Formation Of Dimeric Structure Research Articles

Mechanism of the Aryl-F Bond-Forming Step from Bi(V) Fluorides.

In this article, we describe a combined experimental and theoretical mechanistic investigation of the C(sp2)–F bond formation from neutral and cationic high-valent organobismuth(V) fluorides, featuring a dianionic bis-aryl sulfoximine ligand. An exhaustive assessment of the substitution pattern in the ligand, the sulfoximine, and the reactive aryl on neutral triarylbismuth(V) difluorides revealed that formation of dimeric structures in solution promotes facile Ar–F bond formation. Noteworthy, theoretical modeling of reductive elimination from neutral bismuth(V) difluorides agrees with the experimentally determined kinetic and thermodynamic parameters. Moreover, the addition of external fluoride sources leads to inactive octahedral anionic Bi(V) trifluoride salts, which decelerate reductive elimination. On the other hand, a parallel analysis for cationic bismuthonium fluorides revealed the crucial role of tetrafluoroborate anion as fluoride source. Both experimental and theoretical analyses conclude that C–F bond formation occurs through a low-energy five-membered transition-state pathway, where the F anion is delivered to a C(sp2) center, from a BF4 anion, reminiscent of the Balz–Schiemann reaction. The knowledge gathered throughout the investigation permitted a rational assessment of the key parameters of several ligands, identifying the simple sulfone-based ligand family as an improved system for the stoichiometric and catalytic fluorination of arylboronic acid derivatives.

Hydrogen bonding in acetylene containing dichlorodiazaalkadienes

The detailed study of 1-aryl-4,4-dichloro-3-(2-propargyloxyphenyl)-1,2-diazabuta-1,3-dienes revealed the formation of dimeric structures via hydrogen bonding between the terminal acetylene hydrogen atom and the diazo fragment of the second molecule in the solid state, in solution the compounds being monomeric. This effect is confirmed by IR spectra, X-ray data as well as quantum chemical calculations.

Reactivity of lignin subunits: the influence of dehydrogenation and formation of dimeric structures.

Lignin is one of the most abundant natural materials around the world, accounting for about a quarter of the woody tissue. In general, it is well known that these highly branched aromatic bio-polymers are formed from the polymerization of p-coumaryl, coniferyl, and sinapyl alcohols; however, the connection between these structures are still not known in detail. In this work, we have employed electronic structure calculations to investigate local reactivities and details regarding the connectivity between the basic structures of lignin (unmodified mono and dilignols as well as dehydrogenated monolignols). Condensed-to-atoms Fukui indexes, local softness and hard and soft acids and bases principle were employed in the analyses. The results allow identifying reactive sites on the lignin subunits and access details on the synthesis and degradation of this bio-material. In particular, it is possible to identify a strong influence of the dehydrogenation and monomer dimerization on the monolignols reactivities, which activate the O-C4 and C5 positions. Graphical Abstract The local reactivities of lignin subunits were evaluated via DFT calculations.

Electrochemical Properties of Anthracene-Based Lithium-Solvated Electron Solutions.

The conductivity and open-circuit voltage (OCV) of lithium-solvated electron solutions (LiSESs) based on anthracene in tetrahydrofuran were studied by both experimental measurements and density functional theory calculations with a range-separated functional based on the M06 form and the Solvation Model based on Density (SMD). The OCV was found to decrease with increasing temperature and the ratio of lithium to anthracene. The enthalpy change (ΔH) of LiSESs was the internal energy change of the cell reaction. The conductivity of LiSESs exhibited a weakly metallic-like behavior. The electron transport was facilitated by molecular collisions promoted by the formation of dimeric structures as intermediates. The conductivity of LiSESs at 295.15 K presents positive correlation with the entropy change (ΔS) associated with the variation in the ratio of lithium to poly-aromatic hydrocarbon, including p-terphenyl, anthracene, and triphenylene.

Solvent-Induced and Temperature-Promoted Aggregation of Bipyridine Platinum(II) Triangular Metallacycles and Their Near-Infrared Emissive Behaviors.

