Osmium(VIII)-Catalyzed Oxidation of Some Cyclic Amines by Potassium Hexacyanoferrate(III) in Alkaline Media: a Kinetics and Mechanistic Study

Reactions of morpholine, piperidine, and piperazine with Os(VIII)-catalyzed hexacyanoferrate(III) in alkaline media to produce the corresponding lactam have been studied at constant temperature and ionic strength. The reactions followed first-order kinetics with respect to [amine] and [Os(VIII)] but were independent of [Fe(CN)₆ ^3-] and [OH^-]. The effects of introduced electrolytes, potassium hexacyanoferrate(II), relative permitivity, and temperature have also been studied. A mechanism accounting for these results has been proposed.     

Lithium Ferrocyanide Catholyte for High-Energy and Low-cost Aqueous Redox Flow Batteries**

Aqueous redox flow batteries (ARFBs) are a promising technology for grid-scale energy storage, however, their commercial success relies on redox-active materials (RAM) with high electron storage capacity and cost competitiveness. Herein, a redox-active material lithium ferrocyanide (Li₄[Fe(CN)₆]) is designed. Li⁺ ions not only greatly boost the solubility of [Fe(CN)₆]⁴⁻ to 2.32 M at room temperature due to weak intermolecular interactions, but also improves the electrochemical performance of [Fe(CN)₆]^4−/3−. By coupling with Zn, ZIRFBs were built, and the capacity of the batteries was as high as 61.64 Ah L⁻¹ (pH-neutral) and 56.28 Ah L⁻¹ (alkaline) at a [Fe(CN)₆]⁴⁻ concentration of 2.30 M and 2.10 M. These represent unprecedentedly high [Fe(CN)₆]⁴⁻ concentrations and battery energy densities reported to date. Moreover, benefiting from the low cost of Li₄[Fe(CN)₆], the overall chemical cost of alkaline ZIRFB is as low as $11 per kWh, which is one-twentieth that of the state-of-the-art VFB ($211.54 per kWh). This work breaks through the limitations of traditional electrolyte composition optimization and will strongly promote the development of economical [Fe(CN)₆]^4−/3−-based RFBs in the future.     

The Hexacyanodiborane(6) Dianion [B2(CN)6]2−

Diborane(6) dianions with substituents that are bonded to boron via carbon are very reactive and therefore only a few examples are known. Diborane(6) derivatives are the simplest catenated boron compounds with an electron‐precise B–B σ‐bond that are of fundamental interest and of relevance for material applications. The homoleptic hexacyanodiborane(6) dianion [B₂(CN)₆]²⁻ that is chemically very robust is reported. The dianion is air‐stable and resistant against boiling water and anhydrous hydrogen fluoride. Its salts are thermally highly stable, for example, decomposition of (H₃O)₂[B₂(CN)₆] starts at 200 °C. The [B₂(CN)₆]²⁻ dianion is readily accessible starting from 1) B(CN)₃²⁻ and an oxidant, 2) [BF(CN)₃]⁻ and a reductant, or 3) by the reaction of B(CN)₃²⁻ with [BHal(CN)₃]⁻ (Hal=F, Br). The latter reaction was found to proceed via a triply negatively charged transition state according to an S_N2 mechanism.     

