Conclusiones

1.3.1. Metal centres linked by oxygen and nitrogen donors

The incorporation of more than one metal unit within the same coordination framework offers many benefits, especially if the properties of different metals are combined. Many ligands other than the 1,1-dithio species discussed already are known to generate multimetallic systems. For example multimetallic complexes based on dicarboxylic acids and bipyridines are well established in the literature. Such linkers have been used in the construction of coordination polymers^{80, 81} and metal-organic frameworks (MOFs).^82-84 Most examples of these networks are based on symmetrical linkages, forming homopolymetallic complexes.^85-87 Only one group has recently reported a hetreopolymetallic motif based on an isonicotinic acid framework.⁸⁸ The preparation of multimetallic networks featuring two different metal centres has proved to be considerably challenging. To overcome this difficulty either a protection/deprotection strategy must be employed, or the donor combinations of the linker must be carefully tailored to each metal centre. As mentioned earlier (Section 1.1.1, Fig. 5), one end of a piperazine molecule can be converted into a dithiocarbamate while protecting the other end as an ammonium unit in the zwitterion H2NC4H8NCS2. This protecting approach has allowed the successful preparation of heteromultimetallic compounds bearing 2-6 metal units.¹⁹

Mixed donor ligands are particularly useful for generating heteromultimetallic compounds.

They contain at least two different donor groups capable of chelating to metal ions. Such multifunctional ligands fulfil the same role as the 1,1-dithio unit, but use the innate affinity of certain donor combinations for particular metals rather than a protection strategy.

Herein some background information on carboxylates and pyridine ligands is provided since these ligands are commonly used to generate homopolymetallic systems. The possibilities which arise from combining the two can be exploited in mixed-donor ligands and will be discussed subsequently in the Results and Discussion section.

29 1.3.1.1. Carboxylates and pyridines as linkers

The wide array of coordination modes of the carboxylate anion coupled with its high affinity for metals ions, gives rise to metal carboxylate complexes with rich structural chemistry. Some examples of their coordination modes are shown below using the benzoate ligand (Fig. 24).

Figure 24. Binding modes of carboxylates.

Although many carboxylate complexes are known, one of the most interesting classes is one in which two metal centres are bridged by four carboxylate ligands. These have come to be known as

‗paddlewheel‘ complexes (PWCs) through analogy to boats with a paddlewheel (Fig. 25).⁸⁹ This type of linkage of dicarboxylate units leads to well-ordered lattice structures and the framework often allows multiple bonds between the metals and the ordered linkage. The tuneability of the ligands and the (often) coordinatively-unsaturated nature of the metal centre make them good candidates for catalysis.⁹⁰

The ligand 4,4‘-bipyridine (4,4‘-bipy) is an ideal linker between different transition metal centres for the propagation of coordination networks. It has two potential binding sites which are arranged in an opposite (exo) fashion. In principle, the pyridyl groups of 4,4‘-bipy can rotate along a

Figure 25. Molecular structure of molybdenum acetate with the paddlewheel motif Mo (blue), O (red) and C (grey).⁸⁹

Figure 26. An iconic example of a multimetallic compound based on pyridyl bridging ligands, generating a molecular square with a central cavity.

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central C–C bond; however the rotation does not affect the mutual orientation of the two lone pairs.

Therefore 4,4‘-bipy can be regarded as a rigid and classical bridging ligand. Its length and inflexible structure facilitates the construction of networks with metal atoms, which results in the formation of cavities of molecular dimensions (Fig. 26). The 4,4‘-bipy ligand forms a variety of networks ranging from one-dimensional to three-dimensional with several transition metal salts. The geometry of the architectures depends on several factors such as the coordination geometry of the metal atom, the presence of guest molecules, ligand and transition metal ratios and anions.⁹¹

