Cover Feature: Mechanism of Oxidative Activation of Fluorinated Aromatic Compounds by N-Bridged Diiron-Phthalocyanine: What Determines the Reactivity? (Chem. Eur. J. 63/2019)

The biodegradation of compounds with C−F bonds is challenging due to the fact that these bonds are stronger than the C−H bond in methane. In this work, results on the unprecedented reactivity of a biomimetic model complex that contains an N-bridged diiron-phthalocyanine are presented; this model complex is shown to react with perfluorinated arenes under addition of H₂O₂ effectively. To get mechanistic insight into this unusual reactivity, detailed density functional theory calculations on the mechanism of C₆F₆ activation by an iron(IV)-oxo active species of the N-bridged diiron phthalocyanine system were performed. Our studies show that the reaction proceeds through a rate-determining electrophilic C−O addition reaction followed by a 1,2-fluoride shift to give the ketone product, which can further rearrange to the phenol. A thermochemical analysis shows that the weakest C−F bond is the aliphatic C−F bond in the ketone intermediate. The oxidative defluorination of perfluoroaromatics is demonstrated to proceed through a completely different mechanism compared to that of aromatic C−H hydroxylation by iron(IV)-oxo intermediates such as cytochrome P450 Compound I.     

Cover Feature: Redox-Active Guanidines in Proton-Coupled Electron-Transfer Reactions: Real Alternatives to Benzoquinones? (Chem. Eur. J. 70/2019)

Guanidino-functionalized aromatics (GFAs) are readily available, stable organic redox-active compounds. In this work we apply one particular GFA compound, 1,2,4,5-tetrakis(tetramethylguanidino)benzene, in its oxidized form in a variety of oxidation/oxidative coupling reactions to demonstrate the scope of its proton-coupled electron transfer (PCET) reactivity. Addition of an excess of acid boosts its oxidation power, enabling the oxidative coupling of substrates with redox potentials of at least +0.77 V vs. Fc⁺/Fc. The green recyclability by catalytic re-oxidation with dioxygen is also shown. Finally, a direct comparison indicates that GFAs are real alternatives to toxic halo- or cyano-substituted benzoquinones.     

Cover Feature: Coordination-Cage-Catalysed Hydrolysis of Organophosphates: Cavity- or Surface-Based? (Chem. Eur. J. 14/2020)

The hydrophobic central cavity of a water-soluble M₈L₁₂ cubic coordination cage can accommodate a range of phospho-diester and phospho-triester guests such as the insecticide “dichlorvos” (2,2-dichlorovinyl dimethyl phosphate) and the chemical warfare agent analogue di(isopropyl) chlorophosphate. The accumulation of hydroxide ions around the cationic cage surface due to ion-pairing in solution generates a high local pH around the cage, resulting in catalysed hydrolysis of the phospho-triester guests. A series of control experiments unexpectedly demonstrates that—in marked contrast to previous cases—it is not necessary for the phospho-triester substrates to be bound inside the cavity for catalysed hydrolysis to occur. This suggests that catalysis can occur on the exterior surface of the cage as well as the interior surface, with the exterior-binding catalysis pathway dominating here because of the small binding constants for these phospho-triester substrates in the cage cavity. These observations suggest that cationic but hydrophobic surfaces could act as quite general catalysts in water by bringing substrates into contact with the surface (via the hydrophobic effect) where there is also a high local concentration of anions (due to ion pairing/electrostatic effects).     

Cover Feature: The Role of Common Alcoholic Sacrificial Agents in Photocatalysis: Is It Always Trivial? (Chem. Eur. J. 64/2021)

Photocatalytic hydrogen production is proposed as a sustainable energy source. Simultaneous reduction and oxidation of water is a complex multistep reaction with high overpotential. Photocatalytic processes involving semiconductors transfer electrons from the valence band to the conduction band. Sacrificial substrates that react with the photochemically formed holes in the valence band are often used to study the mechanism of H₂ production, as they scavenge the holes and hinder charge carrier recombination (electron-hole pairs). Here, we show that the desired sacrificial agent is one forming a radical that is a fairly strong reducing agent, and whose oxidized form is not a good electron acceptor that might suppress the hydrogen evolution reaction (HER). In an acidic medium, methanol was found to fulfill both these requirements better than ethanol and propan-2-ol in the TiO₂-(M⁰-NPs) (M=Au or Pt) system, whereas in an alkaline medium, the alcohols exhibit a reverse order of activity. Moreover, we report that CH₂(OH)₂ is by far the most efficient sacrificial agent in a nontrivial mechanism in acidic media. Our study provides general guidelines for choosing an appropriate sacrificial substrate and helps to explain the variance in the performance of alcohol scavenger-based photocatalytic systems.     

