Moderate and Advanced Intramolecular Proton Transfer in Urea–Anion Hydrogen‐Bonded Complexes

Abstract

The study of the interactions of the three urea-based receptors AH, BH(+) and CH(2+) with a variety of anions, in MeCN, has made it possible to verify the current view that hydrogen bonding is frozen proton transfer from the donor (the urea N-H fragment in this case) to the acceptor (the anion X(-)). The poorly acidic, neutral receptor AH establishes two equivalent hydrogen bonds N-H···X(-), with all anions, including CH(3)COO(-) and F(-), in which moderate proton transfer from N-H to the anion takes place. The strongly acidic, dicationic receptor CH(2+) forms, with most anions, complexes in which two inequivalent hydrogen bonds are present: one involving moderate proton transfer (N-H···X(-)) and one in which advanced proton transfer has taken place, described as N(-)···H-X. The degree of proton advancement is directly related to the basic tendencies of the anion. The cationic receptor BH(+) of intermediate acidic properties only forms complexes with two inequivalent hydrogen bonds (moderate+advanced proton transfer) with CH(3)COO(-) and F(-), and complexes with two equivalent hydrogen bonds (moderate proton transfer) with all the other anions. Moreover, [B···HF] and [C···HF](+), on addition of a second F(-) ion, lose the bound HF molecule to give HF(2)(-). Release of CH(3)COOH, with the formation of [CH(3)COOH···CH(3)COO](-), also takes place with the [B···CH(3)COOH] complex in the presence of a large excess of anion.

Site-Specific Description of the Enhanced Recognition Between Electrogenerated Nitrobenzene Anions and Dihomooxacalix[4]arene Bidentate Ureas

“…Concerning the anion properties, the partial charge of the atoms involved in the binding process is referred as the most significant factor. 22 Therefore, it becomes necessary to address the influence of particular properties for both host and guest structures, in order to systematically describe the strength of anion binding. For this purpose, reactivity indexes derived from Density Functional Theory (DFT) have been used for evaluating such effects at global and local scales, 23,24 based on the concept of electrophilicity, expressed by 25,26 …”

Naphthalimide‐Containing Isomeric Urea Derivatives: Mechanoluminescence and Fluoride Recognition

“…Next, we sought to explore the mechanism of interaction of ND‐2U with F − ions, that is, involving hydrogen bonding and possible deprotonation of the urea NH group. Under appropriate conditions, the F − ions were known to deprotonate urea and/or amide NH groups, analogous to HO − ions . To validate this point, we titrated ND‐2U , ND‐3U and ND‐4U with HO − ions in dimethylsulfoxide and monitored the changes in the emission spectra.…”

Anions Stabilize Each Other inside Macrocyclic Hosts

“…Nevertheless, there are no direct observations of anion dimers in solution. With good reason, the authors of those works, Reinhoudt, Kubo, Fabbrizzi, Sessler, and Tomisič, took care to circumscribe these claims with caveats such as “tentatively assigned”. The idea that anions can be bound to each other in solution in a complex has yet to be substantiated.…”