Synergy of {Co(H2O)6}2+ with a Polyoxometalate Leads to Aqueous Homogeneous Hydrogen Evolution: Experiments and Computations.

In this work, we have described a polyoxometalate (POM)-based inexpensive and easily synthesizable compound [Co(H2O)6]2[{K(H2O)}2V10O28]·2H2O (1), which exhibits electrocatalytic hydrogen evolution in its aqueous solution without its decomposition (or electrodeposition), acting as a rare homogeneous electrocatalyst. Even though the compound [Co(H2O)6]2[{K(H2O)}2V10O28]·2H2O (1) (soluble in water) shows electrocatalytic hydrogen evolution reaction (HER) activity because of the Coulombic attraction, including H-bonding interactions, between the [Co(H2O)6]2+ cationic species and [{K(H2O)}2V10O28]4-anionic species, the individual homogeneous solutions of [V10O28]6- (source: Na6[V10O28]·18H2O) and [Co(H2O)6]2+ (source: CoCl2·6H2O) do not show any electrocatalytic HER activity. We have thus established that the synergy of [V10O28]6- with [Co(H2O)6]2+ in crystal matrix as well as in the aqueous solution of 1 makes the compound 1 a stable and highly active electrocatalyst for homogeneous HER in an aqueous solution. In order to corroborate these homogeneous HER studies, we performed density functional theory (DFT) calculations to show that decavanadate cluster anion [V10O28]6- interacts with hexa-aqua complex cation [Co(H2O)6]2+ via strong H-bonding interactions, leading to a synergy effect that enables the cobalt center of [Co(H2O)6]2+ to be an active site of HER in the present work.

Hexagonal Mo Bronze: Single Crystal Structures, Electrocatalytic Hydrogen Evolution, and Proton Conductivity.

Molybdenum trioxide (MoO3) is a well-known transition metal oxide that has drawn much attention as a functional material having numerous applications. However, a vast majority of studies have primarily focused on α-MoO3, the thermodynamically stable polymorph of MoO3. This present work encompasses the synthesis of single crystals of two metastable hexagonal MoO3 described by the formulas {Mn0.03Na0.01}@[Mo0.93VIMo0.07VO3] (1) and {Cu0.01Na0.01}@[Mo0.97VIMo0.03VO3] (2), their comprehensive structural characterization by single-crystal X-ray crystallography, and routine spectral and microscopic studies. Interestingly, compound 1 acts as an efficient electrocatalyst for the hydrogen evolution reaction (HER) as well as an effective proton conductor in comparison to the performance of compound 2. The HER activity of compound 1 is characterized by an overpotential of 340 mV@1 mA cm-2 and a low Tafel slope of 75 mV/decade. The catalyst (compound 1) displays a Faradaic efficiency of 88% with a turnover frequency of 2.9 s-1. The proton conductivity value of this compound (1) is determined to be 4.9 × 10-3 S cm-1 at 55 °C under 98% relative humidity; the relevant proton conduction is operated by the Grotthuss mechanism with an activation energy of 0.17 eV.

Hydroxylated Polyoxometalate with Cu(II)- and Cu(I)-Aqua Complexes: A Bifunctional Catalyst for Electrocatalytic Water Splitting at Neutral pH.

