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

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.

Pressure Dependence of the Magnetic Ordering Temperature (Tc) for the Na2Mn[Mn(CN)6] Noncubic Prussian Blue Analogue

The pressure dependence of the magnetic properties of rhombohedral Na₂Mn[Mn(CN)₆] up to 10 kbar has been studied. The magnetic ordering temperature, T_c, for Na₂Mn[Mn(CN)₆] reversibly increases with increasing applied hydrostatic pressure, P, by 9.0 K (15.2%) to 68 K at 10 kbar with an average rate of increase, dT_c/dP, of 0.86 K/kbar. The magnetization at 50 kOe and remanent magnetization, M_r(H), remain constant with an average value of 13,100 ± 200 and 8500 ± 200 emuOe/mol. The coercive field H_cr increases by 12% from 13,400 to 15,000 Oe. The increase and rate of increase of T_c for rhombohedral Na₂Mn[Mn(CN)₆] are reduced with respect to monoclinic A₂Mn[Mn(CN)₆] (A = K and Rb), but they are still greater than those of cubic Cs₂Mn[Mn(CN)₆]. This is attributed to the compression of the MnNC framework bonding without decreasing ∠Mn^II-N≡C, maintaining the unit cell in accord with cubic A = Cs at lower applied pressures, and not due to reduction in ∠Mn^II-N≡C, which correlates with increasing T_c that is reported for A = K and Rb as well as Cs at higher applied pressures.

On the CN-K coordination modes in Kn[M6-n(CN)6]·xH2O: first evidence of CN-K electron-deficient bonding

Based on the determination of single crystal XRD structures of potassium hexacyanidometallates and on IR, and Raman data, here we propose for the first time the occurrence of an electron-deficient bonding between the N end of the CN^- ligand and the K⁺ metal center. The crystal structures of K_n[M^6-n(CN)₆]·xH₂O (M = Fe(ii), Ru(ii), Os(ii), Co(iii), Rh(iii), Ir(iii), Pt(iv)) reveal the presence of four types of CN^-K interactions: (i) a linear CN-K bond, (ii) the N ends in a bipodal coordination involving two K atoms, (iii) the N ends in a tripodal coordination mode involving three K atoms and (iv) the N ends and the K atoms with the largest K-N distances within the subseries that can be attributed to the electrostatic interactions. The bi- and tripodal coordination modes between the N end of the CN^- ligand and K⁺ ions are atypical and their nature is discussed in this contribution. The CN^- ligand N end can behave as a two-electron donor that participates in a three-center two-electron bonding (i.e. Class II μ-L 3c-2e) for a N-bipodal coordination mode or as a two-electron donor that participates in a four-center two-electron bonding (4c-2e) for an N-tripodal coordination mode. Such a possibility is closely related to the π-back donation ability of the CN^- ligand, which results in a charge density accumulation on the N end, which could be partially donated to the K atom through an σ-mechanism. For the divalent metals (Fe, Ru, Os), the solids crystallize with a monoclinic unit cell in the C2/c space group, while for the trivalent ones (Co, Rh, Ir), the crystal structure corresponds to an orthorhombic unit cell in the Pbcn space group. Potassium hexacyanidoplatinate(iv) crystallizes with a trigonal unit cell, in the P3[combining macron]1m space group, where each N end is always found coordinating two K atoms. The finding of these novel coordination modes of the CN^- ligands, relying on an electron-deficient bonding behavior, paves the way for the design of functional materials based on hexacyanidometallates. The experimental results and the proposed electron-deficient bonding model herein discussed were appropriately supported by the computational calculations.