Palladium-arene interactions in catalytic intermediates: an experimental and theoretical investigation of the soft rearrangement between eta1 and eta2 coordination modes

A series of dichloro-bridged arylbicycloheptylpalladium complexes have been synthesized and characterized by means of NMR spectroscopy. The compound [(C16H19)PdCl]2*CH2Cl2 with ortho and para methyl substituents at the arene has been characterized by means of X-ray diffraction techniques. The C(ipso) atom of the arene lies almost at the fourth planar coordination site of the metal [Pd-C(ipso) = 2.22(1) A (average)], and due to the arene's tilting, the substituted C(ortho) atom is relatively close to the metal atom [2.54(1) A (average)]. The coordinated C(ipso)-C(ortho) linkage, in a seemingly dihapto coordination, is anti with respect to the CH2 bridge of the bicycloheptyl unit. Variable-temperature NMR experiments for the para-substituted dimer 9 reveal restricted rotation of the two aryl groups about the corresponding C-C(ipso) bonds (DeltaE < or =17 kcal x mol(-1)). DFT-B3LYP calculations have been carried out on the known and similar monomer (phenylbicycloheptenyl)Pd(PPh3)I (4) and its related substituted derivatives. The essential results are as follows: (i) The potential energy surface for twisting the phenyl ring away from the symmetric eta(1) coordination in 4 is very flat (DeltaE < or = 1 kcal x mol(-1)) whereas an Atoms in Molecules analysis excludes the existence of an actual Pd-C(ortho) bond in the seemingly eta2-type conformer. (ii) Complete rotation of the unsubstituted phenyl ring is not facile but feasible. A significant strain affects the transition-state structure featuring a Pd-HC(aryl) agostic-type bond. The calculated destabilization of 10.3 kcal x mol(-1), with respect to the ground state, can be compared to the experimental barrier of the dimer 9. (iii) Various methyl-substituted derivatives of 4 have been optimized, and their structural and energetic trends are discussed. An almost ideal eta1 coordination is shown by the anti conformer of the C(ortho)-substituted complex due steric effects. For all of the other cases, a slipped eta2 coordination may be described. As a general conclusion, the unsaturated metal center receives pi electron density of the arene mainly through its C(ipso) atom. The effect may be slightly improved if the C(ortho) atom also gets closer to the metal, but in no case, does the slipped eta2 coordination seem to be crucial for the stability of the system.     

A new multi analytical approach for the identification of synthetic and natural dyes mixtures. The case of orcein-mauveine mixture in a historical dress of a Sicilian noblewoman of nineteenth century

Abstract

In this paper, the application of a multi-analytical approach for the characterisation of synthetic and natural dyes in a historical textile is presented. The work is focused on a historical dress of a Sicilian noblewoman, dating from about 1865–1870. Firstly, SERS on fibre was performed, in order to individuate the classes of dyes employed. The SERS spectra suggested the presence of two main dyes: mauveine and orcein. In order to confirm these preliminary results, two different extraction protocols were applied. The extracts obtained were analysed by ESI-MS, MALDI-ToF and UHPCL-MS analyses, confirming the SERS results. In particular, the application of the ammonia mild extraction technique allowed to selectively extract the phenoxazonic dyes, separating them already in the extraction step from the synthetic ones. Thanks to this multi-analytical approach, this dress could be considered as one of the first examples of employment of synthetic dyes in association with natural ones.     

Investigation of polymer-protected noble metal nanoparticles by transmission electron microscopy: control of particle morphology and shape

 Noble metal nanoparticles were prepared by the in situ reduction of the respective metal salt precursors in the presence of various protective polymers. Transmission electron microscopy (TEM) has been used to determine the particle shapes and morphologies. These are strongly influenced by the reduction methods and conditions chosen, but the choice of the protective polymer is equally important for controlling the particle morphologies and for the stabilization of the colloids. A whole spectrum of nanoparticle morphologies and shapes was obtained, ranging from nanoagglomerates which are nevertheless well-defined and well-stabilized to nanosized single crystals with triangular shape. Received: 2 February 1998 Accepted: 29 May 1998     

Can the pi-facial selectivity of solvation be predicted by atomistic simulation?

This work is concerned with the rationalization and prediction of solvent and temperature effects in nucleophilic addition to alpha-chiral carbonyl compounds leading to facial diastereoselectivity. We study, using molecular dynamics simulations, the facial solvation of (R)-2-phenyl-propionaldehyde in n-pentane and n-octane at a number of temperatures and compare it with experimental selectivity data for the nBuLi addition leading to syn- and anti-(2R)-2-phenyl-3-heptanol, which give nonlinear Eyring plots with the presence of inversion temperatures. We have found from simulations that the facial solvation changes with temperature and alkane. Moreover, by introducing a suitable molecular chirality index we have been able to predict break temperatures (T(CI)) for the two solvents within less than 20 degrees of the inversion temperatures experimentally observed in the diastereoselective nBuLi addition. We believe this could lead to a viable approach for predicting inversion temperatures and other subtle solvent effects in a number of stereoselective reactions.     

