The immobilisation and reactivity of Fe(CN)63−/4− in an intrinsically microporous polyamine (PIM-EA-TB)

Protonation of the molecularly rigid polymer of intrinsic microporosity PIM-EA-TB can be coupled to immobilisation of Fe(CN)₆^3−/4− (as well as immobilisation of Prussian blue) into 1–2 nm diameter channels. The resulting films provide redox-active coatings on glassy carbon electrodes. Uptake, transport, and retention of Fe(CN)₆^3−/4− in the microporous polymer are strongly pH dependent requiring protonation of the PIM-EA-TB (pK_A ≈ 4). Both Fe(CN)₆⁴⁻ and Fe(CN)₆³⁻ can be immobilised, but Fe(CN)₆⁴⁻ appears to bind tighter to the polymer backbone presumably via bridging protons. Loss of Fe(CN)₆^3−/4− by leaching into the aqueous solution phase becomes significant only at pH > 9 and is likely to be associated with hydroxide anions directly entering the microporous structure to combine with protons. This and the interaction of Fe(CN)₆^3−/4− and protons within the molecularly rigid PIM-EA-TB host are suggested to be responsible for retention and relatively slow leaching processes. Electrocatalysis with immobilised Fe(CN)₆^3−/4− is demonstrated for the oxidation of ascorbic acid.

Graphical abstract

     

Electrochemical and thermodynamic properties of hexacyanoferrate(II)/(III) redox system on multi-walled carbon nanotubes

Novel films consist of multi-walled carbon nanotubes (MWCNT) were fabricated by means of catalytic chemical vapor deposition (CVD) technique with decomposition of either acetonitrile (ACN) or benzene (BZ) using ferrocene (FeCp₂) as catalyst. The electrochemical and thermodynamic behavior of the ferrocyanide/ferricyanide, [Fe(CN)₆]^3−/4− redox couple on synthesized MWCNT-based films was investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques at T = (278.15, 283.15, 293.15, and 303.15) K. The redox couple [Fe(CN)₆]^3−/4− behaves quasi-reversibly on fabricated MWCNT-based films and its reversibility is enhanced upon increasing temperature. Namely, the findings establish that with the rise in temperature the barrier for interfacial electron transfer decreases, leading, consequently, to an enhancement of the kinetics of the charge transfer process. According to thermodynamics the equilibrium of the redox process is shifted towards the formation of [Fe(CN)₆]³⁻ at elevated temperatures.     

Stereo-electrochemistry by a self-assembled monolayer of sulfur-bridged calixthiophene on gold

Sulfur-bridged calixthiophene formed a self-assembled mono-molecular layer on polycrystalline gold, and it regulated an electrochemical electron transfer by the host–guest interaction between the cavity and reactants. 1,7,13,19,25-Tetrathia[1.₅](2,5)thiophenophane (thiacalix[5]thiophene) perfectly passivated the gold electrode for relatively large reversible metal complexes: [Fe(CN)₆]^4−/3− and [IrCl₆]^3−/2−. However, for mono-atomic ions, such as silver and some of the halogen ions, the electrode behaved reversibly. For copper reduction, a large activation overpotential was observed to induce an initial copper reduction in the cavity.     

Mediated bioelectrocatalytic O2 reduction to water at highly positive electrode potentials near neutral pH

Combinations of bilirubin oxidase and metal complexes: [W(CN)₈]^3−/4−, [Os(CN)₆]^3−/4− and [Mo(CN)₈]^3−/4− (the formal potentials, E^0′(M), being 0.320, 0.448, and 0.584 V vs. Ag|AgCl, respectively, at pH 7.0), allowed bioelectrocatalytic reduction of O₂ to water at their formal potentials near neutral pH. The O₂ reduction current appeared even at the standard potential of the O₂/H₂O redox couple, E^0′(O₂/H₂O), when [Mo(CN)₈]^3−/4− was used at pH 7.4, though the magnitude was small. The magnitude of the bioelectrocatalytic current systematically decreased with the decrease in the potential difference between E^0′(O₂/H₂O) and E^0′(M). A limiting current as large as 17 mA/cm² of a projected electrode surface area was obtained at 0.25 V (−0.37 V vs. E^0′(O₂/H₂O)) for the O₂ reduction at pH 7.0 with a carbon felt electrode modified with electrostatically entrapped bilirubin oxidase and [W(CN)₈]^3−/4− at the electrode rotation rate of 4000 rpm.     

