Heteroleptic Thorium Terphenolate complexes for small molecule activation

Abstract

The chemistry and physical properties of actinide complexes has become increasingly significant and relevant since the dawn of the nuclear age. In addition to increasing the potency of nuclear power and the safety and disposal of its subsequent waste products, exploration of the chemistry of actinide complexes provides a fascinating insight into the increased complexity and divergence of reactivity of these complexes when compared to transition metal complexes. Chapter One provides a brief introduction to the chemistry of actinides and in particular, the major focus of this work, of thorium. This is followed by a survey of examples of rare examples of thorium complexes with a formal oxidation state other than Th (IV). Following this is a review of selected examples of thorium (IV) complexes exhibiting unusual reactivity surveying thorium hydride and alkyl complexes initially. This progresses into reviewing the chemistry of thorium complexes containing multiple bonds to non-metal atoms, beginning with carbon atoms and then progressing to atoms in the chalcogen and pnictogen groups. The introduction finishes with an investigation into the properties of the terphenolate ligands used in this study, including examples of unusual complexes that they have been shown to stabilise. In Chapter Two, an exploration into the catalytic activity of fairly simple actinide amide catalysts, N”2Th (IV) {k2-N(SiMe3)SiMe2CH2, N”2U (IV) {k2-N(SiMe3)SiMe2CH2} and UN”3, upon terminal acetylenes is presented. The chapter begins with a brief introduction summarising the previous reactivity observed in the catalysis of terminal acetylenes, with particular focus on actinide-based catalyst mediated reactions. The catalytic results on a variety of terminal acetylenes with different steric and electronic properties is then reported upon. It is found that high conversions and selectivities can be achieved upon optimisation of the catalytic process. It was also found that the different catalysts and substrates favoured different products, with selective oligomerisation and cyclotrimerisation reactions observed. The differing reactivities lend support to the role of f-electrons upon the catalytic route of the reaction. Conclusions are discussed at the end of the chapter. In Chapter Three, the synthesis and characterisation of heteroleptic terphenolate thorium chloride complexes and their subsequent reactivity was investigated. The synthesis and characterisation of ThCl2(OTerMes)2DME and ThCl2(OTerMes)2(H2O)3 are initially described. The reactivity of these complexes favoured transmetallation of the terphenolate ligands, with the complexes; [Li(OTerMes)THF]2, [Li(OTerMes)]2THF, μ3- (TerMesO)μ3-(CH2SiMe3)3Li4, LiAlH2(OTerMes)2, [(THF)K(OTerMes)]2, MgCl(OTerMes)(THF)2, MgBr(OTerMes)(THF)2 and Fe(OTerMes)2(py)2 synthesised and characterised from reactions attempting to transform the ancillary chlorido-ligands. The reactivity of ThCl2(OTerMes)2DME was found to not be solely transmetallation of the terphenolate ligands as elucidated by the synthesis and characterisation of [Th(OTerMes)2(Cl)2(4,4’- bipyridyl)1.5]∞ and [MgTh2μ2-Cl2μ3-Cl(OTerMes)2(C4H7)2μ-η3:η3(C4H7)H]. The synthesis of [MgTh2μ2-Cl2μ3-Cl(OTerMes)2(C4H7)2μ-η3:η3(C4H7)H] was found to proceed via a reductive elimination route with concomitant formation of a terphenolate transmetallation product Mg(OTerMes)2(THF)2. The formation of[Th(OTerMes)2(Cl)2(4,4’- bipyridyl)1.5]∞ was achieved via reaction with the Lewis base 4-4’ bipyridine. Reactions attempting to form heteroleptic uranium terphenolate complexes were also detailed. Conclusions are discussed at the end of the chapter. In Chapter Four, the synthesis and characterisation of heteroleptic terphenolate thorium borohydride complexes and their subsequent reactivity was investigated. It was found that the conversion of ThCl2(OTerMes)2DME to Th(BH4)2(OTerMes)2DME proceeded smoothly using a precedented reaction route. In contrast to ThCl2(OTerMes)2DME, reaction with a Lewis acid was found to result in abstraction of the solvating DME molecule, resulting in the synthesis and characterisation of Th(BH4)2(OTerMes)2. In similarity to ThCl2(OTerMes)2DME, Th(BH4)2(OTerMes)2DME was found to react with a Lewis base (4-4’ bipyridine) to form Th(BH4)2(OTerMes)2(4,4’ bipyridine)∞. However, despite the increased robustness and versatility of the borohydride complexes, transmetallation of the terphenolate complexes remained an issue as shown by the synthesis and characterisation of Mg(OTerMes)((μ-H)3BH)THF)2. Th(BH4)2(OTerMes)2 was found to be able to facilitate small molecule activation in a variety of substrates, encompassing CO, CO2 and CS2 amongst others. In most cases this small molecule activation favoured the formation of BMe3, with the concomitant formation of HB(OTerMes)2 in the case of CO2 and CS2. Attempts at catalysis of isonitriles and terminal acetylenes by Th(BH4)2(OTerMes)2 are presented with mixed results. Conclusions are discussed at the end of the chapter. In Chapter Five, investigations into the effects of changing the donor atom of the terphenyl moiety were probed. The chapter began by examining the differing properties of a phosphorous atom acting as a ligating atom, as opposed to the oxygen atom seen in Chapters Three and Four. The chapter continued by detailing the result of reactions attempting to synthesise and characterise terphenyl phosphino-actinide complexes. It was found that in the case of actinides with easily accessible lower oxidation states, i.e. U (IV), that reductive elimination was favoured, culminating in the isolation of (TerMesPH)2. Following this result attempts were made to modify the ligand system in an attempt to divert the reaction away from this product, in the hope of isolating a phosphino-actinide complex. Reactions attempting to ligate the terphenyl moiety via the aryl α-carbon to thorium were also detailed, resulting in radicular degeneration and the isolation of nBuTerTrip and ClTerTrip. Conclusions are discussed at the end of the chapter. Experimental and characterising data are provided in Chapter Six.