Influence of an Inner-Sphere K+ Ion on the Magnetic Behavior of N23-Radical-Bridged Dilanthanide Complexes Isolated Using an External Magnetic Field

The synthesis and full magnetic characterization of a new series of N-2(3-) radical-bridged lanthanide complexes [{(R2N)(2)(THF)Ln}(2)(mu(3)-eta(2):eta(2):eta(2)-N-2)K] [1-Ln; Ln = Gd, Tb, Dy; NR2 = N(SiMe3)(2)] are described for comprehensive comparison with the previously reported series [K(18-crown-6)(THF)(2)]{[(R2N)(2)(THF)Ln](2)(mu-eta(2):eta(2)-N-2)} (2-Ln; Ln = Gd, Tb, Dy). Structural characterization of 1-Ln crystals grown with the aid of a Nd2Fe13B magnet reveals inner-sphere coordination of the K+ counterion within 2.9 angstrom of the N-2(3-) bridge, leading to bending of the planar Ln-(N-2(3-))-Ln unit present in 2-Ln. Direct current (dc) magnetic susceptibility measurements performed on 1-Gd reveal antiferromagnetic coupling between the Gd-III centers and the N-2(3-) radical bridge, with a strength matching that obtained previously for 2-Gd at J similar to -27 cm(-1). Unexpectedly, however, a competing antiferromagnetic Gd-III-Gd-III exchange interaction with J similar to -2 cm(-1) also becomes prominent, dramatically changing the magnetic behavior at low temperatures. Alternating current (ac) magnetic susceptibility characterization of 1-Tb and 1-Dy demonstrates these complexes to be single-molecule magnets under zero applied dc field, albeit with relaxation barriers (U-eff = 41.13(4) and 14.95(8) cm(-1), respectively) and blocking temperatures significantly reduced compared to 2-Tb and 2-Dy. These differences are also likely to be a result of the competing antiferromagnetic Ln(III)-Ln(III) exchange interactions of the type quantified in 1-Gd.