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 LnIII-bpym•– exchange coupling is observed for all species, as indicated by the
increases in χMT at low temperatures.
For the GdIII-containing complex, a fit to the data reveals
antiferromagnetic coupling with J = −10 cm–1 to give an S = 13/2 ground state. The TbIII and DyIII congeners
show single-molecule magnet behavior with relaxation barriers of Ueff = 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.