Aqueous emissive cyclometalated iridium photoreductants: synthesis, computational analysis and the photocatalytic reduction of 4-nitrophenol.

Herein, we present luminescent mononuclear iridium complexes [1]3+-[4]3+ using NEt3-appended C^N chelating benzimidazole (L1-L4) and semi-flexible phenanthroline-pyrazine-based (phpy) ligands exhibiting photocatalytic reduction of 4-nitrophenol (4-NP) in the presence of NEt3 in an aqueous medium. The formation of [1]3+-[4]3+ was confirmed by HRMS, 1H-1H COSY, and 13C and 19F NMR spectroscopy. The complex [4]3+ is water soluble, whereas the others ([1]3+-[3]3+) are partially soluble. The complexes are luminescent in both CH3CN and H2O media. The DFT study reveals that the HOMO of [1]3+ resides on the C^N chelating benzimidazole and iridium center. However, it moves to the pyrazine-pyridine of the phpy unit in the case of [2]3+-[4]3+. The LUMOs are localized on the phenanthroline unit of phpy for all the complexes. This suggests an important role of the fluorine atom on electron density distribution. Spin density analysis demonstrates that the emission bands of the complexes arise from 3MLLCT states. The complex [4]3+ displays promising photocatalytic activity towards 4-NP photoreduction, whereas complexes [1]3+-[3]3+ exhibit lower reactivity. The mechanistic study suggests that the reaction proceeds through an oxidative quenching pathway, where 4-NP is reduced by accepting an electron from excited [Ir(III)] and gets oxidized to Ir(IV), which comes back to its original Ir(III) state by accepting an electron from the sacrificial electron donor NEt3.

Aggregation induced emission and volatile acid vapour sensing in acridine appended poly (aryl ether) based low molecular weight organogelator.

Considerable research attention has been devoted to the development of portable and rapid fluorescence sensors that can selectively detect volatile acids, due to the harmful effects of acid vapour on the environment and human health. Although various types of fluorophores have been reported for sensing volatile acid vapours, regulation of the sensory response using aggregation induced emissive (AIE) based gelators has rarely been reported. In this study, we present the design and synthesis of a novel organogelator that is capable of sensing volatile acids through AIE. An acridine-attached poly(aryl ether) dendron molecular system is synthesized through an aldimine coupling reaction, which self-assembles and forms a gel, exhibiting AIE behavior. The synthesized molecule and prepared gel were characterized using NMR, MASS, XRD, HRSEM and rheology techniques. The AIE property of APD was investigated using steady-state absorption and emission spectroscopic techniques. The sensory response of the APD gelator was tested with various analytes, and the results indicated that APD shows rapid response, particularly to acid vapours, where the detection limits (DL) of trifluoroacetic acid (TFA), hydrochloric acid (HCl) and nitric acid (HNO3) vapor were as low as 0.22, 0.9 and 0.30 ppm, respectively. An APD solid film in filter paper shows a visual color change from yellow to red in an aqueous acidic medium, and the effect is reversed in an alkaline medium. These findings suggest that an APD gelator could potentially be utilized to generate a portable acid vapor sensor kit due to its low detection limit and rapid response time, and it could be also be used as a substitute for existing acid indicators.

Asymmetrically Coordinated Heterodimetallic Ir-Ru System: Synthesis, Computational, and Anticancer Aspects.

