From unprecedented 2,2′-bisimidazole-bridged rare earth organometallics to magnetic hysteresis in the dysprosium congener

Abstract

A new series of bisimidazole-bridged rare earth metallocene complexes, [(Cp* ₂RE) ₂[μ-bim] (RE = Y, Gd, and Dy), was isolated and studied by crystallography, magnetometry, spectroscopy, and computations. The Dy congener is a single-molecule magnet.

Abstract

The first use of the bare 2,2′-bisimidazole (H ₂bim) ligand in rare earth metal chemistry is presented. A series of symmetric dinuclear complexes [(Cp* ₂RE) ₂(μ-bim)] were synthesized from the salt metathesis reaction of the lithium salt Li ₂(bim) with Cp* ₂RE(BPh ₄) (RE = Y (1), Gd (2), Dy (3); Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl). The isostructural complexes 1–3 were unambiguously characterized through elemental analysis, NMR, IR and UV/Vis spectroscopy, single-crystal X-ray diffraction, SQUID magnetometry and density functional theory (DFT) calculations. Intriguingly, the compounds are redox-inactive both on the timescale of chemical and electrochemical experiments. Herein, a rationale for the redox innocence of the bim ²⁻ ligand is provided by calculations of the electron affinity and ionization potential, both correlating well with topologically similar structures of comparable complexes. Remarkably, the Dy complex 3 shows open magnetic hysteresis loops up to 5 K which is rare for lanthanide SMMs with bridging diamagnetic entities. AC magnetic susceptibility measurements at zero field revealed slow magnetic relaxation up to 26 K leading to an effective energy barrier to spin reversal of U _eff = 154(2) cm ⁻¹ and τ ₀ = 5(1) × 10 ⁻⁸ s. The lanthanides are weakly antiferromagnetically coupled, where the J value for the Gd-congener 2 was determined to be −0.074(2) cm ⁻¹.