Reductive elimination of alkylamines and ethers: reactions of bisphosphine-ligated palladium(II) complexes

The reductive elimination reactions detailed in this dissertation provide experimental insight into the mechanism of reductive elimination to form the C(sp3)-N bond of benzylamines and the C(sp3)-O bond of benzyl ethers. The stereochemical outcome of the reaction indicates an ionic pathway, but the process lacks many of the effects of electronic and solvent perturbations that typically signal an ionic intermediate. We propose that reductive elimination from benzylpalladium(II) amido and aryloxide complexes occurs by dissociation of the amido or aryloxide ligand, followed by nucleophilic attack on the benzyl ligand. The proposed ionic mechanism is more akin to the reductive elimination reactions that occur from high-valent Pt(IV) and Ni(III) complexes than reductive elimination reactions that occur from other Pd(II) complexes. Our data indicate that substantial differences exist between reductive eliminations to form the C(sp3) bonds in ethers and amines from palladium(II). We prepared alkylpalladium(II) amido complexes to study the C(sp3)-N reductive elimination reaction from complexes containing a non-benzylic hydrocarbyl ligand. We investigated a series of alkylpalladium amido complexes and observed reductive elimination occurs from bisphosphine-ligated neopentylpalladium amido complexes in low yield. Reductive elimination from neopentylpalladium amido complexes occurs most likely by a concerted reductive elimination reaction, and is favored by the increased steric bulk of the neopentyl ligand. We also investigated azametallacyclic palladium complexes with a norbornyl hydrocarbyl ligand, and observed reductive elimination occurs to form a norbornyl indoline product. We found that the yield was slightly improved over neopentylpalladium complexes, but that the yield of reductive elimination was low. Finally we investigated non-metallacyclic complexes containing a norbornyl hydrocarbyl ligand. We discovered that reductive elimination occurs in moderate yield, and the reductive elimination product ratio indicates a balance between a concerted and an ionic mechanism. The data presented in this dissertation demonstrate that C(sp3)-N reductive elimination from benzylpalladium(II) and alkylpalladium(II) complexes can occur. We propose an ionic mechanism for the formation of benzylamines and benzyl ethers by reductive elimination from benzylpalladium(II) complexes. Reductive elimination from neopentyl and metallacyclicpalladium(II) complexes likely occurs by a concerted mechanism, demonstrating the importance of steric bulk and metal geometry, respectively. Finally, reductive elimination from non-metallacyclic norbornylpalladium(II) complexes indicates that a concerted and ionic mechanism may occur simultaneously. Although the yield of the alkylamine products is low, the observation that C(sp3)-N reductive elimination occurs from the alkylpalladium complexes provides the first step toward developing a synthetically useful reaction for the formation of C(sp3)-heteroatom bonds from low-valent group 10 complexes without the addition of an oxidant

Reductive Elimination of Alkylamines from Low-Valent, Alkylpalladium(II) Amido Complexes

This article discusses reductive elimination of alkylamines from low-valent, alkylpalladium(II) amido complexes

Plasmon-enhanced light-driven water oxidation by a dye-sensitized photoanode

Dye-sensitized photoelectrosynthesis cells (DSPECs) provide a basis for artificial photosynthesis and solar fuels production. By combining molecular chromophores and catalysts with high surface area, transparent semiconductor electrodes, a DSPEC provides the basis for light-driven conversion of water to O2 and H2 or for reduction of CO2 to carbon-based fuels. The incorporation of plasmonic cubic silver nanoparticles, with a strongly localized surface plasmon absorbance near 450 nm, to a DSPEC photoanode induces a great increase in the efficiency of water oxidation to O2 at a DSPEC photoanode. The improvement in performance by the molecular components in the photoanode highlights a remarkable advantage for the plasmonic effect in driving the 4e-/4H+ oxidation of water to O2 in the photoanode

A donor-chromophore-catalyst assembly for solar CO2 reduction

We describe here the preparation and characterization of a photocathode assembly for CO2 reduction to CO in 0.1 M LiClO4 acetonitrile. The assembly was formed on 1.0 μm thick mesoporous films of NiO using a layer-by-layer procedure based on Zr(IV)–phosphonate bridging units. The structure of the Zr(IV) bridged assembly, abbreviated as NiO|-DA-RuCP22+-Re(I), where DA is the dianiline-based electron donor (N,N,N′,N′-((CH2)3PO3H2)4-4,4′-dianiline), RuCP2+ is the light absorber [Ru((4,4′-(PO3H2CH2)2-2,2′-bipyridine)(2,2′-bipyridine))2]2+, and Re(I) is the CO2 reduction catalyst, ReI((4,4′-PO3H2CH2)2-2,2′-bipyridine)(CO)3Cl. Visible light excitation of the assembly in CO2 saturated solution resulted in CO2 reduction to CO. A steady-state photocurrent density of 65 μA cm−2 was achieved under one sun illumination and an IPCE value of 1.9% was obtained with 450 nm illumination. The importance of the DA aniline donor in the assembly as an initial site for reduction of the RuCP2+ excited state was demonstrated by an 8 times higher photocurrent generated with DA present in the surface film compared to a control without DA. Nanosecond transient absorption measurements showed that the expected reduced one-electron intermediate, RuCP+, was formed on a sub-nanosecond time scale with back electron transfer to the electrode on the microsecond timescale which competes with forward electron transfer to the Re(I) catalyst at t1/2 = 2.6 μs (kET = 2.7 × 105 s−1)

