Mixed-Valence Aryloxido-Bridged Dilanthanide Complexes with Lanthanide-Dependent Electron Delocalization

Mixed-valence lanthanide complexes of the type (Cp^iPr5)₂Ln₂I₃ (Cp^iPr5 = pentaisopropylcyclopentadienyl; Ln = Gd, Tb, Dy, Ho, Er) featuring Ln-Ln bonding interactions can exhibit strongly coupled high-moment ground states with a large axial magnetic anisotropy. Here, we report the synthesis, structures, and magnetic properties of the aryloxido-bridged mixed-valence dilanthanide complexes (Cp^iPr5)₂Ln₂(OAr^tt)₃ (1-Ln, Ln = Gd, Dy; ^-OAr^tt = 3,5-bis(tert-butyl)phenoxide anion). The solid-state structures of the two complexes are distinct, with 1-Dy exhibiting a nearly symmetric structure and a short Dy-Dy bond of 3.265(1) Å, suggesting valence delocalization, while 1-Gd has an asymmetric structure with each Gd atom in a distinct coordination environment, indicative of valence localization. Static magnetic susceptibility data confirm that 1-Gd is valence localized, with only very weak antiferromagnetic exchange between the Gd^II and Gd^III centers at low temperatures. In contrast, magnetic susceptibility data for 1-Dy reveal strong magnetic coupling to give a large angular momentum ground state with magnetic blocking below 40 K. Solid-state Raman spectra for 1-Dy are indicative of Dy-Dy bonding that persists up to ambient temperatures. Computational analyses suggest that the bonding interaction in 1-Dy becomes highly polarized in the presence of slight structural asymmetry and that this effect becomes more pronounced at higher temperatures, as supported by variable-temperature single-crystal X-ray diffraction data. Detailed exploration of the vibronic coupling is consistent with vibronic-driven valence localization at elevated temperatures in 1-Dy.