Preparation of57Fe enriched K4[Fe(CN)6] and K3[Fe(CN)6]

A simple method to prepare⁵⁷Fe enriched K₄[Fe(CN)₆] and K₃[Fe(CN)₆] is described. The yields of the products are much better than those reported in the literature so far. The enrichment is essential for⁵⁷Fe Mössbauer investigation in a variety of Prussiate type complexes and other inorganic compounds which are conveniently prepared from K₄[Fe(CN)₆] and K₃[Fe(CN)₆]. K₄[Fe(CN)₆] was obtained by reacting freshly prepared Fe(OH)₃ with glacial acetic acid and treating with iron acetate in boiling aqueous solution of KCN. The novel feature of the procedure to obtain K₃[Fe(CN)₆] is that the oxidation of K₄[Fe(CN)₆] has been carried out in the solid state by passing chlorine gas over the powdered specimen. K₃[Fe(CN)₆] was crystallised from alkaline solution of this oxidised powder. The compounds were characterised by Mössbauer spectroscopy.     

The Effect of Adsorption of Ions of the Hexacyanoferrate(II)/(III) Redox Pair on Self-Assembly of Octanethiol at Its Adsorption from Aqueous Solutions on Gold Electrode

The effect of components of the redox pair K₃[Fe(CN)₆]/K₄[Fe(CN)₆] on the dynamics of formation of octanethiol (OT) monolayers from aqueous thiol-containing solutions of 0.1 М NaClO₄ is studied by cyclic voltammetry (CVA). The formation of OT monolayers is shown to depend on the presence of ions of hexacyanoferrate(II)/(III) in solution. Being added to solution, the components of the [Fe(CN)₆]^3–/4– redox pair sharply increase the time of formation of the insulating monolayer OT films and make them less stable. The destabilizing and inhibiting action of [Fe(CN)₆]^3–/4– ions becomes stronger as their concentration in solution increases. The adsorption activity of individual components of the redox pair is assessed. The strong and approximately equal adsorption activity of ions [Fe(CN)₆]^3– and [Fe(CN)₆]^4– on gold in the presence of octanethiol is observed. At the same time, OT and the hexacyanoferrate(II)/(III) ions can be placed in the following row: OT > [Fe(CN)6]^3– ≈ [Fe(CN)₆]^4–. Recommendations are given on how to eliminate the interfering action of the K₃[Fe(CN)₆]/K₄[Fe(CN)₆] redox-pair ions when studying the insulating properties of thiol monolayers on gold.     

Pseudohalogen Chemistry in Ionic Liquids with Non-innocent Cations and Anions

Within the second funding period of the SPP 1708 “Material Synthesis near Room Temperature”,which started in 2017, we were able to synthesize novel anionic species utilizing Ionic Liquids (ILs) both, as reaction media and reactant. ILs, bearing the decomposable and non-innocent methyl carbonate anion [CO₃Me]⁻, served as starting material and enabled facile access to pseudohalide salts by reaction with Me₃Si−X (X=CN, N₃, OCN, SCN). Starting with the synthesized Room temperature Ionic Liquid (RT-IL) [nBu₃MeN][B(OMe)₃(CN)], we were able to crystallize the double salt [nBu₃MeN]₂[B(OMe)₃(CN)](CN). Furthermore, we studied the reaction of [WCC]SCN and [WCC]CN (WCC=weakly coordinating cation) with their corresponding protic acids HX (X=SCN, CN), which resulted in formation of [H(NCS)₂]⁻ and the temperature labile solvate anions [CN(HCN)_n]⁻ (n=2, 3). In addition, the highly labile anionic HCN solvates were obtained from [PPN]X ([PPN]=μ-nitridobis(triphenylphosphonium), X=N₃, OCN, SCN and OCP) and HCN. Crystals of [PPN][X(HCN)₃] (X=N₃, OCN) and [PPN][SCN(HCN)₂] were obtained when the crystallization was carried out at low temperatures. Interestingly, reaction of [PPN]OCP with HCN was noticed, which led to the formation of [P(CN)₂]⁻, crystallizing as HCN disolvate [PPN][P(CN⋅HCN)₂]. Furthermore, we were able to isolate the novel cyanido(halido) silicate dianions of the type [SiCl_0.78(CN)_5.22]²⁻ and [SiF(CN)₅]²⁻ and the hexa-substituted [Si(CN)₆]²⁻ by temperature controlled halide/cyanide exchange reactions. By facile neutralization reactions with the non-innocent cation of [Et₃HN]₂[Si(CN)₆] with MOH (M=Li, K), Li₂[Si(CN)₆] ⋅ 2 H₂O and K₂[Si(CN)₆] were obtained, which form three dimensional coordination polymers. From salt metathesis processes of M₂[Si(CN)₆] with different imidazolium bromides, we were able to isolate new imidazolium salts and the ionic liquid [BMIm]₂[Si(CN)₆]. When reacting [Mes(nBu)Im]₂[Si(CN)₆] with an excess of the strong Lewis acid B(C₆F₅)₃, the voluminous adduct anion {Si[CN⋅B(C₆F₅)₃]₆}²⁻ was obtained.     

