Final Report Summary - EMOMFSSA (Extended metal-organic materials formed via subcomponent self-assembly)
The project aimed to form new extended metallo-supramolecular architectures, and we have successfully developed a range of structures from the self-assembly of metal, aldehyde and amine components. These structures include cubic, tubular and tetrahedral species. A common feature of these materials is that they contain a central cavity in which suitable guest molecules can be bound. A number of the structures are shown.
These new materials have been fully characterised by a variety of techniques and their host-guest chemistry has been extensively explored. Particular highlight included the observation of selective anion binding, the sequestration of sulphur hexafluoride4 and the separation of large aromatic molecules including the higher fullerenes.
Particular highlight of the research undertaken has been the development of a system of synthetic cage molecules that can adapt both physically and chemically to the presence of suitable guest molecules, producing synergistic anion binding. This (Fe4L6)8+ cage exists as a mixture of three diastereomers. Upon the addition of different anions, these cages adapt on the system-wide level to express a new combination of diastereomers (T, S4 and C3) to encapsulate the anionic guests; a higher proportion of guests are bound by the new mixture than would be possible with the original mixture, suggesting that the anion binding is a characteristic of the system rather than individual molecules.
When the cage was prepared from iron triflimide the proportion of diastereomers present was not statistical; it was found to be 32:49:19 (T: S4:C3). In contrast, when the same cage was prepared with tetrafluoroborate only the T-symmetric diastereomer, was observed, while in the perchlorate analogue the ratio was 64:8:28 and in the case of hexfluorophosphate it was 59:26:15, suggesting that the system of cage molecules was adapting to the presence of different anions. Indeed, both NMR experiments and single crystal structures of the complexes showed that anions were encapsulated within the cavity of the cage structure.
Through careful analysis of nuclear magnetic resonance (NMR) titration data the cage molecules was found to exist as a network of equilibria which can be fully described and it responses to anions assessed. Iodide found bind the strongest out of the guests explored (Ka = 1.7(4)×107 M-1). The magnitude of iodide binding represents the strongest 1:1 iodide binding of which we are aware in either synthetic or natural systems. Indeed, this represents the strongest 1:1 host-guest binding for a metal-organic host that can be isolated in the absence of a guest. The combination of strong binding and diastereomeric response upon the addition of a guest, leads to a significant amplification of anion binding. In each of the cases where T was expressed exclusively upon guest addition (BF4, NO3 and halogens), anion binding is amplified by more than 200 % compared to the potential guest binding of the system if the ratio of diastereomers remained static.
The system of cages therefore, acts as a network, reacting to the addition of certain anionic guests, adapting to express a reconfigured arrangement to produce a synergistically enhanced anion binding function.
The determination of guest binding constants, diastereomer ratios and kinetic parameters provides a complete understanding of the system. Not only does the cage adapt on the molecular level providing a tailored guest-binding pocket, it also adapts on a system-wide level while maintaining both its molecular formula and chemical connectivity, producing synergistically enhanced anion binding. This twofold adaptation is a feature of the responses to external stimuli displayed by biological systems, something that has not previously been observed in synthetic systems. Complex and functional synthetic systems of this type will lead to the design of more effective systems for host-guest recognition and the development of systems approaching the complexity of those that exist in nature.
These new materials have been fully characterised by a variety of techniques and their host-guest chemistry has been extensively explored. Particular highlight included the observation of selective anion binding, the sequestration of sulphur hexafluoride4 and the separation of large aromatic molecules including the higher fullerenes.
Particular highlight of the research undertaken has been the development of a system of synthetic cage molecules that can adapt both physically and chemically to the presence of suitable guest molecules, producing synergistic anion binding. This (Fe4L6)8+ cage exists as a mixture of three diastereomers. Upon the addition of different anions, these cages adapt on the system-wide level to express a new combination of diastereomers (T, S4 and C3) to encapsulate the anionic guests; a higher proportion of guests are bound by the new mixture than would be possible with the original mixture, suggesting that the anion binding is a characteristic of the system rather than individual molecules.
When the cage was prepared from iron triflimide the proportion of diastereomers present was not statistical; it was found to be 32:49:19 (T: S4:C3). In contrast, when the same cage was prepared with tetrafluoroborate only the T-symmetric diastereomer, was observed, while in the perchlorate analogue the ratio was 64:8:28 and in the case of hexfluorophosphate it was 59:26:15, suggesting that the system of cage molecules was adapting to the presence of different anions. Indeed, both NMR experiments and single crystal structures of the complexes showed that anions were encapsulated within the cavity of the cage structure.
Through careful analysis of nuclear magnetic resonance (NMR) titration data the cage molecules was found to exist as a network of equilibria which can be fully described and it responses to anions assessed. Iodide found bind the strongest out of the guests explored (Ka = 1.7(4)×107 M-1). The magnitude of iodide binding represents the strongest 1:1 iodide binding of which we are aware in either synthetic or natural systems. Indeed, this represents the strongest 1:1 host-guest binding for a metal-organic host that can be isolated in the absence of a guest. The combination of strong binding and diastereomeric response upon the addition of a guest, leads to a significant amplification of anion binding. In each of the cases where T was expressed exclusively upon guest addition (BF4, NO3 and halogens), anion binding is amplified by more than 200 % compared to the potential guest binding of the system if the ratio of diastereomers remained static.
The system of cages therefore, acts as a network, reacting to the addition of certain anionic guests, adapting to express a reconfigured arrangement to produce a synergistically enhanced anion binding function.
The determination of guest binding constants, diastereomer ratios and kinetic parameters provides a complete understanding of the system. Not only does the cage adapt on the molecular level providing a tailored guest-binding pocket, it also adapts on a system-wide level while maintaining both its molecular formula and chemical connectivity, producing synergistically enhanced anion binding. This twofold adaptation is a feature of the responses to external stimuli displayed by biological systems, something that has not previously been observed in synthetic systems. Complex and functional synthetic systems of this type will lead to the design of more effective systems for host-guest recognition and the development of systems approaching the complexity of those that exist in nature.