1,5-Cyclooctadiene

1,5-Cyclooctadiene, (abb. COD or 1,5-COD), is the chemical compound with the chemical formula C₈H₁₂. It is an important ligand in organometallic chemistry.^[1][2]

Additional recommended knowledge

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Synthesis

1,5-Cyclooctadiene can be prepared by dimerization of 1-butene in the presence of a nickel catalyst.

Reactions

1,5-COD binds to metals in as a bis η² ligand, meaning that it is attached to the metal center via both alkene groups. Complexes of the form M(cod)₂, where M is Ni, Pd, or Pt (see image below), are often used as starting materials for the synthesis of these complexes. Metal-COD complexes are attractive because they are sufficiently stable to be isolated but the COD ligands are easily displaced by other ligands. These complexes are more stable than related ethylene complexes. The stability of COD complexes is attributable to the chelate effect.

Synthesis M(COD)₂

The complex Ni(COD)₂ is prepared by reduction of anhydrous nickel acetylacetonate in the presence of the ligand, using triethylaluminium ^[3]

1/3 [Ni(C₅H₇O₂)₂]₃ + 2 COD + 2 Al(C₂H₅)₃ → Ni(COD)₂ + 2 Al(C₂H₅)₂(C₅H₇O₂) + C₂H₄ + C₂H₆

The related complex, Pt(COD)₂, is prepared by a more circuitous route involving the dilithium cyclooctatetraene:^[4]

2 Li + C₈H₈ → Li₂C₈H₈

Li₂C₈H₈ + PtCl₂(COD) + 3 C₇H₁₀ → [Pt(C₇H₁₀)₃] + 2 LiCl + C₈H₈ + C₈H₁₂

Pt(C₇H₁₀)₃ + 2 COD → Pt(COD)₂ + 3 C₇H₁₀

Uses of M(COD)₂

Extensive work has been reported on complexes of COD, much of which can has been described in Inorganic Syntheses volumes 25, 26 and 28. The platinum complex has been used in many syntheses:

Pt(COD)₂ + 3 C₂H₄ → Pt(C₂H₄)₃ + 2 COD

COD complexes are useful as starting materials, one noteworthy example is the reaction:

Ni(cod)₂ + 4 CO_(g) Ni(CO)₄ + 2 COD

The product Ni(CO)₄ is highly toxic, thus it is advantageous to generate it in the reaction vessel as opposed to being dispensed directly.

Complexes with other metals

Many low-valent metals form stable complexes with COD, examples being Mo(COD)(CO)₄, [RuCl₂(COD)]_n, and Fe(COD)(CO)₃. COD is an especially important in the coordination chemistry of rhodium(I) and iridium(I), examples being Crabtree's catalyst and cyclooctadiene rhodium chloride dimer. The square planar complexes [M(COD)₂]⁺ are known (M = Rh, Ir).

Other reactions

COD reacts with borane to 9-Borabicyclo[3.3.1]nonane, which is an important reagent in organic chemistry for hydroborations.

COD forms in a two-step reaction with first disulfur dichloride and then sulfuryl chloride the compound 2,6-dichloro-9-thiabicyclo[3.3.1]nonane^[5] which can be further modified as the di-azide or di-cyano derivative in a nucleophilic substitution aided by anchimeric assistance.

References

1. ^ Buehler, C; Pearson, D.Survey of Organic Syntheses. Wiley-Intersciene, New York. 1970.
2. ^ Shriver, D; Atkins, P.Inorganic Chemistry. W. H. Freeman and Co., New York. 1999.
3. ^ Schunn, R; Ittel, S., "Bis(1,5-Cyclooctadiene) Nickel(0)", Inorganic Syntheses volume 28, 1990
4. ^ Crascall, L; Spencer, J., "Bis(1,5-Cyclooctadiene) Platinum(0)", Inorganic Syntheses, 1990, volume 28.
5. ^ 2,6-Dichloro-9-thiabicyclo[3.3.1]nonane: Multigram Display of Azide and Cyanide Components on a Versatile Scaffold Molecules 2006, 11, 212-218 Online article