Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 20 de 41
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
Nanoscale ; 12(14): 7914-7920, 2020 Apr 09.
Artigo em Inglês | MEDLINE | ID: mdl-32232235

RESUMO

As a small molecule possessing both strong H-bond donor and acceptor functions, 1H-imidazole can participate in extensive homo- or heteromolecular H-bonding networks. These properties are important in Nature, as imidazole moieties are incorporated in many biologically-relevant compounds. Imidazole also finds applications ranging from corrosion inhibition to fire retardants and photography. We have found a peculiar behaviour of imidazole during scanning tunnelling microscopy-break junction (STM-BJ) experiments, in which oligomeric chains connect the two electrodes and allow efficient charge transport. We attributed this behaviour to the formation of hydrogen-bonding networks, as no evidence of such behaviour was found in 1-methylimidazole (incapable of participating in intramolecular hydrogen bonding). The results are supported by DFT calculations, which confirmed our hypothesis. These findings pave the road to the use of hydrogen-bonding networks for the fabrication of dynamic junctions based on supramolecular interactions.

2.
Angew Chem Int Ed Engl ; 59(29): 12029-12034, 2020 Jul 13.
Artigo em Inglês | MEDLINE | ID: mdl-32271489

RESUMO

Polyoxometalates have been proposed in the literature as nanoelectronic components, where they could offer key advantages with their structural versatility and rich electrochemistry. Apart from a few studies on their ensemble behaviour (as monolayers or thin films), this potential remains largely unexplored. We synthesised a pyridyl-capped Anderson-Evans polyoxometalate and used it to fabricate single-molecule junctions, using the organic termini to chemically "solder" a single cluster to two nanoelectrodes. Operating the device in an electrochemical environment allowed us to probe charge transport through different oxidation states of the polyoxometalate, and we report here an efficient three-state transistor behaviour. Conductance data fits a quantum tunnelling mechanism with different charge-transport probabilities through different charge states. Our results show the promise of polyoxometalates in nanoelectronics and give an insight on their single-entity electrochemical behaviour.

3.
J Am Chem Soc ; 142(2): 847-856, 2020 Jan 15.
Artigo em Inglês | MEDLINE | ID: mdl-31825213

RESUMO

Making new van der Waals materials with electronic or magnetic functionality is a chemical design challenge for the development of two-dimensional nanoelectronic and energy conversion devices. We present the synthesis and properties of the van der Waals material Bi4O4SeCl2, which is a 1:1 superlattice of the structural units present in the van der Waals insulator BiOCl and the three-dimensionally connected semiconductor Bi2O2Se. The presence of three anions gives the new structure both the bridging selenide anion sites that connect pairs of Bi2O2 layers in Bi2O2Se and the terminal chloride sites that produce the van der Waals gap in BiOCl. This retains the electronic properties of Bi2O2Se while reducing the dimensionality of the bonding network connecting the Bi2O2Se units to allow exfoliation of Bi4O4SeCl2 to 1.4 nm height. The superlattice structure is stabilized by the configurational entropy of anion disorder across the terminal and bridging sites. The reduction in connective dimensionality with retention of electronic functionality stems from the expanded anion compositional diversity.

4.
Inorg Chem ; 58(9): 6495-6506, 2019 May 06.
Artigo em Inglês | MEDLINE | ID: mdl-31021620

RESUMO

The exceptional performance of oxobenzene-bridged bis-1,2,3-dithiazolyls 6 as single-component neutral radical conductors arises from the presence of a low-lying π-lowest unoccupied molecular orbital, which reduces the potential barrier to charge transport and increases the kinetic stabilization energy of the metallic state. As part of ongoing efforts to modify the solid-state structures and transport properties of these so-called multiorbital materials, we report the preparation and characterization of the acetoxy, methoxy, and thiomethyl derivatives 6 (R = OAc, OMe, SMe). The crystal structures are based on ribbonlike arrays of radicals laced together by S···N' and S···O' secondary bonding interactions. The steric and electronic effects of the exocyclic ligands varies, affording one-dimensional (1D) π-stacked radicals for R = OAc, 1D cofacial dimer π-stacks for R = SMe, and a pseudo two-dimensional (2D) brick-wall arrangement for R = OMe. Variable-temperature magnetic and conductivity measurements reveal strong antiferromagnetic interactions and Mott insulating behavior for the two radical-based structures (R = OAc, OMe), with lower room-temperature conductivities (σRT ≈ 1 × 10-4 and ∼1 × 10-3 S cm-1, respectively) and higher thermal activation energies ( Eact = 0.24 and 0.21 eV, respectively) than found for the ideal 2D brick-wall structure of 6 (R = F), where σRT ≈ 1 × 10-2 S cm-1 and Eact = 0.10 eV. The performance of R = OMe, OAc relative to that of R = F, is consistent with the results of density functional theory band electronic structure calculations, which indicate a lower kinetic stabilization energy of the putative metallic state arising from their reduced electronic dimensionality.

