Sulfate-induced large amplitude conformational change in a Solomon link.

A doubly-interlocked [2]catenane - or Solomon link - undergoes a complex conformational change upon addition of sulfate in methanol. This transformation generates a single pocket where two SO42- anions bind through multiple hydrogen bonds and electrostatic interactions. Despite the close proximity of the two anions, binding is highly cooperative.

Stacking Interactions: A Supramolecular Approach to Upgrade Weak Halogen Bond Donors.

The co-crystallization of tetracyanobenzene (TCB) with haloarenes ArX provided six new co-crystals TCB ⋅ ArX (ArX=PhCl, PhBr, 4-MeC6 H4 Cl, 4-MeC6 H4 Br, 4-MeOC6 H4 Cl, 1,2-Br2 C6 H4 ) which were studied by X-ray diffraction. In these systems, the strong collective effect of π⋅⋅⋅π stacking interactions and lone pair-(X)⋅⋅⋅π-hole-(C) bondings between TCB and ArX promote the strength of X⋅⋅⋅Ncyano halogen bonding (HaB). Theoretical studies showed that the stacking interactions affect the σ-hole depth of the haloarenes, thus significantly boosting their ability to function as HaB donors. According to the molecular electrostatic potential calculations, the σ- hole-(Cl) value (1.5 kcal/mol) in the haloarene 4-MeOC6 H4 Cl (featuring an electron-rich arene moiety and exhibiting very poor σ-hole-(Cl) ability) increases significantly in the stacked trimer (TCB)2 ⋅ 4-MeOC6 H4 Cl (12.5 kcal/mol). Theoretical DFT calculations demonstrate the dramatic increase of X⋅⋅⋅Ncyano HaB strength for stacked trimers in comparison with parent unstacked haloarenes.

Host-guest complexes vs. supramolecular polymers in chalcogen bonding receptors: an experimental and theoretical study.

We describe here a comparative study between two tripodal anion receptors based on selenophene as the binding motif. The receptors use benzene or perfluorobenzene as a spacer. The presence of the electron-withdrawing ring activates the selenium atom for anion recognition inducing the formation of self-assembled supramolecular structures in the presence of chloride or bromide anions, which are bonded by the cooperative action of hydrogen and chalcogen bonding interactions. DOSY NMR and DLS experiments provided evidence for the formation of the supramolecular structures only in the presence of a perfluorobenzene based anion receptor while the analogous benzene one shows the classical anion/receptor complex without the participation of the selenium atom. The energetic and geometric features of the complexes of both receptors with the Cl and Br anions have been studied in solution. These results combined with the molecular electrostatic potential (MEP) surface plots allow us to rationalize the quite different behaviors of both receptors observed experimentally.

Anion-π Catalysis Enabled by the Mechanical Bond.

We report a series of rotaxane-based anion-π catalysts in which the mechanical bond between a bipyridine macrocycle and an axle containing an NDI unit is intrinsic to the activity observed, including a [3]rotaxane that catalyses an otherwise disfavoured Michael addition in >60 fold selectivity over a competing decarboxylation pathway that dominates under Brønsted base conditions. The results are rationalized by detailed experimental investigations, electrochemical and computational analysis.

Se⋠⋠⋠O/S and S⋠⋠⋠O Chalcogen Bonds in Small Molecules and Proteins: A Combined CSD and PDB Study.

The importance of selenium-centered noncovalent chalcogen bonds represented as Se⋅⋅⋅A (A=O/S) has been explored for short directional contacts in small molecules and proteins. In addition, S⋅⋅⋅O centered contacts have been analyzed. Computational analyses involving the quantitative assessment of the associated energetics, the molecular electrostatic potentials (MEP), and electron density derived topological parameters, namely, quantum theory of atom in molecules (QTAIM) analyses, and NBO (natural bond orbital) based calculations, have been performed to unequivocally establish the strength, stability, and attractive role of chalcogen bonds in the solid-state. This investigation has been performed in molecules from both the Cambridge Structural Database (CSD) and Protein Data Bank (PDB). Thus futuristic materials may be designed keeping in mind the significance of these interactions, including their relevance in biology.

Diastereoselective Amplification of a Mechanically Chiral [2]Catenane.

Achiral [2]catenanes composed of rings with inequivalent sides may adopt chiral co-conformations. Their stereochemistry depends on the relative orientation of the interlocked rings and can be controlled by sterics or an external stimulus (e.g., a chemical stimulus). Herein, we have exploited this stereodynamic property to amplify a mechanically chiral (P)-catenane upon binding to (R)-1,1'-binaphthyl 2,2'-disulfonate, with a diastereomeric excess of 85%. The chirality of the [2]catenane was ascertained in the solid state by single crystal X-ray diffraction and in solution by NMR and CD spectroscopies. This study establishes a robust basis for the development of a new synthetic approach to access enantioenriched mechanically chiral [2]catenanes.