A sole inorganic framework material [Li(H2O)4][{CuI(H2O)1.5} {CuII(H2O)3}2{WVI12O36(OH)6}]·N2·H2S·3H2O (1) consisting of a hydroxylated polyoxometalate (POM) anion, {WVI12O36(OH)6}6-, a mixed-valent Cu(II)- and Cu(I)-aqua cationic complex species, [{CuI(H2O)1.5}{CuII(H2O)3}2]5+, a Li(I)-aqua complex cation, and three solvent molecules, has been synthesized and structurally characterized. During its synthesis, the POM cluster anion gets functionalized with six hydroxyl groups, i.e., six WVI-OH groups per cluster unit. Moreover, structural and spectral analyses have shown the presence of H2S and N2 molecules in the concerned crystal lattice, formed from "sulfate-reducing ammonium oxidation (SRAO)". Compound 1 functions as a bifunctional electrocatalyst exhibiting oxygen evolution reaction (OER) by water oxidation and hydrogen evolution reaction (HER) by water reduction at the neutral pH. We could identify that the hydroxylated POM anion and copper-aqua complex cations are the functional sites for HER and OER, respectively. The overpotential, required to achieve a current density of 1 mA/cm2 in the case of HER (water reduction), is found to be 443 mV with a Faradaic efficiency of 84% and a turnover frequency of 4.66 s-1. In the case of OER (water oxidation), the overpotential needed to achieve a current density of 1 mA/cm2 is obtained to be 418 mV with a Faradaic efficiency of 80% and turnover frequency of 2.81 s-1. Diverse electrochemical controlled experiments have been performed to conclude that the title POM-based material functions as a true bifunctional catalyst for electrocatalytic HER as well as OER at the neutral pH without catalyst reconstruction.

Electrocatalytic Hydrogen Evolution by a Uranium(VI) Polyoxometalate: an Environmental Toxin for Sustainable Energy Generation.

The uranyl ion (UO2)2+, a uranium nuclear waste, is one of the serious contaminants in our ecosystem because of its radioactivity, relevant human activities, and highly mobile and complex nature of living cells. In this article, we have reported the synthesis and structural characterization of an uranyl cation-incorporated polyoxometalate (POM) compound, K10[{K4(H2O)6}{UO2}2(α-PW9O34)2]·13H2O (1), in which the uranyl cations are complexed with an in situ generated [α-PW9O34]9- cluster. Single-crystal X-ray diffraction (SCXRD) analysis of compound 1 reveals that the uranyl-potassium complex cationic species, [{K4(H2O)6}{UO2}2]8+, is sandwiched by two [α-PW9O34]9- clusters resulting in a Dawson type of POM. Compound 1 was further characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis and infrared (IR), Raman, electronic absorption, and solid-state photoluminescence spectral studies. IR stretching vibrations at 895 and 856 cm-1 and the Raman signature peak at 792 cm-1 in the IR and Raman spectra of compound 1 primarily confirm the presence of a trans-[O═U═O]2+ ion. The solid-state photoluminescence spectrum of 1 exhibits a typical vibronic structure, resulting from symmetrical vibrations of [O═U═O]2+ bands, corresponding to the electronic transitions of S11 → S10 and S10 → S0υ (υ = 0-3). Interestingly, title compound 1 shows efficient electrocatalytic hydrogen evolution by water reduction with low Tafel slope values of 186.59 and 114.83 mV dec-1 at 1 mA cm-2 along with optimal Faradaic efficiency values of 82 and 87% at neutral pH and in acidic pH 3, respectively. Detailed electrochemical analyses reveal that the catalytic hydrogen evolution reaction (HER) activity mediated by compound 1 is associated with the UVI/UV redox couple of the POM. The microscopic as well as routine spectral analyses of postelectrode samples and controlled experiments have confirmed that compound 1 behaves like a true molecular electrocatalyst for the HER. To our knowledge, this is the first paradigm of a uranium-containing polyoxometalate that exhibits electrocatalytic water reduction to molecular H2. In a nutshell, an environmental toxin (a uranium-oxo compound) has been demonstrated to be utilized as an efficient electrocatalyst for hydrogen generation from water, a green approach of sustainable energy production.

Single Crystals of α-MoO3-Intercalated {Ni(H2O)6}2+ and Electrocatalytic Water Reduction: Toward a Class of Molybdenum Bronzes.

