The ability of a deprotonated aza-crown ether to allow isolation of soluble lanthanide and yttrium complexes has been investigated. A convenient route to these complexes has been demonstrated by the protonolysis reactions of Ln[(N(SiMe₃)₂]₃ with 4,13-diaza-18-crown-6. NMR spectroscopy and X-ray crystallography revealed a C₂V structure consisting of a basket shaped geometry. The successful protonolysis route has been extended to the preparation of stable alkyls, dialkyls, and alkyl cations of yttrium and zirconium stabilized by deprotonated aza- crown macrocycles. A yttrium, alkyl complex containing deprotonated diaza-18-crown-6 has been prepared by the protonolysis route. The thermal stability and reactivity of this complex were investigated. This alkyl reacts with terminal alkynes to produce a complex equilibrium between the colourless monomeric and dimeric alkynides and a purple Z-butatrienediyl (ie.RC=C=C=CR²⁻) coupling product. NMR studies demonstrate that electron poor alkynes favour coupling and that the carbon-carbon double bond forming process is readily reversible at room temperature. The flexibility of the deprotonated diaza-crown ligand is apparent from the isolation of both cis and trans-zirconium dibenzyl complexes from the protonolysis of tetrabenzyl zirconium with 4,13-diaza-18-crown-6. The structure of both isomers were investigated by NMR spectroscopy and X-ray crystallography. Both the cis and trans-isomers cleanly converted to the stable cation either by protonolysis with [n-Bu₃NH]⁺[BPh₄]⁻ or by alkyl abstraction with B(C₆F₅)₃. The reactivity of the alkyl cation derived from the reaction with B(C₆F₅)₃ was investigated. The reaction of this cation with t-BuNC gave a vinylamide complex following a 1,2-proton rearrangement of an initially formed iminoacyl. Two members of the still rare yttrium dialkyl class of compounds were isolated using monoanionic, deprotonated aza-crown ethers as supporting ligation. The dialkyl complexes were synthesized by protonolysis of Y(CH₂SiMe₃)₃(THF)₂ with either aza-18-crown-6 or aza-15-crown-5. NMR and X-ray analyses of the yttrium dialkyl supported by aza-18-crown-6 indicates a trans-dialkyl geometry while NMR analysis of the aza-15-crown-5 analog indicates a cis-dialkyl geometry. Reaction of the trans-dialkyl complex with CO afforded a trans-dienolate complex formed by the migration of SiMe₃. Alkyl abstraction from the trans-dialkyl complex using B(C₆F₅)₃ allowed generation of the first yttrium, alkyl cation. / Graduate

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No description available.

Galactitol and cis,cis,cis-1,3,5-cyclohexanetriol are polyols that are ideal examples of model compounds for ligands with lanthanide ions as they have their hydroxyl groups in favourable steric arrangement. Several complexes were synthesised with both lanthanide chloride and lanthanide nitrate salts with galactitol, and a variety of structures, both polymeric and monomeric, were observed. In all these complexes, galactitol acted as a bridging molecule between the lanthanide ions. A notable difference was the lanthanum chloride–galactitol complex that showed both chloride and galactitol bridging. The lanthanide nitrate salts formed only polymeric complexes with galactitol. Not all of the complexes showed nitrate ions coordinated to the metal centre, and in the neodymium nitrate– galactitol complex, which shows both monodentate and bidentate coordination of the nitrate groups. The coordination of the nitrate ions was confirmed using both XRD and IR analysis. Two complexes with lanthanide chloride salts and cis,cis,cis-1,3,5-cyclohexanetriol were synthesised and analysed. Lanthanum chloride formed a polymeric complex, which showed extensive chloride bridging between the metal centres. Praseodymium chloride formed a dimeric complex. All complexes were analysed with single-crystal X-ray diffraction, 1H NMR, 13C NMR and IR spectroscopy.

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No description available.

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No description available.

