Strong magnetic exchange coupling in Ln₂ metallocenes attained by the trans-coordination of a tetrazinyl radical ligand

A combination of high-performing lanthanide metallocenes and tetrazine-based radical ligands leads to a new series of radical-bridged dinuclear lanthanide metallocenes; [(Cp*(2)Ln(III))(2)(bpytz(-))][BPh4] (where Ln = Gd (1), Tb (2), Dy (3) and Y (4); Cp* = pentamethylcyclopentadienyl; bpytz = 3,6-bis(3,5-dimethyl-pyrazolyl)-1,2,4,5-tetrazine). The formation of the radical species is achieved via a controlled, stepwise synthesis and verified in all complexes by X-ray crystallography and SQUID magnetometry, as well as EPR spectroscopy of 4. Through the judicious choice of the Cp* ancillary ligands and by taking advantage of the steric effects imposed by their bulkiness, we were able to promote the trans coordination mode of the bpytz(-) radical anion that enables stronger magnetic exchange coupling compared to the cis fashion. This yields a J(Gd-rad) = -14.0 cm(-1) in 1, which is the strongest exchange coupling observed in organic monoanionic radical-bridged lanthanide metallocene systems. The strong Ln-rad exchange coupling was further confirmed by high-frequency EPR (HF-EPR) spectroscopy and broken-symmetry (BS) density functional theory (DFT) calculations. This combined with the highly anisotropic nature of Tb-III and Dy-III ions in 2 and 3, respectively, leads to strong SMM behavior and slow relaxation of the magnetization at zero fields.