62 research outputs found
Highly ordered N-heterocyclic carbene monolayers on Cu(111)
EA and FG acknowledge funding from EPSRC grants (EA: EP/R512199/1; FG: EP/S027270/1). We thank the EaStCHEM for computational support via the EaStCHEM Research Computing facility. CMC thanks the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation (CFI) for funding of the work from her lab described in this article. AJV acknowledges NSERC for a Vanier scholarship and the Walter C. Sumner foundation for additional financial support. IS acknowledges Queenâs University for the RT Mohan Scholarship and the Ontario government for an Ontario Graduate Scholarship.The benzannulated N-heterocyclic carbene, 1,3-dibenzylbenzimidazolylidene (NHCDBZ) forms large, highly ordered domains when adsorbed on Cu(111) under ultrahigh vacuum conditions. A combination of scanning tunnelling microscopy (STM), high resolution electron energy loss spectroscopy (HREELS) and density functional theory (DFT) calculations reveals that the overlayer consists of vertical benzannulated NHC moieties coordinating to Cu adatoms. Long range order results from the placement of the two benzyl substituents on opposite sides of the benzimidazole moiety, with their aromatic rings approximately parallel to the surface. The organization of three surface-bound benzyl substituents from three different NHCs into a triangular array controls the formation of a highly ordered Kagome-like surface lattice. By comparison with earlier studies of NHCs on Cu(111), we show that the binding geometry and self-assembly of NHCDBZ are influenced by intermolecular and adsorbate-substrate interactions and facilitated by the flexibility of the methylene linkage between the N-heterocycle and the aromatic wingtip substituents.Publisher PDFPeer reviewe
Chiral carbeneâborane adducts: precursors for borenium catalysts for asymmetric FLP hydrogenations
The carbene derived from (1R,3S)-camphoric acid was used to prepare the borane adduct with Piersâ borane 7. Subsequent hydride abstraction gave the borenium cation 8. Adducts with 9-BBN and the corresponding (1R,3S)-camphoric acid-derived carbene bearing increasingly sterically demanding N-substituents (R = Me 9, Et 10, i-Pr 11) and the corresponding borenium cations 12â14 were also prepared. These cations were not active as catalysts in hydrogenation, although 9â11 were shown to undergo carbene ring expansion reactions at 50 °C to give species 15â17. The IBOX-carbene precursors 18 and 19 derived from amino alcohols (S)-valinol and (S)-tert-leucinol (R = i-Pr, t-Bu) were used to prepare borane adducts 20â23. Reaction of the carbenes 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-di-iso-propylimidazol-2-ylidene (IPr) 1-benzyl-3-methylimidazol-2-ylidene (IBnMe), 1-methyl-3-phenylimidazol-2-ylidene (IPhMe) and 1-tert-butyl-3-methylimidazol-2-ylidene (ItBuMe) with diisopinocampheylborane (Ipc2BH) gave chiral adducts: (IMe)(Ipc2BH) 24, (IPr)(Ipc2BH) 25, (IBnMe)(Ipc2BH) 26, (IPhMe)(Ipc2BH) 27, and (ItBuMe)(Ipc2BH) 28. Triazolylidene-type adducts including the (10)-phenyl-9-borabicyclo [3.3.2]decane adduct of 1,3,4-triphenyl-1H-1,2,3-triazolium, rac-29 and the 9-BBN derivative of (S)-2-amino-2â˛-methoxy-1,1â˛-binaphthalene-1,2,3-triazolium 34a/b were also prepared. In catalytic studies of these systems, while several species were competent catalysts for imine reduction, in general, low enantioselectivities, ranging from 1â20% ee, were obtained. The implications for chiral borenium cation catalyst design are considered
N-heterocyclic carbene self-assembled monolayers on copper and gold : dramatic effect of wingtip groups on binding, orientation and assembly
Funding: EPSRC PhD studentship (EP/M506631/1).Selfâassembled monolayers of Nâheterocyclic carbenes (NHCs) on copper are reported. The monolayer structure is highly dependent on the N,Nâsubstituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surface while methylâ or ethylâsubstituted NHCs lie flat. Temperatureâprogrammed desorption studies show that the NHC binds to Cu(111) with a desorption energy of Edes=152Âą10â
kJâmolâ1. NHCs that bind upright desorb cleanly, while flatâlying NHCs decompose leaving adsorbed organic residues. Scanning tunneling microscopy of methylated NHCs reveals arrays of covalently linked dimers which transform into adsorbed (NHC)2Cu species by extraction of a copper atom from the surface after annealing.Publisher PDFPeer reviewe
Charting a course for chemistry
To mark the occasion of Nature Chemistry turning 10 years old, we asked scientists working in different areas of chemistry to tell us what they thought the most exciting, interesting or challenging aspects related to the development of their main field of research will be â here is what they said
Chiral Hybrid Mesoporous Silicas: Assembly of Uniform Hollow Nanospheres and Helical Nanotubes with Tunable Diameters
Uniform helical silica nanotubes and hollow silica nanostructures
with adjustable diameters have been prepared through the self-assembly
of sodium dodecyl sulfate (SDS) as the surfactant, <i>N</i>-trimethoxysilylpropyl-<i>N,N,N</i>-trimethylammoniumchloride
(TMAPS) as a costructure directing agent (CSDA), a binapthyl-based
chiral dopant, and TEOS (SiÂ(OEt)<sub>4</sub>) as the bulk silica constituent.
Depending on the ratio of anionic surfactant to cationic costructure
directing agent, the morphology can be tuned from hollow spheres to
hollow nanotubes. At a 1:1 ratio of TMAPS/SDS, in the presence of
the axially chiral dopant molecule, uniformly helical structures are
obtained. The chirality of the dopant is shown to affect the sense
of helicity. Under identical conditions, a monosilylated chiral dopant
only leads to the formation of well dispersed uniform hollow spheres
rather than helical nanotubes, which further demonstrates the importance
of incorporating the chiral dopant as an integral component of the
siloxane network, rather than merely as a surface group
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