Electronic structure of a binuclear nickel complex of relevance to [NiFe] hydrogenase

The binuclear complex [Ni(2)(L)(MeCN)(2)](3+) (L(2-) = compartmental macrocycle incorporating imine N and thiolate S donors) has a Ni(III) center bridged via two thiolate S-donors to a diamagnetic Ni(II) center. The ground-state has dominant 3d(z)(1)(2) character similar to that observed for [NiFe] hydrogenases in which Ni(III) is bridged via two thiolate donors to a diamagnetic center (Fe(II)). The system has been studied by X-ray crystallography and pulse EPR, ESEEM, and ENDOR spectroscopy in order to determine the extent of spin-delocalization onto the macrocycle L(2-). The hyperfine coupling constants of six nitrogen atoms have been identified and divided into three sets of two equivalent nitrogens. The most strongly coupled nitrogen atoms (a(iso) approximately 53 MHz) stem from axially bound solvent acetonitrile molecules. The two macrocycle nitrogens on the Ni(III) side have a coupling of a(iso) approximately 11 MHz, and those on the Ni(II) side have a coupling of a(iso) approximately 1-2 MHz. Density functional theory (DFT) calculations confirm this assignment, while comparison of the calculated and experimental (14)N hyperfine coupling constants yields a complete picture of the electron-spin density distribution. In total, 91% spin density is found at the Ni(III) of which 72% is in the 3d(z)(2) orbital and 16% in the 3d(xy) orbital. The Ni(II) contains -3.5% spin density, and 7.5% spin density is found at the axial MeCN ligands. In analogy to hydrogenases, it becomes apparent that binding of a substrate to Ni at the axial positions causes a redistribution of the electron charge and spin density, and this redistribution polarizes the chemical bonds of the axial ligand. For [NiFe] hydrogenases this implies that the H(2) bond becomes polarized upon binding of the substrate, which may facilitate its heterolytic splitting.     

Reactions of phosphinites with oxide surfaces: a new method for anchoring organic and organometallic complexes

When a pincer-ligated iridium complex with a phosphinite substituent in the para-position of the aromatic backbone is immobilized on γ-alumina, it becomes a highly effective supported catalyst for the transfer-dehydrogenation of alkanes. The nature of the interaction between the organometallic complex and the support was investigated using solid-state (31)P MAS NMR spectroscopy, solution-state (1)H and (31)P{(1)H} NMR spectroscopy, IR and GC/MS analysis of extracted reaction products. The phosphinite substituent is cleaved from the pincer ligand by its reaction with hydroxyl groups on the γ-alumina surface, resulting in covalent anchoring of the complex via the aryl ring. A similar reaction occurs on silica, allowing for ready grafting onto this support as well. A strategy for anchoring homogeneous catalysts on hydroxyl-terminated oxide supports though the selective cleavage of [POR]-containing ligand substituents is suggested.     

Synthesis and aromatic nucleophilic N-N, N-S and N-O exchange reactions of N,N-dimethyl-2-trifluoroacetyl-1-naphthylamine

We succeeded in the synthesis of N,N-dimethyl-2-trifluoroacetyl-1-naphthylamine (10) by the regioselective deacylation of N,N-dimethyl-2,4-bis(trifluoroacetyl)-1-naphthylamine with trifluoroacetic acid and water. The aromatic nucleophilic substitutions of 10 with various amines, thiols and alcohols proceeded cleanly to give the corresponding N-N, N-S and N-O exchanged products in moderate to excellent yields.     

The Crystal Structure of Ba58Ga22F180O

The crystal structure of Ba₅₈Ga₂₂F₁₈₀O is established by means of X‐ray single crystal diffraction. It is tetragonal: a = 22.033(1) Å, c = 17.626(1) Å, Z = 2. The structure is solved in the space group I4/mmm (n° 139), using 3219 independent reflections. It is mainly built from a deficient arrangement of fluorite‐type [FBa₄] tetrahedra connected by edges and vertices which constitutes the skeleton of the structure, giving rise to large cavities in which lie isolated fluorine ions in tetrahedral and octahedral barium environment, isolated [F₂Ba₆] bitetrahedra, isolated barium ions in eight‐coordination of fluorine and a complex arrangement of isolated [GaF₆] octahedra and isolated [Ga₂F₁₀O] bioctahedra.     

