Exchange couplings in one-dimensionally arrayed 4f-3d heterometallic [Ln2Cu2]n compounds: A chemical trend of the coupling parameter

Heterometallic polymeric coordination compounds {Ln(hfac)₂(CH₃OH)}₂{Cu(dmg)(Hdmg)}₂]_n (abbreviated as Ln₂Cu₂]_n) involving light lanthanoid ions, Pr, Nd, Sm, and Eu, were synthesized, where H₂dmg and Hhfac stand for dimethylglyoxime and 1,1,1,5,5,5-hexafluoropentane-2,4-dione, respectively. The X-ray crystallographic analysis shows that their structures are isomorphous to those of the known heavy lanthanoid analogs. The exchange couplings were evaluated by high-frequency electron paramagnetic resonance and pulsed-field magnetization studies, giving ferromagnetic exchange parameters: J_Pr-Cu/k_B = 0.039(1) K, J_Nd-Cu/k_B = 0.38(1) K, J_Sm-Cu/k_B = 0.95(2) K, and no interaction between Eu and Cu ions. A significant trend is found in the order of the periodic table. The ferro- and antiferromagnetic 4f-3d couplings were characterized for the light and heavy lanthanoid derivatives, respectively. The magnitude of J_Ln-Cu decreases with a decrease of the 4f-spin portion, i.e., the number of unpaired electrons, whether the coupling is ferro- or antiferromagnetic.