Naturally Occurring Allotropes of Carbon.

Carbon is one of the most important chemical elements, forming a wide range of important allotropes, ranging from diamond over graphite to nanostructural materials such as graphene, fullerenes, and carbon nanotubes (CNTs). Especially these nanomaterials play an important role in technology and are commonly formed in laborious synthetic processes that often are of high energy demand. Recently, fullerenes and their building blocks (buckybowls) have been found in natural fossil materials formed under geological conditions. The question arises of how diverse nature can be in forming different types of natural allotropes of carbon. This is investigated here, using modern analytical methods such as ultrahigh-resolution mass spectrometry and transmission electron microscopy, which facilitate a detailed understanding of the diversity of natural carbon allotropes. Large fullerenes, fullertubes, graphene sheets, and double- and multiwalled CNTs together with single-walled CNTs were detected in natural heavy fossil materials while theoretical calculations on the B3LYP/6-31G(d) level of theory using the ORCA software package support the findings.

The Structure of Maneb, An Important Manganese-Containing Bis(dithiocarbamate) Fungicide.

Maneb is a manganese(II)-containing fungicide with a multi-site effect and no resistance, therefore it is widely applied in many parts of the world. There is, however, mounting evidence for neurotoxic effects with Parkinson-like symptoms (manganism) related to usage of Maneb. Due to its insolubility in most solvents and its paramagnetism, structural elucidation is not trivial, and thus its exact molecular structure remains unknown. We report herein a synthesis procedure to prepare Maneb reproducibly in pure form and the use of various analytical techniques including X-ray diffraction, X-ray absorption spectroscopy and electron diffraction to determine the molecular structure of Maneb in the solid state and also in solution.

Circularly Polarized Light Probes Excited-State Delocalization in Rectangular Ladder-type Pentaphenyl Helices.

Ladder-type pentaphenyl chromophores have a rigid, planar π-system and show bright fluorescence featuring pronounced vibrational structure. Such moieties are ideal for studying interchromophoric interactions and delocalization of electronic excitations. We report the synthesis of helical polymers with a rigid square structure based on spiro-linked ladder-type pentaphenyl units. The variation of circular dichroism with increasing chain length provides direct evidence for delocalization of electronic excitations over at least 10 monomeric units. The change in the degree of circular polarization of the fluorescence across the vibronic side bands shows that vibrational motion can localize the excitation dynamically to almost one single unit through breakdown of the Born-Oppenheimer approximation. The dynamic conversion between delocalized and localized excited states provides a new paradigm for interpreting circular dichroism in helical polymers such as proteins and polynucleic acids.

Green Textile Materials for Surface Enhanced Raman Spectroscopy Identification of Pesticides Using a Raman Handheld Spectrometer for In-Field Detection.

Surface enhanced Raman spectroscopy (SERS) has evolved into a powerful analytical method in food and environmental analytical sciences due to its high sensitivity. Pesticide analysis is a major discipline therein. Using sustainable materials has become increasingly important to adhere to Green Chemistry principles. Hence, the green textiles poly-(L-lactic acid) (PLA) and the mixed fabric polyethylene terephthalate polyamide (PET/PA) were investigated for their applicability as solid supports for gold nanoparticles to yield SERS substrates. Gold nanoparticle solutions and green textile supports were prepared after preparation optimization. Particle size, dispersity, and particle distribution over the textiles were characterized by absorption spectroscopy and transmission electron imaging. The performance of the SERS substrates was tested using the three pesticides imidacloprid, paraquat, and thiram and a handheld Raman spectrometer with a laser wavelength of 785 nm. The resulting SERS spectra possessed an intra-substrate variation of 7-8% in terms of the residual standard deviation. The inter-substrate variations amounted to 15% for PET/PA and to 27% for PLA. Substrate background signals were smaller with PLA but more enhanced through PET/PA. The pesticides could be detected at 1 pg on PET/PA and at 3 ng on PLA. Hence, PET/PA woven textile soaked with gold nanoparticle solution provides green SERS substrates and might prove, in combination with fieldable Raman spectrometers, suitable for in-field analytics for pesticide identification.

Synthesis, Reactivity and Antimicrobial Activity of a Series of 2-Arylamino-1,3-selenazoles.

A series of 2-arylamino-1,3-selenazoles was synthesized and their reactivity was studied. The 2-arylamino-1,3-selenazoles and their reaction products were characterized by various spectroscopic methods and X-ray diffraction. In addition, the antimicrobial activity of the 2-arylamino-1,3-selenazoles in a panel of seven bacteria and fungi was examined.

