Institut Parisien de Chimie Moléculaire, UPMC Univ. Paris 06, 4 place Jussieu, 75252 Paris, cedex 05, France.
Chemistry. 2011 Oct 4;17(41):11567-75. doi: 10.1002/chem.201100778. Epub 2011 Aug 31.
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)]·xH(2)O and M(II)(3)M(III)(CN)(6)·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) vacancies, the second type of PBA exhibits M(III)(CN)(6) 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(II)(3)Fe(III)(x)Co(III)(1-x)(CN)(6)·15H(2)O with x=0 (1), 0.25 (2), 0.5 (3), 0.75 (4) and 1 (5), and CsCd(II)[Fe(III)(x)Co(III)(1-x)(CN)(6)]·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.
魔角旋转（MAS）NMR 光谱用于研究普鲁士蓝类似物（PBAs）中的局部结构和自旋离域。我们选择了两种常见的 PBA 原型（A(I)M(II)[M(III)(CN)6]·xH2O 和 M(II)(3)M(III)(CN)6·xH2O，其中 A(I)是碱金属离子，M(II)和 M(III)是过渡金属离子），它们具有相似的立方框架，但微观结构不同。第一种 PBA 含有间隙碱金属离子，不存在任何M(III)(CN)6空位，而第二种 PBA 存在M(III)(CN)6空位，但不包含插入的碱金属离子。在这项研究中，我们选择 Cd(II)作为二价金属，以便使用(113)Cd 核（I=1/2）作为局部结构的探针。在这里，我们对两种系列的 PBAs 进行了完整的 MAS NMR 研究，它们的化学式为 Cd(II)(3)Fe(III)(x)Co(III)(1-x)(CN)6·15H2O，其中 x=0(1)、0.25(2)、0.5(3)、0.75(4)和 1(5)，以及 CsCd(II)[Fe(III)(x)Co(III)(1-x)(CN)6]·0.5H2O，其中 x=0(6)、0.25(7)、0.5(8)、0.75(9)和 1(10)。有趣的是，在镉位附近存在 Fe(III)磁性中心对 NMR 谱具有放大镜效应：它诱导了引人注目的信号展宽，使得分辨率与顺磁 PBAs 相比得到显著提高。通过在样品中掺杂不同量的顺磁 Co(III)，并比较两种类型的 PBAs 的 NMR 谱，我们能够提供一种互补的结构视图，通常可以从 X 射线衍射研究中获得。特别是，这项研究表明，空位在介孔 PBAs 中不是随机分布的。此外，镉的化学位移是超精细耦合的度量，允许估计镉核上的自旋密度，从而阐明这些化合物中的自旋离域机制及其对结构参数的依赖性。
