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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.

The neutral dimethyl pentamethylcyclopentadienylzirconium acetamidinate, (eta(5)-C(5)Me(5))ZrMe(2)[N(t-Bu)C(Me)N(Et)], can serve as a highly active initiator for the living Ziegler-Natta polymerization of alpha-olefins to produce polyolefins of narrow polydispersity (D < or = 1.05) when "activated" through mono demethylation by a substoichiometric amount of the borate, [PhNMe(2)H][B(C(6)F(5))(4)]. The mechanism by which this living polymerization proceeds is through a process of degenerative transfer involving rapid and reversible methyl group exchange between cationic (active) zirconium propagating centers and neutral (dormant) methyl, polymeryl zirconium end groups. Facile metal-centered epimerization of the dormant species is responsible for a loss of stereocontrol during propagation that produces iso-rich material in contrast to the pure isotactic polymer microstructure obtained when degenerative transfer is not present. By turning degenerative transfer "on" and "off" between successive monomer polymerizations, a successful strategy for the production of monomodal stereoblock polyolefins of narrow polydispersity and tunable block length has been demonstrated.

A canary yellow block with dimensions 0.261 x 0.167 x 0.154mm 3 was placed and optically centered on the Bruker SMART CCD system at -80°C. The initial unit cell was indexed using a least-squares analysis of a random set of reflections collected from three series of 0.3° wide ω scans (25 frames/series) that were well distributed in reciprocal space. Data frames were collected [MoKα] with 0.3° wide ω-scans, 40 seconds per frame, 606 frames per series. Five complete series were collected with an additional 160 frames a repeat of the first series for redundancy and decay purposes, with a crystal to detector distance of 4.556cm, thus providing a complete sphere of data to 2θ max =50°. A total of 38144 reflections were collected and corrected for Lorentz and polarization effects and absorption using Blessing's method as incorporated into the program SADABS 1,2 with 10464 unique [R(int)=0.0693]. Structural determination and Refinement:All crystallographic calculations were performed on a Personal computer (PC) with dual Pentium 450MHz processors and 384MB of extended memory. The SHELXTL 3 program package was now implemented, XPREP, to determine the probable space group and set up the initial files. System symmetry, lack systematic absences and intensity statistics indicated the centrosymmetric triclinic group P-1 (no.2). The structure was determined by use of Patterson methods with the successful location of the central Zirconium atom using the program XS 4 . The structure was refined with XL 5 . After the initial refinement difference-Fourier cycle, additional atoms were located and input. After several of these refinement difference-Fourier cycles, all of the atoms were refined isotropically, then anisotropically. A disordered chlorobenzene molecule was modeled and refined with three probable orientations. A centroid was calculated for the pentamethylcyclopentadienyl group. Hydrogen atoms were placed in calculated positions. The final structure was refined to convergence [∆/σ ≤ 0.001] with R(F)=8.87%, wR(F 2 )=11.43%, GOF=1.009 for all 10464 unique reflections [R(F)=4.51%, wR(F 2 )=10.21% for those 6868 data with Fo > 4σ(Fo)]. A final difference-Fourier map was featureless indicating the structure is therefore both correct and complete.The function minimized during the full-matrix least-squares refinement was Σw(Fo 2-Fc 2 ) where w=1/[σ 2 (Fo 2 )+(0.0590*P) 2 +0.0*P] and P=(max(Fo 2 ,0)+2*Fc 2 )/3. An empirical correction for extinction was also attempted but found to be negative and therefore not applied.53.78(11) N(1)-Zr(1)-C( 16) 30.17(11) CT1-Zr(1)-C( 16) 122.21( 9) O(1)-Zr(1)-C( 16) 107.16(10) C(11)-Zr(1)-C(16) 107.43(13) N(2)-Zr(1)-C(16) 29.95(10) C(5)-Zr(1)-C(16) 97.80(10) C(1)-Zr(1)-C(16) 98.40(11) C(3)-Zr(1)-C(16) 150.75(11) C(4)-Zr(1)-C(16) 125.78(11) C(2)-Zr(1)-C(16) 126.86(12) C(2)-C(1)-C(5) 107.6(3) C(2)-C(1)-C(6) 126.2(4) C(5)-C(1)-C(6) 125.9(4) C(2)-C(1)-Zr(1) 74.4(2) C(5)-C(1)-Zr(1) 73.07(19) C(6)-C(1)-Zr(1) 123.1(2) C(1)-C(2)-C(3) 108.7(3) C(1)-C(2)-C(7) 126.4(4) C(3)-C(2)-C(7) 124.3(4) C(1)-C(2)-Zr(...