Carbon-carbon coupling on tetrahedral iridium clusters: X-ray molecular structures and multinuclear NMR studies of the two isomeric forms of [Ir4(CO)6(μ3-η-HCCPh)(μ2-η-C4H2Ph2)(μ-PPh2)2]

The reactions of [HIr₄(CO)₉(Ph₂PCCPh)(μ-PPh₂)] (1) or [Ir₄(CO)₈(μ₃-η²-HCCPh)(μ-PPh₂)₂] (2) with HCCPh gave two isomeric forms of [Ir₄(CO)₆(μ₃-η²-HCCPh)(μ₂-η⁴-C₄H₂Ph₂)(μ-PPh₂)₂] (3 and 4) in good yields as the only products. These compounds were characterized with analytical and spectroscopic data including ¹H, ¹³C and ³¹P NMR (1 and 2D) spectroscopy and their molecular structures were established by X-ray diffraction studies. Compounds 3 and 4 exhibit the same distorted butterfly metal polyhedral arrangement of metal atoms with two μ-PPh₂ that occupy different positions in the structures of the two isomers. Both molecules contain a HCCPh ligand bonded in a μ₃-η²-// mode to one of the wings of the butterfly and a metallacyclic ring, which resulted from head-to-tail coupling, in the case of [Ir₄(CO)₆(μ₃-η²-HCCPh){μ₂-η⁴-(H)CC(Ph)C(H)C(Ph)}(μ-PPh₂)₂] (3) and tail-to-tail coupling, in that of [Ir₄(CO)₆(μ₃-η²-HCCPh){μ₂-η⁴-(H)CC(Ph)C(Ph)C(H)}(μ-PPh₂)₂] (4), and which is linked to two metal atoms of the second wing of the butterfly.     

Computational studies of carbon nanotube-hydrocarbon bond strengths at nanotube ends: effect of link heteroatom and hydrocarbon structure

Semiempirical and density functional electronic structure theory methods were used to study SWNT-X--R bond strengths, where the single-walled carbon nanotube (SWNT) had an armchair or zigzag structure, the link heteroatom X was O, N(H), or S and the hydrocarbon chain R was CH(2)CH(3), CH(OH)CH(3), CHCH(2), or CH(CF(3))CH(3). In all systems the hydrocarbon was bonded to the end of the nanotube. The SWNT-X--R bond (that is, the bond joining the link atom to the hydrocarbon) is more than 0.4 eV stronger for armchair than for zigzag nanotubes with the same diameters, irrespective of whether O, N, or S are used as link atoms or whether OH, C==C, or CF(3) groups are present in the hydrocarbon chain. This raises the possibility for selective manipulation of armchair/zigzag nanotubes using a variety of link atoms and hydrocarbon structures. The SWNT-O--CH(CF(3))CH(3) bond is weaker than the SWNT-O--CH(2)CH(3) bond (for both armchair and zigzag nanotubes), while inclusion of a double bond in the ethyl chain increases the bond strengths. Also, SWNT-S--CH(2)CH(3) and SWNT-N(H)--CH(2)CH(3) bonds are stronger than SWNT-O--CH(2)CH(3) bonds.     

Uronic (hexenuronic) acid profile of ethanol–alkali delignification of giant reed Arundo donax L.

The effect of process variables on uronic acids (UAs) and hexenuronic acids (HexAs) in the annual crop Arundo donax L. during ethanol–alkali pulping has been examined. A substantial loss of UA moieties (up to 90%) was observed by the end of pulping (target kappa number 18) performed with 25% NaOH and 40% EtOH (by volume) within the temperature range of 130–150 °C. At the same time, the progressive formation of HexA in pulp was detected from the early phases of delignification. The proportion of HexA in the residual UA of the final pulp was found to be 84%, indicating almost complete conversion of 4-O-methylglucuronic acid side groups (MeGlcA) of heteroxylan into HexA. The kinetics of UA degradation and HexA formation has been described in terms of three consecutive first-order reaction stages. The overall rate of UA degradation was one order higher than the rate of UA conversion into HexA. The values of apparent activation energy were estimated as 68.6 and 94.7 kJ mol^–1, respectively. The reaction medium alkalinity was shown to be the controlling factor for UA and HexA stability during ethanol–alkali pulping. An increase in alkali charge from 5% to 35% (as NaOH) led to UA loss of 40%, but promoted HexA formation from 11.8 to 20.1 mol g^–1. The addition of organic solvent to the alkaline pulping solution had a similar effect, and about 10% of UA was lost and the content of HexA increased from 6.9 to 10.9 mol g^–1 with an increase in ethanol proportion in the liquor from 20% to 60%.     

Structural and Transport Properties of Mg1−xMnxMn2O4±δ Spinels

The synthesis and structural properties of Mg_1−xMn_2+xO₄, for 0≤x≤1 are described. Complete miscibility in the solid state exists for this system. For the material with the correct stoichiometry, i.e. MgMn₂O₄, the effect of temperature on the cation distribution was investigated= above 600°C the inversion degree (m) starts increasing. The electrical conductivity shows a small dependence on P(O₂) which is consistent with the small oxygen non-stoichiometry determined by means of thermogravimetry. The main contribution to the transport properties arises from the inversion equilibrium. Two distinct conductivity regimes, below and above the inversion threshold, can be assumed to explain the electrical conductivity and thermoelectric power results.     

