Rhodium Complex with Ethylene Ligands Supported on Highly Dehydroxylated MgO:  Synthesis, Characterization, and Reactivity

Abstract

Mononuclear rhodium complexes with reactive olefin ligands, supported on MgO powder, were synthesized by chemisorption of Rh(C(2)H(4))(2)(C(5)H(7)O(2)) and characterized by infrared (IR), (13)C MAS NMR, and extended X-ray absorption fine structure (EXAFS) spectroscopies. IR spectra show that the precursor adsorbed on MgO with dissociation of acetylacetonate ligand from rhodium, with the ethylene ligands remaining bound to the rhodium, as confirmed by the NMR spectra. EXAFS spectra give no evidence of Rh-Rh contributions, indicating that site-isolated mononuclear rhodium species formed on the support. The EXAFS data also show that the mononuclear complex was bonded to the support by two Rh-O bonds, at a distance of 2.18 A, which is typical of group 8 metals bonded to oxide supports. This is the first simple and nearly uniform supported mononuclear rhodium-olefin complex, and it appears to be a close analogue of molecular catalysts for olefin hydrogenation in solution. Correspondingly, the ethylene ligands bonded to rhodium in the supported complex were observed to react with H(2) to form ethane, and the supported complex was catalytically active for the ethylene hydrogenation at 298 K. The ethylene ligands also underwent facile exchange with C(2)D(4), and exposure of the sample to carbon monoxide led to the formation of rhodium gem dicarbonyls.

Simulating the Complexities of Heterogeneous Catalysis with Model Systems: Case studies of SiO2 Supported Pt-Group Metals

“…For example, studies of alkene hydrogenation on supported Rh 6 and Ir 6 clusters [30] have indicated lower TOF than corresponding bulk surfaces. While it has been shown via catalytic studies that carefully prepared supported mononuclear Rh [28,29] and Rh clusters [30] are active for C 2 H 4 hydrogenation [28,30] and ligand exchange (CO $ C 2 H 4 ) reactions [28,29], it may be that the Rh atom ensembles, the binding environment, or the electronic structure present on larger NPs may assist in facilitating the activity of multi-adsorbate reactions such as CO insertion.…”

“…Clusters can be deposited via mononuclear precursor techniques (such as Rh(CO) 2 (acac)) or via molecular cluster precursors to generate surfaces preserving a specific metal cluster structure (for example Ir 4 (CO) 12 to generate Ir 4 clusters) [12,13]. These synthesized surfaces have enabled insightful investigations of reactivity, cluster coordination, support interaction, and adsorbates under relevant reaction condtions on well-defined metal clusters [12][13][14][15][28][29][30]. Size-selected clusters can also be generated on planar oxide supports under UHV conditions by utilizing a metal evaporation source coupled with a quadrupole setup to selectively guide clusters to the substrate surface based on size [25][26][27].…”

Planar oxide supported rhodium nanoparticles as model catalysts

“…While it has been shown via catalytic studies that carefully prepared supported mononuclear Rh (39,40) and Rh clusters (41) are active for C 2 H 4 hydrogenation (39, 41) and ligand exchange (CO ↔ C 2 H 4 ) reactions (39,40), it may be that Rh atom ensembles or the binding environment present on larger NPs may assist in facilitating the activity of multiadsorbate reactions such as CO insertion.…”

Observation of Cluster Formation from MgO-supported Mononuclear Ir-Complexes by Aberration-Corrected STEM

“…The formation of extremely small supported metal clusters, starting from the simplest case of essentially molecular, atomically dispersed catalysts -in our case, supported mononuclear iridium complexes, has been recorded by high-resolution STEM imaging at the atomic scale in real time. This investigation is one of the very first atomic-scale investigations of such structural changes, and thus it points the way to determination of significant information about the dependence of catalytic activity on structure of supported catalysts.The starting material was a MgO-supported mononuclear catalyst containing 1 wt% Ir prepared by the reaction of Ir(C 2 H 4 ) 2 (acac) [acac is CH 3 COCHCOCH 3 ] with dehydroxylated high-area MgO powder [1]. Individual Ir atoms were observed via high angle annular dark-field (HAADF) STEM imaging.…”

“…The starting material was a MgO-supported mononuclear catalyst containing 1 wt% Ir prepared by the reaction of Ir(C 2 H 4 ) 2 (acac) [acac is CH 3 COCHCOCH 3 ] with dehydroxylated high-area MgO powder [1]. Individual Ir atoms were observed via high angle annular dark-field (HAADF) STEM imaging.…”