A series of bipyridine platinum(II) complexes with different sizes of triangular metallacycles and alkyl/oligoether chains has been synthesized and characterized. They are packed in a zig-zag fashion with the formation of dimeric structures according to their X-ray crystal structures. Different emission origins are observed due to the different sizes of the triangular ligands. Their morphologies could be tuned by the modification of the molecular structures with different metallacyclic alkynyl ligands and alkyl/oligoether chains and solvents. More interestingly, unusual electronic absorption changes and upfield shifts of the aromatic proton resonances are observed upon increasing the temperature, suggesting further aggregation of the architectures. Near-infrared (NIR) emission is also realized through the tuning of the π-π stacking, Pt⋅⋅⋅Pt interactions, and the packing of planar metallacycles.

Intermolecular hydrogen bonding, structural and vibrational assignments of 2, 3, 4, 5-tetrafluorobenzoic acid using density functional theory

The conformational study of the 2,3,4,5 tetrafluorobenzoic acid molecule (TFBA) was performed by density functional theory (DFT) using B3LYP functional with 6–311++G(d,p) basis set. Vibrational frequencies were calculated at the same level of theory for the optimized stable structure. The existence of hydrogen bonding was predicted through experimental FTIR spectrum and interpreted with the formation of dimer structure. The natural bond orbital (NBO) analysis and molecular electrostatic potential (MEP) were made on DFT/B3LYP/6-311++G(d,p) level for an extensive analysis on hydrogen bonding. The calculated vibrational frequencies of TFBA monomer were compared with experimental frequencies and were found to be in good agreement. Intermolecular hydrogen bonding energy has also been calculated by topological study.

Hydrogen bond and steric effect directed preparation and structures of dioxovanadium(V) complexes with tridentate Schiff bases

Three new dioxovanadium(V) complexes, [VO2L1]2 (1), [VO2L2] (2) and [VO2L3] (3) (HL1 = 2-[(2-ethylaminoethylimino)methyl]-4-methylphenol, HL2 = 2-[(2-isopropylaminoethylimino)methyl]-6-methylphenol, HL3 = 5-methoxy-2-[(pyridin-2-ylmethylimino)methyl]phenol), have been prepared and structurally characterized by physico-chemical methods and single crystal X-ray diffraction. Complex 1 is a centrosymmetric dimeric dioxovanadium(V) complex with VV distance of 3.187(2) Å. V in 1 is octahedral. Complexes 2 and 3 are mononuclear dioxovanadium(V) complexes. V in 2 is in a distorted square-pyramidal coordination and that in 3 is intermediate between square-pyramidal and trigonal bipyramidal. During the synthesis and crystallization of the complexes from oxovanadium(IV) starting material, aerial oxygen probably acts as the oxidizing agent. The final structures of the complexes are directed by hydrogen bonds and steric effects among the Schiff base ligands. The existence of amino groups and small steric effects such as ethyl group can result in the formation of dimeric structures.

An NMR study of comenic acid and its salts with Li+ and Na+ in aqueous solution and solid state

The forms and structure of comenic acid and its salts with Na+ and Li+ in solution and solid state are studied by multinuclear NMR spectroscopy. The formation of dimeric structures of comenic acid in the solid state is shown. It is found that disubstituted sodium and lithium salts have different structures.

Charged States of α,ω-Dicyano β,β′-Dibutylquaterthiophene as Studied by in Situ ESR UV−Vis NIR Spectroelectrochemistry

The influence of the molecular structure on the stabilization of charged states was studied in detail by in situ ESR UV-vis NIR spectroelectrochemistry at a novel α,ω-dicyano substituted β,β'-dibutylquaterthiophene (DCNDBQT) and the electrochemically generated cation and anion radicals have been proved for the first time. The voltammetry of DCNDBQT results in two separate oxidation steps with the reversible first one. The experimental absorption maxima at 646 and 1052 nm together with the calculated ones (by DFT method) as well as an ESR signal at the first anodic step prove the presence of a radical cation. Three additional optical bands (554, 906, and 1294 nm for CT-transition) can be attributed to the formation of cation radical dimer. The dicationic structure formed in the second oxidation step is not stable. The stabilization proceeds via a dimer formation in two chemical follow-up reactions. The existence of the dimeric structures was proved by ex situ MALDI TOF mass spectrometry. As the substitution by cyano groups opens the route to cathodic reductions, DCNDBQT shows a single quasi-reversible reduction step. Here, the in situ ESR UV-vis NIR spectroelectrochemical measurements and theoretical calculations let us confirm the electrochemical generation of an anion radical. As we found a low number of anion radicals by quantitative ESR spectroelectrochemistry and an appearance of additional bands in the UV-vis NIR absorption spectra, the formation of dimeric structures must be considered and was corroborated by mass spectrometry. The role of dimerization in the reaction mechanism of the DCNDBQT oxidation and reduction are discussed in general. The experimental results were interpreted using the quantum chemical calculations based on density functional theory.

