Solid-State Electrochemistry of a Semiconducting MMX-Type Diplatinum Iodide Chain Complex

Electron-transfer-facilitated dissolution, ion insertion, and desorption associated with an MMX-type quasi-one-dimensional iodide-bridged dinuclear Pt complex (MMX chain) have been investigated for the first time. K₂(NC₃N)­[Pt₂(pop)₄I]·4H₂O (<b>1</b>) (NC₃N²⁺ = (H₃NC₃H₆NH₃)²⁺; pop = P₂H₂O₅^2–) is a semiconductor with a three-dimensional coordination-bond and hydrogen-bond network included in the chain. The cyclic voltammetry of <b>1</b> was studied by using <b>1</b>-modified electrodes in contact with acetonitrile solutions containing electrolyte. The chemical reversibility for oxidation of <b>1</b> depended on the electrolyte cation size, with large cations such as tetrabutylammonium (Bu₄N⁺) being too large to penetrate the pores formed by the loss of K⁺ and NC₃N²⁺ upon oxidation. The potential for reduction of <b>1</b> decreased as the cation size increased. The presence of the acid induced additional well-defined processes but with gradual solid dissolution, attributed to the breaking of the coordination-bond networks

Redox and Acid–Base Chemistry of 7,7,8,8-Tetracyanoquinodimethane, 7,7,8,8-Tetracyanoquinodimethane Radical Anion, 7,7,8,8-Tetracyanoquinodimethane Dianion, and Dihydro-7,7,8,8-Tetracyanoquinodimethane in Acetonitrile

The chemistry and electrochemistry of TCNQ (7,7,8,8-tetracyanoquinodimethane), TCNQ^•–, TCNQ^2–, and H₂TCNQ in acetonitrile (0.1 M Bu₄NPF₆) solution containing trifluoroacetic acid (TFA) has been studied by transient and steady-state voltammetric methods with the interrelationship between the redox and the acid–base chemistry being supported by simulations of the cyclic voltammograms. In the absence of acid, TCNQ and its anions undergo two electrochemically and chemically reversible one-electron processes. However, in the presence of TFA, the voltammetry is considerably more complex. The TCNQ^2– dianion is protonated to form HTCNQ^–, which is oxidized to HTCNQ^•, and H₂TCNQ which is electroinactive over the potential range of −1.0 to +1.0 V versus Ag/Ag⁺. The monoreduced TCNQ^•– radical anion is weakly protonated to give HTCNQ^•, which disproportionates to TCNQ and H₂TCNQ. In acetonitrile, H₂TCNQ deprotonates slowly, whereas in <i>N</i>,<i>N</i>-dimethylformamide or tetrahydrofuran, rapid deprotonation occurs to yield HTCNQ^– as the major species. H₂TCNQ is fully deprotonated to the TCNQ^2– dianion in the presence of an excess concentration of the weak base, CH₃COOLi. Differences in the redox and acid–base chemistry relative to the fluorinated derivative TCNQF₄ are discussed in terms of structural and electronic factors

Role of Water in the Dynamic Disproportionation of Zn-Based TCNQ(F₄) Coordination Polymers (TCNQ = Tetracyanoquinodimethane)

