Nanoparticles

To enhance the dehydrogenation/rehydrogenation kinetic behavior of the LiBH₄–MgH₂ composite system, TiF₄ is used as an additive. The effect of this additive on the hydride composite system has been studied by means of laboratory and advanced synchrotron techniques. Investigations on the synthesis and mechanism upon hydrogen interaction show that the addition of TiF₄ to the LiBH₄–MgH₂ composite system during the milling procedure leads to the in situ formation of well-distributed nanosized TiB₂ particles. These TiB₂ nanoparticles act as nucleation agents for the formation of MgB₂ upon dehydrogenation process of the hydride composite system. The effect of TiB₂ nanoparticles is maintained upon cycling.

New Rh-NPs stabilised by N-Heterocyclic Carbenes (NHC) were synthesized by decomposition of [Rh(η³-C₃H₅)₃] under H₂ atmosphere and fully characterized. Surface studies by FT-IR and NMR spectroscopy employing isotopically labelled ligands were also performed. The Rh^0.2 NPs are active catalysts in the reduction of various aromatic substrates. In the reduction of phenol, high selectivities to cyclohexanone or cyclohexanol were obtained depending on the reaction conditions. However, this catalytic system exhibited much lower activity in the hydrogenation of substituted phenols. Pyridine was easily hydrogenated under mild conditions and interestingly, the hydrogenation of 4-methyl and 4-trifluoromethylpyridine resulted slower than that of 2-methylpyridine. The hydrogenation of 1-(pyridin-2-yl)propan-2-one provided the β-enaminone 13a in high yield as a consequence of the partial reduction of the pyridine ring followed by isomerization. Quinoline could be either partially hydrogenated to 1,2,3,4-tetrahydroquinoline or fully reduced to decahydroquinoline by adjusting the reaction conditions.

In this work, we describe the synthesis of a new N-heterocyclic carbene (NHC) ligand, derived from a hybrid pyrazole-imidazolium scaffold, namely 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-3H-imidazol-2-ylidene (L). This ligand has been used as a stabilizer for the organometallic synthesis of palladium(0) nanoparticles (Pd NPs). L presents a better stabilizing effect than its pre-carbenic HLCl counterpart, allowing the formation of isolated Pd NPs while HLCl yields aggregated ones. Additionally, molecular Pd(II) coordination compounds of L and HLCl were synthesized and characterized to better understand the coordination modes of these ligands. Both molecular and colloidal Pd systems have been further tested in catalytic C–C coupling processes. Three different types of reactions have been observed depending on the catalytic system: (i) the Suzuki–Miyaura reaction takes place with Pd molecular complexes; (ii) a secondary reaction, the dehalogenation of the substrate, is always detected and (iii) the C–C homocoupling between two molecules of bromoarenes is observed with colloidal catalysts.

Various ligands not forming monometalliccomplexes were used for Ru nanoparticle stabilization,enabling the control of size, shape, and electronic properties.HRMAS NMR spectroscopy allowed us to study surface-bound molecules, evidencing ligand hydrogenation anddecomposition of THFduring the RuNP synthesis. Catalysisstudies underscore the importance of the nature of the ligands.The RuNPs were tested in the hydrogenation of aromatics,showing very high activities (TOF > 60 000 h−1, 40 bar, 393K). A pronounced ligand effect was found, and dialkylarylphosphine ligands gave the fastest catalyst.

Two diastereomeric series of hybridc,c-pep-tides derived from conveniently protected derivatives of(1R,2S)- and (1S,2R)-3-amino-2,2-dimethylcyclobutane-1-carboxylic acid andcis-4-amino-L-proline joined in alter-nation have efficiently been prepared through convergent synthesis. High-resolution NMR experiments show that these compounds present defined conformations in solution affording very compact structures as the result of intra and inter residue hydrogen-bonded ring formation. (R,S)-cyclobutane containing peptides adopt more twisted con-formations than (S,R) diastereomers. In addition, all the sec-peptides have high tendency to aggregation providing vesicles of nanometric size, which were stable when allowed to stand for several days, as verified by transmission electron microscopy.