Pyrrolyl-Bridged Metallocene Complexes: From Synthesis, Electronic Structure, to Single-Molecule Magnetism

The π- and σ-basicity of the pyrrolyl ligand affords several coordination modes. A sterically encumbering coordination sphere around metal centers may foster new coordination modes for the pyrrolyl ligand. Here, we present three dinuclear rare earth complexes [Cp*₂RE(μ-pyr)]₂, [RE = Y (1), La (2), Dy (3); Cp* = pentamethylcyclopentadienyl, pyr = pyrrolyl], which were synthesized through a protonolysis reaction between allyl complexes and H-pyrrole. Each metal is ligated by two Cp* ligands and the N atom of the pyrrolyl ring while interacting with the π-system of the other pyrrolyl ligand, yielding an unprecedented coordination mode for pyrrolyl best described as [((η⁵-Cp*)₂RE)₂(μ-1η²-pyr-2κN)(μ-2η²-pyr-1κN)]. The steric congestion implemented by the Cp* ligands forces this asymmetric coordination of the pyrrolyl ligand. 1-3 were characterized by crystallography, electrochemistry, and spectroscopy. Density functional theory calculations on 1 uncovered the bonding situation between the pyrrolyl ligand and the yttrium(III) ion. Excitingly, 3 displays slow magnetic relaxation under zero dc field with U_eff = 98.9(7) cm^-1 and τ_o = 6.7(1) × 10^-8 s, placing it among coveted dinuclear metallocene single-molecule magnets. CASSCF calculations provided the energy of the crystal field states of Dy^III and confirmed the barrier height.