Molecular modeling of interlayer catalytic sites for aniline polymerization in a zirconium mixed phosphonate phosphate

Theoretical modeling of a mixed zirconium 3-carboxypropylphosphonate phosphate system, Zr(O₃P(CH₂)₃COOH)_x (O₃POH)_2-x, is reported both for a series of stoichiometric compounds and for an aniline-intercalated system. Modeling of the interlayer spacing variation of selected stoichiometries of the host series predicts linear behavior for the intermediate compositions (0.5 ≥ × ≥ 1.5) accompanied by a significant contraction at the terminal stoichiometries (× = 0.0 and 2.0). Our results demonstrate that such behavior is a general feature of layered zirconium phosphonates whose pendant groups possess conformational degrees of freedom. For the host-guest series, the results have provided insight into the energetically stable orientations of the aniline molecule within the interlayer. It was found that the most probable geometry was one in which the aniline C₂ axis is in a tilted orientation with respect to the zirconium planes for the host systems studied. However, at slightly higher energy, molecular modeling predicts aniline to be in an orientation corresponding to the C₂ axis being parallel to the zirconium planes, which would be favorable for the formation of polyaniline within the interlayer. These results are interpreted in the context of experimental findings published earlier.