Mechanistic insights into the reversible formation of iodosylarene-iron porphyrin complexes in the reactions of oxoiron(IV) porphyrin pi-cation radicals and iodoarenes: equilibrium, epoxidizing intermediate, and oxygen exchange

We have shown previously that iodosylbenzene-iron(III) porphyrin intermediates (2) are generated in the reactions of oxoiron(IV) porphyrin pi-cation radicals (1) and iodobenzene (PhI), that 1 and 2 are at equilibrium in the presence of PhI, and that the epoxidation of olefins by 2 affords high yields of epoxide products. In the present work, we report detailed mechanistic studies on the nature of the equilibrium between 1 and 2 in the presence of iodoarenes (ArI), the determination of reactive species responsible for olefin epoxidation when two intermediates (i.e., 1 and 2) are present in a reaction solution, and the fast oxygen exchange between 1 and H(2)18O in the presence of ArI. In the first part, we have provided strong evidence that 1 and 2 are indeed at equilibrium and that the equilibrium is controlled by factors such as the electronic nature of iron porphyrins, the electron richness of ArI, and the concentration of ArI. Secondly, we have demonstrated that 1 is the sole active oxidant in olefin epoxidation when 1 and 2 are present concurrently in a reaction solution. Finally, we have shown that the presence of ArI in a reaction solution containing 1 and H(2)18O facilitates the oxygen exchange between the oxo group of 1 and H(2)18O and that the oxygen exchange is markedly influenced by factors such as ArI incubation time, the amounts of ArI and H(2)18O used, and the electronic nature of ArI. The latter results are rationalized by the formation of an undetectable amount of 2 from the reaction of 1 and ArI through equilibrium that leads to a fast oxygen exchange between 2 and H(2)18O.     

Remarkable Solvent,Porphyrin Ligand,and Substrate Effects on Participation of Multiple Active Oxidants in Manganese(III) Porphyrin Catalyzed Oxidation Reactions

The participation of multiple active oxidants generated from the reactions of two manganese(III) porphyrin complexes containing electron‐withdrawing and ‐donating substituents with peroxyphenylacetic acid (PPAA) as a mechanistic probe was studied by carrying out catalytic oxidations of cyclohexene, 1‐octene, and ethylbenzene in various solvent systems, namely, toluene, CH₂Cl₂, CH₃CN, and H₂O/CH₃CN (1:4). With an increase in the concentration of the easy‐to‐oxidize substrate cyclohexene in the presence of [(TMP)MnCl] ( 1 a ) with electron‐donating substituents, the ratio of heterolysis to homolysis increased gradually in all solvent systems, suggesting that [(TMP)Mn? OOC(O)R] species 2 a is the major active species. When the substrate was changed from the easy‐to‐oxidize one (cyclohexene) to difficult‐to‐oxidize ones (1‐octene and ethylbenzene), the ratio of heterolysis to homolysis increased a little or did not change. [(F₂₀TPP)Mn? OOC(O)R] species 2 b generated from the reaction of [(F₂₀TPP)MnCl] ( 1 b ) with electron‐withdrawing substituents and PPAA also gradually becomes involved in olefin epoxidation (although to a much lesser degree than with [(TMP)Mn? OOR] 2 a ) depending on the concentration of the easy‐to‐oxidize substrate cyclohexene in all aprotic solvent systems except for CH₃CN, whereas Mn^V?O species is the major active oxidant in the protic solvent system. With difficult‐to‐oxidize substrates, the ratio of heterolysis to homolysis did not vary except for 1‐octene in toluene, indicating that a Mn^V?O intermediate generated from the heterolytic cleavage of 2 b becomes a major reactive species. We also studied the competitive epoxidations of cis‐2‐octene and trans‐2‐octene with two manganese(III) porphyrin complexes by meta‐chloroperbenzoic acid (MCPBA) in various solvents under catalytic reaction conditions. The ratios of cis‐ to trans‐2‐octene oxide formed in the reactions of MCPBA varied depending on the substrate concentration, further supporting the contention that the reactions of manganese porphyrin complexes with peracids generate multiple reactive oxidizing intermediates.     

