Stimuli-responsive and self-healing supramolecular Zn(II)-guanosine metal-organic gel for Schottky barrier diode application.

Spontaneous formation of a supramolecular metal-organic hydrogel using unsubstituted guanosine as a ligand and Zn2+ ions is reported. Guanosine, in the presence of NaOH, self-assembled into a stable G-quadruplex structure, which underwent crosslinking through Zn2+ ions to afford a stable hydrogel. The gel has been characterized using several spectroscopic as well as microscopic studies. The hydrogel demonstrated excellent stimuli responsiveness towards various chemicals and pH. Furthermore, the gel exhibited intrinsic thixotropic behavior and showed self-healing and injectable properties. The optical properties of the Zn-guanosine metallo-hydrogel suggested a semiconducting nature of the gel, which has been exploited for fabricating a thin film device based on a Schottky diode interface between metal and a semiconductor. The fabricated device shows excellent charge transport characteristics and linear rectifying behavior. The findings are likely to pave the way for newer research in the area of soft electronic devices fabricated using materials synthesized by employing simple biomolecules.

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.

Evolution of metal organic frameworks as electrocatalysts for water oxidation.

In the last two decades, metal organic frameworks (MOFs) have been extensively investigated to develop heterogeneous electrocatalysts for water oxidation (WO). The scope of reticular synthesis, enormous surface area and accessible internal volume of MOFs make them promising candidates for catalysis. However, low electrical conductivity, slow mass transport and lack of stability restrict the scope of MOF-based WO. In recent times, various material designing approaches, e.g., the introduction of mixed metal and multi-metal systems, ligand engineering, guest@MOF composite formation, preparation of thin films, MOF composite formation with conducting carbon-based materials, metal oxides, polymers and layered compounds, etc. have emerged as an effective means to counteract the aforementioned limitations. This feature article critically discusses the common MOF-based material designing strategies with respect to electrochemical WO and provides a platform to understand the potential of MOFs to prepare a sophisticated hybrid electrocatalyst for WO.

Probing through-space and through-bond magnetic exchange couplings in a new benzotriazinyl radical and its metal complexes.

The synthesis and characterization of a new chelating benzotriazinyl radical (Rad2) are described. Crystallographic studies coupled with SQUID magnetometry on Rad2 reveal the presence of discrete radical pairs which are antiferromagnetically coupled. The reaction of Rad2 with the 3d transition metal complexes M(hfac)2·xH2O (hfac- = hexafluoroacetylacetonate) led to mononuclear metal complexes of general formula M(hfac)2(Rad2) [M = Zn(ii) (1); Ni(ii) (2) and Co(ii) (3)] whose structures have been determined by single crystal X-ray diffraction. Compounds 1-3 are isostructural and crystallize in the monoclinic space group P21/n with two molecules in the asymmetric unit. In the case of the Zn(ii) complex (1) through-space intermolecular radicalradical antiferromagnetic exchange interactions viaπ*π* contacts are observed, whereas strong intramolecular through-bond metal-radical ferromagnetic interactions [J = +59.3(9) cm-1] are observed for the Ni(ii) complex (2). For the Co(ii) complex (3), computational and magnetic studies reveal substantial zero field splitting and ferromagnetic metal-radical interactions.

Targeted water soluble copper-tetrazolate complexes: interactions with biomolecules and catecholase like activities.

Two new mononuclear water soluble copper(II) complexes, [Cu{(5-pyrazinyl)tetrazolate}2(1,10-phenanthroline)] 1 and [Cu{(5-pyrazinyl)tetrazolate}(1,10-phenanthroline)2](NO3)0.5(N3)0.5 2, have been synthesized using the metal mediated [2 + 3] cycloaddition reaction between copper bound azide and pyrazinecarbonitrile. The interactions of these copper tetrazolate complexes 1 and 2 with biomolecules like DNA and bovine serum albumin (BSA) are studied and the catecholase like catalytic activity of compound 2 is also explored. Structural determination reveals that both compounds 1 and 2 are octahedral in nature. Screening tests were conducted to quantify the binding ability of complexes (1 and 2) towards DNA and it was revealed that complex 2 has a stronger affinity to bind to CT-DNA. DFT studies indicated that a lower HOMO-LUMO energy gap between the DNA fragment and metal complexes might be the reason for this type of stronger interaction. DNA cleavage activity was explored by gel-electrophoresis and moderate to strong DNA cleavage properties were observed in the presence and absence of co-reagents. Inhibition of cleavage in the presence of sodium azide indicates the propagation of the activity through the production of singlet molecular oxygen. Furthermore enzyme kinetic studies reflect that complex 2 is also effective in mimicking catecholase like activities. An ESI-MS spectral study indicates the probable involvement of dimeric species [(phen)2Cu-(OH)2-Cu(phen)2](2+) in the catalytic cycle.

Greener Selective Cycloalkane Oxidations with Hydrogen Peroxide Catalyzed by Copper-5-(4-pyridyl)tetrazolate Metal-Organic Frameworks.

Microwave assisted synthesis of the Cu(I) compound [Cu(µ4-4-ptz)]n [1, 4-ptz=5-(4-pyridyl)tetrazolate] has been performed by employing a relatively easy method and within a shorter period of time compared to its sister compounds. The syntheses of the Cu(II) compounds [Cu3(µ3-4-ptz)4(µ2-N3)2(DMF)2]n∙(DMF)2n (2) and [Cu(µ2-4-ptz)2(H2O)2]n (3) using a similar method were reported previously by us. MOFs 1-3 revealed high catalytic activity toward oxidation of cyclic alkanes (cyclopentane, -hexane and -octane) with aqueous hydrogen peroxide, under very mild conditions (at room temperature), without any added solvent or additive. The most efficient system (2/H2O2) showed, for the oxidation of cyclohexane, a turnover number (TON) of 396 (TOF of 40 h(-1)), with an overall product yield (cyclohexanol and cyclohexanone) of 40% relative to the substrate. Moreover, the heterogeneous catalytic systems 1-3 allowed an easy catalyst recovery and reuse, at least for four consecutive cycles, maintaining ca. 90% of the initial high activity and concomitant high selectivity.

