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Circular and Chainlike Copper(II)–Lanthanide(III) Complexes Generated by Assembly Reactions of Racemic and Chiral Copper(II) Cross-Linking Ligand Complexes with LnIII(NO3)3·6H2O (LnIII = GdIII, TbIII, DyIII)

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posted on 18.01.2017, 12:49 by Takahiro Ueno, Takeshi Fujinami, Naohide Matsumoto, Masaki Furusawa, Ryo Irie, Nazzareno Re, Takuya Kanetomo, Takayuki Ishida, Yukinari Sunatsuki
The 1:1 assembly reaction of the racemic form of the cross-linking ligand complex Na­[CuIILdpen(1R2R/1S2S)] with LnIII(NO3)3·6H2O gave the centrosymmetric circular (CuIILnIII)2 complex [CuIILdpen(1R2R/1S2S)LnIII(NO3)2]2 (1Ln: Ln = Gd, Tb, Dy), while the reaction of the enantiopure form Na­[CuIILdpen(1R2R)] with LnIII(NO3)3·6H2O gave the chiral chainlike (CuIILnIII)1∞ complex [CuIILdpen(1R2R)LnIII(NO3)2(CH3CN)]1∞·CH3CN (2Ln: Ln = Gd, Tb, Dy), where {CuIILdpen(1R2R)} is (N-((1R,2R)-2-(((E)-3-ethoxy-2-oxybenzylidene)­amino)-1,2-diphenylethyl)-2-oxybenzamide)­copper­(II) and {CuIILdpen(1R2R/1S2S)} is the racemic mixture of {CuIILdpen(1R2R)} and {CuIILdpen(1S2S)}. The copper­(II) component functions as a cross-linking ligand complex and bridges two LnIII ions at two phenoxo oxygen atoms and one ethoxy oxygen atom, as well as at an amido oxygen atom. For 1Ln, two binuclear species of [CuIILdpen(1R2R)LnIII(NO3)2] and [CuIILdpen(1S2S)LnIII(NO3)2] with opposite chiralities are linked by two amido oxygen atoms O3 and O3* to form a centrosymmetric circular structure with Gd–Cu = 3.370(1) Å and Gd–Cu* = 5.627(1) Å. For 2Ln, binuclear species with the same chirality are bridged by Gd–O3* = 2.228(5) Å to form a chiral chainlike structure with Gd–Cu = 3.3348(9) Å and Gd–Cu* = 6.2326(9) Å. The bridged angles through the amido group of Gd–O3*C7* are 133.9(5) and 177.6(4)° for 1Gd and 2Gd, respectively. The magnetic susceptibilities of 1Gd and 2Gd were analyzed by the spin-only Hamiltonian on the basis of the circular tetranuclear (−CuIIGdIII−)2 and linear chainlike (−CuIIGdIII−)1∞ structures, respectively. The CuII–GdIII magnetic interactions through two phenoxo bridges and a three-atom N–CO bridge, J1 and J2, are both ferromagnetic to be J1 = +4.6 cm–1 and J2 = +1.8 cm–1 for 1Gd and J1 = +4.2 cm–1 and J2 = +0.037 cm–1 for 2Gd. The J2 value of 2Gd is much smaller than that of 1Gd. When the temperature was lowered, 1Ln and 2Ln (Ln = Tb, Dy) showed a decrease in the χMT vs T plot due to crystal field effects on the LnIII ion (Stark splitting) and an increase due to the ferromagnetic CuII–LnIII interaction. The magnetization values of 1Ln and 2Ln (Ln = Tb, Dy) without liquid paraffin are considerably larger than the corresponding values with liquid paraffin, indicating the presence of strong magnetic anisotropy. 1Tb and 1Dy showed frequency dependence of ac magnetic susceptibility under zero external dc magnetic field, showing the behavior of single-molecule magnets (SMMs). 2Tb and 2Dy showed no frequency dependence under a zero external magnetic field but showed a meaningful frequency dependence under an external magnetic field. Their energy barriers, Δ/kB, estimated by the Arrhenius plots are 29.4(6) and 20.6(3) K for 1Tb and 2Tb under dc bias fields of 0 and 1000 Oe, respectively, and those of 1Dy and 2Dy are 13.1(9) K and 16.4(2) K under dc bias fields of 0 and 1000 Oe, respectively.