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

The syntheses and magnetic properties of six new compounds featuring the radical-bridged dilanthanide complexes [(Cp*(2)Ln)(2)(mu-tppz(center dot))](+) (Ln = Gd, 1; Tb, 2; Dy, 3; tppz = 2,3,5,6-tetra(2-pyridyl)pyrazine) and [(Cp*(2)Ln)(2)(mu-tppz(center dot))](-) (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(center dot-)) and trianionic (tppz(center dot 3-)) forms, respectively. Substantial Ln(III)-tppz(center dot-) exchange coupling is found for the cationic tppz(center dot-) radical-bridged species of 1-3, as suggested by a rise in chi T-M 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(center dot 3-) radical-bridged Gd compound 4 is of similar magnitude to that of the tppz(center dot-)-bridged analogue 1. However, the anionic tppz(center dot 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(center dot 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.