Chemistry of Transition Metal Cyanide Compounds: Modern Perspectives

Prussian Blue Analogue Glasses for Photoinduced CO2 Conversion

Crystal-to-glass transformation is a powerful approach to modulating the chemical and physical properties of crystals. Here we demonstrate that the glass transformation of cobalt hexacyanoferrate crystals, one of the Prussian blue analogues, increased the concentration of open metal sites and altered the electronic state while maintaining coordination geometries and short-range ordering in the structure. The compositional and structural changes were characterized by X-ray absorption fine structure, energy dispersive X-ray spectroscopy, and X-ray total scattering. The changes contribute to the flat band potential of the glass becoming closer to the redox potential of CO2 reduction. The valence band energy of the glass also shifts, resulting in lower band gap energy. Both the increased open metal sites and the optimal electronic structure upon vitrification enhance photocatalytic activity toward CO2-to-CO conversions (9.9 μmol h-1 CO production) and selectivity (72.4%) in comparison with the crystalline counterpart (3.9 μmol h-1 and 42.8%).

On the Synthesis and Structure of 'Naked' Ga(I) and In(I) Salts and the Surprising Stability of Simple Ga(I) and In(I) Salts in the Coordinating Solvents Ether and Acetonitrile

In this work, we present the solid-state structures of solvent-free Ga[pf] and In[pf] salts ([pf]-=[Al(ORF)4]-; RF=C(CF3)3), which are very rare examples of salts with truly 'naked' metal cations. Both salts may serve as starting materials for subvalent gallium and indium chemistry with very weakly coordinating ligands providing the freedom of choice for solvents and ligands for the future. On the other hand, we report and rationalize the formation and isolation of [M(OEt2)2][pf] and [M(MeCN)2][pf] (M=Ga, In), underlining the surprising stability of these subvalent group 13 M+ ions against disproportionation. Unexpectedly, dicoordinate and carbene analogous [M(L)2]+ ions with the [pf]- counterion are stable in L=acetonitrile and diethyl ether at room temperature, opening up possible applications for example in organic synthesis and catalysis.

Cadmium Dicyanoaurates and Their Reaction with Ammonia

The synthesis and crystal structures of two anionic cadmium dicyanoaurate coordination polymers, [nBu4N]6[(Cd4Cl4)2(Au(CN)2)12][CdCl4] (TCCA) and [nBu4N]2[Cd(Au(CN)2)4], and their reaction with ammonia vapour is reported. TCCA and the isostructural [nBu4N]6[(Cd4Br4)2(Au(CN)2)12][CdBr4] form 3-D arrays with [Cd4X4]4+ (X=Cl, Br) cubane clusters linked from each octahedral Cd(II) centre by three bridging [Au(CN)2]- units. TCCA reacts with ammonia with concentrations of 1000 ppm or higher to give a product with a quantum yield of 0.88, while [nBu4N]2[Cd(Au(CN)2)4], which forms a 2-D anionic Cd[Au(CN)2]2 sheet structure with axially pendant [Au(CN)2]- units, reacts with concentrated ammonia vapour to generate Cd(NH3)2[Au(CN)2]2; this has a similar 2-D sheet structure but with axial NH3 units. Vibrational spectroscopy illustrated that the reaction of both Cd/[Au(CN)2]-based materials with ammonia proceeded by breaking Cd-NC bonds. For [nBu4N]2[Cd(Au(CN)2)4], this results in decomposition into [nBu4N][Au(CN)2] ⋅ 0.5H2O and Cd(NH3)2[Au(CN)2]2, while the reaction of ammonia with TCCA is reversible by heating the ammonia-bound sample above 110 °C. Cd[Au(CN)2]2 can be prepared by thermal removal of NH3 units from Cd(NH3)2[Au(CN)2]2.

