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PubMed2006

Dehydrogenative silane coupling on silicon surfaces via early transition metal catalysis.

Derivatization of silicon surfaces is an area of intense interest due to the centrality of silicon in the microelectronics industry and because of potential promise for a myriad of other applications. In this paper, we investigate the feasibility of Si-Si bond formation directly on the surface to contrast with the more widely studied Si-C and Si-O bond forming reactions. Functionalization of hydride-terminated silicon surfaces with silanes is carried out via early transition metal mediated dehydrogenative silane coupling reactions. Zirconocene and titanocene catalyst systems were evaluated for heterocoupling of a molecular silane, RSiH3, with a surface Si-H group on Si(s). The zirconocene catalysts proved to be much more reactive than the titanium system, and so the former was examined exclusively. The silanes, aromatic or aliphatic, are bonded to the silicon surface through direct Si-Si bonds, although the level of incorporation of the trihydroarylsilanes was substantially higher than that of the aliphatic silanes. The reaction proceeds on nanocrystalline hydride-terminated porous silicon surfaces, as well as flat Si(100)-H(x) and Si(111)-H interfaces. The reactions were studied by a variety of techniques, including FTIR, SIMS, and XPS.

Catalysis↗

PubMed1997

Heterobimetallic Complexes with Phenylcyclopentadienyl Ligand: Syntheses and Structures of Tricarbonylchromium-eta(6),eta(5)-Phenylcyclopentadienyl-Transition Metal Complexes(1).

A new synthetic method of heterobimetallic complexes bridging with a pi,pi-phenylcyclopentadienyl ligand, tricarbonylchromium-eta(6),eta(5)-phenylcyclopentadienyl-transition metal complexes, was developed through the reactions of (eta(6)-C(6)H(5)C(5)H(5))Cr(CO)(3) or its sodium salt with transition metal complexes. The method is suitable for most transition metal elements with the advantages of easy manipulation, mild reaction conditions, and moderate to high yields. As indicated by (1)H NMR spectra, the interactions between the two pi systems of phenyl and cyclopentadienyl rings were weak. X-ray crystal structures of seven complexes were studied. Compound 23, [eta(6)-C(6)H(5)Cr(CO)(3)](2)C(10)H(10), crystallizes in orthorhombic space group Pbca with cell constants a = 29.616(8) Å, b = 12.861(5) Å, c = 12.397(7) Å, V = 4721(6) Å(3), Z = 8, R = 0.045, and R(w) = 0.045. Compound 3, (eta(6)-C(6)H(5)C(9)H(7))Cr(CO)(3), crystallizes in monoclinic space group P2(1)/c with cell constants a = 7.901(3) Å, b = 14.799(2) Å, c = 12.917(2) Å, beta = 99.72(2) degrees, V = 1488.7(4) Å(3), Z = 4, R = 0.044, and R(w) = 0.051. Compound 7, Cr(CO)(3)(eta(6),eta(5)-C(6)H(5)C(5)H(4))Ti(CO)(2)(eta(5)-C(5)H(5)), crystallizes in monoclinic space group P2(1)/c with cell constants a = 12.361(4) Å, b = 12.487(6) Å, c = 12.531(7) Å, beta = 93.48(4) degrees, V = 1930 Å(3), Z = 4, R = 0.047, and R(w) = 0.048. Compound 13, Cr(CO)(3)(eta(6),eta(5)-C(6)H(5)C(5)H(4))Mo(CO)(3)Br, crystallizes in monoclinic space group P2(1)/c with cell constants a = 14.685(2) Å, b = 8.509(3) Å, c = 14.960(3) Å, beta = 104.46(1) degrees, V = 1810.0(7) Å(3), Z = 4, R = 0.037, and R(w) = 0.037. Compound 19, Cr(CO)(3)(eta(6),eta(5)-C(6)H(5)C(9)H(6))Mn(CO)(3), crystallizes in triclinic space group P(-)(1) with cell constants a = 11.660(4) Å, b = 12.578(5) Å, c = 6.987(2) Å, alpha = 100.03(3) degrees, beta = 104.19(2) degrees, gamma = 71.99(3) degrees, V = 939.3(6) Å(3), Z = 2, R = 0.048, and R(w) = 0.065. Compound 21, Cr(CO)(3)(eta(6),eta(5)-C(6)H(5)C(5)H(4))Ru(PPh(3))(2)Cl, crystallizes with one molecule of EtOH in triclinic space group P(-)(1) with cell constants a = 14.033(4) Å, b = 16.163(6) Å, c = 10.411(3) Å, alpha = 104.22(3) degrees, beta = 103.50(2) degrees, gamma = 90.81(3) degrees, V = 2219(1) Å(3), Z = 2, R = 0.081, and R(w) = 0.096. Compound 22, Cr(CO)(3)(eta(6),eta(5)-C(6)H(5)C(5)H(4))Co(CO)(2), crystallizes in orthorhombic space group P2(1)2(1)2(1) with cell constants a = 15.083(5) Å, b = 16.391(5) Å, c = 6.351(4) Å, V = 1570(1) Å(3), Z = 4, R = 0.036, and R(w) = 0.039. The trans configurations of the two metal atoms in these molecules were found as expected; the phenyl and cyclopentadienyl or indenyl rings in the molecules were found to be not coplanar.

