Exchange Coupling and Magnetic Blocking in Bipyrimidyl Radical-Bridged Dilanthanide Complexes

The synthesis and magnetic properties of three new bipyrimidyl radical-bridged dilanthanide complexes, [(Cp*₂Ln)₂(μ-bpym^•)]⁺ (Ln = Gd, Tb, Dy), are reported. Strong Ln^III-bpym^•– exchange coupling is observed for all species, as indicated by the increases in χ_MT at low temperatures. For the Gd^III-containing complex, a fit to the data reveals antiferromagnetic coupling with J = −10 cm^–1 to give an S = ¹³/₂ ground state. The Tb^III and Dy^III congeners show single-molecule magnet behavior with relaxation barriers of U_eff = 44(2) and 87.8(3) cm^–1, respectively, a consequence of the large magnetic anisotropies imparted by these ions. Significantly, the latter complex exhibits a divergence of the field-cooled and zero-field-cooled dc susceptibility data at 6.5 K and magnetic hysteresis below this temperature.