Researchers in the United States have synthesized the first macrocycle N.-Heterocylcarbene (NHC) complexes for incorporation of elements from the f-block and investigation of their unusual electronic structures.
Despite the widespread use of NHC ligands in organometallic complexes, examples of the actinides are rare because an electronic mismatch between them hinders the ligation of neutral NHCs. Polly Arnold, now at the University of California at Berkeley, and her group developed synthetic strategies to avoid these problems by using chelating ligands with anionic groups. Others have used bis or tris NHC borate ligands with a delocalized negative charge over a polydentate backbone.
David Jenkins from the University of Tennessee and his team recently developed dianionic tetra-NHC complexes for catalytic and transition metal group transfer reactions. Now a team surrounding both Arnold and Jenkins has tested whether macrocyclic NHC ligands can better stabilize larger actinide cations. The combination of the expertise of both groups resulted in octa-NHC-actinide complexes with interesting electronic structures due to the strong σ-donor ability of the NHC ligand.
The synthesis of the octa-NHC-actinide complexes was challenging. analogous strategies for transition metal systems using strong bases such as n-Butyllithium was not translated into the actinide variants. Instead, they added sodium bis (trimethylsilyl) amide to deprotonate the ligand, followed by the actinide species and the heat.
They created three complexes with different geometries depending on the macro cycle used. Complexes with square antiprismatic uranium or thorium between two 16-atom macrocycles were air stable, and a complex with square prismatic uranium between two 18-atom macrocycles was reactive and unstable.
The researchers examined the complexes using density functional theory and compared their results to the original actinide sandwiches uranocene and thorocene. They found that the orbital image for the new uranium complexes and the Uranocene was quite similar. However, the HOMO-LUMO gap for the new thorium complex was larger than that of thorocene, indicating that the σ-donor NHC ligand has a stronger ligand field than the π-aromatic cyclooctatetraene ligand of thorocene.