[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

Abstract

The kinetic inertness of the hexaaquachromium(III) (kH2O=2.4x10(-6) s(-1)) has led to challenges with respect to incorporating CrIII ions into Prussian blue-type materials; however, hexakis(acetonitrile)chromium(III) was shown to be substantially more labile (approximately 10(4) 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 [CrIII(NCMe)6]3+ and the corresponding [MIII(CN)6]3- are described. All these compounds {(NEt4)0.02CrIII[VIII(CN)6]0.98(BF4)(0.08).0.10MeCN (1), CrIII[CrIII(CN)6].0.16MeCN (2), CrIII[MnIII(CN)6].0.10MeCN (3), and (NEt4)0.04CrIII0.64CrIV0.40[FeII(CN)6]0.40[FeIII(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 (theta) ranging from -132 to -524 K; and in 2, where the strongest coupling is observed (theta=-524 K), the highest Tc (110 K) value was observed. Weak antiferromagnetic coupling was observed for M=Fe (theta=-12 K) leading to the lowest Tc (3 K) value in this series. Weak coupling and the low Tc value observed in 4 were additionally contributed by the presence of both [FeII(CN)6]4- and [FeIII(CN)6]3- as confirmed by 57Fe-Mössbauer spectroscopy.

Structure and Properties of [CrIIIF(NCMe)5](BF4)2·MeCN: A Nonaqueous Source of CrIIIF2+and a Building Block for New Prussian-Blue-Like Magnetic Materials

“…18 In the solid state, 2 exhibits a broad ν CN absorption at 2168 cm -1 (hwhh: 22 cm -1 ; Figure S5, Supporting Information) that is slightly shifted to lower energy from that observed for Cr III [Cr III (CN) 6 ] • 0.16(MeCN) (ν CN ) 2173 cm -1 ; hwhh: 26 cm -1 ). 12 The 2168 cm -1 absorption is shifted to higher energy with respect to [Cr III (CN) 6 ] 3-(ν CN ) 2111 cm -1 ). 12 This shift is consistent with that of other PB-like materials and is due to the loss of electron density from the 5s orbital and the lack of backbonding via the N atom.…”

“…31 The color is in contrast to the recently reported, very similar red-brown Cr III [Cr III (CN) 6 ] • 0.16(MeCN). 12 Complex 2 is a new PB-like material with fluoride occupying one of the chromium's coordination sites, that is, forming [FCr III (NC-Cr III ) 5 ] moieties, resulting in only five cyanides bridging to the Cr III -F species. To the best of our knowledge, this type of binding in a Prussian blue analog (PBA) has not been previously reported; however, several dinuclear complexes containing Cr III F 2+ and a µ-cyano bridging ligand are reported.…”

“…11 [Cr III (NCMe) 6 ] 3+ was characterized to be ∼10 4 more labile than [Cr III (OH 2 ) 6 ] 3+ and enabled the formation of Cr III [M III (CN) 6 ] (M ) V, Cr, Mn, Fe) Prussian-blue (PB)like materials, with a magnetic ordering Ne ´el temperature, T N , as high as 126 K being observed for Cr III [Cr III (CN) 6 ] • 0.16(MeCN). 12 Upon reinvestigating [Cr III (NCMe) 6 ](BF 4 ) 3 , a dark purple crystalline material was isolated and characterized to be [Cr III F(NCMe) 5 ](BF 4 ) 2 • MeCN (1), whose aqueous analogue, [Cr III F(OH 2 ) 5 ] 2+ , was previously reported in the 1960s. 13 In addition, several kinetic studies on the rate of aquation, 14 the electronic structure, 15 and redox potentials have been performed.…”

Crystal growth and physical properties of an antiferromagnetic molecule: trans-dibromidotetrakis(acetonitrile)chromium(III) tribromide, [CrBr2(NCCH3)4](Br3)

“…First-row transition-metal organonitrile complexes were synthesized decades ago. One reason that inorganic chemists were interested in these compounds is their potential application as a non-aqueous source of metal-ion synthons in the development of other novel materials in various fields (Nortia et al, 1984;Getsis & Mudring, 2008;Nelson et al, 2007;Henriques et al, 1998). The focus on transition-metal halide acetonitrile complexes increased when it was discovered that compounds like divalent NiX 2 ÁxCH 3 CN show catalytic properties towards diolefin polymerization (Getsis & Mudring, 2008;Zhao et al, 1996).…”

Magnetostructural and EPR Studies of Anisotropic Vanadium trans-Dicyanide Molecules

“…Despite the advantages that one could obtain by incorporating V III ions into molecular magnets containing cyanide, there are only a few well-known vanadium cyanide building blocks, − specifically K 4 [V­(CN) 6 ], K 3 [VO­(CN) 5 ], K 4 [V­(CN) 7 ]·2H 2 O, and (Et 4 N) 3 [V­(CN) 6 ], most likely due to the tendency for low valent vanadium complexes to decompose in the presence of oxygen and water. − Indeed, the incorporation of the hexacyano-vanadate­(III) anion [V III (CN) 6 ] 3– into PB-type materials has proven to be difficult because of the ease of oxidation of V III to V IV , but the room temperature cyanide-based magnet, V II 0.42 ­V III 0.58 ­[Cr III ­(CN) 6 ] 0.86 ·2.8H 2 O ( T C = 315 K), reported by Verdaguer’s group is a notable example of a vanadium containing molecule-based extended magnet . It is also important to point out that the first room temperature molecule-based magnet, V­(TCNE) 2 ( T C = 350 K), reported by Miller et al is also based on vanadium, albeit V II and not V III …”