Anionen bindende Resorcinaren-Cavitanden: die Bedeutung von CH⋅⋅⋅Anion-Wechselwirkungen

Unusually Large Effects of Charge-assisted C-H⋅⋅⋅F Hydrogen Bonds to Anionic Fluorine in Organic Solvents: Computational Study of 19 F NMR Shifts versus Thermochemistry

A comparison of computed ¹⁹ F NMR chemical shifts and experiment provides evidence for large specific solvent effects for fluoride-type anions interacting with the σ*(C-H) orbitals in organic solvents like MeCN or CH₂ Cl₂ . We show this for systems ranging from the fluoride ion and the bifluoride ion [FHF]^- to polyhalogen anions [ClF_x ]^- . Discrepancies between computed and experimental shifts when using continuum solvent models like COSMO or force-field-based descriptions like the 3D-RISM-SCF model show specific orbital interactions that require a quantum-mechanical treatment of the solvent molecules. This is confirmed by orbital analyses of the shielding constants, while less negatively charged fluorine atoms (e. g., in [EF₄ ]^- ) do not require such quantum-mechanical treatments to achieve reasonable accuracy. The larger ¹⁹ F solvent shift of fluoride in MeCN compared to water is due to the larger coordination number in the former. These observations are due to unusually strong charge-assisted C-H⋅⋅⋅F^- hydrogen bonds, which manifest beyond some threshold negative natural charge on fluorine of ca. < -0.6 e. The interactions are accompanied by sizable free energies of solvation, in the order F^- ≫[FHF]^- >[ClF₂ ]^- >[ClF₄ ]^- . COSMO-RS solvation free energies tend to moderately underestimate those from the micro-solvated cluster treatment. Red-shifted and intense vibrational C-H stretching bands, potentially accessible in bulk solution, are further spectroscopic finger prints.

Simultaneous endo and exo Complex Formation of Pyridine[4]arene Dimers with Neutral and Anionic Guests

The formation of complexes between hexafluorophosphate (PF₆^- ) and tetraisobutyloctahydroxypyridine[4]arene has been thoroughly studied in the gas phase (ESI-QTOF-MS, IM-MS, DFT calculations), in the solid state (X-ray crystallography), and in chloroform solution (¹ H, ¹⁹ F, and DOSY NMR spectroscopy). In all states of matter, simultaneous endo complexation of solvent molecules and exo complexation of a PF₆^- anion within a pyridine[4]arene dimer was observed. While similar ternary complexes are often observed in the solid state, this is a unique example of such behavior in the gas phase.

Octulene: A Hyperbolic Molecular Belt that Binds Chloride Anions

Octulene, the higher homologue of kekulene and septulene, was synthesized using the fold-in method. This new hydrocarbon macrocycle contains a large 24-membered inner circuit, which is peripherally fused to 24 benzene rings. Such an arrangement produces considerable hyperbolic distortion of the π-conjugated surface. The consequences of distortion in octulene were explored using photophysical methods, which revealed a reduced electronic band gap and greater flexibility of the π system. Octulene contains a functional cavity with a diameter larger than 5.5 Å that is capable of efficiently binding the chloride anion in a nonpolar solvent (K_a = 2.2(4)×10⁴  m^-1 , 1 % dichloromethane (DCM) in benzene). The octulene-chloride interaction is stabilized by eight weak C(sp² )H⋅⋅⋅Cl bonds, providing the first example of a hydrocarbon-based anion receptor.

Cracking cavitands: metal-directed scission of phosphinyl-substituted resorcinarenes

Resorcinarene-derived tetramethylene cavitands bearing a diphenylphosphino group grafted to their wider rim undergo facile, directed C-O bond breaking upon reaction with transition-metal ions in the presence of nucleophiles. One possible reaction mechanism involves formation of a P,O-chelate complex, which weakens the adjacent O-CH2 bond, leading to the formation of an oxacarbenium intermediate.

Binding mechanisms in supramolecular complexes

Supramolecular chemistry has expanded dramatically in recent years both in terms of potential applications and in its relevance to analogous biological systems. The formation and function of supramolecular complexes occur through a multiplicity of often difficult to differentiate noncovalent forces. The aim of this Review is to describe the crucial interaction mechanisms in context, and thus classify the entire subject. In most cases, organic host-guest complexes have been selected as examples, but biologically relevant problems are also considered. An understanding and quantification of intermolecular interactions is of importance both for the rational planning of new supramolecular systems, including intelligent materials, as well as for developing new biologically active agents.