Alexander D. Ryabov

Bis-amido-bis-sulfonamido, [Fe{4-NO2C6H3-1,2-(NCOCMe2NSO2)2CR1R2}(OH2)]− [2; R1/R2 = H/H (2a), H/CH3 (2b), CH3/CH3 (2c)], are NewTAML (NT) iron(III) oxidation catalysts. Herein we identify and mechanistically characterize a ligand-centered, catalyst-lifetime-controlling site (CR1R2), termed a KillSwitch (KS), that enables balancing technical performance with potential safety constraints in NT/Oxidant process design. Under catalytic conditions, CR1R2 sites containing two or one hydrogen(s), (2a, 2b, respectively), undergo deprotonation to form electron-rich ylides that accelerate NT oxidative degradation. The 2a–c series defines a KS susceptibility gradient arising from differential C–H bond acidities controlling the ylide-forming deprotonation─2a, 2b, and 2c represent high-susceptibility-, intermediate-susceptibility-, and null-susceptibility KS states, respectively, thereby providing substantial catalyst functional lifetime control with 2a < 2b ≪ 2c. Kinetic analysis gives similar 2a–c rate constants for H2O2 activation (kI) and substrate oxidation (kII), while catalyst inactivation rate constants (ki) increase markedly and differentially with increasing pH for 2a and 2b, but remain pH-independent for 2c, which degrades more slowly through a separate pathway. Access to null-KS 2c─essential for testing the KS hypothesis─required a decade-long synthetic effort culminating in the quantitative methylation of deprotonated 2b. The KS is expected to be transferable to other NTs with oxidative reactivities distinct from 2a–c, enabling a new dimension in catalyst design that broadly decouples catalytic oxidative aggression from catalyst lifetime. The KS thus provides a practical, safe, and sustainable by design strategy for balancing technical performances against catalyst lifetimes, should shorter-lived NTs become desirable in response to discoveries of postrelease NT adverse environmental effects.

In neutral aqueous media under ambient temperature, the bis-amido-bis-sulfonamido-ligated [Fe^III{4-NO₂C₆H₃-1,2-(NCOCMe₂NSO₂)₂CHMe}(OH₂)]^- NewTAML catalyst (2), activates hydrogen peroxide to mimic and greatly surpass enzymatic activity (biotranscendence) by sweeping fluoroquinolone antibiotics rapidly toward minerals. Stepwise identification of intermediates was paired with mechanistic examination of key steps. For ofloxacin, the 2/H₂O₂ activated catalyst (AC) first attacks ofloxacin's piperazine unit in two enzyme-mimicking parallel pathways, cleaving secondary (desaturation) and primary (N-demethylation) sp³ C-H bonds via H atom abstraction to form dehydro- and desmethyl-ofloxacin, respectively; desaturation is a novel expression of TAML peroxidase-like reactivity. Kinetic data obtained with selectively deuterated ofloxacin compounds revealed for desaturation and N-demethylation, respectively, kinetic isotope effects (k_H/k_D) of 8.6 and 3.8; ΔH^‡ and ΔS^‡ are presented. Dehydro- and desmethyl-ofloxacin are further oxidized through several identified intermediates toward minerals; ion chromatography revealed F^-, CH₃COO^-, HCOO^-, NO₂^-, and NO₃^- as final products. 2/H₂O₂ similarly oxidizes ciprofloxacin and norfloxacin. Desaturation by 2/H₂O₂ of 1-methyl-4-phenylpiperazine delivered a rate that is comprehensibly half that of ofloxacin. DFT calculations provided compelling evidence that the 2/H₂O₂ effective AC is oxidized by two-electrons above the ferric state producing ofloxacin desaturation via sequential hydrogen atom abstraction, single electron transfer and hydrolysis events. Iron(IV) compounds, once considered plausible as ACs in TAML/peroxide C-H bond activations, cannot oxidize ofloxacin sp³ C-H bonds. The temporal toxicity profile of ofloxacin's degradation intermediates based on in silico toxicities highlights that 2/H₂O₂ can serve as a powerful safe-and-sustainable-by-design (SSbD) fluoroquinolone detoxifying water treatment.

The neutral macrocyclic iron(III) complex Fe(H₂O)(L_PYA) (3) is a homogeneous catalyst for the oxidation of organic micropollutants with hydrogen peroxide. The macrocyclic ligand incorporates two hybrid pyridinium amidate/amidate (hybrid PYA/A) N-donors and two sulfonamidate N-donors with an -N(CH₃)- group bridging the two sulfonamidate functions. Anionic analogues of 3 in which the pyridinium group is replaced by o-phenylene (K[Fe(H₂O)(L_o-P)] (1)) or by nitro-substituted o-phenylene (K[Fe(H₂O)(L_o-P*)] (2)) were also synthesized. Complexes 1-3 were investigated as potential catalysts for the oxidative destruction by hydrogen peroxide of persistent micropollutants that remain in wastewater after conventional treatment regimens. Complexes 1-3 catalyzed the oxidation of the surrogate micropollutants Orange II and Safranine O, and kinetic parameters for these reactions were obtained. 3 also catalyzed the oxidation of carbamazepine (CBZ), which is an actual persistent micropollutant under conditions that are relevant for practical applications ([3] 50 nM, [CBZ] 50 μM, [H₂O₂] 250 μM, i.e., 8.5 ppm, 25 °C, pH 7.0). Complex 3 catalyzed the oxidation of each of the above substrates at rates three to four times greater than those of the benchmark homogeneous catalyst for reactions of this type.

The electron transfer from naphthalene at an oxidized iron TAML species (TAML = tetraamido macrocyclic ligand) is a key step of its environmentally relevant deep degradation by hydrogen peroxide in water leading first to naphthoquinones which are further converted to smaller fragments. Other polycyclic arenes are also oxidized, often faster than naphthalene consistent with their lower ionization potentials than that of naphthalene.

Four new arene–ruthenium(II) complexes [(η⁶-p-cymene)RuCl(dpm)], where dpm are hexa-(L3–L5) and meso-substituted (L6) dipyrromethene ligands, were synthesized. These ligands and the corresponding complexes were thoroughly characterized by elemental analysis and spectroscopic techniques (MS, IR, ¹H, ¹³C NMR, and UV–vis), and the structures of one ligand and three ruthenium complexes were determined by X-ray single-crystal analysis. The DNA-binding ability of the Ru-3–Ru-6 complexes was evaluated by UV–vis DNA titration. Compound Ru-3 exhibited the highest binding energy, outperforming the complexes containing a dipyrrin ligand substituted by chlorides (Ru-4 and Ru-5) or a meso-substituted dipyrrin (Ru-6). Molecular docking revealed that the hypothetical Ru-1 and Ru-2 complexes, which contain iodide ligands in the dipyrrin structures, showed higher DNA-binding affinities than Ru-3. Computational calculations supported the experimental results, confirming that Ru-3 has a higher affinity for DNA than the other complexes.