Two Modifications of Nitrilotris(methylenephenylphosphinic) Acid: A Polymeric Network with Intermolecular (O=P–O–H)3 vs. Monomeric Molecules with Intramolecular (O=P–O–H)3 Hydrogen Bond Cyclotrimers

Nitrilotris(methylenephenylphosphinic) acid (NTPAH3) was silylated using hexamethyldisilazane to produce the tris(trimethylsilyl) derivative NTPA(SiMe3)3. From the latter, upon alcoholysis in chloroform, NTPAH3 could be recovered. Thus, a new modification of that phosphinic acid formed. Meanwhile, NTPAH3 synthesized in aqueous hydrochloric acid crystallized in the space group P3c1 with the formation of O-H⋅⋅⋅O H-bonded networks (NTPAH3P), in chloroform crystals in the space group R3c formed (NTPAH3M), the constituents of which are individual molecules with exclusively intramolecular O-H⋅⋅⋅O hydrogen bonds. Both solids, NTPAH3P and NTPAH3M, were characterized by single-crystal X-ray diffraction, multi-nuclear (1H, 13C, 31P) solid-state NMR spectroscopy, and IR spectroscopy as well as quantum chemical calculations (both of their individual constituents as isolated molecules as well as in the periodic crystal environment). In spite of the different stabilities of their constituting molecular conformers, the different crystal packing interactions rendered the modifications of NTPAH3P and NTPAH3M similarly stable. In both solids, the protons of the acid are engaged in cyclic (O=P–O–H)3 H-bond trimers. Thus, the trialkylamine N atom of this compound is not protonated. IR and 1H NMR spectroscopy of these solids indicated stronger H-bonds in the (O=P–O–H)3 H-bond trimers of NTPAH3M over those in NTPAH3P.