Effects of Charge and Substituent on the S∙∙∙N Chalcogen Bond

Neutral complexes containing a S···N chalcogen bond are compared with similar systems in which a positive charge has been added to the S-containing electron acceptor, using high-level ab initio calculations. The effects on both XS···N and XS+···N bonds are evaluated for a range of different substituents X = CH3, CF3, NH2, NO2, OH, Cl, and F, using NH3 as the common electron donor. The binding energy of XMeS···NH3 varies between 2.3 and 4.3 kcal/mol, with the strongest interaction occurring for X = F. The binding is strengthened by a factor of 2–10 in charged XH2S+···NH3 complexes, reaching a maximum of 37 kcal/mol for X = F. The binding is weakened to some degree when the H atoms are replaced by methyl groups in XMe2S+···NH3. The source of the interaction in the charged systems, like their neutral counterparts, is derived from a charge transfer from the N lone pair into the σ*(SX) antibonding orbital, supplemented by a strong electrostatic and smaller dispersion component. The binding is also derived from small contributions from a CH···N H-bond involving the methyl groups, which is most notable in the weaker complexes

Host-Guest Interactions of Sodiumsulfonatomethyleneresorcinarene and Quaternary Ammonium Halides : An Experimental-Computational Analysis of the Guest Inclusion Properties

The molecular recognition of nine quaternary alkyl- and aryl-ammonium halides (Bn) by two different receptors, Calkyl-tetrasodiumsulfonatomethyleneresorcinarene (An), were studied in solution using 1H NMR spectroscopy. Substitution of methylenesulfonate groups at 2-positions of resorcinol units resulted in an increase of cavity depth by ∼2.80 Å and a narrow cavity aperture compared to Calkyl-2-H-resorcinarenes. The effect of alkyl chain lengths on the endo-complexation, that is the ability to incorporate other than N-methyl chains inside the cavities, were investigated using ammonium cations of the type ⁺NH2(R1)(R2), (R1 = Me, Et, Bu, R2 = Bu, Ph, Bz ). The C−H⋯ interactions between guests and hosts are the key driving forces for 14 out of 16 observed endo-complexes. In case of N-butyl-N-benzylammonium cation, the hydrogen bonding between -NH2 and sulfonate oxygens and the larger size hamper the N-butyl and N-benzyl groups from entering the host cavity. Association constants derived from isothermal calorimetry titrations confirm 1:1 host-guest complexes highlighting guest affinity, based on size and orientation. X-Ray crystallographic analysis revealed two types of complexes viz sodium-containing co-crystals, [(An)4⁻･m(Bn)⁺･qNa⁺], and sodium-free, [(An)4⁻･4(Bn)⁺]. Both types accommodate (Bn⁺) guests in their cavities. The N-methylated heterocycle guests and host form capsule-like structures in which the two-halves were joined by O−Na coordination bonds and self-assembles in to 2-D polymeric sheets. From the crystal structures, different conformations of methylenesulfonate groups with respect to cavity arising due to tetrahedral geometry of methylene linker were observed. Density Functional Theory (DFT) computations were used to analyze the effects of endo-guests on host conformations and to estimate the relative strengths of host-guest interactions.peerReviewe

Encapsulation of xenon by bridged resorcinarene cages with high ¹²⁹Xe NMR chemical shift and efficient exchange dynamics

Abstract Functionalized cages encapsulating xenon atoms enable highly sensitive, background-free molecular imaging through a technique known as HyperCEST ¹²⁹Xe MRI. Here, we introduce a class of potential biosensor cage structures based on two resorcinarene macrocycles bridged either by aliphatic carbon chains or piperazines. First-principles-based modeling predicts a high chemical shift (about 345 ppm) outside the typical experimental observation window for ¹²⁹Xe encapsulated by the aliphatically bridged cage and two ¹²⁹Xe resonances for the piperazine-bridged cages corresponding to single and double loading. Based on the computational predictions as well as ¹²⁹Xe chemical exchange saturation transfer (CEST) and T₂ relaxation nuclear magnetic resonance experiments, we confirm Xe encapsulation in the aliphatically bridged and double encapsulation in the piperazine-bridged resorcinarene in methanol. The cages show fast Xe exchange rates (12,000–49,000 s⁻¹), resulting in a high CEST response regardless of the relatively low binding constant (0.09–3 M⁻¹)

Supramolecular macrocycles reversibly assembled by Te…O chalcogen bonding

Organic molecules with heavy main-group elements frequently form supramolecular links to electron-rich centres. One particular case of such interactions is halogen bonding. Most studies of this phenomenon have been concerned with either dimers or infinitely extended structures (polymers and lattices) but well-defined cyclic structures remain elusive. Here we present oligomeric aggregates of heterocycles that are linked by chalcogen-centered interactions and behave as genuine macrocyclic species. The molecules of 3-methyl-5-phenyl-1,2-tellurazole 2-oxide assemble a variety of supramolecular aggregates that includes cyclic tetramers and hexamers, as well as a helical polymer. In all these aggregates, the building blocks are connected by Te(…)O–N bridges. Nuclear magnetic resonance spectroscopic experiments demonstrate that the two types of annular aggregates are persistent in solution. These self-assembled structures form coordination complexes with transition-metal ions, act as fullerene receptors and host small molecules in a crystal

Pseudopeptidic Cages as Receptors for N-Protected Dipeptides

The molecular recognition of short peptides is a challenge in supramolecular chemistry, and the use of peptide-like cage receptors represents a promising approach. Here we report the synthesis and characterization of a diverse family of pseudopeptidic macrobicycles, as well as their binding abilities toward N-protected dipeptides using a combination of different techniques (NMR, ESI-MS, and fluorescence spectroscopy). The cage hosts were assayed for dipeptide binding using competition ESI-MS experiments as high-throughput screening to obtain general trends for the recognition phenomena. Selected hosts were additionally studied by NMR spectroscopy (1H NMR titration and diffusion-ordered spectroscopy experiments) in different solvents. The results unambiguously demonstrated the formation of the [cage·dipeptide] supramolecular complexes and rendered quantitative information about the strength of the interaction (Kass). The structural variables within the pseudopeptidic cage framework that produced a stronger and more selective recognition were thus identified. The cages showed a remarkable selectivity for N-protected dipeptides with an aromatic amino acid at the carboxylic terminus, which prompted us to propose a mode of binding based on polar and nonpolar noncovalent interactions. Accordingly, we faced the molecular recognition of a target dipeptide (Ac-EY-OH) mimicking a biologically relevant sequence by NMR and fluorescence spectroscopy in highly competitive media