[Cr^III(NCMe)₆]³⁺a Labile Cr^III Source Enabling Formation of Cr[M(CN)₆] (M = V, Cr, Mn, Fe) Prussian Blue-Type Magnetic Materials

The kinetic inertness of the hexaaquachromium(III) (k_H2O= 2.4 × 10^-6 s^-1) has led to challenges with respect to incorporating Cr^III ions into Prussian blue-type materials; however, hexakis(acetonitrile)chromium(III) was shown to be substantially more labile (∼10⁴ times) and enables a new synthetic route for the synthesis of these materials via nonaqueous solvents. The synthesis, spectroscopic, and physical properties of Cr[M(CN)₆] (M = V, Cr, Mn, Fe) Prussian blue analogues synthesized from [Cr^III(NCMe)₆]³⁺ and the corresponding [M^III(CN)₆]^3- are described. All these compounds {(NEt₄)_0.02Cr^III[V^III(CN)₆]_0.98(BF₄)_0.08·0.10MeCN (1), Cr^III[Cr^III(CN)₆]·0.16MeCN (2), Cr^III[Mn^III(CN)₆]·0.10MeCN (3), and (NEt₄)_0.04Cr^III_0.64Cr^IV_0.40[Fe^II(CN)₆]_0.40[Fe^III(CN)₆]_0.60(BF₄)_0.16·1.02MeCN (4)} are ferrimagnets exhibiting cluster-glass behavior. Strong antiferromagnetic coupling was observed for M = V, Cr, and Mn with Weiss constants (θ) ranging from −132 to −524 K; and in 2, where the strongest coupling is observed (θ = −524 K), the highest T_c (110 K) value was observed. Weak antiferromagnetic coupling was observed for M = Fe (θ = −12 K) leading to the lowest T_c (3 K) value in this series. Weak coupling and the low T_c value observed in 4 were additionally contributed by the presence of both [Fe^II(CN)₆]^4- and [Fe^III(CN)₆]^3- as confirmed by ⁵⁷Fe−Mössbauer spectroscopy.