Long relegated to the background by the pre-eminence of magnesium-based, stoichiometric Grignard reagents, a distinct chemistry of the heavier alkaline earth metals, calcium, strontium and barium, is only now starting to emerge. As similarities have been drawn between the large, electropositive, redox-inert and d0 alkaline earth Ae2+ dications and the Ln3+ cations of the lanthanide series, a growing group 2-mediated catalytic chemistry has developed over the last decade, including polymerisation reactions, heterofunctionalisation reactions of multiple bonds and some rare examples of dehydrocoupling reactions. Among these catalytic reactions the magnesium- and calcium-catalysed intramolecular hydroamination of aminoalkenes has attracted particular interest. Mechanistic studies have demonstrated many parallels with the lanthanide-mediated catalytic cycle based upon successive σ-bond metathesis and insertion steps. In the first part of this thesis, further investigations into the hydroamination/cyclisation reaction have demonstrated the prominent role of the charge density of the catalytic group 2 cation (M = Mg, Ca, Sr, Ba), the beneficial influence of stabilising spectator ligands, and the importance of the choice of the reactive co-ligand for efficient catalyst initiation. Kinetic analyses of reactions monitored by NMR spectroscopy have given new insight into activation energies, entropic effects, substrate and product inhibition, and kinetic isotope effects, leading to a review of the previously suggested lanthanide-mimetic mechanism. In a second part, this study seeks to address two of the main challenges posed by the intramolecular hydroamination reaction in particular, and heavier alkaline earth-catalysed reactions in general: (i) The need to design new monoanionic spectator ligands capable of stabilising heteroleptic heavier alkaline earth complexes and preventing deleterious Schlenk-type ligand redistribution processes in solution; (ii) The stabilisation of highly reactive heteroleptic group 2 alkyl functionalities for fast, irreversible catalyst initiation and novel reactivity.

Graduation date: 1993

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The work in this thesis relates to the preparation and structural and electrical characterisation of calcium and strontium hydrides, imides and nitride hydrides. Conventional solid state methods in controlled atmospheres were used to synthesise these materials. High temperature neutron diffraction, thermal analysis and conductivity studies performed on calcium and strontium hydride suggest an order – disorder transition in these materials at 350 – 450°C. Disordering is believed to involve rapid exchange of hydride ions across two crystallographic sites. This manifests itself in a lowering of the activation energy for bulk hydride ion conduction. The hydride ion conduction is good in these undoped materials: σ[total]subscript = 0.01 S/cm for CaH₂ at 1000K; for SrH₂, σ[total]subscript = 0.01 S/cm at 830K. Doping of SrH₂ with NaH causes a significant increase in the low temperature conductivity, due to presence of extrinsic defects. The high temperature conductivity is negatively affected by NaH doping. Calcium nitride hydride (Ca₂NH) was obtained as a single phase material by reacting either calcium metal or calcium hydride (CaH₂) in an argon atmosphere containing 5 – 7% H₂ and 1 – 7% N₂. Imide ions substituting for hydride and nitride ions constitute a major chemical defect in this material. Long range ordering of the nitride and hydride ions occurs, giving rise to a double cubic crystal symmetry. This order breaks down at 600 – 650°C. Applying the same reaction conditions to strontium metal results in a mixed phase of strontium nitride hydride and imide. No long range order in the nitride hydride phase could be observed. Doping Ca₂NH with lithium hydride (LiH) causes the appearance of a second calcium imide phase, whereas doping with sodium hydride (NaH) increases the amount of imide ions as a defect in the nitride hydride structure, thereby decreasing the long range ordering of nitride and hydride ions.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 1979 / Includes bibliographical references. / Ph. D. / Ph. D. Massachusetts Institute of Technology, Department of Chemistry

The effect of magnesium, strontium, and other alkaline earths on the formation and persistence of metastable carbonates in the natural environment was investigated to determine the nature of the controlling mechanism. Barium and beryllium were studied to evaluate the effect of ionic radius; magnesium and strontium, in order to determine if the results correlate with the usual order of stability for complexes and adsorbed species. Known weights of aragonite were placed in contact with solutions of beryllium, magnesium, calcium, strontium, and barium. Samples were covered and periodically both pH and percent composition of aragonite determined; supernatant liquids and precipitates were analyzed for cation concentrations by atomic absorption spectroscopy and titrimetric methods. Results indicated that the order of effectiveness of alkaline earth metals in inhibiting recrystallization is : Be > Mg > Sr > Ba. This is the expected order of effectiveness for both surface and solution effects. A solution effect (i.e., sequestration of bicarbonate ions) is strongly suggested by the chemical behavior of each cation.