Intriguingly, coordination polymers containing TCNQ^2– and TCNQF₄^2– (TCNQ = 7,7,8,8-tetracyanoquinodimethane, TCNQF₄ = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, both designated as TCNQ­(F₄)^2–) may be generated from reaction of metal ions with TCNQ­(F₄)^•–. An explanation is now provided in terms of a solvent-dependent dynamic disproportionation reaction. A systematic study of reactions associated with TCNQ­(F₄) and electrochemically generated TCNQ­(F₄)_MeCN^•– and TCNQ­(F₄)_MeCN^2– revealed that disproportionation of TCNQ­(F₄)_MeCN^•– radical anions in acetonitrile containing a low concentration of water is facilitated by the presence of Zn_MeCN²⁺. Thus, while the disproportionation reaction 2TCNQ­(F₄)_MeCN^•– ⇌ TCNQ­(F₄)_MeCN + TCNQ­(F₄)_MeCN^2– is thermodynamically very unfavorable in this medium (<i>K</i>_eq ≈ 9 × 10^–10; TCNQF₄), the preferential precipitation of ZnTCNQ­(F₄)_(s) drives the reaction: Zn_MeCN²⁺ + 2 TCNQ­(F₄)_MeCN^•– ⇌ ZnTCNQ­(F₄)_(s) + TCNQ­(F₄)_MeCN. The concomitant formation of soluble TCNQ­(F₄)_MeCN and insoluble ZnTCNQ­(F₄)_(s) and the loss of TCNQ­(F₄)_MeCN^•– were verified by UV–visible and infrared spectroscopy and steady-state voltammetry. Importantly, the reverse reaction of comproportionation rather than disproportionation becomes the favored process in the presence of ≥3% (v/v) water, due to the increased solubility of solid ZnTCNQ­(F₄)_(s). Thus, in this “wet” environment, Zn_MeCN²⁺ and TCNQ­(F₄)_MeCN^•– are produced from a mixture of ZnTCNQ­(F₄)_(s) and TCNQ­(F₄)_MeCN and with the addition of water provides a medium for synthesis of [Zn­(TCNQ­(F₄))₂(H₂O)₂]. An important conclusion from this work is that the redox level of TCNQ­(F₄)-based materials, synthesized from a mixture of metal cations and TCNQ­(F₄)^•–, is controlled by a solvent-dependent disproportionation/comproportionation reaction that may be tuned to favor formation of solids containing the monoanion radical, the dianion, or even a mixture of both

MnO_<i>x</i> Nanoparticle-Dispersed CeO₂ Nanocubes: A Remarkable Heteronanostructured System with Unusual Structural Characteristics and Superior Catalytic Performance

Understanding the interface-induced effects of heteronanostructured catalysts remains a significant challenge due to their structural complexity, but it is crucial for developing novel applied catalytic materials. This work reports a systematic characterization and catalytic evaluation of MnO_<i>x</i> nanoparticle-dispersed CeO₂ nanocubes for two important industrial applications, namely, diesel soot oxidation and continuous-flow benzylamine oxidation. The X-ray diffraction and Raman studies reveal an unusual lattice expansion in CeO₂ after the addition of MnO_<i>x</i>. This interesting observation is due to conversion of smaller sized Ce⁴⁺ (0.097 nm) to larger sized Ce³⁺ (0.114 nm) in cerium oxide led by the strong interaction between MnO_<i>x</i> and CeO₂ at their interface. Another striking observation noticed from transmission electron microscopy, high angle annular dark-field scanning transmission electron microscopy, and electron energy loss spectroscopy studies is that the MnO_<i>x</i> species are well-dispersed along the edges of the CeO₂ nanocubes. This remarkable decoration leads to an enhanced reducible nature of the cerium oxide at the MnO_<i>x</i>/CeO₂ interface. It was found that MnO_<i>x</i>/CeO₂ heteronanostructures efficiently catalyze soot oxidation at lower temperatures (50% soot conversion, <i>T</i>₅₀ ∼660 K) compared with that of bare CeO₂ nanocubes (<i>T</i>₅₀ ∼723 K). Importantly, the MnO_<i>x</i>/CeO₂ heteronanostructures exhibit a noticeable steady performance in the oxidation of benzylamine with a high selectivity of the dibenzylimine product (∼94–98%) compared with that of CeO₂ nanocubes (∼69–91%). The existence of a strong synergistic effect at the interface sites between the CeO₂ and MnO_<i>x</i> components is a key factor for outstanding catalytic efficiency of the MnO_<i>x</i>/CeO₂ heteronanostructures

Nanowire Morphology of Mono- and Bidoped α‑MnO₂ Catalysts for Remarkable Enhancement in Soot Oxidation

