Diiron bis-cyclopentadienyl complexes as transfer hydrogenation catalysts: The key role of the bridging aminocarbyne ligand

The catalytic activity of a series of diiron complexes based on the {Fe2Cp2(CO)(x)} core (x = 2-3) and containing a bridging aminocarbyne ligand was screened in transfer hydrogenation reaction of cyclohexanone from isopropanol. The series includes cationic tricarbonyl complexes, [1a-d]CF3SO3, and neutral derivatives obtained by substitution of one carbonyl with hydride (2a-c), cyanide (3a-d) or chloride (4a) ligands. The novel compounds 2a-b, 3a-b and 4a were characterized by analytical and spectroscopic techniques, and the single crystal X-ray structure of one isomer of 4a was determined. In general, diiron complexes exhibited a moderate activity in combination with potassium hydroxide; [Fe2Cp2(CN)(CO)(mu-CO){mu-CN (Me)(4-C6H4OMe)}], 3a, emerged as the best catalyst, and the study of its activity was extended to a range of other ketones. DFT calculations suggest an unusual carbyne-centred mechanism, and the better performance displayed by 3a is ascribable to the stabilizing effect provided by the cyanide co-ligand, which is experimentally supported by IR analyses

Bypassing the Inertness of Aziridine/CO2 Systems to Access 5-Aryl-2-Oxazolidinones: Catalyst-Free Synthesis Under Ambient Conditions

The development of sustainable synthetic routes to access valuable oxazolidinones via CO2 fixation is an active research area, and the aziridine/carbon dioxide coupling has aroused a considerable interest. This reaction features a high activation barrier and thus requires a catalytic system, and may present some other critical issues. Here, the straightforward gram-scale synthesis of a series of 5-aryl-2-oxazolidinones was developed at ambient temperature and atmospheric CO2 pressure, in the absence of any catalyst/co-catalyst. The key to this innovative procedure consists in the direct transfer of the pre-formed amine/CO2 adduct (carbamate) to common aziridine precursors (dimethylsulfonium salts), replacing the classical sequential addition of amine (intermediate isolation of aziridine) and then CO2. The reaction mechanism was investigated by NMR spectroscopy and DFT calculations applied to model cases

Drafting a prioritized checklist of Crop Wild Relatives and Wild Harvested Plants of Italy: problems and solutions

The National checklists of Crop Wild Relatives (CWR) and Wild Harvested Plants (WHP) are the basic tools for the development of in situ and ex situ conservation strategies of plant genetic resources. Here we discuss the methodologies and the prioritization process we previously used in the preparation of the prioritized checklist of CWR and WHP for Italy. The starting point were the most up-to-date Italian checklists of native and alien flora with their updates used as a nomenclatural and distributive source of data. Sardinia and Sicily were kept separate from peninsular Italy to perform detailed analyses focused on the taxa of the two major islands. The origin, the endemic status, cultivation, economic importance, uses, gene pool or taxon group, and the Red List status information were added. The WHP status was attributed to all the taxa with known direct uses. A qualitative approach was adopted in the prioritization process, the main criteria used were: 1) the inclusion of wild relative taxa of crops listed in Annex I of the International Treaty on Plant Genetic Resources for Food and Agriculture (ITPGRFA) and/or by the Italian Institute of Statistics (ISTAT) for cultivated areas and yield in the last 5 years; 2) the threatened taxa occurring in national or global Red lists; 3) the endemism. This prioritization process generated 102 taxa as most in need of specific protection and /or monitoring measures, 57 taxa requiring monitoring because of their restricted distribution although not requiring specific protection measures and 735 taxa not requiring any specific protection. However, different prioritization processes could have been applied to the Italian checklists yielding different results. Here we discuss the matter

Ruthenium(II) Tris-Pyrazolylmethane Complexes in Transfer Hydrogenation Reactions

While ruthenium(II) arene complexes have been widely investigated for their potential in catalytic transfer hydrogenation, studies on homologous compounds replacing the arene ligand with the six-electron donor tris(1-pyrazolyl)methane (tpm) are almost absent in the literature. The reactions of [RuCl(κ3-tpm)(PPh3)2]Cl, 1, with a series of nitrogen ligands (L) proceeded with selective PPh3 mono-substitution, affording the novel complexes [RuCl(κ3-tpm)(PPh3)(L)]Cl (L=NCMe, 2; NCPh, 3; imidazole, 4) in almost quantitative yields. Products 2–4 were fully characterized by IR and multinuclear NMR spectroscopy, moreover the molecular structure of 4 was ascertained by single crystal X-ray diffraction. Compounds 2–4 were evaluated as catalytic precursors in the transfer hydrogenation of a series of ketones with isopropanol as the hydrogen source, and 2 exhibited the highest activity. Extensive NMR experiments and DFT calculations allowed to elucidate the mechanism of the transfer hydrogenation process, suggesting the crucial role played by the tpm ligand, reversibly switching from tri- to bidentate coordination during the catalytic cycle

