Electronically tunable anion−π interactions in pyrylium complexes: experimental and theoretical studies

Abstract

Noncovalent interactions of anions with electron-deficient aromatic rings that have been studied so far involve non-heteroaromatic or nitrogen-based heteroaromatic systems. Here we report the first case of an organic oxygenated aromatic system, in particular the tri-aryl-pyrylium tetrafluoroborate system, for which noncovalent anion-π interactions of the pyrylium cation with the tetrafluoroborate anion have been experimentally detected and demonstrated by means of (19)F NMR spectroscopy in solution. A series of pyrylium tetrafluoroborate salts were synthesized in the presence of BF3·Et2O, by direct reaction of 4-substituted benzaldehydes with 4-substituted acetophenones or via the previously obtained chalcone of the less reactive ketone. Correlations of (19)F NMR chemical shifts of tetrafluoroborate anion for most of the synthesized tri-arylpyrylium tetrafluoroborate complexes with both the pyrylium cation molecular weight and the standard substituent Hammett constants, demonstrate anion-π(+) interaction to act between the polyatomic anion BF4(-) and the pyrylium aromatic system. DFT calculations reveal that an additional (C-H)(+)-anion hydrogen bond involving the H(5) of pyrylium ring exists for these fluorescent dyes that show a tunable cup-to-cap shape cavity. The strong fluorescence emission observed for some representative pyrylium tetrafluoroborates described herein, makes them a promising class of tunable emission wavelength dyes for laser technology applications.

Fingerprinting the Nature of Anions in Pyrylium Complexes: Dual Binding Mode for Anion–π Interactions

“…Thus, the dihedral angles ( φ ) between the pyrylium core and the phenyl rings at the C‐2 and C‐6 positions are −40.0 and 43.7°, respectively. Taking into account the structure of the monomer (Figure S8b), these values are far higher than for previous pyrylium salts, but far lower than pyridinium derivatives . Theoretical calculations for compound 15 revealed a stabilizing dual anion–π interaction (Δ E bind =−205.5 kcal mol −1 ) with both pyrylium moieties in which two CN groups point towards both pyrylium centroids (Figure ).…”

“…These systems have the same cation (R 1 =OMe, R 2 =R 3 =F), which was electronically characterized by a Hammett constant (Σ σ p ) value of −0.15. Therefore, these substituents demonstrate a moderate dependence of the Q zz value compared with other pyrylium salts synthesized previously …”

“…Pyrylium salts, six‐membered heterocycles with aromatic character that have a charged oxygen atom, are electronically modulated through the anion–π interaction . This type of cationic receptor molecule also exhibits electrostatic or charge‐transfer interactions.…”

“…[14] Pyrylium salts, six-membered heterocyclesw ith aromatic character that have ac harged oxygen atom, are electronically modulated through the anion-p interaction. [15,16] This type of cationic receptor molecule also exhibits electrostatic or chargetransfer interactions. As ac onsequence, an intense chargetransfer absorption band, corresponding to the electron transfer in the charge-transfer p complex, is observed in the visible region.…”

From Pyrylium to Pyridinium Salts: Understanding Physicochemical Features

“…5 The factors that govern the reactivity of these cations have been previously reported. 6 In this communication, we have focused on the design and synthesis of different 2,4,6-tri-arylpyridinium salts. Mechanistic considerations, acid and base catalyst or steric hindrance are some general requirements that have been explored.…”