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*(2)Ln)(2)(mu-bpym(center dot))](+) (Ln = Gd, Tb, Dy), are reported. Strong Ln(III)-bpym(center dot-) exchange coupling is observed for all species, as indicated by the increases in chi T-M at low temperatures. For the (GdI)-I-II-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.