Rhodium Colloidal Suspensions Stabilised by Poly-N-donor Ligands in Non-Aqueous Ionic Liquids: Preliminary Investigation into the CatalyticHydrogenation of Arenes

摘要

Ionic liquids (ILs), and more particularly imidazolium salts, haverecently been used as templates for the preparation of stablenoble-metal nanoparticles.111 These particles stabilised in ILshave proved to be efficient multiphase catalytic systems forvarious reactions,t21 such as hydrogenation reactions.131 ILs arethus promising as they could play the dual role of solvent andprotective agent. In that area, Dupont and co-workers havelargely reported the synthesis of active metallic colloids incommon ILs such as I-n-butyl-3-methylimidazolium hexafluoro-phosphate or tetrafluoroborate ([13MI][PF61 or [BMI][BF4]) in vari-ous catalytic reactions.vn However, in some cases, the aggrega-tion of Rh° or Ir°nanoparticles has been observed during thecatalytic hydrogenation of arene compounds:5'2bl Consequent-ly, the addition of a stabiliser could be required to avoid theagglomeration of rhodium nanospecies and to increase theirstability in ILs. Different protective agents have already provedtheir efficiency, such as polyRN-vinyl-2-pyrrolidone)-co-(1-vinyl-3-alkylimidazolium halide)] copolymers,161 or poly(N-vinyl-2-pyr-rolidone)P3 This combination of ionic liquid and extra stabiliserhas shown synergistic catalytic effects in terms of activity, se-lectivity and catalytic lifetime. Another approach relies on theuse of ligands on the metal surface, such as phenanthroline-protected palladium nanocatalysts for olefin hydrogenation in[BMI][PF6].181 More recently, our group reported the efficientstabilisation of zero-valent Rh°nanoparticles in different non-aqueous ionic liquids with 2,2'-bipyridine (2,2'-Bipy) as an effi-cient N-donor ligand and their interesting activities and selec-tivities in arene hydrogenation reactions.191 We have also ex-tended this study to bipyridine isomers (3,3'-Bipy and 4,4'-Bipy) and showed the influence of the coordination mode ofthe ligand at the nanoparticle surface on the catalytic activi-ty.[10] Moreover, to avoid the formation of carbenes on the sur-face, we have also investigated the use of various salts such asmethylbutylpyridinium or pyrrolidinium salts.191°1 In fact, thepresence of carbenes has recently been reported to justify thatBMI can be potent poisons of nanocluster catalysts." In ourcase, no poisoning effect in terms of activity has been ob-served with the use of RhCI3·H20 as precursor of nanocatalysts in [BMI][PF6]. On the basis of the efficient results obtained withBipy ligands, we report herein the stabilisation by polynitrogenligands with more complex structures, such as triazine apd-pyr-azine derivatives and particularly 2,4,6-tris(2-pyridyl)-s-triazine(TPTZ) and tetra-2-pyridinylpyrazine (TPPZ; Figure 1), and:com-pare their catalytic activity and selectivity with those obtainedfor 2,2'-Bipy-protected systems.