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

The study of the interactions of the three urea-based receptors AH, BH+ and CH2+ 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 center dot center dot center dot X-, with all anions, including CH3COO- and F-, in which moderate proton transfer from N-H to the anion takes place. The strongly acidic, dicationic receptor CH2+ forms, with most anions, complexes in which two inequivalent hydrogen bonds are present: one involving moderate proton transfer (N-H center dot center dot center dot X-) and one in which advanced proton transfer has taken place, described as N-center dot center dot center dot 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 CH3COO- and F-, and complexes with two equivalent hydrogen bonds (moderate proton transfer) with all the other anions. Moreover, [B center dot center dot center dot HF] and [C center dot center dot center dot HF](+), on addition of a second F- ion, lose the bound HF molecule to give HF2-. Release of CH3COOH, with the formation of [CH3COOH center dot center dot center dot CH3COO](-), also takes place with the [B center dot center dot center dot CH3COOH] complex in the presence of a large excess of anion.