Mixed anion lithium complexes : models for superbases
Creators: name: Lockhart-Gillett, Bryce
Publisher: name: University Of Tasmania
Year: 2023

description: The work reported in this thesis describes the synthesis, characterisation, and reactivity of several mixed anion lithium complexes. Chapter 2 is concerned with the design, synthesis, and characterisation of the mixed phenol/amine containing ligands, which serve as the backbone for mixed O/N anion lithium complexes also described. N-phenylsalicylaldamine, (`ONPhH_2`), N-2,6-diisopropylsalicylaldamine (`ONDIPPH_2`), and N-t-butylsalicylaldamine (`ONtBuPhH_2`), were prepared from their corresponding imine precursors, which were themselves prepared via imine condensation of the appropriate primary amines with salicylaldehyde. Solid state structures were obtained for the amine compounds, which display intermolecular H-bonding networks. Chapter 2 also details the synthesis, and characterisation of mono- and some dilithiated complexes of the O/N ligands. The monolithiated complexes were observed to exclusively form tetrameric species. The two complexes `[{Li(ONPhH)}_4]` and `[{Li(ONDIPPH)}_4]` were prepared by lithiation of the corresponding amine ligands with n-BuLi in 40-60¬¨‚àûC petroleum spirits. The dilithiated complexes were observed to preferentially form dimers in the solid state. Two dimeric dilithiated complexes `[{Li_2(ONPh)}_2(THF)_6]` and `[{Li_2(ONDIPP)}_2(THF)_4]` were prepared by lithiation of the amine ligands with n-BuLi in THF. Several of the lithiated complexes underwent various solvent exchange reactions yielding related complexes; the THF solvated monolithiated tetrameric complex `[{Li(ONPhH)}_4(THF)_3]` was obtained by exposing `[{Li(ONPhH)}_4]` to THF, and the dilithiated complexes `[{Li(ONPh)}_2(TMEDA)_3]`, `[{Li_2(ONDIPP)}_2(TMEDA)_2]`, `[{Li(ONPh)}_2(DME)_2(THF)_2]`, and `[{Li_2(ONDIPP)}_2(DME)_2]` were prepared by exposing the dilithiated THF complexes to TMEDA and DME respectively. (TMEDA = N,N,N',N'-tetramethylethylenediamine, DME = 1,2-dimethoxyethane). A related tetrameric dilithiated complex `[{Li_2(ONPh)}_4(THF)_4]` was obtained from `[{Li_2(ONPh)}_2(THF)_6]` by heating in benzene. Chapter 3 describes further solvent exchanged dilithiated complexes. The complexes `[{Li_2(ONPh)}_2(MeOCH_2CH_2Ot-Bu)_2(THF)_2]`, `[{Li_2(ONDIPP)}_2(MeOCH_2CH_2Ot-Bu)_2]`, and `([{Li_2(ONDIPP)}_2`\\((dioxane)(THF)])_‚Äöv†vª\\) were obtained by exposing the dilithiated THF complexes to `MeOCH_2CH_2Ot-Bu` and dioxane respectively. This chapter also describes the reactivity of some of the dilithiated complexes towards ether type substrates. Cleavage of both aliphatic and aromatic ether molecules was observed, with predictable stereospecificity correlated to the observed geometry of the dilithiated complex. From this reactivity a further monolithiated complex containing a modified N-methylated ligand was obtained `[{Li(ON(Me)PhH)}_4]`. Chapter 4 reports the synthesis and characterisation of mixed amide/alkoxide ligands and their complexes. The ligands N-phenyl(2-trimethylsilylmethyl)benzyl amine `(NCPhH_2)` and N-2,6-diisopropylphenyl(2-trimethylsilylmethyl)benzyl amine `(NCDIPPH_2)` were prepared, and lithiated with n-BuLi to yield the monolithiated complexes `[({Li(NCPhH)}_2(NCPhH))_2]` and \\([(N-TMS 2,6-diisopropyllithiumamide)(THF)_3]\\). Chapter 5 is a compilation of some serendipitous compounds that were isolated from reactions described in Chapter 2 and Chapter 3. Each compound contains unexpected and serendipitous inclusion of molecular fragments, either into the aggregated complex as in the complex `[{Li(ONtBu)}_3Li(OEt)(Et_2O)_3]`, or a portion of silicon grease incorporated into the dilithiated ligand backbone as in `[{Li_2OODIPPSi_2}(DME)_2]`, `[{Li_2(OODIPPSi)}_2(TMEDA)_2]` and `[K_2{Li_2(OODIPPSi)}_2{Li(ONDIPPH)}_2{LiOSi(Me)_2O}_2(DME)_4]`. The complex `[{Li_2(OODIPPSi)}_2{Li(OEt)}_2(Et_2O)_2]` was also observed, which contains both a molecular fragment as well as a modified ligand. Some theoretical work was undertaken to investigate the preference of the dimeric complexes in Chapter 2 towards particular core geometries observed in the solid state. In addition, a possible reaction pathway for the cleavage of DME was modelled in Chapter 3, and supports the observed stereospecificity of the reaction.