Correlation of spin states and spin delocalization with the dioxygen reactivity of catecholatoiron(III) complexes.
A series of catecholatoiron(III) complexes, [Fe(III)L(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.
Higuchi
Masakazu
M
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.
Hitomi
Yutaka
Y
Minami
Hisataka
H
Tanaka
Tsunehiro
T
Funabiki
Takuzo
T
D002396:Catechols / Q000138:chemical synthesis / Q000737:chemistry*; D015392:Chemistry, Inorganic / Q000379:methods; D018360:Crystallography, X-Ray; D004563:Electrochemistry / Q000379:methods; D004578:Electron Spin Resonance Spectroscopy; D007501:Iron / Q000737:chemistry*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008280:Magnetics; D010100:Oxygen / Q000737:chemistry*; D012996:Solutions; D013056:Spectrophotometry, Ultraviolet; D019265:Spectroscopy, Near-Infrared
D002396:Catechols; D008024:Ligands; D012996:Solutions; D007501:Iron; D010100:Oxygen