Database Survey of Single-and-Half Phosphorus–Oxygen Bonds in Salts with the C₂PO₂ Segment: Crystal Structure of [NH₂C₅H₄NH][(C₆H₅)₂P(O)(O)]⋅2H₂O

Abstract—

For structures with a C₂PO₂ skeleton, a comparison of “single-and-half” phosphorus–oxygen bonds with single and double phosphorus–oxygen bonds is presented. The study is performed by considering the title structure, 63 analogous salt/zwitterionic structures with the [O–P=O]^– moiety and 445 structures with a neutral O–P=O moiety retrieved from the Cambridge Structural Database. The average lengths of the single, “single-and-half ” and double phosphorus-oxygen bonds were obtained as 1.58, 1.50, and 1.48 Å, which indicates the proximity of “single-and-half ” bond lengths to double phosphorus–oxygen bond lengths. In the crystal structure of the title compound, the [NH₂C₅H₄NH]⁺ cation, [(C₆H₅)₂P(O)(O)]^– anion and two independent water molecules are interconnected through relatively strong O–H⋅⋅⋅O and N–H⋅⋅⋅O hydrogen bonds, building a two-dimensional assembly along the ab plane. In this hydrogen-bond pattern, the oxygen atoms of the [O–P=O]^– moiety act as double hydrogen-bond acceptors.