Sequential electron-transfer and proton-transfer pathways in hydride-transfer reactions from dihydronicotinamide adenine dinucleotide analogues to non-heme oxoiron(IV) complexes and p-chloranil. Detection of radical cations of NADH analogues in acid-promoted hydride-transfer reactions

Hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) analogues, such as 10-methyl-9,10-dihydroacridine (AcrH 2) and its derivatives, 1-benzyl-1,4-dihydronicotinamide (BNAH), and their deuterated compounds, to non-heme oxoiron(IV) complexes such as [(L)Fe (IV)(O)] (2+) (L = N4Py, Bn-TPEN, and TMC) occurs to yield the corresponding NAD (+) analogues and non-heme iron(II) complexes in acetonitrile. Hydride transfer from the NADH analogues to p-chloranil (Cl 4Q) also occurs to produce the corresponding NAD (+) analogues and the hydroquinone anion (Cl 4QH (-)). The logarithms of the observed second-order rate constants (log k H) of hydride transfer from NADH analogues to non-heme oxoiron(IV) complexes are linearly correlated with those of hydride transfer from the same series of NADH analogues to Cl 4Q, including similar kinetic deuterium isotope effects. The log k H values of hydride transfer from NADH analogues to non-heme oxoiron(IV) complexes are also linearly correlated with those of deprotonation of the radical cations of NADH analogues. Such linear correlations indicate that overall hydride-transfer reactions of NADH analogues to both non-heme oxoiron(IV) complexes and Cl 4Q occur via electron transfer from NADH analogues to the oxoiron(IV) complexes, followed by rate-limiting deprotonation from the radical cations of NADH analogues and subsequent rapid electron transfer from the deprotonated radicals to the Fe(III) complexes to yield the corresponding NAD (+) analogues and the Fe(II) complexes. The electron-transfer pathway was accelerated by the presence of perchloric acid, and the resulting radical cations of NADH analogues were detected by electron spin resonance spectroscopy and UV-vis spectrophotometry in the acid-promoted hydride-transfer reactions from NADH analogues to non-heme oxoiron(IV) complexes. This result provides the first direct evidence that a hydride transfer from NADH analogues to non-heme oxoiron(IV) complexes proceeds via an electron-transfer pathway.     

"Umpolung" photoinduced charge separation in an anion-bound supramolecular complex

A new supramolecular system, consisting of an expanded porphyrin, cyclo[8]pyrrole (C8) and a pyrene carboxylate (Py) is capable not only of electron transfer upon photoexcitation, but results in "umpolung" or a reversal of the predicted charge-separation behavior in that the higher energy radical ion pair, C8.+-Py.- (2.58 eV) is formed as opposed to C8.--Py.+ (1.31 eV). This observation is rationalized in terms of the latter radical ion pair, C8.--Py.+ being in the Marcus inverted region. Transient absorption spectral studies reveal a charge-separated lifetime of 300 micros, which decays to the triplet state of cyclo[8]pyrrole, also an exceedingly long-lived species. The novel features of this noncovalent dyad highlight the potential utility of anion binding in the construction of supramolecular electron transfer systems.     

Spectroscopic characterization of photolytically generated radical ion pairs in single-wall carbon nanotubes bearing surface-immobilized tetrathiafulvalenes

We succeeded in establishing for the first time a conclusive spectroscopic signature for reduced single-wall carbon nanotubes (SWNT), which evolves from electron donor-acceptor interactions between SWNT and electron-donating pi-extended tetrathiafulvalene (exTTF). In particular, pi-pi interactions were employed to anchor the electron donor to the surface of SWNT. New conduction band electrons, injected from photoexcited exTTF, shift the transitions that are associated with the van Hove singularities to lower energies.     

Face-to-face pacman-type porphyrin-fullerene dyads: design, synthesis, charge-transfer interactions, and photophysical studies

Pacman-type face-to-face zinc-porphyrin-fullerene dyads have been newly synthesized and studied. Owing to the close proximity of the donor and acceptor entities, strong pi-pi intramolecular interactions between the porphyrin and fullerene entities resulted in modulating the spectral and electrochemical properties of the dyads. New absorption and emission bands that correspond to the charge-transfer interactions were observed in the near-IR region. Time-resolved transient absorption studies revealed efficient photoinduced electron transfer from the singlet excited porphyrin to the fullerene entity. The rate constants for photoinduced electron transfer are analyzed in terms of the Marcus theory of electron transfer, which afforded a large electron coupling matrix element (V=140 cm(-1)) for the face-to-face dyads. As a consequence of the large charge-recombination driving force in the Marcus inverted region, a relatively long lifetime of the charge-separated state has been achieved.     

