Molecular and electronic structure of (η-pentaalkyl-2,3-dihydro-1,3-diborolyl)(η-pentamethylcyclopentadienyl)metal complexes, M = Fe, Ru

The X-ray diffraction study of the violet ruthenium sandwich derivative 2b revealed a folding along the B?B vector of the 1,3-diborolyl heterocycle of 40.7°, which is very similar to that in the green iron complex 1b. The molecular and electronic structure of the 1,3-diborolyl complexes of iron and ruthenium have been studied by density functional theory (DFT) with the B3LYP functional and extended triple-ζ basis sets. It is shown that the folding of the diborolyl ligand in the title complexes is due to electronic factors. The model complexes with a planar diborolyl ligand 1′ (M = Fe) and 2′ (M = Ru) are by 24.9 and 24.5 kcal/mol less stable than the equilibrium folded structures of 1 and 2, respectively. The electronic structures of 1 and 2 show similarities to those of the 18 VE species due to the participation of the σ(B-C2) bonds in the stabilizing diborolyl-metal bonding. The electronic spectra of 1 and 2 have been studied with the time-dependent DFT method. The absorptions observed in the visible range in the electronic spectrum of the title complexes are assigned as spin-allowed d-d transition with an admixture of metal to diborolyl (π^∗) charge transfer (MLCT).     

Socles of Buchsbaum modules, complexes and posets

The socle of a graded Buchsbaum module is studied and is related to its local cohomology modules. This algebraic result is then applied to face enumeration of Buchsbaum simplicial complexes and posets. In particular, new necessary conditions on face numbers and Betti numbers of such complexes and posets are established. These conditions are used to settle in the affirmative Kühnel's conjecture for the maximum value of the Euler characteristic of a 2k-dimensional simplicial manifold on n vertices as well as Kalai's conjecture providing a lower bound on the number of edges of a simplicial manifold in terms of its dimension, number of vertices, and the first Betti number.     

Alternative Fast and Slow Primary Charge-Separation Pathways in Photosystem II

Photosystem-II (PSII) is a multi-subunit protein complex that harvests sunlight to perform oxygenic photosynthesis. Initial light-activated charge separation takes place at a reaction centre consisting of four chlorophylls and two pheophytins. Understanding the processes following light excitation remains elusive due to spectral congestion, the ultrafast nature, and multi-component behaviour of the charge-separation process. Here, using advanced computational multiscale approaches which take into account the large-scale configurational flexibility of the system, we identify two possible primary pathways to radical-pair formation that differ by three orders of magnitude in their kinetics. The fast (short-range) pathway is dominant, but the existence of an alternative slow (long-range) charge-separation pathway hints at the evolution of redundancy that may serve other purposes, adaptive or protective, related to formation of the unique oxidative species that drives water oxidation in PSII.     

New polyolefin elastomers from metallocenes

In this paper, new polyolefin elastomers, obtained from metallocene based catalytic systems, are presented. The potentialities of metallocenes for the preparation of polyolefin elastomers are discussed with reference to the traditional vanadium and titanium based catalysts. The role played by a Ziegler - Natta catalyst, either stereospecific or aspecific, is discussed.     

Gold-Catalyzed Formal [4+2] Cycloaddition as Access to Antitumor-Active Spirocyclic Oxindoles from Alkynes and Isatin-Derived Ketimines

Due to its excellent bioactivity profile, which is increasingly utilized in pharmaceutical and synthetic chemistry, spirooxindole is an important core scaffold. We herein describe an efficient method for the construction of highly functionalized new spirooxindolocarbamates via a gold-catalyzed cycloaddition reaction of terminal alkynes or ynamides with isatin-derived ketimines. This protocol has a good functional group compatibility, uses readily available starting materials, mild reaction conditions, low catalyst loadings and no additives. It enables the transformation of various functionalized alkyne groups into cyclic carbamates. Gram-scale synthesis was achieved and DFT calculations verify the feasibility of the mechanistic proposal. Some of the target products exhibit good to excellent antiproliferative activity on human tumor cell lines. In addition, one of the most active compounds displayed a remarkable selectivity towards tumor cells over normal ones.