Magnetic Blocking in Fluoflavine Radical-Bridged Dilanthanide Complexes

Magnetic exchange coupling is difficult to foster in polynuclear lanthanide (Ln) complexes and poorly understood. While coupling Ln ions through closed-shell ligands is inherently weak due to the contracted 4f orbitals, placing open-shell ligands instead has proven to promote orders of magnitude stronger coupling, giving rise to single-molecule magnets (SMMs) innate to real magnetic memory effect in the case of the anisotropic Ln ions. Notably, the impact of radical bridges with differing oxidation states on magnetic blocking remains unexplored due to lack of Ln SMMs with radicals in two distinct charge states. Herein, the first dilanthanide complexes (Ln = Gd, Dy) containing fluoflavine (flv) bridges, [(Cp*₂Ln)₂(μ-flv^z)]X, (where X = [Al(OC{CF₃}₃)₄]^- (z = 1-•), 1-Ln; X = 0 (z = 2-), 2-Ln; X =[K(crypt-222)]⁺ (z = 3-•), 3-Ln) are reported. 1-Ln and 3-Ln, comprising the flv^1-• and flv^3-• radical bridges, were investigated via single-crystal X-ray diffraction (SCXRD), ultraviolet-visible (UV-vis) spectroscopy, Superconducting Quantum Interference Device (SQUID) magnetometry, high-field electron paramagnetic resonance (HF-EPR) spectroscopy and broken-symmetry density functional theory (BS-DFT) calculations. 1-Dy and 3-Dy constitute the first SMMs innate to radicals in two differing oxidation states. 1-Dy exhibits a spin-reversal barrier U_eff of 28.36 cm^-1 and open magnetic hysteresis loops below 3 K. By contrast, 3-Dy displays a much higher U_eff of 143(2) cm^-1 and open hysteresis loops until 9.5 K, representing a record for dilanthanide SMMs containing an organic radical bridge. The boost in SMM properties in 3-Dy is attributed to spin-phonon coupling and improved frontier orbital structure. This study paves the way for advanced design strategies of polynuclear Ln SMMs.