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Cyclopentadienylcobalt dicarbonyl

Cyclopentadienylcobalt dicarbonyl is an organocobalt compound with formula (C<sub>5</sub>H<sub>5</sub>)Co(CO)<sub>2</sub>, abbreviated CpCo(CO)<sub>2</sub>. It is an example of a half-sandwich complex. It is a dark red air sensitive liquid. This compound features one cyclopentadienyl ring that is bound in an η<sup>5</sup>-manner and two carbonyl ligands. The compound is soluble in common organic solvents.

Preparation

CpCo(CO)<sub>2</sub> was first reported in 1954 by Piper, Cotton, and Wilkinson who produced it by the reaction of cobalt carbonyl with cyclopentadiene. It is prepared commercially by the same method:

Co<sub>2</sub>(CO)<sub>8</sub> + 2 C<sub>5</sub>H<sub>6</sub> → 2 C<sub>5</sub>H<sub>5</sub>Co(CO)<sub>2</sub> + H<sub>2</sub> + 4 CO

Alternatively, it is generated by the high pressure carbonylation of bis(cyclopentadienyl)cobalt (cobaltocene) at elevated temperature and pressures:

Co(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub> + 2 CO → C<sub>5</sub>H<sub>5</sub>Co(CO)<sub>2</sub> + "C<sub>5</sub>H<sub>5</sub>"

The compound is identified by strong bands in its IR spectrum at 2030 and 1960&nbsp;cm<sup>−1</sup>.

Reactions

CpCo(CO)<sub>2</sub> catalyzes the cyclotrimerization of alkynes. The catalytic cycle begins with dissociation of one CO ligand forming bis(alkyne) intermediate.

CpCo(CO)<sub>2</sub> + 2 R<sub>2</sub>C<sub>2</sub> → CpCo(R<sub>2</sub>C<sub>2</sub>)<sub>2</sub> + 2 CO

This reaction proceeds by formation of metal-alkyne complexes by dissociation of CO. Although monoalkyne complexes CpCo(CO)(R<sup>1</sup>C<sub>2</sub>R<sup>2</sup>) have not been isolated, their analogues, CpCo(PPh<sub>3</sub>)(R<sup>1</sup>C<sub>2</sub>R<sup>2</sup>) are made by the following reactions:

CpCo(CO)<sub>2</sub> + PR<sub>3</sub> → CO + CpCo(CO)(PR<sub>3</sub>)
CpCoL(PR<sub>3</sub>) + R<sub>2</sub>C<sub>2</sub> → L + CpCo(PR<sub>3</sub>)(R<sub>2</sub>C<sub>2</sub>) (where L = CO or PR<sub>3</sub>)

CpCo(CO)<sub>2</sub> catalyzes the formation of pyridines from a mixture of alkynes and nitriles. Reduction of CpCo(CO)<sub>2</sub> with sodium yields the dinuclear radical [Cp<sub>2</sub>Co<sub>2</sub>(CO)<sub>2</sub>]<sup>−</sup>, which reacts with alkyl halides to give the dialkyl complexes [Cp<sub>2</sub>Co<sub>2</sub>(CO)<sub>2</sub>R<sub>2</sub>]. Ketones are produced by carbonylation of these dialkyl complexes, regenerating CpCo(CO)<sub>2</sub>.

Related compounds

The pentamethylcyclopentadienyl analogue Cp*Co(CO)<sub>2</sub> (CAS RN#12129-77-0) is well studied. The Rh and Ir analogues, CpRh(CO)<sub>2</sub> (CAS RN#12192-97-1) and CpIr(CO)<sub>2</sub> (CAS RN#12192-96-0), are also well known.

References