2,2′,2″-Thiotris(acetic acid) betaine, S(CH 2 COOH) 2 (CH 2 COO), and 2,2′,2″-selenotris(acetic acid) betaine, Se(CH 2 COOH) 2 (CH 2 COO)

TThe title compounds, X(CH₂COOH)₂(CH₂COO), X = S, 1, and X = Se, 2, have been characterised by FTIR, NMR and MS and by their crystal structures at 123(2) K. The FTIR spectra show two major peaks, at 1396 and 1731 cm⁻¹ in 1 and at 1390 and 1721 cm⁻¹ in 2. The ⁷⁷Se NMR signal of 2 at 325.5 ppm is 83.4 ppm downfield from the signal of Se(CH₂COOH)₂ indicating a substantial selenonium character of 2. The two compounds are isostructural and have a pyramidal configuration. The C—X—C bond angles range from 99.29 to 103.14° in 1 and from 97.56 to 99.87° in 2. The Xsingle bondCsingle bondCdouble bond; length as m-dashO torsion angles for the three substituents are most different; one of the carboxylic acid groups attains the anti-conformation with rather short S⋯O(H) and Se⋯O(H) distances, 2.744 and 2.750 Å, the other acid group is synclinal and with longer S⋯Odouble bond; length as m-dashC and Se⋯Odouble bond; length as m-dashC distances, 3.063 and 3.090 Å, whereas the carboxylate group is in the Xsingle bondCsingle bondC plane with X⋯Osingle bondC distances of 2.869 and 2.908 Å in 1 and 2. The presence of these strong X⋯O interactions is suggested to be the cause for the very low Bronsted basicity of this class of betaines preventing salts of the corresponding acids, the presently unknown [X(CH₂COOH)₃]⁺ - cations, to be isolated. The molecules are linked together with two fairly strong but different hydrogen bonds to the carboxylate oxygen atoms with O⋯O distances of 2.493 and 2.580 Å in 1 and 2.489 and 2.581 Å in 2 and with one X⋯Odouble bond; length as m-dashC contact, 3.244 Å in 1 and 3.209 Å in 2. The carbonyl oxygen atoms do not participate significantly in intermolecular hydrogen bonding and there are no contacts between the heteroatoms.