Structure and magnetic ordering of the anomalous layered (2D) ferrimagnet [NEt4]2Mn(II)3(CN)8 and 3D bridged-layered antiferromagnet [NEt4]Mn(II)3(CN)7 Prussian blue analogues

Cation Adaptive Structures Based on Manganese Cyanide Prussian Blue Analogues: Application of Powder Diffraction Data to Solve Complex, Unprecedented Stoichiometries and New Structure Types

A Mn(II) salt and A+ CN- under anaerobic conditions react to form 2-D and 3-D extended structured compounds of Am MnII n (CN)m+2n stoichiometry. Here, the creation and characterization of this large family of compounds, for example AMnII 3 (CN)7 , A2 MnII 3 (CN)8 , A2 MnII 5 (CN)12 , A3 MnII 5 (CN)13 , and A2 MnII [MnII (CN)6 ], where A represents alkali and tetraalkylammonium cations, is reviewed. Cs2 MnII [MnII (CN)6 ] has the typical Prussian blue face centered cubic unit cell. However, the other alkali salts are monoclinic or rhombohedral. This is in accord with smaller alkali cation radii creating void space that is minimized by increasing the van der Waals stabilization energy by reducing ∠Mn-N≡C, which, strengthens the magnetic coupling and increases the magnetic ordering temperatures. This is attributed to the non-rigidity of the framework structure due the significant ionic character associated with the high-spin MnII sites. For larger tetraalkylammonium cations, the high-spin Mn sites lack sufficient electrostatic A+ ⋅⋅⋅NC stabilization and form unexpected 4- and 5-coordinated Mn sites within a flexible, extended framework around the cation; hence, the size, shape, and charge of the cation dictate the unprecedented stoichio-metry and unpredictable cation adaptive structures. Antiferromagnetic coupling between adjacent MnII sites leads to ferrimagnetic ordering, but in some cases antiferromagnetic coupling of ferrimagnetic layers are compensated and synthetic antiferromagnets are observed. The magnetic ordering temperatures for ferrimagnetic A2 MnII [MnII (CN)6 ] with both octahedral high- and low-spin MnII sites increase with decreasing ∠Mn-N≡C. The crystal structures for all of the extended structured materials were obtained by powder diffraction.

Synthesis and magnetic properties of two Mn-based coordination polymers constructed by a mixed-ligand strategy

Two Mn(ii) CPs have been synthesized by mixed-ligand strategy and measured for their magnetic properties. Moreover, analysis of the magnetic structure–effect relationship indicates that 1 and 2 have different Mn chains of magnetic interactions.

Anomalous Stoichiometry and Antiferromagnetic Ordering for the Extended Hydroxymanganese(II) Cubes/Hexacyanometalate-Based 3D-Structured [MnII 4 (OH)4 ][MnII (CN)6 ](OH2 )6 ⋅H2 O

The reaction of Mn^II (O₂ CMe)₂ and NaCN or LiCN in water forms a light green insoluble material. Structural solution and Rietveld refinement of high-resolution synchrotron powder diffraction data for this unprecedented, complicated compound of previously unknown composition revealed a new alkali-free ordered structural motif with [Mn^II ₄ (μ₃ -OH)₄ ]⁴⁺ cubes and octahedral [Mn^II (CN)₆ ]^4- ions interconnected in 3D by Mn^II -N≡C-Mn^II linkages. The composition is {[Mn^II (OH₂ )₃ ][Mn^II (OH₂ )]₃ }(μ₃ -OH)₄ ][Mn^II (μ-CN)₂ (CN)₄ ]⋅H₂ O=[Mn^II ₄ (μ₃ -OH)₄ (OH₂ )₆ ][Mn^II (μ-CN)₂ (CN)₄ ]⋅H₂ O, which is further simplified to [Mn₄ (OH)₄ ][Mn(CN)₆ ](OH₂ )₇ (1). 1 has four high-spin (S=5/2) Mn^II sites that are antiferromagnetically coupled within the cube and are antiferromagnetically coupled to six low-spin (S=1/2) octahedral [Mn^II (CN)₆ ]^4- ions. Above 40 K the magnetic susceptibility, χ(T), can be fitted to the Curie-Weiss expression, χ ∝(T-θ)^-1 , with θ=-13.4 K, indicative of significant antiferromagnetic coupling and 1 orders as an antiferromagnet at T_c =7.8 K.

