4-Nitrophenyl phosphoric acid and its four different potassium salts: a solid state structure and kinetic study

Abstract

The structures of 4-nitrophenyl phosphoric acid, H₂NPP (1), and its four different potassium salts, K(H₂NPP)(HNPP) (2), K(HNPP)·CH₃OH (3), K(HNPP)·2H₂O (4), and K₂(NPP)·4H₂O (5), were determined by X-ray diffraction methods. These investigations, together with the study of the role of potassium ions in the hydrolysis of 4-nitrophenyl phosphate, provided information about interactions between the acid, monoanionic and dianionic forms of 4-nitrophenyl phosphate and the potassium cations. In the crystalline state, the H₂NPP molecules form one-dimensional chains of bifurcated O–H⋯O hydrogen bonds between phosphate groups. A different network, derived from ladder-like chains of the O–H⋯O hydrogen bonds linking the phosphate group of the H₂NPP molecules and (HNPP)⁻ monoanions, is formed in 2. By contrast, in 4, the phosphate groups of the (HNPP)⁻ monoanions are donors and acceptors of the O–H⋯O hydrogen bonds for the water molecules only. The 4-nitrophenyl phosphate ligands differ in their coordination modes. The potassium cations are chelated by the nitro group in 2 and 4, whereas in 3 and 5 the potassium cations are chelated by the phosphate group, with the ester oxygen atom engaging in the chelating mode.