Pi-bonded quinonoid transition-metal complexes

Coordination of the carbocyclic ring of hydroquinones to electrophilic transition-metal fragments such as Mn(CO)3+ and Rh(COD)+ produces stable pi-bonded eta6-complexes that are activated to facile reversible deprotonation of the -OH groups. The deprotonations are accompanied by electron transfer to the transition metal, which acts as an internal oxidizing agent or electron sink. With manganese as the metal, the resulting eta5-semiquinone and eta4-quinone complexes have been used to synthesize one- two- and three-dimensional polymeric metal-organometallic coordination networks. With rhodium as the metal, the pi-quinonoid complexes have been demonstrated to play a unique role in multifunctional C-C coupling catalysis and in the synthesis of new organolithium reagents. Both classes of pi-quinonoid complexes appear to have significant applications in nanochemistry by providing an excellent vehicle for templating the directed self-assembly of nanoparticles into functional materials.     

Organometallic crystal engineering of [(1,4- and 1,3-hydroquinone)Rh(P(OPh)3)2]BF4 by charge assisted hydrogen bonding

The ionic complexes [(1,4- and 1,3-hydroquinone)Rh(P(OPh)3)2]BF4 form porous organometallic structures dictated by charge assisted hydrogen bonding.     

Ab initio optical rotatory dispersion and electronic circular dichroism spectra of (S)-2-chloropropionitrile

Coupled-cluster and density-functional methods have been used to determine specific rotations and electronic circular dichroism (ECD) rotational strengths for (S)-2-chloropropionitrile. Coupled-cluster specific rotations using both the length- and velocity-gauge representations of the electric-dipole operator, computed with basis sets of triple-zeta quality containing up to 326 functions, compare very well with recently reported gas-phase cavity-ring-down polarimetry data. ECD rotational strengths for the six lowest-lying excited states are found to vary in sign, and the second excited state, which has a larger rotational strength than the first by a factor of 4, was found to yield a much larger contribution (by a factor of 10) to the overall negative specific rotation observed both experimentally and theoretically. However, both valence and Rydberg states appear to make substantial contributions to the total rotation, often of opposite sign from the converged/linear-response result. Furthermore, the sum-over-states approach was found to be inadequate for reproducing the specific rotations derived from the linear-response approach, even when 100 excited states (well beyond the estimated ionization limit) were included in the summation. Density-functional specific rotations using the B3LYP functional with basis sets of quadruple-zeta quality containing up to 588 functions are found to be too large compared to experiment by approximately a factor of 2. This error appears to be related to both the underestimation of the electronic excitation energies, as well as concomitant overestimation of the corresponding ECD rotational strengths. Although earlier studies reported good agreement between density-functional specific rotations and experiment when electric-field-dependent functions were used in conjunction with a double-zeta-quality basis set, the results reported here, which are near the basis-set limit, suggest that this agreement may be fortuitous.     

Observation of the semileptonic decays B-->D*tau-nutau and evidence for B-->Dtau-nutau

We present measurements of the semileptonic decays B--->D0tau-nutau, B--->D*0tau-nutau, B0-->D+tau-nutau, and B0-->D*+tau-nutau, which are potentially sensitive to non-standard model amplitudes. The data sample comprises 232x10(6) Upsilon(4S)-->BB decays collected with the BABAR detector. From a combined fit to B- and B0 channels, we obtain the branching fractions B(B-->Dtau-nutau)=(0.86+/-0.24+/-0.11+/-0.06)% and B(B-->D*tau-nutau)=(1.62+/-0.31+/-0.10+/-0.05)% (normalized for the B0), where the uncertainties are statistical, systematic, and normalization-mode-related.     

Redox Communication within Multinuclear Iron–Sulfur Complexes Related to Electronic Interplay in the Active Site of [FeFe]Hydrogenase

The one‐electron oxidations of a Fe₂ complex lead to the formation of a persistent metal‐stabilized thiyl radical Fe₂ species, mixed‐valent Fe₄, and Fe₈ complexes. The unpaired spin in the Fe₂ radical species delocalizes over the Fe₂ and the aromatic dithiolate, mostly on the terminal sulfur. The subsequent dimerization of the singly oxidized Fe₂ to the Fe₄ retains the partial thiyl radical character. For an analogue with less steric hindrance, the π–π stacking interaction between the dithiolato aromatic rings induces generation of the Fe₈, in which process electronic structures of the species are modulated through reducing the thiyl radical to the thiolate. Electronic reorganization repeats when the Fe₈ is converted to Fe₄. Electronic interplay in the complexes decreases the energy gap of frontier MOs and buffers electronic impacts upon redox events. Easier accessible redox potentials and increased stability of the species are facilitated. The results demonstrate that electronic versatility of the benzenedithiolate exerts pronounced influences on electronic and coordination structure of the metal complexes.     

Transition Metal Free Cycloamination of Prenyl Carbamates and Ureas Promoted by Aryldiazonium Salts

Upon treatment with aryldiazonium salts, prenyl carbamates and ureas undergo redox‐neutral azocycloamination. In general, N‐aryl O‐prenyl carbamates cyclize in a photocatalytic reaction with visible light and an organic dye. With electron‐deficient diazonium salts, electronic matching with an electron‐rich N‐aryl substituent results in a reaction proceeding in the ground state, without either light or photocatalyst. Cyclic voltammetry suggests that this radical reaction is initiated by hydrogen‐atom abstraction mediated by an aryl radical, followed by a radical addition cascade and proton‐coupled hole propagation. The reaction proceeds at room temperature in short reaction times, and a range of functional groups is tolerated.     

An Extremal Characterization of the Incidence Graphs of Projective Planes

Let G be a 4-cycle free, bipartite graph on 2n vertices with partitions of equal cardinality n. Let c₆(G) denote the number of cycles of length 6 in G. We prove that for n 3, c₆(G) , where , with equality if and only if G is the incidence point-line graph of a projective plane.     

Transport properties of copper-doped carbon aerogels

The magnetic susceptibility, conductivity, magnetoresistance (MR) and Hall effect of copper-doped carbon aerogels are measured and compared with corresponding results from the original carbon aerogels. The experimental results indicate that the temperature-dependent magnetic susceptibility of the copper-doped and of the original carbon aerogels is well fit by a Curie function at low temperatures. The copper-doped carbon aerogels show a higher susceptibility and spin concentration than the original carbon aerogel. After doping by copper, the materials exhibit a more linear current-voltage curve than the original carbon aerogel under the same measurement conditions. The electrical resistance of the copper-doped carbon aerogels is strikingly lower than that of the original carbon aerogels, and decreases with increasing copper content in the samples. The temperature-dependent resistivity ρ(T) of all of the copper-doped and original carbon aerogels can be fitted by an exp(T^−1/2) dependence for T<100 K. The copper-doped and pristine carbon aerogels follow a quadratic MR behavior Δρ/ρ=AB² in the magnetic field range B investigated (up to 5 T), except at very low temperatures (T<4 K).