Selenium carboxylic acids betaine; 3,3′,3″-selenotris(propanoic acid) betaine, Se(CH 2 CH 2 COOH) 2 (CH 2 CH 2 COO)

Attempts to prepare [Se(CH₂CH₂COOH)₃]⁺Cl⁻ from Se(CH₂CH₂COOH)₂ and H₂C=CHCOOH in concentrated hydrochloricacid, for the corresponding sulfonium salt, led exclusively to the Se-betaine,Se(CH₂CH₂COOH)₂(CH₂CH₂COO). The Se-betaine crystallises in the space group P2_l/c with the cell dimensions at 223 K, a = 5.5717(1), b = 24.6358(4), c = 8.4361(1) Å, β = 104.762(1)°, V = 1119.74(3) Å3, Z = 4, D_calc = 1.763 Mgm^{− 3},μ = 3.364 Mm⁻¹. The structure refined to R_I = 0.0223 for 2801 reflections with F₀ > 4σ(F₀). In the crystalline state the molecule is intermolecularly linked to neighbouring molecules by a number of hydrogen bonds; a very strong carboxylic-carboxylate bond with an O⋯O distance of 2.4435(16) Å, a medium strong carboxylic-carboxylate bond with an O⋯O distance of 2.6431(16) Å and several weak O⋯H(CH₂) with O⋯C distances between 3.2 and 3.3 Å. In the carboxylicgroup involved in the very strong hydrogen bond the O⋯H bond is antiperiplanar to the C O bond while the OH bond is periplanar to the C=O bond in the second carboxylic group. Based upon the C-O bond lengths and theelongation of the O-H bond involved in the strong hydrogen bond one may describe the compound as strongly linked units of Se(CH₂CH₂COOH)(CH₂CH₂COO)₂ rather than Se(CH₂CH₂COOH)₂(CH₂CH₂COO). The selenium atom forms two strong intramolecular 1,5-Se⋯O contacts, with a carboxylate oxygen atom, 2.9385(12) Å, and with a carboxylicoxygen atom, 2.8979(11) Å. To allow for these contacts the two organic fragments have been forced into the periplanar conformation. The molecule is only slightly asymmetric with regard to the C-Se-C bond angles but is very asymmetric with regard to the torsion angles.