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 [( SiO)(2)Al-CH2CH(CH3) (2)] 1a, silicon isobutyl [ Si-CH2CH(CH3) (2)] 1b and a silicon hydride [ Si-H] 1c. Their structural identity was characterized by FT-IR and advanced solid-state NMR spectroscopies (H-1, C-13, Si-29, Al-27 and 2D multiple quantum), elemental and gas phase analysis, and DFT calculations. The reaction involves the formation of a highly reactive monopodal intermediate: [ SiO-Al[CH2CH(CH3)(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, Si-O-Si 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 beta-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 degrees C) under high vacuum (10(-5) mbar) generates Al-H through a beta-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-27-H-1 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?
Cavallo, Luigi;
2015-01-01
Abstract
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 [( SiO)(2)Al-CH2CH(CH3) (2)] 1a, silicon isobutyl [ Si-CH2CH(CH3) (2)] 1b and a silicon hydride [ Si-H] 1c. Their structural identity was characterized by FT-IR and advanced solid-state NMR spectroscopies (H-1, C-13, Si-29, Al-27 and 2D multiple quantum), elemental and gas phase analysis, and DFT calculations. The reaction involves the formation of a highly reactive monopodal intermediate: [ SiO-Al[CH2CH(CH3)(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, Si-O-Si 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 beta-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 degrees C) under high vacuum (10(-5) mbar) generates Al-H through a beta-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-27-H-1 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