Heavy quarkonium: progress, puzzles, and opportunities

A golden age for heavy quarkonium physics dawned a decade ago, initiated by the confluence of exciting advances in quantum chromodynamics (QCD) and an explosion of related experimental activity. The early years of this period were chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in 2004, which presented a comprehensive review of the status of the field at that time and provided specific recommendations for further progress. However, the broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles could only be partially anticipated. Since the release of the YR, the BESII program concluded only to give birth to BESIII; the $B$-factories and CLEO-c flourished; quarkonium production and polarization measurements at HERA and the Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the deconfinement regime. All these experiments leave legacies of quality, precision, and unsolved mysteries for quarkonium physics, and therefore beg for continuing investigations. The plethora of newly-found quarkonium-like states unleashed a flood of theoretical investigations into new forms of matter such as quark-gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the spectroscopy, decays, production, and in-medium behavior of c\bar{c}, b\bar{b}, and b\bar{c} bound states have been shown to validate some theoretical approaches to QCD and highlight lack of quantitative success for others. The intriguing details of quarkonium suppression in heavy-ion collisions that have emerged from RHIC have elevated the importance of separating hot- and cold-nuclear-matter effects in quark-gluon plasma studies. This review systematically addresses all these matters and concludes by prioritizing directions for ongoing and future efforts.Comment: 182 pages, 112 figures. Editors: N. Brambilla, S. Eidelman, B. K. Heltsley, R. Vogt. Section Coordinators: G. T. Bodwin, E. Eichten, A. D. Frawley, A. B. Meyer, R. E. Mitchell, V. Papadimitriou, P. Petreczky, A. A. Petrov, P. Robbe, A. Vair

Evidence that ΔS‡ Controls Interfacial Electron Transfer Dynamics from Anatase TiO2 to Molecular Acceptors

Recombination of electrons injected into TiO2 with molecular acceptors present at the interface represents an important loss mechanism in dye-sensitized water oxidation and electrical power generation. Herein, the kinetics for this interfacial electron transfer reaction to oxidized triphenylamine (TPA) acceptors was quantified over a 70° temperature range for para-methyl-TPA (Me-TPA) dissolved in acetonitrile solution, 4-[N,N-di(p-tolyl)amino]benzylphosphonic acid (a-TPA) anchored to the TiO2, and a TPA covalently bound to a ruthenium sensitizer, [Ru(tpy-C6H4-PO3H2)(tpy-TPA)]2+ “RuTPA”, where tpy is 2,2′:6′,2′′-terpyridine. Activation energies extracted from an Arrhenius analysis were found to be 11 ± 1 kJ mol–1 for Me-TPA and 22 ± 1 kJ mol–1 for a-TPA, values that were insensitive to the identity of different sensitizers. Recombination to RuTPA+ proceeded with Ea = 27 ± 1 kJ mol–1 that decreased to 19 ± 1 kJ mol–1 when recombination occurred to an oxidized para-methoxy TPA (MeO-TPA) dissolved in CH3CN. Eyring analysis revealed a smaller entropy of activation |ΔS‡| when the a-TPA was anchored to the surface or covalently linked to the sensitizer, compared to that when Me-TPA was dissolved in CH3CN. In all cases, Eyring analysis provided large and negative ΔS‡ values that point toward unfavorable entropic factors as the key contributor to the barrier that underlies the slow recombination kinetics that are generally observed at dye-sensitized TiO2 interfaces

Reductive Elimination of Alkylamines from Low-Valent, Alkylpalladium(II) Amido Complexes

A series of three-coordinate norbornylpalladium amido complexes ligated by bulky N-heterocyclic carbene (NHC) ligands were prepared that undergo reductive eliminations to form the alkyl–nitrogen bond of alkylamine products. The rates of reductive elimination reveal that complexes containing more-electron-donating amido groups react faster than those with less-electron-donating amido groups, and complexes containing more-sterically bulky amido groups undergo reductive elimination more slowly than complexes containing less-sterically bulky amido groups. Complexes ligated by more-electron-donating ancillary NHC ligands undergo reductive elimination faster than complexes ligated by less-electron-donating NHC ligands. In contrast to the reductive elimination of benzylamines from bisphosphine-ligated palladium amides, these reactions occur with retention of configuration at the alkyl group, indicating that these reductive eliminations proceed by a concerted pathway. The experimentally determined free energy barrier of 26 kcal/mol is close to the computed free energy barrier of 23.9 kcal/mol (363 K) for a concerted reductive elimination from the isolated, three-coordinate NHC-ligated palladium anilido complex

Reductive Elimination of Alkylamines from Low-Valent, Alkylpalladium(II) Amido Complexes

A series of three-coordinate norbornylpalladium amido complexes ligated by bulky N-heterocyclic carbene (NHC) ligands were prepared that undergo reductive eliminations to form the alkyl–nitrogen bond of alkylamine products. The rates of reductive elimination reveal that complexes containing more-electron-donating amido groups react faster than those with less-electron-donating amido groups, and complexes containing more-sterically bulky amido groups undergo reductive elimination more slowly than complexes containing less-sterically bulky amido groups. Complexes ligated by more-electron-donating ancillary NHC ligands undergo reductive elimination faster than complexes ligated by less-electron-donating NHC ligands. In contrast to the reductive elimination of benzylamines from bisphosphine-ligated palladium amides, these reactions occur with retention of configuration at the alkyl group, indicating that these reductive eliminations proceed by a concerted pathway. The experimentally determined free energy barrier of 26 kcal/mol is close to the computed free energy barrier of 23.9 kcal/mol (363 K) for a concerted reductive elimination from the isolated, three-coordinate NHC-ligated palladium anilido complex