TAML- and Buffer-Catalyzed Oxidation of Picric Acid by H₂O₂: Products, Kinetics, DFT, and the Mechanism of Dual Catalysis

Abstract

Studies of the oxidative degradation of picric acid (2,4,6trinitrophenol) 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-(NCOCMe 2 NCO) 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-(NCOCMe 2 NCO) 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.

On TAML Catalyst Resting State Lifetimes: Kinetic, Mechanistic, and Theoretical Insight into Phosphate-Induced Demetalation of an Iron(III) Bis(sulfonamido)bis(amido)-TAML Catalyst

“…Conversely, H 2 PO 4 – is moving away from the first coordination sphere, leaving the option for a weak interaction within the second coordination sphere. Such a scenario has been considered in detail in our previous work . In our opinion, this experiment presents even stronger evidence for a higher affinity of HPO 4 2– versus H 2 PO 4 – toward Fe III of TAML 1 .…”

“…For the prototype TAML activator, 2a , protons and general acids , at appropriate concentrations cause Fe III ejection with liberation of the corresponding macrocyclic ligand. General acids such as mono- and dihydrogen phosphate are often components of buffer solutions in which the catalytic activity of TAMLs is explored. , On rare occasions, TAML catalysis is not observed in the absence of phosphate ions; i.e., phosphate can be more than a pH controller and also serves as a promoter of catalysis. Thus, in building an extensive understanding of the catalytic cycle, it is important to understand the effects of phosphate on the latest generation of TAML activators, among which 1 is the optimal performer thus far.…”

Bleach catalysis in aqueous medium by iron(III)-isoindoline complexes and hydrogen peroxide

“…Similarly to our system, the non-innocent role of the phosphate and sodium carbonate-bicarbonate (CO 2− 3 /HCO − 3 ) buffers has been established in the H 2 O 2 oxidative decomposition of various aromatic pollutants, catalyzed by iron(III) tetrasulfophthalocyanine [37] and iron(III) TAML complexes [38]. In the latter case, it was clearly revealed that picric acid was not oxidized in their absence.…”

Transformative Catalysis Purifies Municipal Wastewater of Micropollutants

“…42 2.6. (6,12,20,30,40,60,120,240, and 360 min) using five treatment conditions (three separate experiments for each condition set and time) with two H 2 O 2 control conditions (1 h heat map in Figure 2; for other times, see Figure S1). The 2/ H 2 O 2 MPPRs were then analyzed to determine the conditions that best balance technical and likely cost performances and are compared with those of ozone (2 and 4 ppm) on the same water in Figure 2 and with higher ozone concentrations (6 and 8 ppm) (Figure S1).…”

“…TAML catalysts − (Figure ) are bioinspired, miniaturized replicas (typically <1% by mass) of the peroxidase enzymes that faithfully mimic the efficient enzymatic catalytic cycle . The latest-generation TAMLs (e.g., 2 is the current best overall technical performer) outperform their predecessors (e.g., 1 ), which have been previously shown to activate peroxide to effectively degrade MPs, including pharmaceuticals, pesticides, natural and synthetic estrogens and testosterone, antimicrobials, bacterial spores, explosives, , dyes, , industrial chemicals including bisphenol A, and many more. , In a study on wastewater samples from a London Municipal Wastewater Treatment Plant (MWWTP), the highest-performance earlier-generation TAML system, 1 (40 and 80 nM) with H 2 O 2 (20 ppm), impressively degraded 11 priority MPs of the U.K. water industry present initially in parts per trillion to low parts per billion concentrations . However, 1 is an organofluorine catalyst.…”