Redox-Active α-Diimines and Novel Schiff Base Ligands for Uranium Coordination Chemistry

Abstract

The use of nuclear power for electricity generation plays a large role in our highly energy-dependent economy, and provides a source of low-emission, renewable energy that is used worldwide. Despite the positive impacts that it has had in lessening our dependence on fossil fuels, it is still a controversial energy source as any accidents that occur in storage, transportation, or on-site at facilities could result in large-scale environmental contamination requiring extensive remediation efforts. Thus, the continued exploration of the chemistry of the actinides is imperative to developing technologies which can improve the safety of nuclear power generation and waste storage. In this context, the coordination chemistry of uranyl (UO22+), the most prevalent and environmentally relevant uranium species, is of great interest, especially its selective coordination and bonding interactions. Towards this end, mixed-donor ligands such as salophens have been investigated for the sensing and extraction of uranyl. The need to further investigate fundamental properties of actinide species, such as their redox properties, has prompted the study of several ligand frameworks presented here. The α-diimine ligand “phen-BIAN”which integrates a redox-active backbone with a salophen-like binding pocket, was developed, and its bonding interactions with uranyl studied and compared to uranyl complexes of less-conjugated analogues. These species have rich electrochemistry and intriguing solid-state interactions, indicative of the contribution of ligand conjugation to supporting covalent bonding with uranium. Studies of the ligand “naphthylsalophen” demonstrate that the extension of the π-system leads to an increase in metal-ligand communication, and actinide complexes of bis-salophen ligands have also been characterized. The latter complexes are unique supramolecular actinide complexes, including a dinuclear uranyl species and trinuclear thorium helicate. Another avenue that has been pursued is the use of the hexadentate pyrrole-containing Schiff base system “pyrrophen” as a softer-donor ligand for favorable coordination of uranyl over other metal ions. This work has shown that this ligand is very well-suited to the linear geometry of the uranyl ion, and its modularity can be exploited for further improvements in its molecular recognition. The findings of these studies provide insight into the some of the unique behaviors exhibited by uranyl that may inform strategic coordination of tetravalent or transuranic actinide species.