Hydrate isomerism in [Cu(en)2(H2O)1.935]2[Fe(CN)6]·4H2O

The title compound, bis[di­aqua­bis­(ethyl­enedi­amine‐κ²N,N′)copper(II)­] hexa­cyano­iron(II) tetrahydrate, [Cu(C₂H₈N₂)₂(H₂O)_1.935]₂[Fe(CN)₆]·4H₂O, was crystallized from an aqueous reaction mixture initially containing CuSO₄, K₃[Fe(CN)₆] and ethyl­enedi­amine (en) in a 3:2:6 molar ratio. Its structure is ionic and is built up of two crystallographically different cations, viz. [Cu(en)₂(H₂O)₂]²⁺ and [Cu(en)₂(H₂O)_1.87]²⁺, there being a deficiency of aqua ligands in the latter, [Fe(CN)₆]⁴⁻ anions and disordered solvent water mol­ecules. All the metal atoms lie on centres of inversion. The Cu atom is octahedrally coordinated by two chelate‐bonded en mol­ecules [mean Cu—N = 2.016 (2) Å] in the equatorial plane, and by axial aqua ligands, showing very long distances due to the Jahn–Teller effect [mean Cu—O = 2.611 (2) Å]. In one of the cations, significant underoccupation of the O‐atom site is observed, correlated with the appearance of a non‐coordinated water mol­ecule. This is interpreted as the partial contribution of a hydrate isomer. The [Fe(CN)₆]⁴⁻ anions form quite regular octahedra, with a mean Fe—C distance of 1.913 (2) Å. The dominant intermolecular interactions are cation–anion O—H⋯N hydrogen bonds and these inter­actions form layers parallel to (001).     

Influence of the chloride counterion on the redox reactivity of tetraammineplatinum(II) cation with octacyanotungstate(V) anion: Crystal structure of [Pt(NH3)4]2[W(CN)8]

The redox reaction between [Pt(NH₃)₄]²⁺ and [W(CN)₈]³⁻ in the presence of Cl⁻ anions in aqueous solution affords single crystals of [Pt^II(NH₃)₄]₂[W^IV(CN)₈] and [Pt^IV(NH₃)₄Cl₂]Cl₂. Trapped cyano ligands of [W(CN)₈]⁴⁻ rectangular antiprisms of D₂ point symmetry between parallel Pt(II) square planes show that the inner-sphere redox pathway is prohibited. The presence of Cl⁻ counterions enables the formation of [Pt(NH₃)₄Cl₂]Cl₂ as the product of the rare outer-sphere pathway of the oxidation of Pt(II) by [W(CN)₈]³⁻.     

A charge‐transfer salt composed of methyl viologen and hexacyanidoferrate(II)

The methyl viologen dication, used under the name Paraquat as an agricultural reagent, is a well‐known electron‐acceptor species that can participate in charge‐transfer (CT) interactions. The determination of the crystal structure of this species is important for accessing the CT interaction and CT‐based properties. The title hydrated salt, bis(1,1′‐dimethyl‐4,4′‐bipyridine‐1,1′‐diium) hexacyanidoferrate(II) octahydrate, (C₁₂H₁₄N₂)₂[Fe(CN)₆]·8H₂O or (MV)₂[Fe(CN)₆]·8H₂O [MV²⁺ is the 1,1′‐dimethyl‐4,4′‐bipyridine‐1,1′‐diium (methyl viologen) dication], crystallizes in the space group P 2₁/c with one MV²⁺ cation, half of an [Fe(CN)₆]⁴⁻ anion and four water molecules in the asymmetric unit. The Fe^II atom of the [Fe(CN)₆]⁴⁻ anion lies on an inversion centre and has an octahedral coordination sphere defined by six cyanide ligands. The MV²⁺ cation is located on a general position and adopts a noncoplanar structure, with a dihedral angle of 40.32 (7)° between the planes of the pyridine rings. In the crystal, layers of electron‐donor [Fe(CN)₆]⁴⁻ anions and layers of electron‐acceptor MV²⁺ cations are formed and are stacked in an alternating manner parallel to the direction of the −2a + c axis, resulting in an alternate layered structure.     

