η6-Benzene Tetra-Anion Complexes of Early and Late Rare-Earth Metals

A novel synthetic route to the triple-decker benzene tetra-anion complexes [(η5-C5iPr5)M(μ:η6:η6-C6H6)M(η5-C5iPr5)] is reported for a range of early and late rare-earth elements, i.e., M = Y, La, Sm, Gd, and Dy (1M). The lanthanum complex 1La 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 1Y and 1La. Analysis of the bonding in 1Y and 1La 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 1Sm, 1Gd, and 1Dy are also consistent with the presence of a benzene tetra-anion ligand. The origins of the appreciable exchange coupling constant of Jexch = −3.35 cm–1 (−2J formalism) in 1Gd are established through a computational study of the interacting magnetic orbitals. The dynamic magnetic properties of 1Dy 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.