Effect of water and UV passivation on the luminescence of suspensions of silicon quantum dots

This article presents the evolution of the photo-luminescence (PL) of silicon quantum dots (QDs) with an average diameter of 5–6 nm dispersed in alcohol under different conditions. Two samples were considered after alcohol dispersion: freshly synthesized (kept in air for 2 days) QDs which do not exhibit luminescence and air-aged (kept in air for 2 years) QDs exhibiting red-IR luminescence. Experiments performed with addition of a small volume of water, followed by heating for different times showed that the oxidation occurs gradually until transforming totally the initial material in SiO₂. The oxidation process does not enable the appearance of PL from the Si core for dispersed non-aged powders, while it results in a blue shift of the PL maximum intensity for the aged ones. The results obtained after UV illumination clearly indicate an effect of the UV irradiation on the luminescence of QDs dispersed in aqueous environment, and the treatments with acidic water lead to the conclusion of a possible enhancement of the PL by hydrogen passivation of the non-radiative defects. This result should be taken into account for post-production treatments and applications, more particularly, considering a controlled and safe use of luminescent Si QDs.     

The Monoacetalization of Glyoxal: A Direct Synthesis of 2, 2-Dimethoxy and Diethoxy Ethanals

Direct monoacetalization of glyoxal by methanol or ethanol is described. Yields of 50 to 70% are obtained.     

Synthesis of E and Z Diastereoisomers of Benzyl Methyl-2-[3-(N-Substituted)indolyl] Cyclopropane-1,1-dicarboxylates and Their Stereoselective Reductive Cleavage

A synthesis and separation procedure is described for the E and Z diastereoisomers of indolyl-cyclopropane dicarboxylates, IV and V, potential intermediates in the synthesis of the corresponding E and Z cyclopropyl analogs of the amino acid tryptophan and for other ring expansion reactions bearing the indole moiety. Also we have observed a stereoselective cleavage of the benzyl group of the E isomer, without affecting the cyclopropane ring.     

Toward Libraries of Biotinylated Chondroitin Sulfate Analogues: From Synthesis to In Vivo Studies

Chondroitin sulfate‐E (CS‐E) oligosaccharidic analogues (di to hexa) were prepared from lactose. In these compounds, the 2‐acetamido group was replaced by a hydroxyl group. This modification speeded up the synthesis, and large oligosaccharides were constructed in a few steps from a lactose‐originated block. The protecting groups used were as follows; Fmoc for hydroxyl groups to be glycosylated, allyl group for anomeric position protection, and trichoroacetimidate leaving groups were used to prepare up to octasaccharides. We took advantage of the presence of allyl group to develop a click biotinylation, through its transformation into a 3‐azido‐2‐hydroxyl propyl group in two steps (epoxidation and sodium azide epoxide opening). The biotinylating agent was a water‐soluble propargylated and biotinylated triethylene glycol (PEG). By using surface plasmon resonance (SPR), it was shown that the di‐, tetra‐, and hexasaccharides display a binding affinity and selectivity toward HSF/GSF and CXCL12 similar to that of CS‐E. A parallel study confirmed their mimicry of natural compounds, based on the hexasaccharide interaction with Otx2, a homeodomain protein involved in brain maturation, thus validating our simplification approach to synthesize bioactive GAG.     

Reactions between alkaloids and bismuth iodide,the compounds formed and their analytical application : A heterometric study

The reactions between nitron, quinine, cinchonine, strychnine, sparteine, papaverine, rivanol, acriflavine, phenanthroline, antipyrine and atropine with bismuth nitrate in potassium iodide solution were studied heterometrically. The individual structure of the N-compounds was discussed on the basis of the reactions and the large salt-compounds obtained 0.5 to 1 mg of N-compound was necessary for a determination which lasted 10–15 min. The errors in almost all cases were negligible 0.3–0.5 mg bismuth could also be determined by the titration of bismuth with N-compounds and the error was negligible.