5.
Chemistry ; 25(17): 4345-4357, 2019 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-30714235

RESUMO

Oxidative cleavage of the Csp3 -O bond in 1-arylisochromans with stoichiometric oxidants, such as CrO3 /H2 SO4 , has been practiced for decades in synthetic chemistry. Herein, we report that a structurally well-defined FeII -pyridyl(bis-imidazolidine) catalyst promotes the aerobic oxygenation of 1-arylisochromans, affording highly selectively 2-(hydroxyethyl)benzophenones, compounds of potential for neuroprotective agents. Key intermediates have been isolated, indicating that the reaction proceeds through dehydrogenative oxygenation of the isochromans at the 1-position, Csp3 -O bond cleavage at the iron centre and hydrogenolysis of the resulting Fe-O bond with the H2 generated from the dehydrogenation step. In the absence of H2 but under iron catalysis, the peroxide intermediate is converted into an unexpected ketal compound, which transfers into a 2-(hydroxyethyl)benzophenone when both O2 and H2 are admitted. The unique ability of the iron catalyst for oxygenation and hydrogenation in the same catalytic process under mild conditions allows for the stepwise preparation of a variety of isolable oxygenated products on a preparative scale, circumventing the need for using wasteful and/or toxic oxidants.

6.
Inorg Chem ; 58(5): 3355-3363, 2019 Mar 04.
Artigo em Inglês | MEDLINE | ID: mdl-30767521

RESUMO

Trimethylaluminum finds widespread applications in chemical and materials synthesis, most prominently in its partially hydrolyzed form of methylalumoxane (MAO), which is used as a cocatalyst in the polymerization of olefins. This work investigates the sequential reactions of trimethylaluminum with hexaprotic phosphazenes (RNH)6P3N3 (=XH6) equipped with substituents R of varied steric bulk including tert-butyl (1H6), cyclohexyl (2H6), isopropyl (3H6), isobutyl (4H6), ethyl (5H6), propyl (6H6), methyl (7H6), and benzyl (8H6). Similar to MAO, the resulting complexes of polyanionic phosphazenates [XH n] n-6 accommodate multinuclear arrays of [AlMe2]+ and [AlMe]2+. Reactions were monitored by 31P NMR spectroscopy, and structures were determined by single-crystal X-ray diffraction. They included 1H4(AlMe2)2, 1H3(AlMe2)3, 2H3(AlMe2)3, 3(AlMe2)4AlMe, 4H(AlMe2)5, 4(AlMe2)6, {5H(AlMe2)4}2AlMe, 5(AlMe2)6, 6(AlMe2)6, {7(AlMe2)4AlMe}2, and 8(AlMe2)6. The study shows that subtle variations of the steric properties of the R groups influence the reaction pathways, levels of aggregation, and fluxional behavior. While [AlMe2]+ is the primary product of the metalation, [AlMe]2+ is utilized to alleviate overcrowding or to aid aggregation. At the later stages of metalation, [AlMe2]+ groups start to scramble around congested sites. The ligands proved to be very robust and extremely flexible, offering a unique platform to study complex multinuclear metal arrangements.

7.
Chem Commun (Camb) ; 55(4): 497-500, 2019 Jan 03.
Artigo em Inglês | MEDLINE | ID: mdl-30548047

RESUMO

In the presence of a nickel catalyst, the intermolecular (4+2) cycloaddition of potassium alkynyltrifluoroborates with 3-azetidinones and 3-oxetanone leads to the formation of borylated dihydropyridinones and dihydropyranones without unwanted carbon-boron bond cleavage. The regioselectivity is influenced only by the trifluoroborate group, and only one regioisomer is obtained, whether the other alkyne substituent is an alkyl, vinyl, or (hetero)aryl group.