Water-Mediated Charge Transfer and Electron Localization in a Co3Fe2 Cyanide-Bridged Trigonal Bipyramidal Complex

The effects of temperature and chemical environment on a pentanuclear cyanide-bridged, trigonal bipyramidal molecular paramagnet have been investigated. Using element- and oxidation state-specific near-ambient pressure X-ray photoemission spectroscopy (NAP-XPS) to probe charge transfer and second order, nonlinear vibrational spectroscopy, which is sensitive to symmetry changes based on charge (de)localization coupled with DFT, a detailed picture of environmental effects on charge-transfer-induced spin transitions is presented. The molecular cluster, Co3Fe2(tmphen)6(μ-CN)6(t-CN)6, abbrev. Co3Fe2, shows changes in electronic behavior depending on the chemical environment. NAP-XPS shows that temperature changes induce a metal-to-metal charge transfer (MMCT) in Co3Fe2 between a Co and Fe center, while cycling between ultrahigh vacuum and 2 mbar of water at constant temperature causes oxidation state changes not fully captured by the MMCT picture. Sum frequency generation vibrational spectroscopy (SFG-VS) probes the role of the cyanide ligand, which controls the electron (de)localization via the superexchange coupling. Spectral shifts and intensity changes indicate a change from a charge delocalized, Robin-Day class II/III high spin state to a charge-localized, class I low spin state consistent with DFT. In the presence of a H-bonding solvent, the complex adopts a localized electronic structure, while removal of the solvent delocalizes the charges and drives an MMCT. This change in Robin-Day classification of the complex as a function of chemical environment results in reversible switching of the dipole moment, analogous to molecular multiferroics. These results illustrate the important role of the chemical environment and solvation on underlying charge and spin transitions in this and related complexes.

Low-Valent Transition Metalate Anions in Synthesis, Small Molecule Activation, and Catalysis

This review surveys the synthesis and reactivity of low-oxidation state metalate anions of the d-block elements, with an emphasis on contributions reported between 2006 and 2022. Although the field has a long and rich history, the chemistry of transition metalate anions has been greatly enhanced in the last 15 years by the application of advanced concepts in complex synthesis and ligand design. In recent years, the potential of highly reactive metalate complexes in the fields of small molecule activation and homogeneous catalysis has become increasingly evident. Consequently, exciting applications in small molecule activation have been developed, including in catalytic transformations. This article intends to guide the reader through the fascinating world of low-valent transition metalates. The first part of the review describes the synthesis and reactivity of d-block metalates stabilized by an assortment of ligand frameworks, including carbonyls, isocyanides, alkenes and polyarenes, phosphines and phosphorus heterocycles, amides, and redox-active nitrogen-based ligands. Thereby, the reader will be familiarized with the impact of different ligand types on the physical and chemical properties of metalates. In addition, ion-pairing interactions and metal-metal bonding may have a dramatic influence on metalate structures and reactivities. The complex ramifications of these effects are examined in a separate section. The second part of the review is devoted to the reactivity of the metalates toward small inorganic molecules such as H2, N2, CO, CO2, P4 and related species. It is shown that the use of highly electron-rich and reactive metalates in small molecule activation translates into impressive catalytic properties in the hydrogenation of organic molecules and the reduction of N2, CO, and CO2. The results discussed in this review illustrate that the potential of transition metalate anions is increasingly being tapped for challenging catalytic processes with relevance to organic synthesis and energy conversion. Therefore, it is hoped that this review will serve as a useful resource to inspire further developments in this dynamic research field.

Predicting Distortion Magnitudes in Prussian Blue Analogues

Based on simple electrostatic and harmonic potential considerations, we derive a straightforward expression linking the composition of a Prussian blue analogue (PBA) to its propensity to undergo collective structural distortions. We demonstrate the existence of a threshold value, below which PBAs are undistorted and above which PBAs distort by a degree that is controlled by a geometric tolerance factor. Our analysis rationalizes the presence, absence, and magnitude of distortions in a wide range of PBAs and distinguishes their structural chemistry from that of other hybrid perovskites.

Metal-Metal Charge Transfer Properties of a Series of Trinuclear Fe2 Ru and Corresponding Pentanuclear Fe2 Ru2 Ag Cyanido-Bridged Complexes

