Molecular isomerism induced Fe(ii) spin state difference based on the tautomerization of the 4(5)-methylimidazole group

Abstract

Two iron(II) molecular isomers, namely face-Λ-[Fe(L₁)₃](ClO₄)₂ (1a) (L₁ = R-1-phenyl-N-((1-hexyl-5-methyl-1H-imidazol-2-ylmethylene)ethanamine)) and face-Λ-[Fe(L₂)₃](ClO₄)₂ (1b) (L₂ = R-1-phenyl-N-(1-hexyl-4-methyl-1H-imidazol-2-ylmethylene)ethanamine), are obtained via a well-designed strategy based on the tautomerization of the 4(5)-methylimidazole group. Structural investigations reveal that the two isomers are extremely similar with only differences in the methyl group position of the ligands. The iron(II) center is surrounded by three bidentate imidazole Schiff-base ligands in the face-Λ conformation, which affords a distorted [FeN₆] octahedral coordination sphere. The average Fe–N bond length of 1a (1.971 Å) is shorter than that of 1b (2.185 Å). Spectroscopy analyses, X-ray crystal structures and magnetic investigations show that 1a is in the low-spin state at room temperature due to the strong ligand field imparted by the electron-donating methyl groups at the 5-position on the imidazole moieties and undergoes a partial spin transition with an estimated T_1/2 = 390 K. In contrast, 1b is stabilized in the high-spin state because of the strong steric hindrance when the three methyl “legs” are changed from the 5-position to the 4-position on the imidazole moieties. In addition, a new complex, 2, without methyl groups attached to the imidazole rings is introduced and characterized to further corroborate the steric influence on the spin state. Complex 2 exhibits a gradual spin-crossover behaviour with T_1/2 = 258 K. Moreover, the diverse spin states of these complexes are computationally studied using the DFT method. The results of the calculations are consistent with the experiments, which prove that the competition of the electronic effect and steric crowding influence the spin states.