A new family of heterometallic pentanuclear complexes of formulas [Mn^IVMn^III₂Ln^III₂O₂(benz)₄(mdea)₃(NO₃)₂(MeOH)] (Ln = Dy (1-Dy), Tb (2-Tb), Gd (3-Gd), Eu (4-Eu), Sm (5-Sm), Nd (6-Nd), Pr (7-Pr); benz(H) = benzoic acid; mdeaH₂= N-methyldiethanolamine) and [Mn^IVMn^III₂Ln^III₂O₂(o-tol)₄(mdea)₃(NO₃)₂(MeOH)] (Ln = Gd (8-Gd), Eu (9-Eu); o-tol(H) = o-toluic acid) have been isolated and structurally, magnetically, and theoretically characterized. dc magnetic susceptibility measurements reveal dominant antiferromagnetic magnetic interactions for each complex, except for 2-Tb and 3-Gd, which reveal an upturn in the χ_MT product at low temperatures. The magnetic interactions between the spin centers in the Gd derivatives, 3-Gd and 8-Gd, which display markedly different χ_MT vs T profiles, were found to be due to the interactions of the Gd^III–Gd^III ions which change from ferromagnetic (3-Gd) to antiferromagnetic (8-Gd) due to structural differences. ac magnetic susceptibility measurements reveal a nonzero out-of-phase component for 1-Dy and 7-Pr, but no maxima were observed above 2 K (H_dc = 0 Oe), which suggests single-molecule magnet (SMM) behavior. Out-of-phase signals were observed for complexes 2-Tb, 4-Eu, 8-Gd, and 9-Eu, in the presence of a static dc field (H_dc = 2000, 3000 Oe). The anisotropic nature of the lanthanide ions in the benzoate series (1-Dy, 2-Tb, 5-Sm, 6-Nd, and 7-Pr) were thoroughly investigated using ab initio methods. CASSCF calculations predict that the origin of SMM behavior in 1-Dy and 7-Pr and the applied field SMM behavior in 2-Tb does not solely originate from the single-ion anisotropy of the lanthanide ions. To fully understand the relaxation mechanism, we have employed the Lines model to fit the susceptibility data using the POLY_ANISO program, which suggests that the zero-field SMM behavior observed in complexes 1-Dy and 7-Pr is due to weak Mn^III/IV–Ln^III and Ln^III–Ln^III couplings and an unfavorable Ln^III/Mn^III/Mn^IV anisotropy. In complexes 4-Eu, 8-Gd, and 9-Eu ab initio calculations indicate that the anisotropy of the Mn^III ions solely gives rise to the possibility of SMM behavior. Complex 7-Pr is a Pr(III)-containing complex that displays zero-field SMM behavior, which is rare, and our study suggests the possibility of coupling weak SOC lanthanide metal ions to anisotropic transition-metal ions to derive SMM characteristics; however, enhancing the exchange coupling in {3d–4f} complexes is still a stubborn hurdle in harnessing new generation {3d–4f} SMMs.

中文翻译：

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了解五核{Mn ^IV Mn ^III ₂ Ln ^III ₂ }单分子磁体中磁弛豫的机理

分子式为[Mn ^IV Mn ^III ₂ Ln ^III ₂ O ₂（苯）₄（mdea）₃（NO ₃）₂（MeOH）的杂金属五核配合物的新家族]（Ln = Dy（1-Dy），Tb（2 -Tb），Gd（3-Gd），Eu（4-Eu），Sm（5-Sm），Nd（6-Nd），Pr（7-Pr）;苯（H）=苯甲酸; mdeaH ₂ = N-甲基二乙醇胺）和[Mn ^IV Mn ^III ₂ Ln ^III ₂ O₂（邻甲苯）₄（mdea）₃（NO ₃）₂（MeOH）]（Ln = Gd（8-Gd），Eu（9-Eu）;邻甲苯（H）=邻甲苯甲酸）被隔离并在结构，磁性和理论上进行了表征。直流磁化率测量表明主导反铁磁磁相互作用为每个复杂，除了2-Tb的和3的Gd，这揭示在χ好转_中号Ť在低温下的产物。Gd衍生物3-Gd和8-Gd中自旋中心之间的磁相互作用，其显示显着不同的χ_中号Ť VS Ť型材，被认为是由于钆的相互作用^III -Gd ^III离子，其变化从铁磁（3-钆）到反铁磁（8钆）由于结构上的差异。交流磁化率测量显示1-Dy和7-Pr的非零异相分量，但在2 K（H _dc = 0 Oe）以上未观察到最大值，这表明单分子磁体（SMM）行为。对于复合物2-Tb，4-Eu，8-Gd和9-Eu观察到异相信号，存在一个静态直流场（H _dc = 2000，3000 Oe）。使用从头算方法彻底研究了苯甲酸酯系列（1-Dy，2-Tb，5-Sm，6-Nd和7-Pr）中镧系离子的各向异性。CASSCF计算预测出1-Dy和7-Pr中SMM行为的起源以及2-Tb中所应用的场SMM行为并不完全源自镧系离子的单离子各向异性。为了充分理解弛豫机理，我们使用Lines模型使用POLY_ANISO程序拟合磁化率数据，这表明在配合物1-Dy和7-Pr中观察到的零场SMM行为是由于弱的Mn ^{III / IV引起的。} -Ln ^III和Ln ^III -Ln ^III偶联以及不利的Ln ^III / Mn ^III / Mn ^IV各向异性。在配合物4-Eu，8-Gd和9-Eu中，从头算计算表明Mn ^III的各向异性离子仅引起SMM行为的可能性。配合物7-Pr是一种含Pr（III）的配合物，显示零场SMM行为，这种情况很少见，我们的研究表明，将弱SOC镧系元素金属离子与各向异性过渡金属离子偶合以产生SMM特性的可能性。但是，增强{3d–4f}复合物中的交换耦合仍然是利用新一代{3d–4f} SMM的顽固障碍。