Abstract
Reaction of triisobutylaluminum with SBA15
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)
Al-CH
CH(CH
)
]
, silicon isobutyl [[triple bond, length as m-dash]Si-CH
CH(CH
)
]
and a silicon hydride [[triple bond, length as m-dash]Si-H]
. Their structural identity was characterized by FT-IR and advanced solid-state NMR spectroscopies (
H,
C,
Si,
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
CH(CH
)
]
], 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
a simple metathesis between the Al-isobutyl and O-Si bonds, while hydride transfer occurs
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
(at 250 °C) under high vacuum (10
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
Al-
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.