Sharon Lai-Fung Chan

Highly selective metal catalysts for intermolecular carbenoid insertion into primary C-H bonds and enantioselective C-C bond formation.

Direct functionalization of C H bonds is an appealing strategy in organic synthesis but its practical application has so far been difficult to realize. The selective functionalization of primary C H bonds of alkanes that also contain secondary and/or tertiary C H bonds is a great challenge, as C H bond energy follows an order primary> secondary> tertiary. In seminal works by Bergman, Jones, and their respective co-workers, stoichiometric reactions of alkanes with [Cp*(Me3P)M] (Cp* = C5Me5; M = Rh, Ir) resulted in the formation of C M bonds by selective activation of primary C H bonds. Subsequent work by Hartwig and coworkers 2] demonstrated C B bond formation by stoichiometric and catalytic functionalization of primary C H bonds mediated by tungsten, rhodium, or ruthenium complexes. The high selectivity for primary C H bond functionalization in these C M or C B bond-formation reactions (Scheme S1 in the Supporting Information) is considered to result from kinetic factors or steric interaction between the metal complexes and alkanes. 3] A well-established process in C C bond formation by direct C H bond functionalization is the metal-catalyzed intraand intermolecular carbenoid insertion into C H bonds, with diazo compounds as the carbene source. 4] These catalytic C C bond-formation reactions generally feature lower selectivity for primary C H bonds than for secondary and tertiary C H bonds. For example, a selectivity order of primary< secondary< tertiary C H bonds has been observed for the extensively investigated carbene insertion catalyzed by rhodium complexes, 5] possibly because of the electron density order of primary< secondary< tertiary C H bonds, which renders primary C H bonds the least susceptible to attack by electrophilic rhodium–carbene intermediates. By manipulating the steric or electronic properties of the metal catalysts, a selectivity for primary C H bonds of alkanes comparable to that for secondary or tertiary C H bonds was observed, with the highest primary/secondary and primary/tertiary ratio per C H bond being 1.17:1.0 and 1.0:0.9, respectively. Herein we report a highly selective primary C H bond functionalization by metal-catalyzed carbenoid insertion into the C H bonds of alkanes (Scheme 1), which features a

Tissue-spray ionization mass spectrometry for raw herb analysis

Tissue-spray ionization mass spectrometry is developed for the in situ chemical analysis of raw herbs under ambient conditions. We demonstrated that analyte molecules could be directly sprayed and ionized from solvent-wetted ginseng tissues upon the application of high electrical voltage to the tissue sample. Abundant phytochemicals/ metabolites, including ginsenosides, amino acids and oligosaccharides, could be detected from ginseng tissues when the tissue-spray experiments were conducted in positive ion mode. Thermally labile and easily hydrolyzed malonyl-ginsenosides were also detected in negative ion mode. The tissue-spray ionization method enables the direct detection of analytes from raw herb samples and preserves the sample integrity for subsequent morphological and/ or microscopic examination. In addition, this method is simple and fast for chemical profiling of wild-type and cultivated-type American ginsengs with differentiation.

Highly conducting two-dimensional copper(I) 4-hydroxythiophenolate network

An unprecedented 2-D Cu-S network is constructed by self-assembly coordination between Cu(I) and 4-hydroxythiophenol and its structure was determined by using powder X-ray diffraction data. The self-assembled coordination network displays ionic behavior with a bulk electrical conductivity value of 120 S cm(-1).

Ytterbium(III) Porpholactones: β-Lactonization of Porphyrin Ligands Enhances Sensitization Efficiency of Lanthanide Near-Infrared Luminescence

The near-infrared (NIR) luminescence efficiency of lanthanide complexes is largely dependent on the electronic and photophysical properties of antenna ligands. Although porphyrin ligands are efficient sensitizers of lanthanide NIR luminescence, non-pyrrolic porphyrin analogues, which have unusual symmetry and electronic states, have been much less studied. In this work, we used porpholactones, a class of β-pyrrolic-modified porphyrins, as ligands and investigated the photophysical properties of lanthanide porpholactones Yb-1 a-5 a. Compared with Yb porphyrin complexes, the porpholactone complexes displayed remarkable enhancement of NIR emission (50-120 %). Estimating the triplet-state levels of porphyrin and porpholactone in Gd complexes revealed that β-lactonization of porphyrinic ligands lowers the ligand T1 state and results in a narrow energy gap between this state and the lowest excited state of Yb(3+) . Transient absorption spectra showed that Yb(III) porpholactone has a longer transient decay lifetime at the Soret band than the porphyrin analogue (30.8 versus 17.0 μs). Thus, the narrower energy gap and longer lifetime arising from β-lactonization are assumed to enhance NIR emission of Yb porpholactones. To demonstrate the potential applications of Yb porpholactone, a water-soluble Yb bioprobe was constructed by conjugating glucose to Yb-1 a. Interestingly, the NIR emission of this Yb porpholactone could be specifically switched on in the presence of glucose oxidase and then switched off by addition of glucose. This is the first demonstration that non-pyrrolic porphyrin ligands enhance the sensitization efficiency of lanthanide luminescence and also display switchable NIR emission in the region of biological analytes (800-1400 nm).

