Front Cover: Can Cyanuric Acid and 2,4,6-Triaminopyrimidine Containing Ribonucleosides be Components of Prebiotic RNA? Insights from QM Calculations and MD Simulations (ChemPhysChem 11/2019)

As a step toward assessing their fitness as pre-RNA nucleobases, we employ DFT and MD simulations to analyze the noncovalent interactions of cyanuric acid (CA) and 2,4,6-triaminopyrimidine (TAP), and the structural properties of the associated ribonucleosides (rNs) and oligonucleotides. Our calculations reveal that the TAP : CA pair has a comparable hydrogen-bond strength to the canonical A : U pair. This strengthens the candidature of CA and TAP as prebiotic nucleobases. Further, the stacking between two canonical nucleobases is stronger than those between TAP or CA and a canonical base, as well as those between two TAP and/or CA, which indicates that enhanced stacking may have served as a driving force for the evolution from prebiotic to canonical nucleobases. Similarities in the DFT-derived anti/syn rotational barriers and MD-derived (anti) glycosidic conformation of the CA and TAP rNs and canonical rNs further substantiate their candidature as pre-RNA components. Greater deglycosylation barriers (as obtained by DFT calculations) for TAP rNs compared to canonical rNs suggest TAP rNs indicate higher resistance to environmental factors, while lower barriers indicate that CA rNs were likely more suitable for less-challenging locations. Finally, the tight packing in narrow CA:TAP-containing helices suggests that the prebiotic polymers were shielded from water, which would aid their evolution into self-replicating systems. Our calculations thus support proposals that CA and TAP can act as nucleobases of pre-RNA.     

Front Cover: Understanding Local-Field Correction Factors in the Framework of the Onsager−Böttcher Model (ChemPhysChem 3/2019)

The determination of the appropriate local-field factor for quantifying the response of a molecule to an external electric field is of major importance in optical spectroscopy. Although numerous studies have dealt with the evolution of the optical properties of emitters as a function of their environment, the choice of the model used to quantify local fields is still ambiguous, and sometimes even arbitrary. In this paper, we review the Onsager–Böttcher model, which introduces the polarizability of the probe molecule as the determinant parameter for the local field factor, and we establish a simple conceptual framework encompassing all commonly used models. Finally, a discussion of published experimental research illustrates the potential of the measurement of local electric fields in dense dielectric media, as well as the subtleties involved in their interpretation.     

Cover Feature: Hg(II)⋅d8[M] Interactions: Are they Metallophilic Interactions or Spodium Bonds? (ChemPhysChem 24/2023)

Some literature reports have shown the existence of short Hg(II)⋅⋅⋅d⁸[M] (M=Pd, Pt) contacts between linear Hg(II) and square planar d⁸[M] complexes that have been defined as heterometallophilic interactions. Linear L−Hg(II)−L complexes exhibit a π-hole or positive belt of electrostatic potential at the Hg atom, whereas late transition metals can serve as effective electron donors through their filled d_z2 orbitals. This study provides compelling evidence that Hg(II)⋅⋅⋅d⁸[M] interactions should be more appropriately termed spodium bonds.     

Cover Feature: Formation of Eigen or Zundel Features at Protonated Water Cluster–Aromatic Interfaces (ChemPhysChem 16/2023)

Interfacial interactions of protonated water clusters adsorbed at aromatic surfaces play an important role in biology, and in atmospheric, chemical and materials sciences. Here, we investigate the interaction of protonated water clusters ((H⁺H₂O)_n (where n=1–3)) with benzene (Bz), coronene (Cor) and dodecabenzocoronene (Dbc)). To study the structure, stability and spectral features of these complexes, computations are done using DFT-PBE0(+D3) and SAPT0 methods. These interactions are probed by AIM electron density topography and non-covalent interactions index (NCI) analyses. We suggest that the excess proton plays a crucial role in the stability of these model interfaces through strong inductive effects and the formation of Eigen or Zundel features. Also, computations reveal that the extension of the π-aromatic system and the increase of the number of water molecules in the H-bounded water network led to a strengthening of the interactions between the corresponding aromatic compound and protonated water molecules, except when a Zundel ion is formed. The present findings may serve to understand in-depth the role of proton localized at aqueous medium interacting with large aromatic surfaces such as graphene interacting with acidic liquid water. Besides, we give the IR and UV-Vis spectra of these complexes, which may help for their identification in laboratory.     

