Domenica Marabello

An experimental (120 K) and theoretical electron- density study of KMnO4 and KClO4

The experimental electron density rho(r) of the two isomorphic salts KMnO(4) and KClO(4) was determined by a multipole analysis of accurate X-ray diffraction data at 120 K. The quantum theory of atoms in molecules was applied to rho(r) and to its Laplacian nabla(2)rho(r). The bonds were characterized using the topological parameters at the bond critical points of the density rho(r), nabla(2)rho(r), G(r) (kinetic energy density), V(r) (potential energy density) and H(r) (total energy density). According to the classification recently proposed by Espinosa, Alkorta, Elguero & Molins [J. Chem. Phys. (2002), 117, 5529-5542], the K-O and Cl-O bonds have a pure ionic and covalent character, respectively, while the Mn-O bonds show an intermediate behaviour. The results of the topological analysis of the experimental and theoretical (fully periodic Hartree-Fock and density functional calculations) electron density are in good agreement, even on a quantitative level. The atomic charges, determined by performing an integration over the topological basins, are about +2 e for Mn and Cl atoms. The ionic radius, estimated with the distance of the bond critical point from the nucleus, is in agreement with a charge of +2 e for the Mn atom.

Formation of an allylic cluster in the reactions of [Ru3(CO)12] with diethylamino-propyne and trimethylsilyl propargyl alcohol. Crystal structure of [(μ-H)Ru3(CO)9(μ3-η3-C3H3)]

Abstract The title complex (μ-H)Ru3(CO)9(μ3-η3-C3H3) has been obtained following two different reaction pathways: one is the deamination of diethylaminopropyne, HCCCH2NEt2, in the presence of Ru3(CO)12 under thermal conditions. The other is the reaction of trimethylsilylpropargyl alcohol, (HO)H2CCC(SiMe3) (TSPA), with Ru3(CO)12. The complex is obtained both under thermal conditions and in the reaction of the carbonyl with TSPA in CH3OH/KOH (followed by acidification). Other reaction products, deriving from the loss of Me3Si and of HCHO fragments from TSPA, have also been characterized. Complex (μ-H)Ru3(CO)9(μ3-η3-C3H3) has been spectroscopically characterized and its crystal structure was determined by an X-ray analysis. An isosceles triangle of Ru atoms with an edge bridged by an H atom, is coordinated by an allylic moiety σ-bonded to two Ru atoms and π-interacting with the third Ru atom.

An Experimental Evidence of a Metal−Metal Bond in μ-Carbonylhexacarbonyl[μ-(5-oxofuran-2(5H)-ylidene-κC,κC)]-dicobalt(Co−Co)[Co2(CO)6(μ-CO)(μ-C4O2H2)]

The orthorhombic crystal structure of [Co2(CO)6(μ-CO)(μ-C4O2H2)] (1) was determined at 150 K (Fig. 1). Two C−H⋅⋅⋅O bonds connect the molecules, forming waving ribbons along the b axis. The experimental electron density, determined with the aspherical-atom formalism, was analyzed with the topological theory of molecular structure. The presence of the Co−Co bond critical point indicates for the first time the existence of a metal−metal bond in a system with bridged ligands. The bond critical properties of the intramolecular bonds and of the intermolecular interactions show features similar to those found in [Mn2(CO)10], confirming our previously established bonding classification for organometallic and coordination compounds.

Stereochemical Assignment of Strigolactone Analogues Confirms Their Selective Biological Activity

Strigolactones (SLs) are new plant hormones with various developmental functions. They are also soil signaling chemicals that are required for establishing beneficial mycorrhizal plant/fungus symbiosis. In addition, SLs play an essential role in inducing seed germination in root-parasitic weeds, which are one of the seven most serious biological threats to food security. There are around 20 natural SLs that are produced by plants in very low quantities. Therefore, most of the knowledge on SL signal transduction and associated molecular events is based on the application of synthetic analogues. Stereochemistry plays a crucial role in the structure-activity relationship of SLs, as compounds with an unnatural D-ring configuration may induce biological effects that are unrelated to SLs. We have synthesized a series of strigolactone analogues, whose absolute configuration has been elucidated and related with their biological activity, thus confirming the high specificity of the response. Analogues bearing the R-configured butenolide moiety showed enhanced biological activity, which highlights the importance of this stereochemical motif.

HOMOGENEOUS HYDROGENATION OF DIPHENYLACETYLENE IN THE PRESENCE OF RU3(CO)9L3 (L = PPH3, PET3). THE CRYSTAL STRUCTURE OF RU3(CO)10(PET3)2 : A REACTION INTERMEDIATE?

