Procedure for Kinetic Analysis. Pseudo-first-order conditions were accessed in the study of formation of 2 by using a limited amount of complex 1 (0.1 mM) and an excess amount of O₂. The absorption change at 325 nm was fit to Eq. 1, and k_obs was obtained as a pseudo-first-order rate constant. The concentration of O₂ varied from 2.30 to 5.76 mM by diluting the saturated solution with degassed CH₂Cl₂. To plot k_obs against concentration of O₂, a straight line passing through the origin gave a slope as the second-order rate constant for the formation of 2, k₁ = k_obs/[O₂]. Temperature was varied from 183 to 203 K, and an Eyring plot offered the activation parameters according to Eq. 2.

By raising the temperature of 2 prepared at 193 or 183 K, the conversion of 2 to 4 was recorded by using a UV-vis spectrophotometer. The absorbance change in the range of 400 to 700 nm indicated a biphasic reaction. The absorbance change at 640 nm was fit to Eq. 3 and gave two first-order rate constants ka and kb (ka < kb ). The Eyring plots were achieved by varying the final temperature. Beyond 218 K, the measurement of the rate constant of the fast step becomes unavailable because of the tiny absorbance change and the longer time required by the temperature change. On the contrary, the slow step is too slow to be evaluated at 193 K.

Procedure for the Titration of 2 by 2,4-di-tert-butylphenol (DTBP). A 0.1 M stock solution was prepared by dissolving DTBP (20.6 mg) to dry CH₂Cl₂ (1.00 ml). To a solution of 2 prepared according to the procedure described previously, was added 5 m l of DTBP stock solution to give a reaction solution with [2]₀ = 1 mM, [DTBP]₀ = 0.167 mM. After completion of the reaction, a UV-visible spectrum was recorded and another 5 m l of DTBP stock solution was added. This procedure was repeated until no absorbance change was observed. Consumption of 2 was indicated by a gradual absorbance decrease at the range from 400 and 700 nm. A Job’s law plot of the absorbance change at 500 nm indicated total consumption of 2 required upon addition of 0.65 equivalent of DTBP.

Analysis of Oxidation Product of DTBP. The reaction between 2 and DTBP was performed under the following conditions: 10 mM 2 in 2 ml of CH₂Cl₂ was mixed with 2 equivalent of DTBP at 193 K. After 30 min, the solvent was removed by vacuum at low temperature, and brown residue was obtained. The oxidized product was abstracted from the residue by using ether (2 ml ´ 3). After evaporating ether, the remaining solid was dissolved in CDCl₃. NMR and GC-MS spectra were taken by Varian VXR 300 spectrometer and a HP-5898 GC (Hewlett–Packard) equipped with a DB-5 capillary column and a Finnigan–MAT 95 mass detector (San Jose, CA), by using NH₃ as ionization gas.