Structural characterization of Al10O6iBu16(mu-H)2, a high aluminum content cluster: further studies of methylaluminoxane (MAO) and related aluminum complexes.

The first structurally characterized isobutyl-containing aluminoxane compound is presented. The Al10O6iBu16(mu-H)2 (I) cluster is produced from neat octakis-isobutyltetraluminoxane (Al4O2iBu8) at 80 degrees C in 6-8 h followed by slow crystallization. The crystal is triclinic (space group P1) with the molecule lying on an inversion center. This aluminoxane contains both nearly linear, 154(2) degrees, aluminum-bridging hydrides and three-coordinate aluminum sites. Solid-state 27Al magic-angle spinning (MAS) NMR experiments were done at 19.6 and 40 T (833 MHz and 1.703 GHz, 1H) and at 30-35 kHz spinning speeds, leading to the determination of the Cq and eta values for the two four-coordinate Al sites and a lower limit of Cq for the three-coordinate Al site. Geometry-optimized restricted Hartree-Fock calculations at the double-zeta level of an idealized structure (methyl substituted, D2h geometry) yielded Cq and eta in close agreement with experiment; Cq agrees within 3 MHz.

Activation of metallocenes for olefin polymerization as monitored by IR spectroscopy.

Binary mixtures of Cp2ZrMe2, Cp2ZrCl2, dimethylaluminum chloride, trimethylaluminum, and methylaluminoxane (MAO), as well as Cp2ZrMe2 with boron-based activators, have been studied by in situ IR spectroscopy (Cp = cyclopentadienyl, Me = methyl). The position of a strong band near 800 cm(-1), corresponding to the out-of-plane vibration of the Cp hydrogen atoms, is sensitive to the bonding environment around Zr and can be used to monitor reactions and the formation of new products in these mixtures. Harmonic frequencies determined by density functional theory correlate well with experimental values and have been used to assist in the interpretation of the data. The frequency of the Cp out-of-plane vibration, ranging from 797 to 832 cm(-1) in our experiments, is found to increase with increasing electron density on the Cp ring and decreasing Zr-Cp distance. In the mixture of MAO and Cp2ZrMe2, a stable complex is rapidly formed at low Al/Zr ratios. A mechanism that may explain the need for a large MAO excess is proposed for the activation of metallocenes with MAO. The proposed mechanism involves the formation of dimers or oligomers of MAO cages that tend to dissipate the charge of the anion. This destabilization of the Cp2ZrMe2-MAO complex facilitates the formation of the catalytically active cation.