Redox-active ligands and their coordination complexes

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Compounds based on the transition metals are used in an incredibly broad spectrum of applications. They are used as reagents and as catalysts to make new drugs, polymers, and other important classes of chemicals. They also possess physical properties (light absorption, emission, magnetism…) that are exploited in many high-tech applications ranging from MRI to batteries to alternative energy. Whatever the particular application, chemists typically work to tailor the properties of metal complexes by carefully selecting the ligands (or designing new ones) that are bound to the metal. The idea of “ligand design” is endemic to many, many aspects of transition metal chemistry. By and large these efforts are based on the premise that the properties of metal complexes stem principally from the metal, and the ligands play a supporting role in fine tuning these properties. An alternative approach involves using ligand which themselves have some active function. The ligand property of interest in my research program is redox activity. As their name suggests, "redox-active ligands" (RALs) can be oxidized or reduced analogously to many transition metals. These sorts of ligands have been known for about 50 years but until recently the main focus was on simply trying to understand the electronic structure of RAL complexes. In the past decade there has been growing recognition that RAL complexes could possess very useful applications: Several technologically important physical properties (optical, magnetic, molecular switching) stem from ligand-based redox activity. And the possibility of harnessing ligand-centered redox activity to make new/more effective/cheaper reagents and catalysts is beginning to receive worldwide attention. These general ideas are still in early stages of exploration; more work is needed to determine the potential uses of RAL complexes. This proposal outlines research into the synthesis, properties and uses of RAL complexes based on new kinds of redox-active ligands. Other researchers in this field are focusing on well-established types of RALs; my goals are to try to broaden the “RAL toolkit” by developing non-traditional systems under development in my lab. We have discovered two fundamentally new types of redox-active ligand, one based on a class of stable radical and the other based on an industrially important organic dye. Both are straightforward to make, and both have unusual/distinctive properties which can be modulated through conventional substituent effects. The proposed research is principally discovery-oriented, in that we will examine a range of properties for our RAL complexes. This work involves a lot of chemical synthesis (designing and making target molecules, based on hypotheses of what they might "do"). In some cases the “property” is reactivity/catalysis, whereas in others the focus is on electro/optical properties. We will assess the strengths and weaknesses of our systems in relation to other RAL complexes (and more generally, other metal complexes not based on RALs) and in so doing we hope to (i) contribute to the collective understanding of the fundamental nature of redox-active ligands and (ii) identify new opportunities for development in catalysis and/or advanced materials.