Planar Benzene Trianion-Bridged Dilanthanide Complexes Exhibiting Strong Ferromagnetic Exchange via Metal-Radical Covalency

The synthesis of molecules exhibiting strong magnetic exchange coupling between contracted lanthanide 4f electrons and a radical spin presents an important challenge relevant to the design of single-molecule magnets and solid-state magnetic materials. Concurrently, the isolation of a planar benzene trianion radical remains an unrealized target. Here, we report the synthesis and characterization of the dilanthanide compounds [(CpiPr5Ln)2(μ-η6:η6-C6H6)][Fe(N(SiMe3)2)3] (Ln = Y, Gd, Tb, Dy, Ho, Er, Tm; CpiPr5 = pentaisopropylcyclopentadienyl), featuring cationic inverse sandwich complexes in which two Ln centers are bridged by a planar benzene trianion radical. These compounds are synthesized via one-electron oxidation of (CpiPr5Ln)2(μ-η6:η6-C6H6) using Fe(N(SiMe3)2)3 as a mild outer-sphere oxidant. Single-crystal X-ray diffraction reveals inverse sandwich structures featuring a planar benzene bridge and structural signatures of extended δ-bonding along the Cp-Ln-Bz-Ln-Cp axis. Continuous-wave electron paramagnetic resonance spectroscopy of the congener containing diamagnetic Y3+ supports assignment of an S = 1/2 benzene trianion radical. Magnetometry measurements on the Gd analogue reveal the presence of remarkably strong ferromagnetic coupling (J = +55.6(6) cm-1) between the S = 7/2 Gd3+ centers and the benzene radical, giving rise to a well-isolated S = 15/2 ground state. These observations are explained through a molecular orbital analysis, which indicates that the δ-symmetric (CpiPr5)- ligands lower the energy of the Ln3+ dδ (dxy and dx2-y2) orbitals. Importantly, this electronic structure facilitates enhanced covalency with the benzene trianion radical, leading to greater than 30% metal character in the radical-based singly occupied molecular orbital (SOMO). The results establish a new regime of radical-mediated lanthanide exchange coupling in which substantial ground-state metal character within the radical SOMO drives strong intraatomic-like ferromagnetic exchange akin to Hund's first rule.