Exchange coupling in TbCu and DyCu single-molecule magnets and related lanthanide and vanadium analogs

Heterometallic coordination compounds [Cu^II(L)(C ₃H₆O)Ln^III(NO₃)₃] and [V^IVO(L)(C₃H₆O)Ln^III(NO ₃)₃] (abbreviated as LnCu and LnV, respectively; H ₂L = N,N′-bis(3-methoxysalicylidene)-1,3-diamino-2,2- dimethylpropane; Ln = Gd, Tb, Dy, Ho, and Er) were synthesized, and the X-ray crystallographic analysis shows that their structures are isomorphous for each series. The single-molecule magnet behavior was observed for TbCu and DyCu, and the activation energies of magnetization reversal were 42.3(4) and 11.5(10) K, respectively. The magnetic exchange couplings in LnCu and LnV were precisely evaluated by means of combined high-frequency EPR and pulsed-field magnetization studies, to give J_Tb-Cu/k_B ≥ 3.3 K, J _Dy-Cu/k_B = 1.63(1) K, J_Ho-Cu/k_B = 1.09(2) K, and J_Er-Cu/k_B = 0.24(1) K. A monotonic decrease of ferromagnetic J_Ln-Cu was found in the order of the atomic number, ₆₄Gd to ₆₈Er. The corresponding exchange parameters in LnV are smaller than those of the Cu derivatives, and J_Gd-V was antiferromagnetic (-3.0 K determined from the magnetization jump). A possible mechanism for the exchange coupling and chemical trend is discussed.