Herein, we present an unprecedented formation of a heterodinuclear complex [{(ppy)2IrIII}(µ-phpy){RuII(tpy)}](ClO4)2 {[1](ClO4)2} using terpyridyl/phenylpyridine as ancillary ligands and asymmetric phpy as a bridging ligand. The asymmetric binding mode (N∧N-∩-N∧N∧C-) of the phpy ligand in {[1](ClO4)2} is confirmed by 1H, 13C, 1H-1H correlated spectroscopy (COSY), high-resolution mass spectrum (HRMS), single-crystal X-ray crystallography techniques, and solution conductivity measurements. Theoretical investigation suggests that the highest occupied molecular orbital (HOMO) and the least unoccupied molecular orbital (LUMO) of [1]2+ are located on iridium/ppy and phpy, respectively. The complex displays a broad low energy charge transfer (CT) band within 450-575 nm. The time-dependent density functional theory (TDDFT) analysis suggests this as a mixture of metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT), where both ruthenium, iridium, and ligands are involved. Complex {[1](ClO4)2} exhibits RuIIIrIII/RuIIIIrIII- and RuIIIIrIII/RuIIIIrIV-based oxidative couples at 0.83 and 1.39 V, respectively. The complex shows anticancer activity and selectivity toward human breast cancer cells (IC50; MCF-7: 9.3 ± 1.2 µM, and MDA-MB-231: 8.6 ± 1.2 µM) over normal breast cells (MCF 10A: IC50 ≈ 21 ± 1.3 µM). The Western blot analysis and fluorescence microscopy images suggest that combined apoptosis and autophagy are responsible for cancer cell death.

A magnetically retrievable mixed-valent Fe3O4@SiO2/Pd0/PdII nanocomposite exhibiting facile tandem Suzuki coupling/transfer hydrogenation reaction.

Herein, we report a magnetically retrievable mixed-valent Fe3O4@SiO2/Pd0/PdIINP (5) nanocomposite system for tandem Suzuki coupling/transfer hydrogenation reaction. The nanocomposite 5 was prepared first by making a layer of [Formula: see text] on [Formula: see text] followed by deposition of [Formula: see text] and sorption of [Formula: see text] ions successively onto the surface of Fe3O4@SiO2NP. The nanocomposite was characterized by powder XRD, electron microscopy (SEM-EDS and TEM-EDS) and XPS spectroscopy techniques. The mixed-valent [Formula: see text] present onto the surface of nanocomposite 5 was confirmed by XPS technique. Interestingly, the mixed-valent nanocomposite Fe3O4@SiO2/Pd0/PdIINP (5) exhibited tandem Suzuki coupling/transfer hydrogenation reaction during the reaction of aryl bromide with aryl boronic acid (90% of C). The nanocomposite 5 displayed much better reactivity as compared to the monovalent Fe3O4@SiO2/Pd0NP (3) (25% of C) and Fe3O4@SiO2/PdIINP (4) (15% of C) nanocomposites. Further, because of the presence of magnetic [Formula: see text], the nanocomposite displayed its facile separation from the reaction mixture and reused at least for five catalytic cycles.

Fly ash supported Pd-Ag bimetallic nanoparticles exhibiting a synergistic catalytic effect for the reduction of nitrophenol.

Coal fly ash (FA) supported Pd-Ag bimetallic nanoparticles (FA-Pd-Ag) were prepared by reducing Pd(II) and Ag(I) salts together onto the dispersed solid support in aqueous medium. Electron microscope analysis (FE-SEM, HRTEM) in combination with elemental mapping (EDS) suggests that the nanoparticles are well dispersed on fly ash with an average diameter of 6-8 nm. The powder XRD analysis indicates that alloying of the interface occurs between Pd and Ag nanoparticles in FA-Pd-Ag, while XPS reveals that charge transfer takes place between the Pd and Ag moieties that come into contact with each other. The FA-Pd-Ag in aqueous NaBH4 solution exhibits an efficient catalytic reduction of 4-nitrophenol into 4-aminophenol and follows pseudo-first-order reaction kinetics (kPd-Ag = 0.7176 min-1). The higher rate constant for FA-Pd-Ag compared to that for their monometallic analogues (FA-Pd (kPd = 0.5449 min-1)) and (FA-Ag (kAg = 0.5572 min-1)) as well as their physical mixture ((FA-Pd + FA-Ag) (kPd+Ag = 0.4075 min-1)) suggests the synergistic catalytic effect of the bimetallic system. Moreover, the present bimetallic nanocatalyst exhibits the highest normalized rate constant (KPd-Ag ≈ 51 100 min-1 mmol-1) compared to the reported bimetallic Pd-Ag nanocatalysts.

A magnetically separable and recyclable g-C3N4/Fe3O4/porous ruthenium nanocatalyst for the photocatalytic degradation of water-soluble aromatic amines and azo dyes.