Molecular imaging of banknote and questioned document using solvent-free gold nanoparticle-assisted laser desorption/ionization imaging mass spectrometry.

Direct chemical analysis and molecular imaging of questioned documents in a non/minimal-destructive manner is important in forensic science. Here, we demonstrate that solvent-free gold-nanoparticle-assisted laser desorption/ionization mass spectrometry is a sensitive and minimal destructive method for direct detection and imaging of ink and visible and/or fluorescent dyes printed on banknotes or written on questioned documents. Argon ion sputtering of a gold foil allows homogeneous coating of a thin layer of gold nanoparticles on banknotes and checks in a dry state without delocalizing spatial distributions of the analytes. Upon N(2) laser irradiation of the gold nanoparticle-coated banknotes or checks, abundant ions are desorbed and detected. Recording the spatial distributions of the ions can reveal the molecular images of visible and fluorescent ink printed on banknotes and determine the printing order of different ink which may be useful in differentiating real banknotes from fakes. The method can also be applied to identify forged parts in questioned documents, such as number/writing alteration on a check, by tracing different writing patterns that come from different pens.

The effects of chelating N4 ligand coordination on Co(II)-catalysed photochemical conversion of CO2 to CO: reaction mechanism and DFT calculations

Here we describe the synthesis and X-ray crystal structures of a panel of cis-[CoII(N4)Cl2] complexes (N4 = tetradentate N atom donor ligand). We also examine the catalytic activities of these complexes in the photochemical and electrochemical reduction of CO2 to CO using [Ir(ppy)3] as the photosensitizer. Among the complexes studied, cis-[CoII(PDP)Cl2] (C1) (PDP = 1,1′-bis(2-pyridinylmethyl)-2,2′-bipyrrolidine) displayed the highest catalytic activity. This Co(II) complex was able to effectively mediate the reduction of CO2 to CO under either electrochemical or visible light photocatalytic conditions. For the electrocatalysis, C1 catalysed CO2 to CO with up to 96% Faraday efficiency at −1.70 V (vs. SCE, SCE = saturated calomel electrode). A selectivity of up to 95% for CO production was achieved in a photocatalytic CO2 reduction system by using C1 as the catalyst, Ir(ppy)3 as the photosensitizer and triethylamine as the electron donor. The Co(I) species in situ generated by the one electron reduction of cis-[CoII(PDP)Cl]+ is suggested to be directly responsible for CO2 activation. Ultrafast time (ns) resolved absorption spectroscopy revealed that the photoinduced electron transfer from the triplet excited state of Ir(ppy)3 to C1 is a key step in the generation of active Co(I) species. The electronic structure and redox properties of the Co(I) species, [CoI(N4)Cl], as well as its role in the catalytic reaction were investigated by DFT calculations. The presence of one chloride ligand cis to the CO2 coordination site neutralizes the positive charge on the Co(I) centre, therefore assisting the bound CO2 molecule in attracting protons. The reaction mechanism for CO2 reduction to CO catalysed by the recently reported [CoII(TPA)Cl]+ (TPA = tris(2-pyridylmethyl)amine) catalyst was also computed. Subtle modifications of the chelating N4 ligand from cis-[CoII(N4)Cl2] were found to have a profound effect on the efficiency of CO2 reduction by DFT calculations.

Phosphorescent Cyclometalated Iridium(III) Complexes That Contain Substituted 2-Acetylbenzo[b]thiophen-3-olate Ligand for Red Organic Light-Emitting Devices

We report the synthesis of a new class of thermally stable and strongly luminescent cyclometalated iridium(III) complexes 1-6, which contain the 2-acetylbenzo[b]thiophene-3-olate (bt) ligand, and their application in organic light-emitting diodes (OLEDs). These heteroleptic iridium(III) complexes with bt as the ancillary ligand have a decomposition temperature that is 10-20 % higher and lower emission self-quenching constants than those of their corresponding complexes with acetylacetonate (acac). The luminescent color of these iridium(III) complexes could be fine-tuned from orange (e.g., 2-phenyl-6-(trifluoromethyl)benzo[d]thiazole (cf3 bta) for 4) to pure red (e.g., lpt (Hlpt=4-methyl-2-(thiophen-2-yl)quinolone) for 6) by varying the cyclometalating ligands (C-deprotonated C^N). In particular, highly efficient OLEDs based on 6 as dopant (emitter) and 1,3-bis(carbazol-9-yl)benzene (mCP) as host that exhibit stable red emission over a wide range of brightness with CIE chromaticity coordinates of (0.67, 0.33) well matched to the National Television System Committee (NTSC) standard have been fabricated along with an external quantum efficiency (EQE) and current efficiency of 9 % and 10 cd A(-1) , respectively. A further 50 % increase in EQE (>13 %) by replacing mCP with bis[4-(6H-indolo[2,3-b]quinoxalin-6-yl)phenyl]diphenylsilane (BIQS) as host for 6 in the red OLED is demonstrated. The performance of OLEDs fabricated with 6 (i.e., [(lpt)2Ir(bt)]) was comparable to that of the analogous iridium(III) complex that bore acac (i.e., [(lpt)2 Ir(acac)]; 6a in this work) [Adv. Mater.- 2011, 23, 2981] fabricated under similar conditions. By using ntt (Hnnt=3-hydroxynaphtho[2,3-b]thiophen-2-yl)(thiophen-2-yl)methanone) ligand, a substituted derivative of bt, the [(cf3bta)2Ir(ntt)] was prepared and found to display deep red emission at around 700 nm with a quantum yield of 12 % in mCP thin film.

