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Dive into the research topics where Fook S. Tham is active.

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Featured researches published by Fook S. Tham.


Organic Letters | 2008

Air- and Water-Stable Catalysts for Hydroamination/Cyclization. Synthesis and Application of CCC−NHC Pincer Complexes of Rh and Ir

Eike B. Bauer; G. T. Senthil Andavan; T. Keith Hollis; Ramel J. Rubio; Joon Cho; Glenn Kuchenbeiser; Theodore R. Helgert; Christopher S. Letko; Fook S. Tham

The scope of CCC-NHC pincer complex synthetic methodology by metalation/transmetalation has been extended to Ir. Structural characterization revealed that it is isomorphous with the Rh complex. Both Rh and Ir complexes are efficient catalysts for the hydroamination/cyclization of secondary amines in the presence of air and/or water.


Nature Structural & Molecular Biology | 2010

Identification and mechanism of ABA receptor antagonism

Karsten Melcher; Yong Xu; Ley-Moy Ng; X. Edward Zhou; Fen-Fen Soon; Viswanathan Chinnusamy; Kelly Suino-Powell; Amanda Kovach; Fook S. Tham; Sean R. Cutler; Jun Li; Eu Leong Yong; Jian-Kang Zhu; H. Eric Xu

The phytohormone abscisic acid (ABA) functions through a family of fourteen PYR/PYL receptors, which were identified by resistance to pyrabactin, a synthetic inhibitor of seed germination. ABA activates these receptors to inhibit type 2C protein phosphatases, such as ABI1, yet it remains unclear whether these receptors can be antagonized. Here we demonstrate that pyrabactin is an agonist of PYR1 and PYL1 but is unexpectedly an antagonist of PYL2. Crystal structures of the PYL2–pyrabactin and PYL1–pyrabactin–ABI1 complexes reveal the mechanism responsible for receptor-selective activation and inhibition, which enables us to design mutations that convert PYL1 to a pyrabactin-inhibited receptor and PYL2 to a pyrabactin-activated receptor and to identify new pyrabactin-based ABA receptor agonists. Together, our results establish a new concept of ABA receptor antagonism, illustrate its underlying mechanisms and provide a rational framework for discovering novel ABA receptor ligands.


Proceedings of the National Academy of Sciences of the United States of America | 2002

Extending supramolecular fullerene-porphyrin chemistry to pillared metal-organic frameworks

Dayong Sun; Fook S. Tham; Christopher A. Reed; Peter D. W. Boyd

Porphyrins and fullerenes are spontaneously attracted to each other. This supramolecular recognition element can be exploited to produce ordered arrays of interleaved porphyrins and fullerenes. C60⋅H2TpyP⋅Pb(NO3)2⋅1.5TCE (H2TpyP = tetra-4-pyridylporphyrin; TCE = 1,1,2,2-tetrachloroethane) crystallizes in the tetragonal P4/n space group and the structure has been solved to high resolution. The Pb2+ ions connect the pyridylporphyrins in infinite sheets with an interlayer spacing of 12.1 Å. The fullerenes are intercalated between these layers, acting as pillars. The 6:6 ring juncture bonds of C60 are centered over the porphyrins, bringing the layers into strict tetragonal register. This arranagement identifies the fullerene–porphyrin interaction as a structure-defining element. The same motif is seen in a related ribbon structure having C70 intercalated into HgI2-linked H2TpyTP. The supramolecular design principles involved in assembling these chromophores may have applications in materials science.


Angewandte Chemie | 2010

C-F activation of fluorobenzene by silylium carboranes: evidence for incipient phenyl cation reactivity.

Simon Duttwyler; Christos Douvris; Nathanael L. P. Fackler; Fook S. Tham; Christopher A. Reed; Kim K. Baldridge; Jay S. Siegel

Author(s): Duttwyler, Simon; Douvris, Christos; Fackler, Nathanael LP; Tham, Fook S; Reed, Christopher A; Baldridge, Kim K; Siegel, Jay S | Abstract: Si(mply) rips it apart: C-F activation of fluorobenzene has been achieved using the extremely strong silyl Lewis acids [Et3Si(X)]+ (X=PhF or Et3SiH) and [(2,6-dixylyl-C6H 3)SiMe2] + paired with the anion CHB 11Cl11-. They abstract fluoride from unactivated fluorobenzene to give arylated products, consistent with phenyl-cation-like reactivity (see scheme).


Angewandte Chemie | 2009

Superacidity of boron acids H2(B12X12) (X = Cl, Br).

Amy Avelar; Fook S. Tham; Christopher A. Reed

Acid remarks: The anhydrous diprotic boron acids H(2)(B(12)X(12)) (X = Cl, Br; see picture, B orange, X green) are the first examples of diprotic superacids and may be the strongest acids yet isolated. Both protons protonate benzene to give benzenium ion salts that are stable at room temperature. These acids owe their existence to the stability of the icosahedral B(12) cluster with its dinegative charge buried beneath a layer of halide substituents.


Journal of the American Chemical Society | 2011

Planar subporphyrin borenium cations.

