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

Mixed-valence lanthanide complexes of the type (CpiPr5)2Ln2I3 (CpiPr5 = 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 (CpiPr5)2Ln2(OArtt)3 (1-Ln, Ln = Gd, Dy; –OArtt = 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 GdII and GdIII 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.