A thorough study of the surface wax of apple fruits

The composition of the surface waxes of three apple ( Malus domestica L.) cultivars ("Florina", "Golden B" and "Ozark Gold") has been studied by means of spectroscopic and GC–MS analysis of the class-fractionated mixture of components. Odd n -alkanes, mainly C₂₇ and C₂₉ molecules, are prevalent in the saturated fraction. Small concentrations of alkenes were also found; the C_28:1 component is strongly (72%) in excess over the other 1-alkenes. Straight-chain esters (mainly of palmitic acid) of saturated primary alcohols (C₁₈–C₃₀) were also detected; whereas the acyl moiety is made up essentially of an even number of carbons, the alcohol counterpart does not exhibit this characteristic. Aldehydes are present (C₂₀–C₃₀) with the homologue patterns C₂₆–C₃₀ most strongly represented. Straight-chain free secondary alcohols characterize the waxes of "Florina" and "Ozark Gold"; the hydroxy function is located far from the extremity of the carbon framework. Outstanding is the presence of three alcohols with 29 carbon centres. These alcohols are accompanied by free straight-chain primary alcohols, mainly with even-numbered carbon chains in the range C₂₆–C₃₀. Free fatty acids are present; all of have a framework of even-numbered carbon chains mainly in the range C₁₆–C₂₀. C_18:1 (oleic acid) is well represented.     

Structure of the hydrated and dimethyl sulfoxide solvated rubidium ions in solution

The structure of the hydrated and the dimethyl sulfoxide solvated rubidium ions in solution has been determined by means of large-angle X-ray scattering (LAXS) and extended X-ray absorption fine structure (EXAFS) studies. The models of the hydrated and dimethyl sulfoxide solvated rubidium ions fitting the experimental data best are square antiprisms with Rb-O bond distances of 2.98(2) and 2.98(3) A, respectively. The EXAFS data show a significant asymmetry in the Rb-O bond distance distribution with C(3) values of 0.0076 and 0.015 A(3), respectively. No second hydration sphere is observed around the hydrated rubidium ion. The dimethyl sulfoxide solvated rubidium ion displays a Rb-O-S bond angle of ca. 130 degrees, which is typical for a medium hard electron acceptor such as rubidium.     

Electron-transfer in molecular functional materials

We discuss electron-transfer processes that govern the physics of several materials or systems of interest for advanced applications. The discussion touches upon several topics, ranging from solvatochromism to solvent-induced symmetry breaking, from excitonic to cooperative effects in molecular crystals, from phase transitions to vibrational contributions to the dielectric constant in organic materials, from spectroscopy to molecular transport. In all these diverse systems electron transfer (ET) plays a major role and is discussed with reference to simple models for delocalized charges.     

Light-emitting molecular machines: pH-induced intramolecular motions in a fluorescent nickel(II) scorpionate complex

A cyclam-like macrocycle has been synthesized with a pendant arm containing a dansylamide group. In the corresponding nickel(II) complex, binding of the pendant arm to the metal is pH controlled. In particular, at pH 4.3, the sulfonamide group deprotonates and coordinates the NiII center, giving rise to a complex of trigonal bipyramidal stereochemistry, as shown by X-ray diffraction studies performed on the crystalline complex salt. At pH 7.5, an OH- ion binds the metal and a six-coordinate species forms. The binding-detachment of the pendant arm to/from the NiII center is signaled by changes in the emission properties of the dansyl subunit in the side chain; the fluorescence of this side chain is high when the pendant arm is not coordinated and low when the sulfonamide group is bound to the metal. The system investigated represents the prototype of a light-emitting molecular machine, driven by a pH change.     

Measurement of the carbonaceous component in the Milan urban particulate matter

The carbonaceous component in the Milan urban particulate matter, i.e. the two components black carbon (BC) and organic carbon (OC), has been measured by means of a thermogravimetric analyzer combined with an infrared spectrophotometer (TGA/FT-IR). While black carbon may be considered a primary pollutant, organic carbon includes both primary emissions and secondary organic aerosols. Since carbonaceous aerosol (including a small quantity of inorganic carbon, too) makes up roughly from 25% to 50% of the average annual PM 2.5 mass concentration, a deeper understanding of this component is required. The TGA/FT-IR technique, employed for the first time to our knowledge for the quantification of the particulate matter carbonaceous component, allows, thought the results here presented are preliminary, to assess the two components BC and OC in a simple way especially if compared with the methods reported in the literature. The total carbon (TC) determinations performed by TGA/FT-IR on Milan urban particulate matter are in good agreement with the results obtained by a total organic carbon (TOC) analyzer operating directly on the solid sample.