Mononuclear metal complex catalysts on supports: foundations in organometallic and surface chemistry and insights into structure, reactivity, and catalysis.

Catalysts that consist of isolated metal atoms bonded to solid supports have drawn wide attention by researchers, with recent work emphasizing noble metals on metal oxide and zeolite supports. Progress has been facilitated by methods for atomic-scale imaging the metals and spectroscopic characterization of the supported structures and the nature of metal-support bonding, even with catalysts in the working state. Because of the intrinsic heterogeneity of support surface sites for bonding of metals and the tendency of noble metal cations on supports to be reduced and aggregated, it is challenging to determine structures of individual metal complexes among the mixtures that may be present and to determine structures of catalytically active species and reactive intermediates. A central premise of this perspective is that synthesis of supported metal complexes that have nearly uniform structures-on supports such as dealuminated HY zeolite, chosen to have relatively uniform surfaces-is a key to fundamental understanding, facilitating progress toward determining the roles of the ligands on the metals, which include the supports and reactive intermediates in catalysis. Characterization of relatively uniform and well-defined samples nonetheless requires multiple spectroscopic, microscopic, and theory-based techniques used in concert and still leaves open many questions about the nature of reactive intermediates and catalytic reaction mechanisms

Chiral analysis by NMR spectroscopy

The analysis of the enantiomeric purity of chiral carboxylic acids requires a reagent to give acceptable NMR chemical shift non-equivalence with a wide range of substrate acids. Extensive studies of the behaviour of N-mono- methyl, N,N-dimethyl and cyclic amines as chiral solvating agents led to the finding that 1,2 diphenyl-1,2-diaminoethane can induce substantial non- equivalence in the diastereomeric salts of chiral a-phenyl and a-halo carboxylic acids. The diastereoisomeric complexes of the diamine with primary carboxylic acids (RCH(_2)CO(_2)H) presents an unusual case in which the internally enantiotopic methylene protons are rendered internally diasteretopic by an external non-covalently bonded reagent. Investigations of the physical parameters determining non-equivalence (stoichiometry, concentration, temperature and substrate enantiomeric purity), combined with NOE observations of the diastereomeric pairs and the crystal structure of the mono- hydrobromide salt were used to suggest the structure for the conformation responsible for shift non-equivalence. The zero valent platinum complex, 3-0-isopropylidene-2,3-dihydroxy-1,4- bis(diphenyl-phosphino)butane-platinum(0)-ethene (DlOP-Pt-ethene) was shown to be a versatile chiral derivatising agent for electron poor and strained η(^2)-donors. This was demonstrated by the enantiomeric purity determinations for alkynes, enones and norbornene derivatives. The crystal structure of DIOP-Pt-ethene was determined and found to be similar to the palladium analogue. If the achiral rhodium complex rhodium(I)-acetylacetone-diethene undergoes a reaction with 2 equivalents of a suitable chiral η(^2)-donor, it will result in the formation of 4 stereoisomers, two meso forms and a pair of enantiomers. The diasteroisomers should display chemical shift non-equivalence in the NMR spectrum of the product, reflecting the enantiomeric purity of the η(^2)-donor (self recognition). The derivatisation of rhodium(l)-acetylacetone-diethene with chiral η(^2)-donors was attempted

Zeolite- and MgO-Supported Molecular Iridium Complexes: Support and Ligand Effects in Catalysis of Ethene Hydrogenation and H-D Exchange in the Conversion of H2 + D2

