New Prussian Blue Analogues Constructed from the [Cr(CN)5NO3−] and [Cr(CN)64−] Anions. A Building Block Approach to High TNFerrimagnets

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Abstract
The synthesis and characterization of Prussian blue analogues derived from the pentacyanonitrosylchromate(I) and hexacyanochromate(II) ions are discussed. By addition of triflate salts of either MnII or CrII, occasionally in the presence of a cesium source, four new molecular magnets have been obtained: K0.5Mn[Cr(CN)5NO]0.83·4H2O·1,5MeOH (1), Cs0.5Cr[Cr(CN)5NO]0.83·2H2O·0.6MeOH (2), Cr[Cr(CN)5NO]0.5·2H2O-·1.8MeOH (3), and Cr[Cr(CN)6]0.5·3H2O (4). The powder XRD patterns for (1) and (4) can be indexed to face-centered cubic unit cells with a = 10.63(4) and 10.37(4) å, respectively. IR spectra are consistent with the view that the nitrosyl ligands in the pentacyanonitrosylchromate salts are bridging: the vno stretching frequency is ca. 60–110 cm−1 higher than that of the potassium salt K3[Cr(CN)5NO]. The compounds undergo spontaneous ferrimagnetic ordering at temperatures of 31, 89, 127, and 238 K, respectively, and they exhibit hysteretic behavior at 4.5 K with coercive fields of 8, 10, 11, and 330 G. For the solids containing chromium(II) centers, saturation magnetization and room temperature susceptibility measurements suggest that these centers are low-spin (S=l). Overall, the [Cr(CN)5N03−] anion affords Prussian blue analogues with lower magnetic ordering temperatures than does the related [Cr(CN)63−] anion; this probably reflects the lower symmetry of the pentacyanonitrosylchromate ion and the fact that it contains fewer unpaired electrons.