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Reaction of triisobutylaluminum with SBA15 700 at room temperature occurs by two parallel pathways involving either silanol or siloxane bridges. It leads to the formation of a well-defined bipodal [([triple bond, length as m-dash]SiO) 2 Al-CH 2 CH(CH 3 ) 2 ] 1a , silicon isobutyl [[triple bond, length as m-dash]Si-CH 2 CH(CH 3 ) 2 ] 1b and a silicon hydride [[triple bond, length as m-dash]Si-H] 1c . Their structural identity was characterized by FT-IR and advanced solid-state NMR spectroscopies ( 1 H, 13 C, 29 Si, 27 Al and 2D multiple quantum), elemental and gas phase analysis, and DFT calculations. The reaction involves the formation of a highly reactive monopodal intermediate: [[triple bond, length as m-dash]SiO-Al-[CH 2 CH(CH 3 ) 2 ] 2 ], with evolution of isobutane. This intermediate undergoes two parallel routes: transfer of either one isobutyl fragment or of one hydride to an adjacent silicon atom. Both processes occur by opening of a strained siloxane bridge, [triple bond, length as m-dash]Si-O-Si[triple bond, length as m-dash] but with two different mechanisms, showing that the reality of "single site" catalyst may be an utopia: DFT calculations indicate that isobutyl transfer occurs via a simple metathesis between the Al-isobutyl and O-Si bonds, while hydride transfer occurs via a two steps mechanism, the first one is a β-H elimination to Al with elimination of isobutene, whereas the second is a metathesis step between the formed Al-H bond and a O-Si bond. Thermal treatment of 1a (at 250 °C) under high vacuum (10 -5 mbar) generates Al-H through a β-H elimination of isobutyl fragment. These supported well-defined Al-H which are highly stable with time, are tetra, penta and octa coordinated as demonstrated by IR and 27 Al- 1 H J-HMQC NMR spectroscopy. All these observations indicate that surfaces atoms around the site of grafting play a considerable role in the reactivity of a single site system.

Well-defined silica supported aluminum hydride: another step towards the utopian single site dream?