A series of trimetallic cyanidometal-bridged compounds [Men Cp(dppe)FeII -(μ-NC)-RuII (MeOpy)4 -(μ-CN)-FeII (dppe)CpMen ] - [PF6 ]2 (N[PF6 ]2 , n=0, N =1; n=1, N=2; n=3, N=3; Cp=cyclopentadiene, dppe=1,2-bis(diphenylphosphino)ethane, MeOpy=4-methoxypyridine) and their one- and two-electron oxidized compounds N3+ and N4+ were synthesized and characterized. Meanwhile, a series of corresponding linear cyanido-bridged pentanuclear compounds [Men Cp(dppe)FeIII -(μ-NC)-RuII (MeOpy)4 -(μ-NC)-AgI -(μ-CN)-RuII (MeOpy)4 -(μ-CN)-FeIII (dppe)CpMen ][BF4 ]5 (M[BF4 ]5 , n=0, M=4; n=1, M=5; n=3, M=6) were also obtained and well characterized. The investigations suggest that in the trinuclear system there exists remote interaction between the two Fe centers, but no significant interactions exist across the central silver unit between the metals on the two sides of the silver center in the pentanuclear system. In both the trinuclear N4+ and the pentanuclear M5+ complexes, there exists the neighboring RuII →FeIII MM'CT transitions, and the MM'CT energy in the corresponding trinuclear system is higher than those in the pentanuclear system in which no remote metal-metal interaction occurs. Meanwhile, as the substituted methyl groups on the cyclopentadiene increases, the redox potential of the ruthenium in the trinuclear N4+ series increases, but that in the pentanuclear M5+ complexes decreases.

Where Are Sodium Ions in AOT Reverse Micelles? Fluoride Anion Probes Nanoconfined Ions by 19F Nuclear Magnetic Resonance Spectroscopy

Confining water to nanosized spaces creates a unique environment that can change water's structural and dynamic properties. When ions are present in these nanoscopic spaces, the limited number of water molecules and short screening length can dramatically affect how ions are distributed compared to the homogeneous distribution assumed in bulk aqueous solution. Here, we demonstrate that the chemical shift observed in ¹⁹F NMR spectroscopy of fluoride anion, F^-, probes the location of sodium ions, Na⁺, confined in reverse micelles prepared from AOT (sodium dioctyl sulfosuccinate) surfactants. Our measurements show that the nanoconfined environment of reverse micelles can lead to extremely high apparent ion concentrations and ionic strength, beyond the limit in bulk aqueous solutions. Most notably, the ¹⁹F NMR chemical shift trends we observe for F^- in the reverse micelles indicate that the AOT sodium counterions remain at or near the interior interface between surfactant and water, thus providing the first experimental support for this hypothesis.

A Ferroelectric Aminophosphonium Cyanoferrate with a Large Electrostrictive Coefficient as a Piezoelectric Nanogenerator

Hybrid materials possessing piezo- and ferroelectric properties emerge as excellent alternatives to conventional piezoceramics due to their merits of facile synthesis, lightweight nature, ease of fabrication and mechanical flexibility. Inspired by the structural stability of aminophosphonium compounds, here we report the first A₃ BX₆ type cyanometallate [Ph₂ (ⁱ PrNH)₂ P]₃ [Fe(CN)₆ ] (1), which shows a ferroelectric saturation polarization (P_s ) of 3.71 μC cm^-2 . Compound 1 exhibits a high electrostrictive coefficient (Q₃₃ ) of 0.73 m⁴  C^-2 , far exceeding those of piezoceramics (0.034-0.096 m⁴  C^-2 ). Piezoresponse force microscopy (PFM) analysis demonstrates the polarization switching and domain structure of 1 further confirming its ferroelectric nature. Furthermore, thermoplastic polyurethane (TPU) polymer composite films of 1 were prepared and employed as piezoelectric nanogenerators. Notably, the 15 wt % 1-TPU device gave a maximum output voltage of 13.57 V and a power density of 6.03 μW cm^-2 .

Uncovering the Interplay of Competing Distortions in the Prussian Blue Analogue K2Cu[Fe(CN)6]

We report the synthesis, crystal structure, thermal response, and electrochemical behavior of the Prussian blue analogue (PBA) K2Cu[Fe(CN)6]. From a structural perspective, this is the most complex PBA yet characterized: its triclinic crystal structure results from an interplay of cooperative Jahn-Teller order, octahedral tilts, and a collective "slide" distortion involving K-ion displacements. These different distortions give rise to two crystallographically distinct K-ion channels with different mobilities. Variable-temperature X-ray powder diffraction measurements show that K-ion slides are the lowest-energy distortion mechanism at play, as they are the only distortion to be switched off with increasing temperature. Electrochemically, the material operates as a K-ion cathode with a high operating voltage and an improved initial capacity relative to higher-vacancy PBA alternatives. On charging, K+ ions are selectively removed from a single K-ion channel type, and the slide distortions are again switched on and off accordingly. We discuss the functional importance of various aspects of structural complexity in this system, placing our discussion in the context of other related PBAs.