Pnictogen-bonding catalysis: brevetoxin-type polyether cyclizations.

Pnictogen-bond donors are attractive for use in catalysis because of deep σ holes, high multivalency, rich hypervalency, and chiral binding pockets. We here report natural product inspired epoxide-opening polyether cyclizations catalyzed by fluoroarylated Sb(v) > Sb(iii) > Bi > Sn > Ge. The distinctive characteristic found for pnictogen-bonding catalysis is the breaking of the Baldwin rules, that is selective endo cyclization into the trans-fused ladder oligomers known from the brevetoxins. Moreover, tris(3,4,5-trifluorophenyl)stibines and their hypervalent stiborane catecholates afford different anti-Baldwin stereoselectivity. Lewis (SbCl3), Brønsted (AcOH) and π acids fail to provide similar access to these forbidden rings. Like hydrogen-bonding catalysis differs from Brønsted acid catalysis, pnictogen-bonding catalysis thus emerges as the supramolecular counterpart of covalent Lewis acid catalysis.

Unravelling the electronic nature of C-FO-C non-covalent interaction in proteins and small molecules in the solid state.

The participation of organic fluorine as a halogen bond donor is rare and is sensitive to the electronic environment in the vicinity of the fluorine atom. The enhancement in the electropositive character (the σ-hole formalism) in fluorine is established by the presence of electron withdrawing groups and this has been examined in the solid-state structures in small molecules and proteins. Short, directional FO contacts have been observed and physical insights obtained, from quantum mechanical calculations, via the molecular electrostatic potential, an analysis of their topological features from atoms-in-molecules, and donor-acceptor characteristics from natural bond orbital analyses. It was observed that such contacts, cooperatively act in the presence of other interactions, and the formed aggregates are stabilizing in nature. In addition, the FO has a bonding character and is attractive in nature. The halogen bonding character of fluorine is relevant in supramolecular chemistry.

Radicalradical chalcogen bonds: CSD analysis and DFT calculations.

This manuscript reports a combination of crystallographic analysis (Cambridge Structural Database) and theoretical DFT calculations in chalcogen bonding interactions involving radicals in both the Ch bond (ChB) donor and acceptor. As a radical ChB acceptor (nucleophile) we have used benzodithiazolyl radical (BDTA) and as Ch bond donors (electrophile) we have used dithiadiazolyl and diselenadiazolyl radicals of the general formula p-X-C6F4-CNChChN (Ch = S, and Se). We have evaluated how the para substituent (X) affects the interaction energy, spin density and charge/spin transfer from the electron rich BDTA radical to the electron poor dichalcogenadiazolyl ring. The ability of the latter rings to form ChBs in the solid state has been examined by a comprehensive search in the CSD; several cases are used to exemplify the preferred geometric features of the complexes and they are compared with the theory. The molecular surface electrostatic potentials calculated for these ChB donors allow for a very precise rationalization of the self-assembly motifs most frequently adopted in the crystalline state and of their relative robustness.

Anion Recognition by Neutral Chalcogen Bonding Receptors: Experimental and Theoretical Investigation.

Invited for the cover of this issue are the groups of Antonio Frontera at the Universitat de les Illes Balears and Antonio Caballero at the Universidad de Murcia. The image depicts one of the tellurophene based receptors described in this work. Read the full text of the article at 10.1002/chem.201905786.

Anion Recognition by Neutral Chalcogen Bonding Receptors: Experimental and Theoretical Investigations.

The utilization of neutral receptors for the molecular recognition of anions based on chalcogen bonding (ChB) is an undeveloped area of host-guest chemistry. In this manuscript, the synthesis of two new families of sulfur, selenium, and tellurium-based ChB binding motifs are reported. The stability of the thiophene, selenophene, and tellurophene binding motifs has enabled the determination of the association constants for ChB halide anion binding in the polar aprotic solvent THF by 1 H, 77 Se, and 125 Te NMR experiments. Two different aromatic cores are used and one or two Ch-binding motifs are incorporated with the purpose of encapsulating the anion, offering up to two concurrent chalcogen bonds. Theoretical calculations and NMR experiments reveal that, for S and Se receptors, hydrogen-bonding interactions involving the acidic H atom adjacent to the chalcogen atom are energetically favored over the ChB interaction. However, for the tellurophene binding motif, the σ-hole interaction is competitive and more favored than the hydrogen bond.

Charge-Assisted Chalcogen Bonds: CSD and DFT Analyses and Biological Implication in Glucosidase Inhibitors.

