Electrochemical Reduction of CO2 Catalyzed by Re(pyridine-oxazoline)(CO)3Cl Complexes.

A series of rhenium tricarbonyl complexes coordinated by asymmetric diimine ligands containing a pyridine moiety bound to an oxazoline ring were synthesized, structurally and electrochemically characterized, and screened for CO2 reduction ability. The reported complexes are of the type Re(N-N)(CO)3Cl, with N-N = 2-(pyridin-2-yl)-4,5-dihydrooxazole (1), 5-methyl-2-(pyridin-2-yl)-4,5-dihydrooxazole (2), and 5-phenyl-2-(pyridin-2-yl)-4,5-dihydrooxazole (3). The electrocatalytic reduction of CO2 by these complexes was observed in a variety of solvents and proceeds more quickly in acetonitrile than in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The analysis of the catalytic cycle for electrochemical CO2 reduction by 1 in acetonitrile using density functional theory (DFT) supports the C-O bond cleavage step being the rate-determining step (RDS) (ΔG⧧ = 27.2 kcal mol-1). The dependency of the turnover frequencies (TOFs) on the donor number (DN) of the solvent also supports that C-O bond cleavage is the rate-determining step. Moreover, the calculations using explicit solvent molecules indicate that the solvent dependence likely arises from a protonation-first mechanism. Unlike other complexes derived from fac-Re(bpy)(CO)3Cl (I; bpy = 2,2'-bipyridine), in which one of the pyridyl moieties in the bpy ligand is replaced by another imine, no catalytic enhancement occurs during the first reduction potential. Remarkably, catalysts 1 and 2 display relative turnover frequencies, (icat/ip)2, up to 7 times larger than that of I.

Ligand- and Brønsted acid/base-switchable reaction pathways in gold(I)-catalyzed cycloisomerizations of allenoic acids.

Gold-promoted cyclizations of 2,2-diaryl substituted γ-allenoic acids were found to give three isomeric lactone products, each of which could be obtained selectively by exploiting Brønsted acid/base and ligand effects. Simple 5-exo-trig cyclization products were favored by strong donor ligands or base additives, whereas weak donor ligands and a Brønsted acid additive gave isomeric enelactones resulting from double bond migration. Further optimization afforded a class of medicinally relevant bridged tricyclic lactones via a tandem hydroacyloxylation/hydroarylation process. Kinetic studies and control experiments indicated that the initial 5-exo-trig cyclization product serves as a branch point for further isomerization to the other lactone products via cooperative gold(I)/Brønsted acid catalysis.

C-shaped diastereomers containing cofacial thiophene-substituted quinoxaline rings: synthesis, photophysical properties, and X-ray crystallography.

Synthesis and characterization of two diastereomeric C-shaped molecules containing cofacial thiophene-substituted quinoxaline rings are described. A previously known bis-α-diketone was condensed with an excess of 4-bromo-1,2-diaminobenzene in the presence of zinc acetate to give a mixture of two C-shaped diastereomers with cofacial bromine-substituted quinoxaline rings. After chromatographic separation, thiophene rings were installed by a microwave-assisted Suzuki coupling reaction, resulting in highly emissive diastereomeric compounds that were studied by UV-vis, fluorescence, and NMR spectroscopy, as well as X-ray crystallography. The unique symmetry of each diastereomer was confirmed by NMR spectroscopy. NMR data indicated that the syn isomer has restricted rotation about the bond connecting the thiophene and quinoxaline rings, which was also observed in the solid state. The spectroscopic properties of the C-shaped diastereomers were compared to a model compound containing only a single thiophene-substituted quinoxaline ring. Ground state intramolecular π-π interactions in solution were detected by NMR and UV-vis spectroscopy. Red-shifted emission bands, band broadening, and large Stokes shifts were observed, which collectively suggest excited state π-π interactions that produce excimer-like emissions, as well as a remarkable positive emission solvatochromism, indicating charge-transfer character in the excited state.

Crystal structure of 4-chloro-2-iodo-aniline.

