Switching the Jahn-Teller Distortion of Copper(II) Compounds - Towards an EPR Reporter Group for Sensor Applications

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Octahedral copper(II) compounds usually show a strongly distorted molecular structure, that is elongated along one axis of the octahedron (the 'Jahn-Teller effect'). This distortion is reflected in the spectra shown by these compounds, particularly with the Electron Paramagnetic Resonance technique (EPR, aka ESR). We have found that a different, compressed molecular structure can be imposed onto an octahedral copper(II) complex compound that has been suitably designed. These were the first molecular copper(II) compounds to have this property. The two types of distorted structure (elongated and compressed) can be readily distinguished by the aforementioned EPR method. Importantly, our results have implied that the Jahn-Teller distortion of some of our compounds might be switched in response to an external stimulus. We are applying to try and do this, by designing a copper compound whose structural distortion might be changed when a second, guest molecule bonds to it. If successful, we will have made a chemical sensor for the guest species, that can be measured by EPR. We have designed two copper(II) complexes, based on our earlier work, to do just that. One is intendeded to interact with carboxylic acid guests, including some peptides. The other is designed to bind alkali metal ions, with a particular preference for Cs+. Through EPR titrations, and UV/vis and NMR titrations of analogous nickel(II) and zinc(II) complex hosts, we will prove whether these guests bind to our compounds and, if so, whether they trigger the expected change in the EPR spectra of the copper reporter centres. The EPR response should be selective for guest species of the correct size and shape. This would be a very novel use of EPR to probe host:guest binding.