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-C₅Pr₅)M(μ:η⁶:η⁶-C₆H₆)M(η⁵-C₅Pr₅)] is reported for a range of early and late rare-earth elements, i.e., M = Y, La, Sm, Gd, and Dy (1). The lanthanum complex 1 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 1 and 1. Analysis of the bonding in 1 and 1 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 1, 1, and 1 are also consistent with the presence of a benzene tetra-anion ligand. The origins of the appreciable exchange coupling constant of J_exch = -3.35 cm^-1 (-2J formalism) in 1 are established through a computational study of the interacting magnetic orbitals. The dynamic magnetic properties of 1 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.