Isocyanide insertion reactivity of dinuclear niobium and tantalum imido complexes: X-ray crystal structure of [{Nb(η-C5H4SiMe3)(CH2Ph)2}2(μ-1,4-NC6H4N)]

The reactivity of dinuclear niobium and tantalum imido complexes with the isocyanide compound 2,6-Me₂C₆H₃NC has been studied. The trialkyl complexes [{NbR₃(CH₃CN)}₂(μ-1,3-NC₆H₄N)], [{NbR₃(CH₃CN)}₂(μ-1,4-NC₆H₄N)] and [{TaR₃(CH₃CN)}₂(μ-1,4-NC₆H₄N)] (R=CH₂SiMe₃) gave [{Nb(η²-RCNAr)₂R}₂(μ-1,3-NC₆H₄N)] (1), [{Nb(η²-RCNAr)₂R}₂(μ-1,4-NC₆H₄N)] (2) and [{Ta(η²-RCNAr)₂R}₂(μ-1,4-NC₆H₄N)] (3) (R=CH₂SiMe₃; Ar=2,6-Me₂C₆H₃), from the isocyanide insertion in two of the metal alkyl carbon bonds. The reaction of the isocyanide reagent with the di-alkyl mono-cyclopentadienyl derivatives [{Nb(η⁵-C₅H₄SiMe₃)R₂}₂(μ-1,3-NC₆H₄N)] (R=Me, CH₂Ph, CH₂SiMe₃), [{Nb(η⁵-C₅H₄SiMe₃)R₂}₂(μ-1,4-NC₆H₄N)] (R=Me, CH₂Ph (4), CH₂SiMe₃) and [{Ta(η⁵-C₅Me₅)(CH₂SiMe₃)₂}₂(μ-1,4-NC₆H₄N)] yielded [{Nb(η⁵-C₅H₄SiMe₃)(η²-RCNAr)R}₂(μ-1,3-NC₆H₄N)] (R=Me (5), CH₂Ph (6), CH₂SiMe₃ (7)), [{Nb(η⁵-C₅H₄SiMe₃)(η²-RCNAr)R}₂(μ-1,4-NC₆H₄N)] (R=Me (8), CH₂Ph (9), CH₂SiMe₃ (10)) and [{Ta(η⁵-C₅Me₅)(η²-Me₃SiCH₂CNAr)CH₂SiMe₃}₂(μ-1,4-NC₆H₄N)] (11) (Ar=2,6-Me₂C₆H₃), respectively, from a single insertion process. The reaction with the mono-alkyl complex [{Nb(η⁵-C₅H₄SiMe₃)(Me)Cl}₂(μ-1,4-NC₆H₄N)] gave [{Nb(η⁵-C₅H₄SiMe₃)(η²-MeCNAr)Cl}₂(μ-1,4-NC₆H₄N)] (12), produced from the isocyanide insertion in the metal-alkyl carbon bond. The alkyl-amido complex [{Nb(η⁵-C₅H₄SiMe₃)(Me)NMe₂}₂(μ-1,4-NC₆H₄N)] gave, from the preferential isocyanide insertion in the metal-amide nitrogen bond, [{Nb(η⁵-C₅H₄SiMe₃)(η²-Me₂NCNAr)Me}₂(μ-1,4-NC₆H₄N)] (13). The molecular structure of one of the alkyl precursors, [{Nb(η⁵-C₅H₄SiMe₃)(CH₂Ph)₂}₂(μ-1,4-NC₆H₄N)] (4), has been determined.     

Synthesis and reactivity of alkynyl niobocene complexes

The synthesis of alkynyl containing niobocene complexes with differing auxiliary ligands is described. The niobium(III) derivatives of the general formula [Nb(η⁵-C₅H₄SiMe₃)₂(CCR)(L)] where L is either carbonyl, phosphine, phosphite, or isocyanide, were prepared by the reaction of the bis(alkynyl)magnesium reagent with the corresponding cloro-niobocene precursor. In a similar manner the niobium(V) imido compounds, of the general formula, [Nb(=NR′)(η⁵-C₅H₄R″)₂(CCR)], were prepared. The characterization of these complexes is discussed. The reactivity of the alkynyl compounds towards oxidation and protonation has been studied.     

Thermal depolarisation current study of electron irradiated fluorinated ethylene propylene

Thermally stimulated current (TSC) studies have been reported in the co-polymer of tetrafluoro ethylene and hexafluoropropylene films. Depolarisation current peaks are obtained atα ₁,α ₂ andβ relaxation temperatures of the polymer and the detrapping process is explained on the basis of its molecular motion. A cross-over electron energy of 18 keV is observed where the nature of TSC spectra undergoes a remarkable change. This is explained in relation to the surface states in FEP. Five groups of trapping levels, 0.25±0.08, 0.57±0.10, 1.07±0.1, 1.3±0.25 and 2.3±0.4 eV are obtained.     

Impurity band for screened impurities in many-valley semiconductors

For a random distribution of screened donor impurities in many-valley semiconductors we study the impurity band as a function of carrier concentration. The results show that the often used assumption of an isolated screened impurity is unjustified, particularly for the critical concentration for which the binding energy becomes zero.     

Magnetic band structure in the valence band of CdSb

Starting from the band structure calculations of Yamada, we have undertaken to calculate in analytic form the Landau levels, de Haas-Shubnikov oscillation periods and effective masses in the valence band of CdSb. Because of the anomalous character of the E — k relation in the x-direction, it is necessaryto use different approximation methods for low and high hole energies; between the two is a region for which calculation is very difficult. Analytic forms have been obtained in both regimes, which should facilitate the calculation of magneto-transport properties and thus the determination of band parameters.     

