Heterogeneous nucleation and shape transformation of multicomponent metallic nanostructures.

To be able to control the functions of engineered multicomponent nanomaterials, a detailed understanding of heterogeneous nucleation at the nanoscale is essential. Here, by using in situ synchrotron X-ray scattering, we show that in the heterogeneous nucleation and growth of Au on Pt or Pt-alloy seeds the heteroepitaxial growth of the Au shell exerts high stress (∼2 GPa) on the seed by forming a core/shell structure in the early stage of the reaction. The development of lattice strain and subsequent strain relaxation, which we show using atomic-resolution transmission electron microscopy to occur through the slip of {111} layers, induces morphological changes from a core/shell to a dumbbell structure, and governs the nucleation and growth kinetics. We also propose a thermodynamic model for the nucleation and growth of dumbbell metallic heteronanostructures.

Evidence of Cu(I)/Cu(II) Redox Process by X-ray Absorption and EPR Spectroscopy: Direct Synthesis of Dihydrofurans from ß-Ketocarbonyl Derivatives and Olefins.

The Cu(I)/Cu(II) and Cu(I)/Cu(III) catalytic cycles have been subject to intense debate in the field of copper-catalyzed oxidative coupling reactions. A mechanistic study on the Cu(I)/Cu(II) redox process, by X-ray absorption (XAS) and electron paramagnetic resonance (EPR) spectroscopies, has elucidated the reduction mechanism of Cu(II) to Cu(I) by 1,3-diketone and detailed investigation revealed that the halide ion is important for the reduction process. The oxidative nature of the thereby-formed Cu(I) has also been studied by XAS and EPR spectroscopy. This mechanistic information is applicable to the copper-catalyzed oxidative cyclization of ß-ketocarbonyl derivatives to dihydrofurans. This protocol provides an ideal route to highly substituted dihydrofuran rings from easily available 1,3-dicarbonyls and olefins.

Direct observation of reduction of Cu(II) to Cu(I) by terminal alkynes.

X-ray absorption spectroscopy and in situ electron paramagnetic resonance evidence were provided for the reduction of Cu(II) to Cu(I) species by alkynes in the presence of tetramethylethylenediamine (TMEDA), in which TMEDA plays dual roles as both ligand and base. The structures of the starting Cu(II) species and the obtained Cu(I) species were determined as (TMEDA)CuCl2 and [(TMEDA)CuCl]2 dimer, respectively.

Cu(II)-Cu(I) synergistic cooperation to lead the alkyne C-H activation.

An efficient alkyne C-H activation and homocoupling procedure has been studied which indicates that a Cu(II)/Cu(I) synergistic cooperation might be involved. In situ Raman spectroscopy was employed to study kinetic behavior, drawing the conclusion that Cu(I) rather than Cu(II) participates in the rate-determining step. IR, EPR, and X-ray absorption spectroscopy evidence were provided for structural information, indicating that Cu(I) has a stronger interaction with alkyne than Cu(II) in the C-H activation step. Kinetics study showed Cu(II) plays a role as oxidant in C-C bond construction step, which was a fast step in the reaction. X-band EPR spectroscopy showed that the coordination environment of CuCl2(TMEDA) was affected by Cu(I). A putative mechanism with Cu(I)-Cu(II) synergistic cooperation procedure is proposed for the reaction.

Discovery of highly selective alkyne semihydrogenation catalysts based on first-row transition-metallated porous organic polymers.

Five different first-row transition metal precursors (V(III), Cr(III), Mn(II), Co(II), Ni(II)) were successfully incorporated into a catechol porous organic polymer (POP) and characterized using ATR-IR and XAS analysis. The resulting metallated POPs were then evaluated for catalytic alkyne hydrogenation using high-throughput screening techniques. All POPs were unexpectedly found to be active and selective catalysts for alkyne semihydrogenation. Three of the metallated POPs (V, Cr, Mn) are the first of their kind to be active single-site hydrogenation catalysts. These results highlight the advantages of using a POP platform to develop new catalysts which are otherwise difficult to achieve through traditional heterogeneous and homogeneous routes.

Capping ligands as selectivity switchers in hydrogenation reactions.

We systematically investigated the role of surface modification of nanoparticles catalyst in alkyne hydrogenation reactions and proposed the general explanation of effect of surface ligands on the selectivity and activity of Pt and Co/Pt nanoparticles (NPs) using experimental and computational approaches. We show that the proper balance between adsorption energetics of alkenes at the surface of NPs as compared to that of capping ligands defines the selectivity of the nanocatalyst for alkene in alkyne hydrogenation reaction. We report that addition of primary alkylamines to Pt and CoPt(3) NPs can drastically increase selectivity for alkene from 0 to more than 90% with ~99.9% conversion. Increasing the primary alkylamine coverage on the NP surface leads to the decrease in the binding energy of octenes and eventual competition between octene and primary alkylamines for adsorption sites. At sufficiently high coverage of catalysts with primary alkylamine, the alkylamines win, which prevents further hydrogenation of alkenes into alkanes. Primary amines with different lengths of carbon chains have similar adsorption energies at the surface of catalysts and, consequently, the same effect on selectivity. When the adsorption energy of capping ligands at the catalytic surface is lower than adsorption energy of alkenes, the ligands do not affect the selectivity of hydrogenation of alkyne to alkene. On the other hand, capping ligands with adsorption energies at the catalytic surface higher than that of alkyne reduce its activity resulting in low conversion of alkynes.

