Thorarinsdottir Agnes E, Bjornsson Ragnar, Harris T David
Department of Chemistry, Northwestern University, Evanston 60208, Illinois, United States.
Department of Inorganic Spectroscopy, Max-Planck-Institut für Chemische Energiekonversion, Mülheim an der Ruhr 45470, Germany.
Inorg Chem. 2020 Apr 6;59(7):4634-4649. doi: 10.1021/acs.inorgchem.9b03736. Epub 2020 Mar 20.
The elucidation of magnetostructural correlations between bridging ligand substitution and strength of magnetic coupling is essential to the development of high-temperature molecule-based magnetic materials. Toward this end, we report the series of tetraoxolene-bridged Fe complexes [(MeTPyA)Fe(L)] (MeTPyA = tris(6-methyl-2-pyridylmethyl)amine; = 2: LH = 3,6-dimethoxy-2,5-dihydroxo-1,4-benzoquinone, LH = 3,6-dichloro-2,5-dihydroxo-1,4-benzoquinone, Na[L] = sodium 3,6-dinitro-2,5-dihydroxo-1,4-benzoquinone; = 4: L = 3,6-bis(dimethylsulfonium)-2,5-dihydroxo-1,4-benzoquinone diylide) and their one-electron-reduced analogues. Variable-temperature dc magnetic susceptibility data reveal the presence of weak ferromagnetic superexchange between Fe centers in the oxidized species, with exchange constants of = +1.2(2) (R = OMe, Cl) and +0.3(1) (R = NO, SMe) cm. In contrast, X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy establish a ligand-centered radical in the reduced complexes. Magnetic measurements for the radical-bridged species reveal the presence of strong antiferromagnetic metal-radical coupling, with = -57(10), -60(7), -58(6), and -65(8) cm for R = OMe, Cl, NO, and SMe, respectively. The minimal effects of substituents in the 3- and 6-positions of L on the magnetic coupling strength is understood through electronic structure calculations, which show negligible spin density on the substituents and associated C atoms of the ring. Finally, the radical-bridged complexes are single-molecule magnets, with relaxation barriers of = 50(1), 41(1), 38(1), and 33(1) cm for R = OMe, Cl, NO, and SMe, respectively. Taken together, these results provide the first examination of how bridging ligand substitution influences magnetic coupling in semiquinoid-bridged compounds, and they establish design criteria for the synthesis of semiquinoid-based molecules and materials.
阐明桥联配体取代与磁耦合强度之间的磁结构相关性对于高温分子基磁性材料的开发至关重要。为此，我们报道了一系列四氧杂环戊二烯桥联的铁配合物[(MeTPyA)Fe(L)]（MeTPyA = 三(6-甲基-2-吡啶甲基)胺； = 2：LH = 3,6-二甲氧基-2,5-二羟基-1,4-苯醌，LH = 3,6-二氯-2,5-二羟基-1,4-苯醌，Na[L] = 3,6-二硝基-2,5-二羟基-1,4-苯醌钠； = 4：L = 3,6-双(二甲基锍)-2,5-二羟基-1,4-苯醌二叶立德）及其单电子还原类似物。变温直流磁化率数据表明，氧化态物种中Fe中心之间存在弱铁磁超交换，交换常数为 = +1.2(2)（R = OMe，Cl）和 +0.3(1)（R = NO，SMe）cm。相比之下，X射线衍射、循环伏安法和穆斯堡尔光谱表明还原配合物中存在以配体为中心的自由基。对自由基桥联物种的磁性测量表明存在强反铁磁金属-自由基耦合，对于R = OMe、Cl、NO和SMe， = -57(10)、-60(7)、-58(6)和 -65(8) cm。通过电子结构计算可以理解L的3-和6-位取代基对磁耦合强度的最小影响，计算表明取代基和环上相关C原子的自旋密度可忽略不计。最后，自由基桥联配合物是单分子磁体，对于R = OMe、Cl、NO和SMe，弛豫势垒分别为 = 50(1)、41(1)、38(1)和33(1) cm。综上所述，这些结果首次研究了桥联配体取代如何影响半醌桥联化合物中的磁耦合，并且它们为基于半醌的分子和材料的合成建立了设计标准。
