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Wenbin Lin

Publications and source records attributed to Wenbin Lin.

At least 19 recordsLinked to original sources
PubMed2006

Uniaxial and biaxial nematic liquid crystals.

The unusual exhibition of a biaxial nematic phase in nonlinear thermotropic mesogens derived from the 2,5-oxadiazole biphenol (ODBP) core is placed in a general context; the uniaxial nematic phase of the prototypical rod-like mesogen para-quinquephenyl does not follow the classical mean-field behaviour of nematics, thus questioning the utility of such theories for quantitative predictions about biaxial nematics. The nuclear magnetic resonance spectra of labelled probe molecules dissolved in ODBP biaxial nematic phases suggest that a second critical rotation frequency, related to the differences in the transverse diamagnetic susceptibilities of the biaxial nematic, must be exceeded in order to create an aligned two-dimensional powder sample. Efforts to find higher viscosity and lower temperature biaxial nematics (with lower critical rotation rates) to confirm the above conjecture are described. Several chemical modifications of the ODBP mesogenic core are presented.

Journal Article↗
PubMed2006

1D and 2D homochiral metal-organic frameworks built from a new chiral elongated binaphthalene-derived bipyridine.

Six homochiral coordination polymers 1-6 based on a new enantiopure elongated (S)-2,2'-diethoxy-1,1'-binaphthyl-6,6'-bis(4-vinylpyridine) ligand (L) and divalent metal (Zn, Cd, and Ni) connecting points were synthesized and characterized by single-crystal X-ray diffraction studies. These new homochiral coordination polymers adopt two distinct framework structures: a one-dimensional infinite chain structure with bridging L ligands occupying the axial positions of the metal centers and a two-dimensional rhombic grid structure formed by linking octahedrally coordinated metal centers with four pyridyl groups of bridging L ligands in the equatorial positions. The structures of these coordination polymers are sensitive to the nature of the anions as well as the solvents from which the coordination polymer crystals were grown. Powder X-ray diffraction studies showed that the two-dimensional chiral rhombic grids exhibited porosity, which could potentially find applications in enantioselective separations and catalysis.

2,2'-Dipyridyl↗
PubMed2006

Directed assembly of mesoscopic metallocycles with controllable size, chirality, and functionality based on the robust Pt-alkynyl linkage.

This paper describes expeditious stepwise directed assembly of large homochiral metallocycles with up to 38 6,6'-bis(alkynyl)-1,1'-binaphthalene bridging ligands (L) and 38 trans-Pt(PEt(3))(2) ([Pt]) centers and with cavities as large as 22 nm in diameter. These unprecedented mesoscopic metallocycles are synthesized by cyclization of different lengths of oligomeric building blocks, L(m)()[Pt](m)()(+1)Cl(2) (m = 1, 2, 3, 5, 7, 11, 19, and 31) and [Pt](n)()L(n)()(+1)H(2) (n = 1, 2, 3, 4, 5, 6, 10, 18, and 30), and have been characterized by a variety of techniques, including (1)H{(31)P}, (13)C{(1)H}, and (31)P{(1)H} NMR spectroscopy, MALDI-TOF MS, elemental analysis, FT-IR, UV-vis, CD, size-exclusion chromatography, and diffusion-ordered NMR spectroscopy. The present synthetic methodology was also extended to the synthesis of non-homochiral metallocycles of very different topologies and macrocyclic structures with additional functional groups precisely placed at different positions. This work provides a general strategy for the construction of nanoscopic and mesoscopic functional supramolecular architectures of controllable size, chirality, and functionality that cannot be accessed from the existing synthetic approaches.

Cyclization↗
PubMed2006

Homochiral porous solids based on 1D coordination polymers built from 46-membered macrocycles.

Six homochiral coordination polymers 1-6 based on an enantiopure elongated and bent bipyridine ligand were synthesized and characterized by single-crystal X-ray diffraction studies. The framework structures of all six compounds were built up from similar 1D polymeric chains composed of 46-membered metallomacrocycles. Four distinct packing patterns were observed for this family of coordination polymers. With the exception of 1, the anions do not coordinate to the metal centers and reside in the open channels. Single-crystal X-ray diffraction studies show that the structures of these coordination polymers are sensitive to the anions even though they do not coordinate to the metal centers. The framework structures are somewhat tolerant of the change of metal centers and their local coordination environments. Gas sorption measurements on 1 suggest that chiral porous solids can be obtained with the present 1D coordination polymeric building blocks.