We have successfully intercalated {NiII(H2O)6}2+ into the α-MoO3 layer, leading to the isolation of green single crystals of [MoVI2O6(CH3COO){NiII(H2O)6}0.5]·H2O (1). The homogeneous electrochemistry of 1 in its aqueous solution exhibits electrocatalytic hydrogen evolution reaction (HER) with concomitant electrochemical deposition of [HMo3VIMoVO12(CH3COO){NiII(H2O)5(OH)}] (2). Compound 2, a new molybdenum bronze, acts as an efficient and stable heterogeneous electrocatalyst for water reduction to molecular hydrogen. This work represents the first paradigm of a molybdenum bronze intercalating a transition metal-aqua ion.

Barrel-Shaped-Polyoxometalates Exhibiting Electrocatalytic Water Reduction at Neutral pH: A Synergy Effect.

Two copper-based barrel-shaped polyoxometalates (POMs), namely, [{H3O}4{Na6(H2O)22}][{CuI (H2O)3}2{CuII (H2O)}3{B-α-BiIIIWVI9O33}2]·7H2O (NaCu-POM) and Li4[{NH4}2{H3O}3{Li(H2O)5}][{CuII(SH)}{(CuIICuI1.5)(B-α-BiIIIWVI9O33)}2]·9H2O (LiCu-POM) have been synthesized and structurally characterized. The single-crystal X-ray diffraction analyses of NaCu-POM and LiCu-POM reveal the presence of penta- and hexa-nuclear copper wheels per formula units, respectively; these copper wheels are sandwiched between two lacunary Keggin anions {B-α-BiIIIWVI9O33}9- (BiW9) to form the barrel-shaped title POM compounds. In both the compounds NaCu-POM and LiCu-POM, the mixed-valent copper centers are present in their respective penta- and hexa-nuclear copper wheels, established by X-ray photoelectron spectroscopy (XPS) as well as by bond valence sum (BVS) calculations. Compound LiCu-POM additionally shows the presence of a sulfhydryl ligand (SH-), coordinated to one of the copper centers of its {Cu6}-wheel, that is expected to be generated from the in situ reduction of sulfate anion present in the concerned reaction mixture (lithium-ion in ammonia solution may be the reducing agent). Interestingly, the title compounds, NaCu-POM and LiCu-POM exhibit an efficient electrocatalytic hydrogen evolution reaction (HER) by reducing water at neutral pH. Detailed electrochemical studies including controlled experiments indicate that the active sites for this electrocatalysis are the W(VI) centers of the title compounds, not the copper centers. However, a relevant tri-lacunary Keggin cluster anion {PVWVI9O33}7- (devoid of copper ion) does not show comparable HER as shown by the title compounds. The intra-cluster cooperative interactions of the mixed-valent copper centers (CuII/CuI) with the tungsten centers (W6+) make the overall system electrocatalytically active toward water reduction to molecular hydrogen at neutral pH. High Faradaic efficiencies (89 and 92%) and turnover frequencies (1.598 s-1 and 1.117 s-1) make the title compounds NaCu-POM and LiCu-POM efficient catalysts toward electrochemical water reduction to molecular hydrogen.

Nanoblackberries of {W72Fe33} and {Mo72Fe30}: Electrocatalytic Water Reduction.