Synthesis and Characterization of Iron(III) Complexes of 5‐(8‐Carboxy‐1‐naphthyl)‐10, 15, 20‐tritolyl Porphyrin

5‐(8‐Carboxy‐1‐naphthyl)‐10, 5, 20‐tritolyl porphyrin (H₃CNTTP) and its iron(III) complexes, [Fe(CNTTP)]₂ and [Fe(CNTTP)(N‐MeIm)₂], were synthesized and characterized. X‐ray crystallography revealed that the carboxylate group is “hanging” over the porphyrin plane. The rigid framework makes the distance between the carboxylate oxygen and iron in the same porphyrin too long to form a coordination bond. On the other hand, the carboxylate group is not bulky enough to block the axial binding site. In the presence of OH^–, the carboxylate oxygen is coordinated to iron in the symmetry‐related unit, which led to the dimeric structure, [Fe(CNTTP)]₂. In the presence of excess N‐methylimidazole, a six‐coordinate species, [Fe(CNTTP)(N‐MeIm)₂], was obtained. In such a structure, CH ··· O interactions between the carboxylate group and imidazole probably play an important role to determine the orientation of imidazole plane. Two imidazole planes have relative parallel orientation. For [Fe(CNTTP)(N‐MeIm)₂], ¹H NMR shows pyrrole protons at the region –10 to –25 ppm. EPR shows rhombic spectrum. Those suggest [Fe(CNTTP)(N‐MeIm)₂] is a type II low‐spin iron(III) porphyrinate.     

Robust and efficient amide-based nonheme manganese(III) hydrocarbon oxidation catalysts: substrate and solvent effects on involvement and partition of multiple active oxidants

Two new mononuclear nonheme manganese(III) complexes of tetradentate ligands containing two deprotonated amide moieties, [Mn(bpc)Cl(H₂O)] ( 1 ) and [Mn(Me₂bpb)Cl(H₂O)] ? CH₃OH ( 2 ), were prepared and characterized. Complex 2 has also been characterized by X‐ray crystallography. Magnetic measurements revealed that the complexes are high spin (S=5/2) Mn^III species with typical magnetic moments of 4.76 and 4.95 μ_B, respectively. These nonheme Mn^III complexes efficiently catalyzed olefin epoxidation and alcohol oxidation upon treatment with MCPBA under mild experimental conditions. Olefin epoxidation by these catalysts is proposed to involve the multiple active oxidants Mn^V?O, Mn^IV?O, and Mn^III? OO(O)CR. Evidence for this approach was derived from reactivity and Hammett studies, KIE (k_H/k_D) values, H₂¹⁸O‐exchange experiments, and the use of peroxyphenylacetic acid as a mechanistic probe. In addition, it has been proposed that the participation of Mn^V?O, Mn^IV?O, and Mn^III? OOR could be controlled by changing the substrate concentration, and that partitioning between heterolysis and homolysis of the O? O bond of a Mn‐acylperoxo intermediate (Mn? OOC(O)R) might be significantly affected by the nature of solvent, and that the O? O bond of the Mn? OOC(O)R might proceed predominantly by heterolytic cleavage in protic solvent. Therefore, a discrete Mn^V?O intermediate appeared to be the dominant reactive species in protic solvents. Furthermore, we have observed close similarities between these nonheme Mn^III complex systems and Mn(saloph) catalysts previously reported, suggesting that this simultaneous operation of the three active oxidants might prevail in all the manganese‐catalyzed olefin epoxidations, including Mn(salen), Mn(nonheme), and even Mn(porphyrin) complexes. This mechanism provides the greatest congruity with related oxidation reactions by using certain Mn complexes as catalysts.     

Isotope exchange between NO and H2O on a platinum-containing catalyst based on fiberglass

The dynamics of ¹⁸O isotope exchange between NO or H₂O and a catalyst and the dynamics of ¹⁸O label transfer from NO to H₂O have been studied under conditions of sorption-desorption equilibrium. The occurrence of a reaction of oxygen exchange between NO and water sorbed in the bulk of the catalyst was detected. This reaction occurs at platinum sites with the participation of acid sites of the glass matrix. The rate constants of the reaction of NO with platinum sites and the diffusion coefficients of water in the bulk of the glass matrix are evaluated.     

