Metal complexes containing non-innocent ligands for functional materials
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Date
29/06/2013Author
Reinhardt, Maxwell James
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Abstract
The existence of complexes of that display non-innocence has been of interest
in the field of coordination chemistry since the investigations of square-planar
dithiolene complexes of the late transition metals in the 1960s. The ligands used in
these systems are termed “non-innocent” when bound to a number of the late
transition metals, because the orbital energy levels are similar to those of the central
metal ion. This allows there to be significant electron delocalisation over the
complex with the potential for the complexes to exist in a number of electronic states
due to the combined electrochemical activity. In 1966, Jørgensen classified
innocence as ligands that “allow oxidation states of the central atoms to be defined”,
thus by this logic non-innocent ligands are defined as complexes where the precise
oxidation states of the ligand and metal are ambiguously assigned. However it should
be noted that no ligand is inherently non-innocent, but rather the ligand may behave
in a non-innocent manner under the right circumstances. The qualification of non-innocence
should therefore only be applied to combinations of metal and ligand that
result in the aforementioned properties. In this thesis, the term “non-innocent” will
be used to define ligands that often possess non-innocent behaviour when complexed
to the metal centres they are bound to. A general form of ligand that displays non-innocent behaviour is that of the
1,2-bidentate moiety with an unsaturated carbon backbone. The chelating donor
groups (X) are either O, NH, S, or a combination of the three. The central transition
metal is generally a late metal that favours a square-planar geometry, because the
planarity of the complex is crucial for electron delocalisation within the molecule
and molecular interactions in the solid material. When the metal is nickel or platinum
for example, their square-planar complexes with such ligands have shown threemembered
electron-transfer series. Specific examples of ligands that have been shown to display non-innocent
behaviour are those of catechol (1,2-dihydroxybenzene) and 1,2-diaminobenzene,
where the unsaturated backbone is provided by a phenyl group. The electronic nature
of these compounds has been extensively investigated by the groups of Pierpont
and Lever, with focus on their redox and magnetic properties. The combined
metal and ligand redox activity results in interesting magnetic behaviour, with
potential for magnetic exchange interactions between a paramagnetic metal centre
and the radical ligand or between two radical ligands mediated by a diamagnetic
metal centre. This research has been advanced by Wieghardt and co-workers
who have performed experimental and theoretical examination of non-innocent
complexes of 1,2-substituted phenyl chelates, where the donor group is a
combination of O and NH. These studies have focused on the understanding the
nature of the metal-ligand interactions to apply to biological systems, such as those
observed at the active site of enzymes that act upon molecules with similar moieties
to the non-innocent ligands. Compounds of catechol may be referred to as dioxolenes in analogy to the
sulfur-based dithiolenes. The deprotonated, dianionic form of catechol is known as
catecholate (cat), which can be readily oxidised to the monoanionic o-semiquinone
(SQ) and neutral o-benzoquinone (Q) forms. It has been seen that
catecholate compounds can be described by localised electronic states with defined
oxidation states, unlike many of the dithiolene class of molecules. However these
states can exist in equilibrium with each other when the metal and ligand orbitals are
close in energy, with differences in the charge and spin definition in what has been
described as “valence tautomerism”. Therefore, although the complexes may not be
seen as non-innocent by definition that their oxidation states are not ambiguous, it is
still a useful description due to the potential for easily accessible charge states. Metal dithiolene complexes, where the metal is coordinated by one or more
ligands with two S-donor atoms linked by a conjugated backbone, are one of the best
researched of the non-innocent class of molecules. The square-planar bis-dithiolenes
of the late transition metals have interesting magnetic, optical and electrical
properties arising from the delocalised nature of the constituent metal and ligand
orbitals, which has enabled their use for a wide range of applications such as
non-linear optics, transistors and near-infrared switches. Of particular interest
is the ability to fine tune the electrical properties to fit the application by changing
the substituents on the core dithiolene moiety. For example, Anthopoulos has
shown that by lowering the energy of the lowest unoccupied molecular orbital
(LUMO), stable n-channel conductivity can be observed in field-effect transistors
(FETs). Materials based on square-planar non-innocent complexes have been tested in
FETs, and been seen to display field-effect mobilities as high as 10˗2 cm2 V˗1
s˗1 as with Ni bis(o-diiminobenzo-semiquinonate) complexes. Most of these
molecules are based on conjugated, chelating ligands such as 1,2-diaminobenzene
and the dithiolene class of molecules. Field-effects have also been observed in
square-planar Pt complexes, where the conductivity is thought to arise from
beneficial Pt-Pt bonds in addition to the π-stacking between molecules. Despite the
similarity to the diaminobenzene and dithiolene counterpart, there are no reports of
catechol-based materials displaying field-effect properties in the literature. Catechol
compounds are well-researched in the field of biological chemistry due to the
prevalence of the catechol moiety and enzymes with which it interacts in nature.
However they have not been examined far beyond their simple coordination
chemistry or chemical characterisation.