Old Materials for New Functions: Recent Progress on Metal Cyanide Based Porous Materials

Cyanide is the simplest ligand with strong basicity to construct open frameworks including some of the oldest compounds reported in the history of coordination chemistry. Cyanide can form numerous cyanometallates with different transition metal ions showing diverse geometries. Rational design of robust extended networks is enabled by the strong bonding nature and high directionality of cyanide ligand. By virtue of a combination of cyanometallates and/or organic linkers, multifunctional framework materials can be targeted and readily synthesized for various applications, ranging from molecular adsorptions/separations to energy conversion and storage, and spin-crossover materials. External guest- and stimuli-responsive behaviors in cyanide-based materials are also highlighted for the development of the next-generation smart materials. In this review, an overview of the recent progress of cyanide-based multifunctional materials is presented to demonstrate the great potential of cyanide ligands in the development of modern coordination chemistry and material science.

Photochemical C(sp)-C(sp2) Bond Activation in Phosphaalkynes: A New Route to Reactive Terminal Cyaphido Complexes LnM-C≡P

The photochemical activation of the C(sp)-C(sp²) bond in Pt(0)-η²-aryl-phosphaalkyne complexes leads selectively to coordination compounds of the type L_nPt(aryl)(C≡P). The oxidative addition reaction is a novel, clean, and atom-economic route for the synthesis of reactive terminal Pt(II)-cyaphido complexes, which can undergo [3 + 2] cycloaddition reactions with organic azides, yielding the corresponding Pt(II)-triazaphospholato complexes. The C-C bond cleavage reaction is thermodynamically uphill. Upon heating, the reverse and quantitative reductive elimination toward the Pt(0)-phosphaalkyne-π-complex is observed.

K2 Mn3 [FeII (CN)6 ]2 NPs with High T1 -Relaxivity Attributable to Water Coordination on the Mn(II) Center for Gastrointestinal Tract MR Imaging

The lack of acid stability in the stomach and of temporal stability when moving through the gastrointestinal (GI) tract has made the development of oral magnetic resonance imaging (MRI) contrast agents based on the platform of Gd³⁺ -complexes problematic.On the other hand, the negative contrast enhancement produced by the T₂ -weighted magnetic metal oxide nanoparticles (NPs) often renders the image readout difficult. Biocompatible NPs of the manganese Prussian blue analog K₂ Mn₃ [Fe^II (CN)₆ ]₂ exhibit extremely high stability under the acidic conditions of the gastric juice. Additionally, the high r₁ relaxivity, low toxicity, and high temporal stability of such NPs offer great potential for the development of a true T₁ -weighted oral contrast agent for MRI of the entire GI tract.

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.

Tricyanidoferrates(-IV) and Ruthenates(-IV) with Non-Innocent Cyanido Ligands

Exceptionally electron‐rich, nearly trigonal‐planar tricyanidometalate anions [Fe(CN)₃]⁷⁻ and [Ru(CN)₃]⁷⁻ were stabilized in LiSr₃[Fe(CN)₃] and AE_3.5[M(CN)₃] (AE=Sr, Ba; M=Fe, Ru). They are the first examples of group 8 elements with the oxidation state of −IV. Microcrystalline powders were obtained by a solid‐state route, single crystals from alkali metal flux. While LiSr₃[Fe(CN)₃] crystallizes in P6₃/m, the polar space group P6₃ with three‐fold cell volume for AE_3.5[M(CN)₃] is confirmed by second harmonic generation. X‐ray diffraction, IR and Raman spectroscopy reveal longer C−N distances (124–128 pm) and much lower stretching frequencies (1484–1634 cm⁻¹) than in classical cyanidometalates. Weak C−N bonds in combination with strong M−C π‐bonding is a scheme also known for carbonylmetalates. Instead of the formal notation [Fe^−IV(CN⁻)₃]⁷⁻, quantum chemical calculations reveal non‐innocent intermediate‐valent CN^1.67− ligands and a closed‐shell d¹⁰ configuration for Fe, that is, Fe²⁻.

The state of the field: from inception to commercialization of metal–organic frameworks

We provide a brief overview of the state of the MOF field from their inception to their synthesis, potential applications, and finally, to their commercialization.

General Approach for Constructing Mechanoresponsive and Redox-Active Metal-Organic and Covalent Organic Frameworks by Solid-Liquid Reaction: Ferrocene as the Versatile Function Unit

We report for the first time the construction of mechanoresponsive and redox-active metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) by anchoring ferrocene (Fc) pendants as mechanophores in the pore wall. This work outlines a simple, general, and low-cost route to tailor MOFs and COFs by a Fc unit for mechanoresponsive nature, the release of Fe ions, redox behavior, and modulation of the skeleton charge together.