Journal Article↗

PubMed2004

Hydration structure and free energy of biomolecularly specific aqueous dications, including Zn2+ and first transition row metals.

The hydration of some of the alkaline earth divalent metal cations and first row transition metal cations is considered within the quasi-chemical theory of solutions. Quantum chemical calculations provide information on the chemically important interactions between the ion and its first-shell water molecules. A dielectric continuum model supplies the outer-shell contribution. The theory then provides the framework to mesh these quantities together. The agreement between the calculated and experimental quantities is good. For the transition metal cations, it is seen that the ligand field contributions play an important role in the physics of hydration. Removing these bonding contributions from the computed hydration free energy results in a linear decrease in the hydration free energy along the period. It is precisely such effects that molecular mechanics force fields have not captured. The implications and extensions of this study to metal atoms in proteins are suggested.

Calcium↗

PubMed2005

Synthesis and electrochemical characterization of a transition-metal-modified ligand-receptor pair.

The energetics of weak interactions (van der Waals forces, hydrogen bonding) are difficult to quantify in biological ligand-receptor pairs. Insight into the biochemical role these forces play is critical to an understanding of signal transduction events and the drug discovery process. Ruthenium pentaammine and iron tetracyano complexes modified with either biotin or desthiobiotin have been synthesized and characterized. These modified biological ligands bind to the protein avidin in a manner similar to that of native biotin. Experiments using redox mediators show that the avidin-bound complexes are electrochemically accessible.

Avidin↗

PubMed2004

Understanding strong two-photon absorption in pi-conjugated porphyrin dimers via double-resonance enhancement in a three-level model.

We present the two-photon absorption (2PA) spectra of a series of conjugated porphyrin dimers and show that they possess extremely large intrinsic (femtosecond) peak 2PA cross sections, up to sigma2 = 1 x 104 GM in the near-IR region; these are among the highest values measured for any organic molecule. Moreover, we demonstrate that the second-order perturbation theory applied to a simple three-level model gives a perfect quantitative description of the observed 2PA cross section. By comparing all the factors of the three-level model for dimers with those of corresponding monomer (for which sigma2 = 20 GM), we explain an approximately 500-fold cooperative enhancement in sigma2 and find that the most important factor is the strength of excited-state transition. The matrix element of dipole moment of this transition amounts gigantic values of 30-40 D for conjugated porphyrin dimers, which can be accounted for a large delocalization radius (large electron-hole separation) in this state. We also demonstrate efficient generation of singlet oxygen upon one- and two-photon excitation of these porphyrin dimers, which can be useful for two-photon initiated photodynamic therapy of cancer.

Journal Article↗

PubMed2005

Synthesis and crystal structure of an open capsule-type octanuclear heterometallic sulfide cluster with a linked incomplete double cubane framework without an intramolecular inversion center.