Anomalous Stoichiometry, 3-D Bridged Triangular/Pentagonal Layered Structured Artificial Antiferromagnet for the Prussian Blue Analogue A3MnII5(CN)13 (A = NMe4, NEtMe3). A Cation Adaptive Structure

The size of the organic cation dictates both the composition and the extended 3-D structure for hybrid organic/inorganic Prussian blue analogues (PBAs) of A _aMn^II _b(CN) _{a+2 b} (A = cation) stoichiometry. Alkali PBAs are typically cubic with both MC₆ and M'N₆ octahedral coordination sites and the alkali cation content depends on the M and M' oxidation states. The reaction of Mn^II(O₂CCH₃)₂ and A⁺CN^- (A = NMe₄, NEtMe₃) forms a hydrated material of A₃Mn^II₅(CN)₁₃ composition. A₃Mn^II₅(CN)₁₃ forms a complex, 3-D extended structural motif with octahedral and rarely observed square pyramidal and trigonal bipyramidal Mn^II sites with a single layer motif of three pentagonal and one triangular fused rings. A complex pattern of Mn^IICN chains bridge the layers. (NMe₄)₃Mn^II₅(CN)₁₃ possesses one low-spin octahedral and four high-spin pentacoordinate Mn^II sites and orders as an antiferromagnet at 11 K due to the layers being bridged and antiferromagnetically coupled by the nonmagnetic cyanides. These are rare examples of intrinsic, chemically prepared and controlled artificial antiferromagnets and have the advantage of having controlled uniform spacing between the layers as they are not physically prepared via deposition methods. A₃Mn₅(CN)₁₃ (A = NMe₄, NEtMe₃) along with [NEt₄]₂Mn^II₃(CN)₈, [NEt₄]Mn^II₃(CN)₇, and Mn(CN)₂ form stoichiometrically related A _aMn^II _b(CN) _{a+2 b} ( a = 0, b = 1; a = 2, b = 3; a = 1, b = 3; and a = 3, b = 5) series possessing unprecedented stoichiometries and lattice motifs. These unusual structures and stoichiometries are attributed to the very ionic nature of the high-spin N-bonded Mn^II ion that enables the maximization of the attractive van der Waals interactions via minimization of void space via a reduced ∠MnNC. This A _aMn^II _b(CN) _{a+2 b} family of compounds are referred to as being cation adaptive in which size and shape dictate both the stoichiometry and structure.

A nickel(ii)–manganese(ii)-azido layered coordination polymer showing a three-dimensional ferrimagnetic order at 35 K

The synthesis, crystal structure and magnetic properties of a two-dimensional nickel(ii)–manganese(ii)–azido system are described. It is an interesting molecular magnet because it exhibits long-range ferrimagnetic ordering at a T_c of 35 K and a hysteresis loop up to 20 K.

Intrinsic organic-based synthetic/artificial antiferromagnets

Mn(TCNE)[C4(CN)8]1/2 (TCNE = tetracyanoethylene) and [NEt4]Mn(II)3(CN)7 have extended layers with nearest neighbor intralayer S = 5/2 and S = 1/2 spin sites that couple antiferromagnetically forming ferrimagnetic layers. These layers are uniformly connected via diamagnetic (nonmagnetic) bridging μ4-[(C4(CN)8](2-) (8.77 Å) or μ-CN (5.48 Å) ligands, respectively, that antiferromagnetic couple the ferrimagnetic layers resulting in an antiferromagnet. The Jinter/kB is -1.0 and -1.8 K (H=-JSi⋅Sj) for Mn(TCNE)[C4(CN)8]1/2 and [NEt4]Mn(II)3(CN)7, respectively. Albeit intrinsically multilayered, these antiferromagnets have the same motif as that for artificial/synthetic antiferromagnets that exhibit giant magnetoresistance (GMR) and are commercially used in many magnetic memory applications.

Thermodynamic investigation by heat capacity measurements of ferrimagnetic A2Mn[Mn(CN)6] (A=K, Rb, Cs) Prussian blue compounds

Heat capacity measurements of a new series of Prussian blue analogs of A2Mn[Mn(CN)6] (A=K, Rb, Cs) composition were performed using thermal relaxation calorimetry. The Cs compound has a face-centered cubic structure with a linear Mn-C≡N-Mn linkage, while the monoclinic Rb and K compounds have nonlinear Mn-C≡N-Mn linkages. For all of the compounds, large broad thermal anomalies associated with magnetic transitions were observed in the temperature dependence of the heat capacity. The systematic changes in the heat capacity for the three compounds under magnetic fields of up to 7 T were found to be consistent with ferrimagnetic ordering with large spontaneous magnetization. Although the peak temperatures were slightly lower than reported values obtained by magnetic susceptibility measurements, the magnetic entropy was evaluated to be 22.0 ± 2.5 J K(-1) mol(-1). This value is consistent with an entropy of Rln12 corresponding to full entropy of one low-spin and one high-spin Mn(II) ion in the formula unit, though some ambiguity remains in lattice estimation. Broadening of the peak width of the magnetic heat capacity divided by the temperature was observed as the size of the alkali ions decreased from Cs to K. This behavior is consistent with an increase in the lattice distortion produced by the bending of the C≡N-Mn angles.