In the present work, nanowire morphologies of α-MnO₂, cobalt monodoped α-MnO₂, Cu and Co bidoped α-MnO₂, and Ni and Co bidoped α-MnO₂ samples were prepared by a facile hydrothermal synthesis. The structural, morphological, surface, and redox properties of all the as-prepared samples were investigated by various characterization techniques, namely, scanning electron microscopy (SEM), transmission and high resolution electron microscopy (TEM and HR-TEM), powder X-ray diffraction (XRD), N₂ sorption surface area measurements, X-ray photoelectron spectroscopy (XPS), hydrogen-temperature-programmed reduction (H₂-TPR), and oxygen-temperature-programmed desorption (O₂-TPD). The soot oxidation performance was found to be significantly improved via metal mono- and bidoping. In particular, Cu and Co bidoped α-MnO₂ nanowires showed a remarkable improvement in soot oxidation performance, with its <i>T</i>₅₀ (50% soot conversion) values of 279 and 431 °C under tight and loose contact conditions, respectively. The soot combustion activation energy for the Cu and Co bidoped MnO₂ nanowires is 121 kJ/mol. The increased oxygen vacancies, greater number of active sites, facile redox behavior, and strong synergistic interaction were the key factors for the excellent catalytic activity. The longevity of Cu and Co bidoped α-MnO₂ nanowires was analyzed, and it was found that the Cu/Co bidoped α-MnO₂ nanowires were highly stable after five successive cycles and showed an insignificant decrease in soot oxidation activity. Furthermore, the HR-TEM analysis of a spent catalyst after five cycles indicated that the (310) crystal plane of α-MnO₂ interacts with the soot particles; therefore, we can assume that more-reactive exposed surfaces positively affect the reaction of soot oxidation. Thus, the Cu and Co bidoped α-MnO₂ nanowires provide promise as a highly effective alternative to precious metal based automotive catalysts

Controlling Core/Shell Formation of Nanocubic <i>p</i>‑Cu₂O/<i>n</i>‑ZnO Toward Enhanced Photocatalytic Performance

p-Type Cu₂O/n-type ZnO core/shell photocatalysts has been demonstrated to be an efficient photocatalyst as a result of their interfacial structure tendency to reduce the recombination rate of photogenerated electron–hole pairs. Monodispersed Cu₂O nanocubes were synthesized and functioned as the core, on which ZnO nanoparticles were coated as the shells having varying morphologies. The evenly distributed ZnO decoration as well as assembled nanospheres of ZnO were carried out by changing the molar concentration ratio of Zn/Cu. The results indicate that the photocatalytic performance is initially increased, owing to formation of small ZnO nanoparticles and production of efficient p–n junction heterostructures. However, with increasing Zn concentration, the decorated ZnO nanoparticles tend to form large spherical assemblies resulting in decreased photocatalytic activity due to the interparticle recombination between the agglomerated ZnO nanoparticles. Therefore, photocatalytic activity of Cu₂O/ZnO heterostructures can be optimized by controlling the assembly and morphology of the ZnO shell

Isoreticular Contraction of Cage-like Metal–Organic Frameworks with Optimized Pore Space for Enhanced C₂H₂/CO₂ and C₂H₂/C₂H₄ Separations

The C2H2 separation from CO2 and C2H4 is of great importance yet highly challenging in the petrochemical industry, owing to their similar physical and chemical properties. Herein, the pore nanospace engineering of cage-like mixed-ligand MFOF-1 has been accomplished via contracting the size of the pyridine- and carboxylic acid-functionalized linkers and introducing a fluoride- and sulfate-bridging cobalt cluster, based on a reticular chemistry strategy. Compared with the prototypical MFOF-1, the constructed FJUT-1 with the same topology presents significantly improved C2H2 adsorption capacity, and selective C2H2 separation performance due to the reduced cage cavity size, functionalized pore surface, and appropriate pore volume. The introduction of fluoride- and sulfate-bridging cubane-type tetranuclear cobalt clusters bestows FJUT-1 with exceptional chemical stability under harsh conditions while providing multiple potential C2H2 binding sites, thus rendering the adequate ability for practical C2H2 separation application as confirmed by the dynamic breakthrough experiments under dry and humid conditions. Additionally, the distinct binding mechanism is suggested by theoretical calculations in which the multiple supramolecular interactions involving C–H···O, C–H···F, and other van der Waals forces play a critical role in the selective C2H2 separation

