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Title: New insights into homo- and heterometallic alkali metal amide chemistry
Author: Robertson, Gemma Margaret
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2012
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Currently a hot topic in organometallic chemistry; bimetallic alkali metal magnesium (or zinc) reagents exhibit a unique ‘synergic’ chemistry which cannot be replicated by either monometallic component on its own. This research project focused on enhancing this chemistry by introducing chiral ligands into these bimetallic systems. The metallate chemistry of achiral cis-2,6-dimethylpiperidide (cis-DMP) is explored through the synthesis and characterisation of a series of bimetallic bases which contain this ligand. The bis(alkyl)amido lithium and sodium zincates [(TMEDA)·Li(μ-cis-DMP)Zn(tBu)2], 75 and [(TMEDA)·Na(μ-cis-DMP)(μ-tBu)Zn(tBu)], 76, were successfully prepared and characterised in both the solid- and solution-states, along with the bis(amido)alkyl sodium zincate [(TMEDA)·Na(μ-cis-DMP)2Zn(tBu)], 77, which was prepared from 76 via a ligand reorganisation process. In addition, the tris(amido) sodium magnesiates [(TMEDA)·Na(μ-cis-DMP)2Mg(cis-DMP)], 79 and [{cis-DMP(H)}·Na(μ-cis-DMP)2Mg(cis-DMP)], 80, are also presented. Surprisingly, little attention has been paid to cis-DMP despite its structural similarity to diisopropylamide (DA) and 2,2,6,6-tetramethylpiperidide (TMP). By comparison of the complexes produced herein with appropriate complexes from the literature, it has been possible to experimentally determine that the steric bulk of cis-DMP closely resembles that of DA but is considerably less bulky than TMP. Focusing on introducing the chiral diamines (−)-sparteine and N,N,Nʹ,Nʹ-(1R,2R)-tetramethylcyclohexane-1,2-diamine [(R,R)-TMCDA] into the molecular framework of bimetallic alkali metal zinc reagents, three bis(alkyl)amido sodium zincates were successfully prepared and characterised ‒ two of the form [{chiral diamine}·Na(μ-TMP)(μ-nBu)Zn(tBu)] [chiral diamine = (−)-sparteine for 96, (R,R)-TMCDA for 97], and the third [{(R,R)-TMCDA}·Na(μ-TMP)(μ-tBu)Zn(tBu)], 98. These complexes represent the first examples of (−)-sparteine or (R,R)-TMCDA being successfully incorporated within the molecular framework of an alkali metal/zinc synergic system [or indeed any alkali metal/divalent metal synergic system for (R,R)-TMCDA], and perhaps most significantly complex 98 is a chiral variant of a synthetically important utility ate base. Having investigated heterobimetallic systems containing chiral diamines, it was also deemed important to study the alkali metal building blocks, specifically chiral diamine adducts of the synthetically important lithium and sodium bis(trimethylsilyl)amides. ‘Conventional' (−)-sparteine adducts of lithium and sodium 1,1,1,3,3,3-hexamethyldisilazide (HMDS) were prepared and characterised, [MHMDS·(−)-sparteine] (M = Li for 106, Na for 107), along with an unexpected and ‘unconventional’ hydroxyl-incorporated solvent-separated hexanuclear sodium sodiate, [(−)-sparteine·Na(μ-HMDS)Na·(−)-sparteine]+[Na4(μ-HMDS)4(μ4-OH)]−, 108. Following this unusual result, a similar complex containing (R,R)-TMCDA was prepared and characterised, namely the hydroxyl-incorporated solvent-separated pentanuclear sodium sodiate [Na{(R,R)-TMCDA}2]+[Na4(μ-HMDS)4(μ4-OH)]−, 109. Given that both of these diamine-NaHMDS systems have formally captured monomeric NaOH, we envisaged that similar systems could encapture substoichiometric quantities of other salts, particularly the Lewis amphoteric metal halides, and in doing so, develop a new Group 1 macrocyclic/supramolecular family of complexes. Initially concentrating on LiHMDS systems and investigating several approaches in reaching this goal, we successfully isolated four solvent-separated ion pair complexes; two of the form [Li·{(R,R)-TMCDA}2]+[Li5(μ-HMDS)5(μ5-X)]− (where X = Cl for 115, Br for 116); and two of the form [Me6-TREN·Li(μ-X)Li·Me6-TREN]+[Li5(μ-HMDS)5(μ5-X)]− [where Me6-TREN = tris[2-(dimethylamino)ethyl]amine and X = Cl for 117, Br for118]. The former two complexes have captured one LiX unit and the latter two complexes two LiX units (bearing parallels with 108 where an additional monomeric NaHMDS unit has been trapped). The anions of these complexes ‒ ten-membered (LiN)5 rings (which host halide guests) ‒ are unprecedented.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available