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: 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: Rare Spin Avoided σ-σ Diradical Planar Tetracoordinate Boron Cluster: A Proto-Star for Planar Pentacoordination (ChemPhysChem 18/2023)

We report a global planar star-like cluster B₃Li₃ featuring three planar tetracoordinate boron centres with a rare spin avoided σ-σ diradical character. The cluster was found to be stable towards dissociation into different fragments. The spin density was found to be localized solely on the three boron atoms in the molecular plane. This spin avoided σ-σ diradical character leads to the extension of the coordination number to yield a neutral B₃Li₃H₃ and a cationic B₃Li₃H₃⁺ cluster with three planar pentacoordinate boron centres in their global minimum structures. The planar geometry of the aninonic B₃Li₃H₃⁻ cluster is slightly higher in energy. The planar global clusters were found to maintain planarity in their ligand protected benzene bound complexes, B₃Li₃(Bz)₃, B₃Li₃H₃(Bz)₃ and B₃Li₃H₃(Bz)₃⁺ with high ligand dissociation energies offering candidature for experimental detection.     

Cover Feature: Effects of Microhydration on the Mechanisms of Hydrolysis and Cl− Substitution in Reactions of N2O5 and Seawater (ChemPhysChem 5/2023)

The reaction of N₂O₅ at atmospheric interfaces has recently received considerable attention due to its importance in atmospheric chemistry. N₂O₅ reacts preferentially with Cl⁻ to form ClNO₂/NO₃⁻ (Cl⁻ substitution), but can also react with H₂O to form 2HNO₃ (hydrolysis). In this paper, we explore these competing reactions in a theoretical study of the clusters N₂O₅/Cl⁻/nH₂O (n=2–5), resulting in the identification of three reaction motifs. First, we uncovered an S_N2-type Cl⁻ substitution reaction of N₂O₅ that occurs very quickly due to low barriers to reaction. Second, we found a low-lying pathway to hydrolysis via a ClNO₂ intermediate (two-step hydrolysis). Finally, we found a direct hydrolysis pathway where H₂O attacks N₂O₅ (one-step hydrolysis). We find that Cl⁻ substitution is the fastest reaction in every cluster. Between one-step and two-step hydrolysis, we find that one-step hydrolysis barriers are lower, making two-step hydrolysis (via ClNO₂ intermediate) likely only when concentrations of Cl⁻ are high.     

Front Cover: Synthesis of Substituted Indazole Acetic Acids by N−N Bond Forming Reactions (Eur. J. Org. Chem. 29/2023)

Herein, we report on the discovery and development of novel cascade N−N bond forming reactions for the synthesis of rare indazole acetic acid scaffolds. This approach allows for convenient synthesis of three distinct indazole acetic acid derivatives (unsubstituted, hydroxy, and alkoxy) by heating 3-amino-3-(2-nitroaryl)propanoic acids with an appropriate nucleophile/solvent under basic conditions. The reaction tolerates a range of functional groups and electronic effects and, in total, 23 novel indazole acetic acids were synthesized and characterized. This work offers a valuable strategy for the synthesis of useful scaffolds for drug discovery programs.     

Front Cover: Safe Entry to Ag(I)/Ag(III) Mixed Valence Salts Containing the Homoleptic Ag(III) Anion [Ag(CF3)4]− (Eur. J. Inorg. Chem. 19/2023)

The synthesis of Naumann's Ag^I/Ag^III mixed valence salt [Ag^I]⁺[Ag^III(CF₃)₄]⁻ ( Ag-1 ) is revisited. Ag-1 is now safely available in half gram scale upon 2e⁻ oxidation of AgF in presence of CF₃SiMe₃ and ambient air. In addition to its unprecedented crystallographic characterization, the use of Ag-1 to build the novel Ag^I/Ag^III salts [ Ag (bpy)₂] -1 , [ Ag (18-crown-6)₂] -1 , [ Ag -crypt-222] -1 and [ Ag (PCy₃)₂] -1 is herein reported, alongside their characterization by NMR, single crystal X-ray diffraction (Sc-XRD) and elemental analysis (EA). The utility of the currently affordable Ag-1 in gold(I) catalysis was demonstrated by the excellent catalytic activity displayed by [{ Au (PPh₃)}₂(μ-Cl)] -1 and [ Au (PPh₃)] -1 in the 5-exo-dig cyclization of N-propargylbenzamide ( 2 ). These cationic Au^I catalysts are accessible from (PPh₃)AuCl and Ag-1 , and outperform the activity of the well-known benchmark catalyst (PPh₃)AuNTf₂.