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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(...

Hydrozirconation of cis-2-butene with Cp*ZrHCl[N(t-Bu)C(Me)N(Et)], generated in situ through hydrogenolysis of Cp*ZrCl(SiMe2Ph)[N(t-Bu)C(Me)N(Et)] (5), proceeds in high yield to produce a 1:2 mixture of the kinetically stable, diastereomeric sec-butyl complexes, 3a and 3b. Hydrozirconation of trans-2-butene under identical conditions provides a 2:1 mixture of 3a and 3b. Isolation of diastereomerically pure 3a was achieved through reaction of Cp*ZrCl2[N(t-Bu)C(Me)N(Et)] (4) with sec-butyllithium to provide a 2:1 ratio of 3a and 3b, followed by fractional crystallization. Crystallographic analysis of 3a establishes the relative configuration of the sec-butyl group with respect to the chiral zirconium center, thereby permitting construction of diastereomeric transition states that explain the origin of high face selectivity in the hydrozirconation of cis-2-butene. Finally, both iodinative zirconium-carbon bond cleavage and insertion of tert-butyl isocyanide into the zirconium-carbon bond of the sec-butyl group of 3a were found to proceed in high yield and with retention of the secondary alkyl structure. Together, these results provide a critical platform upon which efforts directed toward the asymmetric hydrozirconation of alkenes can be based.

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We recently reported evidence that a cyclometalated intermediate can facilitate the polymerization of 1-hexene to append polymer chains to the termini of propyl groups of the Me2Hf­(Cp n‑Propyl)2 catalyst precursor. Herein we provide further mechanistic details on the activation of Me2Hf­(Cp n‑Propyl)2 by B­(C6F5)3 and the polymerization of 1-hexene mainly by applying a battery of mass spectrometry-based techniques. First, a combination of MALDI and CID fragmentation is used to characterize the high molecular mass region (up to 6 kDa) of the isolated poly­(1-hexene) material with attached metallocene. The CID fragmentation patterns are explained by relatively low-energy ligand losses and higher energy hydrocarbon chain degradation via C–C bond cleavage and 1,3-hydrogen shift reactions. Further mechanistic insights are gained by investigating 1-hexene polymerization reaction employing a properly 13C-labeled catalyst activated by B­(C6F5)3. Mass spectrometry analyses, along with supporting NMR experiments, indicate that polymer chain growth from the propylcyclopentadienyl ligand proceeds via a series of 2,1-insertion ring expansions of the hafnium metallacycle. In contrast, free poly­(1-hexene) chains are generated by conventional 1,2-insertions. In addition, six boron-containing species were identified from negative ion mode ESI-QqTOF: [B­(C6F5)3]−•, [H–B­(C6F5)3]−, [CH3–B­(C6F5)3]−, [HO–B­(C6F5)3]−, [C6H13–B­(C6F5)3]−, and [B­(C6F5)4]−.