Similarities and Differences in Benzene Reduction with Ca, Sr, Yb and Sm: Strong Evidence for Tetra-Anionic Benzene.

Inverse sandwich complexes of Yb and Sm stabilized by a bulky β-diketiminate (BDI) ligand have been prepared: (BDI)Ln(η ⁶ ,η ⁶ -C ₆ H ₆ )Ln(BDI); Ln=lanthanide. Coordinated benzene ligands can be neutral, di-anionic or, often controversially discussed, even tetra-anionic. The formal charge on benzene is correlated to assignment of the metal oxidation state which generally poses a problem. Herein, we take advantage of the structural similarities found when comparing Ca ^II with Yb ^II , and Sr ^II with Sm ^II complexes. In this work, we found an excellent overlap of the Ca/Yb inverse sandwich structures but a striking difference for the Sr/Sm pair. The much shorter Sm-N and Sm-C ₆ H ₆ distances are strong evidence for a Sm ^III -benzene ^-4 -Sm ^III assignment. This was further supported by NMR spectroscopy, magnetic susceptibility, reactivity and comprehensive computational investigation.
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