Trigonal kristallisierende Metall(II)-hexacyanoferrate(II)M2II[Fe(CN)6]

Trigonal Crystallizing Metal(II) Hexacyanoferrates(II) M₂^II[Fe(CN)₆] According to X-ray powder diagrams, Ca₂[Fe(CN)₆], Cd₂[Fe(CN)₆], Zn₂[Fe(CN)₆] · 2 H₂O, Pb₂[Fe(CN)₆] and the firstly described compounds Zn₂[Fe(CN)₆] · 2 NH₃ and Sn₂[Fe(CN)₆] crystallize trigonal containing one formula unit in the unit cell. Ca₂[Fe(CN)₆] and Cd₂[Fe(CN)₆] are belonging to the space group D—P3 1m, the other compounds to D—P3 m1. The latters are described as coordination polymers with a coordination number 4 for Zn and 3 for Sn and Pb, respectively.     

CO Oxidation by Group 3 Metal Monoxide Cations Supported on [Fe(CO)4]2−

Infrared photodissociation spectroscopy of mass‐selected heteronuclear cluster anions in the form of OMFe(CO)₅⁻ (M=Sc, Y, La) indicates that all these anions involve an 18‐electron [Fe(CO)₄]²⁻ building block that is bonded with the M center through two bridged carbonyl ligands. The OLaFe(CO)₅⁻ anion is determined to be a CO‐tagged complex involving a [Fe(CO)₄]²⁻[LaO]⁺ anion core. In contrast, the OYFe(CO)₅⁻ anion is characterized to have a [Fe(CO)₄]²⁻[Y(η²‐CO₂)]⁺ structure involving a side‐on bonded CO₂ ligand. The CO‐tagged complex and the [Fe(CO)₄]²⁻[Sc(η²‐CO₂)]⁺ isomer co‐exist for the OScFe(CO)₅⁻ anion. These observations indicate that both the ScO⁺ and YO⁺ cations supported on [Fe(CO)₄]²⁻ are able to oxidize CO to CO₂. Theoretical analyses show that [Fe(CO)₄]²⁻ coordination significantly weakens the MO⁺ bond and decreases the energy gap of the interacting valence orbitals between MO⁺ and CO, leading to the CO oxidation reactions being both thermodynamically exothermic and kinetically facile.     

An Electrochemical Aptamer Biosensor for Bisphenol A on a Carbon Nanofibre-silver Nanoparticle Immobilisation Platform

A nanocomposite platform of silver nanoparticles and carbon nanofibres (AgCNFs) was used to immobilise a bisphenol A specific 63-mer ssDNA aptamer to form a biosensor. The fabrication process of the biosensor was studied with electrochemical impedance spectroscopy and cyclic voltammetry in the presence of [Fe(CN)₆]^3−/4− as redox probe. The biosensor detected bisphenol A in a linear range of 0.1–10 nM, with a limit of detection of 0.39 nM using square wave voltammetry (SWV). The biosensor exhibited good selectivity in the presence of interfering species at 100-fold concentrations and was used to detect BPA in real water sample.     

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.     

Slow Magnetic Relaxation,Antiferromagnetic Ordering,and Metamagnetism in MnII(H2dapsc)-FeIII(CN)6 Chain Complex with Highly Anisotropic Fe-CN-Mn Spin Coupling

Reactions of [Mn(H₂dapsc)Cl₂] ⋅ H₂O (dapsc=2,6- diacetylpyridine bis(semicarbazone)) with K₃[Fe(CN)₆] and (PPh₄)₃[Fe(CN)₆] lead to the formation of the chain polymeric complex {[Mn(H₂dapsc)][Fe(CN)₆][K(H₂O)_3.5]}_n ⋅ 1.5n H₂O ( 1 ) and the discrete pentanuclear complex {[Mn(H₂dapsc)]₃[Fe(CN)₆]₂(H₂O)₂} ⋅ 4 CH₃OH ⋅ 3.4 H₂O ( 2 ), respectively. In the crystal structure of 1 the high-spin [Mn^II(H₂dapsc)]²⁺ cations and low-spin hexacyanoferrate(III) anions are assembled into alternating heterometallic cyano-bridged chains. The K⁺ ions are located between the chains and are coordinated by oxygen atoms of the H₂dapsc ligand and water molecules. The magnetic structure of 1 is built from ferrimagnetic chains, which are antiferromagnetically coupled. The complex exhibits metamagnetism and frequency-dependent ac magnetic susceptibility, indicating single-chain magnetic behavior with a Mydosh-parameter φ=0.12 and an effective energy barrier (U_eff/k_B) of 36.0 K with τ₀=2.34×10⁻¹¹ s for the spin relaxation. Detailed theoretical analysis showed highly anisotropic intra-chain spin coupling between [Fe^III(CN)₆]³⁻ and [Mn^II(H₂dapsc)]²⁺ units resulting from orbital degeneracy and unquenched orbital momentum of [Fe^III(CN)₆]³⁻ complexes. The origin of the metamagnetic transition is discussed in terms of strong magnetic anisotropy and weak AF interchain spin coupling.     

