Characterization of surface hydride hafnium complexes on alumina by a combination of experiments and DFT calculations

The hydrogenolysis at 150 °C of the surface complexes obtained by grafting Hf(CH₂tBu)₄ with A-alumina yields several hydride complexes. They have been characterized by combining various experimental techniques (IR, DQ ¹H NMR, and EXAFS) and DFT calculations. Two main conclusions are drawn from experiments: first, all Hf-CH₂tBu bonds are hydrogenolized in Hf-H ones, while most of the Al-CH₂tBu bonds remain intact. Second, there is mainly formation of monohydride surface complex either as a trialuminoxy monohydride (AlO)₃HfH or a cationic bisaluminoxy [(Al_IVO)(Al_SO))HfH]⁺[Al-CH ₂tBu]^-. The DFT calculations of the possible species show that all the considered transformations are thermodynamically favorable. Moreover, the calculated bond lengths are in good agreement with the EXAFS values confirming the nature of the hydride complexes obtained at this temperature.