Probing Spin Density and Local Structure in the Prussian Blue Analogues CsCd[Fe/Co(CN)6]•0.5H2O and Cd3[Fe/Co(CN)6]2•15H2O with Solid-State MAS NMR Spectroscopy

Magic-angle spinning (MAS) NMR spectroscopy is used to study the local structure and spin delocalisation in Prussian blue analogues (PBAs). We selected two common archetypes of PBAs (A(I)M(II)[M-III(CN)(6)]center dot xH(2)O and M-3(II)[M-III(CN)(6)](2)center dot xH(2)O, in which A(I) is an alkali ion, and M-II and M-III are transition-metal ions) that exhibit similar cubic frameworks but different microscopic structures. Whereas the first type of PBA contains interstitial alkali ions and does not exhibit any [M-III(CN)(6)](3-) vacancies, the second type of PBA exhibits [M-III(CN)(6)](3-) vacancies, but does not contain inserted alkali ions. In this study, we selected Cd-II as a divalent metal in order to use the Cd-113 nuclei (I=1/2) as a probe of the local structure. Here, we present a complete MAS NMR study on two series of PBAs of the formulas Cd-3(II)[(FexCo1-xIII)-Co-III(CN)(6)](2)center dot 15H(2)O with x=0 (1), 0.25 (2), 0.5 (3), 0.75 (4) and 1 (5), and CsCdII[(FexCo1-xIII)-Co-III(CN)(6)]center dot 0.5H(2)O with x=0 (6), 0.25 (7), 0.5 (8), 0.75 (9) and 1 (10). Interestingly, the presence of Fe-III magnetic centres in the vicinity of the cadmium sites has a magnifying-glass effect on the NMR spectrum: it induces a striking signal spread such that the resolution is notably improved compared to that achieved for the diamagnetic PBAs. By doping the sample with varying amounts of diamagnetic Co-III and comparing the NMR spectra of both types of PBAs, we have been able to give a view of the structure which is complementary to that usually obtained from X-ray diffraction studies. In particular, this study has shown that the vacancies are not randomly distributed in the mesoporous PBAs. Moreover the cadmium chemical shift, which is a measure of the hyperfine coupling, allows the estimation of the spin density on the cadmium nucleus, and consequently, the elucidation of the spin delocalisation mechanism in these compounds along with its dependency on structural parameters.