Hexa-Fe(III) Carboxylate Complexes Facilitate Aerobic Hydrocarbon Oxidative Functionalization: Rh Catalyzed Oxidative Coupling of Benzene and Ethylene to Form Styrene.

Fe(II) carboxylates react with dioxygen and carboxylic acid to form Fe6(µ-OH)2(µ3-O)2(µ-X)12(HX)2 (X = acetate or pivalate), which is an active oxidant for Rh-catalyzed arene alkenylation. Heating (150-200 °C) the catalyst precursor [(η2-C2H4)2Rh(µ-OAc)]2 with ethylene, benzene, Fe(II) carboxylate, and dioxygen yields styrene >30-fold faster than the reaction with dioxygen in the absence of the Fe(II) carboxylate additive. It is also demonstrated that Fe6(µ-OH)2(µ3-O)2(µ-X)12(HX)2 is an active oxidant under anaerobic conditions, and the reduced material can be reoxidized to Fe6(µ-OH)2(µ3-O)2(µ-X)12(HX)2 by dioxygen. At optimized conditions, a turnover frequency of ∼0.2 s-1 is achieved. Unlike analogous reactions with Cu(II) carboxylate oxidants, which undergo stoichiometric Cu(II)-mediated production of phenyl esters (e.g., phenyl acetate) as side products at temperatures ≥150 °C, no phenyl ester side product is observed when Fe carboxylate additives are used. Kinetic isotope effect experiments using C6H6 and C6D6 give k H/k D = 3.5(3), while the use of protio or monodeutero pivalic acid reveals a small KIE with k H/k D = 1.19(2). First-order dependencies on Fe(II) carboxylate and dioxygen concentration are observed in addition to complicated kinetic dependencies on the concentration of carboxylic acid and ethylene, both of which inhibit the reaction rate at a high concentration. Mechanistic studies are consistent with irreversible benzene C-H activation, ethylene insertion into the formed Rh-Ph bond, ß-hydride elimination, and reaction of Rh-H with Fe6(µ-OH)2(µ3-O)2(µ-X)12(HX)2 to regenerate a Rh-carboxylate complex.

Pd(II) and Rh(I) Catalytic Precursors for Arene Alkenylation: Comparative Evaluation of Reactivity and Mechanism Based on Experimental and Computational Studies.

We combine experimental and computational investigations to compare and understand catalytic arene alkenylation using the Pd(II) and Rh(I) precursors Pd(OAc)2 and [(η2-C2H4)2Rh(µ-OAc)]2 with arene, olefin, and Cu(II) carboxylate at elevated temperatures (>120 °C). Under specific conditions, previous computational and experimental efforts have identified heterotrimetallic cyclic PdCu2(η2-C2H4)3(µ-OPiv)6 and [(η2-C2H4)2Rh(µ-OPiv)2]2(µ-Cu) (OPiv = pivalate) species as likely active catalysts for these processes. Further studies of catalyst speciation suggest a complicated equilibrium between Cu(II)-containing complexes containing one Rh or Pd atom with complexes containing two Rh or Pd atoms. At 120 °C, Rh catalysis produces styrene >20-fold more rapidly than Pd. Also, at 120 °C, Rh is ∼98% selective for styrene formation, while Pd is ∼82% selective. Our studies indicate that Pd catalysis has a higher predilection toward olefin functionalization to form undesired vinyl ester, while Rh catalysis is more selective for arene/olefin coupling. However, at elevated temperatures, Pd converts vinyl ester and arene to vinyl arene, which is proposed to occur through low-valent Pd(0) clusters that are formed in situ. Regardless of arene functionality, the regioselectivity for alkenylation of mono-substituted arenes with the Rh catalyst gives an approximate 2:1 meta/para ratio with minimal ortho C-H activation. In contrast, Pd selectivity is significantly influenced by arene electronics, with electron-rich arenes giving an approximate 1:2:2 ortho/meta/para ratio, while the electron-deficient (α,α,α)-trifluorotoluene gives a 3:1 meta/para ratio with minimal ortho functionalization. Kinetic intermolecular arene ethenylation competition experiments find that Rh reacts most rapidly with benzene, and the rate of mono-substituted arene alkenylation does not correlate with arene electronics. In contrast, with Pd catalysis, electron-rich arenes react more rapidly than benzene, while electron-deficient arenes react less rapidly than benzene. These experimental findings, in combination with computational results, are consistent with the arene C-H activation step for Pd catalysis involving significant η1-arenium character due to Pd-mediated electrophilic aromatic substitution character. In contrast, the mechanism for Rh catalysis is not sensitive to arene-substituent electronics, which we propose indicates less electrophilic aromatic substitution character for the Rh-mediated arene C-H activation.