In the crystal structure of the title compound, C6H5ClIN, the amino group engages in N-H⋯N hydrogen bonding, creating [100] chains. A Cl⋯I contact is observed [3.7850 (16) Å]. The parallel planes of neigbouring mol-ecules reveal highly offset π-stacking characterized by a centroid-centroid distance of 4.154 (1), a centroid-to-plane distance of 3.553 (3) and ring-offset slippage of 2.151 (6) Å.

4-Chloro-3-ethyl-phenol.

The title compound, C8H9ClO, packs with two independent mol-ecules in the asymmetric unit, without significant differences in corresponding bond lengths and angles, with the ethyl group in each oriented nearly perpendicular to the aromatic ring having ring-to-side chain torsion angles of 81.14 (18) and -81.06 (19)°. In the crystal, mol-ecules form an O-H⋯O hydrogen-bonded chain extending along the b-axis direction, through the phenol groups in which the H atoms are disordered. These chains pack together in the solid state, giving a sheet lying parallel to (001), via an offset face-to-face π-stacking inter-action characterized by a centroid-centroid distance of 3.580 (1) Å, together with a short inter-molecular Cl⋯Cl contact [3.412 (1) Å].

6-Fluoro-indan-1-one.

The title compound, C9H7FO, crystallizes with two independent mol-ecules in the asymmetric unit, in which corresponding bond lengths are the same within experimental error. The five-membered ring in each molecule is almost planar, with r.m.s. deviations of 0.016 and 0.029 Å. In the crystal, mol-ecules form sheets parallel to (1 0 0) via C-H⋯O and C-H⋯F inter-actions with F⋯F contacts [3.1788 (16) and 3.2490 (16) Å] between the sheets.

Crystal structures of three uranyl-acetate-bipyridine complexes crystallized from hydraulic fracking fluid.

Hydraulic fracking exposes shale plays to acidic hydraulic fracking fluid (HFF), releasing toxic uranium (U) along with the desired oil and gas. With no existing methods to ensure U remains sequestered in the shale, this study sought to add organic ligands to HFF to explore potential U retention in shale plays. To test this possibility, incubations were set up in which uranyl acetate and one organic bipyridine ligand (either 2,2'-, 2,3'-, 2,4'-, or 4,4'-bipyridine) were added to pristine HFF as the crystallization medium. After several months and complete evaporation of all volatiles, bulk yellow crystalline material was obtained from the incubations, three of which yielded crystals suitable for single-crystal analysis, resulting in two novel structures and a high-quality structure of a previously described compound. The UO2VI acetate complexes bis(acetato-κ2O,O')(2,2'-bipyridine-κ2N,N')dioxidouranium(VI), [U(C2H3O2)2O2(C10H8N2)2] or [2,2'-bipyridine]UVIO2(CH3CO2)2, (I), and bis(acetato-κ2O,O')(2,4'-bipyridine-κN1')dioxidouranium(VI), [U(C2H3O2)2O2(C10H8N2)2] or [2,4'-bipyridine]2UVIO2(CH3CO2)2, (III), contain eight-coordinate UVI in a pseudo-hexagonal bipyramidal coordination geometry and are molecular, packing via weak C-H...O/N interactions, whereas catena-poly[bis(2,3'-bipyridinium) [di-µ-acetato-µ3-hydroxido-µ-hydroxido-di-µ3-oxido-hexaoxidotriuranium(VI)]-2,3'-bipyridine-water (1/1/1)], (C10H9N2)2[U3(C2H3O2)2O8(OH)2]·C10H8N2·H2O or {[2,3'-bipyridinium]2[2,3'-bipyridine][(UVIO2)3(O)2(OH)2(CH3CO2)2·H2O]}n, (II), forms an ionic one-dimensional polymer with seven-coordinate pentagonal bipyramidal UVI centers and hydrogen-bonding interactions within each chain. The formation of these crystals could indicate the potential for bipyridine to bind with U in shale during fracking, which will be explored in a future study via ICP-MS (inductively coupled plasma mass spectrometry) analyses of U concentration in HFF/bipyridine/shale incubations. The variation seen here between the molecular structures may indicate variance in the ability of bipyridine isomers to form complexes with U, which could impact their ability to retain U within shale in the context of fracking.

