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Friday, August 18, 2017

Preparation of η5-pentamethylcyclopentadienyl rhodium(III) dichloride [Cp*RhCl2]2 - Lu Le Laboratory

Preparation of η5-pentamethylcyclopentadienyl rhodium(III) dichloride

Pentamethylcyclopentadienyl rhodium dichloride is an organometallic compound with the formula [(C5(CH3)5RhCl2)]2, commonly abbreviated [Cp*RhCl2]2 This dark red air-stable diamagnetic solid is a reagent in organometallic chemistry.1

Cp*RhCl2 dimer has been widely used as a homogeneous catalyst for C-H bond activation,2 C-C bond formation,3 and alkenylation and annulation of arenes and heteroarenes with alkynes.4, 5 The synthesis of the catalyst can be readily achieved by reacting rhodium trichloride trihydrate with 1,2,3,4,5-pentamethylcyclopentadiene in hot methanol.


Chemicals

1.      Rhodium trichloride trihydrate (2.0 g, 8.4 mmol, 国药集团 Sinopharm Group Co. Ltd.)
2.      pentamethylcyclopentadiene (1.2 g or 1.380 mL, 8.8 mmol, Alfa Aesar)
3.      methanol (60 mL)

Procedure

Rhodium trichloride trihydrate (2.0 g, 8.4 mmol, 国药集团 Sinopharm Group Co. Ltd.), pentamethylcyclopentadiene (1.2 g or 1.380 mL, 8.8 mmol, Alfa Aesar), methanol (60 mL) and a magnetic stirring bar are placed in a 100-mL round-bottomed flask fitted with a reflux condenser.

The mixture is refluxed gently under nitrogen for 48h with stirring. The reaction mixture is allowed to cool to room temperature and the dark red precipitate is filtered off in air through a glass sinter. 


 The red filtrate is reduced involume to 10-mL using a rotary evaporator to give more red crystals that were combined with the first crop and washed with diethyl ether (3 x 10 mL).


Air drying gives 2.25g (95% yield) of [Rh(η5-C5Me5)C12]2, which is pure enough for most purposes. If required, the product may be recrystallized by dissolving in a minimum volume of chloroform, filtering if necessary, and slowly adding twice that volume of hexane.
(The procedure was modified form White et. al. )


Note: The procedure was modified from White’s et. al. work1 

References
(1) White, C.; Yates, A.; Maitlis, Peter M. (1992). "(η5-Pentamethylcyclopentadienyl)Rhodium and -Iridium Compounds". Inorg. Synth., 29, 2007, 228-234. doi:10.1002/9780470132609.ch53.  
(2) Colby, D. A., Tsai, A. S., Bergman, R. G., & Ellman, J. A., "Rhodium catalyzed chelation-assisted C–H bond functionalization reactions." Acc.Chem. Res., 2012, 45 (6), pp 814–825
(3) Colby, D. A., Bergman, R. G., & Ellman, J. A., “Rhodium-catalyzed C− C bond formation via heteroatom-directed C− H bond activation.” Chem.Rev., 2010, 110 (2),pp 624–655
(4) Boyarskiy, V. P., Ryabukhin, D. S., Bokach, N. A., & Vasilyev, A. V., “Alkenylation of arenes and heteroarenes with alkynes.” Chem. Rev., 2016, 116 (10),pp 5894–5986
(5) Manan, R. S., & Zhao, P., “Merging rhodium-catalysed CH activation and hydroamination in a highly selective [lsqb] 4+ 2 [rsqb] imine/alkyne annulation.” Nat. Commun., 2016, 7, 11506. doi:10.1038/ncomms11506






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