2,2′-selenobis(acetic acid), Se(CH2C(O)OH)2:: an old compound with a novel structure

2,2'-Selenobis(acetic acid), Se(CH2C(O)OH)(2), crystallises in the space group P2(1)/c with the cell dimensions at 253 K, a = 15.8812(2) Angstrom, beta =94.908(1)degrees, V = 629.95(2) Angstrom (3), Z = 4, D-calc = 2.078 Mg m(-3), lambda (MoKalpha) 0.71073 Angstrom, mu = 5.90 mm(-1). The structure refined to R-1 = 0.0177 for 1727 reflections with F-0 > 4 sigma (F-0). In the crystalline state the two carboxylic groups are cis to each other forming dimers with four fairly parallel and linear hydrogen bonds with O . . .O distances ranging from 2.628 to 2.702 Angstrom. The two Se-C-C bond angles are distinctly different, 110.2(1) and 116.4(1)degrees. The selenium atom is surrounded by an intramolecular O (OH) atom, the Se . . .O distance being 3.081 A, and by two inter molecular Se-atoms, both 3.752 Angstrom. A re-determination of the crystal structure of S(CH2C(O)OH)(2) at 253 K gave the following parameters: a = 5.0572(1), b = 17.7896(2), c = 6.6726(1) Angstrom, V = 600.30(2) Angstrom (3), Z = 4, D-calc = 1.661 Mg m(-3), in space group Pnma. The structure was solved as for the Se-compound and previous results by Paul [Acta Cryst. 23 (1967) 491] were confirmed. Theoretical calculations based upon density functional theory suggest that the energy difference between a closed dimer as observed for Se(CH2C(O)OH)(2), and a linear chain as observed for S(CH2C(O)OH)(2), is small. This indicates that the distinct differences observed between these structures may be due to minor differences in weak intermolecular forces, rather than a preference of the Se atom to make intramolecular non-bonding interactions with hydroxy O atoms. (C) 2000 Elsevier Science B.V. All rights reserved.