Origin of the oxygen atom in metalloporphyrin-catalyzed epoxidations with LiOCl as oxidant

Abstract – The origin of the oxygen atom incorporated into epoxides during the oxidation of olefins with the catalytic system Mn(TPP)Cl/LiOCl in the presence of ¹⁸O-labelled water has been investigated. One molar equivalent of H₂¹⁸O with respect to Li¹⁶OCl led to a 53 % ¹⁸O-incorporation within the epoxide formed, and near complete incorporation of ¹⁸O from water was achieved when no limiting amount of water was used. These results clearly indicate that the epoxidic oxygen atom arose from the high-valent metal-oxo species generated by LiOCl, but the exchange of the oxygen atom of hypochlorite with bulk water is fast, compared to the oxidation of the manganese(III) porphyrin by LiOCl. Therefore hypochlorite is not a suitable oxidant for mechanistic studies of metal-catalyzed oxygenation reactions involving high-valent metal-oxo species. Potassium monopersulfate is the adequate oxidant for such studies, because of its slow oxygen atom exchange with bulk water.     

Singlet Oxygen Generation by Cyclometalated Complexes and Applications

While cyclometalated complexes have been extensively studied for optoelectronic applications, these compounds also represent a relatively new class of photosensitizers for the production of singlet oxygen. Thus far, singlet oxygen generation from cyclometalated Ir and Pt complexes has been studied in detail. In this review, photophysical data for singlet oxygen generation from these complexes are presented, and the mechanism of ¹O₂ generation is discussed, including evidence for singlet oxygen generation via an electron‐transfer mechanism for some of cyclometalated Ir complexes. The period from the first report of singlet oxygen generation by a cyclometalated Ir complex in 2002 through August 2013 is covered in this review. This new class of singlet oxygen photosensitizers may prove to be rather versatile due to the ease of substitution of ancillary ligands without loss of activity. Several cyclometalated complexes have been tethered to zeolites, polystyrene, or quantum dots. Applications for photooxygenation of organic molecules, including “traditional” singlet oxygen reactions (ene reaction, [4 + 2] and [2 + 2] cycloadditions) as well as oxidative coupling of amines are presented. Potential biomedical applications are also reviewed.     

Metalloporphyrins‐Al3+ porous coordination polymers: Preparations,Characterizations and Catalytic Properties

In this work, metalloporphyrins‐based MOGs were synthesized as precursor to prepare porous coordination polymers (PCPs). Firstly, the carboxyl metalloporphyrins MCPp (M = Zn (II), Co (II), Cu (II), Mn (III)OAc; CPp = 5,10,15,20‐tetra(4‐(3‐carboxy)phenoxy)phenylporphyrin) were synthesized as gelator. Then, the metalloporphyrin‐based MOGs were prepared with MCPp and Al(NO₃)₃·9H₂O by sol–gel method. Moreover, the residual reactants and solvent molecules in MOGs were removed by Soxhlet extraction and supercritical CO₂ extraction to get the finial porous coordination polymers. This methodology effectually avoids the collapse of frame construction. Compared with the reference material prepared by the oven‐dry method, these PCPs exhibit much higher BET surface ranging from 398 to 439 m² g^?1. The PCPs were carefully characterized by FT‐IR, UV–vis, SEM, TEM and PXRD, and exhibit excellent catalytic activities and stabilities for the oxidation of ethylbenzene.     

Singlet oxygen oxidation of 2′-deoxyguanosine. Formation and mechanistic insights

Emphasis was placed in this work on the delineation of mechanistic aspects of the singlet oxygen-mediated oxidation reactions of 2′-deoxyguanosine 1 used as a DNA model compound in aerated aqueous solution. For this purpose a thermolabile naphthalene endoperoxide derivative was used allowing the generation of [¹⁸O]-labeled singlet oxygen for dedicated mechanistic studies. The analysis and characterization of the oxidized nucleosides of the ¹O₂ reactions were achieved on the basis of accurate HPLC-tandem mass spectrometry measurements. Thus it was found that primary oxidation products include, in addition to the previously identified 8-oxo-7,8-dihydro-2′-deoxyguanosine 5 and the two diastereomers of spiroiminodihydantoin 8, two relatively minor nucleosides, namely the two diastereomers of 4-hydroxy-8-oxo-4,8-dihydro-2′-deoxyguanosine 9.     