Ferrate(VI) and ferrate(V) oxidation of cyanide,thiocyanate, and copper(I) cyanide

Cyanide (CN⁻), thiocyanate (SCN⁻), and copper(I) cyanide (Cu(CN)₄³⁻) are common constituents in the wastes of many industrial processes such as metal finishing and gold mining, and their treatment is required before the safe discharge of effluent. The oxidation of CN⁻, SCN⁻, and Cu(CN)₄³⁻ by ferrate(VI) (Fe^VIO₄²⁻; Fe(VI)) and ferrate(V) (Fe^VO₄³⁻; Fe(V)) has been studied using stopped-flow and premix pulse radiolysis techniques. The rate laws for the oxidation of cyanides were found to be first-order with respect to each reactant. The second-order rate constants decreased with increasing pH because the deprotonated species, FeO₄²⁻, is less reactive than the protonated Fe(VI) species, HFeO₄⁻. Cyanides react 10³–10⁵ times faster with Fe(V) than with Fe(VI). The Fe(V) reaction with CN⁻ proceeds by sequential one-electron reductions from Fe(V) to Fe(IV) to Fe(III). However, a two-electron transfer process from Fe(V) to Fe(III) occurs in the reaction of Fe(V) with SCN⁻ and Cu(CN)₄³⁻. The toxic CN⁻ species of cyanide wastes is converted into relatively non-toxic cyanate (NCO⁻). Results indicate that Fe(VI) is highly efficient in removing cyanides from electroplating rinse water and gold mill effluent.     

Potassium bis(ethylenediamine)­copper(II) ferricyanide

The title compound, potassium bis(ethylenediamine‐N,N′)copper(II) hexacyanoferrate(III), K[Cu(C₂H₈N₂)₂]‐[Fe(CN)₆], contains [Cu(en)₂]²⁺ and [Fe(CN)₆]^3? complex ions, where en is ethylenediamine. The Fe^III and K⁺ ions lie on twofold axes and the Cu^II atom lies on an inversion center. The [Cu(en)₂]²⁺ ion has square‐planar coordination with a mean Cu—N distance of 1.992 (2) Å and the [Fe(CN)₆]^3? ion has distorted octahedral coordination with a mean Fe—C distance of 1.947 (2) Å.     

The dynamic behaviour of the silver-silver hexacyanoferrate(II) electrode: coulometric production of hexacyanoferrate(II)

The electrochemical behaviour of the silver-silver hexacyanoferrate(II) elec-trode was studied. The reaction Ag₄[Fe(CN)₆] + 4e^- → 4Ag + [Fe(CN)₆]^4- was shown to be useful for the coulometric production of hexacyanoferrate(II) ions in titrations of zinc(II). Coulometric titrations of organometallic compounds such as R₂Sn(ClO₄)₂, with electrically generated hexacyanoferrate(II) are also reported.     

A novel three-dimensional mixed bridging–ligand complex with an unexpected 1,3-diaminopropane bridge

Reaction of K₃[Fe(CN)₆] with [Cu(tn)₂](ClO₄)₂ (tn=1,3-diaminopropane) leads to a novel mixed cyano and tn bridged three-dimensional (3D) bimetallic assembly (1), in which each [Fe(CN)₆]⁴⁻ anion connects six copper(II) cations via six CN⁻ groups, whereas each copper(II) cation is linked to three [Fe(CN)₆]⁴⁻ ions and two other copper(II) ions through Cu–NC–Fe and Cu–tn–Cu linkages, respectively. Magnetic studies reveal weak antiferromagnetic interactions between the nearest Cu^II (S=1/2) ions through the diamagnetic [Fe(CN)₆]⁴⁻ anion.     

Anomalous Stoichiometry and Antiferromagnetic Ordering for the Extended Hydroxymanganese(II) Cubes/Hexacyanometalate-Based 3D-Structured [MnII4(OH)4][MnII(CN)6](OH2)6⋅H2O

The reaction of Mn^II(O₂CMe)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^II₄(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] ⋅ H₂O=[Mn^II₄(μ₃-OH)₄(OH₂)₆][Mn^II(μ-CN)₂(CN)₄] ⋅ H₂O, 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.     

