The Interplay of Single Ion Anisotropy and Magnetic 3 d-4 f Interactions in VIII2LnIII2 Butterfly Complexes

Within the framework of 3d–4f molecular magnets, the most thoroughly investigated architecture is that of butterfly-shaped coordination clusters as it provides an ideal testbed to study fundamental magnetic interactions. Here, we report the synthesis and characterisation of a series of isostructural V^III₂Ln^III₂ butterfly complexes, where Ln=Y (1_Y), Tb (2_Tb), Dy (3_Dy), Ho (4_Ho), Er (5_Er), Tm (6_Tm), Yb (7_Yb), which extends the previous study on isostructural butterflies with Cr^III, Mn^III and Fe^III. In zero external field, compounds 2_Tb, 3_Dy and 4_Ho show clear maxima in the out-of-phase component of the ac susceptibility whereas small magnetic fields are needed to suppress quantum tunnelling in 6_Tm. Combined high-field electron paramagnetic resonance spectroscopy and magnetisation measurements unambiguously reveal an easy-plane anisotropy of the V^III ion and antiferromagnetic Ising-like 3d–4f exchange couplings. The strength of J_3d-4f is shown to decrease upon variation of the 4 f ion from Tb to Ho, while increasing antiferromagnetic interaction can be observed from Ho to Tm. The exact inverse chemical trend is found for the relative angle between the 3d and 4 f main anisotropy axes, which highlights the important role of the lanthanide 4 f electron distribution anisotropy for 3d–4f exchange.