Abstract
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*(2)Dy)(2)(μ-tan)] (1, Cp* = pentamethylcyclopentadienyl) containing a diamagnetic tan(2-) bridge from a salt metathesis reaction of Cp*(2)DyBPh(4) and K(2)(tan). Second, we chemically oxidised 1 to [(Cp*(2)Dy)(2)(μ-tan˙)][BArF(20)] (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 (H (C)) of 1.373 T at 1.8 K, which represents a notable record as H (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 via 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.