Magnesium Cyanide or Isocyanide?

Preference for the binding mode of the CN- ligand to Mg (Mg-CN vs. Mg-NC) is investigated. A monomeric Mg complex with a terminal CN ligand was prepared using the dipyrromethene ligand Mes DPM which successfully blocks dimerization. While reaction of (Mes DPM)MgN(SiMe3 )2 with Me3 SiCN gave the coordination complex (Mes DPM)MgN(SiMe3 )2 ⋅NCSiMe3 , reaction with (Mes DPM)Mg(nBu) led to (Mes DPM)MgNC⋅(THF)2 . A Mg-NC/Mg-CN ratio of ≈95:5 was established by crystal-structure determination and DFT calculations. IR studies show absorbances for CN stretching at 2085 cm-1 (Mg-NC) and 2162 cm-1 (Mg-CN) as confirmed by 13 C labeling. In solution and in the solid state, the CN ligand rotates within the pocket. The calculated isomerization barrier is only 12.0 kcal mol-1 and the 13 C NMR signal for CN decoalesces at -85 °C (Mg-NC: 175.9 ppm, Mg-CN: 144.3 ppm). Experiment and theory both indicate that Mg complexes with the CN- ligand should not be named cyanides but are more properly defined as isocyanides.

Prospects for electroactive and conducting framework materials

Electroactive and conducting framework materials, encompassing coordination polymers and metal-organic frameworks, have captured the imagination of the scientific community owing to their highly designable nanoporous structures and their potential applications in electrochromic devices, electrocatalysts, porous conductors, batteries and solar energy harvesting systems, among many others. While they are now considered integral members of the broader field of inorganic materials, it is timely to reflect upon their strengths and challenges compared with 'traditional' solid-state materials such as minerals, pigments and zeolites. Indeed, the latter have been known since ancient times and have been prized for centuries in fields as diverse as art, archaeology and industrial catalysis. This opinion piece considers a brief historical perspective of traditional electroactive and conducting inorganic materials, with a view towards very recent experimental progress and new directions for future progress in the burgeoning area of coordination polymers and metal-organic frameworks. Overall, this article bears testament to the rich history of electroactive solids and looks at the challenges inspiring a new generation of scientists. This article is part of the theme issue 'Mineralomimesis: natural and synthetic frameworks in science and technology'.

Humidity-Induced Switching between Two Magnetic and Structural Phases in a CoII -[WV (CN)8 ] Molecular Magnet

The self-assembly of cobalt(II) with purine and octacyanidotungstate(V) results in the formation of the three-dimensional Co₃ [W(CN)₈ ]₂ (purine)₂ ⋅8.5H₂ O (1) coordination polymer. This compound exhibits humidity-induced variation of the number of water molecules of crystallisation leading to a reversible structural phase transition and the alternation of the long-range ferromagnetic ordering temperature from T_C =29 K for the pristine assembly (1) to T_C =49 K for the sample stored in a low-humidity atmosphere (1-deh). This phenomenon can be attributed to a reversible change in the hydrogen-bonding network resulting in the modification of the local geometries of cobalt(II) as well as the cyanido bridges.

Characterization of Trinuclear Oxo Bridged Cobalt Complexes in Isolation

This study elucidates molecular structures, fragmentation pathways and relative stabilities of isolated trinuclear oxo bridged cobalt complexes of the structural type [Co3O(OAc)6(Py)n]+ (OAc=acetate, Py=pyridine, n=0, 1, 2, 3). We present infrared multiple photon dissociation (IR-MPD) spectra in combination with quantum chemical calculations. They indicate that the coordination of axial pyridine ligands to the [Co3O(OAc)6]+ subunit disturbs the triangular geometry of the Co3O core. [Co3O(OAc)6]+ exhibits a nearly equilateral triangular Co3O core geometry. The coordination of one or two pyridine ligands disturbs this arrangement resulting in isosceles triangular Co3O core geometries (in the cases of n=1 and 2). Coordination of three pyridine ligands (n=3) results in an equilateral triangular Co3O core geometry as in the case of n=0. Collision induced dissociation (CID) studies reveal that the complexes undergo a consecutive elimination of pyridine and acetate ligands with increasing excitation energy. Relative stabilities of the complexes decrease with the number of coordinated pyridine ligands. The presented results help to gain a fundamental insight into the molecular structure of trinuclear oxo bridged cobalt complexes void of any external effects such as crystal packing or solvation.