Designing CuO_<i>x</i> Nanoparticle-Decorated CeO₂ Nanocubes for Catalytic Soot Oxidation: Role of the Nanointerface in the Catalytic Performance of Heterostructured Nanomaterials

This work investigates the structure–activity properties of CuO_<i>x</i>-decorated CeO₂ nanocubes with a meticulous scrutiny on the role of the CuO_<i>x</i>/CeO₂ nanointerface in the catalytic oxidation of diesel soot, a critical environmental problem all over the world. For this, a systematic characterization of the materials has been undertaken using transmission electron microscopy (TEM), transmission electron microscopy–energy-dispersive X-ray spectroscopy (TEM–EDS), high-angle annular dark-field–scanning transmission electron microscopy (HAADF–STEM), scanning transmission electron microscopy–electron energy loss spectroscopy (STEM–EELS), X-ray diffraction (XRD), Raman, N₂ adsorption–desorption, and X-ray photoelectron spectroscopy (XPS) techniques. The TEM images show the formation of nanosized CeO₂ cubes (∼25 nm) and CuO_<i>x</i> nanoparticles (∼8.5 nm). The TEM–EDS elemental mapping images reveal the uniform decoration of CuO_<i>x</i> nanoparticles on CeO₂ nanocubes. The XPS and Raman studies show that the decoration of CuO_<i>x</i> on CeO₂ nanocubes leads to improved structural defects, such as higher concentrations of Ce³⁺ ions and abundant oxygen vacancies. It was found that CuO_<i>x</i>-decorated CeO₂ nanocubes efficiently catalyze soot oxidation at a much lower temperature (<i>T</i>₅₀ = 646 K, temperature at which 50% soot conversion is achieved) compared to that of pristine CeO₂ nanocubes (<i>T</i>₅₀ = 725 K) under tight contact conditions. Similarly, a huge 91 K difference in the <i>T</i>₅₀ values of CuO_<i>x</i>/CeO₂ (<i>T</i>₅₀ = 744 K) and pristine CeO₂ (<i>T</i>₅₀ = 835 K) was found in the loose-contact soot oxidation studies. The superior catalytic performance of CuO_<i>x</i>-decorated CeO₂ nanocubes is mainly attributed to the improved redox efficiency of CeO₂ at the nanointerface sites of CuO_<i>x</i>–CeO₂, as evidenced by Ce M_5,4 EELS analysis, supported by XRD, Raman, and XPS studies, a clear proof for the role of nanointerfaces in the performance of heterostructured nanocatalysts

Electrochemically Directed Synthesis of Cu₂^I(TCNQF₄^II–)(MeCN)₂ (TCNQF₄ = 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane): Voltammetry, Simulations, Bulk Electrolysis, Spectroscopy, Photoactivity, and X‑ray Crystal Structure of the Cu₂^I(TCNQF₄^II–)(EtCN)₂ Analogue