M.Sc. / Aspects of electron-electron correlations have for the past few decades been at the forefront of research in Solid State Physics. More traditional concepts under this topic have been phenomena such as superconductivity, and magnetic ordering in its many forms including long-range ordering and spin-glass freezing. The class of so-called strongly correlated electron systems has been a particularly active field of study, as witnessed by for instance the series of annual international conferences held under this topic since 1992. Compounds and alloys of strong electronic correlations have proved a very rich field of new and anomalous physical behaviours in metallic and semiconducting compounds and alloys of especially 4f- and 5f-electron systems, together with ceramics characterized as the so-called ¡§high-TC¡¨ superconductors. The f-electron systems have revealed a variety of behaviours such as ~ 1000-fold enhanced effective electron masses at low temperature, coexistence of superconductivity and magnetic ordering in systems where the magnetic interactions are far too strong to allow for Cooper-pair formation within the well-established BCS-interpretation, and electron transport and thermodynamic behaviour at low temperatures that completely defy our conventional Fermi-liquid paradigm of understanding the ground states of metals. The series of pseudo-ternary compounds Ce1-xLaxRu4Sn6 that were synthesized and characterized in this work for the first time are formed by substituting La for Ce in CeRu4Sn6, the parent compound. CeRu4Sn6 exhibits a number of properties which have been associated with a special class, the Kondo semiconductors of strongly correlated electron systems. CeRu4Sn6 has very recently been shown [A. M. Strydom et al. (2004)] to comprise an intriguing combination of characteristics that are thus far unique among the Kondo semiconductors: At low temperature (T 10 K) the specific heat proves the development of very strong electronic correlation out of an already low density of charge carriers (as shown by Hall-effect, resistivity, and the Sommerfeld coefficient of the specific heat). Furthermore, the specific heat follows a logarithmic increase as temperature is decreased below ~ 2 K, in a range where thermal transport shows the presence of an energy gap in the electronic density of states. The aim of this work was to investigate the intermediate and higher temperature (4.01 K „T T „T 300 K) behaviour of the electrical resistivity of the Ce1-xLaxRu4Sn6 series of compounds in which the concentration of the 4f-electron magnetic ion Ce is progressively being reduced. A steady but slow decrease of the energy gap with increasing La concentration was found in this work in contrast with what is usually the case in Kondo semiconductors. Both the presence of an energy gap and the low density of charge carriers are found to be connected to the presence of Ce in the unit cell, and are therefore not an artefact of the peculiar filledƒ{cageƒ{like tetragonal crystal structure of these compounds. An interesting strong anisotropy was found in the way in which the tetragonal unit cell expands preferentially within the aƒ{b plane, compared to the elongation along the cƒ{axis, upon moving from CeRu4Sn6 to LaRu4Sn6. / Prof. A.M. Strydom

This Thesis describes the synthesis and characterisation of a series ofbisphenolate supported samarium borohydride, amide and zwitterionic rare earth complexes and their ability to effect the ring opening polymerisation (ROP) of cyclic esters and methylmethacrylate (MMA). Chapter 1 introduces ROP from both an industrial and an academic perspective and describes in detail the research in this area, with emphasis on rare earth initiators. The lanthanide elements and the bisphenolate ligand are also introduced. Chapter 2 describes the synthesis and characterisation ofbisphenolate supported samarium borohydride and silylamide complexes. Chapter 3 describes the ability of a selection of samarium borohydride and amide complexes to effect the ROP of the cyclic esters s-caprolactone (f-CL) and rac- lactide (rac-LA). Emphasis is placed on the effect that the nature of the bisphenolate pendant arm and the initiating moiety has on the polymerisation process. Chapter 4 describes the synthesis and characterisation of rare earth zwitterionic complexes and the ability ofa range of these complexes to effect the ROP of s-Cl. and rac-lactide. Mechanistic aspects ofthe ROP process will be discussed, as will the ability of these complexes to yield amide functionalised poly(rac-LA). Chapter 5 describes the ability ofbisphenolate samarium borohydride complexes to initiate the polymerisation of MMA. The experimental work conducted as part of this study is further supported computationally by calculations at the DFT level, both aspects will be described. Aspects concerning the synthesis and characterisation of the related borohydride derivative [Sm(N2siMe3NNPY)(BH4)2Li]oo will also be emphasised. Chapter 6 contains full experimental and characterising data for all 0 f the new compounds reported in this Thesis. Appendices A- T contain tables of selected crystallographic data for all new crystallographically characterised complexes described in this Thesis (partially on CD).