Cooperative Co-Activation of Water and Hypochlorite by a Non-Heme Diiron(III) Complex

Water-Soluble Iron Porphyrins as Catalysts for Suppressing Chlorinated Disinfection Byproducts in Hypochlorite-Dependent Water Remediation

We are facing a world-wide shortage of clean drinking water which will only be further exacerbated by climate change. The development of reliable and affordable methods for water remediation is thus of utmost importance. Chlorine (which forms active hypochlorites in solution) is the most commonly used disinfectant due to its reliability and low cost. One drawback is that it reacts with organic pollutants to generate toxic chlorinated byproducts. To mitigate chlorination in water remediation, we have investigated the use of catalytic amounts of charged water-soluble iron porphyrins. These are known to activate hypochlorite to generate high valent oxoiron species. We studied the depletion of the model micropollutant phenol and the accumulation of chlorinated disinfection byproducts under water remediation conditions, using iron porphyrins [(TMPyP)FeCl]Cl4 and (NH4)4[(TPPS)FeCl] as catalysts, by membrane inlet mass spectrometry. Despite bearing opposite charges on the meso-substituent, both iron porphyrins suppress the formation of chlorinated disinfection by-products equally well. To gain further insight, spectroscopic studies were performed. These showed the transient formation of Compound II, followed by either regeneration of the iron(III) porphyrin at low NaOCl concentrations, or total decomposition of the porphyrin complex at high NaOCl concentrations. Potential future directions for modifications of porphyrin-based catalysts are discussed.

Periodate-Mediated Aerobic Oxidation of Sulfides over a Bifunctional Porphyrin-polyoxometalate Catalyst: Photosensitized Singlet Oxygen Oxidation of Iodate to Periodate

Regeneration of terminal oxidants by molecular oxygen in metal-catalyzed oxidations of organic substrates has the advantage of avoiding the use of stoichiometric amounts of hazardous and/or expensive reagents to meet (some of) the green chemistry requirements. In the present study, photosensitized singlet oxygen oxidation of iodate to periodate has been used to regenerate the oxidant in polyoxometalate (POM)-catalyzed oxidation of sulfides to sulfoxides with periodate in water. To the best of our knowledge, it is the first report on singlet oxygen oxidation of iodate to periodate. In order to determine the contribution of photooxidation and oxidation pathways in the formation of sulfoxide, the oxidation of diphenyl sulfide with a very low reactivity toward aerobic photooxidation was studied; a sevenfold increase in the conversion of the sulfide to the diphenyl sulfoxide was observed for the reaction conducted in the presence of H2TMPyP-PW12O40/IO3-/O2/hν compared to that in the presence of H2TMPyP-PW12O40/O2/hν. Also, under the same conditions, a ca. 1.5-fold increase was observed in the case of methyl phenyl sulfide, which shows high reactivity toward both the oxidation and photooxidation reactions. A porphyrin-POM nanocomposite formed by the electrostatic immobilization of meso-tetra(N-methylpyridinium-4-yl)porphyrin (H2TMPyP) on PW12O40 was employed for the one-pot oxidation and photooxidation reactions. Field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), diffuse-reflectance UV-vis spectroscopy, thermal gravimetric analysis, and Fourier transform infrared were used to characterize the formation of the hybrid compound. An average particle size of 42 nm was estimated for H2TMPyP-PW12O40 from XRD peak broadening using the Scherrer equation. Also, FESEM images showed the formation of nearly spherical nanoparticles with a size of ca. 200 nm. The redshift of the Soret band of H2TMPyP upon immobilization on POM was attributed to strong N-H···O hydrogen-bond interactions between POM and porphyrin.

Probing Noncovalent Interactions in [3,3]Metaparacyclophanes

Arene-arene interactions are fundamentally important in molecular recognition. To precisely probe arene-arene interactions in cyclophanes, we designed and synthesized (2,6-phenol)paracyclophanes and (2,6-aniline)paracyclophanes that possess two aromatic rings in close proximity. Fine-tuning the aromatic character of one aromatic ring by fluorine substituents enables investigations on the intramolecular interactions between the electron-rich phenol and aniline with tetra-H- and tetra-F-substituted benzene. pK_a measurements revealed that the tetra-F-template increases the acidity of the phenol (ΔpK_a = 0.55). X-ray crystallography and computational analyses demonstrated that all [3,3]metaparacyclophanes adopt cofacial parallel conformations, implying the presence of π-π stacking interactions. Advanced quantum chemical analyses furthermore revealed that both electrostatic interactions and orbital interactions provide the key contribution to the structure and stability of [3,3]metaparacyclophanes.