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Rhodium-catalyzed hydrogenation
Rhodium catalyzed hydrogenation is a chemical reaction that typically involves the addition of pairs of hydrogen atoms to another compound or element in the presence of a Rhodium complex catalyst. The addition of hydrogen to double or triple bonds in hydrocarbons is a type of redox reaction that can be thermodynamically favorable without a catalyst. For example, the addition of hydrogen to an alkene has a Gibbs free energy change of -101 kJ·mol-1.〔Atkins, et al. Shriver and Atkins’ Inorganic Chemistry W.H. Freeman & Co., 5th Ed., 2010. ISBN 978-1-4292-1820-7.〕 However, the reaction rate for most hydrogenation reactions is negligible without extremely high temperatures or pressures. Rhodium (Rh) is a transition metal, and the complexes it can form act as catalysts to allow hydrogenation and other chemical reactions to occur at lower temperatures and lower pressures. The rhodium complex catalyst is able to bind both the unsaturated substrate as well as the H2 in order to facilitate the transfer of hydrogen to substrate. Other transition metals such as platinum, palladium, and nickel can also act as catalysts in the hydrogenation process.
== Preparation of rhodium hydrogenation catalysts ==
Rhodium hydrogenation catalysts can be derived from cationic complexes such as ()+, where L is a neutral donor ligand and n is either 2 or 3.〔R.R. Schrock, J.A. Osborn: J. Am. Chem. Soc., 98 (1976).〕 Examples of the dienes found in these complexes could be 1,3-cyclohexadiene, norbornadiene, 1,3-butadiene, or 1,5-clycooctadiene. With hydrogen, these species will all yield catalytically active solutions by means of reductive elimination of the diene. The rhodium catalytic precursors are generally yellow or orange in color. As the hydrogen reduces the diene, the color fades and the solution becomes colorless.〔
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