RhCl(1,5-cyclooctadiene) dimer is a tremendously useful Rh(I) source due to its solubility in organic solvents and easy removal of the cyclooctadiene ligand for further derivatization. It is largely used for catalytic hydrogenations. The complex can be synthesized in high yields by following an Inorganic Syntheses procedure which starts from RhCl3 trihydrate, ethanol, and 1,5-cyclooctadiene. Product precipitates after heating to reflux overnight.[1]
Notes:
- RhCl(1,5-cyclooctadiene) dimer is commercially available. Beware of many vendors charging upwards of $800 (USD) per gram. I purchased far more cheaply from Pressure Chemicals. They sell the dimer as well as RhCl3 trihydrate at similar prices (roughly $165 USD per gram).
- If you are going to make it via the Inorganic Syntheses procedure, don’t use sodium carbonate in the reaction. Although HCl forms as byproduct, it does not need to be mopped up for the reaction to occur. There are instances where addition of sodium carbonate resulted in the formation of off-colored products.
- RhCl(1,5-cyclooctadiene) dimer is yellowish orange in color and can be handled in air. I prefer this Rh(I) source to the ethene analog because the latter is less stable and needs to be stored in the freezer. The former can be stored at room temperature indefinitely (we kept it in glovebox).
[1] Giordano, G.; Crabtree, R. H. Inorg. Synth. 1990, 28, 88-90. DOI: 10.1002/9780470132593.ch22.