Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan.
Molecules. 2022 Aug 2;27(15):4930. doi: 10.3390/molecules27154930.
A new ground triplet biradical 2′,4′,6′-triisopropylbiphenyl-3,5-diyl bis(tert-butyl nitroxide) (iPr3BPBN) was prepared and characterized by means of room-temperature ESR spectroscopy displaying a zero-field splitting pattern together with a half-field signal. Complex formation with gadolinium(III) 1,1,1,5,5,5-hexafluoropentane-2,4-dionate (hfac) afforded a macrocycle [{Gd(hfac)3(μ-iPr3BPBN)}2]. As the X-ray crystallographic analysis clarified, the biradical serves as a bridging ligand, giving a 16-membered ring, where each nitroxide radical oxygen atom is directly bonded to a Gd3+ ion. The magnetic study revealed that the iPr3BPBN bridge behaved as a practically triplet biradical and that the Gd3+-radical magnetic coupling was weakly ferromagnetic. The exchange parameters were estimated as 2jrad-rad/kB > 300 K and 2JGd-rad/kB = 1.2 K in the H = −2J S1•S2 convention. The DFT calculation based on the atomic coordinates clarified the ground triplet nature in metal-free iPr3BPBN and the enhanced triplet character upon coordination. The calculation also suggests that ferromagnetic coupling would be favorable when the Gd-O-N-C(sp2) torsion comes around 100°. The present results are compatible with the proposed magneto-structure relationship on the nitroxide-Gd compounds.
一种新的三重态双自由基 2′,4′,6′-三异丙基联苯-3,5-二基双(叔丁基氮氧自由基)（iPr3BPBN）通过室温 ESR 光谱法进行了制备和表征，显示出零场分裂图案和半场信号。与钆（III）1,1,1,5,5,5-六氟戊烷-2,4-二酮（hfac）形成配合物，得到大环[ {Gd(hfac)3(μ-iPr3BPBN)}2]。正如 X 射线晶体学分析所阐明的那样，双自由基作为桥联配体，形成 16 元环，其中每个氮氧自由基氧原子直接与 Gd3+离子键合。磁性研究表明，iPr3BPBN 桥表现为实际上的三重态双自由基，并且 Gd3+-自由基磁耦合是弱铁磁性的。在 H = -2J S1•S2 约定下，交换参数估计为 2jrad-rad/kB > 300 K 和 2JGd-rad/kB = 1.2 K。基于原子坐标的 DFT 计算阐明了无金属 iPr3BPBN 中的基态三重态性质以及配位后增强的三重态性质。该计算还表明，当 Gd-O-N-C(sp2)扭转约 100°时，铁磁耦合将是有利的。目前的结果与关于氮氧自由基-Gd 化合物的磁结构关系的建议是一致的。
