This Thesis describes the synthesis and characterisation of new Main Group and Rare Earth alkyl, amide, alkoxide and borohydride complexes and their use as catalysts for the ring-opening polymerisation (ROP) of &epsilon;-caprolactone and rac-lactide. <strong>Chapter 1</strong> introduces ROP from an industrial and academic perspective, as well as polymer characterisation techniques. A literature review is given, with an emphasis placed on Main Group catalysts. <strong>Chapter 2</strong> describes the synthesis and characterisation of new homo- and hetero-scorpionate Main Group complexes. An introduction to homo- and hetero-scorpionate ligands is given, as well as a discussion of the ε-caprolactone and rac-lactide ROP activity displayed by the new complexes. <strong>Chapter 3</strong> describes the synthesis and characterisation of new neutral and cationic Main Group borohydride complexes supported by the tris(pyrazolyl)methane and tris(pyrazolyl)hydroborate ligands. A review of borohydride complexes is also given. The ε-caprolactone and rac-lactide ROP activity shown by the complexes presented is also discussed. <strong>Chapter 4</strong> describes the synthesis and characterisation of new mono- and di-cationic yttrium complexes supported by the tris(pyrazolyl)methane and triazacyclononane ligands. An introduction to the synthesis of neutral and cationic Rare Earth complexes is given. An overview of immortal ROP is also provided. The activity of the new complexes towards the immortal ROP of rac-lactide is also discussed. <strong>Chapter 5</strong> contains experimental details and characterising data for the new complexes reported in this thesis. CD Appendix</strong> contains .cif files for all of the new crystallographically characterised complexes.

This thesis describes the synthesis and characterisation of a series of cationic rhodium bis-phosphine complexes. The reactivity of these new complexes in the solid-state and in solution is reported. In <b>Chapter 2</b> the synthesis of a series of rhodium bis-phosphine diene complexes is presented and the reactions of these complexes with hydrogen in the solid-state are investigated. Several examples of zwitterionic complexes coordinating the [BAr^F4]^─ anion are produced by hydrogenation. A rare example of a sigma-alkane complex, [Rh(ⁱBu₂PCH₂CH₂PⁱBu₂)(eta²-_CH-eta²-_CH-NBA][BAr^F4]<sup>─</sup], is also formed in the solid-state, by a single crystal to single crystal transition driven by hydrogen. This complex is crystallographically characterised and displays two short Rh∙∙∙H−C sigma-interactions. Deuteration studies indicate that the agostic complex [Rh(ⁱBu₂PCH₂CH₂PⁱBu₂)(eta²-_CH-eta²-_CH-NBE][BAr^F4] may form as a short lived intermediate prior to the formation of the sigma-alkane complex. The temporal evolution of the solid-state hydrogenation reactions is monitored by powder X-ray diffraction methods. In <b>Chapter 3</b> the C−X activation of various aryl halides using the [Rh(ⁱBu₂PCH₂CH₂PⁱBu₂)]⁺ fragment is reported. The 'ligand innocence' of the phosphine with respect to intramolecular C−H activation is also discussed. A rare example of C−X activation in the solid-state is presented, which shows the formation of an isomer that is not observed by analogous solution routes. <b>Chapter 4</b> investigates solid-state ligand exchange reactions using ethene, butadiene, CO and NH3 gases. A solid-state transfer dehydrogenation reaction is reported within single crystals of [Rh(ⁱBu₂PCH₂CH₂PⁱBu₂)(C₂H₄)₂][BAr^F4]. H/D exchange of NH3 can also occur in the solid state in the bis-ammonia complex [Rh(ⁱBu₂PCH₂CH₂PⁱBu₂)(NH₃)₂][BAr^F4]. A variety of rhodium complexes are tested as heterogeneous catalysts for the hydrogenation of ethene and the isomerisation of butene. In <b>Chapter 5</b> the binding affinity of a variety of fluorinated arenes to rhodium bis-phosphine fragments is presented using ESI-MS methods. The dependence upon the arene substituents, phosphine substituents and phosphine bite angle are discussed.

