A tetraazanaphthalene radical-bridged dysprosium single-molecule magnet with a large coercive field

Generating strong magnetic coupling poses a fundamental challenge in the design of multinuclear lanthanide complexes. The inherently contracted nature of the valence 4f orbitals precludes the lanthanides from engaging in covalent bonding with closed-shell ligands. The employment of open-shell bridging ligands instead allows efficient interaction of the diffuse radical spin orbitals with the 4f shell of the lanthanides. Herein, we introduce the azaacene ligand, 1,4,5,8-tetraazanaphthalene (tan), into rare earth chemistry: first, we synthesized [(Cp*₂Dy)₂(μ-tan)] (1, Cp* = pentamethylcyclopentadienyl) containing a diamagnetic tan^2- bridge from a salt metathesis reaction of Cp*₂DyBPh₄ and K₂(tan). Second, we chemically oxidised 1 to [(Cp*₂Dy)₂(μ-tan˙)][BArF₂₀] (2) comprising a tan^1-˙ radical bridge. 2 is a rare radical-bridged single-molecule magnet (SMM) with open hysteresis loops below 3.75 K with a maximum coercive field (<i>H</i> _C) of 1.373 T at 1.8 K, which represents a notable record as <i>H</i> _C is approximately doubled compared to all known dinuclear lanthanide SMMs innate to organic radical bridges. A close match of the tan^1-˙/tan^2- and Dy^III/Dy^II redox potentials may be the origin for the impressive hysteresis loops at low temperatures, while the magnetic behaviour at higher temperatures is likely impacted from spin-phonon coupling. The outlined design strategy of matching reduction potentials of the ligand with the metal ions to amplify magnetic coupling, was proposed <i>via</i> prior computations, but is within this study for the first time experimentally confirmed. In sum, highly-tunable azaacene radicals have immense potential not only for radical-containing SMMs but for high-performance magnetic materials at large.