Dr Nicholas C Fletcher

Research

Supramolecular Coordination Chemistry

The study of polymetallic 'supramolecular' assemblies is a rapidly expanding field of chemistry. In particular, the complexes of the tris-diimine complexes of ruthenium(II) have been extensively studied over recent years due to their unique combination of chemical stability, redox and photo-physical properties combined with their ability to control the ground and excited states by ligand modification. This has led to their potential application in materials for such diverse purposes as photochemical molecular devices and as photo-probes into the design and function of polynucleotides such as DNA. However, the development of such species is hampered by the inadequate methods for stereochemical control at the metal centres. The ability to construct multi-component molecular architectures is frustrated by isomeric considerations; to build a unique structure the stereochemistry of the individual components must be controlled. To this end, not only must the metal centred chirality be taken into consideration, but also the issue of mer / fac-isomerism in such species. There are currently a number of threads to the research within the group towards the isolation of isomerically pure compounds for a number of applications including molecular sensors and DNA targeting agents.

 

 

The isolation of enantiomerically pure tris-bipyridine pseudo-octahedral coordination complexes has become almost routine, with a large number of racemic transition metal complexes having been separated into their two chiral components; L and D. The consideration of mer and fac-isomers, prepared from asymmetrically substituted 2,2'-bipyridine ligands has received much less attention. Over the last few years, we have demonstrate a number of techniques that can be used to either separate the two isomers from a mixture of products, or target the synthesis of a single isomer. From examination of the sequential addition of three unsymmetrical bidentate ligand to a metal centre in an uncontrolled synthesis the ratio of isomers should be three mer to one fac. Our results indicate though that other factors can influence the geometric ratio; for example a bulky 5-substituted ligand featuring a tert-butyl group appeared to favour the mer-isomer.

 

1. Separation of the mer - / fac-isomers has been demonstrated in a number of our studies.

Cation-exchange chromatography on SP Sephadex C-25 columns – aliphatic substituents.
Preparative plate silica chromatography – ester and amide substiuents.

 

2. Selective preparation of fac-isomers has been achieved using a new synthetic methodology. If the ligands are tethered together prior to complexation, only a single isomer is able to form. Selective removal of the tether allows the isolation of the fac-isomer. Several new "building blocks" have then been further functionalised by traditional synthetic routes leading to a new stepise synthetic procedure to kinetically inert helicates.

 

 

 

 

 

3. Ligand Control has been used in the preparation of self-assembled polynuclear helical coordination complexes (or helicates). A new range of ligand systems have been prepared possessing chiral groups linking two bipyridine ligands. By careful consideration of the flexibility of the ligand strand:

 

 

It is possible to form a triple stranded dinuclear structure. Either D or L.
It is possible to form single stranded dinuclear structures, but of opposite chirality to the triple stranded analogues.

 

4. DNA Interactions. A remarkable affinity for sites where the minor groove of DNA is relatively wide has been noticed with a series of ruthenium(II) complexes bearing benzoxazole and thiazole moieties. Work is currently in progress to develop compounds that will selectively bind to secondary structures within DNA such as hairpin loops. In addition, these compounds appear to have a remarkable cytotoxicity against a range of human cancer cell lines.

 

 

 

5. Anion Interactions. In recent years we have returned to investigate anion recognition with a range of new rhenium(I) and ruthenium(II) pyridine complexes, with the aim of understanding selectivity for a number of environmentally important oxo anions. Significant progress has been achieved in the selected photo-detection of phosphate and current studies are investigating the detection of nitrate.