Exploring the substitution effect on the magnetic coupling of tetrazinyl-bridged Ln2 single-molecule magnets

Abstract

The design of new radical bridging ligands that can effectively promote strong magnetic coupling with Ln^III ions needs to focus on radicals that are susceptible to synthetic modifications and bear diffuse spin density on their donor atoms. To probe this, we introduced various substituents possessing different electron-withdrawing/donating capabilities into the redox-active s-tetrazinyl centre. This allowed for the systematic tuning of the redox and optoelectronic properties of the tetrazinyl ring. The effect of substitution on the strength of Ln–rad magnetic coupling was investigated on a series of radical-bridged Ln metallocene complexes featuring the 3,6-dimethyl-1,2,4,5-tetrazine (dmtz) and the 3,6-dimethoxy-1,2,4,5-tetrazine (dmeotz) ligands; [(Cp*₂Ln)₂(dmtz˙⁻)(THF)₂][BPh₄]·THF (Ln = Gd (1-Gd) or Dy (1-Dy); Cp* = pentamethylcyclopentadienyl; THF = tetrahydrofuran) and [(Cp*₂Ln)₂(dmeotz˙⁻)(THF)][BPh₄] (Ln = Gd (2-Gd) or Dy (2-Dy)). Cyclic voltammetry, UV-Vis absorption spectroscopy, SQUID magnetometry and ab initio as well as density functional theory (DFT) calculations are combined to underline the trends observed in this study, while comparisons with the unsubstituted 1,2,4,5-tetrazine (tz) and the 3,6-dichloro-1,2,4,5-tetrazine (dctz) are made. Notably, an intricate interplay between orbital overlap, ligand substituent effects and changes in the coordination environment is found to collectively dictate the magnitude of J_Gd–rad in the investigated systems. The strong magnetic coupling combined with highly anisotropic Dy^III ions makes 1-Dy and 2-Dy exhibit slow magnetic relaxation in the absence of an external applied field. For 1-Dy, an opening of the hysteresis loop is observed with H_c = ∼5000 Oe, one of the highest coercivities for a dinuclear organic radical-bridged single-molecule magnet.