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Bimetallic complexes of d- and f-block metals with pentalene ligands

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posted on 2023-06-08, 20:43 authored by Alexander F R Kilpatrick
The focus of this thesis is the synthesis and characterisation of organometallic complexes incorporating the silylated pentalene ligand, [C8H4{SiiPr3-1,4}2]2- (= Pn†), bound to more than one metal centre. In general, metals in low oxidation states from the d- and f-block of the periodic table have been selected for these bimetallic complexes, as they are potentially reactive with small molecule substrates. Chapter One introduces the pentalene molecule and its derivatives, and discusses their use as ligands in organometallic chemistry. Particular emphasis is given to the application of organometallic pentalene complexes, ranging from conducting polymers in materials chemistry to small molecule activation and catalysis. In Chapter Two the silylated pentalene ligand Pn† is used to bridge two lanthanide(II) centres in anti-bimetallic sandwich complexes of the type [Cp*Ln]2(µ-Pn†) (Ln = Yb, Eu and Sm). Magnetic measurements and electrochemical methods are used to investigate the extent of intermetallic communication in some of these systems, which show potential for the design of organometallic 'molecular-wires'. Chemical oxidation of [Cp*Yb]2(µ-Pn†) leads to dissociation into mononuclear fragments (?8-Pn†)YbCp* and [Cp*Yb]+, and reaction of [Cp*Sm]2(µ-Pn†) with CO yields (?8-Pn†)SmCp*. Rational synthetic routes to mononuclear mixed-sandwich Pn†/Cp* compounds with trivalent f-block ions (Dy, Tb and U) are also developed, and their magnetic properties are studied by SQUID magnetometry including variable-frequency ac susceptibility measurements. These studies identified (?8-Pn†)DyCp* as the first known example of a pentalene based single molecule magnet, with a closed-waist hysteresis loop observed up to 2 K. Chapter Three describes the synthesis of iron(II) complexes with silylated pentalene ligands, and efforts towards incorporating them into extended organometallic arrays and heteronuclear anti-bimetallic complexes. Six complexes have been structurally characterised including the triple-decker homobimetallic [Cp*Fe]2(µ-Pn†), and the potassium salt [Cp*Fe(?5-Pn†)][K] which is an organometallic polymer in the solid state. Chapter Four documents efforts towards the synthesis of syn-bimetallic pentalene complexes, including the first row d-block metals V, Ti and Sc. A novel synthetic route to the di-titanium bis(pentalene) 'double-sandwich' complex (Pn†)2Ti2 is developed, via chloride-bridged dimers [(?8-Pn†)Ti]2(µ-Cl)x (x = 2 and 3). The electronic and magnetic properties of the latter are investigated using EPR spectroscopy and SQUID magnetometry, and the structure and bonding in (Pn†)2Ti2 is examined using spectroscopic, crystallographic, electrochemical and computational techniques. Preliminary studies toward the synthesis of an analogous di-scandium complex were unsuccessful, however three novel complexes have been synthesised including (?8- Pn†)ScCp* which is first example of a Sc complex bearing a Pn† ligand to be characterised by X-ray diffraction. Chapter Five explores the reactivity of the double-sandwich compound (Pn†)2Ti2 prepared in Chapter Four, with small molecules which are of industrial and environmental importance. The relatively open structure of (Pn†)2Ti2 allows the formation of adducts with unsaturated small molecules CO, MeNC and CO2. In the latter case the adduct formed is unstable at room temperature and the coordinated CO2 molecule is reduced to give a bis(oxo) bridged dimer and a di-carbonyl complex. This provides the first example of small molecule activation by a di-metal bis(pentalene) double-sandwich complex. The reactivity survey of (Pn†)2Ti2 is extended in Chapter Six to other substrates; including unsaturated heteroallenes as model molecules for CO2. In the case of nonpolar heteroallenes CS2 and carbodiimide, thermally stable adducts are isolated and have been structurally characterised. Polar heteroallenes COS and organic isocyanates undergo reductive transformations to give sulfide- and carbonimidate-bridged complexes respectively. The reactivity of (Pn†)2Ti2 with organic molecules containing heteroatom-heteroatom bonds is also described; the reactions with diphenyldichalcogenides and azobenzene show the ability of the double-sandwich complex to act as a 2e- and 4e- reducing agent respectively. The rich and varied chemistry shown by (Pn†)2Ti2 is evaluated and future work is suggested.


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