Bulky cyclopentadienyl ligands
In the area of organometallic chemistry, a bulky cyclopentadienyl ligand is jargon for a ligand of the type C5H5−nRn− where R is larger than n-alkyl and n = 3 or 4. Representative examples are the tetraisopropyl derivative C5H4− and the tris tert-butyl derivative 1,2,4-C5H23−. These ligands are so large that their complexes behave differently from the pentamethylcyclopentadienyl analogues. Because they cannot closely approach the metal, these bulky ligands stabilize high spin complexes, e.g. 2Fe2I2. These large ligands stabilize highly unsaturated derivatives such as 2Fe2N2.The cyclopentadiene is prepared by alkylation of cyclopentadiene with tert-butyl bromide in the presence of sodium hydride and dibenzo-18-crown-6. The intermediate in this synthesis is di-tert-butylcyclopentadiene. This compound is conveniently prepared by alkylation of cyclobutadiene with tert-butyl bromide under phase-transfer conditions.
Illustrative of the unusual complexes made possible with these bulky ligands is molecular iron nitrido complex 2Fe2N2. In contrast to 2Ir2Cl4, IrCl2 is monomeric.
