Hydrogenation of Arenes over Catalysts that Combine a Metal Phase and a Grafted Metal Complex: Role of the Single‐Site Catalyst

Abstract

In comparison with the separate components, the combination on the same support material of dispersed palladium nanoparticles and grafted molecular rhodium complexes has provided evidence of improved activity in the hydrogenation of arenes.[1] It was proposed that the catalytic efficiency is a consequence of a hydrogen-spillover process that would enhance specifically the hydrogenation activity of the molecular catalyst.[1] A recent study of the hydrogenation of arenes with a catalyst obtained by silica sol-gel coentrapment of metallic palladium and [Rh(cod)(m-Cl)] 2 disagrees with the hydrogen-spillover hypothesis and suggests that the action of both metals is caused by a synergistic effect; [2] however, no explanation of the nature of this effect has been forthcoming. Herein, we provide a rationale for this synergistic effect, and show that the grafted rhodium complex speeds up the hydrogenation of arenes to cyclic dienes.Three different heterogeneous catalysts were tested in the hydrogenation of some arenes in n-pentane at relatively low temperature (40-60 8C) and 30 bar H 2 . A highly dispersed palladium metallic phase and the Rh I complex [Rh(cod)(sulphos)] (cod = cyclooctadiene; sulphos = À O 3 S(C 6 H 4 )CH 2 -C(CH 2 PPh 2 ) 3 ) were separately supported on high-surfacearea porous silica . A calcination/reduction procedure of silica-supported PdCl 2 was employed to prepare Pd 0 /SiO 2 , 1, with a metal content of about 10 wt %, while the Rh I complex was grafted by using a known procedure involving hydrogen bonds between silanols of the support and sulfonate groups from the sulphos ligand.[3] The single-site catalyst, [Rh(cod)-(sulphos)]/SiO 2 , 2 (Figure 1 A) with a metal content of about 0.5 wt %, was previously employed to catalyze the hydrogenation and hydroformylation of olefins in either gas-solid or liquid-solid phase.[3a] Upon hydrogenation, the cod ligand

Hydrogenation of Arenes and Heteroaromatics

“…A synergistic effect operating at the level of the first H 2 addition (e.g., conversion of benzene to cyclohexadiene) was demonstrated by Bianchini and coworkers for the hydrogenation of various benzenes to cyclohexanes by means of a different class of TCSM catalysts [42]. These differ substantially from Angelici's catalysts for the bonding interaction of the molecular complexes to the support material.…”

Surface-Mediated Design and Catalytic Properties of Active Metal Complexes for Advanced Catalysis Creation

“…Figure 8b shows a series of in situ FT-IR spectra in the¯N O region during the adsorption of NO at 443 K in the absence (b1) and presence (b2) of gas-phase CO. Two peaks of 1849 and 1769 cm ¹1 ( Figure 8b1) were attributed to symmetric and asymmetric stretching vibrations (¯N Osym and¯N Oasym ) of dinitrosyl species, respectively, as suggested by the spectrum of a mixture of 14 NO and 15 NO. The intensities of these two peaks increased with adsorption time and were saturated after 20 min of NO exposure as shown in Figure 8b1.…”

Shaping and Enforcing Coordination Spheres: The Implications of C3 and C1 Chirality in the Coordination Chemistry of 1,1,1‐Tris(oxazolinyl)ethane (“Trisox”)

“…However, there are cases in which this is "overruled" by either the strong ligating tendency of the donor functions or inter-ligand angles of significantly below 908 which are enforced by a chelating coordination. An example for the former is the well established Rh I complex chemistry with the triphos ligand and many of its derivatives and variants, [21] whereas nitrogen donor based tripods tend to favor a simple bidentate chelating coordination to rhodium(I), leaving one ligand "arm" non-coordinated. This has been observed both for neutral trispyridylmethane- [22] and trispyrazolylmethane-rhodium(I) complexes [23] as well as for their monoanionic trispyrazolylborate analogues.…”