Two polymorphs of (2-carboxyethyl)(phenyl)phosphinic acid

Two polymorphs of (2-carboxyethyl)(phenyl)phosphinic acid, C₉H₁₁O₄P, crystallize in the chiral P2₁2₁2₁ space group with similar unit-cell parameters. They feature an essentially similar hydrogen-bonding motif but differ slightly in their detailed geometric parameters. For both polymorphs, the unequivocal location of the hydroxy H atoms together with the expected differences in the P—O bond lengths establish unequivocally that both forms contain the S isomer; the protonated phosphinic acid and carboxy O atoms serve as hydrogen-bond donors, while the second phosphinic acid O atom acts as a double hydrogen-bond acceptor and the remaining carboxy O atom is not involved in hydrogen bonding. Thus, an undulating two-dimensional supramolecular layer aggregate is formed based on an R₄³(20) ring unit. Such polymorphism derives from the rotation of the C—C single bonds between the two hydrogen-bond-involved carboxy and phosphinic acid moieties.