8.
Inorg Chem ; 58(1): 419-427, 2019 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-30570252

RESUMO

The N-methyl-4-phenyl-pyridine-bridged bisdithiazolyl radical PhBPMe is polymorphic, crystallizing from cold acetonitrile in a trigonal α-phase, space group P3121, and from hot dichloroethane in an orthorhombic ß-phase, space group Pca21. The crystal structures of both phases consist of slipped π-stacks of undimerized radicals aligned laterally into herringbone arrays. In the ß-phase, there are two independent radicals in the asymmetric unit, and the resulting π-stacks form corrugated layers interspersed by methyl and phenyl groups which block the approach of neighboring radicals. In the α-phase, the methyl/phenyl groups and the radical π-stacks separately form spirals about 31 axes, the latter giving rise to a 3D network of close radical/radical contacts. Variable temperature magnetic susceptibility measurements on the ß-phase indicate strong antiferromagnetic coupling. Weaker but predominantly antiferromagnetic interactions (θ = -20.7 K) are observed in the α-phase. A high temperature series expansion analysis of the magnetic data for the α-phase affords antiferromagnetic exchange energies for the one- and two-step radical/radical interactions about the 31 spirals ( J1 = -1.2 K, J2 = -10.9 K, respectively), with weak ferromagnetic interactions along the π-stacks ( Jπ = +1.8 K). Despite the presence of a 3D network based on the dominant J2 interactions, which affords two independent bipartite sublattices, no evidence of bulk antiferromagnetic order has been observed above T = 2 K. The magnetic results are discussed in light of exchange energies calculated using density functional theory broken symmetry methods.

9.
Inorg Chem ; 57(21): 13901-13911, 2018 Nov 05.
Artigo em Inglês | MEDLINE | ID: mdl-30351085

RESUMO

Condensation of persilylated nicotinimideamide and isonicotinimideamide with sulfur monochloride affords double salts of the 3-, 4-pyridyl-substituted 1,2,3,5-dithiadiazolylium DTDA cations of the general formula [3-, 4-pyDTDA][Cl][HCl] in which the pyridyl nitrogen serves as a noninnocent base. Reduction of these salts with triphenylantimony followed by deprotonation of the intermediate-protonated radical affords the free base radicals [3-, 4-pyDTDA], the crystal structures of which, along with those of their diselenadiazolyl analogues [3-, 4-pyDSDA], have been characterized by powder or single-crystal X-ray diffraction. The crystal structures consist of "pancake" π-dimers linked head-to-tail into ribbonlike arrays by η2-S2---N(py) intermolecular secondary bonding interactions. Methylation of the persilylated (iso)nicotinimide-amides prior to condensation with sulfur monochloride leads to N-methylated double chloride salts Me[3-, 4-pyDTDA][Cl]2, which can be converted by metathesis into the corresponding triflates Me[3-, 4-pyDTDA][OTf]2 and then reduced to the N-methylated radical triflates Me[3-, 4-pyDTDA][OTf]. The crystal structures of both the N-methylated double triflate and radical triflate salts have been determined by single-crystal X-ray diffraction. The latter consist of trans-cofacial π-dimers strongly ion-paired with triflate anions. Variable temperature magnetic susceptibility measurements on both the neutral and radical ion dimers indicate that they are diamagnetic over the temperature range 2-300 K.

10.
Chem Commun (Camb) ; 54(83): 11805-11808, 2018 Oct 16.
Artigo em Inglês | MEDLINE | ID: mdl-30280153

RESUMO

A cyclometalated rhodium complex has been shown to perform highly selective and efficient reduction of aldehydes, deriving the hydrogen from methanol. With methanol as both the solvent and hydrogen donor under mild conditions and an open atmosphere, a wide range of aromatic aldehydes were reduced to the corresponding alcohols, without affecting other functional groups.

11.
Chem Commun (Camb) ; 54(68): 9450-9453, 2018 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-30083669

RESUMO

Platinum-[diaminocarbene] complexes work as transition-metal photocatalysts for the hydrosilylation of alkynes. A catalytic system operates under visible light irradiation (blue LED) enabling the conversion of a range of terminal and internal alkynes to respective vinyl silanes in excellent yields.

12.
Chem Commun (Camb) ; 54(47): 6012-6015, 2018 Jun 08.
Artigo em Inglês | MEDLINE | ID: mdl-29796532

RESUMO

Diethylamine is the smallest and simplest molecule that features a supramolecular helix as its lowest energy aggregate. Structural studies and large scale sampling simulations show that the helical arrangement is more stable than cyclic structures, which are the dominant species for other small hydrogen bonding molecules.

