Triangular (C ₅ <i> ⁱ </i> Pr ₅ ) ₃ Ln ₃ H ₃ I ₂ (Ln = Tb, Dy, Ho, Er, Tm) Clusters with Lanthanide-Dependent Bonding, Valence Delocalization, and Magnetic Anisotropy

Mixed-valence complexes featuring lanthanide-lanthanide bonding have recently been shown to act as single-molecule magnets with unprecedented operating temperatures and magnetic coercivities. Here, we present the synthesis and detailed examination of the electronic structure, bonding, and magnetic properties of mixed-valence trinuclear clusters (C₅^<i>i</i>Pr₅)₃Ln₃H₃I₂ (Ln = Tb, Dy, Ho, Er, and Tm). Near-infrared and X-ray absorption spectra, together with computational results, confirm these clusters possess a three-center, one-electron σ bond. This metal-metal bonding leads to strong intermetal exchange coupling, resulting in magnetic behaviors that starkly contrast with typical multinuclear lanthanide complexes. Notably, structural, spectroscopic, and computational studies of the thulium cluster reveal valence delocalization through a bonding orbital of 5d-parentage between the three Tm centers. This observation represents the first example of a nontraditional electronic structure for thulium involving 5d rather than 4f orbitals. Magnetic analysis reveals a complex interplay between single-ion magnetic anisotropy and ferromagnetic exchange, governing the overall magnetic anisotropy of these clusters. Magnetic susceptibility measurements for Ln = Tb-Er indicate thermally well-isolated high-moment ground states arising from strong magnetic coupling, although the maximum values are lower than those expected for complete parallel alignment of the σ and 4f electrons. Computational analyses suggest that collinear alignment of the local anisotropy axes results in out-of-plane anisotropy for Ln = Er and Tm, whereas noncollinear alignment induces in-plane anisotropy for Ln = Tb, Dy, leading to distinct magnetic relaxation properties. Together, the results highlight the diverse magnetic behaviors that can be realized through lanthanide-lanthanide bonding and outline a synthetic path forward toward maximizing the magnetic anisotropy in f-element clusters.