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

摘要

The focus of this thesis is the synthesis and characterisation of organometallicudcomplexes incorporating the silylated pentalene ligand, [C8H4{SiiPr3-1,4}2]2- (= Pn†),udbound to more than one metal centre. In general, metals in low oxidation states from theudd- and f-block of the periodic table have been selected for these bimetallic complexes,udas they are potentially reactive with small molecule substrates.ududChapter One introduces the pentalene molecule and its derivatives, and discusses theiruduse as ligands in organometallic chemistry. Particular emphasis is given to theudapplication of organometallic pentalene complexes, ranging from conducting polymersudin materials chemistry to small molecule activation and catalysis.ududIn Chapter Two the silylated pentalene ligand Pn† is used to bridge two lanthanide(II)udcentres in anti-bimetallic sandwich complexes of the type [Cp*Ln]2(μ-Pn†) (Ln = Yb,udEu and Sm). Magnetic measurements and electrochemical methods are used toudinvestigate the extent of intermetallic communication in some of these systems, whichudshow potential for the design of organometallic 'molecular-wires'. Chemical oxidationudof [Cp*Yb]2(μ-Pn†) leads to dissociation into mononuclear fragments (η8-Pn†)YbCp*udand [Cp*Yb]+, and reaction of [Cp*Sm]2(μ-Pn†) with CO yields (η8-Pn†)SmCp*.udRational synthetic routes to mononuclear mixed-sandwich Pn†/Cp* compounds withudtrivalent f-block ions (Dy, Tb and U) are also developed, and their magnetic propertiesudare studied by SQUID magnetometry including variable-frequency ac susceptibilityudmeasurements. These studies identified (η8-Pn†)DyCp* as the first known example of audpentalene based single molecule magnet, with a closed-waist hysteresis loop observedudup to 2 K.ududChapter Three describes the synthesis of iron(II) complexes with silylated pentaleneudligands, and efforts towards incorporating them into extended organometallic arrays andudheteronuclear anti-bimetallic complexes. Six complexes have been structurally characterised including the triple-decker homobimetallic [Cp*Fe]2(μ-Pn†), and theudpotassium salt [Cp*Fe(η5-Pn†)][K] which is an organometallic polymer in the solidudstate.ududChapter Four documents efforts towards the synthesis of syn-bimetallic pentaleneudcomplexes, including the first row d-block metals V, Ti and Sc. A novel synthetic routeudto the di-titanium bis(pentalene) 'double-sandwich' complex (Pn†)2Ti2 is developed, viaudchloride-bridged dimers [(η8-Pn†)Ti]2(μ-Cl)x (x = 2 and 3). The electronic and magneticudproperties of the latter are investigated using EPR spectroscopy and SQUIDudmagnetometry, and the structure and bonding in (Pn†)2Ti2 is examined usingudspectroscopic, crystallographic, electrochemical and computational techniques.udPreliminary studies toward the synthesis of an analogous di-scandium complex wereudunsuccessful, however three novel complexes have been synthesised including (η8-udPn†)ScCp* which is first example of a Sc complex bearing a Pn† ligand to beudcharacterised by X-ray diffraction.ududChapter Five explores the reactivity of the double-sandwich compound (Pn†)2Ti2udprepared in Chapter Four, with small molecules which are of industrial andudenvironmental importance. The relatively open structure of (Pn†)2Ti2 allows theudformation of adducts with unsaturated small molecules CO, MeNC and CO2. In theudlatter case the adduct formed is unstable at room temperature and the coordinated CO2udmolecule is reduced to give a bis(oxo) bridged dimer and a di-carbonyl complex. Thisudprovides the first example of small molecule activation by a di-metal bis(pentalene)uddouble-sandwich complex.ududThe reactivity survey of (Pn†)2Ti2 is extended in Chapter Six to other substrates;udincluding unsaturated heteroallenes as model molecules for CO2. In the case of nonpolarudheteroallenes CS2 and carbodiimide, thermally stable adducts are isolated andudhave been structurally characterised. Polar heteroallenes COS and organic isocyanatesudundergo reductive transformations to give sulfide- and carbonimidate-bridgedudcomplexes respectively. The reactivity of (Pn†)2Ti2 with organic molecules containingudheteroatom-heteroatom bonds is also described; the reactions withuddiphenyldichalcogenides and azobenzene show the ability of the double-sandwichudcomplex to act as a 2e- and 4e- reducing agent respectively. The rich and variedudchemistry shown by (Pn†)2Ti2 is evaluated and future work is suggested.