Post-Modification of the Electronic Properties by Addition of π-Stacking Additives in N-Heterocyclic Carbene Complexes with Extended Polyaromatic Systems

A series of iridium complexes containing phenanthro[4,5-abc]phenazino[11,12-d]imidazol-2-ylidene and acetonaphtho[1,2-b]quinoxaline[11,12-d]imidazol-2-ylidene ligands have been obtained and fully characterized. These complexes display highly extended polyaromatic systems attached to the backbone of the N-heterocyclic carbene. The presence of this extended polyaromatic system makes the electron-donating character of these ligands sensitive to the presence of π-stacking additives, such as pyrene and hexafluorobenzene. The computational studies predict that the addition of pyrene affords an increase of the electron-donating character of the polyaromatic ligand (TEP decreases), while the addition of hexafluorobenzene has the opposite effect (TEP increases). This prediction is experimentally corroborated by IR spectroscopy, by measuring the shift of the CO stretching bands of a series of IrCl(NHC)(CO)2 complexes, where NHC is the N-heterocyclic carbene ligand with the polyaromatic system. Finally, the energy of the π-stacking interaction of one of the key Ir(I) complexes with pyrene and hexafluorobenzene has been estimated by using the Benesi-Hildebrand treat-ment, based on the δ-shifts observed by 1H NMR spectroscopy.MEC of Spain (CTQ2011-24055/BQU

Carbonyl-isocyanide mono-substitution in [Fe2Cp2(CO)4]: A re-visitation

The reactions of [Fe2Cp2(CO)4] with a series of isocyanides, CNR, were conducted in acetonitrile and afforded, after a thermal treatment, the mono-isocyanide derivatives [Fe2Cp2(CO)3(CNR)] [R = 1H-indol-5-yl, 1; CH2P(O)(OEt)2, 2; Cy = C6H11, 3; 4-C6H4OMe, 4; Xyl = 2,6-C6H3Me2, 5; Me, 6; 2-naphthyl, 7; Bn = CH2Ph, 8]. In order to avoid multiple substitution, the diiron reactant was used in a molar excess with respect to the isocyanide (1.6 equivalents; 1.1 for the synthesis of 8). The products were separated from unreacted [Fe2Cp2(CO)4] by chromatography or via reversible protonation, and finally isolated in 50\u201383% yields. IR and NMR spectroscopy indicate that the isocyanide ligand is bridging coordinated in 2 and 7, terminal in 3 and 5, while in the remaining cases a mixture of terminal- and bridging-CNR isomers is obtained. The molecular structure of 5 was ascertained by X-ray diffraction. In general, the coordination mode of the isocyanide is scarcely influenced by the environment (solvents with different polarities, solid state). Sluggish partial isocyanide migration from terminal to bridging position was recognized for 3 and 5 upon heating in refluxing toluene, a process which was reproduced by DFT calculations

Assessing the effects of covalent, dative and halogen bonds on the electronic structure of selenoamides

Here, the interaction between N,N-dimethyl-selenobenzoamide PhC(=Se)NMe2 (1) and various Lewis acids of different strengths, namely IC6F13(I), B(C6F5)(3) (B) and Met, has been analysed by a combined experimental and theoretical approach. In all cases, an increase of the C-NMe2 rotational barrier has been evidenced and quantified by H-1-variable temperature-exchange NMR spectroscopy (VT-EXSY) in the cases of 1-I and 1-B. For B(C6F5)(3), the structure of the adduct has been elucidated by single crystal X-ray diffraction, allowing the lengthening of the Se-C bond (186 pm) and the consequent double characteristic of the C-NMe2 bond (131 pm) to be measured. Computational studies (mainly natural bond orbitals and natural orbital for chemical valence analyses) gave precious insight into the effect of various Lewis acids on the electronic structure of 1. The advantages and the limitations of this new method to characterize chemical interactions are discussed

An ab initio benchmark and DFT validation study on Gold(I)-catalyzed hydroamination of alkynes

High level ab initio calculations have been carried out on an archetypal gold(I)-catalyzed reaction: hydroamination of ethyne. We studied up to 12 structures of possible gold(I)-coordinated species modeling different intermediates potentially present in a catalytic cycle for the addition of a protic nucleophile to an alkyne. The benchmark is used to evaluate the performances of some popular density functionals for describing geometries and relative energies of stationary points along the reaction profile. Most functionals (including hybrid or meta-hybrid) give accurate structures but large nonsystematic errors (4-12 kcal/mol) along the reaction energy profile. The double hybrid functional B2PLYP outperforms all considered functionals and compares very nicely with our reference ab initio benchmark energies. Moreover, we present an assessment of the accuracy of commonly used approaches to include relativistic effects, such as relativistic effective potentials and a scalar ZORA Hamiltonian, by a comparison with the results obtained using a relativistic all-electron four-component Dirac-Kohn-Sham method. The contribution of nonscalar relativistic effects in gold(I)-catalyzed reactions, as we investigated here, is expected to be on the order of 1 kcal/mol. \ua9 2014 American Chemical Society