TAML- and Buffer-Catalyzed Oxidation of Picric Acid by H 2 O 2 : Products, Kinetics, DFT, and the Mechanism of Dual Catalysis.

Studies of the oxidative degradation of picric acid (2,4,6-trinitrophenol) by H ₂ O ₂ catalyzed by a fluorine-tailed tetraamido macrocyclic ligand (TAML) activator of peroxides [Fe ^III {4,5-Cl ₂ C ₆ H ₂ -1,2-( N COCMe ₂ N CO) ₂ CF ₂ }(OH ₂ )] ^- ( 2 ) in neutral and mildly basic solutions revealed that oxidative degradation of this explosive demands components of phosphate or carbonate buffers and is not oxidized in their absence. The TAML- and buffer-catalyzed oxidation is subject to severe substrate inhibition, which results in at least 1000-fold retardation of the interaction between the iron(III) resting state of 2 and H ₂ O ₂ . The inhibition accounts for a unique pH profile for the TAML catalysis with the highest activity at pH 7. Less aggressive TAMLs such as [Fe ^III {C ₆ H ₄ -1,2-( N COCMe ₂ N CO) ₂ CMe ₂ }(OH ₂ )] ^- are catalytically inactive. The roles of buffer components in modulating catalysis have been clarified through detailed kinetic investigations of the degradation process, which is first order in the concentration of 2 and shows ascending hyperbolic dependencies in the concentrations of all three participants, i.e., H ₂ O ₂ , picrate, and phosphate/carbonate. The reactivity trends are consistent with a mechanism involving the formation of double ([LFe ^III -Q] ^2- ) and triple ([LFe ^III -{Q-H ₂ PO ₄ }] ^3- ) associates, which are unreactive and reactive toward H ₂ O ₂ , respectively. The binding of phosphate converts [LFe ^III -Q] ^2- to the reactive triple associate. Density functional theory suggests that the stability of the double associate is achieved via both Fe-O _phenol binding and π-π stacking. The triple associate is an outer-sphere complex where phosphate binding occurs noncovalently through hydrogen bonds. A linear free energy relationship analysis of the reactivity of the mono-, di-, and trinitro phenols suggests that the rate-limiting step involves an electron transfer from phenolate to an oxidized ironoxo intermediate, giving phenoxy radicals that undergo further rapid oxidation that lead to eventual mineralization.