Modulating magnetic exchange, spin dynamics and intermacrocyclic interactions via an oxo-bridge in dihemes through stepwise oxidations†‡

IF 6.1 1区 化学 Q1 CHEMISTRY, INORGANIC & NUCLEAR
Sayantani Banerjee, Rupesh Kumar Tiwari, Paulami Chakraborty, Gopalan Rajaraman and Sankar Prasad Rath
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Abstract

A bis-Fe(III)-μ-oxo-porphyrin dimer, where a rigid ethene linker covalently connects the two porphyrin units, has been exploited to investigate the effect of intermacrocyclic interactions and spin coupling upon stepwise oxidations. The complex possesses a bent Fe–O–Fe unit bringing two porphyrin macrocycles into close vicinity, which results in strong intermacrocyclic interactions between them. Here, the two high-spin iron(III) centres undergo strong anti-ferromagnetic coupling via the oxo-bridge (JFe–Fe), resulting in a large upfield shift of the protons in the diamagnetic region of its 1H NMR spectrum. 1e-oxidation produces a porphyrin π-cation radical that displays a well-resolved 1H NMR spectrum with large isotropic shifts of the downfield-shifted methylene protons, indicative of much weaker anti-ferromagnetic coupling via the oxo bridge. As observed in the X-ray structure of the complex, 1e-oxidation brings two porphyrin macrocycles much closer to each other. This results in a remarkably bent Fe–O–Fe unit that causes the porphyrin cores to become much more distorted due to stronger intermacrocyclic interactions between them. In addition, the Fe–Npor and Fe–O distances are also significantly decreased and increased, respectively, in the 1e-oxidized complex as compared with the unoxidized one, causing a significant decrease in the JFe–Fe value. The di-cation di-radical complex that results from 2e-oxidation, on the other hand, exhibits a weakly paramagnetic nature with significant upfield shifts of the methylene proton signals as well as smaller isotropic shifts than its 1e-oxidized complex. This suggests that the former has a much stronger anti-ferromagnetic coupling (JFe–Fe) via the oxo group compared to the 1e-oxidized complex. Most interestingly, the 1H NMR chemical shift of the methylene protons of the 1e- and 2e-oxidized complexes behave completely differently upon varying the temperature and follow Curie and anti-Curie behavior, respectively, which has been rationalized by considering the thermal population of the ground and excited states at the recorded temperatures. Variable temperature magnetic investigation in the solid state indicated antiferromagnetic coupling between the iron centres through the oxo bridge, which follows the order (JFe–Fe): oxo-bridged dimer (−130.4 cm−1) > 2e-oxidized complex (−116.1 cm−1) ≫ 1e-oxidized complex (−37.8 cm−1). On the other hand, the spin couplings between unpaired iron spin with porphyrin π-cation radical (JFe–r) are also antiferromagnetic but with relatively smaller values with an order: 2e-oxidized complex (−27.2 cm−1) > 1e-oxidized complex (−16.5 cm−1). The DFT calculations not only reproduce the experimental J values very well but also provide an insight that suggests both the Fe–O–Fe angle and the Fe–O distances determine the overall J value while the bridging ethylene linker promotes ferromagnetic coupling.

Abstract Image

通过氧化桥调制二血红素的磁交换、自旋动力学和大环间相互作用
一个双铁(III)-µ-氧卟啉二聚体,其中一个刚性乙烯连接共价连接两个卟啉单元,已经被用来研究大环间相互作用和自旋耦合对逐步氧化的影响。该配合物具有一个弯曲的Fe、O、O、Fe单元,使两个卟啉大环紧密相连,形成强的大环间相互作用。在这里,两个高自旋铁(III)中心通过氧桥(JFe-Fe)发生了强的反铁磁耦合,导致其1H NMR谱中反磁性区域的质子发生了大的前场位移。在1H NMR谱中,下场移亚甲基质子的各向同性位移较大,表明通过氧产生的反铁磁耦合弱得多。在配合物的x射线结构中观察到,e - e -氧化使两个卟啉大环彼此更接近。这导致Fe-O -O -Fe单元明显弯曲,同时Fe- npor和Fe-O距离也分别显著减小和增加,导致JFe-Fe值显著降低。另一方面,由2e - c氧化形成的二自由基配合物表现出弱顺磁性,亚甲基质子信号的上场位移明显,各向同性位移比其1e - c氧化的配合物小。最有趣的是,随着温度的变化,1e -和2e -氧化配合物的亚甲基质子的1H NMR化学位移表现完全不同,分别遵循居里和反居里行为,考虑到记录温度下基态和激发态的热居族,这已经得到了合理的解释。固态变温磁性研究表明,铁中心之间通过氧桥存在反铁磁耦合,其顺序为(JFe-Fe):氧桥二聚体(-130.4 cm-1) >;2e -氧化配合物(-116.1 cm-1) >;1e -氧化络合物(-37.8 cm-1)。另一方面,未配对铁自旋与卟啉π-阳离子自由基(JFe-r)之间的自旋耦合顺序为:2e -氧化配合物(-27.2 cm-1) >;1e -氧化络合物(-16.5 cm-1)。DFT计算很好地再现了实验J值,但提供了一个见解,即Fe-O -O角度和Fe-O距离决定了总体J,而桥接乙烯连接剂促进了铁磁耦合。
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来源期刊
Inorganic Chemistry Frontiers
Inorganic Chemistry Frontiers CHEMISTRY, INORGANIC & NUCLEAR-
CiteScore
10.40
自引率
7.10%
发文量
587
审稿时长
1.2 months
期刊介绍: The international, high quality journal for interdisciplinary research between inorganic chemistry and related subjects
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