Synthesis and catalytic activity of binuclear Mn(III,III)–BINOL complexes for epoxidation of olefins

The novel binuclear complexes [Mn₂^{(III, III)}(BINOL)₃L₂]2H₂O, where, L = 2, 2′‐bipyridine (Bpy) or 1,10‐phenanthroline (Phen) and BINOL = 1, 1′‐bi‐2‐naphthol were synthesized and characterized by elemental analyses, magnetic susceptibility and various spectral methods. The catalytic activity of these complexes was studied for the epoxidation reaction of unfunctionalized olefins like styrene, 1‐hexene, 1‐octene and 1‐decene. The products thus obtained were analyzed by GC. The epoxidation reactions were carried out, in the presence of catalyst with different oxidants, to study the effect of the nature of the oxidant on the reactions. The different oxidants used were the peroxide oxygen donor (e.g. TBHP and H₂O₂), mono oxygen donor (e.g. PhIO) and dioxygen donor (e.g. molecular O₂). TBHP was found to be the best oxidant for the epoxidation reaction. To study the effect of the solvent on the epoxidation, the reactions were carried out in different media, such as a polar media (e.g. with CH₃OH as solvent), non‐polar media (e.g. with CH₂Cl₂ and C₆H₆ as solvents) and coordinating solvent (e.g. CH₃CN). The maximum epoxide formation was observed in CH₂Cl₂ medium. The epoxidation reactions with optically active BINOL catalysts under optimum established conditions were carried out to examine the enantioselectivity of the catalysts. The complexes were, however, found not to be enantioselective. Copyright © 2006 John Wiley & Sons, Ltd.     

Enhanced Reactivities of Iron(IV)‐Oxo Porphyrin π‐Cation Radicals in Oxygenation Reactions by Electron‐Donating Axial Ligands

The proximal axial ligand in heme iron enzymes plays an important role in tuning the reactivities of iron(IV)‐oxo porphyrin π‐cation radicals in oxidation reactions. The present study reports the effects of axial ligands in olefin epoxidation, aromatic hydroxylation, alcohol oxidation, and alkane hydroxylation, by [(tmp)^+. Fe^IV(O)(p‐Y‐PyO)]⁺ ( 1 ‐Y) (tmp=meso‐tetramesitylporphyrin, p‐Y‐PyO=para‐substituted pyridine N‐oxides, and Y=OCH₃, CH₃, H, Cl). In all of the oxidation reactions, the reactivities of 1 ‐Y are found to follow the order 1 ‐OCH₃ > 1 ‐CH₃ > 1 ‐H > 1 ‐Cl; negative Hammett ρ values of ?1.4 to ?2.7 were obtained by plotting the reaction rates against the σ_p values of the substituents of p‐Y‐PyO. These results, as well as previous ones on the effect of anionic nucleophiles, show that iron(IV)‐oxo porphyrin π‐cation radicals bearing electron‐donating axial ligands are more reactive in oxo‐transfer and hydrogen‐atom abstraction reactions. These results are counterintuitive since iron(IV)‐oxo porphyrin π‐cation radicals are electrophilic species. Theoretical calculations of anionic and neutral ligands reproduced the counterintuitive experimental findings and elucidated the root cause of the axial ligand effects. Thus, in the case of anionic ligands, as the ligand becomes a better electron donor, it strengthens the FeO? H bond and thereby enhances its H‐abstraction activity. In addition, it weakens the Fe?O bond and encourages oxo‐transfer reactivity. Both are Bell–Evans–Polanyi effects, however, in a series of neutral ligands like p‐Y‐PyO, there is a relatively weak trend that appears to originate in two‐state reactivity (TSR). This combination of experiment and theory enabled us to elucidate the factors that control the reactivity patterns of iron(IV)‐oxo porphyrin π‐cation radicals in oxidation reactions and to resolve an enigmatic and fundamental problem.     

Stabilization and Structure of the cis Tautomer of a Free‐Base Porphyrin

Single‐crystal X‐ray analysis of the β‐heptakis(trifluoromethyl)‐meso ‐tetrakis(p ‐fluorophenyl)porphyrin, H₂[(CF₃)₇TpFPP], has revealed the first example of a stable cis tautomer of a free‐base porphyrin, the long‐postulated intermediate of porphyrin tautomerism. The stability of the unique molecule appears to reflect a dual origin: a strongly saddled porphyrin skeleton, which alleviates electrostatic repulsion between the two NH protons, and two polarization‐enhanced, transannular N−H⋅⋅⋅O−H⋅⋅⋅N hydrogen bond chains, each involving a molecule of water. DFT calculations suggest that the observed tautomer has a lower energy than the alternative, doubly hydrated trans tautomer by some 8.3 kcal mol⁻¹. A fascinating prospect thus exists that H₂[(CF₃)₇TpFPP]⋅2 H₂O and cognate structures may act as supramolecular synthons, which, given their chirality, may even be amenable to resolution into optically pure enantiomers.     

