Bicyclic compounds bearing a quaternary stereogenic center have been obtained using asymmetric intramolecular Buchner reaction with excellent yields and level of enantioselectivity. X-ray ...crystallography determination of the absolute configuration of one product has led to the serendipitous observation of an unusual behavior within the crystal structure, with equilibrating norcaradiene and cycloheptatriene valence isomers at the solid state, as well as an even more unexpected intermediate form. DFT calculations were performed to support these observations.
A mononuclear nonheme iron(V)-imido complex bearing a tetraamido macrocyclic ligand (TAML), FeV(NTs)(TAML)− (1), was oxidized by one-electron oxidants, affording formation of an iron(V)-imido TAML ...cation radical species, FeV(NTs)(TAML+•) (2); 2 is a diamagnetic (S = 0) complex, resulting from the antiferromagnetic coupling of the low-spin iron(V) ion (S = 1/2) with the one-electron oxidized ligand (TAML+•). 2 is a competent oxidant in C–H bond functionalization and nitrene transfer reaction, showing that the reactivity of 2 is greater than that of 1.
A mononuclear nonheme iron(IV)–amido complex bearing a tetraamido macrocyclic ligand, (TAML)FeIV(NHTs)− (1), was synthesized via a hydrogen atom (H atom) abstraction reaction of an iron(V)–imido ...complex, (TAML)FeV(NTs)− (2), and fully characterized using various spectroscopies. We then investigated (1) the pK a of 1, (2) the reaction of 1 with a carbon-centered radical, and (3) the H atom abstraction reaction of 1. To the best of our knowledge, the present study reports for the first time the synthesis and chemical properties/reactions of a high-valent iron(IV)–amido complex.
Stable complexes of pentavalent uranyl UO2(salan- t Bu2)(py)K n (3), UO2(salan- t Bu2)(py)K(18C6) (4), and UO2(salophen- t Bu2)(thf)K(thf)2} n (8) have been synthesized from the reaction of the ...complex {UO2py5KI2py2} n (1) with the bulky amine-phenolate ligand potassium salt K2(salan- t Bu2) or the Schiff base ligand potassium salt K2(salophen- t Bu2) in pyridine. They were characterized by NMR, IR, elemental analysis, single crystal X-ray diffraction, UV−vis spectroscopy, cyclic voltammetry, low-temperature EPR, and variable-temperature magnetic susceptibility. X-ray diffraction shows that 3 and 8 are polymeric and 4 is monomeric. Crystals of the monomeric complex UVO2(salan- t Bu2)(py)Cp*2Co, 6, were also isolated from the reduction of UVIO2(salan- t Bu2)(py), 5, with Cp*2Co. Addition of crown ether to 1 afforded the highly soluble pyridine stable species UO2py5I·py (2). The measured redox potentials E 1/2 (UVI/UV) are significantly different for 2 (−0.91 and −0.46 V) in comparison with 3, 4, 5, 7 and 9 (in the range −1.65 to −1.82 V). Temperature-dependent magnetic susceptibility data are reported for 4 and 7 and give μeff of 2.20 and 2.23 μB at 300 K respectively, which is compared with a μeff of 2.6(1) μB (300 K) for 2. Complexes 1 and 2 are EPR silent (4 K) while a rhombic EPR signal (g x = 1.98; g y = 1.25; g z = 0.74 (at 4 K) was measured for 4. The magnetic and the EPR data can be qualitatively analyzed with a simple crystal field model where the f electron has a nonbonding character. However, the temperature dependence of the magnetic susceptibility data suggests that one or more excited states are relatively low-lying. DFT studies show unambiguously the presence of a significant covalent contribution to the metal−ligand interaction in these complexes leading to a significant lowering of the πu*. The presence of a back-bonding interaction is likely to play a role in the observed solution stability of the UO2(salan- t Bu2)(py)K and UO2(salophen- t Bu2)(py)K complexes with respect to disproportionation and hydrolysis.