New air-stable catalysts for general and efficient suzuki-miyaura cross-coupling reactions of heteroaryl chlorides.

[reaction: see text] New air-stable PdCl(2){P(t)Bu(2)(p-R-Ph)}(2) (R = H, NMe(2), CF(3),) complexes represent simple, general, and efficient catalysts for the Suzuki-Miyaura cross-coupling reactions of aryl halides including five-membered heteroaryl halides and heteroatom-substituted six-membered heteroaryl chlorides with a diverse range of arylboronic acids. High product yields (89-99% isolated yields) and turn-over-numbers (10,000 TON) are observed.

Homolytic cleavage of the O-Cu(ii) bond: XAFS and EPR spectroscopy evidence for one electron reduction of Cu(ii) to Cu(i).

The investigation into the active copper(i) catalysts from copper(ii) precursors has become a fundamental and important task in copper catalysis. In this work, we demonstrate that the (t)BuO(-) anion serves not only as a base but also as a mediator to promote the reduction of Cu(ii) to Cu(i) in copper catalysis. XAFS and EPR spectroscopy evidence the [Cu(O(t)Bu)3](-) ate complex as the key intermediate which undergoes homolytic-cleavage of the O-Cu(ii) bond generating [Cu(O(t)Bu)2](-) ate complex.

Revealing the halide effect on the kinetics of the aerobic oxidation of Cu(i) to Cu(ii).

In situ infrared (IR) and X-ray absorption near-edge structure (XANES) spectroscopic investigations reveal that different halide ligands have distinct effects on the aerobic oxidation of Cu(i) to Cu(ii) in the presence of TMEDA (tetramethylethylenediamine). The iodide ligand gives the lowest rate and thus leads to the lowest catalytic reaction rate of aerobic oxidation of hydroquinone to benzoquinone. Further DFT calculations suggest that oxidation of CuI-TMEDA involves a side-on transition state, while oxidation of CuCl-TMEDA involves an end-on transition state which has a lower activation energy.

Bimetallic zinc complex--active species in coupling of terminal alkynes with aldehydes via nucleophilic addition/Oppenauer oxidation.

A mechanistic study on the zinc-promoted coupling between aldehydes and terminal alkynes via nucleophilic addition/Oppenauer oxidation using operando IR, XANES/EXAFS techniques and DFT calculations was demonstrated. It was determined that a bimetallic zinc complex was the active species.

Iron-Catalyzed Oxidative C-H/C-H Cross-Coupling between Electron-Rich Arenes and Alkenes.

A novel oxidative C-H/C-H cross-coupling reaction between electron-rich arenes and alkenes is established utilizing FeCl3 as the catalyst and DDQ as the oxidant. Interestingly, direct arylation products are obtained with diaryl-ethylenes and double arylation products are obtained with styrene derivatives, which show high chemoselectivity and good substrate scope. A radical trapping experiment and EPR (electron paramagnetic resonance) experiments indicate that this reaction proceeds through a radical pathway in which DDQ plays a key role in the aryl radical formation. XAFS (X-ray absorption fine structure) experiments reveal that the oxidation state of the iron catalyst does not change during the reaction, suggesting that FeCl3 might be used as a Lewis acid. Finally, a detailed mechanism is proposed for this transformation.

Copper-catalyzed aerobic oxidative coupling: From ketone and diamine to pyrazine.

Copper-catalyzed aerobic oxidative C-H/N-H coupling between simple ketones and diamines was developed toward the synthesis of a variety of pyrazines. Various substituted ketones were compatible for this transformation. Preliminary mechanistic investigations indicated that radical species were involved. X-ray absorption fine structure experiments elucidated that the Cu(II) species 5 coordinated by two N atoms at a distance of 2.04 Å and two O atoms at a shorter distance of 1.98 Å was a reactive one for this aerobic oxidative coupling reaction. Density functional theory calculations suggested that the intramolecular coupling of cationic radicals was favorable in this transformation.

Structure-kinetic relationship study of organozinc reagents.

Phenylzinc reagents prepared from various zinc halides show distinct kinetic features in the palladium-catalyzed Negishi-type oxidative coupling reaction, in which the phenylzinc reagent prepared from ZnI2 gives the highest rate. In situ infrared and X-ray absorption spectroscopy studies show that the higher reaction rate was observed for longer Zn-C bond distances.

New catalysts for Suzuki-Miyaura coupling reactions of heteroatom-substituted heteroaryl chlorides.

The new air-stable PdCl2[PR2(Ph-R')]2 complexes, readily prepared from commercial reagents, exhibit unique efficiency as catalysts for the Suzuki-Miyaura coupling reactions of a variety of heteroatom-substituted heteroaryl chlorides with a diverse range of aryl/heteroaryl boronic acids. The coupling reactions catalyzed by the new complexes exhibit high product yields (88-99%) and high catalyst turnover numbers (up to 10,000 TON).

Highly selective rhodium-catalyzed conjugate addition reactions of 4-oxobutenamides.

A variety of 4-oxobutenamides 1 were subjected to rhodium-catalyzed conjugate addition with arylboronic acids providing high regio- and enantioselectivity (97:3 to >99:1, >96% ee) and moderate to excellent yields (54-99%). The key to high selectivity is the use of sterically demanding P-chiral diphosphines, such as Tangphos or Duanphos. The product oxobutanamides 2 may be converted to alternate targets by selective derivatization of either the amide or ketone functional group. A stereochemical model predicting the absolute sense of induction was developed based on single-crystal X-ray structures of product and precatalyst.