13.
J Am Chem Soc ; 140(11): 3846-3849, 2018 03 21.
Artigo em Inglês | MEDLINE | ID: mdl-29513996

RESUMO

Crystals of the heterocyclic radical naphtho-1,3,2-dithiazolyl NDTA display magnetic bistability with a well-defined hysteretic phase transition at Tc↓ = 128(2) K and Tc↑ = 188(2) K. The magnetic signature arises from a radical/dimer interconversion involving one of the two independent π-radicals in the P1̅ unit cell. Variable temperature X-ray crystallography has established that while all the radicals in HT-NDTA serve as paramagnetic ( S = 1/2) centers, half of the radicals in LT-NDTA form closed-shell N-N σ-bonded dimers ( S = 0) and half retain their S = 1/2 spin state. The wide window of bistability (60 K) may be attributed to the large structural changes that accompany the phase transition.

14.
J Am Chem Soc ; 139(4): 1625-1635, 2017 02 01.
Artigo em Inglês | MEDLINE | ID: mdl-28117984

RESUMO

A critical feature of the electronic structure of oxobenzene-bridged bisdithiazolyl radicals 2 is the presence of a low-lying LUMO which, in the solid state, improves charge transport by providing additional degrees of freedom for electron transfer. The magnitude of this multiorbital effect can be fine-tuned by variations in the π-electron releasing/accepting nature of the basal ligand. Here we demonstrate that incorporation of a nitro group significantly stabilizes the LUMO, and hence lowers Ueff, the effective Coulombic barrier to charge transfer. The effect is echoed, at the molecular level, in the observed trend in Ecell, the electrochemical cell potential for 2 with R = F, H and NO2. The crystal structures of the MeCN and EtCN solvates of 2 with R = NO2 have been determined. In the EtCN solvate the radicals are dimerized, but in the MeCN solvate the radicals form superimposed and evenly spaced π-stacked arrays. This highly 1D material displays Pauli-like temperature independent paramagnetic behavior, with χTIP = 6 × 10-4 emu mol-1, but its charge transport behavior, with σRT near 0.04 S cm-1 and Eact = 0.05 eV, is more consistent with a Mott insulating ground state. High pressure crystallographic measurements confirm uniform compression of the π-stacked architecture with no phase change apparent up to 8 GPa. High pressure conductivity measurements indicate that the charge gap between the Mott insulator and metallic states can be closed near 6 GPa. These results are discussed in the light of DFT band structure calculations.

15.
J Am Chem Soc ; 138(34): 10738-41, 2016 08 31.
Artigo em Inglês | MEDLINE | ID: mdl-27537064

RESUMO

Electro-oxidation of the quinoidal bisdithiazole BT in dichloroethane in the presence of [Bu4N][GaBr4] affords the 1:1 radical ion salt [BT][GaBr4], crystals of which belong to the trigonal space group P3. The packing pattern of the radical cations provides a rare example of an organic kagome basket structure, with S = 1/2 radical ion chains located at the triangular corners of a trihexagonal lattice. Magnetic measurements over a wide temperature range from 30 mK to 300 K suggest strongly frustrated AFM interactions on the scale of J/kb ∼ 30 K, but reveal no anomalies that would be associated with magnetic order. These observations are discussed in terms of the symmetry allowed magnetic interactions within and between the frustrated layers.

16.
Faraday Discuss ; 183: 147-60, 2015.
Artigo em Inglês | MEDLINE | ID: mdl-26375151

RESUMO

Immobilization of [Mn(bpy)(CO)3Br], (1) and [Mn(bpy((t)Bu)2)(CO)3Br] (2, where (bpy((t)Bu)2) = 4,4'-di-tert-butyl-2,2'-bipyridine) in Nafion/multi-walled carbon nanotubes (MWCNT) on glassy carbon yielded highly active electrodes for the reduction of CO2 to CO in aqueous solutions at pH 7. Films incorporating have significantly improved selectivity towards CO2, with CO : H2 ∼ 1 at -1.4 V vs. SCE, exceeding that for the previously reported /MWCNT/Nafion electrode. Furthermore, we report the synthesis and subsequent electrochemical characterization of two new substituted Mn(i) bipyridine complexes, [Mn(bpy(COOH)2)(CO)3Br] (3) and [Mn(bpy(OH)2)(CO)3Br] (4) (where (bpy(COOH)2) = 4,4'-di-carboxy-2,2'-bipyridine and (bpy(OH)2) = 4,4'-di-hydroxy-2,2'-bipyridine). Both 3 and 4 were found to have some activity towards CO2 in acetonitrile solutions; however once immobilized in Nafion membranes CO2 reduction was found to not occur at significant levels.

