Reduction of CO to Methanol with Recyclable Organic Hydrides.

The reaction steps for the selective conversion of a transition metal carbonyl complex to a hydroxymethyl complex that releases methanol upon irradiation with visible light have been successfully quantified in acetonitrile solution with dihydrobenzimidazole organic hydride reductants. Dihydrobenzimidazole reductants have been shown to be inactive toward H2 generation in the presence of a wide range of proton sources and have been regenerated electrochemically or photochemically. Specifically, the reaction of cis-[Ru(bpy)2(CO)2]2+ (bpy = 2,2'-bipyridine) with one equivalent of a dihydrobenzimidazole quantitatively yields a formyl complex, cis-[Ru(bpy)2(CO)(CHO)]+, and the corresponding benzimidazolium on a seconds time scale. Kinetic experiments revealed a first-order dependence on the benzimidazole hydride concentration and an unusually large kinetic isotope effect, inconsistent with direct hydride transfer and more likely to occur by an electron transfer-proton-coupled electron transfer (EΤ-PCET) or related mechanism. Further reduction/protonation of cis-[Ru(bpy)2(CO)(CHO)]+ with two equivalents of the organic hydride yields the hydroxymethyl complex cis-[Ru(bpy)2(CO)(CH2OH)]+. Visible light excitation of cis-[Ru(bpy)2(CO)(CH2OH)]+ in the presence of excess organic hydride was shown to yield free methanol. Identification and quantification of methanol as the sole CO reduction product was confirmed by 1H NMR spectroscopy and gas chromatography. The high selectivity and mild reaction conditions suggest a viable approach for methanol production from CO, and from CO2 through cascade catalysis, with renewable organic hydrides that bear similarities to Nature's NADPH/NADP+.

Porphyrins as Photoredox Catalysts in Csp2-H Arylations: Batch and Continuous Flow Approaches.

We have investigated both batch and continuous flow photoarylations of enol-acetates to yield different α-arylated aldehyde and ketone building blocks by using diazonium salts as the aryl-radical source. Different porphyrins were used as SET photocatalysts, and photophysical as well as electrochemical studies were performed to rationalize the photoredox properties and suggest mechanistic insights. Notably, the most electron-deficient porphyrin ( meso-tetra(pentafluorophenyl)porphyrin) shows the best photoactivity as an electron donor in the triplet excited state, which was rationalized by the redox potentials of excited states and the turnover of the porphyrins in the photocatalytic cycle. A two-step continuous protocol and multigram-scale reactions are also presented revealing a robust, cost-competitive, and easy methodology, highlighting the significant potential of porphyrins as SET photocatalysts.

Mechanistic Insights into the Stepwise Assembly of Ruthenium(II) Tris-heteroleptic Compounds.

The ruthenium(II) tris-heteroleptic compounds cis-[Ru(NN)(dcbH2)(NCS)2], NN = polypyridyl ligand and dcbH2 = 2,2'-bipyridine-4,4'-dicarboxylic acid, can be synthesized by a one-pot route starting from [Ru( p-cymene)Cl2]2, followed by the sequential addition of ligands. In this work, each synthetic step for the cis-[Ru(R-phen)(dcbH2)(NCS)2] (R = H, Me, Ph, MeO, or Cl) preparation was individually investigated, aiming to identify reaction intermediates and to establish correlations among temperature, reaction time, reactant concentration, and the identity of the substituent of the polypyridyl ligand with the kinetics of the reactions and distribution of the products. The first step is the cleavage of [Ru( p-cymene)Cl2]2, followed by the coordination of R-phen via an associative mechanism and establishment of a direct correlation between the electron-donating or electron-withdrawing character of R and the reaction rates. The second step is the conversion of [Ru(R-phen)( p-cymene)Cl]Cl to cis-[Ru(R-phen)(dcbH2)Cl2], and the rate-determining step is the formation of the intermediate [Ru(R-phen)Cl2], which exhibits a low dependence on R. The last step is the substitution of Cl- by NCS-, and the N-bound isomer is the major product. The reaction temperature, time, and identity of R influence the relative distribution of the linkage isomers. The comprehension of each of these processes is a key factor to develop new strategies to optimize the one-pot synthetic route.

Impact of Molecular Orientation on Lateral and Interfacial Electron Transfer at Oxide Interfaces.