Herein, we present the development of a visible-light-driven magnetically retrievable nanophotocatalyst made of porous ruthenium nanoparticles supported on magnetic carbon nitride (g-C3N4/Fe3O4/p-RuNP) for the facile removal/degradation of aromatic amines and azo dyes from wastewater. Aromatic amines and azo-based dyes in water bodies are highly toxic and carcinogenic even at very low concentrations and are difficult to separate because of their high solubility. Our nanocatalyst can efficiently degrade/decompose the aromatic amines and azo dyes under visible light (LED/sunlight) at room temperature and in a wide pH range (pH 5.0-9.0) without using any external chemicals. The magnetic property of the nanocatalyst facilitates its efficient and facile separation from the reaction mixture for reuse in multiple photocatalytic cycles. The nanocatalyst-based degradation of azo dyes and aromatic amines presented here is simple and convenient in terms of efficiency, energy, reusability and cost. The process also does not require any external chemicals and forms gaseous/less harmful end products.

Alkyl chain-modified cyclometalated iridium complexes as tunable anticancer and imaging agents.

Five mononuclear cyclometalated iridium complexes [1](PF6)-[5](PF6) were prepared using imidazole-based ligands of varying alkyl chain length. The complexes were characterised by various analytical techniques. The single crystal X-ray structures of [2](PF6), [3](PF6) and [4](PF6) revealed the expected distorted Oh structures around the metal centre; however, the chain length was found to play a crucial role in deciding the overall geometry. Theoretical investigations demonstrated that the HOMOs were mainly contributed by iridium and cyclometalated ligands, whereas the LUMOs were constituted from bpy/phen units. The complexes were found to be luminescent with a moderate emission quantum yield and lifetime in CH3CN. The in vitro growth inhibition assay of the complexes with a shorter alkyl chain ([4]+ and [5]+) displayed higher anticancer activity (IC50 < 0.5 µM) compared to the complexes with a longer alkyl chain ([1]+-[3]+) (IC50 < 30 µM) against human breast cancer (MCF-7) cells. The complexes [4]+ and [5]+ also displayed moderate cancer cell selectivity (∼3 times) over normal breast (MCF-10) cells. The flow cytometry assay and fluorescence microscopy analysis suggested that cellular accumulation was primarily responsible for the variation in anticancer activity. Interestingly, without possessing any anticancer activity or toxicity ((IC50 > 50 µM), the complex [1]+ mainly accumulated near the cell membrane outside the cell and displayed a clear image of the cell membrane. The light microscopy images and western blot analysis reveal that complex [4]+ induced combined apoptosis and paraptosis. Thus, tuning the anticancer activity and cellular imaging property mediated by the alkyl chain would be of great importance and would be useful in anticancer research.

Preparation and mechanistic aspect of natural xanthone functionalized gold nanoparticle.

Herein, a facile scale up and shape variable synthesis of gold nanoparticle (AuNP) and reaction mechanism by natural xanthone derivative (mangiferin) has been reported. Mangiferin (C19H18O11; 1,3,6,7-tetrahydroxyxanthone-C2-ß-d-glucoside), a xanthone derivative is isolated from Mangifera indica L. leaves which efficiently reduces Au3+ ions to Au0 and stabilizes the formed AuNP. The structural, optical and plasmonic properties of synthesized AuNP have been investigated through different instrumental techniques like UV-Vis and FTIR spectroscopy, powder XRD, FESEM and TEM analysis. It is observed that variation of the concentration of Au3+ ions and mangiferin has a great effect on controlling size and shape of nanoparticles. The role of reaction temperature is also notable. An interesting observation is that with same concentration ratio of HAuCl4/mangiferin (0.025 mM/0.002%) at the room temperature kidney shaped AuNP is produced, whereas it is spherical at boiling temperature. Moreover, mangiferin allows high scale synthesis of AuNPs (0.025 mM to 10 mM) without changing the particles size and shape. The mechanistic investigation through UV-Vis, FTIR and GCMS analyses reveal the cleavage of glucose unit and oxidation of phenolic OH groups during AuNP formation. Non-toxicity of mangiferin conjugated AuNP on normal human breast cell line (MCF-10A) suggesting its future application as a drug delivery system and other related medicinal purposes.