1.3.1.2. Metal Organic Framework (MOF) complexes Countless research efforts have concentrated in recent years on a wide variety of coordination polymers.⁹² The most recent, high-profile setting for carboxylate linkers is found in metal-organic frameworks (MOFs). This field originated from work on zeolites and many synthetic routes are similar. MOFs are materials which are formed by the coordination of metals to polydentate linkers, leading to porous materials. The internal cavities between the linked metal units provide a huge internal surface area, making them suitable for storage of gases, in particular. (Fig. 27).^82-84 MOFs can not only store hydrogen molecules,⁹³ but also carbon dioxide,⁹⁴ carbon monoxide,⁹⁵ methane,⁹⁶ and oxygen have been reported.⁹⁷ For this reason, MOFs have become an important class of functional materials. The most common

types of connectors used in MOFs are dicarboxylate ligands (oxalate, terephthlalate etc.) and recently research into using molecular PWCs as building blocks for the synthesis of MOFs has even been reported.⁸⁹

A key requirement of the bridging ligands in MOFs is the ability of the ligands to form bonds reversibly so that a thermodynamic product can be achieved. Carboxylate ligands serve MOF formation very well in this respect.⁹⁸

MOFs have found use in many applications other than storage. They can be used in gas purification as strong chemisorption can take place between unwanted molecules (such as amines, phosphines, oxygenates, alcohols, water, or sulphur-containing molecules) and the framework. This allows the desired gas to pass through the MOF, leaving behind the unwanted molecules. Gas separation can be also performed with MOFs because they allow certain molecules to pass through

Figure 27. Depictions of a Metal Organic Framework (MOF) formed by polycarboxylate ligands. The yellow sphere illustrates the cavity created.⁸²

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their pores based on their size. This is particularly important for separating out harmful gases such as carbon dioxide.

MOFs are also used in catalysis because of their shape, size selectivity and their large volume.⁹⁹ The fine structure and nature of the active site can be controlled and it is possible to have a homogeneous distribution of one or more active sites.

1.3.2. Mixed-donor ligands derived from carboxylate and pyridine units

Mixed donor ligands in which both the dicarboxylate and bipyridine ligands are combined offer great potential in the construction of hetero-multimetallic arrays. Isonicotinic acid (IUPAC name: pyridine-carboxylic acid) is a pyridine variant with a pyridine-carboxylic acid unit in the 4-position (Fig. 28). It is the simplest combination of pyridine and carboxylic acid functional groups and is an isomer of nicotinic acid (also known as niacin and vitamin B3) which differs by the fact that the carboxylic acid side chain is present at the 3-position.

Nicotinic acid is an essential human nutrient and acts to reduce cholesterol and triglycerides in the blood. It has also been shown to reduce cardiovascular problems. Isonicotinic acid itself is mainly used in antituberculosis drugs such as isoniazid (isonicotinic acid hydrazide). Isonicotinic acid and its derivatives are also employed in manufacturing pharmaceuticals and agrochemicals.

Since isonicotinic acid contains both oxygen and nitrogen donors and monodentate and (potentially) bidentate functionality at either end, under the right conditions these differences can be exploited to link different metal units to create heterobimetallic compounds. This is in contrast to dicarboxylic acids or 4,4‘-bipyridine, which result in homobimetallic compounds.

The coordination chemistry of isonicotinic acid and its derivatives are varied and they have been used in a number of contexts, including as a structural element in MOFs. For

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dimensional motif based on an isonicotinate framework in a recent paper involving a solvent-free preparation process (Fig. 29).¹⁰⁰ This bifunctional ligand has shown to have great potential in the assembly of MOFs and some further examples of such architectures can be found in literature.^{101, 102}

A recent reportemployed isonicotinic acid to bond to rhodium(III) metal centres. It was found that the ligands coordinated through the nitrogen donors and that the protonated/deprotonated forms could be controlled by adjusting the pH (Fig. 30).¹⁰³ However, under the right conditions the isonicotinic acid ligand can coordinate through either one or both oxygen donors. An example of the former coordination mode (monodentate) is shown in Fig. 31.¹⁰⁴

These examples aside, surprisingly little has been achieved in coordination chemistry using isonicotinic acid or similar bifunctional linkers.

Figure 31. Structure of [Ni(isonic)(C22H34N6)] showing monodentate coordination of the isonicotinate ligands.¹⁰⁴ Figure 30. Structure of [RhCl2(isonicH)4]⁺.¹⁰³

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