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Cyclooctadiene rhodium chloride dimer

Cyclooctadiene rhodium chloride dimer is the organorhodium compound with the formula Rh<sub>2</sub>Cl<sub>2</sub>(C<sub>8</sub>H<sub>12</sub>)<sub>2</sub>, commonly abbreviated [RhCl(COD)]<sub>2</sub> or Rh<sub>2</sub>Cl<sub>2</sub>(COD)<sub>2</sub>. This yellow-orange, air-stable compound is a widely used precursor to homogeneous catalysts.

Preparation and reactions

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

2 RhCl<sub>3</sub>·3H<sub>2</sub>O + 2 COD + 2 CH<sub>3</sub>CH<sub>2</sub>OH + 2 Na<sub>2</sub>CO<sub>3</sub> → [RhCl(COD)]<sub>2</sub> + 2 CH<sub>3</sub>CHO + 8 H<sub>2</sub>O + 2 CO<sub>2</sub> + 4 NaCl

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

[RhCl(COD)]<sub>2</sub> + L → [L<sub></sub>Rh(COD)]<sup>+</sup>Cl<sup>−</sup> (where L = PR<sub>3</sub>, alkene, etc. and = 2 or 3)

In this way, chiral phosphines can be attached to Rh. The resulting chiral complexes are capable of asymmetric hydrogenation. A related but still more reactive complex is chlorobis(cyclooctene)rhodium dimer. The dimer reacts with a variety of Lewis bases (L) to form adducts with the stoichiometry RhCl(L)(COD).

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 Rh<sub>2</sub>Cl<sub>2</sub> core is also approximately planar, in contrast to the highly bent structure of cyclooctadiene iridium chloride dimer where the dihedral angle is 86°.

References