This proposed research involves the investigation of distal anion effects on the properties of transition metal oxo complexes. Transition metal oxo species are invoked as central intermediates in a wide variety of enzymatic oxidations. This centrality has motivated substantial efforts at understanding their structure and function. Molecular model complexes have provided significant insights into oxo complexes by providing systems where hypotheses can be rationally and systematically studied. Nevertheless, it is becoming increasingly apparent that classic systems used to model oxo intermediates, which typically feature strongly donating anionic ligand sets, do not mimic the electronic structures or reactivities of some of the most interesting enzymatic active sites. Against this backdrop, recent results have underscored the importance of secondary coordination sphere effects in the function of oxo species. These studies have primarily focused on hydrogen bonding interactions. In this research program, we aim to investigate an alternative secondary coordination sphere influence, namely that of distal anionic charges. Enzymatic active sites can be highly charged and this effect can strongly influence the reactivity of different oxo species. However, the effect of the incorporation of distal charges has not been systematically investigated.
We aim to rationally incorporate distal anions onto model oxo complexes in order to study the effect of anionic charge on the reactivity and properties of transition metal oxo species. By incorporating distal anion charges that are not in conjugation with transition metal centers we will be able to tune the redox potential of transition metal oxo complexes independent of the electronic structure of the M-O unit. This will allow us to modulate parameters such as O-H BDE's which should directly influence C-H abstraction capabilities. Furthermore, we anticipate that the incorporation of distal anions will enable the isolation and study of high-valent oxo complexes with comparatively weak ligand fields. These weak ligand fields should enable unusual electronic structures, such as high spin states, that have been proposed as important for crucial intermediates but have little to no synthetic precedent. As a final point, we will also target oxo complexes that have no structural precedent, particularly Cu oxo species. This broad strategy will enable fundamental insights into processes such as C-H activation/hydroxylation and oxygen evolution. The importance of charge on enzymatic function is well-established, but has not been investigated in the context of oxo intermediates. As such, our approach offers a great deal of promise in understanding the function of oxo complexes and rationally controlling their properties and reactivity.
Transition metal oxos are invoked as key intermediates in a number of biological processes and have been thoroughly studied via molecular model complexes. Nevertheless, the systems that have been used to stabilize these species typically attenuate their reactivity or properties compared to enzymes. This research program will address this issue by exploring the fundamental effect of distant charges on molecular transition metal oxo complexes to generate new species with unprecedented structure, function, and properties.