Cyanide Ligand Assembly by Carbon Atom Transfer to an Iron Nitride

The new iron(IV) nitride complex PhB(ⁱPr₂Im)₃Fe≡N reacts with 2 equiv of bis(diisopropylamino)cyclopropenylidene (BAC) to provide PhB(ⁱPr₂Im)₃Fe(CN)(N₂)(BAC). This unusual example of a four-electron reaction involves carbon atom transfer from BAC to create a cyanide ligand along with the alkyne ⁱPr₂N-C≡C-NⁱPr₂. The iron complex is in equilibrium with an N₂-free species. Further reaction with CO leads to formation of a CO analogue, which can be independently prepared using NaCN as the cyanide source, while reaction with B(C₆F₅)₃ provides the cyanoborane derivative.

Increase in the Magnetic Ordering Temperature (Tc) as a Function of the Applied Pressure for A2Mn[Mn(CN)6] (A = K, Rb, Cs) Prussian Blue Analogues

Magnetization measurements under pressure reveal that the external hydrostatic pressure significantly increases in the ferrimagnetic transition temperature, T_c, for A₂Mn[Mn(CN)₆] (A = K, Rb, Cs). In the case of monoclinic A = K and Rb, dT_c/dp values are 21.2 and 14.6 K GPa^-1, respectively, and T_c increases by 53 and 39%, respectively, from ambient pressure to 1.0 GPa. The cubic A = Cs compound also shows a monotonous increase with an initial rate of 4.22 K GPa^-1 and about 11.4 K GPa^-1 above 0.6 GPa, and an overall T_c increase by 26% at 1.0 GPa. The increase in T_c is attributed to deformation of the structure such that the Mn^II-N≡C angle decreases with increasing pressure. The smaller the alkali cation, the greater the decrease in the Mn^II-N≡C angle induced by pressure and the larger the increase of dT_c/dp. This is in accordance with the ambient-pressure structures for A₂Mn[Mn(CN)₆] (A = K, Rb, Cs), which have decreasing Mn^II-N≡C angles that correlate to the observed increasing T_cs as K > Rb > Cs. The large increase in T_c for the A = K compound is the highest class among several cyano-bridged metal complexes. The tuning of the transition temperature by such a weak pressure may lead to additional applications such as switching devices.

Thermal Expansion Behavior of MI[AuX2(CN)2]-Based Coordination Polymers (M = Ag, Cu; X = CN, Cl, Br)

Two sets of trans-[AuX₂(CN)₂]^--based coordination polymer materials-M[AuX₂(CN)₂] (M = Ag; X = Cl, Br or M = Cu; X = Br) and M[Au(CN)₄] (M = Ag, Cu)-were synthesized and structurally characterized and their dielectric constants and thermal expansion behavior explored. The M[AuX₂(CN)₂] series crystallized in a tightly packed, mineral-like structure featuring 1-D trans-[AuX₂(CN)₂]^--bridged chains interconnected via a series of intermolecular Au···X and M···X (M = Ag, Cu) interactions. The M[Au(CN)₄] series adopted a 2-fold interpenetrated 3-D cyano-bound framework lacking any weak intermolecular interactions. Despite the tight packing and the presence of intermolecular interactions, these materials exhibited decreased thermal stability over unbound trans-[AuX₂(CN)₂]^- in [ⁿBu₄N][AuX₂(CN)₂]. A significant dielectric constant of up to ε_r = 36 for Ag[AuCl₂(CN)₂] (1 kHz) and a lower ε_r = 9.6 (1 kHz) for Ag[Au(CN)₄] were measured and interpreted in terms of their structures and composition. A systematic analysis of the thermal expansion properties of the M[AuX₂(CN)₂] series revealed a negative thermal expansion (NTE) component along the cyano-bridged chains with a thermal expansion coefficient (α_CN) of -13.7(11), -14.3(5), and -11.36(18) ppm·K^-1 for Ag[AuCl₂(CN)₂], Ag[AuBr₂(CN)₂], and Cu[AuBr₂(CN)₂], respectively. The Au···X and Ag···X interactions affect the thermal expansion similarly to metallophilic Au···Au interactions in M[Au(CN)₂] and AuCN; replacing X = Cl with the larger Br atoms has a less significant effect. A similar analysis for the M[Au(CN)₄] series (where the volume thermal expansion coefficient, α_V, is 41(3) and 68.7(19) ppm·K^-1 for M = Ag, Cu, respectively) underscored the significance of the effect of the atomic radius on the flexibility of the framework and, thus, the thermal expansion properties.

