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 gr oup P2(1)/c with the cell dimensions at 253 K, a = 15.8812(2) Angstrom, bet a =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 r efined to R-1 = 0.0177 for 1727 reflections with F-0 > 4 sigma (F-0). In th e crystalline state the two carboxylic groups are cis to each other forming dimers with four fairly parallel and linear hydrogen bonds with O . . .O d istances ranging from 2.628 to 2.702 Angstrom. The two Se-C-C bond angles a re 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-det ermination of the crystal structure of S(CH2C(O)OH)(2) at 253 K gave the fo llowing 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 calculation s based upon density functional theory suggest that the energy difference b etween 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 differen ces in weak intermolecular forces, rather than a preference of the Se atom to make intramolecular non-bonding interactions with hydroxy O atoms. (C) 2 000 Elsevier Science B.V. All rights reserved.