Cyclooctadiene rhodium chloride dimer

Cyclooctadiene rhodium chloride dimer

Names
IUPAC names di-μ-chlorido- bis[η^2,η^2-(cycloocta-1,5-diene)rhodium]
Other names Cyclooctadiene rhodium chloride dimer
Identifiers
CAS Number	12092-47-6
3D model (Jmol)	Interactive image
ChemSpider	21171524
ECHA InfoCard	100.031.949
InChI InChI=1S/2C8H12.2ClH.2Rh/c2*1-2-4-6-8-7-5-3-1;;;;/h2*1-2,7-8H,3-6H2;2*1H;;/q;;;;2*+1/p-2/b2*2-1-,8-7-;;;; Key: QSUDXYGZLAJAQU-MIXQCLKLSA-L InChI=1/2C8H12.2ClH.2Rh/c2*1-2-4-6-8-7-5-3-1;;;;/h2*1-2,7-8H,3-6H2;2*1H;;/q;;;;2*+1/p-2/b2*2-1-,8-7-;;;;/r2C8H12.2ClRh/c2*1-2-4-6-8-7-5-3-1;2*1-2/h2*1-2,7-8H,3-6H2;;/b2*2-1-,8-7-;; Key: QSUDXYGZLAJAQU-PXXGERDABU
SMILES Cl[Rh].Cl[Rh].C=1CC\C=C/CCC=1.C/1C\C=C/CC\C=C\1
Properties
Chemical formula	C16H24Cl2Rh2
Molar mass	493.0806 g/mol
Density	1.93 g/cm^3
Melting point	243 °C (469 °F; 516 K)
Solubility in other solvents	dichloromethane
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

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]

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 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 → [L_nRh(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.^[2] A closely related but still more reactive complex is chlorobis(cyclooctene)rhodium dimer.

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,^[3] 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. W. S. Knowles (2003). "Asymmetric Hydrogenations (Nobel Lecture 2001)". Advances in Synthesis and Catalysis. 345 (1–2): 3. doi:10.1002/adsc.200390028.
3. "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