This study reports a combined Cambridge Structural Database and theoretical DFT study of charge assisted chalcogen bonds involving sulfonium, selenonium, and telluronium cations. The chalcogen bond has been recently defined by IUPAC as the net attractive interaction between an electrophilic region associated with a chalcogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. Divalent chalcogen atoms typically have up to two σ-holes and forms up to two ChBs; the same holds for tetravalent chalcogens which adopt a seesaw arrangement. In sulfonium, selenonium, and telluronium salts chalcogen atoms form three covalent bonds, three σ-holes are located opposite to these bonds, and up to three charge assisted ChBs can be formed between these holes and the counterions. The covalent bond arrangement around these chalcogen atoms is similar to trivalent pnictogen atoms and translates into a similar pattern of noncovalent interactions. We have found and studied this type of charge-assisted chalcogen bonds in various sulfonium ion-containing inhibitors of glucosidase, for example, salacinol and kotalanol.

Surface Modification of Pseudoboehmite-Coated Aluminum Plates with Squaramic Acid Amphiphiles.

The functionalization of interfaces has become very important for the protection or modification of metal (metal oxides) surfaces. The functionalization of aluminum is particularly interesting because of its relevance in fabricating components for electronic devices. In this work, the utilization of squaramic acids for the functionalization of aluminum substrates is reported for the first time. The physicochemical properties of the interfaces rendered by n-alkyl squaramic acids on aluminum metal substrates coated with pseudoboehmite [Al(O)x(OH)y] layers are characterized by contact angle, grazing-angle Fourier-transform infrared spectroscopy, atomic force microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and matrix-assisted laser desorption ionization time-of-flight. Moreover, we could confirm the squaramic functionalization of the substrates by diffuse reflectance UV-vis spectroscopy, which cannot be used for the characterization of UV-vis-inactive substrates such as carboxylates and phosphonates, commonly used for coating metallic surfaces. Remarkably, the results of sorption experiments indicate that long-chain alkyl squaramic acid desorbs from activated-aluminum substrates at a reduced rate compared to palmitic acid, a carboxylic acid frequently used for the functionalization of metal oxide surfaces. Theoretical calculations indicate that the improved anchoring properties of squaramic acids over carboxylates are probably due to the formation of additional hydrogen bonding interactions on the interface. Accordingly, we propose N-alkyl squaramic acids as new moieties for efficient functionalization of metal oxides.

Nitropyridine-1-Oxides as Excellent π-Hole Donors: Interplay between σ-Hole (Halogen, Hydrogen, Triel, and Coordination Bonds) and π-Hole Interactions.

In this manuscript, we use the primary source of geometrical information, i.e., Cambridge Structural Database (CSD), combined with density functional theory (DFT) calculations (PBE0-D3/def2-TZVP level of theory) to demonstrate the relevance of π-hole interactions in para-nitro substituted pyridine-1-oxides. More importantly, we show that the molecular electrostatic potential (MEP) value above and below the π-hole of the nitro group is largely influenced by the participation of the N-oxide group in several interactions like hydrogen-bonding (HB) halogen-bonding (XB), triel bonding (TrB), and finally, coordination-bonding (CB) (N+-O- coordinated to a transition metal). The CSD search discloses that p-nitro-pyridine-1-oxide derivatives have a strong propensity to participate in π-hole interactions via the nitro group and, concurrently, N-oxide group participates in a series of interactions as electron donor. Remarkably, the DFT calculations show from strong to moderate cooperativity effects between π-hole and HB/XB/TrB/CB interactions (σ-bonding). The synergistic effects between π-hole and σ-hole bonding interactions are studied in terms of cooperativity energies, using MEP surface analysis and the Bader's quantum theory of atoms in molecules (QTAIM).

Theoretical ab Initio Study on Cooperativity Effects between Nitro π-hole and Halogen Bonding Interactions.

This article analyzes the interplay between nitro's π-hole and halogen-bonding (XB) interactions in nitroarenes. Remarkable cooperativity effects are observed when π-hole and XB interactions coexist in the same complex. The nitroarene presents two π-holes, one approximately over the N atom of the nitro group and the other over the aromatic ring, being the former more positive. The interplay between both interactions has been analyzed in terms of energetic and geometric features of the complexes, which are computed at the RI-MP2/def2-TZVPD level of theory. Molecular electrostatic potential (MEP) surface calculations have been used to explore the variation of the MEP values at the π-hole upon the formation of halogen bonding interactions between the nitroarene and CF3 X (X=Cl, Br and I) molecules. In addition, the Bader's theory of atoms in molecules" (AIM) is used to characterize the interactions by means of the distribution of bond critical points and bond paths and to analyze their strengthening or weakening depending upon the variation of charge density at critical points. The aforementioned computational methods are adequate to examine how these interactions mutually influence each other. Natural bond orbital (NBO) and noncovalent interaction plot (NCIPlot) computational tools have been also used in some representative complexes to further analyze cooperativity effects. Finally, the Cambridge Structural Database (CSD) is used to provide some experimental evidence.