Modulating Magnetic Exchange, Spin Dynamics and Intermacrocyclic Interactions via Oxo-bridge in Dihemes Through Stepwise Oxidations

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 interaction and spin coupling upon stepwise oxidations. The complex possesses a bent FeOFe unit bringing two porphyrin macrocycles in close vicinity for strong intermacrocyclic interactions between them. Here, the two high-spin iron(III) centres undergo strong anti-ferromagnetic coupling via oxo-bridge (JFe-Fe), resulting in a large upfield shift of the protons in the diamagnetic region in its 1H NMR spectrum. 1eˉ-oxidation produces porphyrin π-cation radical which displays a well-resolved 1H NMR spectra with large isotropic shift of the downfield-shifted methylene protons, indicative of much weaker anti-ferromagnetic coupling via oxo. As observed in the X-ray structure of the complex, 1eˉ-oxidation brings two porphyrin macrocycles much closer to each other. This results in a remarkably bent FeOFe unit while the Fe-Npor and Fe-O distances are also significantly decreased and increased, respectively, in the 1eˉ-oxidized complex causing a significant decrease in the JFe-Fe value. The dication diradical complex that results from 2eˉ-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 1eˉ-oxidized complex. Most interestingly, 1H NMR chemical shift of the methylene protons of the 1eˉ- and 2eˉ-oxidized complexes behave completely differently upon varying the temperature and follow Curie and anti-Curie behavior, respectively, which have been rationalized 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 ) > 2eˉ-oxidized complex (-116.1 cm-1 ) >> 1eˉ-oxidized complex (-37.8 cm-1 ). On the other hand, the spin couplings between unpaired iron spin with porphyrin π-cation radical (JFe-r) follows an order: 2eˉ-oxidized complex (-27.2 cm-1 ) > 1eˉ-oxidized complex (-16.5 cm-1 ). The DFT calculations reproduce the experimental J values very well but provide an insight that both FeOFe angle and Fe-O distances determine the overall J while the bridging ethylene linker promoting ferromagnetic coupling.