The reversible self-assembly of a {Mo72Fe30} cluster into nanoblackberries in a dilute solution of the relevant crystalline compound [Mo72Fe30O252(CH3COO)12{Mo2O7(H2O)}2{H2Mo2O8(H2O)}(H2O)91]·150H2O ({Mo72Fe30}cryst) was demonstrated by Liu, Müller, and their co-workers as a landmark discovery in the area of polyoxometalate chemistry. We have described, in the present work, how these ∼2.5 nm nano-objects, {M72Fe30} (M = W, Mo) can be self-assembled into nanoblackberries irreversibly, leading to their solid-state isolation as the nanomaterials Fe3[W72Fe30O252(CH3COO)2(OH)25(H2O)103]·180H2O ({W72Fe33}NM) and Na2[Mo72Fe30O252(CH3COO)4(OH)16(H2O)108]·180H2O ({Mo72Fe30}NM), respectively (NM stands for nanomaterial). The formulations of these one-pot-synthesized nanoblackberries of {W72Fe33}NM and {Mo72Fe30}NM have been established by spectral analysis including Raman spectroscopy, elemental analysis including ICP metal analysis, volumetric analysis (for iron), microscopy techniques, and DLS studies. The thermal stability of the tungsten nanoblackberries {W72Fe33}NM is much higher than that of its molybdenum analogue {Mo72Fe30}NM. This might due to the extra three ferric (Fe3+) ions per {W72Fe30} cluster in {W72Fe33}NM, which are not part of the {W72Fe30} cluster cage but are placed between two adjacent clusters (i.e., each cluster has six surrounding 0.5Fe3+) to form this self-assembly. The isolated blackberries behave like an inorganic acid, a water suspension of which shows pH values of 3.9 for {W72Fe33}NM and 3.7 for {Mo72Fe30}NM because of the deprotonation of the hydroxyl groups in them. We have demonstrated, for the first time, a meaningful application of these inexpensive and easily synthesized nanoblackberries by showing that they can act as electrocatalysts for the hydrogen evolution reaction (HER) by reducing water. We have performed detailed kinetic studies for the electrocatalytic water reduction catalyzed by {W72Fe33}NM and {Mo72Fe30}NM in a comparative study. The relevant turnover frequencies (TOFs) of {W72Fe33}NM and {Mo72Fe30}NM (∼0.72 and ∼0.45 s-1, respectively), the overpotential values of {W72Fe33}NM and {Mo72Fe30}NM (527 and 767 mV, respectively at 1 mA cm-2), and the relative stability issues of the catalysts indicate that {W72Fe33}NM is reasonably superior to {Mo72Fe30}NM. We have described a rationale of why {W72Fe33}NM performs better than {Mo72Fe30}NM in terms of catalytic activity and stability.

Devising a Polyoxometalate-Based Functional Material as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction.

Hydrogen is the solution to all the problems associated with the energy crisis. Generating hydrogen from water splitting is one of the greener approaches, but it requires an efficient catalyst that is economical for the bulk production of hydrogen. The transition metal-aqua coordination complexes, which are otherwise inactive/unstable for electrochemical hydrogen evolution reaction (HER) activity, can efficiently be utilized for the same by attaching these metal-aqua species on a stable support. With a similar approach, we have synthesized and structurally characterized a two-dimensional polyoxometalate (POM)-copper complex hybrid that supports a copper(II)-aqua-bypyridine complex with a molecular formula of the overall system, [{CuII(2,2'-bpy)(H2O)2}][{CoIIWVI12O40}{CuII(2,2'-bpy)(H2O)}{CuII(2,2'-bpy)}]·2H2O (1). The bis(aqua)-mono(bipyridine) Cu(II)-complex fragment {CuII(2,2'-bpy)(H2O)2}2+, attached to the two-dimensional POM-Cu-complex support, acts as an active catalytic center that catalyzes the electrochemical HER. The electrochemical studies done for this work enabled us to understand the role of compound 1 as an electrocatalyst for the HER in near-neutral medium (pH 4.8), under buffered conditions (acetate buffer). Through detailed electrochemical experiments including controlled ones, we understand that compound 1 follows a proton-coupled electron transfer (PCET) pathway with one proton and one electron involvement in the HER. The overpotential required to achieve a current density of 1 mA/cm2 is found to be 520 mV with a Faradaic efficiency of 81%.

Efficient Electrocatalytic Water Oxidation by Fe(salen)-MOF Composite: Effect of Modified Microenvironment.