Facile N9-Alkylation of Xanthine Derivatives and Their Use as Precursors for N-Heterocyclic Carbene Complexes.

The xanthine-derivatives 1,3,7-trimethylxanthine, 1,3-dimethyl-7-benzylxanthine and 1,3-dimethyl-7-(4-chlorobenzyl)xanthine are readily ethylated at N9 using the cheap alkylating agents ethyl tosylate or diethyl sulfate. The resulting xanthinium tosylate or ethyl sulfate salts can be converted into the corresponding PF6- and chloride salts. The reaction of these xanthinium salts with silver(I) oxide results in the formation of different silver(I) carbene-complexes. In the presence of ammonia, ammine complexes [Ag(NHC)(NH3)]PF6 are formed, whilst with Et2NH, the bis(carbene) salts [Ag(NHC)2]PF6 were isolated. Using the xanthinium chloride salts neutral silver(I) carbenes [Ag(NHC)Cl] were prepared. These silver complexes were used in a variety of transmetallation reactions to give the corresponding gold(I), ruthenium(II) as well as rhodium(I) and rhodium(III) complexes. The compounds were characterized by various spectroscopic methods as well as X-ray diffraction.

Silver(I) Complexes with Camphorsulfonato and Phosphine Ligands: Structural Diversity and Antibacterial Activity.

A series of silver(I) camphorsulfonato complexes containing various phosphine ligands having the stoichiometry [Ag(camphSO3)(PR3)] [PR3 = PTA (1,3,5-triaza-7-phosphaadamantane), PASO2 (2-thia-1,3,5-triaza-7-phosphaadamantane-2,2-dioxide), PPh3, PCy3, P(CH2CH2CN)3, PPyPh2, or P(o-tol)3] were prepared and fully characterized by NMR spectroscopic methods and X-ray crystallography. Depending on the nature of the phosphine, a variety of different supramolecular structures, including dimers, macrocycles, and coordination polymers, were observed in the solid state. The in vitro antimicrobial activity in seven different pathogens (Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Acinetobacter baumannii, Candida albicans, and Cryptococcus neoformans var. grubii) as well as toxicity in human cells was also examined. While all compounds show some activity against the bacteria, they were especially active against the fungus C. neoformans. The most active and at the same time least toxic compound was found to be the water-soluble complex [Ag(camphSO3)(PTA)2].

Catalytic Asymmetric Fluorination of Copper Carbene Complexes: Preparative Advances and a Mechanistic Rationale.

The Cu-catalyzed reaction of substituted α-diazoesters with fluoride gives α-fluoroesters with ee values of up to 95 %, provided that chiral indane-derived bis(oxazoline) ligands are used that carry bulky benzyl substituents at the bridge and moderately bulky isopropyl groups on their core. The apparently homogeneous solution of CsF in C6 F6 /hexafluoroisopropanol (HFIP) is the best reaction medium, but CsF in the biphasic mixture CH2 Cl2 /HFIP also provides good results. DFT studies suggest that fluoride initially attacks the Cu- rather than the C-atom of the transient donor/acceptor carbene intermediate. This unusual step is followed by 1,2-fluoride shift; for this migratory insertion to occur, the carbene must rotate about the Cu-C bond to ensure orbital overlap. The directionality of this rotatory movement within the C2 -symmetric binding site determines the sense of induction. This model is in excellent accord with the absolute configuration of the resulting product as determined by X-ray diffraction using single crystals of this a priori wax-like material grown by capillary crystallization.

Complete charge separation provoked by full cation encapsulation in the radical mono- and di-anions of 5,6:11,12-di-o-phenylene-tetracene.

Herein, we report the synthesis and molecular structure of the mono- and dianionic aromatic molecules [(B15C5-κ5O)2K+](LDOPT˙-) (1) and [(B15C5-κ5O)2K+]2(LDOPT2-)THFsolv (2) derived from the parent aromatic polyhydrocarbon 5,6:11,12-di-o-phenylenetetracene (DOPT, LDOPT) by a controlled stepwise one and two electron chemical reduction. The effect of single and double electron charge transfer to a polycondensed aromatic hydrocarbon (PAH) without any disturbing influence of an associated metal cation has been demonstrated. This was achieved by fully sandwiching the cationic K+ counterions between two benzo-15-crown-5-ether (B15C5) ligands resulting in a fully encapsulating (κ10O) geometry which ensures a complete separation of the K+ counterions and the bare anionic PAH species [LDOPT˙-] and [LDOPT2-]. The structural changes accompanied by the stepwise reduction from LDOPT to [LDOPT˙-] to [LDOPT2-] are discussed and compared to earlier predictions based on density functional theory (DFT) as well as the results of previous studies of alkaline metal cationic PAH anion interactions of DOPT in which only a partial metal cation encapsulation has been achieved so far.

