Exploring the Structure and Complexation Dynamics of Azide Anion Recognition by Calix[4]pyrroles in Solution

Abstract

The structure and anion recognition dynamics between calix[4]pyrroles and azide (N 3 − ) anions in the form of its TBA + and Na + salts were investigated in dimethyl sulfoxide solutions by Fourier transform infrared (FTIR) spectroscopy and ultrafast IR spectroscopy. Vibrational energy redistribution of the N 3 − anion in the complex is accelerated through hydrogen bonding interactions with the N−H proton of the receptor. Rotational dynamics of the bound N 3 − is greatly restricted, demonstrating a distinct countercation effect. The detailed binding modes of N 3 − with the receptor were further evaluated by the density functional theoretical (DFT) calculations and nuclear magnetic resonance (NMR) spectroscopy. All of these measurements support the notion that the calix [4]pyrroles are capable of capturing the azide anion in solution. However, the calix[4]pyrroles may not necessarily undergo a conformational change to a conelike geometry when they bind to the azide anion in the solution.

Anion Recognition in Solution: Insights from Thermodynamics and Ultrafast Structural Dynamics

“…To further substantiate this hypothesis, we conducted temperature-dependent rotational anisotropy measurements using sodium azide (NaN 3 ) as the guest molecules, and the results are illustrated in Figure S9. Remarkably, the activation energy for N 3 – orientational relaxation was determined to be 8.5 ± 1.0 kJ mol −1 , a value that aligns well with the free Gibbs energy change (Δ G ) associated with the complexation process, despite differences in their signs. To the best of our knowledge, our findings provide strong evidence that the transition state of anion recognition involves structural rearrangement and is mediated by hydrogen bond switching.…”

“…Notably, the pyrrole NH proton signal (H a ) experiences a significant downfield shift upon the addition of NaOCN. Additionally, the H a peak undergoes broadening and flattening to a certain degree, providing compelling evidence of OCN – interaction with the NH groups within the macrocyclic receptors . NMR titration measurements demonstrate a gradual shift of H a from 9.26 to 10.8 as the addition of NaOCN varies from 1.0 to 5.0 equiv, further indicating that the conformation of calix[4]­pyrrole should be affected by the presence of NaOCN (refer to the Supporting Information, Figure S1).…”

“…Additionally, the H a peak undergoes broadening and flattening to a certain degree, providing compelling evidence of OCN − interaction with the NH groups within the macrocyclic receptors. 31 NMR titration measurements demonstrate a gradual shift of H a from 9.26 to 10.8 as the addition of NaOCN varies from 1.0 to 5.0 equiv, further indicating that the conformation of calix [4]pyrrole should be affected by the presence of NaOCN (refer to the Supporting Information, Figure S1). Conversely, the pyrrolic proton signal (H b ) and the methyl group protons (H c ) in the host molecule exhibit only slight shifts.…”

Distinct Hydrogen Bonding Dynamics Underlies the Microheterogeneity in DMF–Water Mixtures

“…Vibrational spectroscopy is sensitive to the molecular structure and can be a valuable tool to report the local environments in the mixture solutions . Ultrafast two-dimensional infrared (2D IR) spectroscopy, a third-order nonlinear optical process, has been developed as a powerful technique to unravel the structure and dynamics in condensed systems with femtosecond (fs, 10 –15 s) time resolution. − In this article, the hydrogen bond dynamics of DMF–water mixtures over the entire composition range have been investigated by the combination of Fourier transform infrared (FT-IR) spectroscopy and ultrafast IR spectroscopy. To determine hydrogen bonding dynamics in binary solutions, the OD stretch and extrinsic probe of thiocyanate (SCN – ) were both chosen as local vibrational reporters.…”

Recent developments in calix[4]pyrrole (C4P)-based supramolecular functional systems

“…[1][2][3][4][5][6][7][8] Among the numerous supramolecular receptors, 9,10 the non-aromatic tetrapyrrolic synthetic macrocyclic receptor (C4P) has emerged as significant receptor and ligand for diverse purposes owing to its structural variety, easy one-pot synthesis, and recognition ability towards anions, ion-pairs, neutral molecules, and metal ions of fundamental importance. [11][12][13][14][15][16][17][18][19][20][21][22][23] Adolf von Baeyer (1835-1917, a German Chemist, Noble Prize in Chemistry, 1905) is credited for the discovery of the parent C4P in 1886 through the acid-mediated condensation of pyrrole with acetone in a single step (Fig. 1).…”