17.
J Am Chem Soc ; 137(3): 1044-7, 2015 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-25588146

RESUMO

The alternating ABABAB π-stacked architecture of the EtCN solvate of the iodo-substituted, oxobenzene-bridged bisdithiazolyl radical IBBO (space group Pnma) gives rise to strong ferromagnetic exchange along the π-stacks, and the material orders as a spin-canted antiferromagnet with T(N) = 35 K, with a spontaneous (canted) moment M(spont) = 1.4 × 10(-3) µB and a coercive field H(c) = 1060 Oe (at 2 K). The observation of spin-canting can only be understood in terms of multiorbital contributions to both isotropic and anisotropic exchange interactions, the magnitude of which are enhanced by spin-orbit effects arising from the heavy-atom iodine substituent. Pseudodipolar interactions lead to a net canted moment along the c-axis, while the sublattice magnetization is predicted to possess an easy a-axis.

18.
J Am Chem Soc ; 136(23): 8350-60, 2014 Jun 11.
Artigo em Inglês | MEDLINE | ID: mdl-24835531

RESUMO

Selective α-oxidation of ethers under aerobic conditions is a long-pursued transformation; however, a green and efficient catalytic version of this reaction remains challenging. Herein, we report a new family of iron catalysts capable of promoting chemoselective α-oxidation of a range of ethers with excellent mass balance and high turnover numbers under 1 atm of O2 with no need for any additives. Unlike metalloenzymes and related biomimetics, the catalyst produces H2 as the only byproduct. Mechanistic investigations provide evidence for an unexpected two-step reaction pathway, which involves dehydrogenative incorporation of O2 into the ether to give a peroxobisether intermediate followed by cleavage of the peroxy bond to form two ester molecules, releasing stoichiometric H2 gas in each step. The operational simplicity and environmental friendliness of this methodology affords a useful alternative for performing oxidation, while the unique ability of the catalyst in oxygenating a substrate via dehydrogenation points to a new direction for understanding metalloenzymes and designing new biomimetic catalysts.

19.
Chemistry ; 20(1): 245-52, 2014 Jan 03.
Artigo em Inglês | MEDLINE | ID: mdl-24516890

RESUMO

Cyclometalated iridium complexes are found to be versatile catalysts for the direct reductive amination (DRA) of carbonyls to give primary amines under transfer-hydrogenation conditions with ammonium formate as both the nitrogen and hydrogen source. These complexes are easy to synthesise and their ligands can be easily tuned. The activity and chemoselectivity of the catalyst towards primary amines is excellent, with a substrate to catalyst ratio (S/C) of 1000 being feasible. Both aromatic and aliphatic primary amines were obtained in high yields. Moreover, a first example of homogeneously catalysed transfer-hydrogenative DRA has been realised for ß-keto ethers, leading to the corresponding ß-amino ethers. In addition, non-natural α-amino acids could also be obtained in excellent yields with this method.


Assuntos
Aminas/química , Irídio/química , Cetonas/química , Aminação , Catálise , Complexos de Coordenação/química , Cristalografia por Raios X , Hidrogenação , Conformação Molecular , Oxirredução
20.
Angew Chem Int Ed Engl ; 53(1): 193-8, 2014 Jan 03.
Artigo em Inglês | MEDLINE | ID: mdl-24302659

RESUMO

The peptide-based porous 3D framework, ZnCar, has been synthesized from Zn(2+) and the natural dipeptide carnosine (ß-alanyl-L-histidine). Unlike previous extended peptide networks, the imidazole side chain of the histidine residue is deprotonated to afford Zn-imidazolate chains, with bonding similar to the zeolitic imidazolate framework (ZIF) family of porous materials. ZnCar exhibits permanent microporosity with a surface area of 448 m(2) g(-1) , and its pores are 1D channels with 5 Šopenings and a characteristic chiral shape. This compound is chemically stable in organic solvents and water. Single-crystal X-ray diffraction (XRD) showed that the ZnCar framework adapts to MeOH and H2 O guests because of the torsional flexibility of the main His-ß-Ala chain, while retaining the rigidity conferred by the Zn-imidazolate chains. The conformation adopted by carnosine is driven by the H bonds formed both to other dipeptides and to the guests, permitting the observed structural transformations.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA