Crystal-Field Properties of Pentalene Ligands in Erbium(III) Sandwich Complexes

The influence of pentalene ligands on the crystal-field properties of erbium-(III) has been examined using the monometallic complexes [Er-(η⁸-Pn^†)₂]^- (1) and [(η⁸-Pn^†)-Er-(η⁸-COT)]^- (2) (Pn^† = [1,4-( ⁱ Pr₃Si)₂C₈H₄]^2-, COT = [C₈H₈]^2-), which were synthesized as [K-(2.2.2-crypt)]⁺ salts. Both compounds display magnetic anisotropy consistent with the prolate ion Er³⁺, but do not exhibit slow magnetic relaxation in zero field. Field-induced AC susceptibility measurements show that relaxation is governed by Raman and quantum tunnelling processes, with no evidence for an Orbach mechanism. Multireference ab initio calculations reveal anisotropic ground Kramers doublets with significant transverse components in both 1 and 2, along with extensive M _J mixing and low-lying excited states that enable efficient relaxation. Structural analysis shows that the pentalene ligands enforce a low-symmetry coordination environment, which is only partially moderated by incorporation of a cyclo-octatetraenyl ligand in 2. These results indicate that the pentalene ligands impose a local symmetry on erbium-(III) that facilitates transverse anisotropy and effectively precludes single-molecule magnet behavior.