Four organic dyes specifically designed for application in dye-sensitized solar cells are reported. These dyes are based on a perfectly identical pi -conjugated backbone and only differ by the number ...and the nature of the alkyl chain substituents employed as solubilizing groups. These sensitizers have outstanding light absorption properties in the visible range up to 750 nm on TiO2 and they exhibit quite similar energy level positions. Solar cells fabricated and characterized under exactly the same conditions with iodine-based liquid electrolytes show performances ranging from 6.53% to 9.05%. We highlight that the nature and the number of solubilizing groups have a tremendous impact on the performances of solar cells. The differences in the performance of the four sensitizers can be correlated with the current generation, the formation of aggregates and the dye-loading on the electrodes. We also report solar cells with ionic liquid electrolytes that demonstrate power conversion efficiencies up to 7.81% and a good stability of the performances under accelerated ageing conditions for 7300 hours. We establish that the number and the type of solubilizing groups attached on the pi -conjugated backbone of the dyes have a strong influence not only on the performances but also on the lifetimes of the devices; i.e. the dyes that contain lower ratios of alkyl groups lead to the more stable cells.
A new strategy for developing dye‐sensitised solar cells (DSSCs) by combining thin porous zinc tin oxide (Zn2SnO4) fiber‐based photoelectrodes with purely organic sensitizers is presented. The ...preparation of highly porous Zn2SnO4 electrodes, which show high specific surface area up to 124 m2/g using electrospinning techniques, is reported. The synthesis of a new organic donor‐conjugate‐acceptor (D‐π‐A) structured orange organic dye with molar extinction coefficient of 44 600 M−1 cm−1 is also presented. This dye and two other reference dyes, one organic and a ruthenium complex, are employed for the fabrication of Zn2SnO4 fiber‐based DSSCs. Remarkably, organic dye‐sensitized DSSCs displayed significantly improved performance compared to the ruthenium complex sensitized DSSCs. The devices based on a 3 μm‐thick Zn2SnO4 electrode using the new sensitizer in conjunction with a liquid electrolyte show promising photovoltaic conversion up to 3.7% under standard AM 1.5G sunlight (100 mW cm−2). This result ranks among the highest reported for devices using ternary metal oxide electrodes.
Dye‐sensitized solar cells (DSSCs) combining Zn2SnO4 fiber‐based photoelectrodes and purely organic sensitizers are fabricated. The highly porous amorphous Zn2SnO4 electrodes are prepared using electrospinning techniques and combined with a new organic dye. Using this strategy, devices that show photovoltaic conversion up to 3.7% are produced. The results rank among the highest reported for devices using ternary metal oxide sensitized electrodes.
The development of iron catalysts for carbon–heteroatom bond formation, which has attracted strong interest in the context of green chemistry and nitrene transfer, has emerged as the most promising ...way to versatile amine synthetic processes. A diiron system was previously developed that proved efficient in catalytic sulfimidations and aziridinations thanks to an FeIIIFeIV active species. To deal with more demanding benzylic and aliphatic substrates, the catalyst was found to activate itself to a FeIIIFeIVL. active species able to catalyze aliphatic amination. Extensive DFT calculations show that this activation event drastically enhances the electron affinity of the active species to match the substrates requirements. Overall this process consists in a redox self‐adaptation of the catalyst to the substrate needs.
Self‐adaptive catalyst: An efficient diiron catalyst mediates nitrene transfer to sulfides through an FeIV active state but self‐activates to FeV when facing aliphatic substrates that are harder to oxidize.
Metal‐catalyzed nitrene transfer reactions arouse intense interest as clean and efficient procedures for amine synthesis. Efficient Rh‐ and Ru‐based catalysts exist but Fe alternatives are actively ...pursued. However, reactive iron imido species can be very short‐lived and getting evidence of their occurrence in efficient nitrene‐transfer reactions is an important challenge. We recently reported that a diiron(III,II) complex is a very efficient nitrene‐transfer catalyst to various substrates. We describe herein how, by combining desorption electrospray ionization mass spectrometry, quantitative chemical quench experiments, and DFT calculations, we obtained conclusive evidence for the occurrence of an {FeIIIFeIVNTosyl} intermediate that is very active in H‐ion and nitrene‐transfer reactions. DFT calculations revealed a strong radical character of the tosyl nitrogen atom in very low‐lying electronic configurations of the FeIV ion which are likely to confer its high reactivity.
Nitrene transfer: An FeIIIFeIV imido intermediate is identified in nitrene‐transfer reactions by desorption electrospray ionization mass spectrometry (DESI‐MS). DFT calculations show that low‐lying FeIIIFeIII‐.N‐tosyl configurations play a major role in the high reactivity of the intermediate.
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