Surface organometallic chemistry of titanium:: Synthesis, characterization, and reactivity of (Si-O)nTi(CH2C(CH3)3)4-n (n = 1, 2) grafted on aerosil silica and MCM-41

The reaction of tetrakisneopentyl titanium, TiNp4 (1), with the surface of partially dehydroxylated Aerosil silica and MCM-41 and the reactivity of the resultant supported titanium alkyl product with water, alcohols, and oxygen are reported. Two methods of preparation have been investigated and compared for the grafting of TiNp4: (i) reaction of the support with the vapor of the sublimed complex and (ii) impregnation of the support with a solution of the complex. The second method appeared to be more reliable for "larger scale" preparations. The surface species thus obtained were characterized by infrared spectroscopy, solid state NMR, XAFS, elemental analysis, and various test reactions. Whereas on an Aerosil silica partially dehydroxylated at 500 degrees C, SiO2-(500), the surface complex is a monopodal titanium trisalkyl complex, SiO-Ti[CH2C(CH3)(3)](3), 2a, a bipodal titanium bisalkyl complex, ( SiO)(2)Ti[CH2C- CH3)(3)](2), 2b, is obtained as the major species (ca. 65%) with 2a on MCM-41((500)). The reason for this difference in behavior is discussed on the basis of the surface structure. The results obtained from hydrolysis confirmed the structure proposed for the supported alkyl complexes. For the reaction of the alkyl surface complexes with alcohols (MeOH, EtOH, (BuOH)-Bu-t), the surface compounds obtained were characterized by the same techniques and by XPS and UV-vis. The results are consistent with the formation of monosiloxytrisalkoxy titanium complexes on SiO2-(500), SiO-Ti(OR)(3), 3a(OR), and of SiO-Ti((OBu)-Bu-t)(3), 3a(OtBu), and ( SiO)(2)Ti((OBu)-Bu-t)(2), 3b(OtBu), on MCM-41((500)), after reaction with (BuOH)-Bu-t. The supported titanium alkyl, 2a, also reacts with oxygen, leading mainly to SiO-Ti[OCH2C(CH3)(3)](3), probably via an unstable surface compound such as SiO-Ti[OCH2C(CH3)(3)](2)[OOCH2C(CH3)(3)], resulting from the incorporation of two molecules of oxygen in 2a.