| Abstract
| - We investigated the relationship between the spin state and the reactivity with O2 of a series of the chlorocatecholatoiron(III) complexes. The study revealed that the low-spin complex has the lower reactivity with O2 than high-spin analogue, despite higher semiquinonate character in the low-spin state than in the high-spin state. This result indicates that the ferric ion plays a key role in the reaction of the complex with O2.
- 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 beencharacterized by magnetic susceptibility measurements and EPR, 1H NMR, and UV−vis−NIR spectroscopies toclarify the correlation of the spin delocalization on the catecholate ligand with the O2 reactivity as well as thespin-state dependence of the O2 reactivity. EPR spectra in frozen CH3CN at 123 K clearly showed that introductionof electron-withdrawing groups effectively shifts the spin equilibrium from a high-spin to a low-spin state. Theeffective 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 delocalizationon the 4Cl-cat ligand by paramagnetic 1H NMR shifts revealed that the semiquinonatoiron(II) character is moresignificant in the low-spin species than in the high-spin species. The logarithm of the reaction rate constant islinearly correlated with the energy gap between the catecholatoiron(III) and semiquinonatoiron(II) states for thehigh-spin complexes 1−3, although complexes 4 and 5 deviate negatively from linearity. The lower reactivity of thelow-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.
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