Porphyrin-diones and porphyrin-tetraones: reversible redox units being localized within the porphyrin macrocycle and their effect on tautomerism

Porphyrin-2,3-diones and porphyrin-2,3,7,8- and porphyrin-2,3,12,13-tetraones were shown to have a redox-active unit that can function independently of the macrocycle at large. Electroreduction of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin-2,3-diones [(P-dione)M] and the corresponding -2,3,12,13-tetraones [L-(P-tetraone)M] and -2,3,7,8-tetraones [C-(P-tetraone)M], where M = 2H, CuII, ZnII, NiII, and PdII was investigated and the products were characterized by ESR and thin-layer UV-visible spectroelectrochemistry. Electrochemical and spectroelectrochemical data show that the first two reductions of the porphyrin-diones and the first three reductions of the porphyrin-tetraones occur at the dione units. This was confirmed by ESR spectra of first reduction products which show that the electron spin is totally localized on a semidione unit, independent of the central metal ion and of the number and location of dione units. ESR spectra of the radical anions derived from free-base porphyrin-2,3-dione [(P-dione)2H] and porphyrin-2,3,12,13-tetraone [L-(P-tetraone)2H] confirm the trans-arrangement of the two inner protons and their location on nonsubstituted pyrrolic rings, thereby maintaining an 18-atom 18-pi electron bacteriochlorin-like aromatic delocalization pathway. The redox unit is not similarly isolated in the corner free-base porphyrin-2,3,7,8-tetraone [C-(P-tetraone)2H]. A one-electron reduction of C-(P-tetraone)2H leads to the formation of a tautomer with trans inner hydrogens with one residing on the N of the ring with the reduced unit as the only detectable product. This process is favorable because it creates a more delocalized 18-atom 18-pi electron aromatic pathway. This result is consistent with the measured redox potentials which show the first reduction of C-(P-tetraone)2H to be substantially easier than (P-dione)2H or L-(P-tetraone)2H.     

Effect of temperature and pressure on the structure, dynamics, and hydrogen bond properties of liquid N-methylacetamide: a molecular dynamics study

The structure and dynamical properties of liquid N-methylacetamides (NMA) are calculated at five different temperatures and at four different pressures using classical molecular dynamics simulations. Our results are analyzed in terms of pressure-induced changes in structural properties by investigating the radial distribution functions of different atoms in NMA molecule. It is found that the first peak and also the second peak of C-O and N-H are well defined even at higher temperature and pressure. It is also observed that the number of hydrogen bonds increase with application of pressure at a given temperature. On the other hand, the calculated hydrogen bond energy (E(HB)) shows that the stability of hydrogen bond decreases with increasing of pressure and temperature. Various dynamical properties associated with translational and rotational motion of neat NMA are calculated and the self-diffusion coefficient of NMA is found to be in excellent agreement with the experiment and the behavior is non-Arrhenius at low temperatures with application of pressures. The single particle orientational relaxation time for dipole vector and N-C vector are also calculated and it is found that the orientational relaxation time follows Arrhenius behavior with a variation of temperature and pressure.     

Anti-fouling chemistry of chiral monolayers: enhancing biofilm resistance on racemic surface

This work reports the resistance to protein adsorption and bacterial biofilm formation by chiral monolayers of polyol-terminated alkanethiols surrounding micrometer-sized patterns of methyl-terminated alkanethiols on gold films. We discover that patterned surfaces surrounded by chiral polyol monolayers can distinguish different stages of biofilm formation. After inoculation on the surfaces, bacteria first reversibly attached on the chiral polyol monolayers. Over time, the bacteria detached from the polyol surfaces, and attached on the hydrophobic micropatterns to form biofilms. Interestingly, while both enantiomers of gulitol- and mannonamide-terminated monolayer resisted adsorption of proteins (bovine serum albumin, lysozyme, and fibrinogen) and confined biofilms formed on the micropatterns, the monolayers formed by the racemic mixture of either pair of enantiomers exhibited stronger antifouling chemistry against both protein adsorption and biofilm formation than monolayers formed by one enantiomer alone. These results reveal the different chemistries that separate the different stages of biofilm formation, and the stereochemical influence on resisting biofoulings at a molecular-level.     

Strong electric field modulation of transport in PVDF/MWCNT nanocomposite near the percolation threshold

A nanocomposite comprising of multiwalled carbon nanotubes (MWCNT) embedded in ferroelectric Poly(vinylidene fluoride) PVDF polymer matrix is examined for electric field induced transport modulation. The pulsed laser deposition (PLD) grown thin films of the nanocomposite with different MWCNT content were characterized. When used as a channel layer in a field effect transistor configuration, a strong electric field modulation of the transport was realized just below the percolation threshold. We believe that this nanocomposite non-percolating channel concept can provide several opportunities for FET devices for organic electronics.     

A light-gated synthetic ion channel

A gated synthetic ion channel with beta-cyclodextrin as the pore and azobenzene as the gate is reported. Irradiation converts a tethered trans-azobenzene to cis-azobenzene which likely transforms the channel from a self-inclusion complex to a dissociated structure. This transformation results in an increase in anion transport and a decrease in cation transport across a phospholipid vesicle membrane.