Crystalline, Mixed-Valence Manganese Analogue of Prussian Blue: Magnetic, Spectroscopic, X-ray and Neutron Diffraction Studies

Post-synthetic modification of Prussian blue type nanoparticles: tailoring the chemical and physical properties

This review focuses on recent advances in the post-synthetic modification of nano-sized Prussian blue and its analogues and compares them with the current strategies used in metal–organic frameworks to give future outlooks in this field.

Correlated disorder in metal–organic frameworks

Metal–organic frameworks host many types of compositional and structural disorder. In this Highlight article we explore cases where this disorder is correlated, rather than random.

The structures of ordered defects in thiocyanate analogues of Prussian Blue

We report the structures of six new divalent transition metal hexathiocyanatobismuthate frameworks with the generic formula , M = Mn, Co, Ni and Zn. These frameworks are defective analogues of the perovskite-derived trivalent transition metal hexathiocyanatobismuthates M^III[Bi(SCN)₆]. The defects in these new thiocyanate frameworks order and produce complex superstructures due to the low symmetry of the parent structure, in contrast to the related and more well-studied cyanide Prussian Blue analogues. Despite the close similarities in the chemistries of these four transition metal cations, we find that each framework contains a different mechanism for accommodating the lowered transition metal charge, making use of some combination of Bi(SCN)₆ ^3- vacancies, M_Bi antisite defects, water substitution for thiocyanate, adventitious extra-framework cations and reduced metal coordination number. These materials provide an unusually clear view of defects in molecular framework materials and their variety suggests that similar richness may be waiting to be uncovered in other hybrid perovskite frameworks.

Hidden diversity of vacancy networks in Prussian blue analogues

Prussian blue analogues (PBAs) are a diverse family of microporous inorganic solids, known for their gas storage ability¹, metal-ion immobilization², proton conduction³, and stimuli-dependent magnetic^4,5, electronic⁶ and optical⁷ properties. This family of materials includes the double-metal cyanide catalysts^8,9 and the hexacyanoferrate/hexacyanomanganate battery materials^10,11. Central to the various physical properties of PBAs is their ability to reversibly transport mass, a process enabled by structural vacancies. Conventionally presumed to be random^12,13, vacancy arrangements are crucial because they control micropore-network characteristics, and hence the diffusivity and adsorption profiles^14,15. The long-standing obstacle to characterizing the vacancy networks of PBAs is the inaccessibility of single crystals¹⁶. Here we report the growth of single crystals of various PBAs and the measurement and interpretation of their X-ray diffuse scattering patterns. We identify a diversity of non-random vacancy arrangements that is hidden from conventional crystallographic powder analysis. Moreover, we explain this unexpected phase complexity in terms of a simple microscopic model that is based on local rules of electroneutrality and centrosymmetry. The hidden phase boundaries that emerge demarcate vacancy-network polymorphs with very different micropore characteristics. Our results establish a foundation for correlated defect engineering in PBAs as a means of controlling storage capacity, anisotropy and transport efficiency.

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.

The effect of the orientation of the Jahn–Teller distortion on the magnetic interactions of trinuclear mixed-valence Mn(ii)/Mn(iii) complexes

The effect of the orientation of the Jahn–Teller distortion on the magnetic interactions in two new mixed-valence trinuclear Mn(iii)–Mn(ii)–Mn(iii) complexes has been investigated.

Syntheses, structures, and magnetic properties of three new cyano-bridged complexes based on the [Mn(CN)6]3−building block

By tuning the reaction conditions and using macrocyclic ligands, three [Mn(CN)₆]³⁻-based magnetic complexes were prepared and characterized.

Syntheses, crystal structures, MMCT and magnetic properties of four one-dimensional cyanide-bridged complexes comprised of MII–CN–FeIII (M = Fe, Ru, Os)

Four 1D cyanide-bridged complexes [cis-M^II(L)₂(CN)₂Fe^III(salen)](PF₆) (3–6) have been synthesized, of which 3 and 4 are Class II mixed-valence complexes.

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

The reaction of Mn(II) and [NEt(4)]CN leads to the isolation of solvated [NEt(4)]Mn(3)(CN)(7) (1) and [NEt(4)](2) Mn(3)(CN)(8) (2), which have hexagonal unit cells [1: R3m, a = 8.0738(1), c = 29.086(1) Å; 2: P3m1, a = 7.9992(3), c = 14.014(1) Å] rather than the face centered cubic lattice that is typical of Prussian blue structured materials. The formula units of both 1 and 2 are composed of one low- and two high-spin Mn(II) ions. Each low-spin, octahedral [Mn(II)(CN)(6)](4-) bonds to six high-spin tetrahedral Mn(II) ions through the N atoms, and each of the tetrahedral Mn(II) ions are bound to three low-spin octahedral [Mn(II)(CN)(6)](4-) moieties. For 2, the fourth cyanide on the tetrahedral Mn(II) site is C bound and is terminal. In contrast, it is orientationally disordered and bridges two tetrahedral Mn(II) centers for 1 forming an extended 3D network structure. The layers of octahedra are separated by 14.01 Å (c axis) for 2, and 9.70 Å (c/3) for 1. The [NEt(4)](+) cations and solvent are disordered and reside between the layers. Both 1 and 2 possess antiferromagnetic superexchange coupling between each low-spin (S = 1/2) octahedral Mn(II) site and two high-spin (S = 5/2) tetrahedral Mn(II) sites within a layer. Analogue 2 orders as a ferrimagnet at 27(±1) K with a coercive field and remanent magnetization of 1140 Oe and 22,800 emuOe mol(-1), respectively, and the magnetization approaches saturation of 49,800 emuOe mol(-1) at 90,000 Oe. In contrast, the bonding via bridging cyanides between the ferrimagnetic layers leads to antiferromagnetic coupling, and 3D structured 1 has a different magnetic behavior to 2. Thus, 1 is a Prussian blue analogue with an antiferromagnetic ground state [T(c) = 27 K from d(χT)/dT].

