Big Squares

A brief overview of metallomacrocycles and their interactions, the subject of my PhD.

PhD Title: Properties of Metallomacrocycles with d6 Metal Centres

Labwork

Supermolecules are to molecules and the intermolecular bond what molecules are to atoms and the covalent bond.

Jean-Marie Lehn, Angew. Chem. Int. Ed., 1990, 20, 1304

Metallomacrocycles are examples of cyclic supermolecules, effectively made up of smaller molecules and a large central cavity. In this case they are organic linker units and metal-centred complexes. They are often referred to as Molecular Squares as the metal complexes form the corners and are linked together via the organic compounds which are usually linear.

The metallomacrocycles I studied contain two metals: ruthenium and rhenium, each on two opposite corners. They are linked together by an interesting C-shaped molecule called quaterpyridine which incorporates the ruthenium corners into its structure. The quaterpyridine also changes the overall shape of the metallomacrocycle into a bowl which results in a more rhombic central cavity. This cavity can accept small guest molecules which can also affect the overall metallomacrocycle structure.

It is these "host-guest" interactions between the metallomacrocycle and samll guests that I studied using quantum modelling. This is a computational technique for simulating the molecules and their interactions on the computer.

The following two papers contain information about the metallomacrocycles I studied: the first is an introduction to them, and the second is one I co-wrote and presents of some of my research. Both papers are co-written by Anthony Meijer and Jim Thomas, my PhD supervisors.

Paul de Wolf, Phil Waywell, Matt Hanson, Sarah L. Heath, Anthony J. H. M. Meijer, Simon J. Teat, Jim A. Thomas

Abstract: By using a “complex as ligand approach,” the metal-ion-templated self-assembly of heterometallic teteranuclear metallomacrocycles containing kinetically locked RuII centers is described. Depending on the metal-ion template employed in the self-assembly process, the final macrocycle can be kinetically labile or inert. Electrochemical studies reveal that the kinetically inert macrocycles display reversible RuIII/II oxidation couples. The crystal structure of a kinetically inert Ru2Re2 macrocycles reveals a structurally complex palmate anion-binding pocket. Host–guest studies carried out with the same macrocyle in organic solvents reveals that the complex functions as a luminescent sensor for anions and that binding affinity and luminescent modulation is dependent on the structural nature and charge of the guest anion. Computational density functional theory (DFT) studies support the hypothesis that the luminescence of the macrocycle is from a 3MLCT state and further suggests that the observed guest-induced luminescence changes are most likely due to modulation of nonradiative decay processes.

Haslina Ahmad, Benedict W. Hazel, Anthony J. H. M. Meijer, Jim A. Thomas, Karl A. Wilkinson

Abstract: Metal-ion-directed self-assembly has been used to construct kinetically inert, water-soluble heterometallic Ru2Re2 hosts that are potential sensors for bioanions. A previously reported metallomacrocycle and a new derivative synthesised by this approach are found to be general sensors for bioanions in water, showing an “off–on” luminescent change that is selective for nucleotides over uncharged nucleobases. Through a change in the ancillary ligands coordinated to the ruthenium centres of the host, an “off–on” sensor has been produced. Whilst this host only shows a modest enhancement in binding affinities for nucleotides relative to the other two host systems, its sensing response is much more specific. Although a distinctive “off–on” luminescence response is observed for the addition of adenosine triphosphosphate (ATP), related structures such as adenine and guanosine triphosphate (GTP) do not induce any emission change in the host. Detailed and demanding DFT studies on the ATP- and GTP-bound host–guest complexes reveal subtle differences in their geometries that modulate the stacking interactions between the nucleotide guests and the ancillary ligands of the host. It is suggested that this change in stacking geometries affects solvent accessibility to the binding pocket of the host and thus leads to observed difference in the host luminescence response to the guests.