Long-Lived Polypyridyl Based Mononuclear Ruthenium Complexes: Synthesis, Structure, and Azo Dye Decomposition.

Two mononuclear ruthenium complexes [(bpy)2RuIIL1/L2](ClO4)2 ([1]2+/[2]2+) (bpy-2,2' bipyridine, L1 = 2,3-di(pyridin-2-yl)pyrazino[2,3-f][1,10]phenanthroline) and L2 = 2,3-di(thiophen-2-yl)pyrazino[2,3-f][1,10]phenanthroline have been synthesized. The complexes have been characterized using various analytical techniques. The complex [1]2+ has further been characterized by its single crystal X-ray structure suggesting ruthenium is coordinating through the N donors of phenanthroline end. Theoretical investigation suggests that the HOMOs of both complexes are composed of pyridine and pyrazine unit of ligands L1 and L2 whereas the LUMOs are formed by the contribution of bipyridine units. The low energy bands at ∼480 nm of the complexes can be assigned as MLCT with partial contribution from ligand transitions, whereas the rest are ligand centered. The complexes have shown RuII/RuIII oxidation couples at E1/2 at 1.26 (70 mV) V and 1.28 (62 mV) V for [1]2+ and [2]2+ vs Ag/AgCl, respectively, suggesting no significant role of distal thiophene or pyridine units of the ligands. The complexes are emissive and display solvent dependent emission properties. Both complexes have shown highest emission quantum yield and lifetime in DMSO (ϕ = 0.05 and τavg = 460 ns and λmaxem at 620 nm for [1]2+; ϕ = 0.043 and τavg = 425 ns and λmaxem at 635 nm for [2]2+). Further, the long luminescent lifetime of these complexes has been utilized to generate reactive oxygen species for efficient azo dye decomposition.

Cyclometallated iridium complexes inducing paraptotic cell death like natural products: synthesis, structure and mechanistic aspects.

Six mononuclear Ir complexes (1-6) using polypyridyl-pyrazine based ligands (L1 and L2) and {[cp*IrCl(µ-Cl)]2 and [(ppy)2Ir(µ-Cl)]2} precursors have been synthesised and characterised. Complexes 1-5 have shown potent anticancer activity against various human cancer cell lines (MCF-7, LNCap, Ishikawa, DU145, PC3 and SKOV3) while complex 6 is found to be inactive. Flow cytometry studies have established that cellular accumulation of the complexes lies in the order 2 > 1 > 5 > 4 > 3 > 6 which is in accordance with their observed cytotoxicity. No changes in the expression of the proteins like PARP, caspase 9 and beclin-1, Atg12 discard apoptosis and autophagy, respectively. Overexpression of CHOP, activation of MAPKs (P38, JNK, and ERK) and massive cytoplasmic vacuolisation collectively suggest a paraptotic mode of cell death induced by proteasomal dysfunction as well as endoplasmic reticulum and mitochondrial stress. An intimate relationship between p53, ROS production and extent of cell death has also been established using p53 wild, null and mutant type cancer cells.

A dinuclear [{(p-cym)Ru(II)Cl}2(µ-bpytz˙(-))](+) complex bridged by a radical anion: synthesis, spectroelectrochemical, EPR and theoretical investigation (bpytz = 3,6-bis(3,5-dimethylpyrazolyl)1,2,4,5-tetrazine; p-cym = p-cymene).

