Stability and reactivity of low-spin ferric hydroperoxo and alkylperoxo complexes with bipyridine and phenantroline ligands

Abstract

In this work the first-order rate constants of self-decomposition of hydroperoxo and alkylperoxo complexes [Fe(bpy) 2(OOH)Py](NO 3) 2 ( 2a-Py), [Fe(phen) 2(OOH)Py](NO 3) 2 ( 2b-Py) and [Fe(bpy) 2(OO tBu)CH 3CN](NO 3) 2 ( 3a-CH 3CN) were determined in the presence of various substrates and at various temperatures. It was observed, that the alkylperoxo species are far less stable than corresponding hydroperoxo intermediates, k=1.2×10 −2 s −1 ( 3a-CH 3CN in CH 3CN at −10°C) and k=2×10 −4 s −1 ( 2a-Py in CH 3CN at −10°C). The sixth ligand (Py in 2a-Py and 2b-Py; CH 3CN in 3a-CH 3CN) can be replaced by other donor molecules B in appropriate solvent systems. Using d 9- tBuOOH, 2D NMR signals of tBuOO moieties of complexes 3a-CH 3CN, 3a-CH 3OH and 3a-H 2O were observed. The rate of decomposition of hydroperoxo complexes [Fe(bpy) 2(OOH)B](NO 3) 2 ( 2a-B), where B are derivatives of Py (3-Br-Py, 3-Me-Py, 4-Me-Py and 4-Me 2N-Py) increases with the growth of basisity of B (push effect). Such effect is markedly smaller for alkylperoxo species [Fe(bpy) 2(OO tBu)B](NO 3) 2 ( 3a-B). The addition of organic substrates (cyclohexane, cyclohexene, methyl phenyl sulfide) in concentrations up to 3 M at −10°C to +20°C does not noticeably change the rate of self-decomposition of 2a-B, [Fe(phen) 2(OOH)B](NO 3) 2 ( 2b-B) and 3a-B. Thus the intermediates concerned do not directly react with organic substrates. The reactivity patterns of 2a-B, 2b-B and 3a-B were characteristic for free radical oxidation. OH · and HO 2 · radicals were trapped in solution containing 2a-Py, and tBuOO · free radicals were detected in solution in the presence of 3a-B. The determined rates of self-decomposition of complexes 2a-B, 2b-B and 3a-B can be used for evaluation of the upper limit for their reactivity towards organic substrates.