Bimetallic uranium and cerium tetraphenolate complexes for the activation and functionalisation of small molecules

thesis describes the synthesis of bimetallic f-element complexes of meta-functionalised tetraphenol arene ligands, (mTP), and their reactivity, primarily towards small molecular substrates such as dinitrogen. Chapter one introduces the principles of uranium chemistry and the suitability of aryloxides as ligands for low oxidation state uranium. The synthesis and reactivity of selected examples of U(III) and U(IV) complexes are described and an overview of dinitrogen activation by selected transition metal complexes is provided. Chapter two reports the synthesis and characterisation of two meta-functionalised tetraphenol arene ligand precursors H4(mTPm) and H4(mTPt) and describes reactions designed to target closely related analogues. In 1:1 salt metathesis or protonolysis reactions, two tetradentate ligands and two U(IV) centres are combined to yield [U2(mTP)2] complexes with a novel ‘letterbox’ architecture. The reduction of some of these complexes under an atmosphere of dinitrogen yields bound [N2H2]2- following an intramolecular reaction of an activated N2 fragment with benzylic C−H bonds provided by the ligand. The [N2H2]2- moiety is susceptible to further functionalisation through reactions with external electrophiles. Stoichiometric and catalytic functionalisation reactions are discussed and a mechanistic pathway for these transformations is proposed. Chapter three presents the synthesis of another set of bimetallic complexes, [U2(mTP)X4] (X = one electron donor ligand), derived from 2:1 reactions of uranium(IV) with H4(mTP). The redox chemistry of these complexes is explored through cyclic voltammetry and the structure and reactivity of these ‘half-letterbox’ complexes is compared to the complexes reported in Chapter two. The work described in Chapter four returns to complexes with a ‘letterbox’ geometry but employs Ce(III) and potassium cations to prepare a set of heterobimetallic analogues, [K][Ce2(mTP)2K]. The oxidation chemistry of these complexes is investigated and an EPR study is used to examine the magnetic behaviour of the two 4f1 Ce(III) centres. Chapter five details the experimental procedures and characterisation data for the work described in the preceding chapters

Dicerium letterbox-shaped tetraphenolates : f-block complexes designed for two-electron chemistry

Rare examples of molecular, dinuclear CeIII and PrIII complexes with robust Ln-coordination are accessible by use of the tetraphenolate pTP as a supporting, chelating O-donor ligand platform, pTP = [{2-(OC6H2R2-2,4)2CH}-C6H4-1,4]4− that favours the higher formal oxidation states accessible to rare earths. Two classes of complexes have been made from the platforms; one metallacyclic 2 + 2 [Ln2(pTP)2] framework with a rigid, letterbox-shaped geometry and [Ln(aryloxide)4] core, and one more flexible [(LnX)2(pTP)] with one rare earth ion at either end of the platform. The LnIII letterbox complexes have two K+ counter-cations, one of which sits inside the letterbox, binding the two central arenes of the platform sufficiently strongly that it cannot be displaced by solvent molecules (THF and pyridine) or crown ethers. Oxidation of the CeIII lettterboxes is facile and forms the unusual neutral molecular (CeIV)2 letterbox in which the CeIV reduction potential is −1.83 V vs. Fc/Fc+. The electronic structure of the Ce(III/IV) complexes was investigated using HERFD-XAS (high energy resolution fluorescence detection X-ray absorption spectroscopy).Publisher PDFPeer reviewe

Immobilising molecular Ru complexes on a protective ultrathin oxide layer of p-Si electrodes towards photoelectrochemical CO2 reduction