A Highly Oxidizing and Isolable Oxoruthenium(V) Complex [RuV(N4O)(O)]2+: Electronic Structure, Redox Properties, and Oxidation Reactions Investigated by DFT Calculations

The electronic structure and redox properties of the highly oxidizing, isolable Ru(V)=O complex [Ru(V)(N4O)(O)](2+), its oxidation reactions with saturated alkanes (cyclohexane and methane) and inorganic substrates (hydrochloric acid and water), and its intermolecular coupling reaction have been examined by DFT calculations. The oxidation reactions with cyclohexane and methane proceed through hydrogen atom transfer in a transition state with a calculated free energy barrier of 10.8 and 23.8 kcal mol(-1), respectively. The overall free energy activation barrier (ΔG(≠)=25.5 kcal mol(-1)) of oxidation of hydrochloric acid can be decomposed into two parts: the formation of [Ru(III)(N4O)(HOCl)](2+) (ΔG=15.0 kcal mol(-1)) and the substitution of HOCl by a water molecule (ΔG(≠)=10.5 kcal mol(-1)). For water oxidation, nucleophilic attack on Ru(V)=O by water, leading to O-O bond formation, has a free energy barrier of 24.0 kcal mol(-1), the major component of which comes from the cleavage of the H-OH bond of water. Intermolecular self-coupling of two molecules of [Ru(V)(N4O)(O)](2+) leads to the [(N4O)Ru(IV)-O2-Ru(III)(N4O)](4+) complex with a calculated free energy barrier of 12.0 kcal mol(-1).

Homoleptic platinum(II) and palladium(II) organothiolates and phenylselenolates: Solvothermal synthesis, structural determination, optical properties, and single-source precursors for PdSe and PdS nanocrystals

Homoleptic d(8)-metal organothiolates and phenylselenolates [M(EC(6)H(5))(2)](infinity) (E=S, M=Pt 1, M=Pd 2, M=Ni 5; E=Se, M=Pt 3, M=Pd 4) were prepared as crystalline solids under solvothermal conditions. Their structures were solved using powder X-ray diffraction data. In each case, the EC(6)H(5) (E=S, Se) ligand binds to two metal ions (M=Pt, Pd, and Ni) to form chain-like structures with planar (in 1) or zig-zag (in 2-5) conformations. The [M(SR)(2)](infinity) complexes (M=Pt, R=4-tert-butylphenyl 6; R=2-naphthyl 8; R=4-nitrophenyl 10 and M=Pd, R=4-tert-butylphenyl 7; R=2-naphthyl 9; R=4-nitrophenyl 11) were prepared under similar solvothermal conditions. Based on the XPS binding energies and elemental analyses, complexes 6-11 have the same [M(SR)(2)](infinity) formulation as 1 and 2. The cyclic complex [Pd(6)(SCH(3))(12)] 12 was prepared as a crystalline solid by solvothermal annealing treatment of the amorphous precipitate. A chain-like polymer structure is proposed for both [Pd(SC(12)H(25))(2)](infinity) 13 and [Pd(SC(16)H(33))(2)](infinity) 14; these polymeric chains self-assemble to give layer-like structures. Solid-state diffuse reflectance spectra reveal that the optical band gap E(g) (eV) of complexes 1, 6, 8, 10 and of 2, 7, 9, 11 are in the range of 2.10-3.00 eV and 2.10-2.63 eV, respectively, and 5 has the lowest E(g) value (1.72 eV). Heating solid samples of 4 and 13 under solvothermal conditions afforded phase-pure Pd(17)Se(15) and PdS nanocrystals, respectively. Field-effect transistors fabricated with a drop-cast thin film made from Pd(17)Se(15) nanocrystals prior treated with an ethanolic solution of 1-hexadecanethiol displayed ambipolar charge transporting properties with hole and electron mobility being 7x10(-2) cm(2) V(-1) s(-1) and 6x10(-2) cm(2) V(-1) s(-1), respectively.

A Novel Tetradentate Ruthenium(II) Complex Containing Tris(2- pyridylmethyl)amine (tpa) as an Inhibitor of Beta-Amyloid Fibrillation

We report herein the synthesis and application of a novel tetradentate ruthenium(II) complex 1 containing a tris(2-pyridylmethyl)amine (tpa) ligand as an inhibitor of β-amyloid fibrillogenesis. [Ru(tpa)(bt)]ClO(4) 1 (bt =2-acetylbenzo[b]thiophene-3-olate) showed significant inhibition of Aβ(1-40) peptide aggregation in vitro, which was confirmed by a Thioflavin T assay and transmission electron microscopy imaging.