Fractal growth mechanism of sp3-bonded 5H-BN microcones by plasma-assisted pulsed-laser chemical vapor deposition

Here we propose a repetitive photochemical reaction and diffusion model for the fractal pattern formation of sp(3)-bonded 5H-BN microcones in laser-assisted plasma chemical vapor deposition, which was observed experimentally and reported previously. This model describing the behavior of the surface density of precursor species gave explanations to (1) the "line-drawing" nature of the patterns, (2) the origin of the scale-invariant self-similarity (fractality) of the pattern, and (3) the temperature-dependent uniform to fractal transition. The results have implications for controlling the self-organized arrangements of electron-emitter cones at the micro-and nanoscale by adjusting macroscopically the boundary condition (L(X),L(Y)) for the deposition, which will be very effective in improving the electron field emission properties.     

Triplet diphenylcarbenes protected by trifluoromethyl and bromine groups. A triplet carbene surviving a day in solution at room temperature

Two types of diphenyldiazomethanes having two trifluoromethyl and two bromine groups at the ortho positions, either in unsymmetrical or in symmetrical fashion, that is, (2,6-dibromo-4-phenylphenyl)[4-phenyl-2,6-bis(trifluoromethyl)phenyl]diazomethane (U-1-N2) and bis(2-bromo-4-phenyl-6-trifluoromethylphenyl)diazomethane (S-1-N2), are prepared. Triplet diphenylcarbenes (U-(3)1 or S-(3)1) are generated from those precursors and are characterized by ESR, UV/vis spectroscopy at low temperature, as well as time-resolved UV/vis spectroscopy at room temperature. Those carbenes are shown to be at least 2 orders of magnitude less reactive than the most stable triplet diphenylcarbene thus far known, that is, bis(2,6-dibromo-4-phenylphenyl)carbene. It has been also shown that S-(3)1 is significantly more stable than U-(3)1 even though both have the same two kinds of substituents. It is suspected that the perpendicular alignment of the two most bulky groups is a more effective way to shield the carbenic center than the planar one. By this way, triplet substituted diphenylcarbene surviving nearly a day in solution at room temperature is realized for the first time.     

Synthesis, isolation, and structural characterization of the C4h isomer of metal(1,2-naphthalocyanine) and its one-dimensional conductor of the axially substituted species

The C4h isomer of the 1,2-Nc (1,2-naphthalocyaninato) ligand has been efficiently isolated as a hydrated magnesium complex by the fractional crystallization from the benzene/acetone solution after treating the crude mixture of the four isomers (C4h, C(s), C2v, D2h) with benzene. The C4h symmetry has been confirmed by X-ray structure analysis. The central metal ion has been demetalated and subsequently converted to the Li2 complex followed by conversion to the cobalt(II) complex. Electrochemical oxidation of the Co(III)(1,2-Nc)(CN)2 anion prepared from the cobalt(II) complex with TPP (tetraphenylphosphonium) has yielded a partially oxidized salt, TPP[Co(III)(1,2-Nc-C4h(CN)2]2. The crystal comprises slipped stacked Co(III)(1,2-Nc-C4h)(CN)2 one-dimensional chains and one-dimensional arrays of TPPs. The conductivity at room temperature is 0.1 S cm(-1), and the temperature dependence is semiconducting with a small activation energy of about 0.05 eV. The positive temperature-independent value of about 60 microV deg(-1) observed in the thermoelectric power measurements suggests that the salt is in the correlated hopping regime.     

Al-MCM-41 catalyzed three-component Strecker-type synthesis of α-aminonitriles

Mesoporous aluminosilicate (Al-MCM-41) efficiently catalyzed the three-component Strecker-type reaction of benzylacetone and aniline with trimethylsilyl cyanide in CH₂Cl₂ at room temperature to afford the corresponding α-aminonitrile in excellent yields (up to 97%). Mesoporous silica (MCM-41), amorphous SiO₂-Al₂O₃, and H-Y and H-ZSM-5 zeolites also catalyzed this reaction, but gave the desired product in lower yields. The Al-MCM-41 catalyzed three-component Strecker-type reaction was applicable to a wide range of ketones, aldehydes, and amines. Furthermore, the Al-MCM-41 catalyst could be applied to a fixed-bed flow reactor: The desired α-aminonitrile derivative was constantly produced in nearly 80% yields for 48 h.     