Effect of Topology on Photodynamic Sterilization of Porphyrinic Metal-Organic Frameworks

Porphyrinic metal-organic frameworks (MOFs) are promising photosensitizers due to the lack of self-aggregation of porphyrin in aqueous solution. However, how the topology of porphyrinic MOFs affects the generation of singlet oxygen (¹O₂) is unclear. Here, the effect of the topology of porphyrinic MOFs on their photodynamic performance is reported. Four porphyrinic zirconium MOFs (MOF-525, MOF-545, PCN-223 and PCN-224 with different topologies: ftw , csq , shp and she , respectively) were selected to study the influence of topology on the photodynamic antibacterial performance. The ¹O₂ generation and the photodynamic antibacterial performance followed an decreasing order of MOF-545>MOF-525>PCN-224>PCN-223. The results reveal that the pore size, the distance between porphyrin, and the number of porphyrin per Zr₆O₈ cluster in MOFs greatly affected ¹O₂ generation. This work provides guidance for designing new MOFs for efficient photodynamic sterilization.     

Spectroscopy,thermal and structural studies of new ZnII coordination polymer, [Zn3(μ-bpa)4.5(AcO)3](ClO4)3·4.26H2O

Three Zn^II coordination polymers with acetate and perchlorate anions, [Zn₃(μ-bpa)_4.5(AcO)₃](ClO₄)₃·4.26H₂O (1), [Zn₂(μ-bpe)₃(AcO)₂](ClO₄)₂ (2) and [Zn₂(bpe)(AcO)₄] (3), bpa = 1,2-bis(4-pyridyl)ethane and bpe = 1,2-bis(4-pyridyl)ethene, have been synthesized and characterized by elemental analysis, IR, ¹H NMR, and ¹³C NMR spectroscopies, and the structure of compound 1 was determined by single-crystal X-ray diffraction. The thermal stabilities of compounds 1–3 were studied by thermal gravimetric (TG) and differential thermal analyses (DTA). The structural studies of compound 1 show that the structure may be considered as a three-dimensional coordination polymer of zinc(II) with large voids filled with disordered water molecules. The stability of the porous networks after removal of the guest water molecules is confirmed by X-ray powder diffraction.     

Z-Scheme In2S3/NU-1000 Heterojunction for Boosting Photo-Oxidation of Sulfide into Sulfoxide under Ambient Conditions

Photocatalytic oxidation of sulfide into sulfoxide has attracted extensive attention as an environmentally friendly strategy for chemical transformations or toxic chemicals degradation. Herein, we construct a series of In₂S₃/NU-1000 heterojunction photocatalysts, which can efficiently catalyze the oxidation of sulfides to form sulfoxides as the sole product under LED lamp (full-spectrum) illumination in air at room temperature. Especially, the sulfur mustard simulant, 2-chloroethyl ethyl sulfide (CEES), can also be photocatalytically oxidized with In₂S₃/NU-1000 to afford nontoxic 2-chloroethyl ethyl sulfoxide (CEESO) selectively and effectively. In contrast, individual NU-1000 and In₂S₃ show very low catalytic activity on this reaction. The significantly improved photocatalytic activity is ascribed to the constructing of an efficient Z-scheme photocatalysts In₂S₃/NU-1000, which exhibits the enhancement of light harvesting, the promotion of photogenerated electron-hole separation, and the retention of high porosity of the parent MOF. Moreover, mechanism studies in photocatalytic oxidation reveal that the superoxide radical (^.O₂⁻) and singlet oxygen (¹O₂) are the main oxidative species in the oxidation system. This work exploits the opportunities for the construction of porous Z-scheme photocatalysts based on the photoactive MOFs materials and inorganic semiconductors for promoting catalytic organic transformations. More importantly, it provides a route to the rational design of efficient photocatalysts for the detoxification of mustard gas.     