Zirconium-mediated synthesis of arsaalkene compounds from arsines and isocyanides.

An atom-economical synthesis of arsaalkenes via a net coupling of aryl arsines with aryl or alkyl isocyanides at zirconium is presented. Reaction of zirconium arsenido complexes (N3N)ZrAsHAr [N3N = N(CH2CH2NSiMe3)3(3-); Ar = Ph, (2) Mes (3)] with aryl and alkyl isocyanides yields arsaalkene products of the general form (N3N)Zr[NRC(H)═As(Ar)]. Two examples (5: R = Mes, Ar = Ph; 6: R = CH2Ph, Ar = Mes) were structurally characterized. Observation of intermediates in the reaction and structural characterization of the previously reported 1,1-insertion product benzylisocyanide with (N3N)ZrAsPh2 (8), (N3N)Zr[η(2)-C(PPh2)=NCH2Ph] (9), support the mechanistic hypothesis that these reactions occur via 1,1-insertion followed by rearrangement.

2-Bromo-5-fluoro-benzaldehyde.

In the title compound, C7H4BrFO, the benzaldehyde O atom is found to be trans to the 2-bromo substituent. In the crystal, short Br⋯F inter-actions between the bromine and fluorine substituents are observed at distances of 3.1878 (14), 3.3641 (13) and 3.3675 (14) Å. Offset face-to-face π-stacking inter-actions are also observed for both of the independent mol-ecules in the asymmetric unit running parallel to the crystallographic b axis, characterized by centroid-centroid distances of 3.8699 (2) and 3.8699 (2) Å.

(E)-Benz-yl(1-phenyl-ethyl-idene)amine.

The title compound, C15H15N, represents an E isomer. The mol-ecule exhibits a minor [9.1 (2)%] disorder with methyl-benzyl-idene and benzyl groups inter-changing their positions. The C=N bond length is 1.292 (2) Å. The mol-ecular geometry is essentially planar, with the maximal twist of 8.5 (3)° for the benzyl group. The herringbone packing arrangement does not exhibit any π-stacking inter-actions.

Crystallographic and spectroscopic characterization of two 1-phenyl-1H-imidazoles: 4-(1H-imidazol-1-yl)benzaldehyde and 1-(4-meth-oxy-phen-yl)-1H-imidazole.

The title compounds, C10H8N2O, (I), and C10H10N2O, (II), are two 1-phenyl-1H-imidazole derivatives, which differ in the substituent para to the imidazole group on the arene ring, i.e. a benzaldehyde, (I), and an anisole, (II). Both mol-ecules pack with different motifs via similar weak C-H⋯N/O inter-actions and differ with respect to the angles between the mean planes of the imidazole and arene rings [24.58 (7)° in (I) and 43.67 (4)° in (II)].

Spectroscopic, crystallographic, and Hirshfeld surface characterization of nine-membered-ring-containing 9-meth-oxy-3,4,5,6-tetra-hydro-1H-benzo[b]azonine-2,7-dione and its parent tetra-hydro-car-ba-zole.

9-Meth-oxy-3,4,5,6-tetra-hydro-1H-benzo[b]azonine-2,7-dione, C13H15NO3, (I), and 6-meth-oxy-1,2,3,4-tetra-hydro-car-ba-zole, C13H15NO, (II), represent the structures of a benzoazonine that contains a nine-membered ring and its parent tetra-hydro-car-ba-zole. The mol-ecules of (I) pack together via strong amide N-H⋯O hydrogen bonding and weak C-H⋯O inter-actions, whereas the parent tetra-hydro-car-ba-zole (II) packs with C/N-H⋯π inter-actions, as visualized by Hirshfeld surface characterization.