Hydrogen Cyanide in the Rhizosphere: Not Suppressing Plant Pathogens, but Rather Regulating Availability of Phosphate

Plant growth promoting rhizobacteria produce chemical compounds with different benefits for the plant. Among them, HCN is recognized as a biocontrol agent, based on its ascribed toxicity against plant pathogens. Based on several past studies questioning the validity of this hypothesis, we have re-addressed the issue by designing a new set of in vitro experiments, to test if HCN-producing rhizobacteria could inhibit the growth of phytopathogens. The level of HCN produced by the rhizobacteria in vitro does not correlate with the observed biocontrol effects, thus disproving the biocontrol hypothesis. We developed a new concept, in which HCN does not act as a biocontrol agent, but rather is involved in geochemical processes in the substrate (e.g., chelation of metals), indirectly increasing the availability of phosphate. Since this scenario can be important for the pioneer plants living in oligotrophic alpine environments, we inoculated HCN producing bacteria into sterile mineral sand together with germinating plants and showed that the growth of the pioneer plant French sorrel was increased on granite-based substrate. No such effect could be observed for maize, where plantlets depend on the nutrients stored in the endosperm. To support our concept, we used KCN and mineral sand and showed that mineral mobilization and phosphate release could be caused by cyanide in vitro. We propose that in oligotrophic alpine environments, and possibly elsewhere, the main contribution of HCN is in the sequestration of metals and the consequential indirect increase of nutrient availability, which is beneficial for the rhizobacteria and their plant hosts.

Dicyanometallates as Model Extended Frameworks

We report the structures of eight new dicyanometallate frameworks containing molecular extra-framework cations. These systems include a number of hybrid inorganic–organic analogues of conventional ceramics, such as Ruddlesden–Popper phases and perovskites. The structure types adopted are rationalized in the broader context of all known dicyanometallate framework structures. We show that the structural diversity of this family can be understood in terms of (i) the charge and coordination preferences of the particular metal cation acting as framework node, and (ii) the size, shape, and extent of incorporation of extra-framework cations. In this way, we suggest that dicyanometallates form a particularly attractive model family of extended frameworks in which to explore the interplay between molecular degrees of freedom, framework topology, and supramolecular interactions.

Aegle marmelos Mediated Green Synthesis of Different Nanostructured Metal Hexacyanoferrates: Activity against Photodegradation of Harmful Organic Dyes

Prussian blue analogue potassium metal hexacyanoferrate (KMHCF) nanoparticles Fe4[Fe(CN)6]3 (FeHCF), K2Cu3[Fe(CN)6]2 (KCuHCF), K2Ni[Fe(CN)6]·3H2O (KNiHCF), and K2Co[Fe(CN)6] (KCoHCF) have been synthesized using plant based biosurfactant Aegle marmelos (Bael) and water as a green solvent. It must be emphasized here that no harmful reagent or solvent was used throughout the study. Plant extracts are easily biodegradable and therefore do not cause any harm to the environment. Hence, the proposed method of synthesis of various KMHCF nanoparticles followed a green path. The synthesized nanoparticles were characterized by powder X-ray diffraction (PXRD), Field-Emission Scanning Electron Microscopy (FE-SEM), Transmission Electron Microscopy (TEM), and Fourier Transform Infrared Spectroscopy (FT-IR). MHCF nanoparticles were used for the photocatalytic degradation of toxic dyes like Malachite Green (MG), Eriochrome Black T (EBT), Methyl Orange (MO), and Methylene Blue (MB). Under optimized reaction conditions, maximum photocatalytic degradation was achieved in case of KCuHCF nanoparticles mediated degradation process (MG: 96.06%, EBT: 83.03%, MB: 94.72%, and MO: 63.71%) followed by KNiHCF (MG: 95%, EBT: 80.32%, MB: 91.35%, and MO: 59.42%), KCoHCF (MG: 91.45%, EBT: 78.84%, MB: 89.28%, and MO: 58.20%).