Spin-Crossover Temperature Predictable from DFT Calculation for Iron(II) Complexes with 4-Substituted Pybox and Related Heteroaromatic Ligands

Abstract

Spin-crossover (SCO) is a reversible transition between low and high spin states by external stimuli such as heat. The SCO behavior and transition temperature (<i>T</i> _1/2) of a series of [Fe^II(X-pybox)₂](ClO₄)₂ were studied to establish a methodology for ligand-field engineering, where X-pybox stands for 2,6-bis(oxazolin-2-yl)pyridine substituted with X at the 4-position of the pyridine ring. We utilized X = MeO, Me, 3-thienyl, Ph, H, MeS, 2-thienyl, N₃, Cl, Br, 3-pyridyl, and 4-pyridyl. The solution susceptometry on five new derivatives with X = Me, 2-thienyl, N₃, Br, and 3-pyridyl was performed in acetone, giving the SCO temperatures of 220, 260, 215, 280, and 270 K, respectively. The density-functional-theory molecular orbital (MO) calculation was performed on the ligands with geometry optimization. The atomic charge on the pyridine nitrogen atom [ρ(N_py)] was extracted from the natural orbital population analysis. Positive correlation appeared in the <i>T</i> _1/2 versus ρ(N_py) plot with <i>R</i> ² = 0.734, being consistent with the analysis using the Hammett substituent constants (σ_p and σ_p ⁺). This finding well agrees with the mechanism proposed: the rich electron density lifts the t_2g energy level through the dπ-pπ interaction, resulting in a narrow t_2g-e_g energy gap and favoring the high-spin state and low <i>T</i> _1/2. The MO method was successfully applied to the known SCO-active iron(II) compounds involving 4-substituted 2,6-bis(pyrazol-1-yl)pyridines. A distinct positive correlation appeared in the <i>T</i> _1/2 versus ρ(N_py) plot. The comparison of correlation coefficients indicates that ρ(N_py) is a more reliable parameter than σ_p or σ_p ⁺ to predict a shift of <i>T</i> _1/2. Furthermore, this method can be more generalized by application to another known SCO family having 3-azinyl-4-<i>p</i>-tolyl-5-phenyl-1,2,4-triazole ligand series, where azinyl stands for a 2-azaaromatic ring. A good linear correlation was found in the <i>T</i> _1/2 versus ρ(N_A) plot (N_A is the ligating nitrogen atom in the azaaromatic ring). Finally, we will state a reason why the present treatment is competent to predict the SCO equilibrium position only by consideration on the electronic perturbation.