The reaction of the chloro-bridged dimeric precursor [{(p-cym)Ru(II)Cl}(µ-Cl)]2 (p-cym = p-cymene) with the bridging ligand 3,6-bis(3,5-dimethylpyrazolyl)-1,2,4,5-tetrazine (bpytz) in ethanol results in the formation of the dinuclear complex [{(p-cym)Ru(II)Cl}2(µ-bpytz˙(-))](+), [1](+). The bridging tetrazine ligand is reduced to the anion radical (bpytz˙(-)) which connects the two Ru(II) centres. Compound [1](PF6) has been characterised by an array of spectroscopic and electrochemical techniques. The radical anion character has been confirmed by magnetic moment (corresponding to one electron paramagnetism) measurement, EPR spectroscopic investigation (tetrazine radical anion based EPR spectrum) as well as density functional theory based calculations. Complex [1](+) displays two successive one electron oxidation processes at 0.66 and 1.56 V versus Ag/AgCl which can be attributed to [{(p-cym)Ru(II)C}2(µ-bpytz˙(-))](+)/[{(p-cym)Ru(II)Cl}2(µ-bpytz)](2+) and [{(p-cym)Ru(II)Cl}2(µ-bpytz)](+)/[{(p-cym)Ru(III)Cl}2(µ-bpytz)](2+) processes (couples I and II), respectively. The reduction processes (couple III-couple V), which are irreversible, likely involve the successive reduction of the bridging ligand and the metal centres together with loss of the coordinated chloride ligands. UV-Vis-NIR spectroelectrochemical investigation reveals typical tetrazine radical anion containing bands for [1](+) and a strong absorption in the visible region for the oxidized form [1](2+), which can be assigned to a Ru(II) → π* (tetrazine) MLCT transition. The assignment of spectroscopic bands was confirmed by theoretical calculations.

Iridium Complexes as a Roadblock for DNA Polymerase during Amplification.

Iridium-based metal complexes containing polypyridyl-pyrazine ligands show properties of DNA intercalation. They serve as roadblocks to DNA polymerase activity, thereby inhibiting the polymerization process. Upon the addition of increasing concentrations of these iridium complexes, a rapid polymerase chain reaction (PCR)-based assay reveals the selective inhibition of the DNA polymerization process. This label-free approach to study the inhibition of fundamental cellular processes via physical roadblock can offer an alternative route toward cancer therapy.

Synthesis, characterisation and antibacterial activity of [(p-cym)RuX(L)](+/2+) (X = Cl, H2O; L = bpmo, bpms) complexes.

Mononuclear half-sandwiched complexes [(p-cym)RuCl(bpmo)](ClO4) {[1](ClO4)} and [(p-cym)RuCl(bpms)](PF6) {[2](PF6)} have been prepared by reacting heteroscorpionate ligands bpmo = 2-methoxyphenyl-bis(3,5-dimethylpyrazol-1-yl)methane and bpms = 2-methylthiophenyl-bis(3,5-dimethylpyrazol-1-yl)methane, respectively, with a dimeric precursor complex [(p-cym)RuCl(µ-Cl)]2 (p-cym = 1-isopropyl-4-methylbenzene) in methanol. The corresponding aqua derivatives [(p-cym)Ru(H2O)(bpmo)](ClO4)2 {[3](ClO4)2} and [(p-cym)Ru(H2O)(bpms)](PF6)2 {[4](PF6)2} are obtained from {[1](ClO4)} and {[2](PF6)}, respectively, via Cl(-)/H2O exchange process in the presence of appropriate equivalents of AgClO4/AgNO3 + KPF6 in a methanol-water mixture. The molecular structures of the complexes {[1]Cl, [3](ClO4)2 and [4](PF6)(NO3)} are authenticated by their single crystal X-ray structures. The complexes show the expected piano-stool geometry with p-cym in the η(6) binding mode. The aqua complexes [3](ClO4)2 and [4](PF6)2 show significantly good antibacterial activity towards E. coli (gram negative) and B. subtilis (gram positive) strains, while chloro derivatives ({[1](ClO4)} and {[2](PF6)} are found to be virtually inactive. The order of antibacterial activity of the complexes according to their MIC values is [1](ClO4) (both 1000 µg mL(-1)) < [2](PF6) (580 µg mL(-1) and 750 µg mL(-1)) < [3](ClO4)2 (both 100 µg mL(-1)) < [4](PF6)2 (30 µg mL(-1) and 60 µg mL(-1)) for E. coli and B. subtilis strains, respectively. Further, the aqua complexes [3](ClO4)2 and [4](PF6)2 show clear zones of inhibition against kanamycin, ampicillin and chloramphenicol resistant E. coli strains. The detailed mechanistic aspects of the aforesaid active aqua complexes [3](ClO4)2 and [4](PF6)2 have been explored, and it reveals that both the complexes inhibit the number of nucleoids per cell in vivo and bind to DNA in vitro. The results indeed demonstrate that both [3](ClO4)2 and [4](PF6)2 facilitate the inhibition of bacterial growth by binding to DNA.

