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 (T-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, N-3, 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 [rho(N-py)] was extracted from the natural orbital population analysis. Positive correlation appeared in the T1/ 2 versus rho(N-py) plot with R-2 = 0.734, being consistent with the analysis using the Hammett substituent constants (sigma(p) and sigma(+)(p)). This finding well agrees with the mechanism proposed: the rich electron density lifts the t(2g) energy level through the d pi-p pi 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 rho(N-py) plot. The comparison of correlation coefficients indicates that.(Npy) is a more reliable parameter than sigma(p) or rho(+)(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 rho(N-py) 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.