Nickel(II) complexes with a flexible piperazinyl moiety: studies on DNA and protein binding and catecholase like properties.

Four new mononuclear Ni(ii) complexes [Ni(L(1))]ClO4 (), [Ni(L(2))]ClO4(), [Ni(SCN)3(CH3OH)(aminoethylpiperazineH)] (), and [Ni(DMSO)4(aminoethylpiperazineH)](ClO4)3()have been synthesized from two Schiff base ligands [L(1) = 1-phenyl-3-((2-(piperidin-4-yl)ethyl)imino)but-1-en-1-ol and L(2) = 4-((2-(piperazin-1-yl)ethyl)imino)pent-2-en-2-ol] by exploiting the flexibility of the piperazinyl moiety. Structural analysis reveals that and are square planar complexes with piperazine rings in boat conformations whereas hydrolysis of Schiff bases (L(1) and L(2)) occurs during formation of octahedral complexes ( and ) with piperazine rings in chair conformations. Screening tests were conducted to quantify the binding ability of complexes (, and ) towards DNA, BSA and HSA and it was found that square planar complexes ( and ) showed more effective binding properties over octahedral complex (). Furthermore, enzyme kinetic studies reflect that square planar complexes ( and ) are also effective in mimicking catecholase like activities over octahedral complex (). Among all the complexes, was found to be the most promising molecule among the series due to its large binding affinity towards different bio-macromolecules and higher T.O.N in the catechol oxidation reaction.

Copper-organic frameworks assembled from in situ generated 5-(4-pyridyl)tetrazole building blocks: synthesis, structural features, topological analysis and catalytic oxidation of alcohols.

Two new metal-organic compounds {[Cu3(µ3-4-ptz)4(µ2-N3)2(DMF)2](DMF)2}n (1) and {[Cu(4-ptz)2(H2O)2]}n (2) {4-ptz = 5-(4-pyridyl)tetrazolate} with 3D and 2D coordination networks, respectively, have been synthesized while studying the effect of reaction conditions on the coordination modes of 4-pytz by employing the [2 + 3] cycloaddition as a tool for generating in situ the 5-substituted tetrazole ligands from 4-pyridinecarbonitrile and NaN3 in the presence of a copper(ii) salt. The obtained compounds have been structurally characterized and the topological analysis of 1 discloses a topologically unique trinodal 3,5,6-connected 3D network which, upon further simplification, results in a uninodal 8-connected underlying net with the bcu (body centred cubic) topology driven by the [Cu3(µ2-N3)2] cluster nodes and µ3-4-ptz linkers. In contrast, the 2D metal-organic network in 2 has been classified as a uninodal 4-connected underlying net with the sql [Shubnikov tetragonal plane net] topology assembled from the Cu nodes and µ2-4-ptz linkers. The catalytic investigations disclosed that 1 and 2 act as active catalyst precursors towards the microwave-assisted homogeneous oxidation of secondary alcohols (1-phenylethanol, cyclohexanol, 2-hexanol, 3-hexanol, 2-octanol and 3-octanol) with tert-butylhydroperoxide, leading to the yields of the corresponding ketones up to 86% (TOF = 430 h(-1)) and 58% (TOF = 290 h(-1)) in the oxidation of 1-phenylethanol and cyclohexanol, respectively, after 1 h under low power (10 W) microwave irradiation, and in the absence of any added solvent or additive.

Limiting nuclearity in formation of polynuclear metal complexes through [2 + 3] cycloaddition: synthesis and magnetic properties of tri- and pentanuclear metal complexes.

A tridentate ligand p-chloro-2-{(2-(dimethylamino)ethylimino)methyl}phenol (HL) was used to generate an octahedral nickel complex [Ni(L)Cl(H2O)2] 1 which was further converted into a square-planar nickel complex [Ni(L)(N3)] 2. The [2 + 3] cycloaddition reaction between metal coordinated azide 2 and different organonitriles under microwave irradiation afforded tri- and pentanuclear nickel(II) complexes 4a-4c. Reaction with benzonitrile and 3-cyano pyridine furnished the trinuclear species [Ni3L2(5-phenyltetrazolato)4(DMF)2] 4a and [Ni3L2{5-(3-pyridyl)-tetrazolato}4(DMF)2]·2H2O 4b, respectively. The nickel centers were found to be linearly disposed to each other and the complex is formed by a 2,3-tetrazolate bridge and a phenoxo bridge between central and terminal nickel atoms. Compound 2 when treated with 1,2-dicyanobenzene under identical conditions furnished a pentanuclear complex [Ni5L4{5-(2-cyanophenyl)-tetrazolato}4(OH)2(H2O)2]·3H2O·DMF 4c. In this pentanuclear compound two dimeric nickel units are connected to the central nickel center by a µ3-hydroxo bridge and a tetrazolate ligand operating via a relatively rare 1,2,3-bridging mode. The compounds were characterized by IR, elemental analysis, thermogravimetric analysis and single crystal X-ray crystallography. The magnetic susceptibility data for compounds 4a-4c show dominant antiferromagnetic interactions between the nickel centers for all the complexes. DFT calculations were performed to investigate the magnetic parameter in one of the complexes 4b by a broken symmetry approach.