Scott Wilcox: Characterising Weak Interactions in Solution.: An NMR Spectroscopic Approach.

Abstract

Detecting and characterising weak interactions in dilute solutions is challenging. This thesis focusses on the development of a new NMR spectroscopic strategy to do so, whilst also investigates a newly proposed type of weak interaction in solution with currently available NMR techniques.

Newly developed lanthanide(III) ion complexes bearing Lewis basic and Lewis acidic functionalities were synthesised, and their paramagnetic properties were successfully exploited. In both instances and in an iterative manner, a series of small molecules of different hydrogen- and halogen-bond strengths were titrated against a paramagnetic Lewis base/acid host in the polar aprotic solvent d-MeCN. In all cases, binding phenomena were detected at 1.82-2.20 mM concentrations of host – around an order of magnitude lower than previously attainable. Titrations against diamagnetic references, with Lu(III) complexes and with untagged Lewis bases, provided small, almost undetectable changes in chemical shift upon binding. Dilutions and titration against non-bonding paramagnetic complexes, as well as with a non-bonding species, proved that the paramagnetic properties were indeed transferred through binding the Lewis base or acid to the respective titrants. Association constants were determined for each binding pair, as well as their relative geometry to one another, providing evidence of these weak interactions over a much lower concentration range than was previously possible. This strategy represents the first use of paramagnetic NMR to study weak, small-molecule interactions in solution, thus opening up a whole new field of research with potential applications as a new method in the supramolecular toolbox.

In this thesis I also disprove the existence of the recently proposed ‘nucleophilic iodonium interaction,’ a hypothetical force between cationic silver(I) and cationic iodine(I) complexes in solution. Through means of 1H,15N HMBC and DOSY NMR experiments, the lack of interaction between iodine(I) and silver(I) in mixtures of both fast-exchanging bis(pyridine) complexes and slow-exchanging 1,2-bis((pyridine-2-ylethynyl)-benzene) complexes of the two is demonstrated. Testing purposefully contaminated samples of each, separately and in a mixture, lead to the conclusion that the initially published results were anomalous due to wet, consequently decomposed samples. DFT calculations corroborate the experimental findings, suggesting a π-π interactions to be responsible for previously reported crystal structure. These results highlight the need for the careful appraisal of new scientific ideas and the critical interpretation of experimental and computational data.