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
posted on 2023-01-04, 21:04authored byHannah
C. Frame, Longzhu Q. Shen, Alexander D. Ryabov, Terrence J. Collins
At ambient temperatures, neutral pH and ultralow concentrations
(low nM), the bis(sulfonamido)bis(amido) oxidation catalyst [Fe{4-NO2C6H3-1,2-(NCOCMe2NSO2)2CHMe}(OH2)]− (1) has been shown to catalyze
the addition of an oxygen atom to microcystin-LR. This persistent
bacterial toxin can contaminate surface waters and render drinking
water sources unusable when nutrient concentrations favor cyanobacterial
blooms. In mechanistic studies of this oxidation, while the pH was
controlled with phosphate buffers, it became apparent that iron ejection
from 1 becomes increasingly problematic with increasing
[phosphate] (0.3–1.0 M); 1 is not noticeably impacted
at low concentrations (0.01 M). At pH < 6.5 and [phosphate] ≥
1.0 M, 1 decays quickly, losing iron from the macrocycle.
Iron ejection is surprisingly mechanistically complex; the pseudo-first-order
rate constant kobs has an unusual dependence
on the total phosphate concentration ([Pt]), kobs = k1[Pt] + k2[Pt]2, indicating two
parallel pathways that are first and second order in [phosphate],
respectively. The pH profiles in the 5.5–8.3 range for k1 and k2 are different:
bell-shaped with a maximum of around pH 7 for k1 and sigmoidal for k2 with higher
values at lower pH. Mechanistic proposals for the k1 and k2 pathways are detailed
based on both the kinetic data and density functional theory analysis.
The major difference between k1 and k2 is the involvement of different phosphate
species, i.e., HPO42– (k1) and H2PO4– (k2); HPO42– is less
acidic but more nucleophilic, which favors intramolecular rate-limiting
Fe–N bond cleavage. Instead, H2PO4– acts intermolecularly, where the kinetics suggest
that [H4P2O8]2– drives degradation.