Crabtree's catalyst
Encyclopedia
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 and graduate student George Morris 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. One advantage of Crabtree's catalyst is that it is about 100 times more active than Wilkinson's and can hydrogenate even tri- and tetrasubstituted alkene
s.
Crabtree's catalyst has also been used as the basis for the development of newer catalysts; by modifying the ligands, one can modulate the properties of the catalyst. For example, use of chiral ligands has led to the development of enantioselective catalysts.
In the hydrogenation
of a certain terpen-4-ol the comparison with traditional catalysts works out as follows.
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. In cyclohexane
as solvent
the distribution changes to 53:47 where the polar side now has a slight preference for the catalyst. The distribution changes completely in favor of the cis isomer 2A when Crabtree's catalyst is used in dichloromethane
. This directing effect is due to a bonding interaction of the hydroxyl group with the iridium center. Carbonyl groups are also known to direct the hydrogenation by the Crabtree catalyst.
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
of iridium
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
with 1,5-cyclooctadiene
1,5-Cyclooctadiene
1,5-Cyclooctadiene is the organic compound with the chemical formula C8H12. Generally abbreviated COD, this diene is a useful precursor to other organic compounds and serves as a ligand in organometallic chemistry.- Synthesis :...
, tris-cyclohexylphosphine, and pyridine
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
. It is a homogeneous catalyst for hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
reactions, developed by Robert H. Crabtree
Robert H. Crabtree
Robert Howard Crabtree is a British chemist. He is presently serving as Whitehead Professor of Chemistry at Yale University in the United States. He is a naturalized citizen of the United States...
, a professor at Yale University
Yale University
Yale University is a private, Ivy League university located in New Haven, Connecticut, United States. Founded in 1701 in the Colony of Connecticut, the university is the third-oldest institution of higher education in the United States...
. The iridium atom in the complex has a square planar molecular geometry, as expected for a d8 complex.
Crabtree and graduate student George Morris discovered this catalyst in the 1970s while working on iridium analogues of Wilkinson's
Wilkinson's catalyst
Wilkinson's catalyst is the common name for chlorotrisrhodium, a coordination compound with the formula RhCl3 . It is named after the late organometallic chemist and 1973 Nobel Laureate, Sir Geoffrey Wilkinson who popularized its use.-Structure and basic properties:The compound is a square planar,...
rhodium
Rhodium
Rhodium is a chemical element that is a rare, silvery-white, hard and chemically inert transition metal and a member of the platinum group. It has the chemical symbol Rh and atomic number 45. It is composed of only one isotope, 103Rh. Naturally occurring rhodium is found as the free metal, alloyed...
-based catalyst at the Institut de Chimie des Substances Naturelles
Institut de Chimie des Substances Naturelles
The Institut de Chimie des Substances Naturelles is part of the Centre national de la recherche scientifique, France's most prominent public research organization. It is located at Gif-sur-Yvette, near Paris....
at Gif-sur-Yvette, near Paris. One advantage of Crabtree's catalyst is that it is about 100 times more active than Wilkinson's and can hydrogenate even tri- and tetrasubstituted alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s.
Crabtree's catalyst has also been used as the basis for the development of newer catalysts; by modifying the ligands, one can modulate the properties of the catalyst. For example, use of chiral ligands has led to the development of enantioselective catalysts.
In the hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
of a certain terpen-4-ol the comparison with traditional catalysts works out as follows.
With palladium on carbon
Palladium on carbon
Palladium on carbon, often referred to as Pd/C, is a form of palladium used for catalysis. It is usually used for catalytic hydrogenations in organic chemistry...
in ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
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. In cyclohexane
Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...
as solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
the distribution changes to 53:47 where the polar side now has a slight preference for the catalyst. The distribution changes completely in favor of the cis isomer 2A when Crabtree's catalyst is used in dichloromethane
Dichloromethane
Dichloromethane is an organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is widely used as a solvent. Although it is not miscible with water, it is miscible with many organic solvents...
. This directing effect is due to a bonding interaction of the hydroxyl group with the iridium center. Carbonyl groups are also known to direct the hydrogenation by the Crabtree catalyst.