The local structure of Rh in new catalysts based on Rhodium N-heterocycled carbenes located on graphene surfaces

Abstract

We here report an x-ray absorption spectroscopic (XAS) study of a set of hybrid catalysts with Rh. This task focuses on the development and characterization of new effective heterogeneous catalysts, easily recoverable, of great interest for industrial processes. In particular, the hydrosilylation of multiple C-C bonds what is one of the most direct and efficient ways of preparing functionalized alkyl/alkenylsilanes. These silanes are quite versatile intermediate compounds in organic synthesis and represent a more sustainable alternative to stannanos or boranes due to its non-toxic nature. Rh(I) complexes based on N-heterocyclic carbenes showed a high efficiency in the hydrosilylation of terminal alkynes. However, this was a homogeneous system with a consequent difficulty in separating the catalyst for later uses. Therefore, the development of heterogeneous catalysts with similar activity would be a breakthrough in these processes. Our strategy to achieve this goal was to anchor Rh-NHC carbenes in functionalized graphene oxides (GO) through a three steps procedure. First, the azolium salts were linked to the GO walls directly through a C-N bond obtained by reaction of the carbon material with azide groups, and secondly, the azolium salts formed by reaction with alkynes were the precursors of these mesoinic NHC-carbenes bonded to the rhodium by a posterior third reaction. Different salts were tested in order to modify the complex solubility in water. Room temperature x-ray absorption measurements at the Rh K-edge were carried at the Claess beam line of the Alba synchrotron facility (Barcelona, Spain). The energy resolution ΔE/E was estimated to be about 8 × 10−5 at the Rh K-edge, and a foil of Rh metal was simultaneously measured for energy calibration. Different homogeneous and heterogeneous catalysts were measured to determine changes in the local structure around Rh atoms. The extended X-ray absorption fine structure (EXAFS) spectra were analyzed using the ARTEMIS program. The fits were carried out in R space using a Hanning window for filtering purposes. Our XAS results confirms an oxidation state of Rh(I) in both, homogeneous and heterogeneous materials. We were successful in refining the local structure from EXAFS spectra and we have related the catalytic activity to the changes of the ligands surrounding the Rh atoms. One surprising result was the fragility of the catalysts made using azolium salts. In spite of the high activity of those hetereogeneous catalysts based on 1,2,3-triazole ligands supported on graphene oxides in hydrosilylation of unsaturated compounds, our measurements revealed a continuous diminution of the supported Rh after each catalytic cycle.