Synthesis, Structure, and Photophysical Properties of Platinum Compounds with Thiophene-Derived Cyclohexyl Diimine Ligands.

Platinum(II) and platinum(IV) compounds were prepared by the stereoselective and regioselective reactions of thiophene-derived cyclohexyl diimine C^N^N-ligands with [Pt2Me4(µ-SMe2)2]. Newly synthesized ligands were characterized by NMR spectroscopy and elemental analysis, and Pt(II)/Pt(IV) compounds were characterized by NMR spectroscopy, elemental analysis, high-resolution mass spectrometry, and single-crystal X-ray diffraction. UV-vis absorbance and photoluminescence measurements were performed on newly synthesized complexes, as well as structurally related Pt(II)/Pt(IV) compounds with benzene-derived cyclohexyl diimine ligands, in dichloromethane solution, as solids, and as 5% by weight PMMA-doped films. DFT and TD-DFT calculations were performed, and the results were compared with the observed spectroscopic properties of the newly synthesized complexes. X-ray total scattering measurements and real space pair distribution function analysis were performed on the synthesized complexes to examine the local- and intermediate-range atomic structures of the emissive solid states.

Hetero-multinuclear ruthenium(III)/platinum(II) complexes that potentially exhibit both antimetastatic and antineoplastic properties.

Hetero-multinuclear, platinum/ruthenium species were synthesized and tested for their effect on the motility of A549 (nonsmall cell lung) and MDA-MB-231 (breast) cancer cells and for their ability to inhibit DNA mobility using gel electrophoresis. It was found that the Ru(2)Pt trinuclear species [Na(2)]{[Ru(III)Cl(4)(DMSO-S)(-µ-pyz)](2)Pt(II)Cl(2)}, AH197, was much more efficient at inhibiting cell motility than [C(3)N(2)H(5)][Ru(III)Cl(4)(DMSO-S)(C(3)N(2)H(4))], NAMI-A, while the dinuclear RuPt species [K][Ru(III)Cl(4)(DMSO-S)(-µ-pyz)Pt(II)(DMSO-S)Cl(2)], IT127, was slightly better than NAMI-A. However, the dinuclear species retarded the electrophoretic mobility of DNA greater than both the trinuclear complex and cisplatin. The metal complexes and their respective BSA protein/metal adducts were studied by X-ray absorption spectroscopy. The spectra led to the conclusion that BSA donor atoms have substituted for the chloride ligands and perhaps the DMSO ligands.

5-Bromo-2-hy-droxy-benzonitrile.

The title compound, C(7)H(4)BrNO, crystallizes with two mol-ecules in the asymmetric unit. The two molecules exhibit nearly linear C-C N nitrile bond angles of 179.1 (4) and 177.1 (4)°. In the crystal, the mol-ecules are linked into a one-dimensional hydrogen-bonded chain by inter-actions between the phenol H atom and the nitrile N atom [N⋯O = 2.805 (4) and 2.810 (4) Å].

[(4R,5R)-(2,2-Dimethyl-1,3-dioxolane-4,5-di-yl)bis-(diphenyl-methano-lato)-κO:O']bis(N-methyl-methanamin-ato)titanium(IV).

In the title four-coordinate complex, [Ti(C(2)H(6)N)(2)(C(31)H(28)O(4))], two symmetry-independent mol-ecules are present in the asymmetric unit. The Ti(IV) atom displays a distorted tetra-hedral geometry, with Ti-O bond lengths ranging from 1.805 (3) to 1.830 (3) Šand O-Ti-O ligand bite angles of 100.16 (12) and 101.36 (12)°. The short Ti-N bond distances, ranging from 1.877 (4) to 1.905 (4) Å, indicate strong bonding between the Ti(IV) atom and the dimethyl-amide ligands.

3-Bromo-2-hy-droxy-benzaldehyde.