Journal Article↗
PubMed2006

Nanoscale metal-organic frameworks as potential multimodal contrast enhancing agents.

Nanoscale metal-organic frameworks (NMOFs) based on Gd3+ centers and benzenedicarboxylate and benzenetricarboxylate bridging ligands were synthesized using reverse microemulsions and characterized using SEM, PXRD, and TGA. These NMOFs exhibit extraordinarily large R1 and R2 relaxivities because of the presence of up to tens of millions of Gd3+ centers in each nanoparticle and are thus efficient T1 and T2 contrast agents for MRI. The NMOFs can also be made highly luminescent by doping with Eu3+ or Tb3+ centers. The results from this work suggest that NMOFs can be used as potential contrast agents for multimodal imaging.

Contrast Media↗
PubMed2005

Magnetically recoverable chiral catalysts immobilized on magnetite nanoparticles for asymmetric hydrogenation of aromatic ketones.

Novel heterogenized asymmetric catalysts were synthesized by immobilizing preformed Ru catalysts on magnetite nanoparticles via the phosphonate functionality and were characterized by a variety of techniques, including TEM, magnetization, and XRD. These nanoparticle-supported chiral catalysts were used for enantioselective heterogeneous asymmetric hydrogenation of aromatic ketones with very high enantiomeric excess values of up to 98.0%. The immobilized catalysts were easily recycled by magnetic decantation and reused for up to 14 times without loss of activity and enantioselectivity. Orthogonal nature of the present catalyst immobilization approach should allow the design of other superparamagnetic nanoparticle-supported asymmetric catalysts for a wide range of organic transformations.

Catalysis↗
PubMed2005

Applications of 4,4'-(Me3Si)2-BINAP in transition-metal-catalyzed asymmetric carbon-carbon bond-forming reactions.

[structure: see text] A recently developed BINAP derivative with trimethylsilyl substituents on the 4- and 4'-positions of the binaphthyl skeleton, 2,2'-bis(diphenylphosphino)-4,4'-bis(trimethylsilyl)-1,1'-binaphthyl (tms-BINAP), was used in a variety of transition-metal-catalyzed asymmetric carbon-carbon bond-forming reactions. In pi-allylpalladium-mediated reactions, tms-BINAP gave better enantioselectivity than the unsubstituted BINAP, and the origin of the improved enantioselectivity was gained from an X-ray structural study of [Pd(eta(3)-C(3)H(5))((R)-tms-BINAP)]ClO(4).

Journal Article↗
PubMed2005

A homochiral porous metal-organic framework for highly enantioselective heterogeneous asymmetric catalysis.

A homochiral porous noninterpenetrating metal-organic framework (MOF), 1, was constructed by linking infinite 1D [Cd(mu-Cl)2]n zigzag chains with axially chiral bipyridine bridging ligands containing orthogonal secondary functional groups. The secondary chiral dihydroxy groups accessible via the large open channels in 1 were utilized to generate a heterogeneous asymmetric catalyst for the addition of diethyzinc to aromatic aldehydes to afford chiral secondary alcohols at up to 93% enantiomeric excess (ee). Control experiments with dendritic aromatic aldehydes of different sizes indicate that the heterogeneous asymmetric catalyst derived from 1 is both highly active and enantioselective as a result of the creation of readily accessible, uniform active catalyst sites inside the porous MOF.

Aldehydes↗
PubMed2005

Self-assembly of homochiral porous solids based on 1D cadmium(II) coordination polymers.