Preparation of bis(diazo) compounds incorporated into butadiyne and thiophene units and generation and characterization of their photoproducts

(2,6-dimethyl-4-tert-butylphenyl)(2,4,6-tribromophenyl)diazomethane(-N(2)) was found to be stable enough to survive under Sonogashira coupling reaction conditions, and aryldiazomethyl substituents were introduced at the 1,4-positions of butadiyne (4-2N(2)) and the 2,5-positions of thiophene(5-2N(2)). Irradiation of those bis(diazo) compounds generated bis(carbenes), which were characterized by using ESR and UV/vis spectroscopic techniques in a matrix at low temperature as well as time-resolved UV/vis spectroscopy in solution at room temperature. These studies revealed that both of the bis(carbenes), 4 and 5, have singlet quinoidal diradical ground states with a very small singlet-triplet energy gap of less than 1 kcal mol(-1). A remarkable increase in the lifetime of bis(carbenes), as opposed to that of the monocarbene (2), was noted and was interpreted to indicate that bis(carbenes) are thermodynamically stabilized as a result of delocalization of unpaired electrons throughout the pi net framework. In spite of the stability, both bis(carbenes) are readily trapped by molecular oxygen to afford bis(ketones). Presumably, the reaction of the upper-lying localized quintet states with oxygen is much faster than that for lower-lying states.     

Tandem reduction-olefination for the stereoselective synthesis of (Z)-α-fluoro-α,β-unsaturated esters

A tandem stereoselective reduction-olefination reaction of ethyl 2-acyl-2-fluoro-2-diethylphosphonoacetate employing NaBH₄ in EtOH was developed. The one-pot reaction gave α-fluoro-α,β-unsaturated esters with excellent (Z)-selectivity. A plausible mechanism involving a diastereoselective reduction predicted by the Felkin-Anh model, followed by olefination similar to the Horner-Wadsworth-Emmons reaction, has been proposed.     

Charge Carrier Doping in the Ni(dmit)2 Simple Salts by Hydrogen-Bonding Pyridinium Cations (dmit=1,3-dithiol-2thione-4,5-dithiolate)

Three kinds of the 1:1 Ni(dmit)₂ salts with 4-(4-pyridyl)pyridinium (PP), 4-[2-(4-pyridyl)ethenyl]pyridinium (P=P), and 4-[2-(4-pyridyl)ethyl]pyridinium (P-P) cations have been prepared and structurally characterized. All of these crystals are composed of a multi-dimensional network of the Ni(dmit)₂ anions and the hydrogen-bonding one-dimensional cation chains. Compared with tight hydrogen bonds in the P=P and P-P chains, that in the PP chain is rather loose. The P=P and P-P salts show semiconducting behavior with high resistivity and large activation energy, while the PP salt shows the op-posite temperature dependence with low resistivity at high temperature. The thermoelectric power indicates that the PP salt is an n-doped semiconductor. The proton defects may occur in the loosely bound PP chain which results in the carrier doping in the conduction band formed by the π-π interaction of the Ni(dmit)₂ anion radicals.     

Reaction of tetramethylpiperidine N-oxides with persistent triplet diphenylcarbenes

Persistent triplet diphenylcarbenes with considerable stability have been shown to be trapped by tetramethylpiperidine N-oxides (TEMPOs) to give the corresponding benzophenones as major products along with tetramethylpiperidine, which indicates that the reaction pattern is essentially identical with that observed for parent triplet diphenylcarbene. The absolute rate constants for the quenching reaction were measured by a laser flash photolysis technique and compared with those for quenching by other typical triplet carbene quenchers. The results showed that the reactivity of TEMPOs toward triplet carbenes was lower than that of oxygen but higher than that of 1,4-cyclohexadiene. The advantages of TEMPOs as a triplet carbene quencher as opposed to the other quenchers are discussed, and TEMPOs are shown to be very convenient reagents to estimate the reactivity of triplet carbenes.