Zirconocene Catalysts: Ion‐pairs,Zwitterions, or Weakly Bound Molecules?

The bonding between electron deficient zirconocene complexes such as [(η⁵‐C₅Me₅)₂ZrR]⁺ and the counterions [(C₆F₅)₃BR]^– (R = CH₃, 1 , R = H, 2 ) has been investigated by DFT and extended Hückel calculations. The molecular bonding analysis has been carried out in the light of the structural information on the inter‐ion interactions available for these and other similar electron deficient coordinatively unsaturated zirconocene complexes. It has been shown that the “anion‐cation” interactions in 1 and 2 are different manifestations of the same need to provide electron density to the metal centre. In 1 this is achieved via the ‐C–H σ‐bonds and has been described as a pseudo‐agostic interaction, whereas in 2 electron donation occurs via the phenyl F‐atoms. The study of the covalent component of the bonding between the ionic components has shown that the weak electron donation, though not sufficient to saturate the metal centre, stabilises the unsaturated species and allows it to have a longer life time. It has been inferred that in the homogenous Ziegler‐Natta catalytic process the approach of a strongly coordinating molecule, such as an olefin in the polymerisation, destroys the weak interaction and allows the reaction to proceed whilst the ion‐pairing maintains the counterion in proximity of the complex.     

Exploring the Role of L10 Loop in New Delhi Metallo-β-lactamase (NDM-1): Kinetic and Dynamic Studies

Four NDM-1 mutants (L218T, L221T, L269H and L221T/Y229W) were generated in order to investigate the role of leucines positioned in L10 loop. A detailed kinetic analysis stated that these amino acid substitutions modified the hydrolytic profile of NDM-1 against some β-lactams. Significant reduction of k_cat values of L218T and L221T for carbapenems, cefazolin, cefoxitin and cefepime was observed. The stability of the NDM-1 and its mutants was explored by thermofluor assay in real-time PCR. The determination of T_mB and T_mD demonstrated that NDM-1 and L218T were the most stable enzymes. Molecular dynamic studies were performed to justify the differences observed in the kinetic behavior of the mutants. In particular, L218T fluctuated more than NDM-1 in L10, whereas L221T would seem to cause a drift between residues 75 and 125. L221T/Y229W double mutant exhibited a decrease in the flexibility with respect to L221T, explaining enzyme activity improvement towards some β-lactams. Distances between Zn1-Zn2 and Zn1-OH- or Zn2-OH- remained unaffected in all systems analysed. Significant changes were found between Zn1/Zn2 and first sphere coordination residues.     

Unexpected one-pot synthesis of highly conjugated pentacyclic diquinoid compounds

A new class of pentacyclic diquinoid compounds has been synthesized with a facile one-pot reaction of two molecules of 2-hydroxynaphthoquinone and 1-bromoalkanes in the presence of ferrocene. These molecules were isolated as enol tautomers that exhibit intramolecular hydrogen bond and extended electronic conjugation as proved by the intense absorption spectrum with a broad band between 400 and 600 nm. The spectroscopic and electrochemical characterization of this new class of compounds has been performed. One of the synthesized diquinoid derivatives showed a significant cytotoxicity with IC(50) values of 25-50 μM against Cisplatin-Resistant SKOV3 and colon carcinoma SW480 cell lines. The results of our study provide a valuable tool to a one-pot synthesis of highly conjugated polyquinones, analogous to important biological systems, with significant antitumoral activity.     

Dodecatwistarene Imides with Zigzag‐Twisted Conformation for Organic Electronics

1D nonplanar graphene nanoribbons generally have three possible conformers: helical, zigzag, and mixed conformations. Now, a kind of 1D nonplanar graphene nanoribbon, namely dodecatwistarene imides featuring twelve linearly fused benzene rings, was obtained by bottom‐up synthesis of palladium‐catalyzed Stille coupling and C?H activation. Single‐crystal X‐ray diffraction analyses revealed that it displays a zigzag‐twisted conformation caused by steric hindrance between imide groups and neighboring annulated benzene rings with the pendulum angle of 53°. This conformation is very stable and could not convert into other conformations even when heated up to 250 °C for 6 h. Despite of the highly twisted topology, organic field‐effect transistor based on it exhibits electron mobility up to 1.5 cm² V^?1 s^?1 after annealing.     

Dodecatwistarene Imides with Zigzag-Twisted Conformation for Organic Electronics

1D nonplanar graphene nanoribbons generally have three possible conformers: helical, zigzag, and mixed conformations. Now, a kind of 1D nonplanar graphene nanoribbon, namely dodecatwistarene imides featuring twelve linearly fused benzene rings, was obtained by bottom-up synthesis of palladium-catalyzed Stille coupling and C−H activation. Single-crystal X-ray diffraction analyses revealed that it displays a zigzag-twisted conformation caused by steric hindrance between imide groups and neighboring annulated benzene rings with the pendulum angle of 53°. This conformation is very stable and could not convert into other conformations even when heated up to 250 °C for 6 h. Despite of the highly twisted topology, organic field-effect transistor based on it exhibits electron mobility up to 1.5 cm² V⁻¹ s⁻¹ after annealing.     

Hetero‐Seeding and Solid Mixture to Obtain New Crystalline Forms

Para‐methyl benzyl alcohol (p‐MeBA II) and para‐chloro benzyl alcohol (p‐ClBA) are quasi‐isostructural and share the same hydrogen‐bond patterns, but their crystals are not isomorphous. No new polymorphs could be obtained by conventional polymorph screening based on different solvents and different crystallization conditions. Formation of a new polymorph of p‐MeBA named p‐MeBA I, isomorphous with the crystal of p‐ClBA, was induced by hetero‐seeding with a small quantity of powdered p‐ClBA added to a supersaturated solution of p‐MeBA in hexane, while seeding of p‐ClBA with p‐MeBA II failed to give a new phase of p‐ClBA isomorphous with known crystalline p‐MeBA II. Mixed crystals of p‐MeBA and p‐ClBA were also prepared with different p‐MeBA/p‐ClBA ratios to understand the role of the different functional groups in the crystal structure. Crystal phases were characterized by combined use of single‐crystal and powder X‐ray diffraction, differential scanning calorimetry, and solid‐state NMR spectroscopy.     

Intramolecular Dynamics of Tetranuclear Iridium Carbonyl Cluster Compounds. Part III. Crystallographic and dynamic evidence for the intermediate of the ‘merry-go-round’ process in nonacarbonyl-μ3-(1,3,5-trithiane)-tetrairidium

The title complex crystallises in two C_3v, isomeric forms differing in carbonyl-ligand arrangement. In solution, the isomer 1b with three edge-bridging carbonyls on a common face of the metal tetrahedron converts via an endothermic equilibrium into the isomer 1u with no bridging carbonyls. The latter was shown by ¹³C-NMR to be the intermediate of the ‘merry-go-round’ process which exchanges the sites of the basal CO's.