Probing spin density and local structure in the prussian blue analogues CsCd[Fe/Co(CN)_6]·0.5 H_2O and Cd_3[Fe/ Co(CN)_6]_2·15 H_2O with solid-state MAS NMR spectroscopy

摘要

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~IM~(II)[M~(III)(CN)_6]·x H_2O and M~(II)3[M~(III)(CN)_6]_2·x H_2O, 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 ~(113)Cd 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)[Fe_x~(III)Co_(1-x)~(III)(CN)_6]_2·15 H_2O with x=0 (1), 0.25 (2), 0.5 (3), 0.75 (4) and 1 (5), and CsCd~(II)[Fe_x~(III)Co_(1-x)~(III)(CN)_6]· 0.5 H_2O with x=0 (6), 0.25 (7), 0.5 (8), 0.75 (9) and 1 (10). Interestingly, the presence of FeIII 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.