Crabtrees catalyst is an organoiridium compound with the formula PF6. It is a homogeneous catalyst for hydrogenation and hydrogen-transfer reactions, developed by Robert H. Crabtree and this air stable orange solid is available commercially. The complex has a planar molecular geometry, as expected for a d8 complex. It is prepared from cyclooctadiene iridium chloride dimer, Crabtree’s catalyst is effective for the hydrogenations of mono-, di-, tri-, and tetra-substituted substrates. Whereas Wilkinson’s catalyst and the Schrock–Osborn catalyst do not catalyze the hydrogenation of a tetrasubstituted olefin, the catalyst is reactive at room temperature. The reaction is robust without drying solvents or meticulous deoxygenation of the hydrogen, the catalyst is tolerant of weakly basic functional groups such as ester, but not alcohols or amines. The catalyst is sensitive to proton-bearing impurities, the catalyst becomes irreversibly deactivated after about ten minutes at room temperature, signaled by appearance of yellow color. One deactivation process involves formation of hydride-bridged dimers, besides hydrogenation, the catalyst catalyzes the isomerization and hydroboration of alkenes. Crabtrees catalyst is 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, the hydrogenation of a terpen-4-ol demonstrates the ability of compounds with directing groups to undergo diastereoselective hydrogenation. With palladium on carbon in ethanol the product distribution is 20,80 favoring the cis isomer, the polar side interacts with the solvent. This is due to slight haptophilicity, an effect in which a functional group binds to the surface of a heterogeneous catalyst, in cyclohexane as solvent, the distribution changes to 53,47 because haptophilicity is no long present. The distribution changes completely in favor of the cis isomer 2A when Crabtrees catalyst is used in dichloromethane and this selectivity is both predictable and practically useful. Carbonyl groups are known to direct the hydrogenation by the Crabtree catalyst to be highly regioselective. The directing effect that causes the stereoselectivity of hydrogenation of terpen-4-ol with Crabtree’s catalyst is shown below, previous hydrogenation catalysts included Wilkinson’s catalyst and a cationic rhodium complex with two phosphine groups developed by Osborn and Schrock. 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 does not occur in heterogeneous catalysis, and so posited that this step was limiting in homogeneous systems. They sought catalysts with “irreversibly creat active sites in a noncoordinating solvent. ”This led to the development of the Crabtree catalyst, and use of the solvent CH2Cl2
An example of isomerization with Crabtree's catalyst. The reaction proceeds 98% to completion in 30 minutes at room temperature.
Crabtree catalyst in hydrogenation
Directing effect of an –OH group on diastereoselectivity of hydrogenation by Crabtree's catalyst. Hydrogen is added from the direction of the iridium atom, selecting for the reactivity shown above. Additional ligands on catalyst not shown.