Abstract The phosphine-substituted clusters Ru3(CO)9L3 (L=PPh3, 1; L=PEt3, 2) are active homogeneous catalysts for the hydrogenation of diphenylacetylene. In the catalytic reactions involving complex 2, formation of Ru3(CO)10(PEt3)2 (3) has been observed. This complex shows a hydrogenating activity greater than that of the parent complex 2. In the light of these results and of the observed effect of dihydrogen pressure and substrate/cluster ratio, reaction mechanisms are discussed. The structure of 3 has been characterized by X-ray diffraction and is compared with those of other phosphine-substituted triruthenium clusters. Compound 3 crystallizes in the monoclinic space group P21 with a=9.368(7), b=12.20(2), c=13.554(9) A, β=102.63(8)° and Z=2. Refinement of 4975 data gave R1=0.0417 and wR2=0.1105. The two phosphorus ligands occupy equatorial positions on adjacent metal atoms so that they are trans to each other at the ends of the RuRu vector. The presence of bent semi-bridging carbonyl groups makes the molecule chiral and its absolute structure was determined.

Transition metal carbonyl clusters with ene /yne ligands

Abstract Ene–yne molecules are found as natural products. Natural and synthetic ene–ynes are actively studied and partly used as anticancer drugs. Conjugated and non-conjugated ene–yne molecules are widely used as synthons in organic reactions promoted by organometallic compounds. Surprisingly, the coordination of ene–ynes to transition metal clusters has not been fully rationalized up to now; the same happens for the (not very many) examples of their reactivity and of their potentials uses. In this paper the synthetic routes leading to (and the structures of) transition metal clusters containing ene–yne ligands are described. Comparisons with the chemistry of di-ynes coordinated to transition metal clusters are made.

1,2,5-Oxadiazole analogues of leflunomide and related compounds

A new series of compounds, structurally related to leflunomide, based on the 1,2,5-oxadiazole ring system (furazan) has been synthesised, and their ability to undergo ring scission at physiological pH to afford the corresponding cyano-oximes has been analyzed. The latter, together with the respective nitro derivatives obtained by oxidation, have been characterised as weak inhibitors of rat dihydroorotate dehydrogenase (DHODH).

Are the large filamentous microfossils preserved in Messinian gypsum colorless sulfide-oxidizing bacteria?

The thick gypsum deposits formed in the Mediterranean Basin during the Messinian salinity crisis incorporate dense mazes of filamentous fossils, which were interpreted as algae or cyanobacteria, thus pointing to a shallow-marine subtidal or intertidal environment. The data presented here reveal that these filaments represent remains of colorless, vacuolated sulfide-oxidizing bacteria. This interpretation is supported by the presence of small crystal aggregates of iron sulfide (pyrite) and associated polysulfide within the filamentous fossils. Pyrite and polysulfide are considered to result from early diagenetic transformation of original zero-valent sulfur globules stored within the cells, which is a clade-diagnostic feature of living and degraded sulfur bacteria. In addition to filamentous fossils, the studied gypsum crystals contain remains of euryhaline and stenohaline diatoms and clay-rich aggregates interpreted as alteration products of marine snow floccules. This peculiar fossil assemblage reflects conditions of increased productivity in the water column, triggered by high fluxes of nutrients into the basin during phases of enhanced riverine runoff and freshwater discharge. This study confirms that gypsum evaporites have great potential to preserve the early stages of the taphonomic alteration of bacterial cells, shedding light on the paleoecology of ancient hypersaline environments.

Synthesis of bench-stable diarylmethylium tetrafluoroborates.

A representative number of bench-stable nonsymmetric diarylcarbenium tetrafluoroborates have been isolated via the direct coupling of aryl (or heteroaryl) aldehydes and N-heteroarenes and fully characterized. They have proven to be highly stable in the presence of both EDG and EWG substituents. An (E)-iminium vinylogous substructure has been shown as the common cation scaffold by X-ray analysis and by NOE determination.

Non-linear optical properties of β-D-fructopyranose calcium chloride MOFs: an experimental and theoretical approach

The second harmonic generation (SHG) properties of two MOFs, obtained from fructose and calcium chloride, were studied using a Non-Linear Optics Multimodal microscope. The first-order hyperpolarizability and the second-order susceptibility were calculated at the DFT level of theory. Moreover, a semi-classical approach to non-linearities in the optical behaviour was used in order to determine the features responsible for the SHG. The MOFs were synthesized both in ethanol and by solid–solid interaction, with a simple, rapid and low-cost methodology with no environmental impact, and were characterized with IR and RAMAN spectroscopy and both the single-crystal and powder X-ray diffraction. Both the metal–carbohydrate-based MOFs show an interesting SHG intensity: in particular, compound 1 shows an average SH intensity more than twice that of sucrose, in agreement with the theoretical results. A favourable combination of optical properties, transparency, thermal and chemical stability makes compound 1 a potential candidate for applications in electro-optics devices.