Efficiency Considerations for SnO2-Based Dye-Sensitized Solar Cells.

A comparative study of mesoporous thin films based on SnO2 (rutile) and TiO2 (anatase) nanocrystallites sensitized to visible light with [Ru(dtb)2(dcb)](PF6)2, where dtb = 4,4'-(tert-butyl)2-2,2'-bipyridine and dcb = 4,4'-(CO2H)2-2,2'-bipyridine, in CH3CN electrolyte solutions is reported to identify the reason(s) for the low efficiency of SnO2-based dye-sensitized solar cells (DSSCs). Pulsed laser excitation resulted in rapid excited state injection (kinj > 108 s-1) followed by sensitizer regeneration through iodide oxidation to yield an interfacial charge separated state abbreviated as MO2(e-)|Ru + I3-. Spectral features associated with I3- and the injected electron MO2(e-) were observed as well as a hypsochromic shift of the metal-to-ligand charge-transfer absorption of the sensitizer attributed to an electric field. The field magnitude ranged from 0.008 to 0.39 MV/cm and was dependent on the electrolyte cation (Mg2+ or Li+) as well as the oxide material. Average MO2(e-) + I3- → recombination rate constants quantified spectroscopically were about 25 times smaller for SnO2 (6.0 ± 0.14 s-1) than for TiO2 (160 ± 10 s-1). Transient photovoltage measurements of operational DSSCs indicated a 78 ms lifetime for electrons injected into SnO2 compared to 27 ms for TiO2; behavior that is at odds with the view that recombination with I3- underlies the low efficiencies of nanocrystalline SnO2-based DSSCs. In contrast, the average rate constant for charge recombination with the oxidized sensitizer, MO2(e-)|-S+ → MO2|-S, was about 2 orders of magnitude larger for SnO2 (k = 9.8 × 104 s-1) than for TiO2 (k = 1.6 × 103 s-1). Sensitizer regeneration through iodide oxidation were similar for both oxide materials (kreg = 6 ± 1 × 1010 M-1 s-1). The data indicate that enhanced efficiency from SnO2-based DSSCs can be achieved by identifying alternative redox mediators that enable rapid sensitizer regeneration and by inhibiting recombination of the injected electron with the oxidized sensitizer.

Electron Localization and Transport in SnO2/TiO2 Mesoporous Thin Films: Evidence for a SnO2/SnxTi1-xO2/TiO2 Structure.

A study of SnO2/TiO2 core/shell films was undertaken to investigate the influences of shell thickness and post deposition sintering on electron localization and transport properties. Electrochemical reduction of the materials resulted in the appearance of a broad visible-near IR absorbance that provided insights into the electronic state(s) within the core/shell structures. As the shell thickness was increased from 0.5 to 5 nm, evidence for the presence of a SnxTi1-xO2 interfacial state emerged that was physically located between the core and the shell. The lifetime of photoinjected electrons increased with the shell thickness. Electron transport occurred through the SnO2 core; however, when materials with shell thicknesses ≥2 nm were annealed at 450 °C, a new electron transport pathway through the shell was evident. The data indicate that these materials are best described as SnO2/SnxTi1-xO2/TiO2 where electrons preferentially localize in a SnxTi1-xO2 interfacial state and transport through SnO2 and annealed TiO2 (if present). The implications of these results for applications in solar energy conversion are discussed.