Porphyrin Nanorods Modified Glassy Carbon Electrode for the Electrocatalysis of Dioxygen,Methanol and Hydrazine

Porphyrin nanorods (PNR) were prepared by ionic self‐assembly of two oppositely charged porphyrin molecules consisting of free base meso‐tetraphenylsulfonate porphyrin (H₄TPPS₄^2?) and meso‐tetra(N‐methyl‐4‐pyridyl) porphyrin (MTMePyP⁴⁺M=Sn, Mn, In, Co). These consist of H₄TPPS₄^2?? SnTMePyP⁴⁺, H₄TPPS₄^2?? CoTMePyP⁴⁺, H₄TPPS₄^2?? InTMePyP⁴⁺ and H₄TPPS₄^2?? MnTMePyP⁴⁺ porphyrin nanorods. The absorption spectra and transmission electron microscopic (TEM) images of these structures were obtained. These porphyrin nanostructures were used to modify a glassy carbon electrode for the electrocatalytic reduction of oxygen, and the oxidation of hydrazine and methanol at low pH. The cyclic voltammogram of PNR‐modified GCE in pH 2 buffer solution has five irreversible processes, two distinct reduction processes and three oxidation processes. The porphyrin nanorods modified GCE produce good responses especially towards oxygen reduction at ?0.50 V vs. Ag|AgCl (3 M KCl). The process of electrocatalytic oxidation of methanol using PNR‐modified GCE begins at 0.71 V vs. Ag|AgCl (3 M KCl). The electrochemical oxidation of hydrazine began at around 0.36 V on H₄TPPS₄^2?? SnTMePyP⁴⁺ modified GCE. The GCE modified with H₄TPPS₄^2?? CoTMePyP⁴⁺ H₄TPPS₄^2?? InTMePyP⁴⁺ and H₄TPPS₄^2?? MnTMePyP⁴⁺ porphyrin nanorods began oxidizing hydrazine at 0.54 V, 0.59 V and 0.56 V, respectively.     

Stable Anticancer Gold(III)–Porphyrin Complexes: Effects of Porphyrin Structure

In the design of physiologically stable anticancer gold(III) complexes, we have employed strongly chelating porphyrinato ligands to stabilize a gold(III) ion [Chem. Commun. 2003 , 1718; Coord. Chem. Rev. 2009 , 253, 1682]. In this work, a family of gold(III) tetraarylporphyrins with porphyrinato ligands containing different peripheral substituents on the meso‐aryl rings were prepared, and these complexes were used to study the structure–bioactivity relationship. The cytotoxic IC₅₀ values of [Au(Por)]⁺ (Por=porphyrinato ligand), which range from 0.033 to >100 μM , correlate with their lipophilicity and cellular uptake. Some of them induce apoptosis and display preferential cytotoxicity toward cancer cells than to normal noncancerous cells. A new gold(III)–porphyrin with saccharide conjugation [Au(4‐glucosyl‐TPP)]Cl ( 2 a ; H₂(4‐glucosyl‐TPP)=meso‐tetrakis(4‐β‐D ‐glucosylphenyl)porphyrin) exhibits significant cytostatic activity to cancer cells (IC₅₀=1.2–9.0 μM ) without causing cell death and is much less toxic to lung fibroblast cells (IC₅₀>100 μM ). The gold(III)–porphyrin complexes induce S‐phase cell‐cycle arrest of cancer cells as indicated by flow cytometric analysis, suggesting that the anticancer activity may be, in part, due to termination of DNA replication. The gold(III)–porphyrin complexes can bind to DNA in vitro with binding constants in the range of 4.9×10⁵ to 4.1×10⁶ dm³ mol^?1 as determined by absorption titration. Complexes 2 a and [Au(TMPyP)]Cl₅ ( 4 a ; [H₂TMPyP]⁴⁺=meso‐tetrakis(N‐methylpyridinium‐4‐yl)porphyrin) interact with DNA in a manner similar to the DNA intercalator ethidium bromide as revealed by gel mobility shift assays and viscosity measurements. Both of them also inhibited the topoisomerase I induced relaxation of supercoiled DNA. Complex 4 a , a gold(III) derivative of the known G‐quadruplex‐interactive porphyrin [H₂TMPyP]⁴⁺, can similarly inhibit the amplification of a DNA substrate containing G‐quadruplex structures in a polymerase chain reaction stop assay. In contrast to these reported complexes, complex 2 a and the parental gold(III)–porphyrin 1 a do not display a significant inhibitory effect (<10 %) on telomerase. Based on the results of protein expression analysis and computational docking experiments, the anti‐apoptotic bcl‐2 protein is a potential target for those gold(III)–porphyrin complexes with apoptosis‐inducing properties. Complex 2 a also displays prominent anti‐angiogenic properties in vitro. Taken together, the enhanced stabilization of the gold(III) ion and the ease of structural modification render porphyrins an attractive ligand system in the development of physiologically stable gold(III) complexes with anticancer and anti‐angiogenic activities.     