With the increase of the the global energy consumption has also been increasing, which is about 18 TW nowadays (Dudley, 2018), the anthropogenic CO2 emissions have also been increasing, which is about 410 ppm nowadays (Dudley, 2018; Tans & Keeling, 2019). Numerous evidences have been reported indicating that high atmospheric CO2 concentration can have significant greenhouse effect and thus lead to global warming and climate change (Pachauri et al, 2014; Hansen et al, 2013). Therefore, measures need to be taken to control and reduce the atmospheric CO2 concentration. In such circumstance, carbon capture, utilization and storage (CCUS) technologies have been proposed and developed to close the carbon cycle. Mineral carbonation (MC) is one of the CCUS technologies, which mimics the natural silicate weathering process to react CO2 with silicate materials so that carbon can be stabilized in the form of insoluble carbonates for permanent carbon storage (Seifritz, 1990; Lackner et al, 1995). Both Ca- or Mg-bearing silicate minerals and alkaline silicate industrial wastes can be employed as the feedstock for mineral carbonation (Sanna et al, 2014; Gadikota et al, 2014; Park, 2005; Park & Fan, 2004; Park et al, 2003; Park & Zhou, 2017; Zhou, 2014; Zhao, 2014; Swanson, 2014). While they share similar chemistries and total Mg and Ca contents, different MC feedstock can lead to different challenges for CCUS. As for silicate minerals, although they have large enough capacity to mineralize all the anthropogenic CO2 emissions, their reactivities are generally very low, and measures should be developed to accelerate the carbonation kinetics of the minerals (Sanna et al, 2014). However, the elemental extraction of the silicate minerals is a relatively complicated kinetic process, because silica-rich passivation layer can form on the particle surface during mineral dissolution process and thus the rate-limiting step of the process can change from chemical reaction to mass transfer. Without a clear understanding of the elemental extraction kinetics, the design and evaluation of different acceleration methods aiming at different rate-limiting steps of the process can be challenging. As for alkaline industrial wastes, they are generally more reactive than silicate minerals, but can be more heterogeneous with more complicated compositions. In such cases, the separation and recovery of other elements should also be integrated with the carbonation process so that the overall sustainability of the mineral carbonation technology can be enhanced. In order to address these challenges, this study focused on the fundamental understanding of dissolution and carbonation behaviors of alkaline silicate materials and integration of step-wise separations of rare-earth elements (REEs). Both experimental and modeling studies were carried out to provide insights into how Mg and Ca as well as REEs are leaching into solvents at different conditions, and the fundamental understandings on mineral dissolution kinetics and mechanisms were also put forward. The fate of REEs in different product streams was also identified, and methods were developed and optimized to recover and concentrate REEs, while producing solid carbonates with highest purities. Hopefully, the findings in this study can not only advance the carbon mineralization technology but also contribute to the utilization and extraction of alkaline metals, as well as REEs, from other complex unconventional resources for the sustainable energy and material future.

This Thesis describes the activity and mechanism of alkaline earth organohydroborate, tetrahydroborate and alkoxide catalysts for the ring-opening polymerisation (ROP) of cyclic esters including rac-, L-, D- and meso-lactide (LA), and rac-β-butyrolactone (rac-BBL). <b>Chapter One</b> introduces cyclic esters and general mechanisms for their ROP to give polyesters. Living and immortal ROP, an overview of stereocontrolled ROP, and determination of polylactide (PLA) stereosequences are given. Various techniques for polymer characterisations are also described. <b>Chapter Two</b> describes the activity and mechanism of heavy alkaline earth organohydroborate complexes for the ROP of LA. The synthesis and characterisation of alkaline earth alkoxide complexes serving as model species are also described, together with their activities for the ROP of LA. <b>Chapter Three</b> describes the activity and mechanism of a cyclic organohydroborate calcium complex for the ROP of LA. The role of borinic esters as chain transfer agents in the ROP of rac-LA is also discussed. <b>Chapter Four</b> describes the activity and mechanism of heavy alkaline earth tetrahydroborate complexes for the ROP of LA. The immortal ROP of rac-LA using heavy alkaline earth alkoxide complexes and borate esters as chain transfer agents is discussed. <b>Chapter Five</b> describes the activity and mechanism of alkaline earth organohydroborate, tetrahydroborate and alkoxide complexes for the ROP of rac-BBL. <b>Chapter Six</b> presents experimental procedures and characterising data for new complexes reported.

Metal oxide thin films are dynamic materials that have revolutionised the nature of semiconductor and electronic thin film devices. Recently, progress has stagnated in some aspects due to the increasingly complex deposition apparatus required, and the dearth of suitable precursor complexes of certain ???difficult??? metals. This thesis seeks to address both of these issues. The application of a precursor complex, Mg6(O2CNEt2)12 to the SSCVD of MgO thin films delivered the highest quality films ever reported with this technique. The resultant films were found to be of purely (111) orientation. Due to the nature of the precursor, the chemical reactions occurring at the surface during SSCVD growth result in a high growth rate, low flux environment and films of (111) orientation have been achieved without the amorphous underlayer. This finding has important implications for buffer layers in perovskite thin film devices. The unprecedented precursor chemistry has been used as a basis for the extremely high quality material produced, along with the unusual, yet beneficial structural morphology it possesses. A new range of barium complexes with single encapsulating ligands have been prepared for use in chemical vapour deposition (CVD) of BaTiO3 thin films. A novel pathway to an unprecedented class of barium carbamates is reported, and also new dianionic bis ??-ketoesterates and their barium, strontium, and calcium analogues were synthesised. High resolution mass spectrometry showed the barium bis ??-ketoesterate derivatives to be monomeric, and preliminary testing indicated some volatility in these species. Insights were gained into the likely successful pathways to building a volatile heterobimetallic precursor complex containing an alkaline earth metal. The knowledge of intimate mixing in heterobimetallic precursor complexes was extended by some novel chemistry to develop the first mixed Zn/Mg carbamato cluster complexes. These complexes were found to be excellent SSCVD precursors for ZnxMg1-xO thin films. Thin films were deposited with these precursors and exhibited a single preferred orientation, with a constant amount of magnesium throughout the bulk of the films. Investigation of the light emission properties of the films revealed significant improvements in the structural order commensurate with the incorporation of magnesium, and the formation of the ZnxMg1-xO alloy.