Kimura Akifumi, Ishida Takayuki
Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan.
ACS Omega. 2018 Jun 21;3(6):6737-6747. doi: 10.1021/acsomega.8b01095. eCollection 2018 Jun 30.
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 ( ) of a series of Fe(X-pybox) 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, Cl, Br, 3-pyridyl, and 4-pyridyl. The solution susceptometry on five new derivatives with X = Me, 2-thienyl, N, 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)] was extracted from the natural orbital population analysis. Positive correlation appeared in the versus ρ(N) plot with = 0.734, being consistent with the analysis using the Hammett substituent constants (σ and σ ). This finding well agrees with the mechanism proposed: the rich electron density lifts the t energy level through the dπ-pπ interaction, resulting in a narrow t-e energy gap and favoring the high-spin state and low . 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 versus ρ(N) plot. The comparison of correlation coefficients indicates that ρ(N) is a more reliable parameter than σ or σ to predict a shift of . Furthermore, this method can be more generalized by application to another known SCO family having 3-azinyl-4--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 versus ρ(N) plot (N 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.
自旋交叉（SCO）是指通过热等外部刺激在低自旋态和高自旋态之间的可逆转变。研究了一系列Fe(X - pybox)的SCO行为和转变温度（ ），以建立配体场工程的方法，其中X - pybox代表在吡啶环4位被X取代的2,6 - 双（恶唑啉 - 2 - 基）吡啶。我们使用了X = MeO、Me、3 - 噻吩基、Ph、H、MeS、2 - 噻吩基、N、Cl、Br、3 - 吡啶基和4 - 吡啶基。对X = Me、2 - 噻吩基、N、Br和3 - 吡啶基的五种新衍生物在丙酮中进行了溶液磁化率测定，其SCO温度分别为220、260、215、280和270 K。对配体进行了密度泛函理论分子轨道（MO）计算并进行了几何优化。从自然轨道布居分析中提取吡啶氮原子上的原子电荷[ρ(N)]。在 与ρ(N)的图中出现正相关，相关系数为0.734，这与使用哈米特取代基常数（σ和σ ）的分析一致。这一发现与所提出的机制非常吻合：丰富的电子密度通过dπ - pπ相互作用提升t能级，导致t - e能隙变窄，有利于高自旋态和低 。MO方法成功应用于涉及4 - 取代的2,6 - 双（吡唑 - 1 - 基）吡啶的已知SCO活性铁（II）化合物。在 与ρ(N)的图中出现明显的正相关。相关系数的比较表明，ρ(N)是比σ或σ 更可靠的预测 位移的参数。此外，通过应用于另一个具有3 - 氮杂芳基 - 4 - 甲苯基 - 5 - 苯基 - 1,2,4 - 三唑配体系列的已知SCO家族，该方法可以更具通用性，其中氮杂芳基代表2 - 氮杂芳环。在 与ρ(N)的图中发现了良好的线性相关性（N是氮杂芳环中的配位氮原子）。最后，我们将阐述为什么目前的处理方法仅通过考虑电子微扰就能预测SCO平衡位置的原因。