An efficient and robust heterogeneous electrocatalyst, FSWZ-8 ((Fe-(salen)(OH) + H4[SiW12O40]·HCl)@ZIF-8) for oxygen evolution reaction (OER) at the neutral pH, was developed by coencapsulation of Fe-salen (i.e., Fe(salen)Cl) and SiW12 (i.e., H4[SiW12O40]) inside the cavity of zeolitic imidazolate framework-8 (ZIF-8) material by an in situ synthesis. Here ZIF-8 functions as a host, Fe-salen as the active catalyst, and SiW12 helps in the charge transport by lowering the overall electrical resistance of the resulting composite system. High turnover frequency (∼5 s-1) and high Faradaic efficiency (∼94%) make the concerned composite an efficient catalyst toward water oxidation. This is the first report of one of the simplest known metal complexes, Fe-salen, to perfrom electrocatalytic OER as a heterogeneous catalyst in the neutral pH. This work also highlights the benefits of coencapsulation of the Keggin polyoxometalate (POM) along with the active catalyst Fe-salen species. Encapsulation of SiW12 results in (i) faster formation of FSWZ-8 composite, (ii) higher loading of Fe-salen, and, most importantly, (iii) lowering of required overpotential for electrochemical OER by more than 150 mV. The Keggin POMs, located as discrete molecular oxides inside the cavity of ZIF-8 as well as on the surface of ZIF-8, facilitate electrical charge conduction in the ZIF-8 matrix and lower the overall charge-transfer resistance.

Mononuclear Ru(II) Complexes of an Arene and Asymmetrically Substituted 2,2'-Bipyridine Ligands: Photophysics, Computation, and NLO Properties.

By using monosubstituted 2,2'-bipyridine asymmetric ancillary ligands with different electron donor moieties and an arene ligand (p-cymene), we successfully designed and synthesized six Ru(II) compounds (RuBPY1-6) that belong to a piano-stool-type system. The NLO properties of the synthesized complexes have been studied in both solution and the solid state. The electronic spectra of these compounds show a broad feature with two absorption bands in the visible window (350-650 nm). RuBPY1-6 complexes exhibit NIR emission spectra in the solution state (at >720 nm), the maxima of which are bathochromically shifted in comparison to those of the concerned ligands. Interestingly, compounds RuBPY1-6 show NIR emission in their solid state too. Title compounds RuBPY1-6 have lifetimes in the range of 0.2 to 0.9 ns. An important feature of this work is the π-association of the p-cymene ligand to Ru(II) in the synthesized complexes; the π complex is formed by breaking the symmetry of p-cymene, found in the starting precursor (Ru2 dimer). This has been established by NMR spectral studies along with DFT calculations on the 1H NMR spectra. We could derive the molecular structure of the cationic part of this system by density functional theory (DFT), associated with 1H NMR spectral studies. The minimum energy structures for RuBPY1 and RuBPY2 have been optimized at DFT/B3LYP along with the LANL2DZ basis set for ruthenium atoms. These optimized structures are further considered to calculate the excited state properties using the TDDFT method. The electrochemical studies of the complexes, investigated in acetonitrile solution, show that this system is associated with a well-defined Ru(III)/Ru(II) reversible couple, rarely observed for a Ru(II) piano-stool-type compound, along with a feature of irreversible ligand oxidation. The absorption cross-section values, obtained from the two-photon absorption studies of title compounds RuBPY1-6, are worth reporting and lie in the range of 3-28 GM (in the femtosecond case).

Polyoxometalate-Supported Bis(2,2'-bipyridine)mono(aqua)nickel(II) Coordination Complex: an Efficient Electrocatalyst for Water Oxidation.

