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

Kilpatrick, Alexander F R (2015) Bimetallic complexes of d- and f-block metals with pentalene ligands. Doctoral thesis (PhD), University of Sussex.

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Abstract

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 (η8-Pn†)YbCp*
and [Cp*Yb]+, and reaction of [Cp*Sm]2(μ-Pn†) with CO yields (η8-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 (η8-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(η5-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 [(η8-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 (η8-
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.

Item Type: Thesis (Doctoral)
Schools and Departments: School of Life Sciences > Chemistry
Subjects: Q Science > QD Chemistry > QD0241 Organic chemistry
Depositing User: Library Cataloguing
Date Deposited: 08 May 2015 07:29
Last Modified: 28 Sep 2015 14:17
URI: http://sro.sussex.ac.uk/id/eprint/53914

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