Role of molecular oxygen and its active forms in generation of electrochemiluminescence

A short review is dedicated to the role of molecular oxygen and its active forms in generation of electrochemiluminescence (ECL). It is shown at the example of such widely used luminogens as luminol, lucigenin, acridine esters, acridane, and indoles that the role of O₂ and its active forms, such as hydrogen peroxide, superoxide and hydroperoxide radicals in generation of ECL by organic compounds is largely reduced to oxidation of the initial luminogen or intermediates of its partial electrochemical preoxidation/prereduction. Such intermediates are most often particles in the doublet state (radical-cation, radical-anion, or free radicals) that form unstable emitter precursors of peroxide (dioxetanone) type under interaction with O₂, hydrogen peroxide, or other active oxygen forms. It is also shown that the product of secondary transformations of active oxygen forms is singlet oxygen that may also be a radiation source as a result of cooperative transitions.     

DNA binding and photocleavage specificities of a group of tricationic metalloporphyrins

The interactions of 5,10,15-tris(1-methylpyridinium-4-yl)-20-(4-hydroxyphenyl)porphyrinatozinc(II) Zn[TMPyHP]³⁺ (2) along with Cu[TMPyHP]³⁺ (3), Co[TMPyHP]⁴⁺ (4), Mn[TMPyHP]⁴⁺ (5) and the free base porphyrin H₂[TMPyHP]³⁺ (1) with duplex DNA have been studied by using a combination of absorption, fluorescence titration, surface-enhanced Raman spectroscopy (SERS), induced circular dichroism (ICD) spectroscopy, thermal DNA denaturation, viscosity measurements as well as gel electrophoresis experiment. Their binding modes and intrinsic binding constants (K_b) to calf DNA (CT DNA) were comparatively studied and were found significantly influenced by different metals coordinated with the porphyrin plane. Except 3, which has four-coordination structure at the metal, all the metal derivatives showed non-intercalative DNA-binding mode and lower K_b than the free base porphyrin 1, most probably due to the steric hindrance results from the axial ligands of the inserted metals which are five or six-coordination structures. Meanwhile, the insertion of metals into cationic porphyrin greatly removed the self-aggregation of the metal-free porphyrins, and thus fully enhanced the singlet oxygen (¹O₂) productivities in the DNA photocleavage experiments. Therefore, these metalloporphyrins have comparable DNA cleavage ability with the free base porphyrin.     

Porphyrin/Platinum(II) C^N^N Acetylide Complexes: Synthesis,Photophysical Properties,and Singlet Oxygen Generation

A new class of substituted porphyrins has been developed in which a different number of cyclometalated Pt^II C^N^N acetylides and polyethylene glycol (PEG) chains are attached to the meso positions of the porphyrin core, which are meant for photophysical, electrochemical, and in vitro light‐induced singlet oxygen (¹O₂) generation studies. All of these Zn^II porphyrin–Pt^II C^N^N acetylide conjugates show moderate to high (Φ_Δ=0.55 to 0.63) singlet oxygen generation efficiency. The complexes are soluble in organic solvents but, despite the PEG substituents, slowly aggregate in aqueous solvent systems. These conjugates also exhibit interesting photophysical properties, including near‐complete photoinduced energy transfer (PEnT) through the rigid acetylenic bond(s) from the Pt^II C^N^N antenna units to the Zn^II porphyrin core, which shows sensitized luminescence, as shown by quenching of Pt^II C^N^N‐based luminescence. Electrochemical measurements show a set of redox processes that are approximately the sum of what is observed for the Pt^II C^N^N acetylide and Zn^II porphyrin units. UV/Vis spectroscopic properties are supported by DFT calculations.     

Triptycene-based porous photoluminescent polymers with dual role: efficient capture of carbon dioxide and sensitive detection of picric acid

Design and synthesis of unique photoluminescent triptycene-based porous polymers (TBP-OH and TBP-NH₂) bearing active functional groups is described herein. Pd catalyzed Sonogashira cross-coupling reaction was utilized to obtain these polymeric networks that are nanoporous and strongly fluorescent in THF. In solid state, these polymers demonstrated CO₂ uptake up to 92 mg g^?1 at 273 K/1bar and H₂ up to 16 mg g^?1 at 77 K/1bar which may be attributed to the presence of 3D robust triptycene and CO₂-philic groups –OH and –NH₂ in their polymeric framework. TBP-OH and TBP-NH₂ also selectively capture CO₂ over nitrogen and methane. CO₂ capture by TBP-OH and TBP-NH₂ is a physisorption process and hence reversible in nature. Suspensions of TBP-OH and TBP-NH₂ in THF are strongly fluorescent and are also capable of detecting picric acid (an environmental pollutant and explosive) in trace amounts. The Stern–Volmer quenching constants (K_sv) for detection of picric acid (PA) are in the order of 10⁵ M^?1.     