An open capsule-type octanuclear heterometallic sulfide cluster without an intramolecular inversion center [Ru(eta(6)-C(6)Me(6)){P(OMe)(3)}{MoO(mu(3)-S)(3)}(CuI)(2)](2) (5) has been synthesized for the first time by stepwise connection of three mononuclear building blocks, i.e., (i) [RuCl(2)(eta(6)-C(6)Me(6)){P(OMe)(3)}] (1a) as an octahedral terminal building block to control the direction of cluster expansion, (ii) [MoOS(3)](2)(-) as a tetrahedral polydentate building block owing to the strong coordination ability of the S atoms, and (iii) a CuI building block to form a trigonal planar (mu-S)(2)CuI unit or to form a linkage unit of two incomplete cubane-type octanuclear frameworks. The stepwise connection was made in the following order: [RuCl(2)(eta(6)-C(6)Me(6)){P(OMe)(3)}] (1a, mononuclear) --> [Ru(eta(6)-C(6)Me(6)){P(OMe)(3)}{MoOS(mu(2)-S)(2)}] (2a, dinuclear) --> [Ru(eta(6)-C(6)Me(6)){P(OMe)(3)}{MoO(mu(2)-S)(2)(mu(3)-S)}CuI] (3a, butterfly-type trinuclear) --> [Ru(eta(6)-C(6)Me(6)){P(OMe)(3)}{MoO(mu(3)-S)(3)}(CuI)(2)](2) (5). When P(OMe)(3) was replaced by P(OEt)(3), which is more bulky than P(OMe)(3), in the starting ruthenium building block [RuCl(2)(eta(6)-C(6)Me(6)){P(OEt)(3)}] (1b, mononuclear), only the tetranuclear incomplete single cubane cluster [Ru(eta(6)-C(6)Me(6)){P(OEt)(3)}{MoO(mu(3)-S)(3)}(CuI)(2)] (6) was generated, owing to the steric effect of P(OEt)(3).

Crystallization↗

PubMed2005

Covalent attachment of a transition metal coordination complex to functionalized oligo(phenylene-ethynylene) self-assembled monolayers.

We have investigated the reaction of tetrakis(dimethylamido)titanium, Ti[N(CH(3))(2)](4), with N-isopropyl-N-[4-(thien-3-ylethynyl) phenyl] amine and N-isopropyl-N-(4-{[4-(thien-3-ylethynyl) phenyl]ethynyl}phenyl) amine self-assembled monolayers (SAMs), on polycrystalline Au substrates. The structure of the SAMs themselves has also been investigated. Both molecules form SAMs on polycrystalline Au bound by the thiophene group. The longer-molecular-backbone molecule forms a denser SAM, with molecules characterized by a smaller tilt angle. X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS have been employed to examine the kinetics of adsorption, the spatial extent of reaction, and the stoichiometry of reaction. For both the SAMs, adsorption is described well by first-order Langmuirian kinetics, and adsorption is self-limiting from T(s) = -50 to 30 degrees C. The use of angle-resolved XPS clearly demonstrates that the Ti[N(CH(3))(2)](4) reacts exclusively with the isopropylamine end group via ligand exchange, and there is no penetration of the SAM, followed by reaction at the SAM-Au interface. Moreover, the SAM molecules remain bound to the Au surface via their thiopene functionalites. From XPS, we have found that, in both cases, approximately one Ti[N(CH(3))(2)](4) is adsorbed per two SAM molecules.

Crystallization↗

PubMed2005

Naked (C5Me5)2M cations (M = Sc, Ti, and V) and their fluoroarene complexes.

The ionic metallocene complexes [Cp*(2)M][BPh(4)] (Cp* = C(5)Me(5)) of the trivalent 3d metals Sc, Ti, and V were synthesized and structurally characterized. For M = Sc, the anion interacts weakly with the metal center through one of the phenyl groups, but for M = Ti and V, the cations are naked. They each contain one strongly distorted Cp* ligand, with one (V) or two (Ti) agostic C-H...M interactions involving the Cp*Me groups. For Sc and Ti, these Lewis acidic species react with fluorobenzene and 1,2-difluorobenzene to yield [Cp*(2)M(kappaF-FC(6)H(5))(n)][BPh(4)] (M = Sc, n = 2; M = Ti, n = 1) and [Cp*(2)M(kappa(2)F-1,2-F(2)C(6)H(4))][BPh(4)], the first examples of kappaF-fluorobenzene and kappa(2)F-1,2-difluorobenzene adducts of transition metals. With the perfluorinated anion [B(C(6)F(5))(4)](-), both Sc and Ti form [Cp*(2)M(kappa(2)F-C(6)F(5))B(C(6)F(5))(3)] contact ion pairs. The nature of the metal-fluoroarene interaction was studied by density functional theory (DFT) calculations and by comparison with the corresponding tetrahydrofuran (THF) adducts and was found to be predominantly electrostatic for all metals studied.

