Development of New Ligand Scaffolds for the Preparation of Heterobimetallic Complexes

There is currently much interest in the development of methods to harness sustainable, CO2 neutral, non-fossil fuel based energy sources due to diminishing worldwide supply of fossil fuels and concerns over historically high levels of CO2 in the atmosphere, which may have a devastating impact on the world’s climate. One such avenue is through the conversion of atmospheric CO2 into useful, high-energy density, organic fuel sources. Photosynthesis is the biological process by which plants convert sunlight, water, and CO2 into the reduced organic materials that we extract from the earth and burn (completing the cycle back to CO2) to release the energy stored in the bonds of the molecules. The development of synthetic methods to mimic the enzymatic processes of photosynthesis in order to utilize CO2 as a carbon feedstock for organic fuels would be of tremendous benefit. The one electron reduction of CO2 to CO2- is a highly unfavorable process as evidenced by the relatively high reduction potential of -1.9V. The two electron reduction of CO2 via proton assisted electron transfers is a more favorable process. Noble metals are known to undergo 2 electron processes though they are generally quite rare, expensive, and toxic to work with. Efforts have been made to use the more abundant first row transition metals (base metals) to mediate 2 electron processes with little success as they are more likely to undergo 1 electron redox processes. One such approach that has shown some success in achieving 2 electron processes with base metals is through the use of bimetallic cooperativity where two separate metal centers, each involved in a 1 electron event, work in tandem to achieve a net 2 electron redox event. This thesis describes investigations into metal complexes of new ligand designs involving a hard-soft approach to preferentially bind different metals in a site specific manner intended for the production of heterobimetallic complexes. Two classes of ligands are explored and described: (1) a series of P2N3 pincer type ligands and (2) a series of N-confused Trispyrazolylmethane (Nc-Tpm’s) ligands