Linear, Electron-Rich Erbium Single-Molecule Magnet with Dibenzocyclooctatetraene Ligands

Judicious design of ligand scaffolds to highly anisotropic lanthanide ions led to substantial advances in molecular spintronics and single-molecule magnetism. Erbium-based single-molecule magnets (SMMs) are rare, which is attributed to the prolate-shaped Er^III ion requiring an equatorial ligand field for enhancing its single-ion magnetic anisotropy. Here, we present an electron-rich mononuclear Er SMM, [K(crypt-222)][Er(dbCOT)₂], 1 (where dbCOT = dibenzocyclooctatetraene), that was obtained from a salt metathesis reaction of ErCl₃ and K₂dbCOT. The dipotassium salt, K₂dbCOT, was generated through a two-electron reduction of the bare dbCOT⁰ ligand employing potassium graphite and was crystallized from DME to give the new solvated complex, [K(DME)]₂[dbCOT], 2. 1 was analyzed through crystallography, electrochemistry, spectroscopy, magnetometry, and CASSCF calculations. The structure of 1 consists of an anionic metallocene complex featuring a linear (180.0°) geometry with an Er^III ion sandwiched between dianionic dbCOT ligands and an outer-sphere K⁺ ion encapsulated in 2.2.2-cryptand. Two pronounced redox events at negative potentials allude to the formation of a trianionic erbocene complex, [Er(dbCOT)₂]^3-, on the electrochemical time scale. 1 shows slow magnetic relaxation with an effective spin-reversal barrier of U_eff = 114(2) cm^-1, which is close in magnitude to the calculated energies of the first and second excited states of 96.9 and 109.13 cm^-1, respectively. 1 exhibits waist-constricted hysteresis loops below 4 K and constitutes the first example of an erbocene-SMM bearing fused aromatic rings to the central COT ligand. Notably, 1 comprises the largest COT scaffold implemented in erbocene SMMs, yielding the most electron-rich homoleptic erbium metallocene SMM.