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 “TiIII[CrIII(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 CrII[CrIII(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 CrII[CrIII(CN)6]0.67 · xH2O and TiO2 in the ratio of 1–1.6. These results suggest that TiIII reduces some of the [CrIII(CN)6]3− ions to generate TiIV and CrII; the former hydrolyzes to amorphous TiO2 · 2H2O, the latter loses its bound CN ligands and reacts with unreacted [CrIII(CN)6]3− ions to generate the crystalline all-chromium PB species. The electrochemical potentials suggest that the [CrIII(CN)6]3− ion should not be reduced by TiIII; 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.

Graphical abstractThe reaction of [NEt4]3[Cr(CN)6] with titanium(III) p-toluenesulfonate affords a gray solid that appeared to be a Prussian blue of stoichiometry “TiIII[CrIII(CN)6] · H2O” but is actually a nanocomposite of CrII[CrIII(CN)6]0.67 · xH2O and TiO2 · nH2O, an amorphous form of rutile. Evidently, TiIII reduces some of the [CrIII(CN)6]3− ions to generate TiIV and CrII; subsequent reactions generate the observed products.Figure optionsDownload full-size imageDownload as PowerPoint slide