Kinetics and mechanism of pentacyanohydroxoferrate(III) formation from [FeL(OH)2]− complex, (L=N-(2-hydroxyethyl) iminodiacetate anion

Summary The kinetics and mechanism of the system [FeHIDA-(OH)₂]^–+5CN^–[Fe(CN)₅OH^–+HIDA^2–+OH^– (HIDA=N-(2-hydroxyethyl) (iminodiacetate) at pH=9.5±0.02, I=0.1 M and at 25±0.1°C have been studied spectrophotometrically at 395 nm ( _max of [Fe(CN)₅OH]^3–]. The reaction has three distinguishable stages; the first is formation of [Fe(CN)₅OH]^3–, the second is conversion of [Fe(CN)₅OH]^3– into [Fe(CN)₆]^3–, and last is the reduction of [Fe(CN)₆]^3– to [Fe(CN)₆]^4– by the HIDA^2– released in the first stage. The first stage shows variable-order dependence on cyanide concentration, unity at high cyanide concentration and zero at low cyanide concentration. The second stage exhibits first-order dependence on the concentration of [Fe(CN)₅OH]^3– as well as on cyanide. The reverse reaction between [Fe(CN)₅OH]^3– and HIDA^2– is first-order in each of these species and inverse first-order in cyanide. On the basis of forward and reverse rate studies, a five-step mechanism has been proposed for the first stage. The first step involves a slow loss of one OH^–, by a cyanide-independent path.     

MII-N-diisopropoxythiophosphorylthiobenzamide complexing processes in M2[Fe(CN)6]-gelatin-immobilized matrices (M = Co,Ni, Cu)

Complexing processes in M^II-N-diisopropoxythiophosphorylthiobenzamide binary systems (M = Co, Ni, Cu) in metal(II) hexacyanoferrate(II) gelatin-immobilized matrices upon contact with aqueous–alkaline (pH = 12.0 ± 0.1) solutions of organic compounds have been studied. It has been shown that, in Co^II and Cu^II, the initial act of complexing involves destruction of the Co^II and Cu^II hexacyanoferrates(II) by OH^– ions, leading to formation of the corresponding hydroxides which react with the ligand indicated. In the both systems, successive addition of two ligand molecules per M(OH)₂ fragment occurs and [MB(OH)(OH₂)] and [MB₂] coordination compounds are formed (B^–-a singly deprotonated ligand form). In the Ni^II-N-diisopropoxythiophosphorylthiobenzamide system, the formation of three complexes, (Ni₂BOH)₂[Fe(CN)₆], [NiB(OH)(OH₂)] and [NiB₂] occurs.     

Darstellung und spektroskopische Charakterisierung von Kupfer(II)- und Nickel(II)-tricyanmethanid-Komplexen mit Imidazol-Derivaten – Kristallstruktur von [Cu{C(CN)3}2(2-meiz)2]

Synthesis and Spectroscopic Characterization of Copper(II) and Nickel(II) Tricyanomethanide Complexes with Imidazoles – Crystal Structure of [Cu{C(CN)₃}₂(2-meiz)₂] The copper(II) and nickel(II) tricyanomethanide complexes with imidazoles of the type [Cu{C(CN)₃}₂L₄], [L = 2- or 4-methylimidazole (meiz)] and [M{C(CN)₃}₂L₂] [M = Cu, L = imidazole (iz), 2- or 4-meiz; M = Ni, L = iz, 2- or 4-meiz] were prepared and characterized by electronic, infrared, and – some of them – by ESR spectroscopy. The structure [Cu{C(CN)₃}₂(2-meiz)₂], solved by X-ray crystallographic analysis, shows a two-dimensional network with unsymmetric C(CN)₃-bridges between the Cu^II atoms. Polymeric structures with bridging C(CN)₃-groups were identified by means of spectroscopic methods also for the other [M{C(CN)₃}₂L₂] complexes. On the other hand, for the complexes [M{C(CN)₃}₂L₄] follow molecular structures, in which monodentate C(CN)₃ ligands are present. All compounds under investigation show a tetragonal-bipyramidal geometry with various degree of tetragonal distortion.     