Palladium‐Catalyzed Synthesis of Heteroarene‐Fused Cyclooctatetraenes through Dehydrogenative Cyclodimerization

Arene‐fused cyclooctatetraenes (COTs) possess unique structural and electronic properties that originate from their saddle‐shaped π‐conjugated architectures. Considerable attention has been paid to the transition‐metal‐mediated synthesis of these cyclic compounds; however, there have been limited achievements to date in the efficient construction of heteroarene‐fused COTs. In this contribution, we report a novel Pd‐catalyzed dehydrogenative cyclodimerization of biheteroarenes through four‐fold C−H activation toward the synthesis of a series of heteroarene‐fused COTs. A set of mechanistic investigations indicated the involvement of high‐valent Pd species prior to the dimerization event in the catalytic cycle. The redox behavior of the obtained COTs is also described briefly.     

ChemInform Abstract: Structural Phase Transitions in EuNbO3 Perovskite.

Polycrystalline EuNbO₃ samples are prepared by solid state reaction of stoichiometric amounts of Eu₂O₃ and Nb₂O₃ (air, 1400 °C, 1 d) to form Eu₃NbO₇ which in a second step is sintered with stoichiometric amounts of NbO₂ and Nb (Mo foil enwrapped in evacuated silica tubes, 1200 °C, 12 h).     

Solvent‐Free One‐Step Photochemical Hydroxylation of Benzene Derivatives by the Singlet Excited State of 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone Acting as a Super Oxidant

Photoinduced hydroxylation of neat deaerated benzene to phenol occurred under visible‐light irradiation of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ), which acts as a super photooxidant in the presence of water. Photocatalytic solvent‐free hydroxylation of benzene derivatives with electron‐withdrawing substituents such as benzonitrile, nitrobenzene, and trifluoromethylbenzene used as neat solvents has been achieved for the first time by using DDQ as a super photooxidant to yield the corresponding phenol derivatives and 2,3‐dichloro‐5,6‐dicyanohydroquinone (DDQH₂) in the presence of water under deaerated conditions. In the presence of dioxygen and tert‐butyl nitrite, the photocatalytic hydroxylation of neat benzene occurred with DDQ as a photocatalyst to produce phenol. The photocatalytic reactions are initiated by oxidation of benzene derivatives with the singlet and triplet excited states of DDQ to form the corresponding radical cations, which associate with benzene derivatives to produce the dimer radical cations, which were detected by the femto‐ and nanosecond laser flash photolysis measurements to clarify the photocatalytic reaction mechanisms. Radical cations of benzene derivatives react with water to yield the OH‐adduct radicals. On the other hand, DDQ ^{. ?} produced by the photoinduced electron transfer from benzene derivatives reacts with the OH‐adduct radicals to yield the corresponding phenol derivatives and DDQH₂. DDQ is recovered by the reaction of DDQH₂ with tert‐butyl nitrite when DDQ acts as a photocatalyst for the hydroxylation of benzene derivatives by dioxygen.     

High Catalytic Activity of Heteropolynuclear Cyanide Complexes Containing Cobalt and Platinum Ions: Visible‐Light Driven Water Oxidation

A near‐stoichiometric amount of O₂ was evolved as observed in the visible‐light irradiation of an aqueous buffer (pH 8) containing [Ru^II(2,2′‐bipyridine)₃] as a photosensitizer, Na₂S₂O₈ as a sacrificial electron acceptor, and a heteropolynuclear cyanide complex as a water‐oxidation catalyst. The heteropolynuclear cyanide complexes exhibited higher catalytic activity than a polynuclear cyanide complex containing only Co^III or Pt^IV ions as C‐bound metal ions. The origin of the synergistic effect between Co and Pt ions is discussed in relation to electronic and local atomic structures of the complexes.     

Modulation of Energy Conversion Processes in Carbonaceous Molecular Bearings

The energetics and photodynamics of carbonaceous molecular bearings with discrete molecular structures were investigated. A series of supramolecular bearings comprising belt‐persistent tubular cycloarylene and fullerene molecules accepted photonic stimuli to afford charge‐separated species via a photoinduced electron transfer process. The energy conversion processes associated with the photoexcitation, however, differed depending on the molecular structure. A π‐lengthened tubular molecule allowed for the emergence of an intermediary triplet excited state at the bearing, which should lead to an energy conversion to thermal energy. On the other hand, low‐lying charge‐separated species induced by an endohedral lithium ion in fullerene enabled back electron transfer processes to occur without involving triplet excited species. The structure–photodynamics relationship was analyzed in terms of the Marcus theory to reveal a large electronic coupling in this dynamic supramolecular system.