High-fidelity Recognition of Organotrifluoroborate Anions (R−BF3−) as Designer Guest Molecules

The recognition of boron compounds is well developed as boronic acids but untapped as organotrifluoroborate anions (R−BF₃⁻). We are exploring the development of these and other designer anions as anion-recognition motifs by considering them as substituted versions of the parent inorganic ion. To this end, we demonstrate strong and reliable binding of organic trifluoroborates, R−BF₃⁻, by cyanostar macrocycles that are size-complementary to the inorganic BF₄⁻ progenitors. We find that recognition is modulated by the substituent's sterics and that the affinities are retained using the common K⁺ salts of R−BF₃⁻ anions.     

Chiral molecular glass: synthesis and characterization of enantiomerically pure thiophene-based [7]helicene

Synthesis of thiophene-based [7]helicenes, which are functionalized for both design of organic chiral glasses with strong chiroptical properties and for further homologation to higher [n]helicenes, is reported. The key synthetic transformations are kinetic resolution of the intermediate diketone and the annelation step forming the center benzene ring by means of an intramolecular McMurry reaction. Based upon X-ray crystallographic determinations of the absolute configurations for (+)-enantiomers of the diketone and the [7]helicene, stereochemical correlation between the (R) axial chirality of the diketone and the (M) helical chirality of the [7]helicene is established. One such enantiopure trimethylsilyl-substituted [7]helicene possesses enchanced chiroptical properties and forms a chiral molecular glass.     

Synthesis and capsule formation of upper rim substituted tetra-acrylamido calix[4]arenes

Upper rim substituted tetraiodo calix[4]arenes are coupled to a variety of acrylamides using the palladium catalysed Heck reaction. Tetra-acrylamido upper rim substituted calix[4]arenes are obtained in good yields with exceptionally high stereoselectivity, to produce the all-trans isomers. Tetra-acrylamido calix[4]arenes derived from secondary acrylamides are shown to dimerise via eight hydrogen bonds to form dimeric capsules, which are able to include small organic molecules.     

A facile approach to a d4 RuN: moiety

Replacement of chloride in (PNP)RuCl, PNP = (tBu2PCH2SiMe2)2N, by Me3SiN3 gives a pre-redox adduct that, already at -30 degrees C, releases N2 to produce the mononuclear nonplanar Ru(IV) nitride (PNP)RuN, characterized by spectroscopic and X-ray methods. DFT calculations show the planar structure to be only 1.6 kcal/mol less stable, which explains the time-averaged simplicity of the 1H NMR spectrum, as well as the large vibrational amplitude of the nitride ligand.     

Parabolic focal conics in self-assembled solid films of cellulose nanocrystals

Suspensions of cellulose nanocrystals form colloidal chiral nematic phases. The liquid crystalline order in these suspensions can be captured in solid films by slow evaporation of the liquid. Studies of the microstructure of such chiral nematic solid films revealed parabolic focal conic (PFC) defects, a symmetric form of focal conic defects in which the line defects form a pair of perpendicular, antiparallel, and confocal parabolas. The cellulose films with PFC defects were characterized by polarized-light and atomic force microscopy. The film surface showed a regular array of large and small elevations resulting from the displacement of the structural layers. Film fracture lines showed a series of layered half-cones. The microstructure of the films was modeled by computer. The model revealed that many structural layers terminate at the film surface.     

Multifunctional behavior by a Bis-(phosphinimino)methanide ligand: eta 2- vs eta 3-coordination vs Bronsted basicity

The reaction of [(Cymene)RuCl2]2 with the chelate LiHC(PPh2NPh)2 occurs to remove both chloride ligands, to furnish a cationic Ru(II) complex with the monoanionic ligand bound eta3, through two N and an sp3 carbon. This cation is also produced from the conjugate acid of the ligand H2C(PPh2NPh)2 because this molecule can serve as a Br?nsted base, to deprotonate the acidic carbon of another molecule of H2C(PPh2NPh)2. DFT calculations show an energy surface where (Cymene)RuHC(PPh2NPh)2L is more stable with a Ru-CH(PPh2NPh)2 bond and with L = Cl- or MeCN not coordinated to Ru, than to an eta2-HC(PPh2NPh)2 structure with coordinated L; this is tested experimentally. The greater tendency for this ligand to be coordinated eta3 vs analogous diketiminates is discussed. The nucleophilicity of Cgamma in structure 1, vs that of donors L = Cl- or MeCN, is evaluated to understand the preference of the bis(phosphinimino)methanide to be bidentate or tridentate.     

Formation and characterization of phospholipid monolayers spontaneously assembled at interfaces between aqueous phases and thermotropic liquid crystals

This paper reports an experimental investigation of the self-assembly of phospholipids (l-alpha-phosphatidylcholine-beta-oleoyl-gamma-palmitoyl (l-POPC), dipalmitoyl phosphatidylcholine (DPPC), and l-alpha-dilauroyl phosphatidylcholine (l-DLPC)) at interfaces between aqueous phases and the nematic liquid crystal (LC) 4'-pentyl-4-cyanobiphenyl. Stable planar interfaces between the aqueous phases and LCs were created by hosting the LCs within gold grids (square pores with widths of 283 microm and depths of 20 microm). At these interfaces, the presence and lateral organization of the phospholipids leads to interface-driven orientational transitions within the LC. By doping the phospholipids with a fluorescently labeled lipid (Texas Red-1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (TR-DPPE)), quantitative epifluorescence microscopy revealed the saturation coverage of phospholipid at the interface to be that of a monolayer with an areal density of approximately 49 +/- 8% relative to hydrated lipid bilayers. By adsorbing phospholipids to the aqueous-LC interface from either vesicles or mixed micelles of dodecyltrimethylammonium and phospholipid, control of the areal density of phospholipid from 42 +/- 10 to 102 +/-18% of saturation monolayer coverage was demonstrated. Fluorescence recovery after photobleaching (FRAP) experiments performed by using laser scanning confocal microscopy (LSCM) revealed the lateral mobility of fluorescently labeled DPPE in l-DLPC assembled at the interface with the liquid crystal to be (6 +/- 1) x 10(-12) m(2)/s for densely packed monolayers. Variation of the surface coverage and composition of phospholipid led to changes in lateral diffusivity between (0.2 +/- 0.1) x 10(-12) and (15 +/- 2) x 10(-12) m(2)/s. We also observed the phospholipid-laden interface to be compartmentalized by the gold grid, thus allowing for the creation of patterned arrays of phospholipids at the LC-aqueous interface.     

Light-induced coumarin cyclopentannelation

[reaction: see text] Whereas cyclopentenylcarbenes resulting from photocycloaddition of 4-alk-1-ynylcoumarins to 2,3-dimethylbut-2-ene undergo tandem cyclization to hitherto unknown tetracyclic (4-hetera)cyclopent[b,c]acenaphthylenes, the corresponding cyclopentenylnitrenes stemming from 4-cyanocoumarins and the same alkene are converted into tricyclic imines via H-abstraction.