16O/18O Exchange of Aldehydes and Ketones caused by H218O in the Mechanistic Investigation of Organocatalyzed Michael,Mannich, and Aldol Reactions

Organocatalyzed Michael, Mannich, and aldol reactions of aldehydes or ketones, as nucleophiles, have triggered several discussions regarding their reaction mechanism. H₂¹⁸O has been utilized to determine if the reaction proceeds through an enamine or enol mechanism by monitoring the ratio of ¹⁸O incorporated into the final product. In this communication, we describe the risk of H₂¹⁸O as an evaluation tool for this mechanistic investigation. We have demonstrated that exchange of ¹⁶O/¹⁸O occurs in the aldehyde or ketone starting material, caused by the presence of H₂¹⁸O and amine catalysts, before the Michael, Mannich, and aldol reactions proceed. Because the newly generated ¹⁸O starting aldehydes or ketones and ¹⁶O water affect the incorporation ratio of ¹⁸O in the final product, the use of H₂¹⁸O would not be appropriate to distinguish the mechanism of these organocatalyzed reactions.     

Axial Ligand and Spin‐State Influence on the Formation and Reactivity of Hydroperoxo–Iron(III) Porphyrin Complexes

The present study focuses on the formation and reactivity of hydroperoxo–iron(III) porphyrin complexes formed in the [Fe^III(tpfpp)X]/H₂O₂/HOO^? system (TPFPP=5,10,15,20‐tetrakis(pentafluorophenyl)‐21H,23H‐porphyrin; X=Cl^? or CF₃SO₃^?) in acetonitrile under basic conditions at ?15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high‐spin [Fe^III(tpfpp)(OOH)] and low‐spin [Fe^III(tpfpp)(OH)(OOH)] could be observed with the application of a low‐temperature rapid‐scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of O? O bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo‐ to heterolytic O? O bond cleavage is observed for high‐ [Fe^III(tpfpp)(OOH)] and low‐spin [Fe^III(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron‐rich porphyrin ligands, electron‐deficient [Fe^III(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [Fe^III(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)–oxo porphyrin π‐cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450.     

Echange d'oxygène entre des p-benzoquinones substituées et l'eau

The velocity of oxygen exchange between several substituted p-benzoquinones and H₂¹⁸O has been measured at 25°C in neutral tetrahydrofuran containing 2 to 5% water. Halogen substituents accelerate the exchange (Cl > Br), methyl groups slow it down. The reaction was found to be second or third order in [H₂O].     

Di‐Benzo‐18‐Crown‐6 and its Derivatives as Ligands in the Search for Ion Channels