Facile tandem Suzuki coupling/transfer hydrogenation reaction with a bis-heteroscorpionate Pd-Ru complex.

Design and synthesis of the bis(pyrazol-1-yl)methane based bis-heteroscorpionate Pd-Ru complex results in efficient tandem Suzuki coupling/transfer hydrogenation reaction with a broad range of substrate reactivity.

Dinuclear [{(p-cym)RuCl}2(µ-phpy)](PF6)2 and heterodinuclear [(ppy)2Ir(µ-phpy)Ru(p-cym)Cl](PF6)2 complexes: synthesis, structure and anticancer activity.

Phpy bridged homodinuclear Ru-Ru () and heterodinuclear Ir-Ru complexes () have been developed. Complex induces autophagy towards the cisplatin resistant human breast cancer (MCF7) cell line, whereas is inactive.

Synthesis, characterisation and biological activities of [(p-cym)RuX(pz4lut)]n+ and [{(p-cym)RuX}2(µ-pz4lut)]n+ (X = Cl, H2O and pz4lut = α,α,α',α'-tetra(pyrazol-1-yl)-2,6-lutidine).

Mononuclear [(p-cym)RuCl(pz4lut)]Cl (1) and dinuclear [{(p-cym)RuCl}2(µ-pz4lut)]Cl2 (2) complexes (p-cym = 1-isopropyl-4-methylbenzene) comprising of bis(pyrazol-1-yl)methane based heteroscorpionate ligand α,α,α',α'-tetra(pyrazol-1-yl)-2,6-lutidine (pz4lut) have been synthesised from pz4lut ligand and dimeric precursor complex [(p-cym)RuCl(µ-Cl)]2 in methanol. The aqua derivatives [(p-cym)Ru(H2O)(pz4lut)](ClO4)2 (3) and [{(p-cym)Ru(H2O)}2(µ-pz4lut)](ClO4)4 (4) are obtained from 1 and 2, respectively, via Cl/H2O exchange process in presence of appropriate equivalents of AgClO4 in methanol­water mixture. The molecular structures of dinuclear complexes, 2 and 4 are authenticated by their single crystal X-ray structures. Cyclic voltammetry reveals negligible electronic communication between the metal centres in the ligand bridged complex 2. All four complexes have been tested for their anticancer activities in human breast (MCF7), lung (A549) and colon (HCT116) cancer cell lines. The complexes show dose dependent suppression of cell viability with moderately good IC50 values ranging from 3.5­92 µM. Experimental results have revealed that the aqua derivatives, 3 and 4 exhibit better cytotoxic effect against all those cell lines as compared to the precursor chlorido complexes, 1 and 2. Results also demonstrate that the complexes are more potent against HCT116 cells as compared to other cell lines.

An artificial enzyme-based assay: DNA detection using a peroxidase-like copper-creatinine complex.