Dicationic phosphonium salts: Lewis acid initiators for the Mukaiyama-aldol reaction.

Two strategies were used to prepare dicationic phosphonium cations. The first method consists of the reaction of 1-chlorocyclopropenium salts with phosphines to obtain cyclopropenium-substituted phosphonium salts 10a-f[BF4]. Anion exchange was performed to access the corresponding [B(C6F5)4]- analogues 10a-f[B(C6F5)4], which showed much higher solubility in organic solvents. In addition, we developed a synthesis of dicationic phosphonium salts containing 2-, 3-, or 4-methylpyridinium substituents 11a-c[TfO], which were converted as well to their [B(C6F5)4]- analogues 11a-c[B(C6F5)4]. Finally, the phenoxy-substituted phosphonium salt 12[B(C6F5)4] was also prepared. All salts demonstrated remarkable stability in air as compared with their fluorinated analogues. The Lewis acidity of these salts was evaluated by means of theoretical calculations and finally, they were shown to be effective in initiating the Mukaiyama-aldol reaction.

Adiponitrile at 100†K.

Adiponitrile, C6H8N2, is a key intermediate in the synthesis of the polyamide Nylon 66 and is produced industrially on a large scale. We have determined the crystal and molecular structure of adiponitrile by single-crystal X-ray analysis at 100 K, a suitable crystal (m.p. 275 K) having been grown from the melt at low temperature. The compound crystallizes in the monoclinic space group P21/c with Z = 2. In the crystal structure, the molecule adopts an exact Ci-symmetric gauche-anti-gauche conformation of the C-C-C-C skeleton about an inversion centre. The molecules are densely packed, with short intermolecular contacts between the α-H and nitrile N atoms.

Enantioselective Synthesis of [6]Carbohelicenes.

The use of α-cationic phosphonites derived from TADDOL as ancillary ligands has allowed a highly regio- and enantioselective synthesis of substituted [6]carbohelicenes by sequential Au-catalyzed intramolecular hydroarylation of diynes. Key for these results is the modular structure of these new ligands, and the enhanced reactivity that they impart to Au(I)-centers after coordination.

Acetic anhydride at 100†K: the first crystal structure determination.

Acetic anhydride (ethanoic anhydride), (CH3CO)2O, is a widely used acetylation reagent in organic synthesis. The crystal and molecular structure, as determined by single-crystal X-ray analysis at 100 K, is reported for the first time. A crystal of the title compound (m.p. 200 K) suitable for X-ray diffraction was grown from the melt at low temperature. The title compound crystallizes in the orthorhombic space group Pbcn, with Z = 4. In the crystal, the molecule adopts an exact C2-symmetric conformation about a crystallographic twofold axis. The molecules are densely packed. Two of the methyl H atoms form short intermolecular contacts to a neighbouring carbonyl O atom, which can be viewed as weak hydrogen bonds.

One-step synthesis of a cyclic 2,17-dioxo[3,3](4,4') biphenylophane and first preparation of a microporous polymer network from a macrocyclic precursor by cyclotrimerization.

One-step synthesis of a cyclic 2,17-dioxo[3,3](4,4')biphenylophane (MC) was achieved in high yield; its structure was verified by single crystal X-ray analysis. As a first example, a microporous polymer network was formed from macrocycle MC via acid-catalysed cyclotrimerization yielding a BET surface area of ca. 570 m(2) g(-1).

Unusual bonding and properties in main group element chemistry: rational synthesis, characterization, and experimental electron density determination of mixed-valent tetraphosphetes.

