Time-Resolved, In Situ X-ray Diffraction Studies of Staging during Phosphonic Acid Intercalation into [LiAl₂(OH)₆]Cl·H₂O

Intercalation of a range of phosphonic acids into [LiAl₂(OH)₆]Cl·H₂O at pH 8 leads to co-intercalation of both mono- and dianionic guest anions. Solid-state ³¹P NMR data can be used to show that these materials exhibit a ³¹P chemical shift that is intermediate between the values for the monoanionic and dianionic forms of the respective acids, suggesting that rapid proton exchange occurs between the intercalated anions. It is possible to calculate the relative amounts of mono- and dianionic species present using the observed averaged chemical shift of the intercalate phase. In the case of methylphosphonic acid, the ratio of mono- and dianionic species (MePO₃H^-/MePO₃^2-) is estimated to be 2.1. The intercalation of methyl-, ethyl-, phenyl-, and benzylphosphonic acids into [LiAl₂(OH)₆]Cl·H₂O was studied using time-resolved, in situ energy-dispersive synchrotron X-ray diffraction. The rates of intercalation are significantly greater for methylphosphonic acid (MPA), ethylphosphonic acid (EPA), and benzylphosphonic acid (BPA) than for phenylphosphonic acid (PPA). The large BPA anions intercalate the most quickly, and MPA reacts more rapidly than EPA. Kinetic analyses of the rate data suggest that these are diffusion-controlled reactions. In situ X-ray diffraction experiments performed with slow addition of the guest have allowed observation of intermediate crystalline phases during the reactions with MPA and BPA. The intermediate phases can be indexed assuming a second-stage intercalation compound in which alternate interlayer regions are occupied by phosphonic acid and Cl^- anions, respectively (Williams et al. Chem. Commun. 2003, 1816). The observation of these second-stage intermediates is very surprising, because staging in rigid layer lattices such as layered double hydroxides (LDHs) is highly unusual.