This thesis expands the scope for using the permethylpentalene ligand and its precursors in the synthesis of organometallic complexes. <strong>Chapter one</strong> begins with a brief review of linked metallocenes, with which multimetallic compounds bridged by pentalene ligands have often been compared, followed by a comprehensive review of the routes used to make pentalenes and substituted pentalenes. Organometallic compounds of pentalenes are introduced, with a focus on bimetallic systems. <strong>Chapter two</strong> explores the diversification of substituents added to the permethylpentalene (Pn*) precursor WeissH₄, to include ethyl and isopropyl groups. Low-symmetry mono-, di-, tri- and tetraalkylated products are formed, eight such organic molecules have been identified by NMR spectroscopy, and two characterised crystallographically. It has been demonstrated that subsequent hydrolysis and decarboxylation of two of these products produces low-symmetry alkylpentalene precursors. The chapter concludes with discussions on the selectivity exhibited in these reactions, and the assignment of stereochemistry. <strong>Chapter three</strong> describes the synthesis of the first homoleptic double metallocene complex of iron. Fe₂Pn*₂ has been characterised by X ray diffraction, and cyclic voltammetry studies demonstrate four accessible oxidation states (-1, 0, +1, +2). Magnetic measurements in the solid and solution state reveal an unusual triplet configuration, and DFT calculations indicate the origin of a high magnetic moment likely resides in unquenched orbital angular momentum contributions from SOMOs which have metal d character. Fe₂Pn*₂ is EPR silent at 5, 40, and 300 K both in solution and the solid state, suggesting a large zero-field splitting parameter. The reaction of the di-iron complex with carbon monoxide, ethylene and H2 is reported; the bimetallic CO adduct, Fe₂(&mu; &eta;⁵,&eta;³ Pn*)(&mu; &eta;⁵,&eta;¹ Pn*)(CO)₂, has been crystallographically characterised, and contains a highly distorted allylic bonding motif, which to the author’s knowledge is believed to be unique among iron complexes. <strong>Chapter four</strong> discusses the interaction of the bidentate Pn* ligand in anti bimetallic fused metallocenes. A new ligand exchange route has been developed to access the complexes (MCp)₂Pn* (M = Co, Ni), and the isostructural complexes (MCp*)₂Pn* have been made for M = Fe, Co, Ni by salt metathesis reactions. All five complexes have been characterised by single crystal X-ray crystallography, and have diamagnetic ground states in solution in common with their Pn bridged analogues. Variable temperature NMR studies reveal a spin-equilibrium between S = 0 and S = 1 in the dinickel complexes. DFT calculations reproduce the spin states found, and suggest the distortion towards &eta;³ coordination observed on crossing from Fe, to Co, to Ni, results from population of orbitals with M―bridgehead antibonding character. The electronic structures show it is important to draw comparisons between isoelectronic linked metallocenes. Electrochemical studies on the diiron, dicobalt, and (NiCp)₂Pn* complexes reveal at least three redox events for each. <strong>Chapter five</strong> documents the successful synthesis and characterisation of monometallic complexes of iron and manganese with Pn*H ligands. The isostructural complexes Fe(Pn*H)₂ and Mn(Pn*H)₂ can have been characterised crystallographically, and are potential precursors for accessing heterometallic, and multimetallic complexes. Mn(Pn*H)₂ is a rare example of a manganese sandwich compound and magnetic studies on a single isomer in the solution and solid states suggest it adopts intermediate spin states of S = 2 in solution, and S = 3/2 in the solid state. <strong>Chapter six</strong> gives experimental details for all syntheses and studies described in the preceding chapters. <strong>Chapter seven</strong> provides characterising data for all new compounds. Fitting data for VT NMR and SQUID studies are provided in the <strong>appendix</strong> at the end of this thesis. Crystallographic data in the form of .cif files, DFT output files, and raw SQUID data, can be found in the <strong>electronic appendix</strong>.

This thesis explores the capacity of sterically and electronically unsaturated boranes to bind substrates of biological and environmental interest, and transduce such binding events into a photo-physical and/or electrochemical response, hence reporting the presence of these substrates. Chapter three details the synthesis of a range of novel ferrocenyl boranes featuring either a proximal hydrogen-bond donor or a second Lewis acidic centre. These novel boranes were shown to be competent at binding both cyanide and fluoride anions, with the role played by a proximal hydrogen-bond donor or a second Lewis acidic centre in anion binding investigated by both NMR and crystallographic studies. Chapter four reports the synthesis of novel pyridinyl and related boronic esters, as well as unexpected mixed alkenyl/aryl boranes. The capacity of both types of system to bind fluoride or cyanide anions in solution was investigated by UV-Vis and NMR studies. The photo-physical responses to these anions were also probed, leading to the establishment of both switch-on and switch-off fluorescent responses. Chapter five extends the knowledge derived from selective anion receptor design and combines this with recent developments in the field of frustrated Lewis pairs (FLPs) to activate, bind and report the presence of nitrous oxide (N₂O) molecule. Thus, the syntheses of novel, highly Lewis acidic ferrocenyl boranes that incorporate a high degree of steric loading around the boron centre are reported. The electrochemical and photo-physical response of an FLP system to the presence of N₂O was investigated leading to the development of a novel N₂O reporting system.

This Thesis describes the synthesis and characterisation of sulfonamide supported titanium, zirconium and aluminium complexes and their use as ring opening polymerisation catalysts for ε-caprolactone and rac-lactide. Chapter 1 introduces polyester use, development and characterisation in general. Metal catalysed ring opening polymerisation of cyclic esters is considered in a literature review of the field. Titanium, zirconium and aluminium complexes supported by polydentate sulfonamide ligands are also discussed. Chapter 2 describes the synthesis and characterisation of new sulfonamide supported titanium amide, isopropoxide and zirconium isopropoxide complexes. Their application as catalysts for the ring opening polymerisation of ε-caprolactone and rac-lactide is discussed and compared with known zirconium isopropoxide complexes supported by bis(phenolate) amine ligands. Chapter 3 describes the synthesis and characterisation of Cs symmetric titanium amide and alkoxide complexes supported by dianionic, tri- and tetradentate sulfonamide ligands. Zirconium alkyl and amide complexes supported by C3- symmetric trianionic ‘tren’ type ligands bearing three different sulfonamide groups are also presented. The application of these complexes for the ring opening polymerisation of ε-caprolactone and rac-lactide is described and compared with the complexes presented in Chapter 2. Chapter 4 provides an overview of the synthesis and characterisation of aluminium alkoxide and alkyl complexes supported by dianionic, tri- and tetradentate sulfonamide ligands. Solution state behaviour and solid state structures are presented and discussed. An assessment of these complexes for the ring opening polymerisation of rac-lactide is presented. Chapter 5 presents full experimental procedures and characterisation data for the new complexes reported. CD Appendix contains .cif files for all new crystallographically characterised complexes described, and additional polymerisation graphs.