Cover Feature: How Many O-Donor Groups in Enterobactin Does It Take to Bind a Metal Cation? (Chem. Eur. J. 28/2019)

The E. coli siderophore enterobactin, the strongest Fe^III chelator known to date, forms hexacoordinate complexes with Si^IV, Ge^IV, and Ti^IV. Synthetic protocols have been developed to prepare non-symmetric enterobactin analogues with varying denticities. Various benzoic acid residues were coupled to the macrocyclic lactone to afford a diverse library of ligands. These enterobactin analogues were bound to Si^IV, Ge^IV, and Ti^IV, and the complexes were investigated through experimental and computational techniques. The binding behavior of the synthesized chelators enabled assessment of the contribution of each of the phenolic hydroxy groups in enterobactin to metal-ion complexation. It was found that at least four O-donors are needed for enterobactin derivatives to act as metal binders. Density functional theory calculations indicate that the strong binding behavior of enterobactin can be ascribed to a diminished translational entropy penalty, a common feature of the chelate effect, coupled with the structural arrangement of the three catechol moieties, which allows the triseryl base to be installed without distorting the preferred local metal-binding geometry of the catecholate ligands.     

Cover Feature: Copper and Nickel Complexes of Oxamate−Phenol Containing Ligands: A Structural Dichotomy in Oxidized Species (Eur. J. Inorg. Chem. 15/2023)

The copper and nickel complexes of two tetradentate ligands derived from bis(aminophenol) and bis(phenol) architectures connected by an oxamate linker were isolated. Depending on the metal and ligand, the complex is isolated with either an intact (deprotonated) ligand ( 1²⁻ ), one-electron oxidized ligand ( 2⁻ ) or quinone form ( 3 ). Surprisingly, the Mannich base is easier to oxidize than the amidophenol derivatives. The complexes were characterized by X-ray diffraction, cyclic voltammetry, UV-Vis-NIR and EPR spectroscopies. Complex 1 shows two reversible oxidation waves assigned to the successive iminosemiquinone/aminophenolate redox systems. Complex 2⁻ shows an intense NIR feature, as well as an EPR signal at g_iso=2.043, consistent with a metallic contribution to the main ligand radical SOMO. Complex 3 shows the typical feature of an isolated Cu(II) complex. Spectro-electrochemistry coupled to DFT calculations demonstrate a ligand-centered oxidative redox chemistry for all the complexes.     

Front Cover: Non-conventional Behavior of a 2,1-Benzazaphosphole: Heterodiene or Hidden Phosphinidene? (Chem. Eur. J. 52/2021)

The titled 2,1-benzazaphosphole ( 1 ) (i. e. ArP, where Ar=2-(DippN=CH)C₆H₄, Dipp=2,6-iPr₂C₆H₃) showed a spectacular reactivity behaving both as a reactive heterodiene in hetero-Diels-Alder (DA) reactions or as a hidden phosphinidene in the coordination toward selected transition metals (TMs). Thus, 1 reacts with electron-deficient alkynes RC≡CR (R=CO₂Me, C₅F₄N) giving 1-phospha-1,4-dihydro-iminonaphthalenes 2 and 3 , that undergo hydrogen migration producing 1-phosphanaphthalenes 4 and 5 . Compound 1 is also able to activate the C=C double bond in selected N-alkyl/aryl-maleimides RN(C(O)CH)₂ (R=Me, tBu, Ph) resulting in the addition products 7–9 with bridged bicyclic [2.2.1] structures. The binding of the maleimides to 1 is semi-reversible upon heating. By contrast, when 1 was treated with selected TM complexes, it serves as a 4e donor bridging two TMs thus producing complexes [μ-ArP(AuCl)₂] ( 10 ), [(μ-ArP)₄Ag₄][X]₄ (X=BF₄ ( 11 ), OTf ( 12 )) and [μ-ArP(Co₂(CO)₆)] ( 13 ). The structure and electron distribution of the starting material 1 as well as of other compounds were also studied from the theoretical point of view.     

Cover Feature: Stereoselective Domino Semipinacol-Schmidt Reaction: Diastereoselective Synthesis of 7 a-epi-(+)-Lepadiformine C and Formal Synthesis of 7 a-epi-(−)-Lepadiformine A (Eur. J. Org. Chem. 19/2023)

A concise approach has been developed for the diastereoselective synthesis of the azatricyclic core of (−)-7 a-epi-lepadiformine A ( 8 a ) and (−)-7 a-epi-lepadiformine C ( 8 b ) using stereoselective domino semipinacol-Schmidt reaction as a key step. In presence of TiCl₄, the oxaspiropentane-azide derivatives underwent stereoselective domino cyclization to furnish the corresponding angularly fused azatricyclic cores in very good yields. Moreover, azatricyclic core of (−)-7 a-epi-lepadiformine A has also been realized, in a stepwise manner, through the intramolecular Schmidt reaction of azido-spirocyclobutanone intermediate. The synthetic utility of domino semipinacol-Schmidt reaction is further shown in the diastereoselective synthesis of (+)-7 a-epi-lepadiformine C ( 7 ).