The betainic form of (imidazol-2-yl)phenylphosphinic acid hydrate

Single crystals of the title compound, (imidazolium-2-yl)phenyl­phosphinate monohydrate, C(9)H(9)N(2)O(2)·H(2)O, were ob­tained from methanol/water after deprotection and oxidation of bis­(1-diethoxy­methyl­imidazol-2-yl)phenyl­phosphane. In the structure, several N–H⋯O and P—O⋯H–O hydrogen bonds are found. π–π inter­actions between the protonated imidazolyl rings [centroid–centroid distance = 3.977 (2) Å] help to establish the crystal packing. The hydrate water mol­ecule builds hydrogen bridges to three mol­ecules of the phosphinic acid by the O and both H atoms.