Anderson-type heteropolyanions of molybdenum(VI) and tungsten(VI)

The previously reported preparation of some Anderson-type molybdopolyanions containing divalent metal ions (Zn, Cu, Co or Mn) as a heteroatom has been reinvestigated. The molybdopolyanions of Zn(II) and Cu(II) were confirmed, although the Cu(II) polyanion was not stable and could not be recrystallized. On the other hand, the polyanions of Co(II) and Mn(II) could not be reproduced. Another type of heteropoly compound, [X(H₂O)_6-x(Mo₇O₂₄)]⁴⁻ [X = Cu(II), Co(II) or Mn(II)], was isolated as solids, which are not stable thermally. The mixed-type Anderson polyanions, [Ni(II)Mo_6-xW_x,O₂₄H₆]⁴⁻, which have been questioned as mixtures of species with different x values, were also reinvestigated using IR, UV absorption and MCD spectra. They are single species, but not mixtures, although some positional isomers may be present for the compounds where x = 2-4. The possibility of oxidation of the heteroatom with the Anderson structure maintained was examined. The oxidation of [Ni(II)Mo₆O₂₄H₆]⁴⁻ by the S₂O²⁻₈ ion in aqueous solution gave the Waugh-type [Ni(IV)Mo₉O₃₂]⁶⁻ polyanion, whereas the oxidation of [Ni(II)W₆O₂₄H₆]⁴⁻ gave no heteropoly compound.     

Bis­[tris(2,2′‐bi­pyridine‐κ2N,N′)­ruthenium(II)] hexa­cyano­ferrate(III) chloride octahydrate

In the title compound, [Ru^II(C₁₀H₈N₂)₃]₂[Fe^III(CN)₆]Cl·8H₂O, the [Ru(bpy)₃]²⁺ (bpy is 2,2′‐bi­pyridine) cations and water mol­ecules afford intriguing microporous honeycomb layers, while the [Fe(CN)₆]³⁻ anions and the remainder of the water mol­ecules form anionic sheets based on extensive hydrogen‐bonding networks. The cationic and anionic layers alternate along the c axis. The Fe atom in [Fe(CN)₆]³⁻ lies on an inversion centre and the axial cyano ligands are hydrogen bonded to the water mol­ecules encapsulated within the micropores [N⋯O = 2.788 (5) Å], giving an unusual interpenetration between the cationic and anionic layers. On the other hand, the in‐plane cyano ligands are relatively weakly hydrogen bonded to the water mol­ecules [N⋯O = 2.855 (7) and 2.881 (8) Å] within the anionic sheets.     

Kinetics and mechanism of a trans-tetraazamacrocyclic chromium(III) complex oxidation by hexacyanoferrate(III) in strongly alkaline media

Oxidation of the trans-[Cr(cyca)(OH)₂]⁺ complex, where cyca = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, by [Fe(CN)₆ ]^3- ion in strongly alkaline media, leading to [Cr^V O(cyca_ox )]³⁺ ion, has been studied using electronic and e.p.r. spectroscopy. The kinetics of the Cr^III → Cr^IV transformation have been studied using a large excess of the reductant and OH^- ion over the oxidant. The reaction is a second order process: first order in [Cr^III] and [Fe^III] at constant [OH^-]. The second order rate constant is higher than linearly dependent on the OH^- concentration. The mechanism of the reaction has been discussed. A relatively inert intermediate chromium(V) species was detected based on characteristic bands in the visible region and the e.p.r. signal at g_iso = 1.987 for the systems where an excess of oxidant was used. The hyperfine structure of the main e.p.r. signal is consistent with the d¹ -electron interactions with four equivalent nitrogen nuclei and [Cr^V = O(cyca_ox)]³⁺ formula, where cyca_ox = oxidized cyca, can be postulated for the intermediate Cr^V complex.     

