No abstract

The monocyclopentadienyl zirconium acetamidinate complexes, (eta(5)-C(5)Me(5))Zr[N(R(1))C(Me)N(R(2))]R(3)R(4) (1-8), have been shown to be remarkably resistant to beta-hydrogen eliminations/abstractions, including the tert-butyl derivative, 3 (R(1) = R(2) = Cy, R(3) = t-Bu, R(4) = Cl), which resists both decomposition and isomerization in solution to temperatures of at least 100 degrees C. Further, two striking examples of an apparent preference for alternative hydrogen-atom abstractions in which complexes 1 and 7/8 that bear isomeric dibutyl substituents are transformed at elevated temperatures to complexes 9 and 10/11 that contain the isomeric butadiene and trimethylenemethane (TMM) C(4) fragments, respectively, are presented. These results serve to not only introduce a new ligand environment for zirconium in which beta-hydrogen elimination/abstraction processes are substantially retarded, but they further document the availability of alternative low-energy hydrogen abstraction pathways for group 4 alkyl complexes.

Insertion of optically pure (R,R)-and meso-(R,S)-1,3-bis(1-phenylethyl)carbodiimide into a Ti-C Me bond of (η 5 -C 5 R 5 )TiMe 3 provides the title compounds (R,R)-3 (R ) H), (R,R)-5 (R ) Me), (R,S)-4 (R ) H), and (R,S)-6 (R ) Me), in high yield. Reaction between (R)-1-tertbutyl, 3-(1-phenylethyl)carbodiimide, and (η 5 -C 5 H 5 )TiMe 3 yields compound 7 in a similar fashion. Variable-temperature 1 H NMR studies unequivocally establish that a low-energy amidinate ring twisting pathway is the exclusive origin of configurational instability in this class of piano-stool complexes. Further, evidence for a dynamic process involving amidinate ring conformation is obtained for (R,S)-6 by a similar study. A conformational analysis of the 1-phenylethyl substituents observed in the solid-state structures of (R,R)-3, (R,R)-5, exo-(R,S)-4, and exo-(R,S)-6, as determined by single-crystal X-ray analysis, provides a rationale for the apparent preferred 1-phenylethyl conformations of these compounds and of the unprecedented amidinate ring conformation (interplane angle of 41.5°) observed for the exo isomer of the meso complex 6.