Regio- and stereospecific syntheses and structural characterization of alkyl-substituted 1,3-diene complexes of cyclopentadienylzirconium amidinates of the general formula: (η5-C5Me5)Zr[N(iPr)C(Me)N(iPr)](σ2,π-C4H5R) (R = Me or Et)

Alkylation of Cp∗Zr[N(iPr)C(Me)N(iPr)](Cl)2 (Cp∗ = η5-C5Me5) (4) with 2 equiv. of n-hexyllithium in diethylether at −30 °C provided Cp∗Zr[N(iPr)C(Me)N(iPr)](n-C6H13)2 (5) in excellent yield. In solution, 5 undergoes thermally induced loss of “C6H16” to provide a 35% isolated yield of the 1,3-hexadiene complex, Cp∗Zr[N(iPr)C(Me)N(iPr)](C4H5Et) (6), in regio- and diastereochemically pure form. Ligand substitution of the η2-styrene group in Cp∗Zr[η2-CH2CH(C6H5)][N(iPr)C(Me)N(iPr)] (3) was achieved using an excess of 2-methyl-1,3-butadiene (isoprene) in toluene at 25 °C to provide Cp∗Zr[N(iPr)C(Me)N(iPr)](isoprene) (7) in regio- and stereochemically pure form. In similar fashion, an excess of cis,trans-1,3-pentadiene was used to separately prepare the isomeric diene complex Cp∗Zr[N(iPr)C(Me)N(iPr)](1,3-pentadiene) (8), once again in regio- and stereospecific fashion. Detailed experimental investigations of the molecular and electronic structures of 6–8, as performed in solution by 1D and 2D NMR spectroscopy and in the solid state by single-crystal X-ray analysis, revealed that the best structural description for these compounds is that of the Zr(IV) σ2,π-metallacyclopent-3-ene limiting resonance form rather than as a Zr(II) η4-diene complex.