A polyoxometalate (POM)-supported nickel(II) coordination complex, [NiII(2,2'-bpy)3]3[{NiII(2,2'-bpy)2(H2O)}{HCoIIWVI12O40}]2·3H2O (1; 2,2'-bpy = 2,2'-bipyridine), has been synthesized and structurally characterized. We could obtain a relatively better resolved structure from dried crystals of 1, NiII(2,2'-bpy)3]3[{NiII(2,2'-bpy)2(H2O)}{HCoIIWVI12O40}]2·H2O (D1). Because the title compound has been characterized with a {NiII(2,2'-bpy)2(H2O)}2+ fragment coordinated to the surface of the Keggin anion ([H(CoIIW12O40]5-) via a terminal oxo group of tungsten and the [NiII(2,2'-bpy)3]2+ coordination complex cation sitting as the lattice component in the concerned crystals, the electronic spectroscopy of compound 1 has been described by comparing its electronic spectral features with those of [NiII(2,2'-bpy)2(H2O)Cl]Cl, [NiII(2,2'-bpy)3]Cl2, and K6[CoIIW12O40]·6H2O. Most importantly, compound 1 can function as a heterogeneous and robust electrochemical water oxidation catalyst (WOC). To gain insights into the water oxidation (WO) protocol and to interpret the nature of the active catalyst, diverse electrochemical experiments have been conducted. The mode of action of the WOC during the electrochemical process is accounted for by confirmation that there was no formation/participation of metal oxide during various controlled experiments. It is found that the title compound acts as a true catalyst that has NiII (coordinated to POM surface) acting as the active catalytic center. It is also found to follow a proton-coupled electron-transfer pathway (two electrons and one proton) for WO catalysis with a high turnover frequency of 18.49 (mol of O2)(mol of NiII)-1 s-1.

A Keggin Polyoxometalate Shows Water Oxidation Activity at Neutral pH: POM@ZIF-8, an Efficient and Robust Electrocatalyst.

Keggin-type polyoxometalate anions [XM12 O40 ]n- are versatile, as their applications in interdisciplinary areas show. The Keggin anion [CoW12 O40 ]6- turns into an efficient and robust electrocatalyst upon its confinement in the well-defined void space of ZIF-8, a metal-organic framework (MOF). [H6 CoW12 O40 ]@ZIF-8 is so stable to water oxidation that it retains its initial activity even after 1000 catalytic cycles. The catalyst has a turnover frequency (TOF) of 10.8 mol O2 (mol Co)-1 s-1 , one of the highest TOFs for electrocatalytic oxygen evolution at neutral pH. Controlled experiments rule out the chances of formation and participation of CoOx in this electrocatalyic water oxidation.

Electrochemical Water Oxidation Catalyzed by an In Situ Generated α-Co(OH)2 Film on Zeolite-Y Surface.

The design and synthesis of an efficient and robust water-oxidation catalyst with inexpensive materials remains an important challenge in the context of artificial photosynthesis. Herein, a simple but unique technique is reported to in situ generate a thin-film of α-Co(OH)2 on the surface of zeolite-Y [hereafter referred to as Y-α-Co(OH)2 ] that acts as an efficient and stable catalyst for electrochemical water oxidation in alkaline medium. Catalyst Y-α-Co(OH)2 is so stable that it retains its catalytic activity even after 2000 cyclic voltammetric cycles of water oxidation. Expectedly, the chemical composition of α-Co(OH)2 on the surface of zeolite-Y remains same as that of parent Y-α-Co(OH)2 after 2000 electrocatalytic cycles. A Tafel slope as low as 59 mV decade-1 in 0.1 m KOH (pH 13) suggests faster oxygen evolution kinetics (overpotential=329 mV; turnover frequency=0.35 mol O2 (mol Co)-1 s-1 at 1 mA cm-2 ) than the existing α-Co(OH)2 -based electrocatalysts operating in alkaline medium.

A Functional Zn(II) Metallacycle Formed from an N-Heterocyclic Carbene Precursor: A Molecular Sensor for Selective Recognition of Fe3+ and IO4- Ions.