Photoelectrochemical CO2 reduction is a promising approach for renewable fuel generation and to reduce greenhouse gas emissions. Owing to their synthetic tunability, molecular catalysts for the CO2 reduction reaction can give rise to high product selectivity. In this context, a Ru-II complex [Ru(HO-tpy)(6-mbpy)(NCCH3)](2+) (HO-tpy = 4 '-hydroxy-2,2 ':6 ',2 ''-terpyridine; 6-mbpy = 6-methyl-2,2 '-bipyridine) was immobilised on a thin SiOx layer of a p-Si electrode that was decorated with a bromide-terminated molecular layer. Following the characterisation of the assembled photocathodes by X-ray photoelectron spectroscopy and ellipsometry, PEC experiments demonstrate electron transfer from the p-Si to the Ru complex through the native oxide layer under illumination and a cathodic bias. A state-of-the-art photovoltage of 570 mV was determined by comparison with an analogous n-type Si assembly. While the photovoltage of the modified photocathode is promising for future photoelectrochemical CO2 reduction and the p-Si/SiOx junction seems to be unchanged during the PEC experiments, a fast desorption of the molecular Ru complex was observed. An in-depth investigation of the cathode degradation by comparison with reference materials highlights the role of the hydroxyl functionality of the Ru complex to ensure its grafting on the substrate. In contrast, no essential role for the bromide function on the Si substrate designed to engage with the hydroxyl group of the Ru complex in an S(N)2-type reaction could be established

Heavier pnictogens - treasures for optical electronic and reactivity tuning

We highlight recent advances in organopnictogen chemistry contrasting the properties of lighter and heavier pnictogens. Exploring new bonding situations, discovering unprecedented reactivities and producing fascinating opto-electronic materials are some of the most prominent directions of current organopnicogen research. Expanding the chemical toolbox towards the heavier group 15 elements will continue to create new opportunities to tailor molecular properties for small molecule activation/reactivity and materials applications alike. This frontier article illustrates the elemental substitution approach in selected literature examples

Reduced quenching effect of pyridine ligands in highly luminescent Ln(iii) complexes : the role of tertiary amide linkers

Luminescent Eu(iii) and Tb(iii) complexes were synthesised from octadentate ligands carrying various carbostyril sensitizing antennae and two bidentate picolinate donors. Antennae were connected to the metal binding site via tertiary amide linkers. Antennae and donors were assembled on a 1,4,7-triazacyclononane (tacn) platform. Solution- and solid-state structures were comparable to those of previously reported complexes with tacn architectures, with nine-coordinate distorted tricapped trigonal prismatic Ln(iii) centres, and distinct from those based on 1,4,7,10-tetraazacyclododecane (cyclen) macrocycles. In contrast, the photophysical properties of these tertiary amide tacn-based complexes were more comparable to those of previously reported systems with cyclen ligands, showing efficient Eu(iii) and Tb(iii) luminescence. This represents an improvement over secondary amide-linked analogues, and is due to a greatly increased sensitization efficiency in the tertiary amide-linked complexes. Tertiary amide-linked Eu(iii) and Tb(iii) emitters were more photostable than their secondary amide-linked analogues due to the suppression of photoinduced electron transfer and back energy transfer

[2+2] Cycloaddition of phosphaalkenes as a key step for the reductive coupling of diaryl ketones to tetraaryl olefins

Procedures for the reductive coupling of carbonyl compounds to alkenes in the literature rely either on a radical coupling strategy, as in the McMurry coupling, or ionic pathways, sometimes catalysed by transition metals, as in more contemporary contributions. Herein, we present the first example of a third strategy that is based on the [2 + 2] cycloaddition of ketone-derived phosphaalkenes. Removal of P-trimethylsilyl groups at the intermediary 1,2-diphosphetane dimer results in its collapse and concomitant release of the tetraaryl-substituted alkene. In fact, the presented strategy is the only alternative to the McMurry coupling in the literature that allows tetraaryl alkene formation from diaryl ketones, with yields as high as 85%. The power of the methodology is illustrated in the reaction of tethered bis-benzophenones which engage in intramolecular reductive carbonyl couplings to form unusual macrocycles without the need for high dilution conditions or templating

Core and double bond functionalisation of cyclopentadithiophene-phosphaalkenes

The heterofulvenoid cyclopentadithiophene-phosphaalkene is a versatile building block for opto-electronic tuning with donor and acceptor moieties. Both the annulated thienyl rings and the phosphaalkene bond can be functionalised using a variety of chemical transformations, e.g. forming C-C, C-E (E=Si, Br) bonds, or oxidation and metal coordination, respectively. Solid-state structures, optical and electronic properties are probed theoretically and experimentally, illustrating the opto-electronic tailoring opportunities at this motif.De två första författarna delar förstaförfattarskapet</p