Molecular dyes, called sensitizers, with a cis-[Ru(LL)(dcb)(NCS)2] structure, where dcb is 4,4'-(CO2H)2-2,2'-bipyridine and LL is dcb or a different diimine ligand, are among the most optimal for application in dye-sensitized solar cells (DSSCs). Herein, a series of five sensitizers, three bearing two dcb ligands and two bearing one dcb ligand, were anchored to mesoporous thin films of conducting tin-doped indium oxide (ITO) or semiconducting TiO2 nanocrystallites. The number of dcb ligands impacts the surface orientation of the sensitizer; density functional theory (DFT) calculations revealed an ∼1.6 Å smaller distance between the oxide surface and the Ru metal center for sensitizers with two dcb ligands. Interfacial electron transfer kinetics from the oxide material to the oxidized sensitizer were measured as a function of the thermodynamic driving force. Analysis of the kinetic data with Marcus-Gerischer theory indicated that the electron coupling matrix element, Hab, was sensitive to distance and ranged from Hab = 0.23 to 0.70 cm-1, indicative of nonadiabatic electron transfer. The reorganization energies, λ, were also sensitive to the sensitizer location within the electric double layer and were smaller, with one exception, for sensitizers bearing two dcb ligands λ = 0.40-0.55 eV relative to those with one λ = 0.63-0.66 eV, in agreement with dielectric continuum theory. Electron transfer from the oxide to the photoexcited sensitizer was observed when the diimine ligand was more easily reduced than the dcb ligand. Lateral self-exchange "hole hopping" electron transfer between surface-anchored sensitizers was found to be absent for sensitizers with two dcb ligands, while those with only one were found to hop with rates similar to those previously reported in the literature, khh = 47-89 µs-1. Collectively, the kinetic data and analysis reveal that interfacial kinetics are highly sensitive to the surface orientation and sensitizers bearing two dcb ligands are most optimal for practical applications of DSSCs.

Inhibiting Charge Recombination in cis-Ru(NCS)2 Diimine Sensitizers with Aromatic Substituents.

A series of cis-[Ru(LL)(dcbH2)(NCS)2] compounds, where dcbH2 = 2,2'-bipyridine-4,4'-dicarboxylic acid and LL = 1,10-phenanthroline (Ru(phen)), 4,7-dipyrrole-1,10-phenanthroline (Ru(pyr)), 4,7-diindole-1,10-phenanthroline (Ru(ind)), or 4,7-dicarbazole-1,10-phenanthroline (Ru(cbz)), was investigated for application as sensitizers in mesoporous TiO2 dye-sensitized solar cells (DSSCs). A systematic increase in the number of rings of the aromatic substituents at the 4,7-positions of the 1,10-phenanthroline allowed tuning of the molecular size of the sensitizers and the energy stored in the excited state while maintaining the same ground-state Ru3+/2+ reduction potentials. These small structural changes had a significant influence on the rates and/or efficiencies of electron injection, back-electron transfer, recombination to oxidized mediators, lateral self-exchange electron transfer, and regeneration through iodide oxidation that were reflected in distinct photoelectrochemical performance of full operating DSSCs. The global efficiencies, open-circuit voltages, and short-circuit current densities of the DSSCs consistently followed the trend Ru(pyr) < Ru(ind) < Ru(phen) < Ru(cbz), and the most optimal performance of Ru(cbz) was ascribed to dramatically slower recombination to the oxidized redox mediators. Transient photovoltage and transient absorption experiments both revealed significantly slower recombination as the size of the aromatic substituents increased with Ru(cbz) providing the most promising behavior for application in dye sensitization.

Correlation Between Charge Recombination and Lateral Hole-Hopping Kinetics in a Series of cis-Ru(phen')(dcb)(NCS)2 Dye-Sensitized Solar Cells.

Four complexes of the general form cis-Ru(phen')(dcb)(NCS)2, where dcb is 4,4'-(CO2H)2-2,2'-bipyridine and phen' is 1,10-phenanthroline (phen), 4,7-(C6H5)2-phen (Ph2-phen), 4,7-(CH3)2-phen (Me2-phen), or 3,4,7,8-(CH3)4-phen (Me4-phen), were anchored to mesoporous TiO2 thin films for applications as sensitizers in dye-sensitized solar cells (DSSCs). The compounds displayed metal based reductions Eo(RuIII/II) = 1.01 ± 0.05 V vs NHE and were potent reductants competent of excited-state electron transfer to TiO2 with yields ϕinj ≥ 0.75 in acetonitrile electrolytes. Average charge recombination rate constants, kcr, abstracted from nanosecond transient absorption measurements, and the apparent diffusion coefficients for lateral hole-hopping, abstracted from chronabsorptometry measurements, showed the same sensitizer dependency: Ru(Me4-phen) > Ru(Ph2-phen) > Ru(Me2-phen) ≈ Ru(phen). When used in operational solar cells, Ru(Ph2-phen) was most optimal with an efficiency of (6.6 ± 0.5)% in ionic liquids under 1 sun illumination. The superior performance of Ru(Ph2-phen) was traced to a higher injection yield and more efficient regeneration due to an unusually small sensitivity of kcr to the number of injected electrons.