Modulating magnetic exchange, spin dynamics and intermacrocyclic interactions via an 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 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 (J_Fe–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–N_por 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 J_Fe–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 (J_Fe–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 (J_Fe–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 (J_Fe–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.