|Molar mass||804.9026 g/mol|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Crabtree's catalyst is the name given to a complex of iridium with 1,5-cyclooctadiene, tris-cyclohexylphosphine, and pyridine. It is a homogeneous catalyst for hydrogenation reactions, developed by Robert H. Crabtree, a professor at Yale University. The iridium atom in the complex has a square planar molecular geometry, as expected for a d8 complex. Crabtree’s catalyst is air stable and available commercially as orange crystals.
Development of Catalyst
Crabtree, graduate student George Morris and John Derek Woollins discovered this catalyst in the 1970s while working on iridium analogues of Wilkinson's rhodium-based catalyst at the Institut de Chimie des Substances Naturelles at Gif-sur-Yvette, near Paris.
Previous hydrogenation catalysts included Wilkinson’s catalyst and a cationic rhodium(I) complex with two phosphine groups developed by Osborn and Schrock. These catalysts accomplished hydrogenation through displacement; after hydrogen addition across the metal, a solvent or a phosphine group dissociated from the rhodium metal so the olefin to be hydrogenated could gain access to the active site. This displacement occurs quickly for rhodium complexes but occurs barely at all for iridium complexes. Because of this, research at the time focused on rhodium compounds instead of compounds involving transition metals of the third row, like iridium. Wilkinson, Osborn, and Schrock also only used coordinating solvents.
Crabtree noted that the ligand dissociation step was not present in heterogeneous catalysis, and so posited that this step was limiting in homogeneous systems. Because of this, he decided to study catalysts with “irreversibly creat[ed] active sites in a noncoordinating solvent.” This led to the development of the Crabtree catalyst, and use of the solvent CH2CL2, which was found to be noncoordinating.
Crabtree’s catalyst has a much higher turnover frequency than competing reactions for the hydrogenations of mono-, di-, tri-, and tetra-substituted substrates. In fact, while Wilkinson’s catalyst and the Schrock-Osborn catalyst are unable to catalyze the hydrogenation of a tetrasubstituted olefin, Crabtree’s catalyst is able to at high turnover frequencies, as displayed in the table.
|Substrate||Wilkinson's Catalyst||Schrock-Osborn Catalyst||Crabtree's Catalyst|
The catalyst is reactive at room temperature. The reaction is robust without drying solvents or meticulously removing air from added hydrogen. The catalyst is tolerant of functional groups like CO2R but not good ligands like CH2OH or strong bases like NR2. The catalyst is sensitive to proton-bearing impurities.
The catalyst becomes deactivated with time (roughly ten minutes, depending on the catalyst) due to an irreversible deactivation process. The catalysts turn yellow upon deactivation. Deactivation occurs due to the formation of coordination complexes with other molecules of the catalyst. The catalyst, as shown in the figure, forms a dimeric cation with another molecule of the catalyst through 3-center, 2-electron bonds between the Iridium metal centers and hydrogen atoms. This can be prevented by keeping catalyst molecules further apart in solution - for example, by adding dropwise quantities of a dilute catalyst solution into the reaction as it proceeds.
An intermediate of the hydrogenation reaction catalysed by crabtree’s catalyst was able to be isolated. After exposure to hydrogen gas, the red solution of catalyst precursor, [Ir(cod)L2]PF6, in dichloromethane turns colorless; this solution is stable at -80C for several hours. The isolated intermediate is cis-[IrH2(cod)L2]PF6. It appears that the rest of the reaction proceeds through [IrH2(olefin)2L2]+, which collapses to form an alkane without further steps. In this way the catalyst resembles a heterogenous catalyst more than a homogeneous one.
Besides hydrogenation, the catalyst has been used for isomerization and hydroboration.
Crabtree's catalyst can also be used in isotope exchange reactions. More specifically, it catalyzes the direct exchange of a hydrogen atom with its isotopes deuterium and tritium, without the use of an intermediate. It has been shown that isotope exchange with Crabtree’s catalyst is highly regioselective.
Hydrogenation by Crabtree’s catalyst is an Enantioselective synthesis. Because Crabtree’s catalyst is coordinatively unsaturated, it is able to bind and be directed by ligating groups, particularly OH.
The hydrogenation of a certain terpen-4-ol is illustrative of this enantioselectivity, and the differences in enantioselectivity of different catalysts. With palladium on carbon in ethanol the product distribution is 20:80 in favor of the cis isomer (2B in scheme 1). The polar side (with the hydroxyl group) interacts with the solvent leaving the apolar to the catalyst surface. This is due to slight haptophilicty, an effect in which a functional group binds to the surface of a heterogeneous catalyst and directs the reaction. In cyclohexane as solvent the distribution changes to 53:47 because haptophilicty is no long present (there is no directing group on cyclohexane). The distribution changes completely in favor of the cis isomer 2A when Crabtree's catalyst is used in dichloromethane. This selectivity is both predictable and practically useful. Carbonyl groups are also known to direct the hydrogenation by the Crabtree catalyst is highly regioselective.
The directing effect that causes the enantioselectivity of hydrogenation of terpen-4-ol with Crabtree’s catalyst is shown below.
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