A family of homochiral 1D cadmium(II) coordination polymers based on the (S)-2,2'-dimethoxy-1,1'-binaphthyl-3,3'-bis(4-vinylpyridine) (L) bridging ligand were synthesized from the same building blocks under slightly different conditions, and characterized by single-crystal X-ray crystallography. While [CdL(DMF)4](ClO4)2 x EtOH x 0.5H2O (1) adopts a 1D zigzag chain structure, [CdL2(ClO4)2] x 3EtOH x H2O (2) and [CdL2(ClO4)(H2O)] (ClO4) x 1.5(o-C6H4Cl2) x 3EtOH x 6H2O (3) both exhibit 1D polymeric structures that are built from 38-membered macrocycles. These 1D coordination polymers further pack into chiral porous frameworks via pi...pi interactions with a large percentage of void spaces that are occupied by solvent molecules and counterions.

Journal Article↗
PubMed2005

Development of 4,4'-substituted-XylBINAP ligands for highly enantioselective hydrogenation of ketones.

[reaction: see text] A family of 4,4'-substituted-xylBINAPs was synthesized in multistep sequences and characterized by NMR spectroscopy and mass spectrometry. Ru(diphosphine)(diamine)Cl(2) complexes based on these 4,4'-substituted-xylBINAPs and chiral diamines (DPEN and DAIPEN) were synthesized by treatment of [(benzene)RuCl(2)](2) with 4,4'-substituted-xylBINAP followed by chiral diamine, and characterized by (1)H and (31)P NMR spectroscopy and mass spectrometry. These Ru complexes were used for asymmetric hydrogenation of aromatic ketones in a highly enantioselective manner with complete conversion. With 0.1% catalyst loading, complete conversion and enantioselectivity greater than 99% were obtained for most of the aromatic ketones examined. These Ru catalysts thus gave the highest ee for asymmetric hydrogenation of aromatic ketones among all of the catalysts reported in the literature. A single-crystal X-ray diffraction study of Ru[(R)-L(4)()][(R,R)-DPEN]Cl(2) indicated that the 4-methyl group of the naphthyl ring and the methyl groups of the two xylyl moieties form a fence on the opposite side of the DPEN ligand of the Ru center. These three methyl groups will have significant repulsive interactions with the bulky aryl ring of the hydrogen-bonded aromatic ketone in the disfavored transition state. These results support our hypothesis of combining dual modes of enantiocontrol (i.e., the substituents on 4,4'-positions of the binaphthyl framework and the methyl groups on the bis(xylyl)phosphino moieties) to achieve higher stereoselectivity in the hydrogenation of aromatic ketones.

Journal Article↗
PubMed2005

Ru-catalyzed asymmetric hydrogenation of alpha-phthalimide ketones and 1,3-diaryl diketones using 4,4'-substituted BINAPs.

[reaction: see text] A family of tunable precatalysts [NH2Et2][{Ru(4,4'-BINAP)Cl}2(mu-Cl)3] was synthesized and used for highly enantioselective hydrogenation of phthalimide-protected amino ketones and 1,3-diaryldiketones. The bulky groups on the 4,4'-positions of BINAP were believed to be responsible for the enhancement of enantioselectivity (and diasteroselectivity) in these reactions.

Journal Article↗
PubMed2005

A highly electroluminescent molecular square.

Chiral molecular triangles and squares containing the Pt(diimine) metallocorners were synthesized and characterized, and used as the triplet MLCT luminophore in highly efficient light-emitting devices.

Journal Article↗
PubMed2004

Chiral molecular squares based on angular bipyridines: self-assembly, characterization, and photophysical properties.

Chiral molecular squares 1-12 based on [M(dppe)](2+) metallocorners (M = Pd or Pt, and dppe = bis(diphenylphosphino)ethane) and new angular bipyridine bridging ligands derived from the 1,1'-binaphthyl framework were readily assembled and characterized by a variety of techniques including infrared, UV-vis, circular dichroism (CD), and NMR spectroscopy, and ESI mass spectrometry. All these chiral metallocycles are highly luminescent in solution at room temperature with quantum efficiency of 0.06-0.63. Interestingly, when equal molar enantiopure molecular squares of opposite handedness were mixed in solution, a new meso dimeric metallocycle with C(2) symmetry formed. This result indicates the lability of the M-pyridyl bonds in these metallocycles, which may hinder their applications in many enantioselective processes.

Journal Article↗