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

Abstract

The first use of the bare 2,2′-bisimidazole (H₂bim) 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 Li₂(bim) with Cp*₂RE(BPh₄) (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 bim²⁻ 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 τ₀ = 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.