Visible Light and Microwave-Mediated Rapid Trapping and Release of Singlet Oxygen Using a Coordination Polymer

Due to its high reactivity and oxidative strength, singlet oxygen (¹O₂) is used in a variety of fields including organic synthesis, biomedicine, photodynamic therapy and materials science. Despite its importance, the controlled trapping and release of ¹O₂ is extremely challenging. Herein, we describe a one-dimensional coordination polymer, CP1 , which upon irradiation with visible light, transforms ³O₂ (triplet oxygen) to ¹O₂. CP1 consists of Cd^II centers bridged by 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene ligands which undergo a [4+2] cycloaddition reaction with ¹O₂, resulting in the generation of CP1−¹O₂ . Using microwave irradiation, CP1−¹O₂ displays efficient release of ¹O₂, over a period of 30 s. In addition, CP1 exhibits enhanced fluorescence and has an oxygen detection limit of 97.4 ppm. Theoretical calculations reveal that the fluorescence behaviour is dominated by unique through-space conjugation. In addition to describing a highly efficient approach for the trapping and controlled release of ¹O₂, using coordination polymers, this work also provides encouragement for the development of efficient fluorescent oxygen sensors.     

Chelating, bridging, and chain structures within Ag(I) and Cu(I) complexes of bis(nicotinyloxy), bis(isonicotinyloxy), and bis(pyrimidine-5-carboxyloxy)-1,8-disubstituted anthraquinone ligands

Reaction of Ag(I) and Cu(I) [M(NCCH₃)₄]X (X = BF₄⁻ and PF₆⁻) salts with 1,8-bis(nicotinyloxy)anthracene-9,10-dione (1), 1,8-bis(isonicotinyloxy)anthracene-9,10-dione (2), and 1,8-bis(pyrimidine-5-carboxyloxy)anthracene-9,10-dione (3), yield new chelating and bridging complexes and two new coordination polymers. The bridging capabilities of ligands 1 and 2 have not been demonstrated before, and ligand 1, by itself, has the flexibility to produce either chelated or bridged structures and an unusual ladder coordination polymer. The tetradentate ligand 3 also produces a one-dimensional coordination polymer in the presence of one equivalent of Ag(I). All complexes have been characterized by X-ray crystallography.     

A novel strapped porphyrin receptor for molecular recognition

A novel strapped porphyrin receptor Zn1, in which two electron-rich bis(p-phenylene)-34-crown ether-10 units are incorporated, has been designed and synthesized from the newly developed intermediate 7 for investigating new chemistry of molecular recognition. ¹H NMR and UV-Vis studies revealed that Zn1 displays relatively weak binding abilities to neutral electron deficient naphthalene-1,8,4,5-tetracarboxydiimide (NDI) derivatives 13 (no simple complexing stoichiometry was observed), 19 (K_a=48(±5) M⁻¹) and 30 (K_a=46(±5) M⁻¹) in chloroform-d, strong binding ability to pyridine derivative 25, (K_a=1.5(±0.12)×10³ M⁻¹) in chloroform, moderately strong binding ability to tetracationic compound 35·4PF₆ (K_a=475(±50) M⁻¹) in acetone-d₆, and very strong binding affinity to compound 22 (K_a=6.5(±0.7)×10⁵ M⁻¹), which consists of one pyridine and two NDI units, in chloroform. Remarkable cooperative effect of the intermolecular metal-ligand coordination and donor-acceptor interactions in complex Zn1·22 was observed by comparing the complexing behaviors between Zn1 and the appropriately designed guests. Complex Zn1·22 possesses an unique three-dimensional tri-site binding feature. For comparison, the complexing affinity of 1 toward compounds 13, 19, and 30 in chloroform-d and 35·4PF₆ in acetone-d₆ has also been investigated and the binding patterns in different complexes were explored. The results demonstrate that strapped porphyrin derivatives are ideal precursors for constructing new generation of three-dimensional multi-site artificial receptors for molecular recognition and host-guest chemistry.     

