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Studies of Paramagnetic Rotaxanes and Related Assemblies

  • Bennett, Tom Stanley
Publication Date
Jun 19, 2023
Manchester eScholar
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The work presented in this thesis centres on the synthesis and measurements of paramagnetic rotaxane molecules consisting of [Cr7NiF8(O2CtBu)16]- {Cr7Ni} octametallic rings. The possibility of using {Cr7Ni} rings as qubits has triggered interest from research chemists. The aim of the project is to synthesise molecules which connect {Cr7Ni} rings and dissimilar qubits via an ammonium thread. Paramagnetic NMR is used to investigate the relative solution stabilities for a series of pseudorotaxanes by exchanging the ammonium thread. An understanding of the binding preferences of the octametallic macrocycles is important in the design of more complex molecular assemblies. This work begins by exploring the binding preferences between a [Cr7CoF8(O2CCH2tBu)16]- macrocycle and dialkylammonium cations through equilibrium constants, quantified using paramagnetic NMR. The study concludes that the relative stability of a pseudorotaxane is dependent upon the structure of the ammonium cation and solvent used. Knowledge of the molecules’ relative stabilities was used to synthesise new pseudorotaxanes via an indirect route in high yields (> 80 %). The observations are the first step in the design of more elaborate [n]rotaxane molecules and dynamic systems. A series of two- and three-spin [2]rotaxanes have been synthesised involving [Cr7NiF8(O2CtBu)16]- {Cr7Ni} macrocycles, charge-balanced by ammonium threads terminated with metal salen groups (where M = Cu, Ni, VO). Each compound has been characterised using single crystal X-ray diffraction. One of the complexes synthesised includes a three-hetero-spin [2]rotaxane comprising three spin = ½ centres. Continuous wave and pulsed electron paramagnetic resonance (EPR) experiments were implemented to investigate the structure of the molecules in solution. The inter-spin distances obtained from pulsed EPR experiments are consistent with those observed in the crystal. A twelve-membered nickel cluster has been used to bring multiple {Cr7Ni} rings together to form a series of [7]rotaxanes. The structure of each molecule differs in the crystal depending on the structure of the coordinated thread. Small angle X-ray scattering supported by molecular dynamic simulations were used to investigate the structure of the molecules in solution. The results indicate that the molecule is stable in solution but adopts conformations which differ to that in the crystal.

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