Nanocomposite of a chromium Prussian blue with TiO₂. Redox reactions and the synthesis of Prussian blue molecule-based magnets

The reaction of [NEt₄]₃[Cr(CN)₆] with titanium(III) p-toluenesulfonate at a pH of 2 affords a gray solid whose metal content and spectroscopic and magnetic properties are fully consistent with it being a Prussian blue material of stoichiometry "Ti^III[Cr^III(CN)₆] · H₂O". The carbon, nitrogen, and hydrogen content, however, are not consistent with this stoichiometry, and further investigation showed that the gray material has a powder X-ray diffraction profile, infrared spectrum, and magnetic properties very similar to those of the "all-chromium" Prussian blue Cr^II[Cr^III(CN)₆]_0.67 · 6H₂O. All data, including the C, H, and N weight percentages, are consistent with the conclusion that the material isolated is a nanocomposite of Cr^II[Cr^III(CN)₆]_0.67 · xH₂O and TiO₂ in the ratio of 1-1.6. These results suggest that Ti^III reduces some of the [Cr^III(CN)₆]^3- ions to generate Ti^IV and Cr^II; the former hydrolyzes to amorphous TiO₂ · 2H₂O, the latter loses its bound CN ligands and reacts with unreacted [Cr^III(CN)₆]^3- ions to generate the crystalline all-chromium PB species. The electrochemical potentials suggest that the [Cr^III(CN)₆]^3- ion should not be reduced by Ti^III; evidently, this unfavorable reaction is driven by the insolubility of the reaction products. The results constitute a cautionary tale in two respects: first, that the characterization of Prussian blue materials must be conducted with care and, second, that the insolubility of Prussian blue analogues can sometimes drive reactions that in solution are thermodynamically unfavorable.