“Cation‐π Interactions” Detected by Mass Spectrometry; Selective Recognition of Alkali Metal Cations by a π‐Basic Molecular Cavity

Abstract

A large, soft Cs+ ion nests in the cavity of [1.1.1]‐paracyclophane (1), giving rise to a stable [1 · Cs]+ complex (space‐filling model on the right), which can be detected in the gas phase by mass spectrometry. Crucial to the formation of this complex is the interaction of the Cs+ ion with all four arene rings. The somewhat smaller Rb+ ion also forms a stable complex with 1, but Li+, Na+, and K+ are too small to interact effectively with the cavity of 1.

Exclusive π Encapsulation of Light Alkali Metal Cations by a Neutral Molecule

“…17 ppm) were observed. However,a ll attempts to isolate the compounds as solids led to crystallization of the alkali metal salt M[BAr 23 Na MAS NMR spectrum of 2b showed ac entral resonance of as econd-order quadruploar powder pattern, which could be fitted to two nearly coincident and equal-intensity signals (d iso = À8.7, À10.3 ppm). Thet wo signals of equal intensity in each of these spectra can be attributed to inequivalent cation sites in 2a and 2b.…”

The π Complexation of Calcium inside themeso-Octaethylporphyrinogen Tetraanion Cavity

“…A limited number of examples comes from the transition metal calix [4]arene chemistry, [4,5] where the transition metal shapes the cavity, thus incapsulating alkali metal cations, even though the alkali metal cation ± oxygen interaction remains by far the strongest one. The macrocyclic tetraanion derived from meso-octaalkylporphyrinogen, owing to the presence of meso sp 3 -hybridized carbon atoms, is particularly suited to arrange the pyrrolyl anions to function as h 3 or h 5 binding sites.…”

Probing alkali metal–π interactions with the side chain residue of tryptophan

“…Most of the experimental evidence that is available falls into three categories. These are computational experiments (2-10), mass spectrometric studies (11)(12)(13)(14)(15), and solid-state structure analyses of charged species. Examples of the latter abound and date from the 1960s (16).…”