Cyclooctadiene rhodium chloride dimer

Cyclooctadiene rhodium chloride dimer is the organorhodium compound with the formula Rh2Cl2(C8H12)2, commonly abbreviated [RhCl(COD)]2 or Rh2Cl2(COD)2. This yellow-orange, air-stable compound is a widely used precursor to homogeneous catalysts.[1]

Cyclooctadiene rhodium chloride dimer
Names
IUPAC name
di-μ-chlorido-bis[η2,η2-(cycloocta-1,5-diene)rhodium]
Other names
Cyclooctadiene rhodium chloride dimer
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.031.949
EC Number
  • 235-157-6
Properties
C16H24Cl2Rh2
Molar mass 493.0806 g/mol
Density 1.93 g/cm3
Melting point 243 °C (469 °F; 516 K)
Solubility in other solvents dichloromethane
Hazards
GHS pictograms
GHS Signal word Warning
GHS hazard statements
 H302, H315, H317, H319, H335, H411
P261, P264, P270, P271, P272, P273, P280, P301+312, P302+352, P304+340, P305+351+338, P312, P321, P330, P332+313, P333+313, P337+313, P362, P363, P391, P403+233, P405, P501
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Preparation and reactions

The synthesis of [RhCl(COD)]2 involves heating a solution of hydrated rhodium trichloride with 1,5-cyclooctadiene in aqueous ethanol in the presence of sodium carbonate:[1][2]

2 RhCl3·3H2O + 2 COD + 2 CH3CH2OH + 2 Na2CO3 → [RhCl(COD)]2 + 2 CH3CHO + 8 H2O + 2 CO2 + 4 NaCl

[RhCl(COD)]2 is principally used as a source of the electrophile "[Rh(COD)]+."

[RhCl(COD)]2 + nL → [LnRh(COD)]+Cl− (where L = PR3, alkene, etc. and n = 2 or 3)

In this way, chiral phosphines such as chiraphos, DIPAMP, and DIOP have been attached to Rh. The resulting chiral catalysts are capable of asymmetrically hydrogenating certain prochiral alkenes.[3] A closely related but still more reactive complex is chlorobis(cyclooctene)rhodium dimer.


The dimer reacts with a variety of Lewis bases (B:) to form adducts B:RhCl(COD). Its reaction with 1-methylimidazole and the corresponding enthalpy are:

1-CH3NC3N: + 1/2Rh2Cl2(COD)2 → 1-CH3NC3N:RhCl(COD)   ΔH= -40.kJ mol-1

This enthalpy corresponds to the enthalpy change for a reaction forming one mole of the product, 1-CH3NC3N:RhCl(COD), from the acid dimer. The dissociation energy for cyclooctadiene rhodium chloride dimer, which is an energy contribution prior to reaction with the donor,

Rh2Cl2(COD)2 →2 RhCl(COD)

has been estimated to be 48 kJ mol-1 by the ECW model.

Structure

The molecule consists of a pair of square planar Rh centers bound to a 1,5-cyclooctadiene and two chloride ligands that are shared between the Rh centers. The Rh2Cl2 core is also approximately planar,[4] in contrast to the highly bent structure of cyclooctadiene iridium chloride dimer where the dihedral angle is 86°.

References
  1. Giordano, G.; Crabtree, R. H. “Di-μ-chloro-bis(η4-1,5-cyclooctadiene)dirhodium(I)” Inorganic Syntheses, 1990, volume 28, pages 88-90. doi:10.1002/9780470132593.ch22
  2. Chatt, J.; Venanzi, L. M. (1956). "Olefin Complexes of Rhodium". Nature. 177: 852–3. doi:10.1038/177852b0.CS1 maint: uses authors parameter (link)
  3. W. S. Knowles (2003). "Asymmetric Hydrogenations (Nobel Lecture 2001)". Advanced Synthesis & Catalysis. 345 (1–2): 3. doi:10.1002/adsc.200390028.
  4. "Di-μ-chloro-bis[(cis,cis-η4-1,5-cyclooctadiene)rhodium(I)]: a redetermination" De Ridder, Kirk J. A. Acta Crystallographica, Section C: Crystal Structure Communications 1994, C50, 1569-72. doi:10.1107/S0108270194001459
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