Exchange coupling and magnetic blocking in dilanthanide complexes bridged by the multi-electron redox-active ligand 2,3,5,6-tetra(2-pyridyl)pyrazine

Abstract

The syntheses and magnetic properties of six new compounds featuring the radical-bridged dilanthanide complexes [(Cp*₂Ln)₂(μ-tppz˙)]⁺ (Ln = Gd, 1; Tb, 2; Dy, 3; tppz = 2,3,5,6-tetra(2-pyridyl)pyrazine) and [(Cp*₂Ln)₂(μ-tppz˙)]⁻ (Ln = Gd, 4; Tb, 5, Dy, 6) are reported. Cyclic voltammograms for compounds 1–3 reveal that the tppz ligand can reversibly undergo multiple redox changes. Hence, in the two sets of compounds isolated, 1–3 and 4–6, the redox-active ligand tppz exists in the monoanionic (tppz˙⁻) and trianionic (tppz˙³⁻) forms, respectively. Substantial Ln^III–tppz˙⁻ exchange coupling is found for the cationic tppz˙⁻ radical-bridged species of 1–3, as suggested by a rise in χ_MT at low temperatures. For the Gd compound 1, fits to the data yielded a coupling constant of J = −6.91(4) cm⁻¹, revealing antiferromagnetic coupling to give an S = 13/2 ground state. Both of the Tb^III and Dy^III-containing compounds 2 and 3 exhibit single-molecule magnet behavior under zero applied dc field. Importantly, the Dy congener shows a divergence of the field-cooled and zero-field-cooled dc susceptibility data at 2.8 K and magnetic hysteresis below 3.25 K. Interestingly, the coupling constant of J = −6.29(3) cm⁻¹ determined for the trianionic tppz˙³⁻ radical-bridged Gd compound 4 is of similar magnitude to that of the tppz˙⁻-bridged analogue 1. However, the anionic tppz˙³⁻-bridged species containing Tb^III and Dy^III centers, compounds 5 and 6, do not exhibit slow magnetization dynamics under zero and applied dc fields. Computational results indicate a doublet ground state for the bridging tppz˙³⁻ unit, with a different distribution for the spin density orientation towards the Ln^III centers. These results have important implications for the future design of molecule-based magnets incorporating exchange-coupled lanthanide-radical species.