Surface organometallic chemistry of Hf(CH₂tBu)₄ on silica and silica-alumina: Reaction of the resulting grafted hafnium neopentyl with dihydrogen

[Hf(CH₂tBu)₄], 1, has been found to react at room temperature with an aerosil silica dehydroxylated at different temperatures θ (θ = 800, 500, 200 °C) to give a modified surface referred to as 1/SiO_2-(θ) and with a silica-alumina to give a modified surface referred to as 1/SiO₂-Al₂O_3-(500). With SiO_2-(800) a single-site monosiloxy surface complex [(≡SiO)Hf(CH₂tBu)₃], 1-SiO_2-(800), is obtained. Contrarily, with SiO_2-(500) a monosiloxy 1-SiO _2-(800) and a bis-siloxy surface complex [(≡SiO) ₂Hf(CH₂tBu)₂], 2-SiO_2-(500), are formed in a ratio of 70%:30%. With SiO_2-(200), there is mainly the formation of the bis-siloxy surface complex (up to 90%) but in different local environments: (≡SiO)₂Hf(CH₂tBu)₂, 2-SiO_2-(200), and (≡SiO)₂(≡SiOH)Hf(CH ₂tBu)₂, 2′-SiO_2-(200). Finally with SiO₂-Al₂O_3.(500), two major neutral surface complexes are formed: the monosiloxy 1-SiO₂-Al₂O _3-(500) and the bisiloxy 2-SiO₂-Al₂O _3.(500), as well as a third complex, 3, which is not as well-defined and may be cationic. Under hydrogen at 150 °C, 17 h, both modified surface 1/SiO_2-(θ) and 1/SiO₂-Al₂O _3-(500) afford the same surface hydrides but in different proportion and diverse surface "local, environments". The formation of these hydrides is concomitant with the formation of [(≡SiO)₂Si(H) ₂] and [(≡SiO)₃SiH]. With 1/SiO₂-Al ₂O_3.(500), the formation of [(≡SiO)_nAlH] is also observed. The major surface hydride in the hydrogenolysis of 1/SiO _2-(800), 1/SiO_2-(500), and l/SiO₂Al ₂O_3-(500) is (≡SiO)₂Hf(H)₂, whereas in the hydrogenolysis of 1/SiO_2-(200), (≡SiO) ₃Hf(H) forms preferentially. All these alkyl and hydride surface complexes have been fully characterized by elemental analysis, labeling experiments, infrared, ¹H/¹³C solid-state NMR, and ¹H DQ solid-state NMR.