt)\(_2\)(OEt\(_2\))\(_2\)] and [Li\(_{10}\)(O’NPhiPr)\(_4\)(OEt)\(_2\)(Et\(_2\)O)\(_4\)] were obtained from the lithiation of O’NR ligand precursors in diethyl ether whilst [Li\(_{10}\)(O’NPhiPr)\(_4\)(OCH=CH\(_2\))\(_2\)(THF)\(_6\)] was obtained from THF. NMR spectroscopic studies showed the ether cleavage was near instantly completed following the addition of a stoichiometric amount of n-butyl lithium to a solution of the O’NRH\(_2\) ligand precursors in diethyl ether.
Chapter 5 details the preliminary investigations into the lithiation of the O’N’RH\(_2\) ligand precursors and the structures that resulted. A single monolithiated tetramer cube [Li\(_4\)(O’N’iPrH)\(_4\)] was obtained alongside three stepped ladders [Li\(_4\)(O’N’Ph)\(_2\)(THF)\(_4\)], [Li\(_4\)(O’N’Mes)\(_2\)(THF)\(_4\)] and [Li\(_4\)(O’N’DIPP)\(_2\)(THF)\(_4\)]. The structure and reactivity of the ladders were compared to analogous structures obtained in a previous project from the Gardiner group.
Chapter 6 discussed aggregation structures formed from reactivity towards contaminating compounds and the O’N’R ligands. The first half of Chapter 6 discusses two structures incorporating serendipitous silicone grease fragments [Li\(_8\)(O’NPh)\(_2\)(O’NPhH)\(_2\)(SiO\(_2\)Me\(_2\))(THF)\(_6\)] and [Li\(_{12}\)(O’NPhMe\(_2\))\(_4\)(SiO\(_2\)Me\(_2\))\(_2\)(Et\(_2\)O)\(_4\)]. The second half of the Chapter reports two aggregates [Li\(_{14}\)(O’N’Mes)\(_4\)(O’NMes-H)\(_2\)(OEt\(_2\))\(_4\)] and [Li\(_{20}\)(O’N’iPr)\(_6\)(O’N’iPr-H)\(_2\)(C4H\(_{10}\))\(_2\)] found to contain unexpected alkyl and aryl lithiation of the ligands.
The compounds isolated in this project successfully highlighted the potential tethered mixed anionic ligands possess in organolithium chemistry, showing both a number of novel aggregation structures as well as numerous cases of superbasic activity.

Item Type: Thesis - PhD
Authors/Creators:Harris, SR
Keywords: Lithium, organolithium, aggregate, crystallography, structure, superbases
DOI / ID Number: 10.25959/100.00045763
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Copyright 2021 the author

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