Cations↗

PubMed2005

Detection of homocysteine and cysteine.

At elevated levels, homocysteine (Hcy, 1) is a risk factor for cardiovascular diseases, Alzheimer's disease, neural tube defects, and osteoporosis. Both 1 and cysteine (Cys, 3) are linked to neurotoxicity. The biochemical mechanisms by which 1 and 3 are involved in disease states are relatively unclear. Herein, we describe simple methods for detecting either Hcy or Cys in the visible spectral region with the highest selectivity reported to date without using biochemical techniques or preparative separations. Simple methods and readily available reagents allow for the detection of Cys and Hcy in the range of their physiologically relevant levels. New HPLC postcolumn detection methods for biological thiols are reported. The potential biomedical relevance of the chemical mechanisms involved in the detection of 1 is described.

Aldehydes↗

PubMed2006

Cyanoesterification of 1,2-dienes: synthesis and transformations of highly functionalized alpha-cyanomethylacrylate esters.

A Ni/PMe2Ph catalyst is found to effect regioselective addition of cyanoformate esters across 1,2-dienes, giving rise to 3-alkoxycarbonyl-3-butenenitriles. Functional groups such as cyano, protected hydroxyl, and amino groups in the 1,2-diene substrates are tolerated. Isomerization of the initial products to thermodynamically more stable isomers takes place possibly through further oxidative addition of the C-CN bond of 3-alkoxycarbonyl-3-butenenitriles to Ni(0) followed by reductive elimination. The cyanoesterification products undergo further addition across alkynes in the presence of a Ni/P(4-CF3-C6H4)3 catalyst.

Alkadienes↗

PubMed2006

Synthesis of aromatic ketones by a transition metal-catalyzed tandem sequence.

Both simple Ag(I) and Au(I) are effective catalysts for a tandem [3,3]-sigmatropic rearrangement/formal Myers-Saito cyclization of propargyl esters to form aromatic ketones. A mechanism in which the metal catalyzes both of these processes through alkyne activation is proposed. By using this method a wide range of aromatic structures including naphthyl, anthracenyl and indole ketones are available from readily available propargyl esters.

Catalysis↗

PubMed2006

Quinone-annulated N-heterocyclic carbene-transition-metal complexes: observation of pi-backbonding using FT-IR spectroscopy and cyclic voltammetry.

A new N-heterocyclic carbene architecture comprising a 1,4-naphthoquinone annulated to 1,3-dimesitylimidazolylidene (NpQ-NHC) was synthesized in two high yielding steps from commercially available starting materials. The free NpQ-NHC was characterized (solution and solid-state) and was used to synthesize various Rh and Ag complexes that ranged in pi-electron density. Enabled by the quinone moiety, the pi-systems of these complexes were analyzed using infrared spectroscopy and cyclic voltammetry. In contrast to previous reports, pi-backbonding was found to be non-negligible and was directly influenced by the metal's electronic character.

Electrochemistry↗

PubMed2002

Conformationally constrained alpha-boc-aminophosphonates via transition metal-catalyzed/curtius rearrangement strategies.

A transition metal-catalyzed/Curtius rearrangement sequence toward the development of conformationally constrained alpha-Boc-aminophosphonates 2-6 is described. An approach using the versatile tert-butylphosphonoacetate moieties 1a and 1b to derive an array of mono- and bicyclic alpha-Boc-aminophosphonate systems is presented. Conformational constraint is incorporated using either the ring-closing metathesis reaction catalyzed by the first generation Grubbs catalyst or intramolecular cyclopropanation mediated by Rh2(OAc)4. Using the tert-butyl ester functionality in 1a or 1b as a potential amino group, the Curtius rearrangement provides an efficient route toward the target alpha-Boc-aminophosphonates.

Amines↗

PubMed2005

Total synthesis of heliophenanthrone.

