No abstract

The reaction of 2 equiv of lithium 1-phenylboratabenzene with ZrCl4 in ether affords bis(1-phenylboratabenzene)zirconium dichloride (2), while [4-tert-C4H9C5H4BPh]2ZrCl2 (3), obtained in a similar manner, was subjected to a single-crystal X-ray diffraction study. The reaction of a solution of 2 and methylaluminoxane (MAO) with C2H4 (1 atm, 25 °C) affords ethylene oligomers (dimers of 1-alkenes as the major product, together with minor amounts of 1- and 2-alkenes).

No abstract

A computational study of olefin polymerization has been performed on 51 zirconocene catalysts. The catalysts can be categorized into three classes according to the ligand framework: class I, Cp2ZrCl2 (10 catalysts), class II, CpIndZrCl2 (38 catalysts), and class III, Ind2ZrCl2 (3 catalysts), Ind = η5-indenyl. Detailed reaction pathways, including chain propagation and chain termination steps, are modeled for ethylene polymerization using zirconocene catalysts. Optimized structures for reaction coordinates indicated the presence of α-agostic interactions in the transition states (TSs) for both the first and second ethylene insertions, as well as in the ethylene π-complex of the ZrnPr cation. However, β-agostic interactions predominate in the cationic n-propyl and n-pentyl intermediates. The calculated activation energy barrier energies show that the TS for the insertion of ethylene into the Zr–CH3 + bond is the highest point on the computed reaction coordinates. Quantitative structure–activity relationship studies were also performed for 38-mixed zirconocene dichlorides. This study, in concert with the previous work, suggests that the type of ring attached to Zr (Cp vs Ind) affects the reaction kinetics and thermodynamics less significantly than the type of substituents attached to the Cp and indenyl rings and that substituent effects are even greater than those arising from changing the metal (Zr vs Hf).

While the synthesis and reactivity of trimethylenemethane (TMM) complexes containing late transition metals is well documented,' the chemistry of high-oxidation-state, earlytransition-metal counterparts remains virtually unexplored. The few known examples display a range of bonding modes; the TMM ligand in Cp*(TMM)TaMeZ2 (Cp* = M e G ) and [ Cp * (TMM)Zr@ -C1)2Li( TMED A)] (TMED A = N, N,N',N'tetramethylethylenediamine) is q4 bound, while in Cp*zZr-(TMM)4 an q3 coordination is observed. Sterically significant substituents on the periphery of the TMM skeleton exert a strong influence. For example, tribenzylidenemethane (TBM) complexes of zirconium, i.e., [Cp*(TBM)ZrClz]-[Li(TMEDA)#, are discrete salts instead of zwitterions and have distorted (intermediate between q3 and q4) TBM-Zr b~n d i n g .~The electronic relationship between the TMM and cyclopentadienyl fragments motivates our early-metal studies: both ligands are formally six-electron donors but differ in their respective charges. Substitution of a cyclopentadienyl ligand for a dianionic six-electron donor is a strategy of current interest in the general area of homogeneous Ziegler-Natta polymerization as it permits construction of neutral group IV counterparts of catalytically important cationic metallocene alkyls as well as incorporation of group V metals into metallocene-like molecules. To this end, ( C~% M~) ( C~B~H I ~)TaMez? Cp*(q4-1,3-butadiene)TaMe2,6 and Cp*(CJ&BNiPr2)TaMez? containing the dianionic dicarbollide, diene, and aminoborollide ligands, respectively, have been prepared. The activities of dicarbollide complexes toward olefin insertion suffer from the hydridic B-H ligand cage constituents, which can bind to electrophilic metal sites.8 Aminoborollide and diene complexes are significantly less active than group IV metallocenes, perhaps as a result of the "reduced" and therefore less electrophilic nature of the metal (Ta(II1) resonance contribution). The TBM ligand is an attractive alternative since it provides considerable steric protection, has no extraneous functionalities, and is strictly dianionic. In this communication we report the synthesis, characterization, and reactivity of (TBM)TaMe3 and complexes of general composition Cp(TBM)TaMez (Cp = C5H5).Slow addition of LiZ(TBM)(TMEDA)z to Me3TaCh in benzene results in the formation of (TBM)TaMe3. Isolation from residual LiCl(TMEDA), species requires efficient removal of TMEDA by three xylene condensation-evaporation cycles on the crude reaction mixture. Extraction with benzene yields (TBM)TaMe3 as an orange powder in 44% isolated yield (l)(a) General review: Jones, M. D.; Kemmit, R. D. W. Adv.(Scheme 1). Similar results are obtained using Liz('Bu-TBM)-(TMEDA)2. These 12-electron complexes are thermally and chemically robust (compare with explosive WMe6).99'0 Structural determination of ('Bu-TBM)TaMes revealed a C3 symmetric molecular geometry (neglecting the tert-butyl group) as shown in Scheme 1." Tantalum is seven coordinate with a "domed" TBM framework containing the three phenyl rings...