One, two, and three methylene phosphonic acid groups (–CH2PO3H2) on a mesitylene ring: synthesis, characterization and aspects of supramolecular aggregation

Abstract

The ease of substitution of –CH₂Br group on the mesitylene ring in a stepwise manner has been exploited to prepare mesitylene mono-, bis-, and tris-methylene phosphonic acids C₆H₂Me₃(CH₂PO₃H₂) (1), C₆HMe₃(CH₂PO₃H₂)₂ (2), and C₆Me₃(CH₂PO₃H₂)₃ (3) in 80–90% yield, through a Michaelis–Arbuzov reaction followed by acid hydrolysis. Compounds 1–4 have been characterized by analytical and spectroscopic (IR, NMR, and MS) techniques. The X-ray structural investigations on all the three compounds reveal that the central mesitylene ring is a robust platform for hosting multiple methylene phosphonic acid groups. Mesitylene mono-methylene phosphonic acid 1, which can be considered as the sterically encumbered modification of benzyl phosphonic acid, interestingly associates as a one-dimensional tubular structure through extensive O–H⋯O hydrogen bonding between adjacent –PO₃H₂ groups, albeit with no such interactions between the tubes due to the highly hydrophobic nature of the surface. The X-ray structure of mesitylene-1,3-diphosphonic acid 2 reveals that the molecules exhibit a syn-orientation of the –CH₂PO₃H₂, which are involved in extensive P–OH⋯O hydrogen bonding to result in a two-dimensional layered structure. The presence of three –PO₃H₂ units on the same side of the mesitylene ring in 3 results in a three-dimensional framework solid. The present studies clearly reveal a linear relationship between the dimensionality of the supramolecular structure and the number of phosphonic acid moieties on the mesitylene ring.