Nicolini Alessio, Affronte Marco, SantaLucia Daniel J, Borsari Marco, Cahier Benjamin, Caleffi Matteo, Ranieri Antonio, Berry John F, Cornia Andrea
Department of Chemical and Geological Sciences, University of Modena and Reggio Emilia & INSTM, I-41125 Modena, Italy.
Department of Physics, Informatics and Mathematics, University of Modena and Reggio Emilia, I-41125 Modena, Italy.
Dalton Trans. 2021 Jun 8;50(22):7571-7589. doi: 10.1039/d1dt01007g.
Iron-based extended metal atom chains (EMACs) are potentially high-spin molecules with axial magnetic anisotropy and thus candidate single-molecule magnets (SMMs). We herein compare the tetrairon(ii), halide-capped complexes [Fe4(tpda)3Cl2] (1Cl) and [Fe4(tpda)3Br2] (1Br), obtained by reacting iron(ii) dihalides with [Fe2(Mes)4] and N2,N6-di(pyridin-2-yl)pyridine-2,6-diamine (H2tpda) in toluene, under strictly anhydrous and anaerobic conditions (HMes = mesitylene). Detailed structural, electrochemical and Mössbauer data are presented along with direct-current (DC) and alternating-current (AC) magnetic characterizations. DC measurements revealed similar static magnetic properties for the two derivatives, with χMT at room temperature above that for independent spin carriers, but much lower at low temperature. The electronic structure of the iron(ii) ions in each derivative was explored by ab initio (CASSCF-NEVPT2-SO) calculations, which showed that the main magnetic axis of all metals is directed close to the axis of the chain. The outer metals, Fe1 and Fe4, have an easy-axis magnetic anisotropy (D = -11 to -19 cm-1, |E/D| = 0.05-0.18), while the internal metals, Fe2 and Fe3, possess weaker hard-axis anisotropy (D = 8-10 cm-1, |E/D| = 0.06-0.21). These single-ion parameters were held constant in the fitting of DC magnetic data, which revealed ferromagnetic Fe1-Fe2 and Fe3-Fe4 interactions and antiferromagnetic Fe2-Fe3 coupling. The competition between super-exchange interactions and the large, noncollinear anisotropies at metal sites results in a weakly magnetic non-Kramers doublet ground state. This explains the SMM behavior displayed by both derivatives in the AC susceptibility data, with slow magnetic relaxation in 1Br being observable even in zero static field.
铁基扩展金属原子链（EMACs）是具有轴向磁各向异性的潜在高自旋分子，因此是单分子磁体（SMMs）的候选者。我们在此比较通过在甲苯中，在严格无水和无氧条件下（HMes = 均三甲苯）使二卤化铁与[Fe2(Mes)4]和N2,N6-二（吡啶-2-基）吡啶-2,6-二胺（H2tpda）反应得到的四铁（II）卤化物封端的配合物[Fe4(tpda)3Cl2]（1Cl）和[Fe4(tpda)3Br2]（1Br）。给出了详细的结构、电化学和穆斯堡尔数据以及直流（DC）和交流（AC）磁性表征。直流测量揭示了这两种衍生物具有相似的静态磁性，室温下的χMT高于独立自旋载流子的χMT，但在低温下要低得多。通过从头算（CASSCF-NEVPT2-SO）计算探索了每种衍生物中铁（II）离子的电子结构，结果表明所有金属的主磁轴都指向靠近链的轴。外层金属Fe1和Fe4具有易轴磁各向异性（D = -11至-19 cm-1，|E/D| = 0.05 - 0.18），而内层金属Fe2和Fe3具有较弱的硬轴各向异性（D = 8 - 10 cm-1，|E/D| = 0.06 - 0.21）。这些单离子参数在直流磁性数据拟合中保持不变，这揭示了铁磁的Fe1 - Fe2和Fe3 - Fe4相互作用以及反铁磁的Fe2 - Fe3耦合。超交换相互作用与金属位点处大且非共线的各向异性之间的竞争导致了弱磁性的非克莱默斯双态基态。这解释了两种衍生物在交流磁化率数据中显示的单分子磁体行为，即使在零静态场中，1Br中的慢磁弛豫也是可观察到的。