Synthesis of copper,nickel and cobalt complexes containing a new N2S2 ligand: benzyl-N,N′-alkylbis(2-amino-1-cyclopentencarbodithioate)

The synthesis of [benzyl-N,N-alkylbis(2-amino-1-cyclopentencarbodithioate)]M^II [alkyl = ethene (L₂) and propene, (L₃) and M = Cu, Ni and Co] complexes and their characterization (u.v.–vis., FT-IR, ¹H-n.m.r., mass spectra and cyclic voltametry) are reported.     

Defect and dopant properties of MgTa2O6

Atomistic computer simulation techniques have been used, for the first time, to reproduce the crystal structure of MgTa₂O₆ and to investigate the defect chemistry and dopant properties of this material. The calculated defect energetics suggest that the concentration of intrinsic atomic defects in this phase is insignificant and that the system is probably stable to both oxidation and reduction. Dopant solution energy versus ion size trends are found for both isovalent and aliovalent dopant incorporation at Mg and Ta sites. Divalent dopants (e.g. Ca, Cu) preferentially occupy the Mg site whereas dopants with higher charge (e.g. Sc, Zr, Nb) are more favorable on the Ta site. High migration activation energies (>2 eV) predict limited ionic conductivity in this material.     

Phytochromes with noncovalently bound chromophores: the ability of apophytochromes to direct tetrapyrrole photoisomerization

Chromophore-apoprotein interactions were studied with recombinant apoproteins, oat phytochrome (phyA) and CphB of the cyanobacterium Calothrix PCC7601, which were both incubated with the bilin compounds biliverdin (BV) IXalpha, phycocyanobilin (PCB) and the 3'-methoxy derivative of PCB. Previously it was shown that CphB and its homolog in Calothrix, CphA, show strong sequence similarities with each other and with the phytochromes of higher and lower plants, despite the fact that CphB carries a leucine instead of a cysteine at the chromophore attachment position and thus holds the chromophore only noncovalently. CphA binds tetrapyrrole chromophores in a covalent, phytochrome-like manner. For both eyanobacterial phytochromes, red and far-red light-induced photochemistry has been reported. Thus, the role of the binding site of CphB in directing the photochemistry of noncovalently bound tetrapyrroles was analyzed in comparison with the apoprotein from phyA phytochrome. Both the aforementioned compounds, which were used as chromophores, are not able to form covalent bonds with a phytochrome-type apoprotein because of their chemical structure (vinyl group at position 3 or methoxy group at position 3'). The BV adducts of both apoproteins showed phytochrome-like photochemistry (formation of red and far-red-absorbing forms of phytochrome [P(r) and P(fr) forms]). However, incubation of the oat apophytochrome with BV primarily yields a 700 nm form from which the P(r)-P(fr) photochemistry can be initiated and to which the system relaxes in the dark after illumination. The results for CphB were compared with a CphB mutant where the chromophore-binding cysteine had been introduced, which, upon incubation with PCB, shows spectral properties nearly identical with its (covalently binding) CphA homolog. A comparison of the spectral properties (P(r) and P(fr) forms) of all the PCB- and BV-containing chromoproteins reveals that the binding site of the cyanobacterial apoprotein is better suited than the plant (oat) phytochrome to noncovalently incorporate the chromophore and to regulate its photochemistry. Our findings support the proposal that the recently identified phytochrome-like prokaryotic photoreceptors, which do not contain a covalently bound chromophore, may trigger a light-induced physiological response.     

The catalytic system [(Cp2TiCl)2]:LiAlH4 in aromatic solvents,Part II. Formation of η3-allyltitanocene derivatives in the presence of dienes — their structure and e.s.r. spectra

Summary The addition of dienes to the system [(Cp₂TiCl)₂] LiAlH₄toluene changes the system so that the complex [Cp₂TiAlH₄] is quantitatively formed instead of a titanocene hydride — aluminium hydride cluster. The complex [Cp₂TiAlH₄] is further converted into ³-allyltitanocene derivatives ([Cp₂TiA]) if the diene structure is suitable for formation of stable [Cp₂TiA] compounds and if the equilibrium [Cp₂TiAlH₄]+diene[Cp₂TiA]+A1H₃ is shifted towards the formation of [Cp₂TiA] by the excess of diene. All the compounds [Cp₂TiA] exhibit high-resolution e.s.r. spectra at g=1.993, showing interaction of the unpaired electron with the cyclopentadienyl and ³-allyl protons. The e.s.r. spectra clearly reveal the presence of alkyl substituents atsyn-1,3-positions of ³-allyl ligand, and show a triplet of multiplets for (³-allyl)titanocene, doublets of multiplets for (1-alkyl-³-allyl)titanocenes and single multiplets for (1,3-dialkyl-³-allyl)-titanocenes. thermal isomerization of (1,3-dimethyl-³-allyl)-titanocene and (1-methyl-3-ethyl-³-allyl)titanocene, hitherto considered as the stable Cp₂TiA compounds, into (1-alkyl-³-allyl)titanocenes was confirmed by e.s.r. and electronic absorption spectroscopy as well as by chemical means.