Highly dealuminated Y zeolite-supported mononuclear iridium complexes with reactive ethylene ligands were synthesized by chemisorption of Ir(C2H4)2(C5H7O2). The resultant structure and its treatment in He, CO, ethylene, and H2 were investigated with infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. The IR spectra show that Ir(C2H4)2(C5H7O2) reacted readily with surface OH groups of the zeolite, leading to the removal of C5H7O2 ligands and the formation of supported mononuclear iridium complexes, confirmed by the lack of Ir−Ir contributions in the EXAFS spectra. The EXAFS data show that each Ir atom was bonded to four carbon atoms at an average distance of 2.10 Å, consistent with the presence of two ethylene ligands per Ir atom and in agreement with the IR spectra indicating π-bonded ethylene ligands. The EXAFS data also indicate that each Ir atom was bonded to two oxygen atoms of the zeolite at a distance of 2.15 Å. The supported iridium−ethylene complex reacted with H2 to give ethane, and it also catalyzed ethylene hydrogenation at atmospheric pressure and 294 K. Treatment of the sample in CO led to the formation of Ir(CO)2 complexes bonded to the zeolite. The sharpness of the υCO bands indicates a high degree of uniformity of these complexes on the support. The iridium−ethylene complex on the crystalline zeolite support is inferred to be one of the most nearly uniform supported metal complex catalysts. The results indicate that it is isostructural with a previously reported rhodium complex on the same zeolite; thus, the results are a start to a family of analogous, structurally well-defined supported metal complex catalysts.

The reaction of Rh(C2H4)2(acac) with the partially dehydroxylated surface of dealuminated zeolite Y (calcined at 773 K) and treatments of the resultant surface species in various atmospheres (He, CO, H2, and D2) were investigated with infrared (IR), extended X-ray absorption fine structure (EXAFS), and 13C NMR spectroscopies. The IR spectra show that Rh(C2H4)2(acac) reacted readily with surface OH groups of the zeolite, leading to loss of acac ligands from the Rh(C2H4)2(acac) and formation of supported mononuclear rhodium complexes, confirmed by the lack of Rh-Rh contributions in the EXAFS spectra; each Rh atom was bonded on average to two oxygen atoms of the zeolite surface with a Rh-O distance of 2.19 A. IR, EXAFS, and 13C NMR spectra show that the ethylene ligands remained bonded to the Rh center in the supported complex. Treatment of the sample in CO led to the formation of site-isolated Rh(CO)2 complexes bonded to the zeolite. The sharpness of the nu(CO) bands in the IR spectrum gives evidence of a nearly uniform supported Rh(CO)2 complex and, by inference, the near uniformity of the mononuclear rhodium complex with ethylene ligands from which it was formed. The supported complex with ethylene ligands reacted with H2 to give ethane, and it also catalyzed ethylene hydrogenation at 294 K.

The best transition-metal complex catalysts are characterized by both high activities and high selectivities, with the latter typically associated with the uniqueness of the catalytic species. Optimal use in technology also demands ease of separation of catalysts from products, and the goal of efficient separation has been a primary motivation for research with supported metal complexes, which offer the added benefit of being resistant to corrosion. General challenges in the synthesis of practical single-site catalysts on supports include the following: a) low catalytic activity associated with nonoptimal ligands, which include the support and ligands remaining from the precursor metal complex that is adsorbed on the support; b) non-uniform distribution of metal sites as a consequence of the intrinsic heterogeneity of the surfaces of the inorganic support; and c) loss of structural integrity of the catalysts during operation, sometimes associated with reduction and sintering of the metal to form species of non-uniform (and often poorly characterized) metal nuclearity and often undesired oxidation states of the metal.Several research groups have pursued the synthesis and testing of organometallic precursors suitable for the preparation of supported catalysts and meet the criteria stated above. [1,2] In particular, Basset and co-workers [1] have developed precursors that, when bonded to silica, closely mimic structures that may exist in catalytic cycles, with the expectation that these surface species will readily enter catalytic cycles once presented with suitable reagents (point (a), above). [1] We have been drawn to zeolites as potential supports because their well-defined, highly uniform adsorption sites seem to address point (b) above. [3,4] As shown below and elsewhere, EXAFS (extended X-ray absorption fine structure) spectroscopy, used in concert with IR and NMR spectroscopic techniques, provides the means to test directly the efficacy of synthetic procedures intended to provide specific mononuclear (or polynuclear) metal sites. [5] Herein, we report the synthesis of the first supported mononuclear Rh + species with exchangeable ethylene ligands; the support is dealuminated zeolite Y. The isotopically unmodified ethylene ligands (natural abundance) introduced in the synthesis of the catalyst are easily and completely exchanged by either 2 H-or 13 C-labeled ethylene, as verified by IR and NMR spectroscopy, respectively. Furthermore, the assynthesized or 13 C-exchanged ethylene ligands readily undergo reduction with autogenous hydrogen (i.e., reverse spillover), as expected for an active catalyst.Central to this report, by using variable-temperature 13 C CP-MAS (cross-polarization magic-angle spinning) NMR spectroscopy we observed that all of the [ 13 C 2 ]ethylene ligands on the zeolite-supported catalyst undergo anisotropic rotation at the same frequency at a given temperature which is characteristic of the precursor compound either in solution or as a crystalline compound. [6] Given the extreme sensitivity ...

Mononuclear La2O3-supported AuIII complexes synthesised from AuIII(CH3)2(C5H7O2) and characterised by X-ray absorption spectroscopy are highly active, stable CO oxidation catalysts at room temperature, demonstrating the importance of the support in stabilizing catalytically active gold species, which need not include zerovalent gold for high activity.

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Oxidative fragmentation of the clusters Os(3)(CO)(12) adsorbed on MgO powder was investigated by X-ray absorption spectroscopy and scanning transmission electron microscopy (STEM). Exposure of the clusters to air leads to their fragmentation, oxidation of the osmium, and formation of ensembles consisting of three Os atoms. X-ray absorption near-edge spectra demonstrate the oxidative nature of the fragmentation process. Extended X-ray absorption fine structure (EXAFS) spectra indicate an average Os-Os distance of 3.33 Angstrom and an Os-Os coordination number of 2, consistent with the formation of ensembles of three Os atoms on the support. STEM images confirm the presence of such trinuclear ensembles, and the diameters of the observed scattering centers (6.0 Angstrom) match that indicated by the EXAFS results.

By anchoring metal complexes to supports, researchers have attempted to combine the high activity and selectivity of molecular homogeneous catalysis with the ease of separation and lack of corrosion of heterogeneous catalysis. However, the intrinsic nonuniformity of supports has limited attempts to make supported catalysts truly uniform. We report the synthesis and performance of such a catalyst, made from [Rh(C(2)H(4))(2)(CH(3)COCHCOCH(3))] and a crystalline support, dealuminated Y zeolite, giving {Rh(C(2)H(4))(2)} groups anchored by bonds to two zeolite oxygen ions, with the structure determined by extended X-ray absorption fine structure (EXAFS) spectroscopy and the uniformity of the supported complex demonstrated by (13)C NMR spectroscopy. When the ethylene ligands are replaced by acetylene, catalytic cyclotrimerization to benzene ensues. Characterizing the working catalyst, we observed evidence of intermediates in the catalytic cycle by NMR spectroscopy. Calculations at the level of density functional theory confirmed the structure of the as-synthesized supported metal complex determined by EXAFS spectroscopy. With this structure as an anchor, we used the computational results to elucidate the catalytic cycle (including transition states), finding results in agreement with the NMR spectra.

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.