Solvent Medium-Induced Changes to Internal Pressure in the Layered Host VOPO₄·2H₂O and the Influence on Intercalation Reactions

Layered vanadium phosphate dihydrate, VOPO₄·2H₂O, has been chosen as a host for ferrocene intercalation to explore the influence of solvent-guest and solvent-host interactions on rates and product selection. After eliminating solvents that react directly with the VOPO₄·2H₂O host, ferrocene intercalation was evaluated in a series of seven solvents that do not themselves intercalate or otherwise alter the host structure. The intercalation kinetics are analyzed in acetone, butanone, pentanone, acetonitrile, propionitrile, butyronitrile, and DMF. No intercalation is observed in DMF, but all other solvents yield a mixture of stage 1 and stage 2 ferrocenium intercalation products with reaction rates and product ratios varying across the series of solvents. Using <i>ex situ</i> as well as <i>in situ</i> PXRD methods, faster reactions yield more stage 1 product, while a higher degree of stage 2 product is seen for slower kinetics. The results are interpreted in the context of competition between intercalation rates and the buildup of elastic strain between intercalated domains and unintercalated host. To understand solvent effects, both solvent-guest and solvent-host effects are considered. None of the typically considered solvent-guest effects, such as guest desolvation energy, dielectric constant, and guest oxidation potential, correlate with the observed reaction kinetics. On the other hand, different solvent environments are shown to change the internal pressure, <i>P</i>_eff, within the layered hosts, a consequence of changing surface energy and surface tension (stress) in particles with nanometer scale dimensions. The VOPO₄·2H₂O interlayer spacing decreases when particles are suspended in DMF and organonitrile solvents, while the layers separate slightly in acetone, butanone, and pentanone. Using calculated elastic constants, the structural changes correspond to changes in <i>P</i>_eff in the range 0.22 GPa > Δ<i>P</i>_eff > -0.11 GPa across the series of solvents. A density functional theory analysis of the influence of pressure on ferrocene diffusion adds support for the idea that intercalation kinetics are altered by changing internal pressure. The results show solvent environments can be responsible for altering the effective pressure within intercalation hosts, influencing intercalation rates and product selection, even if the solvents do not react directly with the host solids.