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Title: Synthesis and coordination chemistry of functionalised phosphazanes
Author: Benson, Callum Giles Maxwell
ISNI:       0000 0004 6348 9675
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2016
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This thesis focuses on the chemistry of novel phosphazane species derived from the chloro-phosphazanes [ClP(μ-NR)]$_2$ and their use as ligands and building blocks for macrocyclic compounds. The introduction (Chapter 1) surveys previous literature in the area, which is pertinent to the new studies. One of the most important issues is the various ways in which dimeric phosph(III)azanes can be employed as precursors for new ligands and in the design of new types of inorganic macrocycles. In Chapter 2 the synthesis of new chloro-dimers of the type [ClP(μ-NR)]$_2$ is described, which are the primary starting materials. Chapter 3 concerns the substitution at the chlorine atoms of [ClP(μ-NR)]$_2$ by LiSH to give a variety of sulfur-containing PV species [S=P(H)(μ-NR)]$_2$. The thermodynamic preference for the cis or trans isomers of the latter are explored by detailed NMR spectroscopic and DFT calculation investigations. Deprotonation of the $^t$Bu derivative [S=P(H)(μ-N$^t$Bu)]$_2$ using organometallic bases (Chapter 3) led to the isolation of s-block salts (Mg$^2+$, Na$^+$, K$^+$) of the P$^III$ dianion [S-P(μ-N$^t$Bu)]$^2_2-$. The Mg$^2+$ salt was found to form a mononuclear complex in the solid state whereas the Na$^+$ salt formed a large cage comprising eight dianions and 16 Na$^+$ ions built around a NaSH core. These s-block metal salts were shown to be useful transfer reagents for the dianion towards complexation with main group metals (e.g., Sn and Ge). Chapter 4 explores the oxidation of the P$^III$ dianions [S-P(μ-N$^t$Bu)]$^2_2-$ by chalcogens to give the more stable P$^V$ species [(E=)P$^V$(-S)(μ-N$^t$Bu)]$^2_2-$ (E = S, Se). The increase in stability and lower reducing ability of the P$^V$ dianion allowed the formation of complexes with transition and main group metals. Reaction of [(E=)PS(μ-N$^t$Bu)]$^2_2-$ with chloro-dimers [ClP(μ-NR)]$_2$ led to the formation of homo- and heteroleptic P$^III$-P$^V$ phosphazane macrocycles of the type [{S=PV(μ-N$^t$Bu)}2(u-E){P$^III$(μ-NR)}$_2$]. These can be oxidised to give air- and moisture-stable all-P$^V$ species [{S=P$^V$(μ-N$^t$Bu)}$_2$(μ-E){S=P$^V$(μ-NR)}$_2$] by reaction with elemental sulfur. Finally, in Chapter 5 the substitution of the chlorine atoms in [ClP(μ-NR$^1$)]$_2$ by amines (R$^2$NH$_2$) is explored, to give a range of bis(amino) cyclophosphazanes [(R$^2$NH)P(-NR$^1$)]$_2$ containing chiral and non-chiral R$^2$ and R$^1$ groups. These species were used in the formation of early transition metal (Zr, Ti, Hf) complexes which are potential pre-catalysts for alkene polymerisation studies.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Chemistry ; Inorganic ; Phosphazane ; Macrocycle