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 t Bu)CH 3 CN](NO 3 ) 2 ( 3a -CH 3 CN) 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 3 CN in CH 3 CN at −10°C) and k =2×10 −4 s −1 ( 2a -Py in CH 3 CN at −10°C). The sixth ligand (Py in 2a -Py and 2b -Py; CH 3 CN in 3a -CH 3 CN) can be replaced by other donor molecules B in appropriate solvent systems. Using d 9 - t BuOOH, 2 D NMR signals of t BuOO moieties of complexes 3a -CH 3 CN, 3a -CH 3 OH and 3a -H 2 O 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 2 N-Py) increases with the growth of basisity of B (push effect). Such effect is markedly smaller for alkylperoxo species [Fe(bpy) 2 (OO t Bu)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 t BuOO · 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.