Piperazine-2,5-dione-oxalic acid-water (1/1/2) and a redetermination of piperazine-2,5-dione, both at 120 K: hydrogen-bonded sheets containing multiple ring types.

In piperazine-2,5-dione-oxalic acid-water (1/1/2), C4H6N2O2.C2H2O4.2H2O, both organic components lie across inversion centres in space group P-1. The molecules are linked by N-H...O and by both two-centre O-H...O and three-centre O-H...(O)2 hydrogen bonds into sheets built from R(1)(2)(5), R(2)(2)(8), R(4)(4)(8) and R(5)(4)(15) rings. In piperazine-2,5-dione, C4H6N2O2, where the molecules lie across centres of inversion in space group P2(1)/c, the molecules are linked by paired N-H...O hydrogen bonds into ribbons of centrosymmetric R(2)(2)(8) rings, which are further linked into sheets by C-H...O hydrogen bonds, generating R(4)(3)(14) rings between the ribbons.