Crabtree's catalyst
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{{Chembox new
| ImageFile = crabtree.png
| ImageSize = 220px
| ImageName = Crabtree's catalyst
| ImageFile1 = Crabtree's-catalyst-cation-3D-sticks.png
| IUPACName = (''SP''-4)tris(cyclohexyl)phosphane<br />[(1-2-η:5-6-η)-cycloocta-1,5-diene]<br />pyridineiridium(1+) hexafluoridophosphate(1−)
| Section1 = {{Chembox Identifiers
| CASNo = 64536-78-3
}}
| Section2 = {{Chembox Properties
| Formula = C<sub>31</sub>H<sub>50</sub>F<sub>6</sub>IrNP<sub>2</sub>
| MolarMass = 804.9026 g/mol}}
}}
'''Crabtree's catalyst''' is the name given to a [[complex (chemistry)|complex]] of [[iridium]] with [[1,5-Cyclooctadiene|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 d<sup>8</sup> complex.<ref>{{cite journal|author = [[Robert H. Crabtree|Crabtree, R. H.]] | title = Iridium compounds in catalysis | journal = Acc. Chem. Res | year = 1979 | volume = 12 | pages = 331–337 | doi = 10.1021/ar50141a005}}</ref><ref>{{cite journal|author=Brown, J. M. |title = Directed Homogeneous Hydrogenation | journal = [[Angew. Chem. Int. Ed.]] | date = 1987 | volume = 26 | pages = 190–203}}</ref>
Crabtree and graduate student George Morris discovered this catalyst in the 1970s while working on iridium analogues of [[Wilkinson's catalyst|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.<ref>{{cite journal|title = Directing effects in homogeneous hydrogenation with [Ir(cod)(PCy3)(py)]PF6 | author = [[Robert H. Crabtree|Crabtree, R.H.]] Davis, M. W. | journal = [[J. Org. Chem.]] | date = 1986 | volume = 51 | issue = 14 | pages = 2655–2661|doi = 10.1021/jo00364a007}}</ref>
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.
[[Image:Crabtree cat reaction.png|500px|Crabtree catalyst in hydrogenation]]
==References==
<references/>
[[Category:catalysts]]
[[Category:Homogeneous catalysis]]
[[Category:Coordination compounds]]
[[Category:iridium compounds]]
[[Category:Organometallic compounds]]