New prussian blue analogues constructed from the [Cr(CN)₅NO^3-] and [Cr(CN)₆^4-] anions. A building block approach to high T_N ferrimagnets

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 Mn^II or Cr^II, occasionally in the presence of a cesium source, four new molecular magnets have been obtained: K_0.5Mn[Cr(CN)₅NO]_0.83·4H ₂O· 1.5MeOH (1), Cs_0.5Cr[Cr(CN)₅NO]_0.83·2H ₂O·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=I). Overall, the [G(CN)₅NO^3-] anion affords Prussian blue analogues with lower magnetic ordering temperatures than does the related [Cr(CN)₆^3-] anion; this probably reflects the lower symmetry of the pentacyanonitrosylchromate ion and the fact that it contains fewer unpaired electrons.