The mol-ecule of the title compound, C(7)H(5)BrO(2), is almost planar (r.m.s. deviation from the plane of all the non-H atoms = 0.0271 Å) and displays intra-molecular O-H⋯O hydrogen bonding between the phenol group and the aldehyde O atom. Packing is directed by weak inter-molecular C-H⋯Br inter-actions and π-stacking between nearly parallel mol-ecules [dihedral angle = 5.30 (6)° and centroid-centroid distance = 3.752 (1) Å].

One pot tandem dehydrogenative cross-coupling of primary and secondary alcohols by ruthenium amido-functionalized 1,2,4-triazole derived N-heterocyclic carbene complexes.

One-pot tandem dehydrogenative cross-coupling of primary and secondary alcohols was catalyzed by three ruthenium complexes [1-(R)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazol-5-ylidene]Ru(p-cymene)Cl [R = Et (1b), i-Pr (2b), Bn (3b)], of amido-functionalized 1,2,4-triazole derived N-heterocyclic carbene (NHC) ligands. Density Functional Theory (DFT) calculations were employed for the ruthenium (1b) precatalyst to understand this reaction mechanism completely, and the mechanisms adapted are divided categorically into three steps (i) nucleophilic substitution of chloride ions by alcohols, (ii) dehydrogenation of primary and secondary alcohols, and (iii) olefin and ketone hydrogenation. Our mechanistic study reveals that the formation of a deprotonated Ru-alcoholate (A) or (E) intermediate is favorable compared to the protonated form (A') or (E') from (1b) by associative nucleophilic substitution. Though an ionic pathway that proceeds through (A') or (E'), has less barriers in the dehydrogenation and olefin/ketone hydrogenation steps than that of the neutral pathway, proceeding through (A) or (E), a steep energy barrier was observed in the first nucleophilic substitution step, prohibiting the reaction to proceed via the intermediate (A') or (E'). Thus, our thorough mechanistic study reveals that the reaction proceeds via deprotonated Ru-alcoholate (A) or (E) species. Furthermore, the 1,4 addition of an α,ß-unsaturated carbonyl compound is kinetically and thermodynamically favorable over the 1,2 addition, and the experiments support these observations. As a testimony towards practical application in synthesizing bio-active flavonoid based natural products, five different flavan derivatives (16-20), were synthesized by the dehydrogenative coupling reaction using the neutral ruthenium (1-3)b complexes.

Aza-crown macrocycles as chiral solvating agents for mandelic acid derivatives.

A series of new chiral macrocycles containing the trans-1,2-diaminocyclohexane (DACH) subunit and arene- and oligoethylene glycol-derived spacers has been prepared in enantiomerically pure form. Four of the macrocycles have been characterized by X-ray crystallography, which reveals a consistent mode of intramolecular N-H···N hydrogen bonding and conformational variations about the N-benzylic bonds. Most of the macrocycles were found to differentiate the enantiomers of mandelic acid (MA) by (1)H NMR spectroscopy in CDCl(3); within the series of macrocycles tested, enantiodiscrimination was promoted by (i) a meta-linkage geometry about the arene spacer, (ii) the presence of naphthalene- rather than phenylene-derived arene spacers, and (iii) increasing length of the oligoethylene glycol bridge. (1)H NMR titrations were performed with optically pure MA samples, and the data were fitted to a simultaneous 1:1 and 2:1 binding model, yielding estimates of 2:1 binding constants between some of the macrocycles and MA enantiomers. In several cases, NOESY spectra of the MA:macrocycle complexes show differential intramolecular correlations between protons adjacent to the amine and carboxylic acid groups of the macrocycles and MA enantiomers, respectively, thus demonstrating geometric differences between the diastereomeric intermolecular complexes. The three most effective macrocycles were employed as chiral solvating agents (CSAs) to determine the enantiomeric excess (ee) of 18 MA samples over a wide ee range and with very high accuracy (1% absolute error).

1-Chloro-2-methyl-3-nitro-benzene.

In the title compound, C(7)H(6)ClNO(2), the chloro, methyl and nitro substituents are situated next to each other in this order on the benzene ring, with the mean plane of the nitro group twisted away from the mean plane of the benzene ring by 38.81 (5)°.