A coordination polymer of manganese (III) porphyrinate with right-handed helicity and opticity

A first example of 1D coordination polymer [MnTHPP·H₂O·(DMF)₂ (1)] with right-handed helicity and opticity was successfully assembled. Crystal analyses indicate that the right-handed helical assembly is based on coordination bonds from peripheral to center, associating with the saddle distortion of the molecular planes. In its crystal architecture, MnTHPP [manganese (III) ion coordinated by 5,10,15,20-tetra(4-hydroxyphenyl) porphyrin] molecules were ligated each other via the axial coordination between –PhO^? → Mn^III (peripheral to center model, from the peripherally deprotonated hydroxyphenyl to the central metal of the neighbor molecule) to construct the right-handed helices, and the helices are sustained by plentiful hydrogen bonding. Meanwhile DMF molecules are entrapped within the voids among the helices. This is a rare example of the multiporphyrin arrays with right-handed helices bearing opticity.     

Phyto – Monoterpene linalool as precursor to synthesis epoxides and hydroperoxides as anti carcinogenic agents via thermal and photo chemical oxidation reactions

Linalool (I) was extracted from Zingiber officinale. The oxidation of linalool using m-chloroperbenzoic acid at room temperature gave the mixture of 2,2,6-trimethyl-6-vinyl-tetrahydro-pyran-3-ol (IIa) & 2-(5-methyl-5-vinyl- tetrahydro-furan-2-yl)-propan-2-ol (IIa′), which cannot be separated. Whereas, the photo-epoxidation of it using hydrogen peroxide gave the above mixture IIa/IIa′, beside 2,2,8-trimethyl-6-oxianyl-tetrahydro-pyran-3-ol (IIb). It was evaluated for anti-oxidant, using erythrocyte hemolysis and ABTS methods. It showed inhibitory effect in case of erythrocyte hemolysis and low inhibitory effect in case of ABTS method. Photosensitization reactions of linalool with tetraphenyl porphyrin or chlorophyll produced a mixture of two isomeric hydroperoxides, 7-hydroperoxy-3,7-dimethylocta-1,5-dien-3-ol (IIIa) & 6-hydroperoxy-3,7-dimethyl octa-1,7-dien-3-ol (IIIb), which can be successfully separated. On the other hand, the cytotoxic activity of linalool (I) was tested against epdermoid carcinoma (HEP2), it has medium effect. Whereas, in case of human prostate cancer (PC-3), it has weak effect. From this study concludes that ginger has good antioxidant potential and this spice can be used to produce novel natural antioxidants and flavoring agent like linalool which recommends it as an active therapy for humans. Monoterpene linalool abstract singlet oxygen (¹O₂) by photosensitization reactions to produce these hydroperoxides, which are caused relatively little oxidative DNA damage.     

A 3D porous indium(III) coordination polymer involving in-situ ligand synthesis

The hydrothermal reaction of In³⁺ and 1,2,4-benzenetricarboxylic acid with the presence of piperazine leads to the generation of a novel 3D porous coordination polymer, [H₃O][In₂(btc)(bdc)(OH)₂]·5.5H₂O (1), (btc=1,2,4-benzenetricarboxylate, bdc=1,4-benzenedicarboxylate). Compound 1 crystallizes in orthorhombic space group Pbca with a=16.216(7) Å, b=13.437(6) Å, c=31.277(14) Å, and Z=8. It is interesting to find that the in-situ decarboxylation reaction of 1,2,4-benzenetricarboxylate (btc) partially transformed into 1,4-benzenedicarboxylate (bdc) occurs. The 16 indium(III) centers were linked by four btc, four bdc and two μ₂-OH ligands to form a box-girder. The adjacent box-girders are further connected by the bdc and btc ligands to generate a novel porous metal–organic framework containing nanotubular open channel with a cross-section of approximately 11.5×11.3 Å². The micropores are occupied by lattice water molecules, and the solvent-accessible volume of the unit cell was estimated to be 3658.6 Å³, which is approximately 53.7% of the unit-cell volume (6815.4 Å³).     

