Exploring the effect of axial ligands, solvation, and redox on structure and inter-metal communication in Fe3-EMACs

Abstract

In the present work, we have studied the effect of the axial ligands, solvation, and oxidation and reduction on the structure and the nature of the Fe–Fe bonding of [Fe₃L₃] complex. The NCS⁻, CN⁻, and NO₃⁻ ions have been placed at the axial position of the [Fe₃L₃], giving [Fe₃L₃X₂], where X represents NCS⁻, CN⁻, and NO₃⁻ ions. Our DFT results show that the structure of the [Fe₃L₃] molecule remains symmetrical regardless of the nature of the capped ligand. The ΔFe–Fe value is 0.01 Å, which falls within the range of symmetrical values (ΔM–M within 0.05 Å) of all the three complexes: [Fe₃L₃CN₂], [Fe₃L₃(NCS)₂], and [Fe₃L₃(NO₃)₂]. Next, we examined the solvation effect using benzene and toluene. Our results demonstrate no effect of solvation on the structure of [Fe₃L₃CN₂] and [Fe₃L₃(NCS)₂], as the ΔFe–Fe values remain consistent with the gas-phase structure and also X-ray value. Finally, in terms of the redox effect, our calculations indicate that the first oxidation process removes an electron from the σ* orbital, resulting in [Fe₃L₃]¹⁺. This oxidation leads to unsymmetrical metal–metal bonds, with Fe₁–Fe₃ and Fe₁–Fe₂ measuring 2.35 Å and 2.52 Å, respectively. The second oxidation process removes another electron, this time from the δ* (HOMO-1) orbital, yielding [Fe₃L₃]²⁺. This electron removal also results in an unsymmetrical structure, with Fe₁–Fe₃ of 2.35 Å and Fe₁-Fe₂ of 2.53 Å, similar to that of the [Fe₃L₃]¹⁺. This similarity is due to the removal of an electron from the weakly overlapped anti-bonding δ* orbital. The reduction process has a less pronounced effect on the structure of [Fe₃L₃] compared to that of the two-oxidation process. The Fe–Fe bond lengths of [Fe₃L₃]¹⁻ are the same as those of the natural molecule, with Fe₁–Fe₃ of 2.39 Å and Fe₁–Fe₂ of 2.42 Å.