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Synthesis, structures and reactivity of rare earth and alkaline earth aryloxides
thesisposted on 16.01.2017, 22:53 by Townley, Joshua
The focus of this Thesis is on the synthesis and characterisation of aryloxides of the lanthanoid and alkaline earth metals with an emphasis on structural characterisation. One of the key objectives has been to explore the boundaries of the redox transmetallation/protolysis (RTP) reaction (formerly known as redox transmetallation/ligand exchange), and this is the focus of Chapter 2. Redox transmetallation/protolysis reactions were performed in the non-donor solvents toluene and hexane, in which success has not previously been achieved, and the reaction rates monitored by 19F NMR spectroscopy. A donor-solvent free pathway is proposed in which an intermediate of the type [Ln(C6F5)2(HOAr)n] is formed, and plays a pivotal role in determining the speed of the reaction. The application of this method for the synthesis of homoleptic products was established by the synthesis of the known homoleptic compounds [Ln(2,6-dpp)3] (Ln = La, Pr, Nd), [Ba2(2,6- dpp)4] (2,6-dppH = 2,6-diphenylphenol) and [La(2,6-dip)3]2 (2,6-dipH = 2,6- diisopropylphenol) as well as the isolation of new compounds [La4(3,5-dbp)12(3,5- dbpH)] and [Nd4(3,5-dbp)12(3,5-dbpH)]/[Nd4(3,5-dbp)10(3,5-dbpH)4(OH)2] (3,5- dbpH = 3,5-di-tert-butylphenol). A series of lanthanoid aryloxide complexes of the type [Ln(2,4-dbp)3(THF)3] (Ln = La, Pr, Nd, Gd, Er) were prepared by RTP reactions with 2,4-di-tert-butylphenol (2,4-dbpH) and structurally characterised (Chapter 3). Reaction in DME with neodymium afforded the seven-coordinate [Nd(2,4-dbp)3(DME)2], while the dinuclear [Yb2(2,4-dbp)6(DME)2] was obtained by an analogous reaction with ytterbium. Recrystallising the THF derivatives from toluene or hexane resulted in the loss of THF and the formation of the dinuclear [Ln2(2,4-dbp)6(THF)2] (Ln = Nd, Er). The structurally similar [Nd2(2,4-dbp)6(2,4-dbpH)2] was the product of a thermally induced protolysis reaction between the metal and the phenol. A RTP reaction between ytterbium metal and 2-tert-butylphenol (2-tbpH) did not give a solid product, however, the complex [Yb2(2-tbp)6(THF)2] was obtained after crystallisation from toluene. Extending this chemistry to the heavy alkaline earth metals, calcium, strontium and barium resulted in the isolation of a group of new and interesting compounds ii (Chapter 4). Reactions in THF afforded [Ca(2,4-dbp)2(THF)4] and [Sr3(2,4- dbp)6(THF)6], while analogous reactions in DME gave the remarkable [Ca2(2,4- dbp)4(DME)5], containing a bridging molecule of DME, and the asymmetrically bridged [Sr2(2,4-dbp)4(DME)3]. Reactions with the larger barium yielded cluster complexes [Ba8(2,4-dbp)12(OH)4(DME)4] and [Ba8(2,4-dbp)12(OH)4]. Chapter 5 details the synthesis and characterisation of a series of lanthanoid and alkaline earth calixarene complexes. Reactions between the lanthanoid metals and p- But-calixarene in the presence of bis(pentafluorophenyl)mercury yielded [Ln(calixOH)(THF)]2 complexes (Ln = Nd, Yb). Attempts to isolate divalent products through the strategic use of the p-But-calix(Et)2(OH)2 were unsuccessful, instead affording a series of chloride-containing complexes of the type [Ln(calixEt2)Cl(THF)]2, in which the presence of the chloride arose from contamination of the starting material. A reaction between strontium metal and p- But-calix(OH)4 in the presence of bis(pentafluorophenyl)mercury afforded [Sr4(calix(OH)2)4(OH2)4(THF)4] after partial hydrolysis. A number of compounds prepared in the course of this work ([Nd(2,4- dbp)3(THF)3], [Gd(2,4-dbp)3(THF)3], [Ca2(2,4-dbp)4(DME)5], [Sr2(2,4- dbp)4(DME)3]) as well as the known compounds [Yb(ttfpz)2(THF)4], [Sm(dippForm)2(THF)2], [La(xylForm)3(THF)], [Sm(xylForm)3] and [Sm(mesForm)3] were taken to Oxford University and tested as potential initiators for the ring-opening polymerisation of rac-lactide. All were found to be capable of initiating polymerisation, albeit with varying degrees of success. Most systems could be improved by the introduction of a co-initiator (or chain-transfer agent) such as benzyl alcohol or benzylamine to promote immortal polymerisation. Chapter 6 provides detailed discussions of the polymer features, as well as kinetics studies of the rate of polymerisation in the cases of [Nd(2,4-dbp)3(THF)3], [Ca2(2,4- dbp)4(DME)5], [Sr2(2,4-dbp)4(DME)3] and [Sm(dippForm)2(THF)2] in the presence of benzylamine. Furthermore a structure-activity relationship for the alkaline earth initiators is discussed.