Oxygen evolution: the mechanism of formation of porous anodic alumina

Abstract  Aluminium anodization behavior in ammonium sebacate solution (w = 4%) in ethylene glycol, and in several H₃PO₄-containing electrolytes, has been investigated. A new mechanism is proposed for the formation of porous anodic films. The model emphasizes the close relationship between pore generation and oxygen evolution. PO₄ ³⁻ ions incorporated in the anodic films behave as the primary source of avalanche electrons. It is the avalanche electronic current through the barrier film that causes oxygen evolution during anodization. When growth of anodic oxide and oxygen evolution occur simultaneously at the aluminium anode, cavities or pores are formed in the resulting films. Accordingly, the mechanisms of growth of barrier and porous films are not very different in nature. These findings are a decisive new step towards full understanding of the nature of anodic alumina films. Graphical abstract        

Metal coordination architectures of triazole-based ligands: Effect of the backbone of bridging ligands on the construction of polymers

In our efforts to investigate the influence of the backbone of different triazole-based bridging ligands on the structure of their metal complexes, four new coordination polymers, {[Cu(L¹)₂(H₂O)₂]Cl₂}_n (1), [Cu(L²)₂Cl₂]_n (2), [Co(L²)₂(SCN)₂]_n (3), and [Cu(L³)₂(NO₃)₂]_n (4), (L¹ = 1,2-bis(triazol-1-ylmethyl)benzene, L² = 1,3-bis(triazol-1-ylmethyl)benzene, L³ = 1,4-bis(triazol-1-ylmethyl)benzene), have been synthesized. All the complexes have been structurally characterized by IR, elemental analysis and single-crystal X-ray diffraction. Structural analyses show that 1 and 4 possess 2D coordination networks with (4,4) topology, and 1 shows a diagonal–diagonal inclined interpenetration. 2 and 3 are isostructural and feature 1D double chain, which further connected by C–H···Cl or π···π weak interactions to form 2D supramolecular frameworks. The results show that the structures of ligands (with different non-coordination backbone spacers) play important roles in the formation of such coordination architectures. Furthermore, EPR (Electron Paramagnetic Resonance) spectra of Cu^II complexes (1, 2, and 4) have been investigated in the solid state at room temperature.     

Photochemical hydrogen production with molecular devices comprising a zinc porphyrin and a cobaloxime catalyst

Two new noble-metal-free molecular devices,[{Co(dmgH) 2 Cl}{Zn(PyTPP)}](1,dmgH = dimethyloxime,PyTPP = 5-(4pyridyl)-10,15,20-triphenylporphyrin) and [{Co(dmgH) 2 Cl}{Zn(apPyTPP)}](2,apPyTPP = 5-[4-(isonicotinamidyl)phenyl]10,15,20-triphenylporphyrin),for light-driven hydrogen generation were prepared and spectroscopically characterized.The zinc porphyrin photosensitizer and the Co III-based catalyst unit are linked by axial coordination of a pyridyl group in the periphery of zinc-porphyrin to the cobalt centre of catalyst with different lengths of bridges.The apparent fluorescence quenching and lifetime decays of 1 and 2 were observed in comparison with their reference chromophores,Zn(PyTPP)(3) and Zn(apPyTPP)(4),suggesting a possibility for an intramolecular electron transfer from the singlet excited state of zinc porphyrin unit to the cobalt centre in the molecular devices.Photochemical H2-evolving studies show that complexes 1 and 2 are efficient molecular photocatalysts for visible light-driven H2 generation from water with triethylamine as a sacrificial electron donor in THF/H2 O,with turnover numbers up to 46 and 35 for 1 and 2,respectively.In contrast to these molecular devices,the multicomponent catalyst of zinc porphyrin and [Co(dmgH) 2 PyCl] did not show any fluorescence quenching and as a consequence,no H2 gas was detected by GC analysis in the presence of triethylamine with irradiation of visible light.The plausible mechanism for the photochemical H2 generation with these molecular devices is discussed.