Bulky cyclopentadienyl ligands
# Bulky cyclopentadienyl ligands

In the area of organometallic chemistry, a bulky cyclopentadienyl ligand is jargon for a ligand of the type C $_5H$ $_{5−n}R^−$ $_n$ where R is a branched alkyl and n = 3 or 4. Representative examples are the tetraisopropyl derivative C $_5H^iPr^−$ $_4$ and the tris(tert-butyl) derivative 1,2,4-C $_5H$ $_2^tBu^−$ $_3.$ 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, such as $(C_5H_2^tBu_3)_2Fe_2I_2.$ These large ligands stabilize highly unsaturated derivatives such as $(C_5H_2^tBu_3)_2Fe_2N_2.$

## Synthesis and reactions

The (tert-butyl)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 $(^tBu_3C_5H_2)_2Fe_2N_2.$ In contrast to $(C_5Me_5)_2Ir_2Cl_4,$ $(^tBu_3C_5H_2)IrCl_2$ is monomeric.