Based on the previously reported one‐dimensional channel system [(H₂O)?(DB18C6)(μ₂‐H₂O)_2/2][(H₃O)?(DB18C6)(μ₂‐H₂O)_2/2]I₃ ( 2 ), which is realized by stacking of crown ether molecules (DB18C6 = dibenzo‐18‐crown‐6), other synthetic approaches towards ionic channels and their results are presented in this paper. The “cutting out” approach using DB18C6 as scissor, applied on NaI, yields the compound [Na?(DB18C6)I(THF)][Na?(DB18C6)(H₂O)₂]I(THF)₂(CHI₃) ( 1 ), in high yield. It is based on a neutral and a cationic complex of sodium by DB18C6 linked via H‐bonding to give short chain fragments. The anion exchange approach, trying to replace I₃^? by Br₃^? leads to the intercalation of a cation into a DB18C6 chain in [(Me₃NPh)(DB18C6)]Br₃ ( 3 ). A similar reaction as for the synthesis of 2 , but replacing iodide with bromine, yields finally a brominated DB18C6 ligand. In the presence of iron, the compound [(H₅O₂)?(Br₄‐DB18C6)₂][FeBr₄], 4 , is observed, in which a H₅O₂⁺‐cation is encapsuled by two brominated crown ether molecules. The absence of Fe and an excess of Br₂ leads to the complexation of H₃O⁺, and co‐crystallisation of bromine in [(H₃O)?(Br₄‐DB18C6)]Br₃Br₂ ( 5 ).     

An improved protocol for the ruthenium(pybox)-catalyzed asymmetric alkene epoxidation

A considerable rate enhancement in the ruthenium-catalyzed asymmetric epoxidation of olefins in the presence of PhI(OAc)₂ is reported. By the addition of H₂O, the rate of the reaction was increased by two orders of magnitude. Reactions of both aliphatic and aromatic olefins were realized for the first time and enantioselectivities up to 71% ee were obtained. In addition an in situ generation of ruthenium pybox catalysts for faster screening of oxidation catalysts was also developed.     

Palladium‐Catalyzed Aerobic Intermolecular Cyclization of Acrylic Acid with 1‐Octene to Afford α‐Methylene‐γ‐butyrolactones: The Remarkable Effect of Continuous Water Removal from the Reaction Mixture and Analysis of the Reaction by Kinetic,ESI‐MS,and XAFS Measurements

Further study of our aerobic intermolecular cyclization of acrylic acid with 1‐octene to afford α‐methylene‐γ‐butyrolactones, catalyzed by the Pd(OCOCF₃)₂/Cu(OAc)₂ ? H₂O system, has clarified that the accumulation of water generated from oxygen during the reaction causes deactivation of the Cu cocatalyst. This prevents regeneration of the active Pd catalyst and, thus, has a harmful influence on the progress of the cyclization. As a result, both the substrate conversion and product yield are efficiently improved by continuous removal of water from the reaction mixture. Detailed analysis of the kinetic and spectroscopic measurements performed under the condition of continuous water removal demonstrates that the cyclization proceeds in four steps: 1) equilibrium coordination of 1‐octene to the Pd acrylate species, 2) Markovnikov‐type acryloxy palladation of 1‐octene (1,2‐addition), 3) intramolecular carbopalladation, and 4) β‐hydride elimination. Byproduct 2‐acryloxy‐1‐octene is formed by β‐hydride elimination after step 2). These cyclization steps fit the Michaelis–Menten equation well and β‐hydride elimination is considered to be a rate‐limiting step in the formation of the products. Spectroscopic data agree sufficiently with the existence of the intermediates bearing acrylate (Pd? O bond), η³‐C₈H₁₅ (Pd? C bond), or C₁₁H₁₉O₂ (Pd? C bond) moieties on the Pd center as the resting‐state compounds. Furthermore, not only Cu^II, but also Cu^I, species are observed during the reaction time of 2–8 h when the reaction proceeds efficiently. This result suggests that the Cu^II species is partially reduced to the Cu^I species when the active Pd catalytic species are regenerated.     

Synthesis of α‐Dawson‐Type Silicotungstate [α‐Si2W18O62]8− and Protonation and Deprotonation Inside the Aperture through Intramolecular Hydrogen Bonds

The design of structurally well‐defined anionic molecular metal–oxygen clusters, polyoxometalates (POMs), leads to inorganic receptors with unique and tunable properties. Herein, an α‐Dawson‐type silicotungstate, TBA₈[α‐Si₂W₁₈O₆₂] ? 3 H₂O ( II ) that possesses a ?8 charge was successfully synthesized by dimerization of a trivacant lacunary α‐Keggin‐type silicotungstate TBA₄H₆[α‐SiW₉O₃₄] ? 2 H₂O ( I ) in an organic solvent. POM II could be reversibly protonated (in the presence of acid) and deprotonated (in the presence of base) inside the aperture by means of intramolecular hydrogen bonds with retention of the POM structure. In contrast, the aperture of phosphorus‐centered POM TBA₆[α‐P₂W₁₈O₆₂]?H₂O ( III ) was not protonated inside the aperture. The density functional theory (DFT) calculations revealed that the basicities and charges of internal μ₃‐oxygen atoms were increased by changing the central heteroatoms from P⁵⁺ to Si⁴⁺, thereby supporting the protonation of II . Additionally, II showed much higher catalytic performance for the Knoevenagel condensation of ethyl cyanoacetate with benzaldehyde than I and III .     