The total synthesis of rac-heliophenanthrone (3a) was achieved by a convergent approach, making use of a transition-metal-catalyzed domino process with an intramolecular Diels-Alder reaction at an isobenzopyrylium cation as key step.

Catalysis↗

PubMed2005

Transition-metal-mediated cascade reactions: the water-accelerated carboalumination-Claisen rearrangement-carbonyl addition reaction.

[Chemical reaction: See text] A three-step cascade reaction involving a water-accelerated catalytic carboalumination, a Claisen rearrangement, and a nucleophilic carbonyl addition converts terminal alkynes and allyl vinyl ethers into allylic alcohols containing up to three contiguous asymmetric carbon centers. Stoichiometric quantities of water as an additive increase the rate of the [3,3] sigmatropic rearrangement as well as the diastereoselectivity of the carbonyl addition process. Reaction products contain 1,6-diene functionalities that are readily cyclized to substituted cyclopentenes. An extension of this methodology to a sequence involving a [1,3] sigmatropic shift was feasible with a cyclopropylmethyl vinyl ether substrate.

Acetylene↗

PubMed2006

Synthesis of marine polyacetylenes callyberynes A-C by transition-metal-catalyzed cross-coupling reactions to sp centers.

Efficient total syntheses of the sponge-derived hydrocarbon polyacetylenes callyberynes A-C have been achieved using metal-catalyzed cross-coupling reactions of highly unsaturated 1,3-diyne fragments as the key steps, namely: Cadiot-Chodkiewicz reaction under Alami's optimized conditions (sp-sp), sequential Sonogashira reaction of a cis,cis-divinyl dihalide (sp2-sp), and Kumada-Corriu reaction of an unactivated alkyl iodide (sp3-sp). This last approach constitutes the first application of a metal-catalyzed sp3-sp Kumada-Corriu cross-coupling reaction to the synthesis of a natural product.

Animals↗

PubMed2005

Imaging nanometer metallocatalysts formed by photosynthetic deposition of water-soluble transition-metal compounds.

A method of imaging nanometer metallocatalysts formed by photosynthetic precipitation of the water-soluble transition-metal compounds [PtCl(6)](2-) and [RuCl(6)](2-) is reported. Hexachloroplatinate and hexachlororuthenate can accept up to four electrons from Photosystem I (PSI) reaction centers in photosynthetic thylakoid membranes, thereby converting [PtCl(6)](2-) and [RuCl(6)](2-) anions to either metallic platinum (Pt) and ruthenium (Ru) and/or partially oxidized nanometer catalysts at the reducing sides of PSI molecules. Use of this method can potentially create nanometer-sized Pt and/or bimetallic catalysts (such as Pt-Ru) on biomembranes and molecules at pH 7 and room temperature with preservation of the biological function of the molecules.

Catalysis↗

PubMed2005

Density functional calculation of the electronic circular dichroism spectra of the transition metal complexes [M(phen)3]2+ (M = Fe, Ru, Os).

The circular dichroism spectra of the tris-bidentate metal complexes Lambda-[M(phen)3]2+, with M = Fe, Ru, Os and phen = 1,10-tris-phenanthroline, are investigated computationally, employing time-dependent density functional theory. Good agreement with experimental spectra is obtained for Ru and Os. The Lambda-[Os(phen)3]2+ spectrum is analyzed in detail. It is shown how relativistic effects red shift CD bands where the Os 5d-orbital participates to a large extent in the excitations. Further, the participation of the metal in the ligand pi --> pi exciton CD is determined to be of the order of 10%. Though solvent effects can have a noticeable effect on individual transitions and rotatory strengths, they are demonstrated to have only a very small overall effect on the resulting simulated CD spectra. For Lambda-[Fe(phen)3]2+, the results are shown to be rather sensitive to the choice of the applied hybrid and nonhybrid density functionals, and the optimized geometries based thereupon. In particular, the sign pattern of the lower-energy part (up to 33 x 10(3) cm(-1)) of the Lambda-[Fe(phen)3]2+ CD spectrum is difficult to reproduce. Some combinations of functionals and geometries yield good agreement with experiment, but no "best" approach can be devised based on the available results. Possible sources of errors in the spectrum of Lambda-[Fe(phen)3]2+ due to deficiencies in the functionals and the exchange-correlation kernels are investigated.

Circular Dichroism↗