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 N23– radical-bridged lanthanide complexes [{(R2N)2(THF)­Ln}2(μ3-η2:η2:η2-N2)­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(μ-η2:η2-N2)} (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 Å of the N23– bridge, leading to bending of the planar Ln–(N23–)–Ln unit present in 2-Ln. Direct current (dc) magnetic susceptibility measurements performed on 1-Gd reveal antiferromagnetic coupling between the GdIII centers and the N23– radical bridge, with a strength matching that obtained previously for 2-Gd at J ∼ −27 cm–1. Unexpectedly, however, a competing antiferromagnetic GdIII–GdIII exchange interaction with J ∼ −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 (Ueff = 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 LnIII–LnIII exchange interactions of the type quantified in 1-Gd.