We have reported the synthesis and structural characterization of a unique Zn(II) metallacycle (1) and its utilization as a fluorescent probe for the shape-specific selective recognition (turn-off) of Fe3+ and IO4- ions. The relevant Stern-Volmer graphs indicate that the recognitions of Fe3+ and IO4- ions are examples of diphasic and monophasic quenchings, respectively. The title metallacycle has been prepared by the reaction of a novel N-heterocyclic carbene precursor, 1,3-bis(2,6-diisopropyl-4-(pyridin-4-yl)phenyl)-1H-imidazol-3-ium chloride/bromide (L), and zinc(II) chloride salt. Notably, the ligand itself did not show any type of recognition for any ions. DFT calculations were performed on L and metallacycle 1 using the geometric parameters, obtained from their single-crystal X-ray diffraction data, to understand the electronic structures of the ligand and macrocycle. The detection limit for the recognition of the Fe3+ ion was determined to be 2.5 × 10-6 mol/L, and that for IO4- ion was found to be 6.3 × 10-5 mol/L.

An Organic Receptor Isolated in an Unusual Intermediate Conformation: Computation, Crystallography, and Hirshfeld Surface Analysis.

1,1″-1,4-Phenylene-bis(methylene)bis-4,4'-bipyridinium cation [C28H24N4]2+ (c), an organic receptor that generally crystallizes in its anti conformation, has recently been shown to be isolated in its syn conformation in an ion-paired compound [C28H24N4][Zn(dmit)2]·2DMF (dmit2- = 1,3-dithiole-2-thione-4,5-dithiolate; DMF = dimethylformamide). In this article, we demonstrated that the same receptor [C28H24N4]2+ (c) can also be stabilized in an unusual intermediate conformation (neither syn nor anti) with PF6- anion in compound [C28H24N4](PF6)2·(1,4-dioxane) (1·(1,4-dioxane)). The energetically favored anti conformation has been described in its nitrate salt [C28H24N4](NO3)2·2H2O (2·2H2O). Compounds 1·(1,4-dioxane) and 2·2H2O, crystallizing in triclinic and monoclinic systems with space groups P1̅ and P21/n, respectively, were additionally characterized by Hirshfeld surface analysis. The density functional theory calculations are performed to understand the internal mechanism of the stability of various conformers of cationic receptor c, compound 1, and compound 2. In conjunction with the electronic stability of the conformers, the natural bond orbital analysis and conformational equilibrium constants at different temperatures are also calculated to find out the sources of the different stability of the various conformers of experimentally synthesized compounds.

Isolation of Blackberry-Shaped Nanoparticles of a Giant {Mo72Fe30} Cluster and Their Transformation to a Crystalline Nanoferric Molybdate.

When an aqueous solution of sodium molybdate is added to an aqueous solution of ferric chloride, acidified with acetic acid, a giant {Mo72Fe30} cluster is instantaneously formed as the amorphous substance Na2[Mo72Fe30O252(CH3COO)4(OH)16(H2O)108]·180 H2O (1). Compound 1 consists of aggregated nanovesicles of {Mo72Fe30} clusters, as confirmed by field-emission scanning electron microscopy and transmission electron microscopy images of 1. An aqueous suspension of 1 upon moderate heating results in the formation of crystalline nanoferric molybdate, which gives insight into understanding the formation of a yellow coating mineral, ferrimolybdite, frequently found on the ores of molybdenum.

Cyclometalated Iridium(III) Complexes Containing 4,4'-π-Conjugated 2,2'-Bipyridine Derivatives as the Ancillary Ligands: Synthesis, Photophysics, and Computational Studies.

This article demonstrates a series of cyclometalated Ir(III) complexes of the type [Ir(III)(C^N)2(N^N)](PF6), where C^N is 2-phenylpyridine, and N^N corresponds to the 4,4'-π-conjugated 2,2'-bipyridine ancillary ligands. All these compounds were synthesized through splitting of the binuclear dichloro-bridged complex precursor, [Ir(C^N)2(µ-Cl)]2, with the appropriate bipyridine ligands followed by the anion exchange reaction. The linear and nonlinear absorption properties of the synthesized complexes were investigated. The absorption spectra of all the title complexes exhibit a broad structureless feature in the spectral region of 350-700 nm with two bands being well-resolved in most of the cases. The structures of all the compounds were modeled in dichloromethane using the density functional theory (DFT) algorithm. The nature of electronic transitions was further comprehended on the basis of time-dependent DFT analysis, which indicates that the origins of various bands are primarily due to intraligand charge transfer transitions along with mixed-metal and ligand-centered transitions. The synthesized compounds are found to be nonemissive at room temperature because of probable nonradiative deactivation pathways of the T1 state that compete with the radiative (phosphorescence) decay modes. However, the frozen solutions of compounds Ir(MS 3) and Ir(MS 5) phosphoresce at the near-IR region, the other complexes remaining nonemissive up to 800 nm wavelength window. The two-photon absorption studies on the synthesized complexes reveal that values of the absorption cross-section are quite notable and lie in the range of 300-1000 GM in the picosecond case and 45-186 GM in the femtosecond case.