We report an artificial enzyme-based DNA assay using a peroxidase-like copper (Cu)-creatinine complex as a catalyst for 3,3',5,5'-tetramethylbenzidine (TMB) oxidation. The assay employs double signal amplification and a homogeneous catalytic reaction: (i) fast catalytic growth of Cu on a gold (Au) nanoparticle (NP) label forms a thick Cu layer (first amplification); (ii) dissolution of the Cu layer generates many Cu-creatinine complexes per NP (generation of homogeneous catalysts); (iii) peroxidase-like Cu-creatinine complexes rapidly convert TMB into a colored product (second amplification). To investigate the effect of ligand on the catalytic activities of Cu complexes, the kinetics of catalytic TMB oxidation is tested with and without using imidazole ring-containing ligands (creatinine, imidazole, and poly(l-histidine)). Both fast oxidation of TMB and slow further oxidation of the colored product are required for high signal-to-background ratios. Cu-creatinine complex allows relatively fast oxidation and slow further oxidation. Fast seed-mediated Cu growth on Au NP and slow Cu autonucleation (i.e., slow formation of Cu NP in the absence of Au NP) are also required for high signal-to-background ratios. In tris-EDTA (tris(hydroxymethyl)aminomethane-ethylenediaminetetraacetic acid) buffer (pH 7.7) containing high concentrations of Cu(2+) (90 mM), ascorbic acid (50mM), and Mg(2+) (200 mM), Cu growth on Au NP is very fast and autonucleation is significantly suppressed. Fast catalytic oxidation by Cu-creatinine complex along with fast Cu growth on Au NP allows a detection limit of 0.1 pM for DNA in a simple microplate format.

A facile method of achieving low surface coverage of Au nanoparticles on an indium tin oxide electrode and its application to protein detection.

Low surface coverage of Au nanoparticles on an indium tin oxide electrode for sensitive electrochemical detection was achieved using electrostatic adsorption of AuCl(4)(-) followed by reduction.

Enhancement of the electrocatalytic activity of gold nanoparticles via NaBH4 treatment.

We report on the enhancement of the electrocatalytic activity of Au nanoparticles after NaBH4 treatment and its application to H2O2 detection.

Mixed-valent metals bridged by a radical ligand: fact or fiction based on structure-oxidation state correlations.

Electron-rich Ru(acac)2 (acac- = 2,4-pentanedionato) binds to the pi electron-deficient bis-chelate ligands L, L = 2,2'-azobispyridine (abpy) or azobis(5-chloropyrimidine) (abcp), with considerable transfer of negative charge. The compounds studied, (abpy)Ru(acac)2 (1), meso-(mu-abpy)[Ru(acac)2]2 (2), rac-(mu-abpy)[Ru(acac)2]2 (3), and (mu-abcp)[Ru(acac)2]2 (4), were calculated by DFT to assess the degree of this metal-to-ligand electron shift. The calculated and experimental structures of 2 and 3 both yield about 1.35 A for the length of the central N-N bond which suggests a monoanion character of the bridging ligand. The NBO analysis confirms this interpretation, and TD-DFT calculations reproduce the observed intense long-wavelength absorptions. While mononuclear 1 is calculated with a lower net ruthenium-to-abpy charge shift as illustrated by the computed 1.30 A for d(N-N), compound 4 with the stronger pi accepting abcp bridge is calculated with a slightly lengthened N-N distance relative to that of 2. The formulation of the dinuclear systems with monoanionic bridging ligands implies an obviously valence-averaged Ru(III)Ru(II) mixed-valent state for the neutral molecules. Mixed valency in conjunction with an anion radical bridging ligand had been discussed before in the discussion of MLCT excited states of symmetrically dinuclear coordination compounds. Whereas 1 still exhibits a conventional electrochemical and spectroelectrochemical behavior with metal centered oxidation and two ligand-based one-electron reduction waves, the two one-electron oxidation and two one-electron reduction processes for each of the dinuclear compounds Ru2.5(L*-)Ru2.5 reveal more unusual features via EPR and UV-vis-NIR spectroelectrochemistry. In spite of intense near-infrared absorptions, the EPR results show that the first reduction leads to Ru(II)(L*-)Ru(II) species, with an increased metal contribution for system 4*-. The second reduction to Ru(II)(L2-)Ru(II) causes the disappearance of the NIR band. One-electron oxidation of the Ru2.5(L*-)Ru2.5 species produces a metal-centered spin for which the alternatives RuIII(L0)Ru(II) or Ru(III)(L*-)Ru(III) can be formulated. The absence of NIR bands as common for mixed-valent species with intervalence charge transfer (IVCT) absorption favors the second alternative. The second one-electron oxidation is likely to produce a dication with Ru(III)(L0)Ru(III) formulation. The usefulness and limitations of the increasingly popular structure/oxidation state correlations for complexes with noninnocent ligands is being discussed.