Probing Spin Density and Local Structure in the Prussian Blue Analogues CsCdA 6 )·0.5 H 2 O and Cd 3 A 6 ) 2 ·15 H 2 O 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 A III (CN)6)·x H2O and M II 3A III (CN)6)2·x H2O, in which A I is an alkali ion, and M II and M III are tran- sition-metal ions) that exhibit similar cubic frameworks but different micro- scopic 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� va- cancies, 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 II 3(Fe III xCo III 1� x(CN)6)2·15 H2O with x = 0( 1), 0.25 (2), 0.5 (3), 0.75 (4) and 1( 5), and CsCd II (Fe III xCo III 1� x(CN)6)· 0.5 H2O 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 nota- bly improved compared to that ach- ieved for the diamagnetic PBAs. By doping the sample with varying amounts of diamagnetic Co III and com- paring the NMR spectra of both types of PBAs, we have been able to give a view of the structure which is comple- mentary 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 cad- mium chemical shift, which is a mea- sure of the hyperfine coupling, allows the estimation of the spin density on the cadmium nucleus, and consequent- ly, the elucidation of the spin delocali- sation mechanism in these compounds along with its dependency on structural parameters.