RESUMO
Rhodium complexes functionalized by N-heterocyclic carbene ligands react with dioxygen to form adducts. Depending on the specifics of the ancillary ligands, oxygen binds to Rh either as a peroxide to form a fully oxidized Rh(III) complex, or as singlet dioxygen in a Rh(I) square planar complex. We have shown through analysis of a series of compounds, some previously published and some novel, that the presence of additional ligands that would support the formation of an octahedral geometry, as typically found with Rh(III) complexes, is critical for formation of the peroxide. In addition, we have demonstrated through DFT studies, that the potential energy surface with regard to the O-O bond length is relatively shallow, which provides a rationale for the distribution of bond lengths observed for apparently similar complexes analyzed by crystallography.
RESUMO
Two triethoxysilyl-functionalized N-heterocyclic carbene ligands have been synthesized and used to prepare the corresponding second-generation ruthenium olefin metathesis catalysts. These complexes were then grafted onto silica gel, and the resulting materials were efficient heterogeneous catalysts for a number of metathesis reactions. The solid-supported catalysts were shown to be recyclable over a number of reaction cycles, and no detectable levels of ruthenium were observed in reaction filtrates (ruthenium concentration of filtrate <5 ppb).
RESUMO
The reaction of oxygen with rhodium complexes containing N-heterocyclic carbenes was found to give dioxygen complexes with rare square planar geometries and unusually short O-O bond lengths. Analysis of the bonding in these complexes by Rh L-edge X-ray absorption spectroscopy (XAS), Raman spectroscopy, and DFT calculations provides evidence for a bonding model in which singlet oxygen is bound to a Rh(I) d8 metal complex, rather than the more common Rh(III) d6 peroxo species with octahedral geometry and O-O bond lengths in the 1.4-1.5 A range.
Assuntos
Compostos Heterocíclicos/química , Metano/análogos & derivados , Compostos Organometálicos/química , Compostos Organometálicos/síntese química , Oxigênio/química , Ródio/química , Metano/química , Modelos Químicos , Modelos Moleculares , Estrutura Molecular , Espectrometria por Raios X/métodosRESUMO
[reaction: see text] The removal of residual ruthenium after olefin metathesis reactions and asymmetric hydrogenations has been described using amine-functionalized mesoporous silicates. Aminopropyltriethoxy silane-derivatized silicates were found to be the most effective. More than 99.99% of RuCl(3) could be removed in a single treatment. Scavenging of Ru after metathesis reactions using a Grubbs generation 1 catalyst gave a product with concentrations of less than 10 microg/5 mg after two treatments with amine-modified silica. Most importantly, no chromatography was required.