Electrochemical detection of asparaginase antibodies using bifunctionalized carbon interfaces

The early detection of anti-asparaginase biomarker can facilitate timely modification of asparaginase chemotherapy, thereby avoiding serious complications. Herein we describe the preparation of a novel electrochemical biosensing interface for rapid detection of anti-asparaginase in the picomolar range (1–10 000 pM). Coimmobilization of ferrocene and asparaginase on a carbon interface (via diazonium grafting) facilitates transduction through attenuation of the surface-bound ferrocene redox couple. The limit of detection of 0.8 pM for this point-of-care applicable method compares favourably to that of traditional faradaic assaying (2.0 pM) where transduction occurs by the target blocking the diffusion of the solution redox probe [Fe(CN)₆]^3−/4−.     

Mössbauer and X-ray crystallographic studies of etraalkylammonium hexacyanoferrates(III)

A hexacyanoferrate(III) salt [N(C₂H₅)₄]₃[Fe(CN)₆]^.5H₂O (1)crystallized in a monoclinic space group (P2₁, Z = 2) with the nearest neighboring Fe-Fe distance of 8.20 Åound 1 distinctly showed magnetically-relaxed ⁵⁷Fe Mössbauer spectra below ca. 40 K. The Mössbauer line width at 4.2 K was much larger than that of K₃[Fe(CN)₆], which is ascribable to the long Fe-Fe distance in 1. Further broadened spectra were observed for [N(n-C₄H₉)₄]₃[Fe(CN)₆]^.xH₂O (2).     

Using an Electrochemical MIP Sensor for Selective Determination of 1-Naphthol in Oilfield Produced Water

A molecularly imprinted polymer (MIP) sensor was successfully constructed on glassy carbon electrode for the determination of 1-naphthol (1-Nph). The sensor was constructed by electropolymerization on bare GCE in the presence of the target molecule. The recognition of 1-Nph was conducted indirectly using [Fe(CN)₆]^3−/4− as redox probe. The MIP sensor presented wide linear working range and limit of detection of 1.5×10⁻⁹ mol L⁻¹. The MIP sensor was applied for the determination of 1-Nph in oilfield produced water. The results obtained showed good selectivity and sensitivity of the proposed sensor in terms of 1-Nph quantification.     

Electrically ‘wired’ tyrosinase enzyme inhibition electrode for the detection of respiratory poisons

The use of the solution redox species, [Os(bpy)₂Cl₂]^+/0, [Os(bpy)₂(MeIm)Cl]^2+/+ and [Fe(CN)₆]^4−/3−, where bpy is 2,2-bipyridine and MeIm is N-methylimidazole, as electron mediators in the enzymatic reduction of oxygen by tyrosinase is investigated. Co-immobilization of both enzyme and an osmium redox mediator in a hydrogel on glassy carbon electrodes results in a biosensor for the ‘reagentless’ addressing of enzyme activity, consuming only oxygen present in solution. Immobilized enzyme inhibition biosensors can thus be constructed for the detection of tyrosinase inhibitors, such as sodium azide, using this approach. The enzyme inhibition biosensor can detect levels of azide as low as 5 × 10⁻⁶ mol dm⁻³ in solution and may be useful in environmental monitoring applications and as an early warning poison sensor.     

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.     

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.     

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⁻.     

Nanoscale ensembles using building blocks inspired by the [FeFe]-hydrogenase active site

The hydrogenase model [Fe₂(S₂C₃H₆)(CN)₂(CO)₄]²⁻ was employed as a molecular tecton for the construction of supramolecular aggregates. IR spectroscopy indicated that cyanide bridged aggregates are formed when [Fe₂(S₂C₃H₆)(CN)₂(CO)₄]²⁻ was treated with Lewis acids such as Zn(tetraphenylporphyrinate), [Cu(NCMe)(2,2′-bipyridine)]PF₆ and [Cu(NCMe)₄]PF₆. Condensation of [Fe₂(S₂C₃H₆)(CN)₂(CO)₄]²⁻ with the tritopic Lewis acid [Cp¹Rh]²⁺ afforded the novel expanded tetrahedron cage, {[Fe₂(S₂C₃H₆)(CN)₂(CO)₄]₆[Cp¹Rh]₄}⁴⁻. The tetrahedron cage was characterized crystallographically as the PPN salt.