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 (BuCH)FeN.  In contrast to (CMe)IrCl, (BuCH)IrCl is monomeric.