Protonation Behavior and Intramolecular Interactions of α,ω-Alkanediaminepolymethylenepolyphosphonates

Abstract

Abstract The protonation equilibria of two types of aminopolyphosphonates ((CH3)2−p(CH2PO3)pN(CH2)mN(CH2PO3)p′(CH3)2−p′) in aqueous solution were investigated by means of potentiometry and 31P NMR spectroscopy. The protonation behavior of diaminotetramethylenetetraphosphonate (p = p′ = 2) is significantly altered by a change in the number of methylene chains (m = 2, 3, 6). The protonations of the imino fragments of hdtmp, which is connected with a six methylene chain (m = 6), are essentially independent of each other, whereas edtmp, which is connected with a two methylene chain (m = 2), forms a hydrogen bond between both nitrogen atoms (NH+···N) in the mono protonated species. The ethylenediaminepolyphosphonates (m = 2), having an iminodiphosphonate group (p = 2, p′ = 0—2), show a peculiar 31P NMR behavior in which the signal shows a downfield shift at higher protonated species. However, eddmp (N,N′), having one methylenephosphonate group at each imino fragment (p = p′ = 1), does not show such an anomaly. These results indicate the existence of an intramolecular interaction between both fragments, such as a hydrogen bond of a protonated nitrogen atom with unprotonated phosphonates of another imino fragment, where two methylenephosphonate groups in one imino fragment are required to operate such an interaction.