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

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 (T1/2) of a series of [FeII(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(Npy)] was extracted from the natural orbital population analysis. Positive correlation appeared in the T1/2 versus ρ­(Npy) plot with R2 = 0.734, being consistent with the analysis using the Hammett substituent constants (σp and σp+). This finding well agrees with the mechanism proposed: the rich electron density lifts the t2g energy level through the dπ–pπ interaction, resulting in a narrow t2g–eg energy gap and favoring the high-spin state and low T1/2. 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 T1/2 versus ρ­(Npy) plot. The comparison of correlation coefficients indicates that ρ­(Npy) is a more reliable parameter than σp or σp+ to predict a shift of T1/2. 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 T1/2 versus ρ­(NA) plot (NA 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.