Lanthanide and Actinide Schiff-Base Complexes

As concerns about climate change grow, so does the interest in alternative green energy sources such as solar, wind, and nuclear energy. While in many ways, nuclear energy is the most developed technology that can meet the current energy needs of the United States, nuclear energy is still considered controversial. The efficiency of the nuclear fuel cycle can be improved by finding more cost-effective ways to recycle the fissile uranium from neutron poisonous fission products, the worst of which are the difficult to separate lanthanide elements. Separating the intermediate-lived and stable fission products from the long-lived fission products to reduce the volume of spent fuel stored in geological waste repositories is a crucial challenge that continues to be researched. Towards these goals, the development of ligand systems that can selectively sense, bind, or extract uranium from mixtures of lanthanides and actinides are of particular interest. In this work, a redox-active backbone was used to synthesize a class of new Schiff-base ligands herein named “pyrasal” that feature a pyrazine ring. These pyrasal ligands were explored with cerium, thorium and uranyl. Cerium showed a switch in coordination geometry due to just changes in pH and temperature. Thorium complexes with pyrasal were planar and showed much less fluorescence than the comparable naphthylsalophen thorium complexes. With uranyl, pyrasal ligands facilitated the lengthening of the U=O bond and enhanced the basicity of the oxo ligand allowing for the oxo ligand to act as a hydrogen bond acceptor. Indeed, the lengthening of the U=O bond was seen to be strongly affected by the solvent choice. Finally, a bis-salophen ligand was synthesized from the condensation of 3,3-diaminobenzidine and 4-(N,N-diethylamino)salicylaldehyde. When the bis-salophen ligand is complexed with thorium nitrate in a 1:1 ratio, then a unique structure herein named “thorium delta” is formed. Thorium deltas are discrete molecular structures with a triangular shape, 3-to-3 metal-to-ligand ratio, and racemic Δ,Δ,Δ-chirality. The center of the structure has a pore that can be used to bind anions and showed affinity for chloride anions.