[EN] Zeolite- and MgO-supported mononuclear iridium diethene complexes were formed by the reaction of Ir(C2H4)2(acac) (acac = acetylacetonate, C5H7O2¿) with each support. Changes in the ligand environment of the supported iridium complexes were characterized by infrared, X-ray absorption near edge structure, and extended X-ray absorption fine structure spectroscopies as various mixtures of H2, C2H4, and CO flowed over the samples. In contrast to the nonuniform metal complexes anchored to metal oxides, our zeolite-supported metal complexes were highly uniform, allowing precise determinations of the chemistry, including the role of the support as a macroligand. Zeolite- and MgO-supported Ir(C2H4)2 complexes are each rapidly converted to Ir(CO)2 upon contact with a pulse of CO, and the ¿CO frequencies indicate that the iridium is more electron-deficient when the support is the zeolite. The Ir(CO)2 complex supported on MgO was highly stable in the presence of various combinations of CO, C2H4, and helium. In contrast, the zeolite-supported Ir(CO)2 complex was found to be highly reactive, forming Ir(CO)3, Ir(CO)(C2H4), Ir(CO)2(C2H4), and Ir(CO)(C2H4)2. The ¿-bonded ethene ligands of the zeolite-supported Ir(C2H4)2 in H2 were facilely converted to ¿-bonded ethyl when treated. However, the stability of the ethene ligands was markedly increased when the support was changed to MgO or when a CO ligand was simultaneously bonded to the iridium. The rates of catalytic ethene hydrogenation and H2/D2 exchange in the presence of a catalyst initially consisting of Ir(C2H4)2 on the zeolite were found to be more than an order of magnitude higher than when MgO was the support. The iridium complexes containing one or more CO ligands were found to be inactive for H2/D2 exchange reactions when the support was MgO, but they were moderately active when it was the zeolite. The effects of the MgO and zeolite supports on reactivity and catalytic activity are attributed to their differences as ligands donating or withdrawing electrons, respectively.Lu, J.; Serna Merino, PM.; Gates, BC. (2011). Zeolite- and MgO-Supported Molecular Iridium Complexes: Support and Ligand Effects in Catalysis of Ethene Hydrogenation and H-D Exchange in the Conversion of H2 + D2. ACS CATALYSIS. 1(11):1549-1561. doi:10.1021/cs200397rS1549156111

Zeolite- and MgO-Supported Molecular Iridium Complexes: Support and Ligand Effects in Catalysis of Ethene Hydrogenation and H–D Exchange in the Conversion of H₂ + D₂

Zeolite- and MgO-supported mononuclear iridium diethene complexes were formed by the reaction of Ir(C₂H₄)₂(acac) (acac = acetylacetonate, C₅H₇O₂^–) with each support. Changes in the ligand environment of the supported iridium complexes were characterized by infrared, X-ray absorption near edge structure, and extended X-ray absorption fine structure spectroscopies as various mixtures of H₂, C₂H₄, and CO flowed over the samples. In contrast to the nonuniform metal complexes anchored to metal oxides, our zeolite-supported metal complexes were highly uniform, allowing precise determinations of the chemistry, including the role of the support as a macroligand. Zeolite- and MgO-supported Ir(C₂H₄)₂ complexes are each rapidly converted to Ir(CO)₂ upon contact with a pulse of CO, and the ν_CO frequencies indicate that the iridium is more electron-deficient when the support is the zeolite. The Ir(CO)₂ complex supported on MgO was highly stable in the presence of various combinations of CO, C₂H₄, and helium. In contrast, the zeolite-supported Ir(CO)₂ complex was found to be highly reactive, forming Ir(CO)₃, Ir(CO)(C₂H₄), Ir(CO)₂(C₂H₄), and Ir(CO)(C₂H₄)₂. The π-bonded ethene ligands of the zeolite-supported Ir(C₂H₄)₂ in H₂ were facilely converted to σ-bonded ethyl when treated. However, the stability of the ethene ligands was markedly increased when the support was changed to MgO or when a CO ligand was simultaneously bonded to the iridium. The rates of catalytic ethene hydrogenation and H₂/D₂ exchange in the presence of a catalyst initially consisting of Ir(C₂H₄)₂ on the zeolite were found to be more than an order of magnitude higher than when MgO was the support. The iridium complexes containing one or more CO ligands were found to be inactive for H₂/D₂ exchange reactions when the support was MgO, but they were moderately active when it was the zeolite. The effects of the MgO and zeolite supports on reactivity and catalytic activity are attributed to their differences as ligands donating or withdrawing electrons, respectively