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

[Image: see text] 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 (T(1/2)) of a series of [Fe(II)(X-pybox)(2)](ClO(4))(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, N(3), Cl, Br, 3-pyridyl, and 4-pyridyl. The solution susceptometry on five new derivatives with X = Me, 2-thienyl, N(3), 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 T(1/2) versus ρ(N(py)) plot with R(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 T(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 T(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 T(1/2). Furthermore, this method can be more generalized by application to another known SCO family having 3-azinyl-4-p-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 T(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.