Die Kristallstruktur der hydratisierten Cyanokomplexe NMe4MnII[(Mn, Cr)III(CN)6] · 3 H2O und NMe4Cd[MIII(CN)6] · 3 H2O (MIII = Fe,Co): Verwandte des Berliner Blaus

The Crystal Structure of the Hydrated Cyano Complexes NMe₄Mn^II[(Mn, Cr)^III(CN)₆] · 3 H₂O and NMe₄Cd[M^III(CN)₆] · 3 H₂O (M^III = Fe, Co): Compounds Related to Prussian Blue The crystal structures of the isotypic tetragonal compounds (space group I4, Z = 10) NMe₄Mn^II · [(Mn, Cr)^III(CN)₆] · 3 H₂O (a = 1653.2(4), c = 1728.8(6) pm), NMe₄Cd[Fe(CN)₆] · 3 H₂O (a = 1642.7(1), c = 1733.1(1) pm) and NMe₄Cd[Co(CN)₆] · 3 H₂O (a = 1632.1(2), c = 1722.4(3) pm) were determined by X‐rays. They exhibit ⊥ c cyanobridged layers of octahedra [M^III(CN)₆] and [M^IIN₄(OH₂)₂], which punctually are interconnected also || c to yield altogether a spaceous framework. The M^II atoms at the positions linking into the third dimension are only five‐coordinated and form square pyramids [M^IIN₅] with angles N–M^II–N near 104° and distances of Mn–N: 1 × 214, 4 × 219 pm; Cd–N: 1 × 220 resp. 222, 4 × 226 resp. 228 pm. Further details and structural relations within the family of Prussian Blue are reported and discussed.     

Kinetics of Oxidation of L-Valine by a Copper(III) Periodate Complex in Alkaline Medium

The kinetics of oxidation of L-valine by a copper(III) periodate complex was studied spectrophotometrically. The inverse second-order dependency on [OH⁻] was due to the formation of the protonated diperiodatocuprate(III) complex ([Cu(H₃IO₆)₂]⁻) from [Cu(H₂IO₆)₂]³⁻. The retarding effect of initially added periodate suggests that the dissociation of copper(III) periodate complex occurs in a pre-equilibrium step in which it loses one periodate ligand. Among the various forms of copper(III) periodate complex occurring in alkaline solutions, the monoperiodatocuprate(III) appears to be the active form of copper(III) periodate complex. The observed second-order dependency of [L-valine] on the rate of reaction appears to result from formation of a complex with monoperiodatocuprate(III) followed by oxidation in a slow step. A suitable mechanism consistent with experimental results was proposed. The rate law was derived as:

Fulleride-metal η5 sandwich and multi-decker sandwich complexes: A DFT prediction

The (C₆₀CN)⁻ formed by the reaction of CN⁻ with fullerene shows high electron rich character, very similar to C₆₀˙⁻, and it behaves as a large anion. Similar to Cp⁻, the bulky anion, (C₆₀CN)⁻, acts as a strong η⁵ ligand towards transition metal centers. Previous studies on η⁵ coordination of fullerene cage are reported for pseudo fullerenes whereas the present study deals with sandwich complexes of (C₆₀CN)⁻ with Fe(II), Ru(II), Cr(II), Mo(II), and Ni(II) and multi-decker sandwich complexes of CN–fullerides with Fe(II). The structural parameters of these complexes and the corresponding Cp⁻ complexes showed very close resemblance. Analysis of the metal-to-carbon bonding molecular orbitals showed that sandwich complex [Fe(η⁵-(C₆₀CN)⁻)₂] exhibit bonding features very similar to that of ferrocene. Also, a 6-fold decrease in the band gap energy is observed for [Fe(η⁵-(C₆₀CN)⁻)₂] compared to ferrocene. The energy of dissociation (ΔE) of the ligand (C₆₀CN)⁻ from [Fe(η⁵-(C₆₀CN)⁻)₂] is slightly lower than the ΔE of a Cp* ligand from a ferrocene derivative wherein each cyclopentadienyl unit is substituted with four tertiary butyl groups. The (C₆₀CN)⁻ ligand behaved as one of the bulkiest ligands in the chemistry of sandwich complexes. Further, the coordinating ability of the dianion, (C₆₀(CN)₂)²⁻ is evaluated which showed strong coordination ability simultaneously with two metal centers leading to the formation of multi-decker sandwich and pearl-necklace type polymeric structures.