Photoinduced Charge Separation in Zinc–Porphyrin/Tungsten–Alkylidyne Dyads: Generation of Reactive Porphyrin and Metallo Radical States

The luminescent tungsten–alkylidyne metalloligand [WCl(≡C‐4,4′‐C₆H₄CC‐py)(dppe)₂] ( 1 ; dppe=1,2‐bis(diphenylphosphino)ethane) and the zinc–tetraarylporphyrins ZnTPP and ZnTP_ClP (TPP=tetraphenylporphyrin, TP_ClP=tetra(p‐chlorophenyl)porphyrin) self‐assemble in fluorobenzene solution to form the dyads ZnTPP( 1 ) and ZnTP_ClP( 1 ), in which the metalloligand is axially coordinated to the porphyrin. Excitation of the porphyrin‐centered S₁ excited states of these dyads initiates intramolecular energy‐transfer (ZnPor→ 1 ) and electron‐transfer ( 1 →ZnPor) processes, which together efficiently quench the S₁ state (～90 %). Transient‐absorption spectroscopy and an associated kinetic analysis reveal that the net product of the energy‐transfer process is the ³[dπ*] state of coordinated 1 , which is formed by S₁→¹[dπ*] singlet–singlet (Förster) energy transfer followed by ¹[dπ*]→³[dπ*] intersystem crossing. The data also demonstrate that coordinated 1 reductively quenches the porphyrin S₁ state to produce the [ZnPor^?][ 1⁺ ] charge‐separated state. This is a rare example of the reductive quenching of zinc porphyrin chromophores. The presence in the [ZnPor^?][ 1⁺ ] charge‐separated states of powerfully reducing zinc–porphyrin radical anions, which are capable of sensitizing a wide range of reductive electrocatalysts, and the 1⁺ ion, which can initiate the oxidation of H₂, produces an integrated photochemical system with the thermodynamic capability of driving photoredox processes that result in the transfer of renewable reducing equivalents instead of the consumption of conventional sacrificial donors.     

Does the oxide layer take part in the oxygen evolution reaction on platinum?: A DEMS study

Using ¹⁸O labelling together with differential electrochemical mass spectroscopy (DEMS) it was found that (i) an ¹⁸O containing Pt-oxide layer does not exchange oxygen with H₂¹⁶O; (ii) only ¹⁶O¹⁶O is evolved from H₂¹⁶O on an ¹⁸O containing oxide layer, both in acid and in alkaline solutions. Consequently, the oxide layer does not take part in the oxygen evolution reaction on Pt electrodes.     

Visible light induced oxygenation of cyclohexene with activation of water sensitized by dihydroxy coordinated tetraphenyloprphyrinatotin(IV)

Visible light irradiation of a reaction mixture containing dihydroxy coordinated tetraphenylporphyrinatotin(IV), cyclohexene and potassium hexachloroplatinate induced oxygenation of the cyclohexene under degassed conditions. In the reaction system, a water molecule served as the oxygen donor. Cyclohex-2-enol, 1,2-dichlorocyclohexane and 2-chlorocyclohexanol were the major oxidation products and the quantum yield was around 0.1. An experiment using H₂ ¹⁸O revealed that an ¹⁸O atom was quantitatively incorporated into the oxygenated products. The reaction was initially induced by an electron transfer from an excited triplet porphyrin to potassium hexachloroplatinate producing a cation radical of the porphyrin. Metal-oxo type complexes formed through deprotonation of the hydroxy group of the porphyrin cation radical were key reactive intermediates reacting with cyclohexene. Two kinds of the metal-oxo type complex reactive intermediate were kinetically demonstrated to be involved in the reaction system, producing different oxidation products from cyclohexene.