A Mononuclear Co(II) Coordination Complex Locked in a Confined Space and Acting as an Electrochemical Water-Oxidation Catalyst: A "Ship-in-a-Bottle" Approach.

Preparing efficient and robust water oxidation catalyst (WOC) with inexpensive materials remains a crucial challenge in artificial photosynthesis and for renewable energy. Existing heterogeneous WOCs are mostly metal oxides/hydroxides immobilized on solid supports. Herein we report a newly synthesized and structurally characterized metal-organic hybrid compound [{Co3 (µ3 -OH)(BTB)2(dpe)2} {Co(H2O)4(DMF)2}0.5]n ⋅n H2O(Co-WOC-1) as an effective and stable water-oxidation electrocatalyst in an alkaline medium. In the crystal structure of Co-WOC-1, a mononuclear Co(II) complex {Co(H2O)4(DMF)2}(2+) is encapsulated in the void space of a 3D framework structure and this translationally rigid complex cation is responsible for a remarkable electrocatalytic WO activity, with a catalytic turnover frequency (TOF) of 0.05 s(-1) at an overpotential of 390 mV (vs. NHE) in 0.1 m KOH along with prolonged stability. This host-guest system can be described as a "ship-in-a-bottle", and is a new class of heterogeneous WOC.

Asymmetrically Substituted and π-Conjugated 2,2'-Bipyridine Derivatives: Synthesis, Spectroscopy, Computation, and Crystallography.

A new series of monosubstituted styryl- and bistyryl-2,2'-bipyridine luminophores (compounds 16-23) have been synthesized via Horner-Wadsworth-Emmons reaction involving a monophosphonate and donor aromatic aldehydes. In the title chromophores, the amino donors are varied between acyclic and cyclic while the alkoxy donors are varied in terms of their number and position. The absorption maxima of these chromophores shift predominantly due to intramolecular charge transfer (ICT) between different donor and acceptor moieties. The title donor-acceptor molecules exhibit intense fluorescence in solution at room temperature, and their emissive behavior has been found to be highly sensitive to solvent polarity. The fluorescence spectra and quantum yields of all the chromophores were recorded in four different solvent media, and the chromophores 16, 17, 19, and 21 exhibit fluorescence in the solid state too. The influence of the nature and position of the donor functionalities in the conjugated backbone of the bipyridine moiety on the electronic absorption properties of the title chromophores (16-23) has been demonstrated, which has further been corroborated by DFT and TD-DFT computation both in gas phase and in solution phase. The crystal structure of compound 18 has been described as a representative member of the family (16-23).

5-Hy-droxy-2-nitro-benzaldehyde thio-semicarbazone (HNBATSC).

The asymmetric unit of the title compound, C8H8N4O3S, consists of two independent mol-ecules. Each mol-ecule is approximately planar with dihedral angles of 8.71 (3) and 1.50 (2)° between the aromatic ring and the thio-semicarbazide moiety while the NO2 group makes dihedral angles of 29.27 (3) and 17.78 (3)° with the benzene ring. In the crystal, the molecules are linked by N-H⋯S, O-H⋯O and N-H⋯O hydrogen bonds, forming two-dimensional networks parallel to (100).