The new compound Cu₂^I(TCNQF₄^II–)­(MeCN)₂ (TCNQF₄^2– = dianion of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) has been synthesized by electrochemically directed synthesis involving reduction of TCNQF₄ to TCNQF₄^2– in acetonitrile containing [Cu­(MeCN)₄]⁺_(MeCN) and 0.1 M Bu₄NPF₆. In one scenario, TCNQF₄^2– is quantitatively formed by reductive electrolysis of TCNQF₄ followed by addition of [Cu­(MeCN)₄]⁺ to form the Cu₂^I(TCNQF₄^II–)­(MeCN)₂ coordination polymer. In a second scenario, TCNQF₄ is reduced in situ at the electrode surface to TCNQF₄^2–, followed by reaction with the [Cu­(MeCN)₄]⁺ present in the solution, to electrocrystallize Cu₂^I(TCNQF₄^II–)­(MeCN)₂. Two distinct phases of Cu₂^I(TCNQF₄^II–)­(MeCN)₂ are formed in this scenario; the kinetically favored form being rapidly converted to the thermodynamically favored Cu₂^I(TCNQF₄^II–)­(MeCN)₂. The postulated mechanism is supported by simulations. The known compound Cu^ITCNQF₄^I– also has been isolated by one electron reduction of TCNQF₄ and reaction with [Cu­(MeCN)₄]⁺. The solubility of both TCNQF₄^2–- and TCNQF₄^•–-derived solids indicates that the higher solubility of Cu^ITCNQF₄^I– prevents its precipitation, and thus Cu₂^I(TCNQF₄^II–)­(MeCN)₂ is formed. UV–visible and vibrational spectroscopies were used to characterize the materials. Cu₂^I(TCNQF₄^II–)­(MeCN)₂ can be photochemically transformed to Cu^ITCNQF₄^I– and Cu⁰. Scanning electron microscopy images reveal that Cu^ITCNQF₄^I– and Cu₂^I(TCNQF₄^II–)­(MeCN)₂ are electrocrystallized with distinctly different morphologies. Thermogravimetric and elemental analysis data confirm the presence of CH₃CN, and single-crystal X-ray diffraction data for the Cu₂^I(TCNQF₄^II−)­(EtCN)₂ analogue shows that this compound is structurally related to Cu₂^I(TCNQF₄^II–)­(MeCN)₂

Observation of Ferromagnetic Exchange, Spin Crossover, Reductively Induced Oxidation, and Field-Induced Slow Magnetic Relaxation in Monomeric Cobalt Nitroxides

The reaction of [Co^II(NO₃)₂]·6H₂O with the nitroxide radical, 4-dimethyl-2,2-di­(2-pyridyl) oxazolidine-<i>N</i>-oxide (L^•), produces the mononuclear transition-metal complex [Co^II(L^•)₂]­(NO₃)₂ (<b>1</b>), which has been investigated using temperature-dependent magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, density functional theory (DFT) calculations, and variable-temperature X-ray structure analysis. Magnetic susceptibility measurements and X-ray diffraction (XRD) analysis reveal a central low-spin octahedral Co²⁺ ion with both ligands in the neutral radical form (L^•) forming a linear L^•···Co­(II)···L^• arrangement. This shows a host of interesting magnetic properties including strong cobalt-radical and radical–radical intramolecular ferromagnetic interactions stabilizing a <i>S</i> = ³/₂ ground state, a thermally induced spin crossover transition above 200 K and field-induced slow magnetic relaxation. This is supported by variable-temperature EPR spectra, which suggest that <b>1</b> has a positive <i>D</i> value and nonzero <i>E</i> values, suggesting the possibility of a field-induced transverse anisotropy barrier. DFT calculations support the parallel alignment of the two radical π*_NO orbitals with a small orbital overlap leading to radical–radical ferromagnetic interactions while the cobalt-radical interaction is computed to be strong and ferromagnetic. In the high-spin (HS) case, the DFT calculations predict a weak antiferromagnetic cobalt-radical interaction, whereas the radical–radical interaction is computed to be large and ferromagnetic. The monocationic complex [Co^III(L^–)₂]­(BPh₄) (<b>2</b>) is formed by a rare, reductively induced oxidation of the Co center and has been fully characterized by X-ray structure analysis and magnetic measurements revealing a diamagnetic ground state. Electrochemical studies on <b>1</b> and <b>2</b> revealed common Co-redox intermediates and the proposed mechanism is compared and contrasted with that of the Fe analogues