Ethane-1,2-diylbis(methylphosphinic acid)

In the title compound, C4H12O4P2, there are two crystallographically independent half-molecules in the asymmetric unit, both molecules lying on centres of symmetry. Each molecule is connected on both sides to neighbouring molecules via strong O—H⋯O hydrogen bonds. The –POOH groups accept and donate one hydrogen bond in interactions with the neighbouring –POOH group of the adjacent molecule, to give one-dimensional chains along [10]. As each phosphinic acid group donates and accepts one hydrogen bond, the connection between the molecules is best described by a ring motif which can be classified by the Etter symbol R 2 2(8).

In the title compound, C 4 H 12 O 4 P 2 , there are two crystallographically independent half-molecules in the asymmetric unit, both molecules lying on centres of symmetry. Each molecule is connected on both sides to neighbouring molecules via strong O-HÁ Á ÁO hydrogen bonds. The -POOH groups accept and donate one hydrogen bond in interactions with the neighbouring -POOH group of the adjacent molecule, to give one-dimensional chains along [101]. As each phosphinic acid group donates and accepts one hydrogen bond, the connection between the molecules is best described by a ring motif which can be classified by the Etter symbol R 2 2 (8).

Comment
The title compound, [HOO(CH 3 )P-(CH 2 ) 2 -P(CH 3 )OOH, crystallizes in the monoclinic centrosymmetric space group P2 1 /c with two crystallographically independent molecules in the asymmetric unit both of them lying on a centre of symmetry.
The molecules are connected on both sides to the next molecules via strong O-H···H hydrogen bonds. The bond lengths and angles in the two crystallographic independent molecules are identical within the ranges of their standard uncertainties.
As each posphinic acid group donates and accepts one hydrogen bond the motif of this connection between the molecules is best described by an eight-membered ring ( Fig.1) which can be classified by the Etter symbol R 2 2 (8) (Etter et al., 1990). A motif which is well known for acetic acid and its derivatives. Each -POOH group accepts and donates one hydrogen bond to the neighbouring -POOH groups of the next molecules to give a one-dimensional chains along [10-1]. This was surprising to us, as the very similar ethane-1,2-diylbis(phosphinic acid) forms a two-dimensional hydrogen bonded network (Bruckmann et al., 1999).

Experimental
The title compound is generally available by methods described in the literature (Sicken et al., 2000). Recrystallization of the raw material from ethanolic solution at room temperature gave block shaped, colourless crystals.