Correlation of Spin States and Spin Delocalization with the Dioxygen Reactivity of Catecholatoiron(III) Complexes

A series of catecholatoiron(III) complexes, [FeIIIL(4Cl-cat)]BPh4 (L = (4-MeO)2TPA (1), TPA (2), (4-Cl)2TPA (3),
(4-NO2)TPA (4), (4-NO2)2TPA (5); TPA = tris(pyridin-2-ylmethyl)amine; 4Cl-cat = 4-chlorocatecholate), have been
characterized by magnetic susceptibility measurements and EPR, 1H NMR, and UV−vis−NIR spectroscopies to
clarify the correlation of the spin delocalization on the catecholate ligand with the O2 reactivity as well as the
spin-state dependence of the O2 reactivity. EPR spectra in frozen CH3CN at 123 K clearly showed that introduction
of electron-withdrawing groups effectively shifts the spin equilibrium from a high-spin to a low-spin state. The
effective magnetic moments determined by the Evans method in a CH3CN solution showed that 5 contains 36%
of low-spin species at 243 K, while 1−4 are predominantly in a high-spin state. Evaluation of spin delocalization
on the 4Cl-cat ligand by paramagnetic 1H NMR shifts revealed that the semiquinonatoiron(II) character is more
significant in the low-spin species than in the high-spin species. The logarithm of the reaction rate constant is
linearly correlated with the energy gap between the catecholatoiron(III) and semiquinonatoiron(II) states for the
high-spin complexes 1−3, although complexes 4 and 5 deviate negatively from linearity. The lower reactivity of the
low-spin complex, despite its higher spin density on the catecholate ligand compared with the high-spin analogues,
suggests the involvement of the iron(III) center, rather than the catecholate ligand, in the reaction with O2.

一系列儿茶酚合铁(III)配合物[FeIIIL(4Cl-cat)]BPh4（其中L分别为(4-MeO)₂TPA（1）、TPA（2）、(4-Cl)₂TPA（3）、(4-NO₂)TPA（4）以及(4-NO₂)₂TPA（5）；TPA为三(吡啶-2-基甲基)胺(tris(pyridin-2-ylmethyl)amine, TPA)，4Cl-cat为4-氯儿茶酚酸根(4-chlorocatecholate, 4Cl-cat)），已通过磁化率测量、电子顺磁共振（electron paramagnetic resonance, EPR）、氢核磁共振（proton nuclear magnetic resonance, ¹H NMR）以及紫外-可见-近红外（ultraviolet-visible-near infrared, UV-Vis-NIR）光谱表征，旨在阐明儿茶酚配体上的自旋离域与氧气反应活性的关联，以及氧气反应活性的自旋态依赖性。
123 K下冷冻乙腈溶液中的EPR谱清晰显示，引入吸电子基团可有效使自旋平衡由高自旋态向低自旋态偏移。
通过埃文斯法（Evans method）在乙腈溶液中测定的有效磁矩结果表明，243 K时配合物5中含有36%的低自旋物种，而配合物1~4则主要以高自旋态存在。
借助顺磁性¹H NMR位移对4Cl-cat配体上的自旋离域程度进行评估，发现低自旋物种中的半醌合铁(II)特征相较于高自旋物种更为显著。
对于高自旋配合物1~3，反应速率常数的对数与儿茶酚合铁(III)态和半醌合铁(II)态之间的能级差呈线性相关，但配合物4和5呈现负偏离。
尽管低自旋配合物在儿茶酚配体上的自旋密度高于高自旋对应物，但其反应活性更低，这表明该氧气反应过程涉及铁(III)中心而非儿茶酚配体。