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.