POLEMICA CON LOS ANARQUISTAS

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2.1 Aims and objectives of present study

The aim of this study is to exploit transition metal directed self-assembly approaches to generate hybrid inorganic-organic materials that display appealing physical properties. Particular attention is focused on the development of metallo- supramolecular assemblies such as molecular helicates and coordination clusters that may exhibit interesting magnetic properties such as spin crossover and superparamagnetism. We further aim to modify our reaction systems to obtain supramolecular architectures with open-framework topologies that may give rise to a degree of porosity that could potentially be exploited for molecular recognition or gas storage purposes. In order to synthesise these metallo-supramolecular assemblies three main types of ligands are employed, bis-bidentate and tris-bidentate Schiff base ligands (Figure 2.1), carboxylic acid substituted aromatic ligands (Figure 2.2a) and iminodiacetic acid substituted phenol ligands (Figure 2.2b).

Figure 2.1: Structure of bis-bidentate and tris-bidentate ligands used in this study.

a) b)

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2.2 Thesis outline

This thesis mainly concentrates on the transition metal directed self-assembly of a series of bis-bidentate and tris-bidentate Schiff base ligands shown in Figure 2.1 (Chapters 3-5). Imine ligands are employed in this study as their modular nature and bidentate binding mode could potentially give rise to the generation of a diverse range of metallo-supramolecular assemblies. These Schiff base ligands are also useful for the formation of transition metal complexes that display interesting magnetic properties. For example, upon complexation with FeII _{ions these imine ligands}

produce an intermediate ligand field which may allow the resultant complexes to exhibit spin crossover behaviour.

In Chapter 3, the coordination chemistry of a series of novel bis-bidentate Schiff base ligands (Figure 2.1a) is investigated. These ligands consist of two methyl- imine-imidazole functionalities linked via a benzene ring. Upon complexation with

transition metal ions, the relative orientation of the imine-imidazole bidentate binding sites influence the topology of the resultant species. We obtain dinuclear CoII and NiII complexes and tetranuclear FeIII and CuII complexes. Attention is also focused on using these imine ligands to stabilise coordination clusters. The applied synthetic approach results in the formation of a nonanuclear Cu oxo-cluster that is stabilised by two carbinolamine ligands. The carbinolamine ligands arise from the partial hydrolysis of the Schiff base ligands under the applied reaction conditions.

Chapter 4 presents a synthetic strategy for the formation of metallo-helicates. The applied approach involves the use of flexible bis-bidentate ligands (Figure 2.1b). These ligands consist of imine-imidazole binding sites linked by 4- 4’diaminodiphenylmethane, 4-4’ oxydianiline or 4-4’ diaminodiphenylamine linkers. The flexible nature of the amine linkers gives rise to the formation of helical architectures upon reaction with transition metal ions. The reaction of the flexible Schiff base ligands with FeII salts is of particular interest as helicate species that display spin crossover behaviour can be generated. The counterions used or the structure of the organic ligand employed in the formation of the FeII _{helicate play a}

significant role in determining the SCO properties of the resultant compound.

In Chapter 5, the coordination chemistry of related tris-bidentate Schiff base ligands is discussed (Figure 2.1c). In this case the imine binding sites are connected by a tri-phenyl benzene backbone. The ligands generate trinuclear CoII complexes that

32 are stabilised by to two Schiff base ligands. These molecular entities further pack in the solid state into a hexagonal network structure which is stabilised by weak H- bonding and CH-halogen bonding interactions. The hexagonally packed coordination assembly contains solvent filled channels.

A further aspect of the presented research project is concerned with the formation of Li coordination networks as these materials have recently been shown to display enhanced hydrogen storage capabilities.162,163 Surprisingly, carboxylic acid substituted aromatic ligands have rarely been employed for the formation of Li-based network structures. In Chapter 6, a synthetic approach for the formation of Li coordination networks that are stabilised by simple carboxylic acid substituted aromatic ligands (Figure 2.2a), is described.

Chapter 7 outlines the use of iminodiacetic acid substituted phenol ligands (Figure 2.2b) for the formation of amphiphilic FeIII coordination assemblies. The resulting architectures originate from the self-assembly of mononuclear FeIII complexes that network via hydrogen bonds involving constitutional water molecules.

The amphiphilic nature of the coordination assemblies that contain well defined hydrophobic organic and hydrophilic inorganic areas, can be exploited for molecular self-assembly purposes to generate non-crystalline, hierarchical nanomaterials, e.g. spherical vesicles. Electron microscopy was employed to characterise the self- assembled materials suggesting that they consist of layered lamellar structural motifs similar to the parent crystalline materials.

Figure 2.2: a) Structure of carboxylic acid substituted aromatic ligands used in the formation of Li

coordination networks. b) The structure of the iminodiacetic acid substituted phenol ligands used in the

generation of amphiphilic FeIII _{coordination assemblies.}

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2.3 Specific aims of this study

In order to accomplish our objectives we initially set out to attain the following:

a) Synthesise and fully characterise assorted organic ligands.

b) Obtain an understanding of the structural directing influences of these ligands in order to generate metallo-helicates, metal-organic frameworks, coordination compounds and oxo-cluster compounds upon complexation with transition metals.

c) Utilise single crystal X-ray diffraction to structurally characterise the compounds.

d) Determine the physiochemical properties (photochemical properties, thermal stability, etc.) of all the compounds.

e) Monitor the formation of these compounds via spectroscopic methods.

f) Investigate the magnetic properties of all relevant compounds with particular emphasis on the study of the FeII complexes that may display SCO behaviour. The temperature dependent magnetic susceptibility of oxo coordination clusters and other molecular species will also be examined where appropriate. g) Gain an insight into the effect of ligand functionality, on both the SCO

behaviour of the FeII complexes and on the inter-atomic coupling and spin ground state of potential cluster compounds.

h) Determine the surface area of all relevant compounds.

i) Investigate if molecular self-assembly can be employed to synthesise hierarchical nanomaterials using H-bonded FeIII coordination networks. j) Utilise electron microscopy techniques (TEM and SEM) to monitor the

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