Microscopic understanding of negative magnetization in Cu, Mn, and Fe based Prussian blue analogues

A crossover of the field-cooled magnetization from positive to negative has been observed below the magnetic ordering temperature (17.9 K) in a multimetal Prussian Blue analogue (PBA), Cu_{0.73}Mn_{0.77}[Fe(CN)_{6}].zH_{2}O. The reverse Monte Carlo (RMC) modeling (using the program RMCPOW) has been used to derive the various scattering contributions (e.g., nuclear diffuse, nuclear Bragg, magnetic diffuse, and magnetic Bragg) from the observed neutron diffraction patterns. The RMC analysis combined with the Rietveld refinement technique show an antiferromagnetic ordering of Mn moments with respect to the Cu as well as the Fe moments. Our study gives the first neutron magnetic structure evidence towards the microscopic understanding of the negative magnetization in the PBAs. This information can be effectively utilized to design suitable PBAs for making multifunctional devices.

Tuning the magnetic behavior via dehydration/hydration treatment of a new ferrimagnet with the composition of K0.2Mn1.4Cr(CN)6 x 6H2O

A new complex (1) of Prussian blue analogue with the composition of K0.2Mn1.4Cr(CN)6 x 6H2O was prepared and characterized structurally as well as magnetically. The crystal structure of complex 1 was determined by X-ray diffraction analysis. The results indicate that complex 1 consists of a 3D cubic lattice similar to those of Mn3[Cr(CN)6]2 x xH2O, Mn3[Co(CN)6]2 x xH2O, Cd3[Cr(CN)6]2 x xH2O, and Cd3[Co(CN)6]2 x xH2O. Magnetic measurements show that complex 1 is a ferrimagnet with T(c) = 66 K. It is interesting to note that the magnetic behavior of complex 1 can be substantially modulated through a dehydration/rehydration treatment. The T(c) value of this ferrimagnet increases to 99 K after dehydration reaching a 23.4% weight loss, and it decreases back to 66 K after the dehydrated sample reabsorbs water molecules.

Structural and Magnetic Diversity in Cyano-Bridged Bi- and Trimetallic Complexes Assembled from Cyanometalates and [M(rac-CTH)]n+ Building Blocks (CTH = d,l-5,5,7,12,12,14-Hexamethyl-1,4,8,11-tetraazacyclotetradecane)

Seven new cyano-bridged heterometallic systems have been prepared by assembling [M'(rac-CTH)]n+ complexes (M' = CrIII, NiII, CuII), which have two cis available coordination positions, and [M(CN)6]3- (M = FeIII, CrIII) and [Fe(CN)2(bpy)2]+ cyanometalate building blocks. The assembled systems, which have been characterized by X-ray crystallography and magnetic investigations, are the molecular squares (meso-CTH-H2)[{Ni(rac-CTH)}2{Fe(CN)6)}2].5H2O (2) and [{Ni(rac-CTH)}2{Fe(CN)2(bpy)2}2](ClO4)4.H2O (5), the bimetallic chain [{Ni(rac-CTH)}2{Cr(CN)6)}2Ni(meso-CTH)].4H2O (3), the trimetallic chain [{Ni(rac-CTH)}2{Fe(CN)6)}2Cu(cyclam)]6H2O (4), the pentanuclear complexes [{Cu(rac-CTH}3{Fe(CN)6}2].2H2O (6) and [{Cu(rac-CTH)}3{Cr(CN)6)}2].2H2O (7), and the dinuclear complex [Cr(rac-CTH)(H2O)Fe(CN)6].2H2O (8). With the exception of 5, all compounds exhibit ferromagnetic interaction between the metal ions (JFeNi = 12.8(2) cm-1 for 2; J1FeCu= 13.8(2) cm-1 and J2FeCu= 3.9(4) cm-1 for 6; J1CrCu= 6.95(3) cm-1 and J2CrCu= 1.9(2)cm-1 for 7; JCrFe = 28.87(3) cm-1 for 8). Compound 5 exhibits the end of a transition from the high-spin to the low-spin state of the octahedral FeII ions. The bimetallic chain 3 behaves as a metamagnet with a critical field Hc = 300 G, which is associated with the occurrence of week antiferromagnetic interactions between the chains. Although the trimetallic chain 4 shows some degree of spin correlation along the chain, magnetic ordering does not occur. The sign and magnitude of the magnetic exchange interaction between CrIII and FeIII in compound 8 have been justified by DFT type calculations.