Eiji Tsurumaki; Shinya Hayashi; Fook S. Tham; Christopher A. Reed; Atsuhiro Osuka

Subporphyrin borenium cations with a carborane counterion have been prepared by treatment of B-methoxy subporphyrins with the silylium reagent Et(3)Si(CH(6)B(11)Br(6)). In contrast to the distinctly domed subphthalocyanine borenium cation, a nearly planar structure with sp(2) hybridized boron is found in the X-ray structure of the triphenylsubporphyrin borenium cation. The cations exhibit absorption and fluorescence spectra that are quite similar to those of B-methoxy subporphyrins. B-phenyl subporphyrins were prepared in good yield by reaction of subporphyrin borenium cations with phenyllithium.


Journal of the American Chemical Society | 2008

The nature of the hydrated proton H(aq)+ in organic solvents.

Evgenii S. Stoyanov; Irina V. Stoyanova; Fook S. Tham; Christopher A. Reed

The nature of H(H2O)n(+) cations for n = 3-8 with weakly basic carborane counterions has been studied by IR spectroscopy in benzene and dichloroethane solution. Contrary to general expectation, neither Eigen-type H3O x 3 H2O(+) nor Zundel-type H5O2(+) x 4 H2O ions are present. Rather, the core species is the H7O3(+) ion.


Journal of the American Chemical Society | 2010

Hysteretic spin and charge delocalization in a phenalenyl-based molecular conductor.

Sushanta K. Pal; Pradip Bag; Arindam Sarkar; Xiaoliu Chi; Mikhail E. Itkis; Fook S. Tham; Bruno Donnadieu; Robert C. Haddon

We have investigated the solid-state electronic structure and properties of a phenalenyl-based butyl-substituted neutral radical, 3, that shows a hysteretic phase transition just above room temperature. We quantitatively analyzed the electron density distribution of this radical throughout both branches of the hysteretic phase transition using solid-state X-ray structures and found two distinct electronic states in the hysteresis loop that accompanies the phase transition. The bistability of the two electronic states was observed through a number of measurements, including IR transmittance spectra of single crystals in the vicinity of the phase transition. By comparing the changes in the crystal structures of 3 and the related ethyl-substituted radical 1 (which exhibits no hysteresis) at various temperatures, we show that the change in the interplanar π-π distance within dimers is the most important structural parameter in determining the physical properties of the radicals. The large change in the C-H···π interaction in 3 occurs in concert with the spin redistribution during the phase transition, but these factors are not responsible for the hysteresis effect. We suggest that the presence of a high-temperature state inside the hysteretic loop during the cooling cycle is due to thermodynamic stability, while the existence of the low-temperature state during the heating cycle is due to the presence of a large energy barrier between the two states (estimated to be greater than 100 kJ/mol) that results from the large-amplitude motion of the phenalenyl rings and the associated lattice reorganization energy that is required at the phase transition.


Journal of the American Chemical Society | 2008

Trisphenalenyl-Based Neutral Radical Molecular Conductor

Sushanta K. Pal; Mikhail E. Itkis; Fook S. Tham; Robert W. Reed; Richard T. Oakley; Robert C. Haddon

We report the preparation, crystallization, and solid-state characterization of the first member of a new family of tris(1,9-disubstituted phenalenyl)silicon neutral radicals. In the solid state, the radical packs as weak partial pi-dimers with intermolecular carbon...carbon contacts that fall at the van der Waals atomic separation. Magnetic susceptibility measurements indicate approximately 0.7 Curie spins per molecule from room temperature down to 50 K, below which antiferromagnetic coupling becomes apparent; the compound has a room-temperature single-crystal conductivity of sigmaRT = 2.4 x 10(-6) S cm(-1).


Journal of the American Chemical Society | 2008

Localization of Spin and Charge in Phenalenyl-Based Neutral Radical Conductors

Robert C. Haddon; Arindam Sarkar; Sushanta K. Pal; Xiaoliu Chi; Mikhail E. Itkis; Fook S. Tham

We report the development of an experimentally based structural analysis to examine the degree of localization of the spin and charge in the phenalenyl-based neutral radical molecular conductors--the results motivate a reinterpretation of the electronic structure of a number of the radicals that we have reported over the past 10 years. The analysis is based on the well-known relationship between bond order and bond length and makes use of the experimental bond distance deviations between the molecular structure of the radical and its corresponding cation. We determined the single crystal X-ray structure of the ethyl radical (1) at 11 temperatures between 90 K and room temperature so that we could follow the evolution of the structure and the electron density distribution through the magnetic phase transition that occurs in the vicinity of 140 K. We show that the enhanced conductivity in the dimeric ethyl (1) and butyl (3) radicals at the magnetic phase transition results from the development of a complex, but highly delocalized electronic structure and not to the formation of a diamagnetic pi-dimer. We find that the monomeric radicals 4, 12, and 13 have an asymmetric electron density distribution in the crystal lattice whereas radical 11 is the only monomeric radical which remains fully delocalized. The pi-chain radicals (7, 8, 14, and 15) retain the strongly delocalized electronic structures expected for a resonating valence bond ground-state structure.

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Ashwani Vij

Air Force Research Laboratory

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