Formation and structures of [1,2-bis(N-tert- butylcarbamoyl)cyclopentadienyl]zirconium complexes - Coordination chemistry of a 'fulvenologous' malonic amide anion ligand system

Article abstract of DOI:10.1002/(SICI)1099-0682(199901)1999:1<11::AID-EJIC11>3.0.CO;2-5

Treatment of sodium cyclopentadienide with two molar equivalents of tert-butyl isocyanate yields sodium 1,2-bis(N-tert- butylcarbamoyl)cyclopentadienide (6). The [C₅H₃(1,2-CONHCMe₃)₂]Na reagent 6 adds to Cp₂Zr(CH₃)Cl (8) to yield Cp₂Zr(CH₃)[C₅H₃(CONHCMe₃)₂] (9). In 9 the C₅H₃(CONHCMe₃)₂ ligand is bonded to zirconium through one of its carboxamido-oxygen atoms (κO-coordination). Treatment of 6 with CpZrCl₃(THF)₂ yields CpZrCl₂[C₅H₃(CONHCMe₃)₂](THF) 11. In 11 the 1,2- bis(N-tert-butylcarbamoyl)cyclopentadienide moiety serves as a C(s)-symmetric chelate ligand, binding to zirconium through both carbamoyl oxygens (κ²O,O'-coordination). The same seven-membered metallacyclic structural type is found in the reaction products of 6 with ZrCl₄(THF)₂ in 1:1 and 2:1 molar ratios. The former yields the distorted octahedral complex ZrCl₃[C₅H₃(CONHCMe₃)₂](THF) (12), the latter gives the chiral octahedral system ZrCl₂[C₅H₃(CONHCMe₃)₂]₂ (13). In solution, complex 13 undergoes a thermally induced enantiomerization process (Λ ? Δ interconversion), for which a Gibbs activation energy of ΔG(+/+)(enant) = 14.0 ± 0.3 kcal mol^{- 1} was determined by dynamic ¹H-NMR spectroscopy. The κ²O,O'-coordination of the [C₅H₃(1,2-CONHCMe₃]^- ligand in the complexes 11, 12, and 13 was secured by X-ray crystal structure analyses.