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.-C2H2O4.2H(2)O, both organic components lie across inversion centres in space group P (1) over bar 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-...O hydrogen bonds, generating R-4(3)(14) rings between the ribbons.