Proteomic analysis of N‐glycosylation in mosquito dopachrome conversion enzyme

A novel dopachrome conversion enzyme (DCE) is present in insects and involved in their melanization pathway. DCE shares no sequence homology with any noninsect species from bacteria to humans. Several DCE sequences have been available, but enzyme structure and catalytic mechanism are unclear. This study concerns DCE PTMs, especially glycosylation. A mosquito DCE was purified and its monosaccharide composition, N-glycosylation site, and oligosaccharide structures were determined. Results showed that N-acetyl D-glucosamine and D-mannose are the major monosaccharides and L-fucose, D-xylose, and D-arabinose are the minor ones in mosquito DCE. Glycosylation site and oligosaccharide structures were elucidated from MS and MS/MS spectra of trypsin-digested DCE glycopeptides. A single N-glycosylation site (Asn285 -Glu-Thr) was identified in DCE and was proven to be fully glycosylated. Man3GlcNAc2, Man3(Fuc)1-2GlcNAc2, and their truncated structures were the dominant oligosaccharides. In addition, high mannose-type structures (Man4-7(Fuc)GlcNAc2) were also identified. Removal of DCE N-oligosaccharides with peptide N-glycosidase (PNGase F) decreased its activity and thermal stability. However, partial DCE deglycosylation with alpha-mannosidase or alpha-fucosidase somewhat stimulated its activity and improved its thermal stability. During mass spectrometric analysis of DCE glycopeptides, their CID patterns were highly intriguing, in that some glycopeptides underwent both C-terminal rearrangement and formation of dimeric structures during CID. Results of this study provide an interesting example in terms of potential complexity of the glycopeptide CID fragmentation pattern.

Polyol Metal Complexes. Part 55 Germanes with Alkylenedioxy Substituents

AbstractWithin this work five novel germanes with alkylenedioxy substituents derived from 1,2‐diols have been synthesized and structurally characterized. Various coordination patterns from tetra‐ to hexacoordination were determined. Both open‐chain and cyclic 1,2‐diols such as 1,2‐ethandiol (Ethd), 1,1′‐bicyclohexyl‐1,1′‐diol (Bhxd), cis‐oxolane‐3,4‐diol (AnEryt) and cis‐1,2‐cyclopentanediol (cis‐1,2‐Cptd) were used. Tetracoordination of the germanium atom in the crystalline state appears to be restricted to sterically demanding substituents.In general, the tendency of germanium to extend its coordination sphere to build penta‐ or hexacoordinate compounds is demonstrated. It is also shown that germanium is able to compensate a lack of substituents by the formation of dimeric structures.

Heterogenized palladium chitosan complexes as potential catalysts in oxidation reactions: study of the structure

Abstract Complexes of palladium with chitosan were prepared by different methods such as coprecipitation and adsorption and studied by EXAFS, FTIR, and X-ray photoelectron spectroscopy (XPS). It was found in the case of the coprecipitation-type complex that a chelate structure is a likely state of palladium. In the case of the adsorption-type system, palladium is coordinated by two chitosan NH2 groups without chelatization. It is evident from the EXAFS and XPS data that the structure of the complexes changes in the course of palladium reduction. In the case of the complex prepared by coprecipitation, Pd(0) atoms presumably form trimers in a polymer matrix; the formation of dimeric structures seems to be most typical of the adsorption complex. XPS data show the possibility of reversible redox transformations of palladium immobilized on a chitosan matrix. The complexes prepared can be considered as potential oxidation catalysts.