Five dispirocyclic λ(3),λ(5)-tetraphosphetes [{R2Si(NR(1))(NR(2))P2}2] (R(1) = R(2) and R(1) ≠ R(2)) are easily prepared in almost quantitative yields via photolysis of the respective bis(trimethylsilyl)phosphanyldiazaphosphasiletidines with intense visible light. These deep-yellow low-coordinate phosphorus compounds can be considered as the first higher congeners of the well-known cyclodiphosphazenes. The tetraphosphetes are remarkably stable in air and show unexpected molecular properties related to the unique bonding situation of the central four-π-electron four-membered phosphorus ring. The extent of rhombic distortion of the central P4 ring is remarkable due to an unusually acute angle at the σ(2)-phosphorus atoms. All of the P-P bonds are approximately equal in length. The distances are in the middle of the range given by phosphorus single and double bonds. The anisotropic absorption of visible light that can easily be observed in the case of the yellow/colorless dichroic crystals of [{Me2Si(NtBu)(NtBu)P2}2] and the exceptional (31)P NMR chemical shift of the σ(2)-phosphorus atoms are the most remarkable features of the λ(3),λ(5)-tetraphosphetes. In the case of [{Me2Si(NtBu)(NtBu)P2}2], the Hansen-Coppens multipole model is applied to extract the electron density from high-resolution X-ray diffraction data obtained at 100 K. Static deformation density and topological analysis reveal a unique bonding situation in the central unsaturated P4 fragment characterized by polar σ-bonding, pronounced out-of-ring non-bonding lone pair density on the σ(2)-phosphorus atoms, and an additional non-classical three-center back-bonding contribution.

Crystal structure of tris-(phenyl-seleno-lato-κSe)tris-(tetra-hydro-furan-κO)thulium(III).

In the title compound, [Tm(C6H5Se)3(C4H8O)3], the Tm(III) atom lies on a threefold rotation axis and is coordinated by three phenyl-seleno-late ligands and three tetra-hydro-furan ligands leading to a distorted fac-octa-hedral coordination environment. The Tm-Se and Tm-O bond lengths are 2.7692 (17) and 2.345 (10) Å, respectively, and the bond angles are 91.32 (6)° for Se-Tm-Se, 92.6 (2) and 94.4 (2)° for Se-Tm-O, and 81.2 (3)° for O-Tm-O. In the crystal, the discrete complexes are linked by van der Waals inter-actions only. The crystal was refined as a non-merohedral twin (ratio = 0.65:0.35).

Toward molecular recognition: three-point halogen bonding in the solid state and in solution.

A well-defined three-point interaction based solely on halogen bonding is presented. X-ray structural analyses of tridentate halogen bond donors (halogen-based Lewis acids) with a carefully chosen triamine illustrate the ideal geometric fit of the Lewis acidic axes of the former with the Lewis basic centers of the latter. Titration experiments reveal that the corresponding binding constant is about 3 orders of magnitude higher than that with a comparable monodentate amine. Other, less perfectly fitting multidentate amines also bind markedly weaker. Multipoint interactions like the one presented herein are the basis of molecular recognition, and we expect this principle to further establish halogen bonding as a reliable tool for solution-phase applications.

Stereochemistry rules: a single stereocenter changes the conformation of a cyclic tetrapeptide.

Two novel cyclo(Boc-Cys-Pro-Leu-Cys-OMe) peptides 1 and 2 containing the enantiomeric amino acids d-Leu and l-Leu, respectively, were synthesized to investigate the effect of chiral centers on peptide conformations. By combining a variety of experimental techniques (X-ray crystallography, 2D NMR spectroscopy, temperature-dependent (1)H NMR and IR spectroscopy, and UV-CD spectroscopy) with replica exchange molecular dynamics (REMD) techniques and quantum mechanics/molecular dynamics (QM/MM) calculations, we establish that the stereochemistry of just one residue can noticeably influence the properties of the whole peptide and rationalize the origins of this effect, with potential implications for the rational design of peptides of chemical and biological relevance.

Synthesis, structure and reactivity of cyclopropenyl-1-ylidene stabilized S(II), Se(II) and Te(II) mono- and dications.

The syntheses and structural characterization of carbene-stabilized chalcogen centred mono- and dications employing a reverse electron demand onio-substitutent transfer strategy are reported. The electronic structures of these compounds were determined by density functional calculations and their reactivity towards Pd(0) centres was evaluated.

The halogen bond made visible: experimental charge density of a very short intermolecular Cl···Cl donor-acceptor contact.

[ZnCl(2)(3,4,5-trichloropyridine)(2)] features short intermolecular Cl···Cl contacts between halogen atoms of different nature, and a charge density study provides experimental evidence for the accepted model of the halogen bonds: an arene-bonded Cl atom acts as a donor of electron density towards the "sigma hole" of a chlorido ligand attached to a neighbouring Zn(II) cation.