η⁶‑Benzene Tetra-Anion Complexesof Early and Late Rare-Earth Metals

A novel synthetic route to the triple-decker benzene tetra-anion complexes [(η⁵-C₅<i>ⁱ</i>Pr₅)M(μ:η⁶:η⁶-C₆H₆)M(η⁵-C₅<i>ⁱ</i>Pr₅)] is reported for a range of early and late rare-earth elements, i.e., M = Y, La, Sm, Gd, and Dy (<b>1</b>_<b>M</b>). The lanthanum complex <b>1</b>_<b>La</b> is the first benzene tetra-anion complex of the largest rare-earth element. Aromaticity in the 10π-electron benzene ligands is confirmed through crystallographic studies of all compounds and nucleus-independent chemical shift calculations on <b>1</b>_<b>Y</b> and <b>1</b>_<b>La</b>. Analysis of the bonding in <b>1</b>_<b>Y</b> and <b>1</b>_<b>La</b> using density functional theory revealed strong covalency in the metal-benzene interactions, with very similar contributions from the metal 4d/5d orbitals, respectively, and the benzene π* orbitals. Magnetic susceptibility measurements on <b>1</b>_<b>Sm</b>, <b>1_Gd</b>, and <b>1</b>_<b>Dy</b> are also consistent with the presence of a benzene tetra-anion ligand. The origins of the appreciable exchange coupling constant of <i>J</i>_exch = −3.35 cm^–1 (−2<i>J</i> formalism) in <b>1</b>_<b>Gd</b> are established through a computational study of the interacting magnetic orbitals. The dynamic magnetic properties of <b>1</b>_<b>Dy</b> are also described. The clear absence of SMM behavior in the dysprosium complex is explained using multireference calculations and an ab initio ligand-field theory description of the 4f orbitals, which clearly show that the benzene tetra-anion ligand provides a dominant equatorial contribution.